ACTA FACULTATIS XYLOLOGIAE ZVOLEN 62 2/2020

TECHNICKÁ UNIVERZITA VO ZVOLENE DREVÁRSKA FAKULTA

ACTA FACULTATIS XYLOLOGIAE ZVOLEN

VEDECKÝ ČASOPIS SCIENTIFIC JOURNAL

62 2/2020

Scientific journal Acta Facultatis Xylologiae Zvolen publishes peer-reviewed scientific papers covering the fields of wood: structure and properties, wood processing, machining and drying, wood modification and preservation, thermal stability, burning and fire protection of lignocelluloses materials, furniture design and construction, wooden constructions, economics and management in wood processing industry. The journal is a platform for presenting reports and reviews of books of domestic and foreign authors.

Vedecký časopis Acta Facultatis Xylologiae Zvolen uverejňuje pôvodné recenzované vedecké práce z oblastí: štruktúra a vlastnosti dreva, procesy spracovania, obrábania, sušenia, modifikácie a ochrany dreva, termickej stability, horenia a protipožiarnej ochrany lignocelulózových materiálov, konštrukcie a dizajnu nábytku, drevených stavebných konštrukcií, ekonomiky a manažmentu drevospracujúceho priemyslu. Poskytuje priestor aj na prezentáciu názorov formou správ a recenzií kníh domácich a zahraničných autorov.

VEDECKÝ ČASOPIS DREVÁRSKEJ FAKULTY, TECHNICKEJ UNIVERZITY VO ZVOLENE 62 2/2020 SCIENTIFIC JOURNAL OF THE FACULTY OF WOOD SCIENCES AND TECHNOLOGY, TECHNICAL UNIVERSITY IN ZVOLEN 62 2/2020 Redakcia (Publisher and Editor’s Office): Drevárska fakulta (Faculty of Wood Sciences and Technology) T. G. Masaryka 24, SK-960 01 Zvolen, Slovakia Redakčná rada (Editorial Board): Predseda (Chairman): Vedecký redaktor (Editor-in-Chief): Členovia (Editors):

Jazykový editor (Proofreader): Technický redaktor (Copy Editor):

prof. Ing. Ján Sedliačik, PhD prof. Ing. Ladislav Dzurenda, PhD. prof. RNDr. František Kačík, PhD. prof. RNDr. Danica Kačíková, PhD. prof. Ing. Jozef Kúdela, CSc. prof. Ing. Ladislav Reinprecht, CSc. prof. Ing. Jozef Štefko, CSc. doc. Ing. Pavol Joščák, CSc. doc. Ing. Hubert Paluš, PhD. Mgr. Žaneta Balážová, PhD. Antónia Malenká

Medzinárodný poradný zbor (International Advisory Editorial Board): Pavlo Bekhta (UA), Nencho Deliiski (BG), Vlado Goglia (HR), Denis Jelačić (HR), Bohumil Kasal (USA), Wojciech Lis (PL), Remy Marchal (FR), Miloslav Milichovský (CZ), Róbert Németh (HU), Peter Niemz (CH), Kazimierz A. Orlowski (PL), Franc Pohleven (SI), František Potůček (CZ), Włodzimierz Prądzyński (PL), Alfréd Teischinger (AT), Jerzy Smardzewski (PL), Mikuláš Šupín (SK), Richard P. Vlosky (USA), Rupert Wimmer (AT) Vydala (Published by): Technická univerzita vo Zvolene, T. G. Masaryka 2117/24, 960 01 Zvolen, IČO 00397440, 2020 Náklad (Edition) 150 výtlačkov, Rozsah (Pages) 174 strán, 15,04 AH, 150,13 VH Tlač (Printed by): Vydavateľstvo Technickej univerzity vo Zvolene Vydanie I. – október 2020 Periodikum s periodicitou dvakrát ročne Evidenčné číslo: 3860/09 Časopis Acta Facultatis Xylologiae Zvolen je registrovaný v databáze (Indexed in): Web of Science, SCOPUS, ProQuest, AGRICOLA, Russian Science Citation Index Za vedeckú úroveň tejto publikácie zodpovedajú autori a recenzenti.

Rukopis neprešiel jazykovou úpravou Všetky práva vyhradené. Nijaká časť textu ani ilustrácie nemôžu byť použité na ďalšie šírenie akoukoľvek formou bez predchádzajúceho súhlasu autorov alebo vydavateľa.

© Copyright by Technical University in Zvolen, Slovak Republic.

ISSN 1336–3824

CONTENTS

01. ANTON GEFFERT – JARMILA GEFFERTOVÁ – EVA VÝBOHOVÁ: SURFACE CHANGES OF BEECH AND PINE WOOD UNDER THE INFLUENCE OF UV RADIATION ....................................................................

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02. RICHARD HRČKA: THE INFLUENCE OF SELECTED MODIFYING TEMPERATURES ON SPRUCE WOOD EMISSIVITY ................................ 19 03 IGOR NOVÁK – IGOR KRUPA – JÁN SEDLIAČIK – ONDREJ ŽIGO – PETER JURKOVIČ – JÁN MATYAŠOVSKÝ: INVESTIGATION INTO MECHANICAL, SURFACE AND ADHESIVE PROPERTIES OF DATE PALM WOOD-POLYOLEFIN MICRO COMPOSITES ................................. 27 04. JOZEF KÚDELA: SURFACE PROPERTIES OF A MEDIUM DENSITY FIBREBOARD EVALUATED FROM THE VIEWPOINT OF ITS SURFACE TREATMENT ................................................................................. 35 05. NENCHO DELIISKI – LADISLAV DZURENDA – NENO TRICHKOV – NATALIA TUMBARKOVA: COMPUTING THE 2D TEMPERATURE FIELD IN NON-FROZEN LOGS AT CHANGING ATMOSPHERIC TEMPERATURE AND DURING THEIR SUBSEQUENT AUTOCLAVE STEAMING ....................................................................................................................... 47 06. GABRIELA SLABEJOVÁ – MÁRIA ŠMIDRIAKOVÁ – JÁN SVOCÁK: INTERLAYER WITH MICROCAPSULES AND ITS INFLUENCE ON THE SURFACE FINISH QUALITY OF WOOD ...................................................... 61 07. IVETA ČABALOVÁ – MARTIN ZACHAR – MICHAL BÉLIK – KRISTÍNA MAJERSKÁ: ASSESSMENT OF THE FLAMGARD RETARDANT EFFICCIENCY DURING THE THERMAL LOADING OF SPRUCE WOOD (PICEA ABIES L.) .............................................................. 75 08. OLENA PINCHEVSKA – JÁN SEDLIAČIK – OLHA BARANOVA – VALENTYN GOLOVACH – MYKOLA VASYLENKO – KONSTANTIN SHEVCHENKO – YURIY LAKYDA: ACOUSTIC DEFECTOSCOPY OF VENEER LAYERED COMPOSITE MATERIALS ... 89 09. PAVLIN VITCHEV – ZHIVKO GOCHEV – GEORGI VUKOV: THE INFLUENCE OF SOME FACTORS ON THE VIBRATIONS GENERATED

BY WOODWORKING SPINDLE MOULDER MACHINE WHEN PROCESSING SPECIMENS FROM BEECH WOOD .................................... 99 10. ZHIVKO GOCHEV – PAVLIN VICHEV: DETERMINATION OF PERFORMANCE INDICATORS OF PCD ABRASIVE WHEELS FOR SHARPENING TUNGSTEN CARBIDE WOOD CUTTING TOOLS ............ 109 11. ANNA MUKANOVA – YURY LOZHKIN – MIKHAIL CHERNYCH – VLADIMIR STOLLMANN: COMBINATION OF WOOD AND GLASS IN THE SET OF DECORATIVE ITEMS FOR INTERIOR DESIGN ............. 115

12. SILVIA LORINCOVÁ – ĽUBICA BAJZÍKOVÁ – IVETA OBORILOVÁ – MILOŠ HITKA: CORPORATE CULTURE IN SMALL AND MEDIUM-SIZED ENTERPRISES OF FORESTRY AND FORESTBASED INDUSTRY IS DIFFERENT ............................................................... 121 13. MARIANA SEDLIAČIKOVÁ – ZUZANA STROKOVÁ – DENISA MALÁ – DANA BENČIKOVÁ – MARCEL BEHÚN: PSYCHOLOGICAL ASPECTS AND EMOTIONS EVOKED BY IMPLEMENTING THE CONTROLLING SYSTEM IN WOOD-PROCESSING ENTREPRISES IN SLOVAKIA ........................................................................................................ 137 14. EMILIA GRZEGORZEWSKA – MARIANA SEDLIAČIKOVÁ – JOSEF DRÁBEK – MARCEL BEHÚN: EVALUATING THE INTERNATIONAL COMPETITIVENESS OF POLISH FURNITURE MANUFACTURING INDUSTRY IN COMPARISON TO THE SELECTED EU COUNTRIES ......... 149 15. ERIKA LOUČANOVÁ – MIRIAM OLŠIAKOVÁ: CONSUMERS' PERCEPTION OF RETRO-INNOVATION OF WOOD PRODUCTS ........... 165

ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 5−18, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.01

SURFACE CHANGES OF BEECH AND PINE WOOD UNDER THE INFLUENCE OF UV RADIATION Anton Geffert - Jarmila Geffertová - Eva Výbohová ABSTRACT The influence of UV radiation on two different types of wood – European beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.) was studied. The chemical composition of the original wood of the monitored woods (TEE, holocellulose, cellulose and lignin) was determined by standard analytical procedures. Changes in color characteristics (brightness, lightness L*, green-red coordinate a*, blue-yellow coordinate b* and total color difference ∆E*) were measured at the irradiation times of 30, 60, 90, 120 and 150 minutes.

Chemical changes in the surface layers of irradiated wood were monitored by ATR-FTIR spectroscopy. Chemical analysis of the monitored wood samples showed higher content of cellulose, lignin and extractives and at the same time lower content of holocellulose and also hemicelluloses in pine wood than in beech wood, which affected the differences in the color changes of irradiated wood. More significant color changes were observed in pine wood compared to beech wood. A decrease in brightness was 37% of the absolute value for pine and 31% for beech, a decrease in lightness L* was 11% for pine and 9.7% for beech, and the total color difference ∆E* was 15.0 for pine and 11.3 for beech. The biggest color changes of color characteristics were recorded after the first 30 minutes of irradiation. The changes in the FTIR spectra of the surface of the irradiated wood showed that UV radiation caused the degradation of lignin and the formation of conjugated and unconjugated carbonyls and quinoid structures responsible for yellowing the wood surface. A decrease in the ratio of the absorbance A1510/A1370 (lignin / carbohydrates) was most significant after the first 30 minutes of irradiation (30% for pine and 43% for beech). However, an increase in carbonyl content was more significant in pine (3-fold increase) compared to beech (2-fold increase).

The dependence of the ratio of "relative" lignin absorbances and carbonyls on the total color difference ∆E* in both monitored woods showed a linear increasing trend in the case of carbonyls and a slightly decreasing trend in the case of lignin.

Key words: beech, pine, UV radiation, chemical characteristics, CIEL*a*b* color space, ATR-FTIR spectroscopy, lignin degradation, carbonyl formation.

INTRODUCTION Wood is a natural material with a cellular structure, which represents a complicated complex of heterogeneous macromolecular substances. The main components are: cellulose (35–50%), hemicellulose (20–35%), lignin (15–35%) and accompanying components (3– 10%) (BUČKO 2001). There is a significant difference between the chemical composition in the wood of broad-leaved and coniferous trees. While the wood of coniferous trees contains 5

more cellulose, lignin and accompanying substances, the wood of broad-leaved trees contains more hemicelluloses.

The wood retains its original appearance for a long time, structure and properties under suitable exposure conditions. While in atmospheric conditions it undergoes permanent degradation processes under the influence of a number of abiotic factors. Sunlight is one of the most aggressive factors in wood damage (REINPRECHT 1998, KUBOVSKÝ et al. 2018).

HON and IFJU (1978) proved that different types of radiation penetrate only to very small depths of wood. While visible radiation (400 to 750 nm) penetrates to a depth of 200 m, UV radiation with wavelengths below 400 nm penetrates to a maximum of 75 m.

After absorption of the light quantum by wood, primary free radicals are formed, which are highly reactive and cause secondary radical chain reactions. These reactions can also cause chemical reactions in the deeper layers of wood. FEIST and HON (1984) state that the secondary degradation process takes place to a depth of about 2,500 m.

The degradation reactions indicated by visible radiation take place on the surface of lignocellulosic materials under long-term exposure. They are manifested by a change in the physical properties and color of their surface. Many wooden buildings, tiles, fences, roof shingles, or facade elements of buildings are long-term exposed to negative external influences, which cause the aging of wood and shorten its shelf-life. The root-cause of these changes is the interaction of wood components with radiation - especially with its ultraviolet (UV) component – as a result of a complex of chemical changes of lignin, polysaccharides and extractives. The magnitude of the changes depends on the chemical composition of the wood and the duration of UV radiation on its surface. Numerous oxidation and degradation reactions result to formation of new chromophores and changes of wood color.

The wood constituents show different capacities with respect to absorbing UV radiation. According to NORRSTRÖM (1969) lignin contributes 80-95%, the carbohydrates 520%, and the extractives about 2% to the absorption coefficient. The UV absorption occurs at chromophoric structural elements within the molecular network of lignin, such as phenolic hydroxyl groups, α-carbonyl group, conjugated double bond, quinone, biphenyl or free radicals.

Photodegradation of wood is related to the interaction of lignin with the UV component of light. Lignin is extremely susceptible to UV radiation. The site of absorption is aromatic ring, unsaturated bonds and lignin carbonyl groups. The absorbed energy quantum is transferred to less stable chemical bonds and causes their homolytic cleavage (e.g. β-alkyl-aryl ether bonds (FENGEL and WEGENER 1984, HON and CHANG 1984, SOLÁR 2004). The resulting radicals – phenoxy-benzyl, hydroperoxide postulate both depolymerization and condensation reactions of lignin. New chromophoric structures are created and the wood darkens from yellow to brown shades.

The interaction of polysaccharides with the UV radiation leads to weight loss, depolymerization and, in the case of cellulose, also to a reduction in the α-cellulose content.

The chromophoric group in the interaction of cellulose with electromagnetic radiation is believed to be the grouping of oxygen atoms on the C1 carbon of the glucopyranose unit (formation of C1, C4 radicals). The resulting radicals react with oxygen to form unstable peroxy radicals causing the formation of other radicals (C2, C3, C5, C6). At the same time, the aldehyde and primary alcohol groups of the cellulose end units are oxidized, which after decarboxylation are converted to D-xylopyranose and D-arabinopyranose units (FENGEL and WEGENER 1984, SOLÁR 2004). High UV absorption and the strong capability of lignin autooxidation retard the photolytic degradation of cellulose (NORRSTRÖM 1969).

New chromophoric groups are formed simultaneously in the wood components (carbonyls, carboxyls, peroxides, hydroperoxides, quinoid structures and conjugated double

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bonds), which increase the possibilities for the absorption of additional light quanta and the intensification of the photodegradation process (REINPRECHT 1998, SOLÁR 2004).

FEIST (1990) points to the high degradation of lignin and hemicelluloses in the surface layers of maple wood during accelerated aging. The erosion of the wood surface is related to the density of the wood and the lignin content.

Extractive wood components can also act as UV absorbers - they scavenge free radicals and are subject to photooxidation reactions. They are also slowing the photodegradation of lignin (CHANG et al. 2010, 2014).

The aim of the work was to compare the effect of UV radiation on two different types of wood – European beech (Fagus sylvatica L.) and Scots pine (Pinus sylvestris L.), which differ significantly in their chemical composition, and to determine the dependence of changes in color characteristics (brightness, L*, a*, b*, ∆E*) with irradiation time as a consequence of chemical changes on the surface of irradiated wood.

MATERIAL AND METHODS The untreated beech and pine wood without defects, i.e. without knots, resin canals, biological damage or other defects was used for the experiments in which the main components of the wood were determined by standard chemical processes: Ethanol–toluene solubility of wood

ASTM D 1107-96. Standard Test Method for Ethanol-Toluene Solubility of Wood – the determination of the ethanol-toluene soluble content of wood, which is a measure of the waxes, fats, resins, and oils, plus tannins and certain other ether-insoluble components Polysaccharide fraction (holocellulose) Method according to Wise – the action of NaClO2 in acetic acid on sawdust after ethanol extraction [KAČÍK and SOLÁR 2000] Cellulose Kürschner–Hoffer method – repeated treatment with a mixture of nitric acid and ethanol [KAČÍK and SOLÁR 2000] Lignin ASTM D 1106 – 96. Standard Test Method for Acid Insoluble Lignin in Wood – two-stage treatment with sulfuric acid.

Samples of beech and pine (heart) wood with the dimension of 100 × 100 × 20 mm were used to monitor color changes depending on the time of exposure to UV radiation.

Wood samples (one piece from each tree species) were exposed to intense UV radiation by device Sirius UVIR with mercury lamp (125W) at time intervals 30, 60, 90, 120 and 150 minutes under normal laboratory conditions. The distance of samples from the UV radiation source was 20 ± 2 cm (STN 50 0376). (Notice: mercury lamp is a strong UV emitter suitable for studying photodegradation employing short exposure times (TIMAR et.al. 2016, TOLVAJ and VARGA 2012).

Brightness of irradiated wood samples was determined by device Leukometer for measuring reflectance according to STN ISO 3688 (brightness is defined by the reflectivity of the surface of the measured sample, expressed in % of the reflectivity of the basic brightness normal - magnesium oxide (MgO) - measured for an effective wavelength of 457 ± 5 nm). Brightness of each sample were measured 10-times.

Surface color of irradiated wood samples was determined using the color reader CR10, which is defined by the coordinates of the CIEL*a*b* color space (L*, a*, b*) and is evenly 7

perceptible over a wide range of colors (ISO 11664-4, HRČKA 2013, DZURENDA 2018). Color parameters L*, a*, b* of each sample were measured 10-times and total color difference E* was determined by the equation:



E*  ( L*2  L*1 ) 2  (a2*  a1* ) 2  b2*  b1*



2

(1)

(L2*-L1*) change the value of the white-black coordinate (lightness) (a2*-a1*) change the value of the green-red coordinate (b2*-b1*) change the value of the blue-yellow coordinate.

The surface of the beech and pine samples was subsequently analysed by the ATRFTIR technique and the chemical changes of the main components on the surface of the wood were characterized by evaluating the obtained spectral records.

Fourier transform-infrared (FTIR) spectroscopy measurements were carried out using a Nicolet iS10 FTIR spectrometer equipped with Smart iTR attenuated total reflectance (ATR) sampling accessory with diamond crystal (Thermo Fisher Scientific). The resolution was set at 4 cm-1, 32 scans were recorded for each analysis in an absorbance mode at the wavenumber range from 4000 cm-1 to 650 cm-1. Four analyses were performed at four locations per sample, the average spectra from obtained spectra were created for each sample. The evaluating of spectra using the OMNIC 8.0 software (Thermo Fisher Scientific) was performed.

RESULTS AND DISSCUSSION Basic chemical analysis was performed for a more detailed characterization of the examined beech and pine wood, which included the determination of the content of extractives (TEE), holocellulose, cellulose and lignin. The content of hemicelluloses was calculated as the difference from the content of holocellulose and cellulose (Table 1).

Tab. 1 Chemical characteristics of beech and pine wood.

Wood sample Beech Pine

TEE (%) 1,8 5,2

Holocellulose (%) 78,6 73,1

Cellulose (%) 45,5 47,3

Hemicelluloses (%) 33,1 25,8

Lignin (%) 22,7 25,6

Chemical analysis of the monitored samples of beech and pine wood confirmed the generally known differences between the wood of coniferous and broad-leaved trees - more cellulose, lignin and extractives, but overall less polysaccharides (holocellulose) in the wood of coniferous trees than in the wood of broad-leaved trees.

The exposure of the wood surface to UV radiation reduced the brightness. The pine sample showed a higher original brightness than the beech sample. Prolonged exposure to UV radiation decreased the brightness values of both trees (Fig. 1). The largest decrease in brightness was recorded after the first 30 minutes of irradiation, while in beech the value of brightness decreased by 4.2% MgO, in pine wood by up to 10.3% MgO. The total decrease in the brightness of the wood surface in the observed time of UV exposure (150 min) was 18.6% MgO for pine and 12.2% MgO for beech.

The evaluation using color coordinates - lightness L* (black-white), a* (green-red) and * b (blue-yellow) in the color space CIEL*a*b* represents more objective expression of color changes on the wood surface.

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The L* coordinate, characterizing the lightness of the measured surface, was higher in the case of pine wood in the entire observed irradiation time range. During the 150-minutes exposure, it decreased by 9.2 (by 11%), while in beech the decrease in lightness was by 7.0 (by 9.7%) (Fig. 2).

Fig. 1 Changes in the brightness of the wood surface during irradiation.

Fig. 2 Changes in the lightness of the wood surface during irradiation.

Changes in the coordinates a* and b* at 30 minute UV intervals are shown in Fig. 3.

Larger changes (shifts in color space) were registered for pine wood.

The a* coordinate, characterizing the position in the green-red part of the color space, reported higher values in the original beech sample and also in the whole monitored irradiation range than in the pine. The a* coordinate shifted to the red area due to UV radiation in both woody plants. Pine increased from an average of 4.5 to 8.7 (1.9-fold increase) and beech from 6.4 to 9.1 (1.4-fold increase). The shift of the a* coordinate to the red region is associated by some authors with the content and photodegradation of extractive substances (PERSZE and TOLVAJ 2012, TIMAR et al. 2016).

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Fig. 3 Changes in color coordinates a* and b* during irradiation.

The average values of the b* coordinate, characterizing the blue-yellow area of the color space, were higher for the original pine wood and also in the entire observed irradiation time range. A shift to the yellow area was noted for both trees. The shift was higher for pine wood by 11.1 (50%), for beech by 8.4 (44%).

According to several authors, yellowing of the wood surface due to UV radiation is associated with the degradation of lignin and the formation of new structures - quinones, quinonemides, stilbenes and diphenyls (HON and GLASSER 1979, GIERER and LIN 1972, GELLERSTEDT and PETTERSSON 1977, MÜLLER et al. 2003, KUBOVSKÝ et al. 2016).

However, some authors also attribute the yellowing to the formation of chromophore groups (carbonyls, carboxyls and hydroperoxides) during cellulose irradiation (KLEINERT and MARRACCINI 1966 a,b, KLEINERT 1969, HON 1979).

It could be concluded from the measured values of the coordinates a* and b* that the blue-yellow coordinate b* shows larger changes comparing to green-red coordinate a* and also more significant changes of both coordinates in the pine wood. In Fig. 4, a gradual increase in the intensity of the yellow-red hues with a prolonged exposure time to UV radiation can be seen on the beech and pine wood samples.

Beech wood

Pine wood

Fig. 4 Color changes on the pine and beech wood surface during the exposure to UV radiation.

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The evaluation of the color coordinates L*, a*, b*, expressed in terms of the total color difference (∆E*), showed that the largest change in the wood surface color in both trees was recorded in the first 30 minutes of irradiation (Fig. 5). This conclusion is in accordance with the views of many authors who state that photodegradation effects of UV radiation occurs rapidly at the beginning of exposure and slows down with the time (BAAR and GRYC 2012, TOLVAJ and FAIX 1995, TOLVAJ and MITSUI 2010, AGRESTI et al. 2001, CALIENNO et al.

2014, GEFFERTOVÁ et al. 2016, KÚDELA and KUBOVSKÝ 2016, TIMAR et al. 2016, GEFFERTOVÁ et al. 2018).

The total color difference ∆E* reached a value of 7.2 for pine wood after 30 minutes of irradiation, and due to the commonly used color change scale (ALLEGRETTI et al. 2009), the change can be classified up to level 5, which is defined as “high color difference“ for 6 < ∆E* < 12.The total color difference increased with increasing irradiation time and after 150 minutes reached the value of 15.0, which according to the given scale is "difference colors". The largest change ∆E* for pine wood in the photodegradation of six woods is also reported by TIMAR et al. (2016). ∆E* of beech wood after 30 minutes of irradiation reached a value of 5.4 and classification into the 4th degree of color change, which is characterized for 3 < ∆E* < 6 as "color difference visible with medium quality screen" and after 150 minutes of irradiation reached color difference value 11.3 (high color difference).

The effect of UV radiation was manifested by a visible darkening and a change in the shade of the surface of the samples of pine and beech wood (Fig. 4), which is related to the chemical composition of the wood (Table 1) and changes in its chemical components. This fact was also reflected in the values of the total color difference ∆E*, where e.g. a value of 7.2 was reached for pine wood after only 30 minutes of exposure to UV radiation, while beech wood has reached the given value of color change until after 60 minutes of irradiation.

Fig. 5 Changes of the color difference E* by UV radiation.

Originally lighter pine wood with L* = 81.9, darkened more due to irradiation (∆L* pine = 9.2) than the original beech wood with L* = 72.0 (∆L* beech = 7.0). More lignin (25.6%) and extractives (5.2%) were determined in pine wood than in beech wood (22.7% and 1.8%). Lignin and phenolic extractives act as absorbers of UV radiation due to their chemical structure. Their aromatic rings, unsaturated bonds and carbonyl groups interact with UV radiation, resulting in homolytic cleavage to form a variety of radicals. The 11

absorbed energy quantum is then transferred to the surrounding, less stable chemical bonds, and causes secondary photodegradation reactions of the other components of the wood. In addition to degradation reactions, condensation reactions also take place and result to the formation of new chromophoric structures that visually manifestby darkening of the wood surface (FENGEL and WEGENER 1984, SOLÁR 2004).

Thin samples of 10 × 10 mm in size were taken from the surface of irradiated beech and pine wood samples after color measurement in order to assess chemical changes by FTIR spectroscopy. The observed differential FTIR spectra are presented in Figures 6 and 7.

Fig. 6 Differential FTIR spectra of beech wood.

Fig. 7 Differential FTIR spectra of pine wood.

They show changes in chemical components in the upper region of the so-called "fingerprint" (1200–1800 cm-1) that occurred in the surface layers of the wood due to UV radiation, compared to the original wood.

The differential spectra of both tree species show the decrease in absorption within the range of 1200–1300 cm–1. In the case of pine, the decrease is more intensive at 1267 cm–1.

This fact indicates degradation of guaiacyl ring, which is the main structural component in the 12

lignin macromolecule of conifers. In the case of beech, the decrease is more intensive at 1229 cm–1. This band is characteristic for syringyl ring in hardwoods lignin and xylan (HON 2001). The decrease in absorption around 1462 cm-1 also indicated the degradation of lignin and hemicelluloses. Hemicelluloses and amorphous cellulose also degraded by UV radiation, but less than lignin (COGULET et al. 2016).

A visible decrease in the absorbance of the aromatic lignin band at 1510 cm–1, in combination with an increase in the carbonyl bands in the range between 1660 and 1800 cm-1, proves the course of the light-induced oxidation reactions. Significant formation of new conjugated and aromatic carbonyls as well as quinones can be observed in the region of 1660–1700 cm–1 and the formation of unconjugated aliphatic carbonyls in the region of 1700–1800 cm–1 (PANDEY 2005, LIU et al. 2016). Production of unsaturated carbonyl compounds (quinones) is related to wood discoloration. According to FEIST and HON (1984) the oxidation of cellulose also contributed to increment of carbonyl groups.

The progress of the chemical changes on the surface of the monitored samples of beech and pine wood was observed on the absorption bands A1510 (for lignin) and A1730 (for unconjugated aliphatic carbonyls) in relation to the duration of UV radiation - Figures 8 and 9. The intensity of the chemical changes was expressed as the ratio of the absorbance of the selected chemical component and the relatively stable carbohydrate band at 1370 cm -1, which was used as a reference.

Fig. 8 Change of A1510/1370 ratio during irradiation.

Fig. 8 shows the changes in the A1510/1370 ratio during irradiation for both monitored trees determined from the difference graphs. The trend in the decrease in lignin content in the wood surface layers was similar in both wood species, while the picture also shows a higher content of lignin in pine wood.

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Fig. 9 Change of A1730/A1370 during irradiation.

Fig. 9 shows changes in the A1730/1370 ratio during irradiation of both monitored trees. The increasing trend of the content of unconjugated carbonyls in the wood surface layers was much more significant in pine, where the increase in the proportion of A1730/1370 was 3 times higher than the original value, while in beech the increase was only 2-fold.

The chemical changes of lignin and unconjugated carbonyls during UV exposure, which caused color changes in the wood surface, are shown in Figures 10 and 11.

Fig. 10 Dependence of the A1510/1370 ratio on the total color difference ∆E*.

The relationship between the total color difference ∆E* and the values that characterize the lignin relative content (A1510/1370) is shown in Fig. 10. The course of dependence showed a decreasing trend of (A1510/1370) ratio related to decomposition of lignin due to photodegradation. It could be concluded that for both wood species it is a linear dependence.

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Fig. 11 Dependence of the A1730/1370 ratio on the total color difference ∆E*.

Figure 11 is showing that the total color difference ∆E* of the two woods is linearly correlated with the ratios of the corrected area of the relevant absorption bands A1730/A1370. The course of dependencies showed an increase in unconjugated carbonyl groups (A1730/A1370) as a result of photooxidation reactions, which is consistent with the conclusions of TIMAR et al. (2016).

The differences between the monitored trees (beech, pine) are resulting from the differences in their chemical composition. This applies to the content of lignin, hemicelluloses and also to the content of extractives, which are significantly higher in pine wood than in beech wood. The faster and uneven increase in the carbonyl content can be attributed to the effect of UV radiation on the non-lignin components of the wood (polysaccharides and extractives), as well as the heterogenity of the wood. Similar conclusions were reached in their work by TIMAR et al. (2016) and MÜLLER et al. (2003), that tried to express the relationship between carbonyls and ∆E* by nonlinear polynomial dependence.

CONCLUSION Chemical analysis of the monitored samples of beech and pine wood confirmed higher content of cellulose, lignin and extractives in pine wood than in beech wood. At the same time, an overall lower content of holocellulose and also hemicelluloses was determined in pine wood.

Although the chemical composition of the original wood of the two monitored woods was different, the trends of changes in color characteristics due to radiation were similar decrease in brightness and lightness, increase in green-red coordinate a*, blue-yellow coordinate b* and total color difference ∆E*, whereas the changes were more significant in pine wood.

The largest changes of color characteristics were recorded after the first 30 minutes of irradiation.The changes of color characteristics in other time intervals were less significant.

While the lightness values L* were decreasing, the coordinate values a* and b* increased. It was observed a shift to the red and yellow areas of the color space with more significant 15

increase in the values of the b* coordinate than the increase in the values of the a* coordinates.

The total color difference ∆E* showed a steady increase with prolonging irradiation time, whereas the b* coordinate contributing the most to its change.

Changes in the FTIR spectra of the surface of the irradiated wood showed that UV radiation caused lignin degradation and the formation of conjugated and unconjugated carbonyls and quinoid structures due to the photooxidation reactions of the wood components responsible for yellowing of the wood surface.

The decrease in lignin content in the surface layers of wood due to UV radiation was similar in both woods, but the increase in carbonyl content was more significant in pine compared to beech.

The dependence of the ratio of "relative" absorbances of lignin and carbonyls on the total color difference ∆E* in both monitored woods showed a linear increasing trend in the case of carbonyls and a slightly decreasing trend in the case of lignin.

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GEFFERTOVÁ, J., GEFFERT, A., VÝBOHOVÁ, E. 2018. The effect of UV irradiation on the colour change of the spruce wood. In Acta Facultatis Xylologiae Zvolen, 2018, 60(1), pp. 41–50.

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MÜLLER, U., RÄTZSCH, M., SCHWANNINGER, M., STEINER, M., ZÖBL, H. 2003. Yellowing and IRchanges of spruce wood as result of UV-irradiation. In Journal of Photochemistry and Photobiology B: Biology, 2003, (69), pp. 97–105.

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SOLÁR, R. 2004. Chémia dreva (Wood Chemistry), 1st ed. Zvolen : Technical University in Zvolen, 2001, 101 p. ISBN 80-228-1420-2.

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ACKNOWLEDGEMENT This work was supported by the Slovak Research and Development Agency under the contract No. APVV-17-0456.

ADDRESSES OF THE AUTHORS prof. Ing. Anton Geffert, CSc. doc. Ing. Jarmila Geffertová, PhD. Ing. Eva Výbohová, PhD. Technical University in Zvolen Faculty of Wood Sciences and Technology Department of chemistry and chemical Technologies T.G.Masaryka 24 96001 Zvolen Slovakia geffert@tuzvo.sk geffertova@tuzvo.sk vybohova@tuzvo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 19−25, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.02

THE INFLUENCE OF SELECTED MODIFYING TEMPERATURES ON SPRUCE WOOD EMISSIVITY Richard Hrčka ABSTRACT The precise and accurate result of the measurement of wood temperature using pyrometer requires the setting of the emissivity of the object. The lack of the information about wood emissivity supports the aim of this contribution. The method of the wood emissivity measurement is based on determining the surface temperature and computing the solution of heat conduction equation. The particular integral is found using the radiative boundary condition. Spruce wood is the second most abundant tree species in Slovakia with a wide use for example in constructions or pulp and paper industry. Adding heat at the modifying temperatures observed from 160°C to 220°C applying for four hours significantly influences the spruce wood thermal properties measured at standard temperature. Mass specific heat capacity and thermal conductivity in tangential direction of spruce wood showed continuous decreasing character with increasing values of the modifying temperatures. Emissivity showed more constant character. Its value significantly differs only in the case of treated wood 𝜀𝜖〈0.74 ± 0.09; 0.79 ± 0.13〉 compared to untreated wood 𝜀 = 0.85 ± 0.04. Thermal diffusivity is the most stable property among the wood thermal properties.

Key words: spruce wood, emissivity, specific heat, thermal conductivity, thermal diffusivity

INTRODUCTION The permeability of spruce wood is very low (POŽGAJ et al. 1993), especially in dry state. Therefore the dominant transport of heat through wood is conduction. Wood substance forms wood, and wood can contain water and air. Every component of wood specimen requires the definition of the domain for solution of heat conduction equation, initial and boundary conditions. Such attitude is time-consuming problem. Therefore wood is treated as continuum. Such continuum is homogeneous in its volume and may be anisotropic. In general, wood is cylindrical orthogonal anisotropic material (STEINHAGEN 1977, POŽGAJ et al. 1993, HRČKA and BABIAK 2016, DELISKII and TUMBARKOVA 2017). When wood is in contact with air, convection occurs on the boundary. Moreover, radiation occurs at temperatures more than absolute zero between the object and its surroundings of different temperatures. The radiation intensity is proportional to the fourth power of temperature. The Stefan-Boltzman law is linearized to match the radiative boundary condition or the boundary condition of the third kind (CARSLAW and JAEGGER 1959, LYKOV 1968). Then, solution of heat conduction equation is used to determine the transient temperature field in wood. The radiative heat intensity is measured by pyrometer and object temperature is computed with setting the value of object emissivity. The inverse problem determines the object emissivity 19

from object surface temperature. The influencing factor is treatment temperature, which can be significant as is indicated in the publication of CZAJKOVSKI (2019). Therefore, the aim of this contribution is reporting the values of spruce wood emissivity. The computing of the surface temperature requires determining the temperature field in wood. Thermal diffusivities in principal directions along with Biot numbers are determined with solution of inversed problem. As far as, the heat intensity is known at the boundary, volume specific heat is determined, which is influenced with moisture content. The mass specific heat capacity is determined from volume specific heat and density at given moisture content.

Spruce wood is the second most abundant tree species in Slovakia with wide use for example in constructions or pulp and paper industry, where spruce wood comes to contact to elevated temperatures. Thermally modified wood exhibits substantially lower moisture content, than untreated (BABIAK and NÉMETH 1998, HRČKA et al. 2018). And finally, question arises if is the value of spruce wood emissivity influenced with treatment temperature?

MATERIAL AND METHOD Spruce wood (Picea abies, Karst.) was obtained from locality Vlčí jarok (Budča, Central Slovakia). The logs were cut into slabs of 70cm × 10cm × 2cm (L × R × T) dimensions. A final moisture content of 10% for flat sawn lumber was achieved by the kiln drying method at the Technical University in Zvolen. The five slabs were stored at the temperature of 10°C. Four of them were thermally modified at the temperatures of 160, 180, 200 and 220°C and one sample remained unmodified. The hydrothermal treatment was performed at the Arboretum of FLD (CZU in Prague) in Kostelec nad Černými lesy (Czech Republic) using heating technology in the LAC S 400/03 chamber KATRES s.r.o.

(HRČKOVÁ et al. 2018). The slabs temperature of 60°C or lower was recorded after removal them from the chamber. Four pieces of slabs were thermally treated according to the following procedure: the 1st period: the lumber was heated to reach target temperature. the 2nd period: the lumber was heated at different treatment temperatures (160, 180, 200, and 220 °C) for 4 h. the 3rd period: the lumber was cooled to an ambient temperature of 60 °C and the target moisture content was reached up to the desired value by the water spraying method.

Tab. 1 The schedule of the thermal treatment.

Temperature (°C) 160 180 200 220

1st Period (h) 10 12 14 16

2nd Period (h) 4 4 4 4

3rd Period (h) 3 4 5 6

The parts of treated spruce timbers were equilibrated with humid air in a climatic chamber Binder KBF 720 (Tuttlingen, Germany). The controlled parameters of humid air were at a relative humidity of 65% and a temperature of 20 °C for half year. The final specimens dimensions 100mm × 50mm x 8mm (L × R × T) were cut off from the slabs and radial surfaces were sanded on wide belt sander (P80). The total number of used different specimens was 100.

The equilibrium was detected according to the constant mass of the moisture specimen of timber using the technical scales Kern KB 1000-2 (Balingen, Germany).

The principle of emissivity method measurement is described in the publication of HRČKA and SLOVÁČKOVÁ (2019). The difference between described method of HRČKA and

20

SLOVÁČKOVÁ (2019) and used method in this contribution is in the utilization of two heating foils (NiCr - Vacronium, thickeness of 0.01mm) symmetrically placed inside of four specimens block (Figure 1).

Source of DC

Datalogger

Pyrometer Heating foils Thermocouples Block of 4 specimens Fig. 1 Arrangement of apparatus.

Then, the position of three thermocouples of type K (Omega, USA) is shown in Figure 2.

Thermocouples

Fig. 2 Position of thermocouples.

The forth thermocouple is placed in the position of one heating foil. The form of the heating foil is structured source which consists in thin strips as is shown in Figure 3.

Cu contacts

Fig. 3 Structured heating foil.

The set current was 0.12mA (QPX1200SP, TTi, UK) and expected maximum temperature rise was 20°C at the position of the heating foil. The initial condition of measurements was 20°C.

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RESULTS AND DISCUSSION Smaller amount of water inside wood influences significantly its specific heat. Also, wood substance must be modified, because reduction of equilibrium moisture content occurred. There is necessity to measure and investigate thermally modified wood thermal properties as a complex set of quantities. The results are shown in Table 2, 3 and 4.

Tab. 2 Equilibrium moisture content and average input parameters of spruce wood thermal properties ̅ – mean, s- standard deviation; t modifying temperature; T, R, L – dimensions in measurement (𝒙 principal anatomic directions;  – density at given moisture content w, equilibrium at relative humidity of 65% and initial temperature 20°C). 𝑥̅ s 𝑥̅ s 𝑥̅ s 𝑥̅ s 𝑥̅ s

t [°C] 20

T [m] 0.0129

R [m] 0.0498

L [m] 0.1010

160

0.0090

0.0498

0.0950

180

0.0088

0.0499

0.0952

200

0.0083

0.0499

0.0954

220

0.0077

0.0498

0.0957

 [kg.m-3] 462.7 29.5 403.5 25.4 429.5 16.5 430.0 8.0 412.0 14.1

w 0.120 0.002 0.096 0.003 0.079 0.001 0.074 0.001 0.060 0.002

Typical temperature increase inside spruce wood block of four specimens is shown in Figure 4. The curves are fitted by using the method of least squares. The four different curves provide at least 7 degree of freedom, and therefore three thermal diffusivities, three Biot numbers and one specific heat can be determine for wood as orthotropic material, when dimensions are oriented in principal anatomical directions. Then, three thermal conductivities and three transfer coefficients are computed. Moreover, if surface temperature and surface flux are recorded, then emissivity of wood is computed, Figure 5.

25

Change of Temperatute [°C]

20 15 10 5 0 0

500

1000

1500

2000

2500

3000

3500

4000

Time [s] Fig. 4 Temperature increase in different thermocouples (squares) and fitted temperatures (crosses).

Black curve belongs to heating foil position, red curve belongs to position of the 1 st thermocouple, green one to position of the 2nd thermocouple, blue to position of the 3 rd thermocouple and gray curve belongs to surface temperature which was measured by pyrometer (squares) with set emissivity to 1.000, surface temperature computed is indicated with grey crosses.

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As far as, the curves do not overlap, heat flows through lateral surfaces to environment.

Therefore, three dimensional inverse problems cannot be replaced by one dimensional problem.

Tab. 3 Thermal diffusivities (a), thermal conductivities () in principal anatomical directions and mass specific heat capacity (c) of spruce wood and its modification form by heat at elevated temperatures (t) ̅ – mean, s- standard deviation). (𝒙 t [°C]

𝑥̅ s 𝑥̅ s 𝑥̅ s 𝑥̅ s 𝑥̅ s

R [W.(m.K)-1]

T [W.(m.K)-1]

L [W.(m.K)-1]

c [kJ.(kg.K)-1]

aR [m2.s-1]

aT [m2.s-1]

aL [m2.s-1]

0.14 0.02 0.10 0.01 0.11 0.01 0.09 0.02 0.09 0.01

0.11 0.01 0.09 0.01 0.09 2·10-3 0.08 4·10-3 0.07 5·10-3

0.32 0.02 0.38 0.06 0.34 0.03 0.37 0.07 0.34 0.07

1.53 0.01 1.42 0.04 1.40 0.03 1.36 0.02 1.25 0.02

1.9·10-7 0.6·10-7 1.8·10-7 0.3·10-7 1.8·10-7 0.1·10-7 1.5·10-7 0.3·10-7 1.7·10-7 0.1·10-7

1.5·10-7 0.4·10-7 1.5·10-7 0.2·10-7 1.4·10-7 0.1·10-7 1.5·10-7 0.1·10-7 1.4·10-7 0.1·10-7

4.5·10-7 0.6·10-7 6.7·10-7 1.3·10-7 5.7·10-7 0.5·10-7 6.4·10-7 1.3·10-7 6.5·10-7 1.2·10-7

20 160 180 200 220

60

Radiative flux [W·m-2]

50 40 30 20 10 0 0

1000

2000 Time [s]

3000

4000

Fig. 5 Radiative flux increase in time as measured by pyrometer (red squares) and computed using adjusted emissivity (grey crosses).

Tab. 4 Emissivity (), Biot number and heat transfer coefficient () on radial surfaces of spruce wood ̅ – mean, s- standard deviation). (𝒙 𝑥̅ s 𝑥̅ s 𝑥̅ s 𝑥̅ s 𝑥̅ s

t [°C] 20 160 180 200 220

 0.85 0.04 0.77 0.07 0.74 0.04 0.79 0.13 0.74 0.09

BiT 0.9 0.2 1.8 0.6 1.7 0.3 2.2 0.8 2.3 0.5

T [W.(m2.K)-1] 7.5 18 16 22 22

The transient method of measurement enables to determine specific heat capacity, eigenvalues of the wood thermal conductivity and diffusivity tensor and finally the quantities 23

related to surface. Spruce wood emissivity differs significantly at temperature of 20°C. The emissivity average value 0.85 is in the range of wood emissivity values which were published in user manual of producer of pyrometers Optris. The emissivity coefficient of variation is always lower than Biot number coefficient of variation. The emissivity coefficient of variation is the same order as other thermal properties coefficients of variation, with exception to coefficient of variation of specific heat. This exception must be explained by performing other experiments with different wood species of substantially different oven dry densities. The values of emissivity of milled spruce wood were published by ZAŤKO et al. (1993). The emissivity values of spruce wood modified at 180°C and 220°C are the same after rounded to two significant digits as were measured by ZAŤKO et al. (1993). The difference between emissivity values at 20°C can be attributed to differences in measured wood sections. ZAŤKO et al. (1993) reported the emissivity values for tangential surfaces. The coefficient of variation of mass specific heat capacity for given modifying temperature is the lowest as expected (REGINÁČ and BABIAK 1977). Its value is comparable to coefficient of variation of equilibrium moisture content. The measured values continually decrease with moisture content and so does in thermally treated wood. The value at 20°C is higher than value measured by KRIŠŤÁK et al.

(2018). KRIŠŤÁK et al. (2018) presented measured values of mass specific heat capacity dependence on anatomical direction. Also, treatment temperature, along with moisture content, influences the measured values of thermal conductivity. The thermal conductivity in longitudinal direction is of the same values as was measured by VAY et al. (2015) after rounded to two significant digits. The same is true in tangential direction values published by PASZTORY et al. (2017). KRIŠŤÁK et al. (2019), SONDEREGGER et al. (2011) published significantly lower values in longitudinal direction. The less variable thermal property is thermal diffusivity. Its value is the less variable with treatment temperature and density at given moisture content.

The agreement of the presented values of thermal diffusivity and published values of KRIŠŤÁK et al. (2018) is mentionable in all anatomical directions after rounded to significant digits.

CONCLUSIONS The presented values of spruce wood thermal properties, Biot numbers and heat transfer coefficients and their comparisons to previously published values declare the suitability of the used method to determine wood emissivity. The values of spruce wood emissivity 𝜀𝜖〈0.74 ± 0.09; 0.79 ± 0.13〉 mentioned for interval of 160°C220°C are the same rounded value to two significant digits as value published by previous researches 𝜀 = 0.74. The value of 𝜀 = 0.85 ± 0.04 at 20°C is significantly higher than values published previously by the same researches. The method proved tensor character of thermal conductivity and diffusivity of wood from one measurement of set of four temperatures at specified position in the block of four specimens. Therefore, the method provides only one value of mass specific heat capacity, which does not depend on anatomical direction. The precise determination of surface temperature enables to determine Biot numbers, heat transfer coefficients and finally wood emissivity. The lowest variable property, due to changed treatment temperature, is thermal diffusivity. The lowest variation coefficient was determined for specific heat. As wood variability is present, every specimen has its own thermal properties.

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HRČKA, R., SLOVÁČKOVÁ, B. 2019. The method of wood emissivity measurement. In Acta Facultatis Xylologiae Zvolen, 61(2): 17−24, DOI: 10.17423/afx.2019.61.2.02 HRČKOVÁ, M., KOLEDA, P., KOLEDA, P., BARCÍK, Š., ŠTEFKOVÁ, J. 2018. Color change of selected wood species affected by thermal treatment and sanding. In BioResources. 13(4), 89568975.

KRIŠŤÁK, L., IGAZ, R., RUŽIAK, I. 2019. Applying the EDPS Method to the Research into Thermophysical Properties of Solid Wood of Coniferous Trees. In Advances in Materials Science and Engineering, 2019, 9 pages, https://doi.org/10.1155/2019/2303720 LYKOV, A.V. 1968. Analytical heat diffusion theory. New York and London : Academic Press.

PÁSZTORY, Z., HORVÁTH, N ., BÖRCSÖK, Z. 2017. Effect of heat treatment duration on the thermal conductivity of spruce and poplar wood. In European Journal of Wood and Wood Products75::843– 845, DOI 10.1007/s00107-017-1170-2 POŽGAJ, A., CHOVANEC, D., KURJATKO, S., BABIAK, M. 1993. Štruktúra a vlastnosti dreva (Wood Structure and Properties). Bratislava : Príroda. ISBN 80-07-00600-1 (in Slovak) REGINÁČ, L., BABIAK, M. 1977. Základné tepelnofyzikálne charakteristiky smrekového dreva pri normálnych podmienkach. In Drevársky výskum, 22(3): 165174. (in Slovak) SONDEREGGER, W., HERING, S., NIEMZ, P. 2015. Thermal behaviour of Norway spruce and European beech in and between the principal anatomical directions. In Holzforschung, 65, pp. 369– 375, DOI 10.1515/HF.2011.036 STEINHAGEN, H. P. 1977. Heating times for frozen veneer logs – new experimental data. In Forest Products Journal, 27(6), 24–28.

WAY, O., DE BORST, K., HANSMANN, CH., TEISCHINGER, A., MÜLLER, U. 2015 Thermal conductivity of wood at angles to the principal anatomical directions. In Wood Science and Technology, 49, 577–589. DOI 10.1007/s00226-015-0716-x.

ZAŤKO, Š., DOLEŽEL, M., FICEK, F. 1993. The factors influencing emissivity of wood in experimental determination of it’s value in atmospherical conditions by thermovision method. In Vacuum drying of wood. Proceedings of the international conference on wood drying, ed. Assoc. Prof. Ing. Pavel Trebula, Ph.D. p. 4552.

ACKNOWLEDGMENTS This work was supported by the Slovak Research and Development Agency under the contracts No. APVV-16-0177.

AUTHOR'S ADDRESS Richard Hrčka Technical University in Zvolen T.G. Masaryka 24 96001 Zvolen Slovakia richard.hrcka@tuzvo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 27−34, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.03

INVESTIGATION INTO MECHANICAL, SURFACE AND ADHESIVE PROPERTIES OF DATE PALM WOOD-POLYOLEFIN MICRO COMPOSITES Igor Novák – Igor Krupa – Ján Sedliačik – Ondrej Žigo  Peter Jurkovič – Ján Matyašovský ABSTRACT Wood-plastic composites are composite materials made of wood fibre or other lignocellulosic materials and thermoplastic(s). Date palms are one of the potential replacements of insufficient timber sources in the Middle East and the Horn of Africa countries. Low density polyethylene (LDPE) was blended with date palm wood (DPW) (Phoenix dactylifera) powder to prepare composites with the concentrations of filler ranging from 10 to 70 wt. %. The Young´s modulus of the composites significantly increased with an increase in the filler content in the entire concentration range. The maximum value of 1933 MPa for the composite filled with 70 wt.% of the filler is approximately 13 times higher than that for the neat LDPE. The incorporation of DPW into the LDPE matrix led to a significant increase in the polarity of composites and to an increase in their adhesion to polar substrates.

Key words: date palm wood, polyethylene, wood-plastic composite, adhesion, shear strength.

INTRODUCTION The date palm (Phoenix dactylifera) consists of six different potential sources of natural fibres, namely bunches, mesh, petiole, fruit (pits), leaves and palm trunk (KELEDI et al. 2012). Mechanical characteristics of palmyra palm (Borassus aethiopum Mart.) were determined and results of this study showed that mechanical properties of the palmyra are very influenced by the number and the mechanical characteristics of the fibres (KIMTANGAR et al. 2019). Both date palm fibres and wood themselves and their composites have been recently investigated (AGOUDJIL et al. 2012).

ALSEWAILEM et al. (2010) studied high density polyethylene (HDPE) and polystyrene (PS) matrices reinforced with powder from date palm pits and their mechanical and thermal properties. Polyethylene based matters were successfully used for a dimensional stabilization of wood (REPÁK and REINPRECHT 2020) or bonding of veneer (BEKHTA and SEDLIAČIK 2019). ALSAEED et al. (2013) investigated epoxy resins reinforced with long date palm fibres. The authors searched for the optimum length of embedded fibres that have controlled interfacial adhesion properties and determined that 10 mm was the optimum length. Similar research focused on the effect of diameters and alkali treatments on the tensile properties of date palm fibre reinforced epoxy composites was performed by ABDAL-HAY et al. (2012).

These authors determined that the ultimate tensile strength and percentage elongation of a 27

single fibre after alkali treatment increased by 57% and 24.7%, respectively. Because the alkali treatment of date palm fibres was able to provide good adhesion within the matrix, the tensile strength, elastic modulus and the fibre-matrix interaction of the composite were improved. Date palm wood powder/glass fibres reinforced hybrid composites of recycled polypropylene were investigated by many researchers (WOLCOTT and ADCOCK 2000, LYUTYY et al. 2018, MIRSKI et al. 2019, AL-OTAIBI et al. 2020). The influence of date palm fibres from different parts of the date palm plant (the trunk, rachis, and the petiole) on the mechanical properties of HDPE-based composites was studied by MAHDAVI et al. (2010).

The highest strengths were achieved in composites with 30 and 40% fibre content, and these gains were dependent on what parts of the original tree were used.

Wetting of wood based materials by standard liquids is a complex process controlled by chemical composition of the liquids used, properties of the substrate, interactions among unsaturated force fields across the phase boundary between wood and liquid, as well as by secondary effects of a range of factors implied by specific properties of the wood and the liquids used (KÚDELA 2014).

The aim of this contribution is determination of selected physical and mechanical properties of composites based on low-density polyethylene (LDPE) and date palm wood powder.

MATERIAL AND METHODS Linear low density polyethylene was used as the matrix (melting point = 110.6 ± 0.1 °C and specific enthalpy of melting = 118 ± 5 J/g) and ground date palm wood powder (DPW) was used as the filler.

Large pieces of DPW were ground using a high energy mill. The obtained filler had a linear fibrous shape with the moisture content of 7%. The average length and the standard deviation were calculated from at least 20 measurements. The majority of the filler particles had the length ranging from 1 to 3 mm. The composites were prepared by mixing both components in the 30 ml mixing chamber of a Brabender Plasticorder PLE 331 at 140 °C for 10 minutes at a mixing speed of 35 rpm. 1-mm thick slabs were prepared by compression moulding of the mixed composites using a Fontijne SRA 100 laboratory press at 140 C for 1 minute. Dog-bone shaped specimens with a working area of 30×4×1 mm were cut from the slabs. The mechanical properties were measured at room temperature using an Instron 3365 universal testing machine at a deformation rate of 50 mm.min-1.

For adhesive tests, Epoxy resin CHS-Epoxy 531 and polyamine hardener Telalit 410 (Spolchemie), mixing ratio of epoxy resin to hardener = 4 : 1 weight parts, dichloromethane (Merck), have been used.

The surface properties of the LDPE/DPW composites were determined by measuring the contact angles of re-distilled water using a Surface Energy Evaluation system coupled with a web camera (Advex) and PC software. The drops of water, which was used as a testing liquid (V = 3 l), were placed on the investigated surface with a micropipette (05 l, Biohit), and the contact angle of the testing liquid was measured just after drop deposition.

The shear strength of the adhesive joints (Pshear) was measured by tensile testing of the single overlapped adhesive joints. The adhesive joints were prepared using LDPE/wood filler slabs with dimensions of 60 × 10 × 2 mm. The thickness of the epoxy/based adhesive layer between slabs was 0.1 mm, the length of the overlap was 15 mm, and the bonded area was 150 mm2. The LDPE/wood filler slabs were bonded together at laboratory temperature in a hand press using an epoxy/based adhesive. The adhesive joints were tested using an Instron 4301 universal testing machine at a constant crosshead speed of 10 mm.min1.

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RESULTS AND DISCUSSION Surface and adhesive properties The surface and adhesive properties of the LDPE/DPW composites were investigated. The dependence of the water contact angle on composite surface vs. the filler content in LDPE is shown in Figure 1. The increase in the filler content results in a more polar nature of the composite material. The dependence of the water contact angle vs. the wood content decreases nonlinearly. The water contact angle on the LDPE/DPW composite surface significantly decreases with the DPW concentration from 93.2 deg (unfilled polyethylene) to 87.8 deg (30 wt. % of wood in composite), and to 78.9 deg (70 wt. %. of the filler).

The results of the shear strength in the adhesive joint LDPE/DPW composite – epoxy vs. filler content are shown in Figure 2. The shear strength of the adhesive joint between the LDPE/DPW composite and the epoxy resin significantly increased from 0.62 MPa (unfilled PE) to 1.37 MPa (LDPE/DPW composite with 70 wt.% of the filler).

94 92

Contact angle (deg)

90 88 86 84 82 80 78 0

10

20

30

40

50

60

70

80

Wood content (wt.%)

Fig. 1 The dependence of the water contact angle on the LDPE/DPW surface vs. filler content.

1.5 1.4 1.3

Pshear (MPa)

1.2 1.1 1.0 0.9 0.8 0.7 0.6 0

10

20

30

40

50

60

70

80

Wood content (wt.%)

Fig. 2 Shear strength of adhesive joint LDPE/DPW composite – epoxy adhesive vs. filler content.

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Mechanical properties The mechanical properties of the composites tested in the tensile mode at room temperature (25 °C) were characterised. The static mechanical properties evaluated from the stress-strain curves included the yield point, stress and elongation at break and Young’s modulus. The stress-strain diagram of LDPE is shown in Figure 3.

20

LDPE yield point 2

15

stress, MPa

yield point 1

10

5

0

-100

0

100

200

300

400

500

600

700

elongation, %

Fig. 3 The tensile stress-strain diagram of neat LDPE.

The LDPE behaviour is common for polyolefins. Fig. 3 reveals significant cold drawing and good deformability up 650%. The materials also undergo strain (orientation) hardening, which results in a high strength at break. There are also two distinguished yield points in the curve, which is a common behaviour of polyolefins that have a broad size distribution of crystallites. A yield point in polymers is conventionally accepted as being the point where the stress-strain curve exhibits a local maximum. For samples that initially deform homogeneously, this maximum occurs as a result of the internal plastic strain rate of the material increasing to a point where it becomes equal to the applied strain rate. In some cases, a maximum in the force also relates to the onset of necking, where the strain hardening of the necked materials is not sufficient to counteract the reduction of the cross-sectional area, which leads to a reduction in load. This maximum may become less defined as the testing temperature is increased or as the strain rate is decreased, until it disappears. The temperature where the local maximum disappears is lowest for the most branched material and highest for the unbranched, high-density material. The yielding phenomenon of semicrystalline polymers is associated with a change in the morphology of the material, where a spherulitic structure transforms into a fibrillar one. During stretching, this change occurs through shearing and fragmentation of the crystalline lamellae into blocks that rearrange into the form of parallel microfibrils.

The stress-strain diagram of composite consisting of 10 wt.% of DPW is shown in Figure 4.

A dramatic decrease in drawability is observed, even at this very low filler content.

The filler particles represent defects and stress concentrators and significantly reduce the drawability of the matrix. The orientation hardening is completely suppressed; however, the material exhibits some extent of plastic deformation. In this case, the rupture is not brittle.

30

10

stress , MPa

8

6

4

2

LDPE/DPW=90/10 w/w

0 0

10

20

30

40

50

60

elongation, %

Fig. 4 The tensile stress-strain diagram of LDPE/DPW=90/10 w/w.

The behaviours of the composites filled with 20 and 30 wt.% of wood are similar.

However, when the matrix is filled with 40 wt.% of the filler and greater, the material becomes brittle. The plastic deformation is fully suppressed, and the material is broken after the yield point. This behaviour is shown in Figure 5.

10

stress, MPa

8

6

4

2

LDPE/DPW=60/40 w/w

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

elongation, %

Fig. 5. The tensile stress-strain diagram of LDPE/DPW=60/40 w/w.

The mechanical properties of the composites determined at 25 °C are summarised in Table 1. The stiffness of the composites, which is characterised by the Young´s modulus, significantly increased with an increase in the filler content in the entire concentration region. The maximum value of 1933 MPa for the specimen filled with 70 wt.% of the filler is approximately 13 times greater than the one of the LDPE. This result indicates that the filler has a very strong reinforcing effect. Similar results were achieved by AMMAR et al.

(2019) who obtained results showed that the sample made from date palm fibres and biomatrix from lignin-glyoxal-resin thermo pressed in the ratio of 50/50 wt.% has the best mechanical properties.

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Tab. 1 Basic statistical variables for the stress and strain characteristics of the composites loaded in tension. The x/y notation represents the LDPE/DPW w/w ratio.

Sample



LDPE

0

90/10 80/20 70/30 60/40 50/50 40/60 30/70

0.069 0.142 0.221 0.306 0.398 0.498 0.608

y (Sy) (%) 15.5a (0.3) 96.0b (0.4) 15.2 (0.3) br br br br br br

y (Sy) (MPa) 8.0 (0.2) 9.4 (0.1) 9.2 (0.2) br br br br br br

b (Sb) (%)

b (Sb) (MPa)

E (SE) (MPa)

633 (20)

18.5 (0.7)

150 (7)

22.0 (1.9) 8.8 (0.6) 4.8 (0.6) 3.2 (0.3) 2.1 (0.1) 1.4 (0.1) 1.1 (0.1)

9.0 (0.3) 9.2 (0.4) 9.4 (0.2) 9.3 (0.5) 9.7 (0.5) 10.2 (0.4) 11.1 (0.6)

285(22) 376 (22) 562 (71) 800 (42) 1064 (83) 1457 (122) 1933 (124)

Where: y, y, b, b, E – elongation at yield, yield stress, elongation at break, stress at break, and Young’s modulus of elasticity, Sy, Sy, Sb, Sb, SE – standard deviations,  – the volume portion of the filler, br – refers to the brittle rupture, a,b – two yield points are observed, as shown in Figure 3.

The stress at the break of the composites and the dependence on the filler fraction varies nonlinearly. We have considered two influences of the filler on the stress at the break.

On the one hand, we have to consider that the reinforcing effect of the filler leads to an increase in the tensile stress values with an increase in the filler fraction and, on the other hand, the orientation strengthening occurs for semi-crystalline polymers at high deformation.

The latter effect is indirectly negatively influenced by the filler presence and by a steep decrease in the deformation such that orientation of the matrix cannot occur. At low filler fractions, the deformation is sufficiently low to prevent the orientation, but the reinforcing effect of the filler presence is marginal. The particles of the filler represent defects and stress concentrators. The behaviour of the stress-strain curve is changed; orientation hardening and cold flow are suppressed. The samples break close to the yield point. Therefore, an initial dramatic decrease of tensile strength was observed. The initial stress at the break of LDPE (18.5  0.7 MPa) decreased to a value of 9.0  0.3 when it was filled with 10 wt. % of the filler. However, in this case, only the orientation hardening was suppressed. The cold flow occurs only up to 10 wt.% of the filler. After this level, material becomes brittle if the filler content increases. The slight increase in the stress at break at higher filler contents is caused by the reinforcing effect of the filler.

The Young’s modulus is often a property of particular interest. The improvement in the Young’s modulus is an expected outcome because of the reinforcement effect of the filler particles. It is generally known that the improvement of the tensile modulus is caused by the good dispersion of particles and good interfacial adhesion between the particles and the matrix; therefore, the mobility of polymer chains is restricted under loading. The effect of different polypropylene (PP) matrices on the mechanical, morphological, and thermal properties of date palm fiber (DPF)-reinforced PP composites was investigated by ALOTAIBI et al. (2020). They concluded the same tendency, the tensile modulus is slightly increasing with increasing the weight percent ratio of added amount of date palm fibres starting at 1850 MPa and growing to 2600 MPa.

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CONCLUSION A fine powder with a fibrous shape was prepared from date palm wood by grinding in a high energy mill. The prepared fibres have a broad size distribution; the majority of the fibres have a length between 1 and 3 mm. Low density polyethylene was used as the matrix for preparing LDPE/DPW composites. The filler concentration ranged from 10 to 70 wt.%.

The stiffness of the composites, which were characterised by the Young´s modulus, significantly increased with an increase in the filler content in the entire concentration range.

The maximum value of 1933 MPa for the composite filled with 70 wt.% of the filler is approximately 13 times greater than that for the LDPE. This result indicates that the filler has a strong reinforcing effect and that there is a good distribution of the filler. Furthermore, the strength limit of the composites and its dependence on the filler fraction varies nonlinearly. The material becomes brittle if filled with more than 10 wt. % of the filler. The incorporation of DPW into the LDPE matrix led to a significant increase in the polarity of composites and to an increase in their adhesion to polar substrates.

REFERENCES ABDAL-HAY, A., SUARDANA, N.P.G., CHOI, K.S., LIM, J.K. 2012. Effect of diameters and alkali treatment on the tensile properties of date palm fiber reinforced epoxy composites. In International Journal of Precision Engineering and Manufacturing 13(7), p. 11991206.

AGOUDJIL, B., BENCHABANE, A., BOUDENNE, A., IBOS, L., FOIS, M. 2011. Renewable materials to reduce building heat loss: Characterization of date palm wood. In Energy and Buildings 43, p.

491497.

ALMAADEED, M.A., KAHRAMAN, R., KHANAM, P.N., MADI, N. 2012. Date palm wood flour/glass fibre reinforced hybrid composites of recycled polypropylene: Mechanical and thermal properties.

In Materials & Design (42), p. 289294.

AL-OTAIBI, M.S., ALOTHMAN, O.Y., ALRASHED, M.M., ANIS, A., NAVEEN, J., JAWAID, M. 2020.

Characterization of date palm fiber-reinforced different polypropylene matrices. In Polymers 12(3), 597.

ALSAEED, T., YOUSIF, B.F., KU, H. 2013. The potential of using date palm fibres as reinforcement for polymeric composites. In Materials & Design (43), p. 177184.

ALSEWAILEM, F.D., BINKHDER, Y.A. 2010. Preparation and characterization of polymer/date pits composites. In Journal of Reinforced Plastics and Composites 29(11), p. 17431749.

AMMAR, M., MECHI, N., SAAD, M.E.K., ELALOUI, E., MOUSSAOUI, Y. 2018. Characterisation of composite panels produced from lignin-glyoxal-resin reinforced by date palm petiole fibers. In European Journal of Wood and Wood Products 76(4), p. 12951302.

BEKHTA, P., SEDLIAČIK, J. 2019. Environmentally-friendly high-density polyethylene-bonded plywood panels. In Polymers 11(7), 1166.

KELEDI, G., SUDAR, A., BURGSTALLER, C., RENNER, K., MOCZO, J., PUKANSZKY, B. 2012. Tensile and impact properties of three-component PP/wood/elastomer composites. In Express Polymer Letters 6(3), p. 224236.

KIMTANGAR, N., TAO, G., NTAMACK, G.E. 2019. Study of the correlation between fiber and mechanical properties of wood Borassus aethiopum Mart. of Chad. In Wood Research 64(2), p.

195204.

KÚDELA, J. 2014. Wetting of wood surface by a liquids of a different polarity. In Wood Research 59(1), p. 1124.

LYUTYY, P., BEKHTA, P., ORTYNSKA, G. 2018. Lightweight flat pressed wood plastic composites: Possibility of manufacture and properties. In Drvna Industrija 69(1), p. 5562.

MAHDAVI, S., KERMANIAN, H., VARSHOEI, A. 2010. Comparison of mechanical properties of date palm fiber-polyethylene composite. In BioResources 5(4), p. 23912403.

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MIRSKI, R., BEKHTA, P., DZIURKA, D. 2019. Relationships between thermoplastic type and properties of polymer-triticale boards. In Polymers 11(11), 1750.

REPÁK, M., REINPRECHT, L. 2020. Physico-mechanical properties of thermally modified beech wood affected by its pre-treatment with polyethylene glycol. In Acta Facultatis Xylologiae Zvolen 62(1), p. 6778.

WOLCOTT, M.P., ADCOCK, T. 2000. New advances in wood-fiber polymer formulations. In Proceedings of Wood-Plastic Conference. Plastics Technology Magazine and Polymer Process Communications. p. 107114.

ACKNOWLEDGEMENT This contribution was supported by Ministry of Education of the Slovak Republic and Slovak Academy of Sciences, project VEGA, Grant No. 1/0570/17.

This research was supported by the Slovak Research and Development Agency under the contracts No. APVV-16-0177, APVV-17-0583 and APVV-18-0378.

AUTHOR’S ADDRESS Ing. Igor Novák, PhD. Polymer Institute Slovak Academy of Sciences 845 41 Bratislava Slovakia igor.novak@savba.sk Ing. Igor Krupa, PhD. Center of Advanced Materials Qatar University P.O. Box 2713, Doha Qatar igor.krupa@qu.edu.qa Prof. Ing. Ján Sedliačik, PhD. Technical University in Zvolen Department of Furniture and Wood Products T.G. Masaryka 24 960 01 Zvolen Slovakia jan.sedliacik@tuzvo.sk Ing. Peter Jurkovič, PhD. Ing. Ján Matyašovský, PhD. VIPO, a.s.

Generála Svobodu 1069/4 958 01 Partizánske Slovakia pjurkovic@vipo.sk jmatyasovsky@vipo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 35–45, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.04

SURFACE PROPERTIES OF A MEDIUM DENSITY FIBREBOARD EVALUATED FROM THE VIEWPOINT OF ITS SURFACE TREATMENT Jozef Kúdela ABSTRACT The paper is focused on the study and analysis of specified surface properties of medium density fibreboards (MDF). The primary purpose was to acquire data expressing the surface properties of these materials. The study results related to the morphology of the MDF surface show structural differences between the MDF surfaces and the core layers. The surface layer with the thickness from 1 to 2 mm, consisted of fine wood fibres impregnated with the glue and paraffin. In the inspected MDF boards, this surface layer in interaction with the pressing technology, induced lower roughness and clearly higher resistance to water and against non-polar liquids. The MDF surfaces also showed low surface free energy with dominating dispersion component. In the case of applying the film-forming substances, this may result in uneven spreading of the substances across and their poorer adhesion to the MDF surface.

Key words: MDF, morphology, roughness, wetting, surface free energy.

INTRODUCTION Medium Density Fibreboards (MDFs) are made up of fibres of lignin-cellulose materials with a density range of 400850 kgm3 (VOJTA et al. 20018). The final board density however, mostly ranges from 690750 kgm3 (GUL et al. 2017). The main MDF component is modified fibres from coniferous or broadleaved woody plants. The second one is glue. The glue type and properties define the product performance under mechanical and moisture loading. Therefore, the properties of glues used at MDF production are extremely important (ŠTEFKA 2006, REINPRECHT 2016, UNER and OLGUN 2017). The third component is a group of secondary, facilitating chemical assistant substances such as hydrophobic and protective substances (fire retarders, fungicides) and similar, modifying purposely the MDF performance.

Today, MDFs are the leading primary material for manufacturing kitchen and bathroom furniture, mainly the doors. Under these circumstances, the MDFs may be exposed to a rather high ambient air humidity, or even be in direct contact with liquid water (de CADEMARTORI et al. 2015). That’s why the MDFs are supplemented with hydrophobic and fungicidal agents.

The most commonly used hydrophobic medium is paraffin, but there are possible alternative hydrophobization modes for MDF treatment (DE CADEMARTORI et al. 2018). The hydrophobization degree of wood materials supplemented with paraffin and the degree of their performance modification depend on the paraffin amount, type and form, as well as on the 35

application mode of this substance into the material (ŠTEFKA 2002, ROFFAEL et al. 2005, GARAI et al. 2005, TORKAMAN 2008, CAI et al. 2016, KÚDELA 2019). Paraffin reduces the sorption capacity of food fibres, and, consequently, the thickness swelling of the boards concerned. On the other hand, an excessive paraffin amount may cause problems concerning surface treatment with coating materials or with gluing foils or veneers (LIPTÁKOVÁ and KÚDELA 1997, AYRILMIS and WINANDY 2009, KÚDELA 2019). Hydrophobization of agglomerated materials, MDFs included, with paraffin, weakens their surface wetting and lowers their surface free energy, which has also been confirmed by LIPTÁKOVÁ and KÚDELA (1997).

According the last cited work, the surface energy of all the tested agglomerated materials with dominant disperse component was significantly lower compared to the corresponding native wood with dominant polar component. This is in accord with (de CADEMARTORI et al. 2015, CAI et al. 2016, KÚDELA 2019).

The study of surface treatment defects in hard fibreboards (KÚDELA 2019) revealed that the spots with more paraffin did not achieve the required desiccation degree. This had negative impact on applying the additional layers, especially in the case of automatized manufacturing lines. These spots exhibited impaired adhesion between the coating and the substrate. The result was many additional defects, including outlook defects (orange peel, bordered spots, coat cracking after drying, and similar). There was even been observed paraffin penetration throughout the prime coating into the second layer applied.

For MDF surface treatment with coating materials and for gluing thin foils, MDF surface morphology is important as well, because this feature affects the final product’s surface outlook. The material morphology is evaluated based on their roughness and waviness. MDF morphology is determined by the fibre structure and processing, MDF manufacturing technology as well as by the mode of the mechanical treatment of the MDF surface. MDF surfaces are machined by sanding or milling. The ground surface morphology is first of all the result of the grain size. The surface morphology in milled surfaces is backedup by multiple factors (the milling machine quality, number of knives, shifting speed, rotation speed and others).  (SINN et al. 2005, LIN et al. 2006, AKBULUT and KOC 2006, AYRILMIS et al. 2010, JARUSOMBUTI et al. 2010, SÜTCÜ and KARAGÖZ 2012, SEDLECKÝ 2017, KMINIAK et al. (2020).

It follows that the MDF surface properties follow from a number of factors (wood species, wood fibre type and size, amounts of ingredients, pressing conditions, surface treatment mode, moisture content, and similar). Key important is wood fibre type and its interactions with the other factors (AKBULUT and KOC 2006).

The surface treatment of these materials is supposed to guarantee good spreading of the applied film-forming substances (coating materials and glues) and forming a compact, decorative protective layer or a stable glued joint after their hardening. KÚDELA and LIPTÁKOVÁ (2006), KÚDELA (2019) demonstrate that the phenomena at the interface between wood or agglomerated materials and film-forming substances in both liquid and solid phase are very complex. The hardening of film-forming materials on the substrate surface is accompanied with physical and chemical phenomena inducing changes in the solid coating (glue) chemical structure. This is, on its turn, reflected on their surface free energy values and on the values of their cohesion and adhesion to the substrate (LIPTÁKOVÁ and KÚDELA 2002, KÚDELA and LIPTÁKOVÁ 2006, AYRILMIS and WINANDY 2009, SLABEJOVÁ et al. 2016, SLABEJOVÁ and ŠMIDRIAKOVÁ 2018). From this viewpoint it is important to know the discussed MDF surface properties as well as the properties of the used film-forming material (KÚDELA and LIPTÁKOVÁ 2006, KÚDELA 2012).

To obtain an as much as possible comprehensive idea about the investigated MDF surface, it is necessary to know a number of its properties (morphology, chemical and thermodynamic properties, etc.). It is also obligatory to recognize the effects of a range of 36

factors on the MDF surface properties. For these reasons, the study of MDF surface properties and on the interactions occurring at the interface MDF – film-forming material is an up-to-date subject, from the viewpoint of improving the surface treatment quality of these materials with coating substances, as well as from the viewpoint of prolonging the correct performance of the glued joint.

The objective of this paper was experimental measuring and evaluation of specific surface properties of commercially produced MDFs, with the aim to assembly data important for these boards´ surface treatment. MDF surface morphology was evaluated from anatomical and physical viewpoint; through values of roughness and waviness, MDF surface wetting with polar and non-polar liquids. There were also determined the surface free energy and its impact on the final MDF surface treatment quality.

MATERIAL AND METHODS Experimental material Surface properties were investigated in raw MDFs produced commercially by the manufacturing company DH Decor Ltd. Humpolec, the Czech Republic, and supplied for manufacturing furniture doors. The tested boards were five, selected randomly. From each board, there were cut four specimens, with surface dimensions of 50  100 mm. The specimen thickness was 18 mm (Fig. 1.). There were together 20 sp. The specimens were examined for their density, roughness, waviness, surface wetting with water and diiodomethane, surface free energy with distinguished the polar and disperse component. .

a)

b) Fig. 1 MDF test specimen: a) board surface structure, b) lateral view with detectable darker top layer.

Determining of MDF density and assessment of MDF surface morphology The density was determined on specimens demonstrated in (Fig. 1). Each specimen was weighed with a precision of 0.01g and measured (all three dimensions) with a precision of 0.01 mm. Then surface layers thick of 1.52 mm were sawn from the upper and lower surface. The density was determined separately for the two surface layers and the core.

MDF surface morphology was assessed with the aid of light microscopy. The appliance used was a microscope Leica MZ 9.5, equipped with a camera Leica EC 3. From the physical viewpoint, the MDF surface was evaluated based on roughness and waviness, measured with a profile-meter „Surfcom 130A“ supplemented with an evaluation unit and a software equipment.

Roughness and waviness were measured on each specimen two times, at two different spots. Altogether, there were completed 40 measurements. There were scanned MDF surface 37

profiles. The roughness evaluation was done in the following way: After filtering away waviness from the measured profile, there was obtained the roughness profile curve. This curve was transferred onto the base line. Then the roughness was filtered away from the curve. The final result was the waviness profile. The traversing length consisted of the startup length, five sampling lengths (cutoff) and the run-off length. The start up and run-off of the measuring equipment served for the elimination of vibrations possible to generate during starting and stopping the measuring eqipment. The sampling length was chosen from the interval 0.0258 mm based on preliminary measured values of roughness parameters Ra and Rz. In the case of MDF, the sampling length was 2.5 mm and the total evaluation length ln was 5  sampling length, making together 12.5 mm. The surface roughness and waviness were evaluated based on parameters: arithmetic mean deviation  Ra, (Wa), maximum height of the assessed profile within a sampling length  Rz, maximum height of the assessed profile within a traversing length  Rt, (Wt) and mean distance between the valley  RSm, (WSm) (EN ISO 4287).

MDF surface wetting with liquids and determining of surface free energy MDF surface resistance to liquids was tested with two liquids differing in polarity– redistilled water and diiodomethane. The two liquids were chosen following KÚDELA (2014).

Diiodomethane is a non-polar liquid with the non-polar surface free energy component higher than the disperse component of wood. The second liquid – redistilled water represents polarnon-polar liquids with the polar surface free energy component higher than the polar component of wood. The parameters for the two liquids can be found in Kúdela et al. 2020).

The MDF wetting process, associated with the measuring contact angle values as far as the complete drop soaking into the substrate was realised with using a goniometer Krüss DSA30 Standard (Krüss, Germany). The drop with a volume of 0.0018 ml, after having reached the MDF surface, wetted the surface and spread over it. The time course of the drop profile during the wetting was scanned with a camera. The scanning frequency was one second. The drop shape analysis was made and the contact angles were determined with using the circle method. Simultaneously, there was measured the drop diameter d (Kúdela at al.

2020). The contact angle value θ0 was determined at the beginning of the wetting process, this means immediately after the drop had reached the board surface. Based on the time-dependent variation of the parameter d (drop diameter), the moment of conversion of the contact angle from advancing to receding one was identified. The contact angle measured at this moment was considered as „equilibrium“ contact angle – e. The contact angle values 0 and equilibrium contact angle values e served for calculation of the contact angle values w for an ideally smooth surface. The calculation followed the methods designed by LIPTÁKOVÁ and KÚDELA (1994). On each test specimen, contact angles were measured at two different spots.

The MDF wetting was investigated with using two wetting liquids; consequently, the wood surface free energy was calculated separately from wetting with water and wetting with diiodomethane, following the adjusted equation proposed originally by NEUMANN et. al. (1974); the disperse and polar components Sd and Sp of this energy were calculated according to KLOUBEK (1974).

RESULTS AND DISCUSSION MDF surface density and morphology The visual observations of the board lateral surfaces as well as microscopic observations of the board structure in its surface layer and in its core layer resulted in finding that the fibre 38

fraction varied across the board cross section. There was confirmed that the MDF producer is using finer fibre fraction for the upper surface cover layer, with the aim to obtain smoother surface. After the pressing, this layer was thick from 1.5 to 2 mm, visible as a narrow darker band on the lateral board surface (Fig. 1b). The average density values for the whole boards and for their upper and core layers are in Table 1. The results confirm that the density across the cross section varied. The upper layer average density was evidently higher than the density of the medium layer (Table 1). In certain cases, the first attained even the values of hard fibre boards. AKBULUT and KOÇ (2006) report MDF density profiles demonstrating the lowest density in the board core, and the highest density on the board surface. The higher density of the surface consisting of fine fibre fraction was also reflected on this surface morphology. Under a 32 and a 60-ply magnification, the very fine particles on the MDF surface are not possible to specify with sufficient precision. The very fine wood particles and wood dust are impregnated with glue and paraffine (Fig. 2a). Under the same magnification, entire wood fibres in core layers were distinctly visible (Fig. 2b).

Tab. 1 Basic statistical characteristics for MDF density.

Basic statistical characteristics x̄ [kg·m3] s [kg·m3] n

Whole MDF 760 30 20

Surface layer 889 46 20

Core layer 714 15 20

x̄  average, s – standard deviation, n – sample size; the symbols are valid for all the tables

a)

b) Fig. 2 MDF structure a) surface layer , b) core layer.

39

The above described fine fibre fraction impregnated with glue and paraffin, in interaction with the pressing technology, determined the MDF surface geometry. This geometry was quantified though roughness and waviness parameters. A representative roughness and waviness profile for MDF surface is in Fig. 3.

Fig. 3 Roughess and waviness profile of MDF surface.

All the measured roughness and waviness profiles were evaluated trough roughness parameters Ra, Rz, Rt and RSm and waviness parameters Wa, Wt, WSm. The average values of these parameters, together with other statistic characteristics are in Table 2. Logically, we may suppose that higher MDF surface density will be associated with lower surface roughness. An example of using two surfaces with different properties is production of milled furniture doors.

Tab. 2 Basic statistic characteristics for roughness and waviness parameters.

Basic statistical characteristics x̄ [m] s [m] v [%] n

Ra 4.29 0.77 18.04 40

Roughness parameters Rz Rt 32.31 42.03 5.02 9.16 15.55 21.80 40 40

RSm 223.19 31.65 14.18 40

Wavines parameters Wa Wt WSm 2.20 13.77 2618.67 0.50 3.36 982.98 22.54 24.38 37.54 40 40 40

The roughness parameter values we measured in MDFs are in accordance with the values reported by AYRILMIS and WINANDY (2009), AYRILMIS et al. (2010) for MDFs subject to appropriate thermal treatment or to final sanding with a paper with a grain size of 120. In our case, the average value of the mean arithmetic deviation Ra was from 2 to 3 times lower than the value reported for various milled MDF surfaces by KMINIAK et al. (2020).

This implies that more distinct roughness is to consider in the case of milled surfaces common in furniture doors production.

The obtained roughness parameters were compared with the corresponding parameter values for spruce and beech  the two wood species most commonly used for MDF production in our region. The comparison showed that the MDF roughness was more distinct than the roughness of the milled surfaces of the two original wood species along the grain.

The MDF roughness and waviness parameter values were the same as the corresponding parameter values recognized for tangential surfaces of the two wood species milled perpendicular to the grain direction (KÚDELA et al. 2018).

40

MDF surface wetting and surface free energy The tested MDF surface exhibited resistance to water, which has also been approved with the measured contact angle values. In all the cases, the contact angle values at the moment where the liquid drop reached the substrate surface were higher than 90°, ranging from 110 to 135°, with an average value of 129°. It follows that the surfaces treated in this way were almost wetting resistant. The spreading rate of the liquid drop over the surface was very slow. The contact angle values had dropped under 90° as late as after 10 to 17 minutes. The contact angle values at the moment of drop application, after one minute and after two minutes of MDF wetting, together with other statistic variables are in Table 3.

Tab. 3 Time-dependent contact angle values  for MDF wetted with water and diiodomethane.

Basic statistical characteristics

Contact angle  for wetting with water

0

60

120

Contact angle  for wetting with diiodomethane

0

60

x̄ [°] 129 123 120 89 83 s [°] 4.8 7.4 9.6 9.4 11.6 n 40 40 40 40 40 The lower index corresponds to the time (0, 60, 120 seconds) of the contact angle measurement.

120 81 4.6 40

at the moment of application

after 22 min. of wetting

after 42 min. of wetting

at the moment of application

after 16 min. of wetting

after 22 min. of wetting

Water

Diiodomethane

High MDF surface resistance to wetting was also confirmed for diiodomethane. Also in this case, the contact angle values at the moment of the drop application were relatively high, ranging from 73 to 106°. The average contact angle values together with other statistical characteristics are in Table 3. Diiodomethane also exhibited a low spreading rate over the MDF substrate surface. After two minutes of wetting, the average contact angle had been reduced to 81° which represents only 8° reduction.

The wetting process with wetting liquids was scrutinised up to the complete drop spreading over and soaking into the substrate. In the case of water, this time was rather variable, lasting from 18 to 30 min. The equilibrium contact angle time was somewhat shorter, but not always possible do determine unambiguously. In the case of diiodomethane, even 42 minutes were not enough for drop spreading and soaking. As diiodomethane is a non-polar liquid, better wetting was expected on the background of interactions between unsaturated non-polar forces occurring at the beginning of the wetting process, mostly immediately after the diiodomethane touching the substrate KÚDELA (2014). The contact angle values varying with time are in Fig. 4. In the case of water, all the contact angle values at the moment of application were higher compared to diiodomethane, but to the end, the water spreading over the substrate surface was faster.

Fig. 4 Wetting-time-dependent profiles of water and diiodomethane drop.

41

Based on the works LIPTÁKOVÁ and KÚDELA (1997), CAI et al. (2016) and KÚDELA (2019) we may suppose that the principal cause resulting in high MDF surface resistance to wetting with liquids is paraffin admixed into the MDFs. The second important factor possibly backing-up lower wetting performance is the impact of pressing temperature, acting for a short time, but at high values. ANDOR (2018) demonstrates a substantial wood surface hydrophobization as the result of thermal treatment at a temperature of 180°C. The last cited work implies that after grinding, the surface hydrophobic performance was reduced significantly. This was also observed for MDFs (AYRILMIS et al. 2010). If the paraffin amount in the fibre board is higher, the paraffin may melt due to heat generated during grinding and, in this way, increase the MDF surface hydrophobic efficiency. Subsequently, this has a negative impact on the surface treatment of such surface with certain types of solvent-based coating substances (KÚDELA 2019).

To determine the equilibrium contact angle was tricky, so the surface free energy values were calculated based on the contact values measured at the moment of the drop application, both in the case of water and diiodomethane. The resulting surface free energy values, together with the disperse and polar components derived from the wetting with water and diiodomethane are in Tab. 4. The surface free energy obtained in this way was very low.

In both cases, the disperse components were dominant. Similar results for surface free energy in insulation fibreboards have been recognised by CAI et al. (2016).

Tab. 4 Surface free energy and its disperse and polar components on MDF surface.

Basic statistical characteristics x̄ [mJm2] s [mJm2] n

Surface free energy and its components derived from wetting with water

Surface free energy and its components derived from wetting with diiodomethane

s

s d

s p

s

s d

s p

7.35

7.28

0.06

17.86

14.16

3,70

2,49 40

2,41 40

0.10 40

4.58 40

4.55 40

0.16 40

The calculated MDF surface free energy was lower than the surface energy of coating substances in liquid phase (KÚDELA and LIPTÁKOVÁ 2006, ŠTRBOVÁ 2015). This may cause poor spreading of film-forming materials across MDF surface, and also have a negative impact on the film-forming material adhesion to the MDF surface. If paraffin distribution across the board surface is not uniform, the local surface tension in the film-forming materials may be weakened during the process of these materials application and drying. In this way, the surface tension gradient originates, causing the polymers from the film-forming material to flow from the areas with lower surface tension to the ones in which this tension is higher. This polymer flow is backing up the creation of orange peel and craters (WITTHE 1999, KÚDELA 2019).

As the MDF hydrophobization is necessary for improving their resistance to water, there is an urgent need for an appropriate surface treatment guaranteeing elimination of negative impact of hydrophobization on the surface treatment in these boards. A viable approach seems surface pre-treatment with plasma, prior to the film-forming substances application (de CADEMARTORI et al. 2015, KLÍMEK et al. 2016).

CONCLUSIONS Our research objective was to study MDF surface properties from the viewpoint of their surface treatment. The evaluation and analysis of the obtained results allow us to 42

deduce the following conclusions: The study of MDF morphology revealed differences in the structure between the MDF surface and core layers. The upper surface layer, thick of 1.5 to 2 mm is composed of fine fibre fraction impregnated with glue and paraffin. The core layer exhibited distinct observable fibrous elements.

The fine wood fibre fraction together with glue and paraffin interacted with wood pressing technology, and in such a way, determined the MDF surface geometry. This altogether resulted in lower roughness, evident from lower values of roughness and waviness parameters.

The tested MDF surfaces manifested high resistance to water and to non-polar liquids. This was documented with the measured contact angle values. In the case of water, the contact angle values at the moment of drop application onto the MDF surface ranged from 110° to 135°; the value range for diiodomethane was from 73° to 106°. MDF surface resistance to liquids was also evident on long time necessary for spreading and soaking the drop into the substrate. This time was several tens of minutes. The main role in the high MDF resistance to liquids has been attributed to paraffin admixed in MDFs. The MDF surfaces were characterized by a low surface energy with dominant disperse component. The MDF free surface energy was lower than the surface free energy of the film-forming materials. This may cause poor spreading of film-forming materials over the MDF surface, with possible negative impact on film-forming material adhesion to the MDF surface.

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ACKNOWLEDGMENTS This work was supported by the Slovak Research and Development Agency under the contracts No. APVV-16-0177. The author would like to acknowledge the support from Antonia Malenká and Bc. Adrián Žitniak concerning the laboratory work.

ADDRESSES OF AUTHOR

Prof. Ing. Jozef Kúdela, CSc. Technical University in Zvolen Faculty of Wood Sciences and Technology Department of Wood Science T. G. Masaryka 24 960 53 Zvolen Slovak Republic kudela@tuzvo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 47−59, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.05

COMPUTING THE 2D TEMPERATURE FIELD IN NON-FROZEN LOGS AT CHANGING ATMOSPHERIC TEMPERATURE AND DURING THEIR SUBSEQUENT AUTOCLAVE STEAMING Nencho Deliiski – Ladislav Dzurenda – Neno Trichkov – Natalia Tumbarkova ABSTRACT An approach for mathematical modelling and research into the 2D non-stationary temperature distribution in logs under an influence of periodically changing atmospheric temperature near them and during their subsequent steaming in autoclaves is described in the paper. Mathematical descriptions of the periodically changing atmospheric temperature and also of the temperature of the steaming medium in the autoclaves and of the conditioning air medium is introduced as boundary conditions in our own 2D non-linear mathematical model of the 2D temperature distribution in non-frozen logs during their heating and cooling.

Numerical solutions of the model in calculation environment of Visual FORTRAN Professional are given as an application of the suggested approach. The results from a simulative investigation into 2D non-stationary temperature distribution in non-frozen beech (Fagus sylvatica L.) logs during their 5 day and night heating and cooling at sinusoidal change of the air temperature with different initial values above 0 °C and different amplitudes, and also during steaming of the logs in autoclave and their subsequent conditioning are graphically presented and analysed. The obtained results can be used for development of energy saving steaming regimes with an optimal duration depending on the precise determining the initial temperature of the logs of each batch subjected to thermal treatment. They can be used also for the creation of a system for optimized model predictive automatic control of the steaming process of logs.

Key words: autoclave steaming, beech logs, 2D model, atmospheric temperature, heating, cooling, model based control.

INTRODUCTION It is known that the steaming of wood materials with different shape is an important part of the technological processes in the production of veneer, plywood, parquet, layered articles (CHUDINOV 1966, SHUBIN 1990, STEINHAGEN 1991, BURTIN et al. 2000, BEKHTA – NIEMZ 2003, DELIISKI 2003, 2009, 2011a, DAGBRO et al. 2010, DELIISKI – DZURENDA 2010, DZURENDA 2014), etc.

Logs from various tree species are subjected to steaming with the aim of plasticization in the production of veneer, plywood, and other items (CHUDINOV 1968, TREBULA – KLEMENT 2002, VIDELOV 2003, PERVAN 2009, DELIISKI et al. 2010). The higher temperature of the steaming medium and the good heat insulation of the autoclaves allow the significant reduction of the duration and the specific energy consumption of the process in comparison 47

with the traditional technologies for steaming wood materials under atmospheric pressure (RIEHL et al. 2002, DELIISKI 2003, 2009, SOKOLOVSKI et al. 2007, DELIISKI et al. 2013, 2015).

For the development and automatic realizing of energy saving steaming regimes with an optimal duration, and also for ensuring an optimal performance of the autoclaves it is very important to know the initial temperature of the logs of each batch subjected to steaming.

The initial temperature of the separate batches depends on the duration of the log storing in an open warehouse at periodically changing air temperature.

The aim of the present work is to suggest an approach to computing the temperature field in logs at periodically changing atmospheric temperature during many days and nights and to study the influence of the computed average mass temperature of such non-frozen logs on the duration of the regimes for their autoclave steaming.

MATERIAL AND METHODS Modelling of the 2D temperature distribution in non-frozen logs subjected to influence of periodically changing atmospheric temperature during days and nights When the length of the logs, L, is larger than their diameter, D, not more than 3 ÷ 4 times, for the calculation of the temperature changes in the logs’ longitudinal sections (i.e. along the coordinates r and z of these sections) during their heating or cooling in air medium the following 2D mathematical model can be used (DELIISKI – TUMBARKOVA 2019): c 

  2T (r , z, ) 1 T (r , z, )   r  T (r , z, )  2 T (r , z, )  r   .      r r r r 2   T 

 p

2

 T (r , z, ) z

with an initial condition:

2



 p  T (r , z, )   T  z

(1)

2

T r , z,0  T01

(2)

and the following boundary conditions: • along the radial coordinate r on the logs’ frontal surface:



 p (r ,0, ) T (r ,0, )  T (r ,0, )  Tair () r  p (r ,0, )



(3)

• along the longitudinal coordinate z on the logs’ cylindrical surface:



 (0, z, ) T (0, z, )  r T (0, z, )  Tair () z  r (0, z, )



(4)

Equations (1) to (4) represent a common form of a mathematical model of 2D heat distribution in non-frozen logs subjected to influence of periodically changing atmospheric temperature.

48

Modelling of the 2D temperature distribution in non-frozen logs during their steaming in an autoclave and subsequent cooling in an air medium The mechanism of 2D change in the temperature in the longitudinal sections of logs during their steaming and subsequent conditioning is mathematically described by eq. (1) (DELIISKI 2003) with an initial condition (5) T r , z,0  T02 and the following boundary conditions: • during the steaming process – at prescribed temperature of the steaming medium:

T (r,0, )  T (0, z, )  Tm ()

(6)

• during the cooling of the steamed logs in air environment – at convective boundary conditions, which are presented above by eqs. (3) and (4).

Equations (1), (5), (6), (3), and (4) represent a common form of a mathematical model of 2D heat distribution in non-frozen logs subjected to steaming and subsequent cooling in air medium.

Mathematical description of the thermo-physical characteristics of the non-frozen logs For solving and practical use of eq. (1) it is necessary to have mathematical descriptions of the radial and longitudinal thermal conductivity of the non-frozen wood, λr and λp respectively, the specific heat capacity of the non-frozen wood, c, and the wood density above the hygroscopic range (i.e. when u > ufsp), ρ.

Mathematical descriptions of the thermal conductivity and the specific heat capacity of non-frozen wood have been suggested by DELIISKI (1990, 1994, 2003, 2013a, 2013b) using the data for their change as a function of t and u experimentally determined in the dissertations by KANTER (1955) and CHUDINOV (1966, 1968). This data find a wide use in both the European (SHUBIN 1990, POŽGAJ et al. 1997, TREBULA – KLEMENT 2002, VIDELOV 2003, PERVAN 2009) and the American specialized literature (STEINHAGEN 1986, 1991, STEINHAGEN – LEE 1988, KHATTABI – STEINHAGEN 1992, 1993) when calculating various processes of the wood thermal treatment.

According to the mathematical description suggested in DELIISKI (1994, 2003, 2013a), the thermal conductivity of the non-frozen wood can be calculated using the following equations for (T , u, b ) above the hygroscopic range:

   0  1    (T  273 .15)

λ0  Kad-    [0.165  1.39  3.8u   (3.3  10 7 2b  1.015  10 3 b )]

(7) (8)

 579    3.65  0.124  10 3  b 

(9)

v  0.1284  0.013u

(10)

The precise values of the coefficient Kad-λ in eq. (8) for various tree species have been determined by DELIISKI (2003, 2013b). For beech (Fagus sylvatica L.) wood, the following values of the coefficient Kad-λ in eq. (8) were obtained: in radial direction Kr-λ = 1.35 and in longitudinal direction Kr-λ = 2.40.

According to the mathematical description suggested by DELIISKI (1990, 2003, 2011b, 2013b), the specific heat capacity of the non-frozen wood above the hygroscopic range can be calculated using the following equation: 49

c

1 (2862 u  2.95T  5.49u  T  0.0036 T 2  555 ) 1 u

(11)

The wood density ρ, which participates in eq. (1), is determined above the hygroscopic range according to the equation (CHUDINOV 1968, PERVAN 2009, DELIISKI 2011b, DELIISKI et al. 2015, HRČKA 2017, HRČKA – BABIAK 2017)

  b  (1  u )

(12)

The heat transfer coefficients αr in eq. (4) and αp in eq. (3) of the horizontally situated logs subjected to heating or cooling at free convection of the periodically changing air near them is equal to (TELEGIN at al. 2002, TUMBARKOVA 2019)

r  1.123  T (0, z, )  Tair () 

0.26

p  2.560  T (r ,0, )  Tair () 

0.26

(13) (14)

The heat transfer coefficients αr in eq. (4) and αp in eq. (3) of the beech logs subjected to air cooling after their autoclave steaming is equal to (DELIISKI 2003b, 2013b) r  0.380  1.026

T (0, z,

p  0.676  1.026

reg

T (r ,0,

reg

) Tair

  Т (0, z, 

 ( )   Т (r ,0, reg )  Tair ()

) Tair ()

reg )  Tair ()

(15) (16)

Mathematical description of the periodically changing atmospheric temperature For the numerical solving of the mathematical model (1) ÷ (4), a mathematical description of the change in the atmospheric temperature near the logs during many days and nights, Tair is needed.

The periodic change of the atmospheric temperature Tair during the time at a constant value of its amplitude Tair-a can be described by the following equation (DELIISKI 1988):

Т air  Т air0  (Tair -a  Tair0 )  sin(   )

(17)

where Tair0 is the initial value of Tair, K; Tair-a – amplitude value of Tair, K; ω – angular frequency of Tair, s1; τ – time, s.

The angular frequency of Tair in eq. (17) is equal to 2  (18) 0 where τ0 is the period of change in Tm, s. For the precise solving of tasks with the participation of eqs. (17) and (18) it is needed to use π = 3.14159.

For a periodic change of the air temperature during one day and night, i.e. at τ 0 = 1 d = 24 h = 86,400 s, according to eq. (18) it is obtained that 

2 2  3.14159   7.2722  105 s1 0 86400

When Tair0 and Tair-a gradually increase or decrease during the time compared to their initial values, Tair0-in and Tair-a-in, respectively, then the temperature Tair can be calculated using the equation

Т air  Т air0-in  (1  Kair0  )  [(Т air -a -in  Tair0 in )  (1  Kair -a  )]  sin(   ) (19)

50

where Kair0 and Kair-a are coefficients determining how much the change in Tair0 and Tair-a (in K) is over a period.

The signs “+” and “–“ on the right side of eq. (19) are used when Tair0 and Tair-a increase or decrease, respectively, during the periodical change in Tm. For the purpose of analysis of the current log temperature condition at the initial temperature of the logs before their steaming, synchronously with the solving of the model (1) to (4), the average mass temperature of the logs, Т avg , for each moment of their periodically heating and cooling is calculated according to the equation Т avg 

1 Sw

Ti,k dSw n

(20)

Sw

RESULTS AND DISCUSSION The mathematical descriptions of the thermo-physical characteristics of non-frozen logs considered above, and also of the periodically changing atmospheric temperature were introduced in the mathematical models (1) to (6).

For numerical solution of the models aimed at computation of 2D temperature fields in logs, a software package was prepared, which was an input in the calculation environment of Visual FORTRAN Professional developed by Microsoft. For transformation of the models in a form suitable for programming an explicit form, the finite-difference method has been used (DELIISKI 2011b, 2013b). The calculation mesh was built on ¼ of the longitudinal section of the logs due to the circumstance that this ¼ was mirror symmetrical towards the remaining ¾ of the same section.

Using that package, computations were made for the determination of the 2D nonstationary temperature distribution in the longitudinal sections of non-frozen beech logs with different dimensions and initial temperatures.

The change in temperature of the processing air medium, tm, log surface and average mass temperature, ts and tavg, respectively, and also t of 4 characteristic points in the logs subjected to atmospheric temperature influence or to autoclave steaming were studied in this work.

The coordinates of the four characteristic points in the longitudinal section of the logs were equal to: Point 1: r = R/4 and z = L/4; Point 2: r = R/2 and z = L/4; Point 3: r = 3R/4 and z = 3L/8; Point 4: r = R and z = L/2. These coordinates of the points allow the determination and analysing the 2D temperature distribution in logs during their heating and cooling.

During the solution of the models, the above presented descriptions of the thermophysical characteristics of non-frozen beech wood with the moisture content of 0.6 kg·kg1 and basic density ρb = 560 kg·m3 (DELIISKI – DZURENDA 2010) were used.

Computation of 2D temperature field in logs at changing atmospheric temperature Three options of 120 h (i.e. 5 d) continuous periodic heating and cooling of non-frozen beech logs with a diameter D = 0.24 m, length L = 0.48 m, moisture content u = 0.6 kg·kg1, and initial temperature t01 = 10 °C (refer to eq. (2)) were studied as follows: • for Log 1: at constant values of tair0 = 20 °C and tair-a = 20 °C; • for Log 2: at gradual increasing of tm0-in = 20 °C by 2 °C/d and synchronously with this gradual decreasing of tair-a-in = 20 °C by 4 °C/d.

51

• for Log 3: at gradual decreasing of tair0-in = 20 °C by 2 °C/d and synchronously with this gradual decreasing of tair-a-in = 20 °C by 4 °C/d.

To provide a change of tair0 by 2 °C/d and of tair-a by 4 °C/d the following values of the coefficients Kair0 and Kair-a in eq. (19) were used: Kair0 = 7.89635·10-8 and Kair-a = 2.3148·10-6.

Fig. 1, Fig. 2 and Fig. 3 present the calculated change in tm, ts, tavg, and t of 4 characteristic points in Log 1, Log 2 and Log 3 during their continuous 5 day and night (i.e.

120 h) periodic heating and cooling under the described above conditions of the atmospheric temperature influence.

The coordinates of the characteristic points were as follows: Point 1 with the temperature t1: r = R/4 = 30 mm and z = L/4 = 120 mm; Point 2 with t2: r = R/2 = 60 mm and z = L/4 = 120 mm; Point 3 with t3: r = 3R/4 = 90 mm and z = 3L/8 = 180 mm; Point 4 with t4: r = R = 120 mm and z = L/2 = 240 mm.

In Fig. 1 it is seen that at constant values of tair0 and tair-a after 48th h, i.e. after the 2nd period of tair, a periodical change in the log temperature with practically constant amplitudes for the separate points is coming. As far as the point is distanced from the log surfaces that much smaller is the amplitude of the periodic change of the temperature in that point.

The amplitudes of tair, tavg, and t in the separate characteristic points after the 2nd period are equal to as follows: tair-a = 20.0 °C, ts-a = 11.4 °C, t1a = 10.9 °C, t2a = 9.7 °C, t3a = 8.8 °C, t4a = 8.6 °C, and tavg-a = 9.6 °C. The average mass temperature of the Log 1, tavg, at 120th h reaches a value equal to 11.2 °C. When tair0 increases and tair-a decreases during the time, the amplitudes of t in the separate points and also of tavg gradually decrease but the minimal values of tavg gradually increase from period to period (see Fig. 2). At 120th h the amplitudes of all log points become equal to 0 when tair0 = 20 °C + 2 °C/d and tair-a = 20 °C – 4 °C/d, and then tavg of Log 2 reaches a value, equal to 29.0 °C. 50

Beech log 1: t 01 = 10 oC, u = 0.6 kg.kg-1 t air0 = 20 oC = const, t air-a = 20 oC = const

tair

40

Temperature t , °C

ts t1

30

t2

20

t3 t4

10

tavg

0 0

12

24

36

48

60

72

84

96

108

120

Time τ, h

Fig. 1 Change in tair, ts, tavg, and t of 4 characteristic points of the Log 1 during its 120 h periodical heating and cooling at constant values of tair0 and tair-a.

52

50

Beech log 2: t 01 = 10 oC, u = 0.6 kg.kg-1 t air0 = 20 oC + 2 oC / d, t air-a = 20 oC - 4 oC / d

tair

40 Temperature t , °C

ts t1

30

t2

20

t3 t4

10

tavg

0 0

12

24

36

48

60

72

84

96

108

120

Time τ, h

Fig. 2 Change in tair, ts, tavg, and t of 4 characteristic points of the Log 2 during its 120 h periodical heating and cooling at increasing values of tair0 and decreasing values of tair-a.

50

Beech log 3: t 01 = 10 oC, u = 0.6 kg.kg-1 t air0 = 20 oC - 2 oC / d, t air-a = 20 oC - 4 oC / d

tair

40 Temperature t , °C

ts t1

30

t2

20

t3 t4

10

tavg

0 0

12

24

36

48

60

72

84

96

108

120

Time τ, h

Fig. 3 Change in tair, ts, tavg, and t of 4 characteristic points of the Log 3 during its 120 h periodical heating and cooling at decreasing values of tair0 and tair-a.

When tair0 and tair-a decrease during the time, the amplitudes of t in the separate points and also of tavg gradually decrease but the maximum values of tavg gradually decrease from period to period (see Fig. 3). At 120th h the amplitudes of all the log’s points become equal to 0 when tair0 = 20 °C – 2 °C/d and tair-a = 20 °C – 4 °C/d, and then tavg of Log 3 reaches a value, equal to 10.7 °C. Computation of 2D temperature field in logs during their steaming in an autoclave Two options of autoclave steaming and subsequent conditioning of non-frozen beech logs (named as Log 4 and Log 5) with a diameter D = 0.4 m, length L = 0. 8 m, and the moisture content 0.6 kg·kg-1 were studied as follows: • Log 4 was with initial temperature t02 = 0 °C (refer to eq. (5)); • Log 5 was with initial temperature t02 = 20 °C. 53

During the solving the mathematical model, 3-stage regimes for autoclave steaming of the logs were used. The typical temperature time profile of the processing medium temperature tm in a steaming autoclave and the air medium for the consequent conditioning of the heated wood materials is shown in (DELIISKI 2003, DELIISKI – DZURENDA 2010).

During the first stage of the steaming regimes input of water steam is accomplished in the autoclave, with logs situated inside, until the temperature of the processing medium tm = 132 °C at the steam pressure of 0.2 MPa was reached. After reaching tm = 132 °C, this temperature was maintained unchanged by reducing the input of steam flux inside the autoclave until the calculated by the model average mass temperature of the wood, tavg, reaches a value of 90 °C. After reaching tavg = 90 °C the input of steam in the autoclave was terminated and the second stage of the steaming regime began. During this stage, by using the accumulated heat in the autoclave, the further heating and plasticizing of the logs was accomplished, thus resulting in gradual reduction of the temperature tm for about 2 hours down to around 110 °C. Afterwards, the cranes directing the steam and condensed water out of the autoclave were opened, which initiates the third stage of the steaming regime. This stage ended after about 2 hours, when tm reached approximate value of around 85 °С. After that, a conditioning of the logs under external aerial medium was conducted. During the time of conditioning of the heated logs a redistribution and equalization of the temperature in their volume took place, which was especially appropriate for the obtaining of quality veneer.

In Fig. 4 and Fig. 5 the calculated change in tm, ts, and t of 4 characteristic points of the Log 4 and Log 5 during their autoclave steaming and subsequent conditioning at tm = 20 °C is presented. The coordinates of the characteristic points of Logs 1 and 2 correspond according to the theory of similarity to the coordinates of the characteristic points in Logs 1 to 3, as follows: Point 1 with temperature t1: r = R/4 = 50 mm and z = L/4 = 200 mm; Point 2 with t2: r = R/2 = 100 mm and z = L/4 = 200 mm; Point 3 with t3: r = 3R/4 = 150 mm and z = 3L/8 = 300 mm; Point 4 with t4: r = R = 200 mm and z = L/2 = 400 mm.

One of the aims of this study was to determine how the average mass temperature of the logs at the beginning of the steaming process influences the duration of the steaming regime. That is why during the simulations with the model, the initial temperature of Log 4 and Log 5, t02, which in the practice is equal to tavg of the logs after their staying in an open warehouse before steaming, were assumed to be equal to t02 = 0 °C and t02 = 20 °C respectively.

In Fig. 4 and Fig. 5 the minimum and maximum values of the temperature, tmin = 62 °C and tmax = 90 °C are also shown. It is well known that for obtaining the quality veneer from plasticized beech wood it is needed that the temperature of all characteristic points of the logs during the veneer cutting process stays between these optimum values of tmin and tmax (DELIISKI 2003, DELIISKI – DZURENDA 2010).

The computation of the temperature fields in the logs was done interconnectedly for the processes of their heating in an autoclave and their subsequent conditioning in an air environment. This means that the calculation of the non-stationary 2D change in temperatures in the longitudinal sections of the logs during the time of their conditioning begins from the already reached during the time of calculations distribution of temperature at the end of the heating. Based on the calculations it can be determined when the moment of reaching in the entire volume of the heated logs occurred for the necessary ideal temperatures (between tmin and tmax on Fig. 4 and Fig. 5) needed for cutting the veneer.

54

160

Log 4: D = 0.4 m, L = 0.8 m, ρ b = 560 kg·m-3, u = 0.6 kg·kg-1, t 02 = 0 oC

tm

120

ts

o

Temperature t , C

140

100

t1

80

t2 t3

60

t4

40

tmax

20

tmin

0 0

2

4

6

8 10 Time τ, h

12

14

16

Fig. 4 Change in tm, ts, and t in 4 characteristic points of the Log 4 with t02 = 0 °C during its steaming in an autoclave and its subsequent conditioning at tair = 20 °C. 160

Log 5: D = 0.4 m, L = 0.8 m, ρ b = 560 kg·m-3, u = 0.6 kg·kg-1, t 02 = 20 oC

tm

120

ts

o

Temperature t , C

140

100

t1

80

t2

60

t3 t4

40

tmax

20

tmin

0 0

2

4

6

8 10 Time τ, h

12

14

16

Fig. 5 Change in tm, ts, and t in 4 characteristic points of the Log 5 with t02 = 20 °C during its steaming in an autoclave and its subsequent conditioning at tair = 20 °C.

It can be seen in Fig. 4 and Fig. 5 that temperatures of all characteristics enter between tmin = 62 °C and tmax = 90 °C after the following duration of the conditioning process of the logs at the air environment: 90 min for Log 4 and 60 min for Log 5.

The analysis of Fig. 4 and Fig. 5 showed that the duration of the steaming regimes of the studied logs was equal to τreg = 10 h for Log 4 and to τreg = 9 h for Log 5. This means that an increase in the initial log temperature t02 by 20 °C caused a decrease in τreg by 1 h, i.e. each increase in t02 by 1 °C in our case causes a decrease in τreg by approximately 3 min.

CONCLUSIONS This paper describes an approach to mathematical modelling and research into the 2D non-stationary temperature distribution in non-frozen logs under influence of periodically changing atmospheric temperature and during their subsequent steaming in autoclaves.

55

Mathematical descriptions of the periodically changing atmospheric temperature and of the temperature of the log steaming regimes and of their subsequent conditioning in an air environment were carried out. These descriptions are introduced as boundary conditions in our own 2D non-linear mathematical model of the 2D temperature distribution in non-frozen logs during their heating and cooling.

A software package for solving the model and computing the 2D temperature field of logs during considered processes were prepared in FORTRAN, which was input in the calculation environment of Visual FORTRAN Professional. The paper shows and analyses, e.g. for the application of the suggested approach, diagrams of the change in tm, ts, tavg, and 2D temperature distribution in non-frozen beech logs with basic density of 560 kg·m-3 and moisture content of 0.6 kg·kg-1 for the following two cases: • during 5 days (i.e. 120 h) of continuous heating and cooling of logs with D = 0.24 m and L = 0.48 m under an influence of periodically changing atmospheric temperature at the constant values of tair0 = 20 °C and tair-a = 20 °C (Log 1); at gradual increasing of tm0-in = 20 °C by 2 °C/d and synchronously with this gradual decrease in tair-a-in = 20 °C by 4 °C/d (Log 2) and at gradual decrease in tair0-in = 20 °C by 2 °C/d and synchronously with this gradual decrease in tair-a-in = 20 °C by 4 °C/d (Log 3). It was calculated that at the end of 120 h periodically heating and cooling of the studied logs their average mass temperature was equal to 11.2 °C for Log 1, to 29.0 °C for Log 2, and to 10.7 °C for Log 3; • during 3-stage regimes for autoclave steaming of two logs with D = 0.4 m, L = 0.8 m, and an initial temperature of 0 °C and 20 °C and during the time of their subsequent conditioning at the air temperature of 20 °C. It was determined that the duration of the steaming regimes of the studied logs was equal to τreg = 10 h for Log 4 and to τreg = 9 h for Log 5, which means that each increase in the initial log temperature by 1 °C in the considered case caused a decrease of τreg by approximately 3 min.

The obtained results can be used for the development of energy saving steaming regimes with an optimum duration depending on the initial temperature of the logs of each batch subjected to thermal treatment. This will ensure optimal productivity of the autoclaves.

The presented approach to the computing the 2D temperature field in logs and their average mass temperature at periodically changing atmospheric temperature can help the accurate determination of the initial temperature of the logs before steaming, depending on the duration of the log storing in an open warehouse. This approach can be used also for the creation of a system for optimized model predictive automatic control (DELIISKI 2004, 2011a, HADJISKI – DELIISKI 2016) of the steaming process of logs and other wood materials.

Symbols c d D L R r S T t u z α

– specific heat capacity, J·kg1·K1 – day and night, 1 d = 24 h = 86,400 s – diameter, m – length, m – radius, m: R = D/2 – radial coordinate: 0  r  R, m – aria (for longitudinal section of the logs), m2 – temperature, K: T = t + 273.15 – temperature, °C: t = T – 273.15 – moisture content, kg·kg-1 = %/100 – longitudinal coordinate: 0  z  L/2, m – heat transfer coefficients between log’s surfaces and the surrounding air medium, W·m-2·K-1

56

  τ ω d

– thermal conductivity, W·m-1·K-1 – density, kg·m-3 – time, s – angular frequency, s-1 – day and night: d = 24 h = 86,400 s

Subscripts: a – amplitude ad – anatomical direction air – air avg – average (for wood mass temperature) b – basic (for wood density, based on dry mass divided to green volume) fsp – fiber saturation point i – knot of the calculation mesh in the direction along the logs’ radius: i = 1, 2, 3,…, 21 k – knot of the calculation mesh in longitudinal direction of the logs: k = 1, 2, 3, …, 41 in – initial m – medium (for temperature of the steaming medium) max – maximal min – minimal p – parallel to the wood fibres r – radial direction reg – regime (for duration of the steaming regimes) s – surface w – wood 0 – period of the change in the atmospheric temperature, or at 0 °C (for λ) 01, 02 – initial (for temperature of logs in the beginning of their thermal treatment) Superscripts: – current number of the step along the time coordinate: n = 0, 1, 2,… n REFERENCES CHUDINOV, B. S. 1966. Theoretical Research of Thermo Physical Properties and Thermal Treatment of Wood. Dissertation for DSc., Krasnojarsk, USSR : SibLTI. CHUDINOV, B. S. 1968. Theory of the Thermal Treatment of Wood. Moscow : Nauka, 255 pp.

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KANTER, K. R. 1955. Investigation of the Thermal Properties of Wood. PhD Thesis, Moscow, USSR : MLTI. KHATTABI, A., STEINHAGEN, H. P. 1992. Numerical Solution to Two-dimensional Heating of Logs. In Holz als Roh- und Werkstoff, 50 (78): 308312.

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POŽGAJ, A., CHOVANEC, D., KURJATKO, S., BABIAK, M. 1997. Structure and Properties of Wood. 2nd edition, Bratislava : Príroda a.s., 485 pp.

RIEHL, T., WELLING, J., FRÜHWALD, A. 2002. Druckdämpfen von Schnittholz, Arbeitsbericht 2002/01: Institut für Holzphysik, Hamburg: Bundesforschungsanstalt für Forst- und Holzwirtschaft.

SHUBIN, G. S. 1990. Drying and Thermal Treatment of Wood. Moscow : Lesnaya Promyshlennost, 337 pp.

SOKOLOVSKI, S., DELIISKI, N., DZURENDA, L. 2007. Constructive Dimensioning of Autoclaves for Treatment of Wood Materials under Pressure. In Woodworking Techniques, Zalesina, Croatia, 117126.

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STEINHAGEN, H. P. 1986. Computerized Finite-difference Method to Calculate Transient Heat Conduction with Thawing. In Wood and Fiber Science 18(3), p. 460467.

STEINHAGEN, H. P. 1991. Heat Transfer Computation for a Long, Frozen Log Heated in Agitated Water or Steam – A Practical Recipe. In Holz Roh- Werkstoff, 49(7–8): 287290.

STEINHAGEN, H. P., LEE, H. W. 1988. Enthalpy Method to Compute Radial Heating and Thawing of Logs. In Wood and Fiber Science, 20(4): 415421.

TELEGIN, A. S., SHVIDKIY, B. S., YAROSHENKO, U. G. 2002. Heat- and Mass Transfer. Moscow : Akademkniga, 456 pp.

TREBULA, P., KLEMENT, I. 2002. Drying and Hydrothermal Treatment of Wood. Zvolen : TU vo Zvolene, 449 pp.

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ACKNOWLEDGEMENTS This document was supported by the APVV Grant Agency as part of the project: APVV-17-0456 as a result of work of authors and the considerable assistance of the APVV agency.

AUTHORS’ ADDRESSES Prof. Dr. Nencho Deliiski, DSc. University of Forestry Faculty of Forest Industry St. Kliment Ohridski Blvd. 10 1797 Sofia Bulgaria deliiski@netbg.com Prof. Ing. Ladislav Dzurenda, PhD. Technical University in Zvolen Faculty of Wood Science and Technology T. G. Masaryka 24 960 01 Zvolen Slovakia dzurenda@tuzvo.sk Assoc. Prof. Neno Trichkov, PhD. Eng. Mag. Natalia Tumbarkova, PhD. University of Forestry Faculty of Forest Industry St. Kliment Ohridski Blvd. 10 1797 Sofia, Bulgaria ntrichkov@gmail.com ntumbarkova@abv.bg

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 61−74, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.06

INTERLAYER WITH MICROCAPSULES AND ITS INFLUENCE ON THE SURFACE FINISH QUALITY OF WOOD Gabriela Slabejová  Mária Šmidriaková  Ján Svocák ABSTRACT The paper deals with the quality of standard and modified surface finishes on beech, oak and spruce wood intended into exterior. Modified surface finish was created by the same coating materials as a standard surface finish, but it contained an interlayer with microcapsules. The quality of the surface finishes was assessed according to the film hardness, impact resistance, the resistance to scuffing, adhesion tests, the Cross-cut test and the Pull-off test for adhesion. The interlayer with microcapsules had no impact on the surface hardness of the surface finishes on beech wood and spruce wood, but it positively affected the film hardness of oak wood. The modified surface finish on beech wood and spruce wood showed a low impact resistance at a low drop height. It was more fragile and the impact resistance test resulted in the cracks formation. The modified surface finish was more resistant to scuffing. The adhesion of the standard and modified surface finishes to individual tree species, tested according to the Cross-cut test, was the same or minimally different.

According to the Pull-off test, the standard surface finish showed higher adhesion to beech wood and oak wood when compared with the modified surface finish. On spruce wood, the Pull-off test for adhesion determined cohesion of wood surface layers.

Key words: adhesion, hardness, impact resistance, modified surface finish, resistance against scuffing

INTRODUCTION The exterior pigmented surface finish is intended to protect wood, enhance the product aesthetically, and cover the substrate. Requirements for exterior wood finishing are put for the weather resistance mainly. In the world, the research is carried out using transparent and pigmented surface finishes. “To extend the lifetime of wood and maintain its natural look, the research and development of clear coatings with minimal use of harmful chemicals has become very important” (MIKLEČIĆ et al. 2017).

The resistance of the surface finish to weathering is not the only property that determines the quality. Many properties – visual, chemical, resistant, physical-mechanical – give information about how the surface finish will behave during the use. The important physical-mechanical property of the exterior surface finishes is the adhesion. In several works, the influences of type of surface finish, parts of wood, moisture, wood-destroying fungi, aging, and the surface pre-treatment on the adhesion were investigated (HAZIR and KOC 2019, SLABEJOVÁ and VIDHOLDOVÁ 2019a, b, MIKLEČIĆ et al. 2017, VIDHOLDOVÁ et al. 2017, COOL and HERNÁNDEZ 2016, UGULINO and HERNÁNDEZ 2016, ŠOMŠÁK and 61

REINPRECHT, 2015, TOLVAJ et al. 2014, PODGORSKI et al. 2010, BULIAN and GRAYSTONE 2009, DELPECH and COUTINHO 2000, DE MEIJER and MILITZ 1998).

The surface finishes created by coating materials were evaluated according to appearance and physical-mechanical properties (HAZIR and KOC 2019, SLABEJOVÁ et al.

2019, SLABEJOVÁ et al. 2018, SLABEJOVÁ and ŠMIDRIAKOVÁ 2018, TESAŘOVÁ et al. 2017, SALCA et al. 2017, YONG et al. 2017, BEKHTA et al. 2014, MODRÁK and MANDULÁK 2013, SCRINZI et al. 2010).

To increase protection of wood and surface finish, the coatings can be adapted. The coatings can be modified with nano-technological products (CATALDI et al. 2017, MIKLEČIĆ et al. 2017, REINPRECHT and VIDHOLDOVÁ 2017, WETHTHIMUNI et al. 2016, KUMAR et al.

2015, KAYGIN and AKGUN 2009, LEE et al. 2003).

The presented work deals with pigmented surface finish intended to exterior. The surface finish contained a modifying interlayer with nano-technological product. The influence of the modifying intermediate layer on selected physical-mechanical properties of the surface finish was monitored.

MATERIALS AND METHODS Test specimens Beech wood (Fagus sylvatica L.), oak wood (Quercus petraea L.), and spruce wood (Picea abies /Karst./ L.) were used in the experiment. The dimensions of the test specimens were 250 mm × 80 mm × 20 mm, and the moisture content of 8 % ± 2 %.

The test specimens’surface was sanded according to the recommendations listed in technical sheets for the coating materials. The test specimens were surface finished on all sides by low-pressure spraying.

Coating materials The following representative coating materials (ADLER) for pigmented surface finishinginto exteriorwere selected:  Aquawood TIG HighRes Weiss – a base coat for wood, water-based protective impregnation with good covering ability; it is designed for a three-layer coating system with Aquawood Intercare SH and Aquawood – Spritzlack XT.  Aquawood Intercare SH – water-based interlayer with low content of solvents, enriched with microcapsules for sealing of cracks caused by weathering.  Aquawood Spritzlack XT – water-based pigmented finishing coat with excellent resistance to weathering.

Two types of surface finish were made:  Standard surface finish – 1 coat with the Aquawood TIG HighRes Weiss (the average film thickness of 100 μm in wet condition) and 1 coat with the AquawoodSpritzlack XT (150-180 μm in wet condition).  Modified surface finish – 1coat of the Aquawood TIG HighRes Weiss (100μmin wet condition), 1 coat of the modifying coating material AquawoodIntercare SH (150-180 μmin wet condition) and 1coat of the AquawoodSpritzlack XT (150-180 μm in wet condition).

Thickness of the coating film Two methods were chosen to measure the film thickness:  Destructive method – using the SuperPig SP 1100 instrument.  Non-destructive method – using the ultrasonic instrument PosiTector 200.

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Film hardness The film hardness was determined by the Pencil test according to the standard STN EN ISO 15184 (2012). The results of the test were evaluated according to the pencil that scratched the surface (Table 1). The test started with the softest pencil – number 1.

Tab. 1 Degrees of film hardness.

Pencil number Pencil hardness

1 3B

2 2B

3 B

4 HB

5 F

6 H

7 3H

8 4H

9 5H

10 6H

11 7H

12 8H

13 9H

Impact resistance The impact resistance of the surface finishes was determined according to the standard STN EN ISO 6272-2 (2011). The intrusion (diameter of the intrusion) was measured and the surface finish was evaluated subjectively according to Table 2.

Tab. 2 Impact resistance: degree and evaluation.

Degree 1 2 3 4 5

Visual evaluation No visible changes No cracks on the surface and the intrusion was only slightly visible Visible light cracks on the surface, typically one to two circular cracks around the intrusion Visible large cracks at the intrusion Visible cracks were also off-site of intrusion, peeling of the coating

Surface resistance against scuffing Evaluation of the surface finish resistance against scuffing was determined according to the standard STN EN ISO 7784-3 (2006). The coefficient of the resistance against scuffing KT was calculated according to the formula: KT = (m1 – m2)/F

(1)

Where: m1 – specimen weight before sanding (g), m2 – specimen weight after sanding (g), F – correction coefficient of the used pair of abrasive papers (F = 1,052).

Sanding number ZT was calculated according to the formula: ZT = n * F

(2)

Where: n – the number of revolutions until the surface is sanded down to the surface, F – correction coefficient of the used abrasive papers (F= 1,052).

The surface sanding number ZT is determined when the tested surface finish is sanded down to the surface to at least 25 % of the sanded surface.

Adhesion tests Adhesion of the coating films to wood was determined by the Pull-off test according to the standard STN EN ISO 4624 (2016) and by theCross-cut test according to the standard STN EN ISO 2409 (2013).

The testing machine PosiTest AT-M (Qualitest, Canada) was used for the Pull-off test.

Small 20 mm diameter dollies were glued to the coating using two-component epoxy resin (Pattex Repair Epoxy). After 24 h of curing at 20 °C and a relative air humidity of 60%, perimeters of glued dollies were carefully incised to prevent propagation of failures outside the tested area. Pulling was carried at a rate of 1 mm/min up to separation of the dolly from the surface. After each test, the fracture was evaluated visually using a stereo-microscope LEICA MZ 9.5 with magnification of 4 ×.

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The Cross-cut test was done as follows: a cross hatch pattern was cut through the coating film to the substrate. The adhesion of the coating film was classified according to the standard STN EN ISO 2409 (2013) (Table 3). The figures are examples for a cross-cut within each step of the classification. The percentages stated are based on the visual impression given by the pictures and the same percentages will not necessarily be reproduced with digital imaging.

Tab. 3 Evaluation of the cross-cut area.

Classification Surface of crosscut area from which flaking has occurred.

(Example for six parallel cuts)

0

1

2

3

4

5

none

< 5%

5% – 15%

15% – 35%

35% – 65%

> 65%

RESULTS AND DISCUSSION Thickness of coating film The film thickness of surface finishes was determined by two methods (destructive, nondestructive, Table 4). The coating film thickness determined by the two different methods was similar. More significant difference was noticed only for the standard surface in the case of spruce wood (destructive method 138 μm; non-destructive method 108 μm). In all cases, the thickness of the coating film determined by the destructive method was slightly higher than the thickness determined by the non-destructive method. These slight differences may be due to the fact that if measured by the destructive method, the part of the film impregnating the surface layer of wood is included in the thickness of the coating film. In the ultrasonic non-destructive method, these surface layers are probably not counted to the film thickness, but are considered as a substrate. Particularly onthe spruce wood, the wood surface layer could be impregnated deeper (from a microscopic point of view).

Knowing the thickness of the coating film is not only needed to determine the adhesion by the Cross-cut test, but the thickness also affects quality of the surface finish.

HUNDHAUSEN et al. (2018) and PALIJA et al. (2018) focused on factors that affect the thickness of a coating film in industrial production; because the effect of thickness on quality is significant.

Tab. 4 Coating films thickness.

Sample Destructive Non- destructive

Beech – 2 166 158

Coating film thickness [μm] Beech – 3 Oak – 2 Oak – 3 256 140 270 237 129 251

Spruce – 2 138 108

Spruce – 3 218 206

Film hardness The surface hardness of the coating film is a degree, which corresponds with the pencil which damaged the surface as the first (Table 5). Both types of surface finish in the case of spruce wood and beech wood showed the film hardness of 6. On oak wood, the standard surface finish reached the hardness of 8 and the modified surface finish the hardness of 9. The results show that higher surface hardness could be reached by a harder substrate, a slightly thicker coating film, or by the interaction of the two factors. The highest hardness of the coating 64

film was achieved by the modified surface finishes on oak wood (Table 5). From comparison of the two types of surface finish in the case of spruce wood and beech wood, we can conclude that the surface hardness was not affected by the interlayer. The results in this work are similar to the statements by HAZIR and KOC (2019) that higher surface hardness may not provide higher impact resistance, but a harder surface may be more fragile.

Tab. 5 Surface hardness of coating films.

Sample Pencil number

Beech– 2 6

Degree of film hardness Beech– 3 Oak– 2 Oak– 3 6 8 9

Spruce – 2 6

Spruce– 3 6

Impact resistance of surface finish The impact resistance, (the diameter of the intrusion) was measured and the damage on the surface finish was evaluated subjectively according to Table 6. The modification of surface finish did not increase the impact resistance if compared to the standard surface finish. On spruce wood, the modified surface finish showed low impact resistance even at a drop height of 50 mm. At this drop height, a semi-circular crack was formed around the intrusionin the surface finish. This was also observed on beech wood at drop heights of 100 mm and 200 mm, and on oak wood at a drop height of 200 mm. We can conclude that the modified surface finishis less resistant than the standard surface finish. It is more fragile and therefore cracks appear on it sooner.

However, the total damage of both types of surface finish was greater in the case of spruce wood than on beech wood or oak wood. The lowest impact resistance (grade 5) was measured for both types of surface finish in the case of spruce wood at the maximum drop height of 400 mm (Fig. 1). In this case, the cracks were formed in the surface finish outside the intrusion. This finding shows that the modified surface finish may be more prone to cracking when hailing or during mechanical damage caused by little rocks. The purpose of the inter-layer is to seal the micro-cracks formed during aging of the surface finish in the exterior. But we found that the modified surface finish is more prone to cracking under mechanical loading.

Tab. 6 Degree of change on the surface and diameter of the intrusions.

Sample BEECH - 2 Ø mm BEECH - 3 Ø mm OAK - 2 Ø mm OAK - 3 Ø mm SPRUCE - 2 Ø mm SPRUCE - 3 Ø mm

10 1 0 1 0 1 0 1 0 1 0 1 0

Drop height [mm] and Degree of change 25 50 100 200 1 2 2 2 1 3 4 4 2 2 3 3 2 2 4 4 2 2 2 2 2 3 4 5 1 2 2 3 1 2 4 4.5 2 2 3 3 1 3 4 5 2 3 3 4 2 4 4.5 5

400 3 5 3 5 3 5 3 5 5 5.5 5 6

Impact resistance of the coating increases with increasing thickness of the coating to some extent (SLABEJOVÁ 2012, SLABEJOVÁ et al. 2018). In our case, the coating film of the modified surface finish had higher thickness than the standard surface finish and this fact also confirms the above mentioned statement. HAZIR and KOC (2019) claim that the impact 65

resistance of the surface finish is significantly influenced by the type of surface finish. That means it is influenced by all the components present in the coating film. This was also confirmed by our results; the modifying interlayer had an effect on the impact resistance of surface finish at some drop heights.

Standard

Modified Beech

Standard

Modified Oak

Standard

Modified Spruce

Fig. 1 Surface finishes after impact resistance test at a drop height of 400 mm; evaluated visually using a stereomicroscope (with magnification of 4 ×).

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Surface finish resistance against scuffing When evaluating the resistance against scuffing, the modified surface finish showed higher resistance than the standard surface finish on all three tree species. The coefficient of the coating film resistance against scuffing KT for the modified surface finish was smaller than the one for the standard surface finish (Table 7).

Tab. 7 The coefficient of resistance against scuffing K T and sanding number ZT. Surfac efinish resistance agains scuffing KT

Beech – 2 0.057

Beech – 3 0.038

Oak – 2 0.057

Oak – 3 0.038

Spruce – 2 0.066

Spruce – 3 0.019

ZT

841.6

1578

736.4

1683.2

946.8

1472.8

Sample

Standard

Modified Beech

Standard

Modified Oak

Standard

Modified Spruce

Fig. 2 Scans of the surface finishes after resistance to scuffing test.

The higher resistance against scuffing was also confirmed by the other coefficient, the sanding number ZT (Table 7). For the standard surface finish, at least 25 % sanding to the substrate (Fig. 2) occurred after only 700 revolutions on oak wood, after 800 revolutions on beech wood, and after 900 revolutions on spruce wood. For the modified surface finish, at 67

least 25% sanding to the substrate (Fig. 2) occurred after 1600 revolutions on oak wood, after 1500 revolutions on beech wood, and after 1400 revolutions on spruce wood.

Adhesion of the coating film according to the Cross-cut test The adhesion determined by the Cross cut test was evaluated according to Table 8. The adhesion of both the modified and standard surface finishes to oak wood and spruce wood was of grade 2. The adhesion of modified surface finish to beech wood was of grade 2 and the adhesion of standard surface finish of grade 1. It can be stated that the adhesion of the standard and modified surface finish to the individual wood surfaces was the same or slightly different (Fig. 3).

Tab. 8 Degree of damage at Cross-cut testing.

Classification

Beech-2 1

Beech-3 2

Oak-2 2

Standard

Oak-3 2

Spruce-2 2

Spruce-3 2

Modified Beech

Standard

Modified Oak

Standard

Modified Spruce

Fig. 3 Surface finishes after the Cross-cut test evaluated visually using a stereomicroscope (with magnification of 4 ×).

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Adhesion of the coating film according to the Pull-off test Statistical evaluation of adhesion is given in Table 9. The results of the two-factor analysis showed that the adhesion of coating film was statistically significantly influenced by the tree species. Also the type of surface finish and the interaction ”tree species – type of surface finish“ showed a high significant impact on the adhesion of the coating film.

Tab. 9 Basic table of analysis of variance for specific adhesion.

2900.808 84.427

Degree of freedom 1 2

25.193

1

25.193

71.557

0.0000000000040

24.165

2

12.083

34.319

0.0000000000606

23.237

66

0.352

Sum of squares Abs. member Tree species Type of surface finish Interaction “tree species – type of surface finish” Error

Variance

F – test

p – significance level F - test

2900.808 42.214

8239.296 119.901

0.0000000000000 0.0000000000000

Fig. 4 shows that the standard surface finish if compared with the modified surface finish reached higher adhesion to both beech wood and oak wood. On spruce wood, there was no difference in the adhesion between the standard and modified surface finishes. The modified surface finish in the case of spruce wood had a higher variance of values of adhesion.

Fig. 4 Surface finishes' adhesion to individual tree species.

The fracture in the system “wood – coating film – metal dolly“ was evaluated visually using a microscope at 4 × magnification. Fig. 6 a), c) and e) show the fracture on the standard surface finish after separation of the dolly. The figures show the surfaces of the metal dollies with the coating film separated from beech, oak, and spruce wood. On the surface of the dolly, areas of the coating film with separated wood fibres can be seen by a microscope. On beech wood and oak wood, separation of wood fibres was minimal and the separated wood fibres did not exceed 10 % of the area under the dolly. On the contrary, on spruce wood, separation of wood fibres was the most significant and, in some cases, reached more than 45 % of the area under the dolly.

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Fig. 5 a) to f) show wood surfaces after separation of the dollies. The separated wood fibres are visible. On beech wood and oak wood, after separation of the dolly, the fracture occurred at the interface “wood – coatingfilm”. The fibres were separated to a small extent, up to 10 % only. Such type of fracture confirms that the measured value of adhesion is the adhesion of coating film to wood surface. On spruce wood, the separation of the surface layers of wood occurred in larger extent. This type of fracture can be considered a cohesive fracture in the surface layers of wood.

Surface of wood a) Standard

Dolly

Surface of wood b) Modified

Dolly

Surface of wood d) Modified

Dolly

Surface of wood f) Modified

Dolly

Beech

Surface of wood c) Standard

Dolly Oak

Surface of wood e) Standard

Dolly Spruce

Fig. 5 Surface finishes after the Pull-off test; evaluated visually using a stereomicroscope (with magnification of 4 ×).

The adhesion of the coating film of the modified surface finish to beech wood and to oak wood was lower if compared with the standard surface finish. The modifying intermediate layer showed a negative effect on the adhesion of coating film to the substrate.

Fig. 5 b) and d) show that after the dolly was separated, there was little separation of the surface wood fibres. The fracture can be described as an adhesion fracture. At these two tree species, the amount of separated wood fibres did not exceed 10 % of the area under the dolly.

Fig. 5 b) and d) show that after the dolly was separated almost no coating film remained on the wood surface. As with the standard surface finish, so the modified surface finish, the largest amount of fibres were separated from spruce wood (more than 45 % of the area under 70

the dolly; Fig. 5 f). Again, it can be stated that, on spruce wood, the fracture can be considered as a cohesive fracture in the surface layers of wood. Fig. 4 show that the adhesion of the standard surface finish and the modified surface finish to beech wood and oak wood was different. On spruce wood, the difference was not registered because the fracture occurred in the surface layers of wood; so the measured values were similar.

The high significant impact of surface finish on the adhesion was also confirmed by HAZIR and KOC (2019), SLABEJOVÁ and VIDHOLDOVÁ (2019b), MIKLEČIĆ, et al. (2017) and DELPECH and COUTINHO (2000).

According to the results by HAZIR and KOC (2019), a type of coating was an effective factor for the adhesion strength, surface coating hardness, layer thickness, and rapid deformation test.

Comparison of the two methods of the adhesion testing shows that the Cross-cut test has less informative value than the Pull-off test. According to the Cross-cut test, the modifying interlayer had no impact on the adhesion of coating film. From the results it is seen that the modifying layer reduced the adhesion of the coating film to hard tree species (oak, beech). Such a conclusion cannot be draw for the surface finish in the case of spruce wood because during the test, a failure occurred in the surface layers of wood, i.e. it was a cohesive fracture.

CONCLUSION From the results of properties of the tested surface finishes, the following conclusions can be drawn:  The surface hardness of the coating film was not affected by the modifying layer.

The interlayer did not increase the surface hardness of surface finish.  The surface hardness of the surface finish was influenced by tree species.  Impact resistance of the surface finish, at a maximum drop height of 400 mm, was not affected by the modifying layer. At a drop height of 200 mm, the modified surface finish was less resistant and more fragile in comparison with the standard surface finish.  The resistance against scuffing of the surface finish was increased by the modifying layer.  The modifying layer in the surface finish had impact on the adhesion of the coating film. If compared the two surface finishes, the standard surface finish reached a statistically significantly higher adhesion to beech wood and to oak wood than the modified surface finish (evaluated according to the Cross-cut test).

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Shellac/nanoparticles dispersions ad protective materials for wood. Applied Physics a-Materials Science&Processing 122(12), 1058. DOI: 10.1007/s00339-016-0577-7 YONG, Q.W., NIAN, F.W., LIAO, B., GUO, Y., HUANG, L.P., WANG, L., PANG, H. 2017. Synthesis and surface analysis of self-matte coating based on waterborne polyurethane resin and study on the matte mechanism. Polymer Bulletin 74(4), 1061-1076. DOI: 10.1007/s00289-016-1763-7 ACKNOWLEDGEMENTS This work was supported by the Slovak Research and Development Agency under the contract No. APVV-16-0177.

AUTHORS’ ADDRESSES Gabriela Slabejová Mária Šmidriaková Technical University in Zvolen Faculty of Wood Sciences and Technology T.G. Masaryka 24 960 01 Zvolen Slovakia slabejova@tuzvo.sk smidriakova@tuzvo.sk

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Ján Svocák Technical University in Zvolen Faculty of Wood Sciences and Technology T.G. Masaryka 24 960 01 Zvolen Slovakia jsvocak@yahoo.com

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 75−87, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.07

ASSESSMENT OF THE FLAMGARD RETARDANT EFFICCIENCY DURING THE THERMAL LOADING OF SPRUCE WOOD (Picea abies L.) Iveta Čabalová – Martin Zachar – Michal Bélik – Kristína Majerská ABSTRACT The aim of the paper was to determine the resistance of spruce wood (Picea abies L.) treated with a flame retardant Flamgard after exposure of its surface to radiant heat. Treated samples were thermally loaded for 30 and 45 minutes and untreated sample were loaded for 30 minutes. The changes in the chemical composition of wood (extractives, lignin, cellulose and hemicelluloses content) in two layers which were removed below the charred layer (layer 1 - thickness up to 20 mm and layer 2 - thickness from 20 to 40 mm) and thickness of the charred layer were evaluated. During the thermal loading of a control sample, the temperature of 300°C in time of 920 seconds was observed. The average thickness of charred layer was 15.41 mm. The limit temperature of 300°C was not reached by both treated samples loaded for 30 and 45 minutes. The average thickness of charred layer of the treated samples was 10.03 mm (30 min.) and 20.04 mm (45 min.). The major chemical changes were recorded in the wood sample untreated with a retardant, mainly in the layer 1 (decrease in lignin by 12.26% and holocellulose content by 16.87%). The experiment showed that the retardant used reduced considerably the degradation of material in terms of changes in its chemical composition. There were no big changes of both lignin and holocellulose content during the thermal loading of the treated samples (decrease in lignin from 1.89 to 6.46% and holocellulose amount from 0 to 4.14%). The experiments also confirmed that the least thermally stable components of wood, namely hemicelluloses, were degraded to the greatest extent, most in the layer 1 of the control sample (decrease in 20.25 %).

Key words: spruce wood, flame retardant, thermal degradation of wood, extractives, cellulose, hemicelluloses, lignin

INTRODUCTION Nowadays, we cannot imagine the building industry without wood. Spruce wood is the most commonly used in the building industry from among the various tree species. It represents a quality and also economically available raw material. This is mainly due to the tree composition of our forests. Spruce is one of the dominated coniferous tree species at our territory for several centuries.

However, wood is combustible material, which is on one hand positive property but also negative property of wood. Fire as undesirable burning is the main negative consequence of wood combustibility. (MAKOVICKÁ OSVALDOVÁ 2009). Burning can be divided into several phases. The most often used is the division into phases according to 75

temperature-time curve, based on which following basic phases are recognised: ignition, propagation, flashover, fully developed fire, and extinction (KARLSSON, QUINTIERE 2000).

In terms of fire protection, information on materials in these phases are the most important, since during them the damage can be minimized to the greatest extent. The point is the thermal decomposition of bonds of its main components such as cellulose, hemicellulose and lignin. When the chemical composition changes, many new products are being created (KAČÍK et al. 2017, LUPTÁKOVÁ et al. 2018).

Thermal decomposition of cellulose is a complex of chemical reactions that take place at different temperatures. Up to a temperature of 150°C, the bounded water evaporates and changes in cellulose structure (formation of free radicals, lactones, later decarbonylation and formation of CO and CO2) begin to occur at temperatures above 150°C. Complete degradation of cellulose, especially in its amorphous areas, appears between the temperatures of 180°C and 270°C. Cellulose become more flexible at temperatures above 300°C, and the main reaction is depolymerisation and formation of levoglucosane (SHEN, GU 2009, KUČEROVÁ et al. 2012, KAČÍKOVÁ et al. 2013). At temperatures above 500°C, levoglucosan begins to transform to combustible gases, CO, CO2, water and tar substances and char residues (REINPRECHT 2016, KUČEROVÁ et al. 2011, MARTINKA et al. 2012a, b, 2018).

Most of hemicellulose decomposition take place at temperatures up to 300°C (REINPRECHT 2016, BOONSTRA et al. 2007b). Their decomposition is much more complicated mainly due to their structure, which is diverse and therefore their decomposition takes place in stages. The first stage is a partial depolymerization (KO et al. 2015, HRČKA et al. 2018). In the second stage, they are degraded to monosaccharides and subsequently to volatile products (BOONSTRA et al. 2007b). In the second stage, the course of degradation is different, it proceeds so fast that it is not possible to detect intermediates. Thermal stability is impacted significantly by acetyl groups, which are also cleaved (SIVONEN et al. 2002, NUOPPONEN et al. 2004, WIKBERG, MANUU 2004, JEBRANE et al. 2018). The final product of hemicellulose decomposition is methanol, acetic acid, furan and valerolacton (KUČEROVÁ et al. 2011).

Changes in lignin during thermal loading are the smallest compared to the other main components of wood because it is the most thermally stable component of phytomass. Lignin degradation begins at relatively lower temperatures, at the same time, new products of its decomposition are formed. At higher temperatures, cross-linking and condensation attains (NUOPPONEN et al. 2004, WINDEISEN et al. 2007, KUČEROVÁ et al. 2016).

Degradation of the main chemical components of wood closely relates to mechanical strength of wood (BOONSTRA et al. 2007a, KAČÍKOVÁ et al. 2011). Therefore, it is necessary to protect wood from effects of thermal exposure. It is important to pay attention to compliance with valid regulations and standards, especially in the area of fire safety of buildings. Protection of wood materials by e.g. flame retardants helps to meet required time criteria, criteria for load carrying capacity and stability in case of fire. Due to measures provided by flame retardants, the load bearing structures are able to resist effect of fire longer. It is important for the safe evacuation of persons and for the safe intervention of firefighting units (KAFKOVÁ 2006).

Some flame retardants (intumescent retardants Flamgard, Flamgard Transparent, etc.) under thermal load create a foam, which is an insulating layer between material and thermally loaded layer or they support the formation rate of charred layer and thus increase the fire resistance of solid wood.

The thickness and speed of charring is one of the most important fire properties of wood and wood product. (BABRAUSKAS 2005). In assessment of fire spread, the technical standard (Eurocode 5) is the most reliable for analysis the charring depth. To determine the 76

time of burning and fire intensity, the charring depth is defined according to the standard STN EN 1995-1-2: 2004 as a distance between the outer surface of the original element and the position of the line between the charred layer and the rest of the cross-section. The standard specifies the line of the charred layer of wooden construction as the place where the temperature reaches 300°C. According to FONSECA a BARREIRA (2009), the charred layer is the dividing line between thermally degraded and non-degraded wood, bounded by a black and a brown wood layer and is characterised by the temperature of 300°C. In accordance with WHITE and NORDHEIM (1992), the charred layer corresponds to the temperature of 288°C. Results of the scientific work of FINDORÁK et al. (2016) confirms above mentioned statements, i.e. rapid thermal decomposition of wood (in case of short-term exposure) starts just below the temperature of 300°C. The aim of the paper is to assess the effectiveness of retardant applied to spruce wood at thermal loading. Degradation of wood was assessed through changes in the course of temperatures in material during its thermal loading, by measuring of charred layers, but also by chemical analysis, where changes of the main wood components i.e. extractives, lignin, cellulose and hemicellulose were detected.

MATERIAL AND METHODS Samples The samples of spruce wood (Picea abies L. Karst.) (harvested in the east part of Slovakia in Dobšiná, trunk diameter approximately of 35 cm, age of 85 years) were cut into the shapes of blocks with the dimensions of 150 × 150 × 1,000 mm (thickness × width × length). The wood surface was treated by 80 grit sandpaper.

The retardant was applied by painting to the air-dry wood (moisture of 8 %) in three coats in a time interval at least of 24 hours, within one week. The overall coating thickness was 500 g/m2 in line with the instructions of producer, which is our case 300 g of retardant per sample. The weight of applied retardant was less than 1% of the weight of the test sample.

Used flame retardant Flamgard Flamgard flame retardant is produced by Stachema company. The substance contains ammonium phosphates, foaming additives, flame retardants, polymers and additives. In case of fire, substance can create harmful gases and vapours (CO2, CO, NOx, PxOx, NH3, vapours of acetic acid (KBÚ, www.stachema.sk).

Radiant heat Three types of samples were thermal loaded, i.e. one control (untreated) sample and two samples treated by retardant Flamgard.

The source of the radiant heat was a ceramic radiant panel used for model testing of the evaluation of building elements from the fire protection point of view. The source of heat can be characterized by the following data: the dimensions of the radiation zone – 480 × 280 mm, max. performance of the radiation zone – 50.5 kW∙m2, the total reached temperature of the radiation zone – 935°C. To achieve the requested performance, the heat flow (30.9 kW∙m2) needed to supply the body of the radiant with propane at the constant flow of 13 l∙hour1. We placed the support holder 20 cm from the radiation panel on which we placed the thermocouples of type K (Ni-Cr-Ni), which measured the temperature on the surface of the block. We also placed in the same room a second thermocouple of type K which measured the temperature of the environment during the experiments (t = 25.0 ± 0.7 °C).

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The first thermocouple (M0.0) was placed on the site, which was directly exposed to the radiant heat from the radiant panel. Another five thermocouples were inserted into drilled holes (75 mm deep below the surface, which is the centre of the sample) on the top side of the wooden block (M1.0 – M5.0) at a distance of 10 mm from each other. Thermocouples on the top side of the sample were covered by mineral wool during the measurement. The last thermocouple was freely placed in the testing laboratory and measured ambient temperature. The samples were gradually placed on the stand in front of the radiation panel (distance 20 cm) and were thermally loaded for 30 and 45 minutes per treated samples, and for 30 minutes per untreated sample. The process was monitored and recorded by digital measurement device (Data logger ALMEMO® 710). The recorded data were then evaluated.

Fig. 1 Emplacement of the thermocouples M 0.0 – M 5.0.

Size and depth of charred area The measurement was conducted using a laboratory measuring instrument and a digital calliper. Firstly, the length of the charred layer was measured. To measure the depth of the charred layer, it was needed to remove this layer. After removing of the charred layer, the depth was measured in nine predetermined points (starting in the middle of the sample and then continuing at the distance of 100 mm from the centre).

Sampling for the purpose of chemical analysis Sawdust was needed to determine chemical composition of wood. Two layers were removed by a circular saw. The first one with the thickness up to 20 mm (layer 1) and the second one with the thickness from 20 to 40 mm (layer 2) below the charred layer.

Chemical composition of wood Samples were disintegrated into sawdust, and fractions in size of 0.5 mm to 1.0 mm were used for the chemical analysis. The extractives content was determined with the use of Soxhlet apparatus with a mixture of ethanol and toluene (2:1) according to ASTM D110796 (2007). The lignin content was determined according to SLUITER et al. (2012), and the cellulose content was determined according to the method by SEIFERT (1956), and the holocellulose content according to the method by WIESE (1946). Measurements were conducted on four replicates per sample. The results were presented as oven-dry wood percentages.

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RESULTS AND DISCUSSION Temperature changes during thermal loading by radiant heat, depth of charred layer Fig. 2, 4 and 5 shows temperature changes on particular thermocouples during the thermal loading of untreated sample of spruce wood and samples treated by flame retardant Flamgard.

Based on the course of temperatures in the sample it is possible to determine the thickness of charred layer not only by its measurement but also by determination of time when the critical temperature 300°C was reached (as mentioned above by several authors).

Control sample Immediately after placing the sample in front of the radiant panel we observed blackening the surface, creation of charred layer, smoke and smouldering. Flame burning occurred in the 19th minute. In Fig. 3, we can observe burning in time of 1140 sec and at the temperature of 489.80°C (M 0.0). Flames were visible from 3rd minute. Burning did not spread to the opposite side of the sample. Nevertheless, we could see higher increase of temperature on thermocouple M 1.0. Sharper increase of the temperature was not recorded on thermocouple M 2.0. On other thermocouples, the temperature increased slowly, however up to a maximum of about 200 °C. M 0.0

M 1.0

M 2.0

M 3.0

M 4.0

M 5.0

M 6.0

Temperature [°C]

600 500 400 300 200 100 0 0

150

300

450

600

750

900

1050 1200 1350 1500 1650 1800

Time[s] Fig. 2 Temperature changes during thermal loading, control sample (M 0.0, M 1.0, M 2.0, M 3.0, M 4.0, M 5.0, M 6.0 - thermocouples).

Fig. 3 Flame combustion of control sample.

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In case of untreated sample, on thermocouple M 1.0, the temperature of 300°C was reached in time of 920 sec which correspond to the thickness of charred layer 10 mm below the thermal loaded surface. On other thermocouples (M2.0 – M5.0), the limit temperature 300°C was not reached in the time interval up to 1800 sec.

The average thickness of charred layer was 15.41 mm, maximum thickness was 18.40 mm and minimum thickness was 10.10 mm (Table 1).

Treated sample, time of thermal loading 30 minutes After a short time from the beginning of thermal loading, stronger smoke, audible cracking and creation of charred layer were observed. In comparison with the control sample, we can see on Fig. 4, that the temperatures in time of 240 seconds on thermocouple M 1.0 are different (130.0 °C in case of control sample, and 79.3°C in case of sample treated by Flamgard). We can conclude that the retardant protected the sample from the rapid increase of temperature and subsequent ignition. The overall course of temperatures increase on particular thermocouples is shown in Fig. 4.

M 0.0

M 1.0

M 2.0

M 3.0

M 4.0

M 5.0

M 6.0

600

Temperature [°C]

500 400 300 200 100

1800

1650

1500

1350

1200

1050

900

750

600

450

300

150

0

0

Time [s] Fig. 4 Temperature changes, treated sample, thermal loading 30 minutes (M 0.0, M 1.0, M 2.0, M 3.0, M 4.0, M 5.0, M 6.0 - thermocouples).

In case of sample treated by retardant Flamgard, thermally loaded for 30 minutes, the limit temperature of 300°C was not reached on any thermocouples (M1.0 – M5.0) (Fig. 4), therefore we can conclude that the retardant protected the sample.

Average thickness of charred layer was 10.03 mm (Table 1), which is approx. 5 mm less than in case of control, untreated sample. Maximum thickness of charred layer was 12.60 mm (18.40 mm in case of untreated sample). Since both samples were exposed to radiation heat for the same time interval, we can conclude that retardant Flamgard increased resistance of spruce wood to its thermal degradation during 30 minutes.

Treated sample, time of thermal loading 45 minutes The course of the experiment was similar as in case of treated sample thermally loaded for 30 minutes. However, in case of the sample thermally loaded for 45 minutes we could see flameless burning (Fig. 6). Small pieces of charred layer fell off. Due to effects of radiant heat with heat flow 30.9 kW∙m-2, we could also observe marked cracks, especially in weak spots of the sample.

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In case of the sample treated by retardant Flamgard, thermally loaded for 45 minutes, the limit temperature of 300°C was not also reached on any thermocouple (M1.0 – M5.0) (Fig. 5). Maximum temperature of 292°C was reached on thermocouple M1.0, in time of 2700 sec from the beginning of thermal loading. Based on that, we can also conclude that the retardant Flamgard protected the sample, only surface charring occurred.

Tab. 1 Thickness of the charred layer of samples.

Treatment Control sample Treated sample

Thickness of charred layer (mm) min max average 10.10 18.40 15.41 7.28 12.60 10.03 15.60 31.10 20.04

Time of thermal loading (min) 30 30 45

M 0.0

M 1.0

M 2.0

M 3.0

M 4.0

M 5.0

M 6.0

600 500 Temperature [°C]

400 300 200 100 2700

2400

2100

1800

1500

1200

900

600

300

0

0

Time [s] Fig. 5 Temperature changes during thermal stress, threated sample, thermal loading 45 minutes (M 0.0, M 1.0, M 2.0, M 3.0, M 4.0, M 5.0, M 6.0 – thermocouples).

Fig. 6 Flameless burning of sample.

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In case of the sample treated by retardant Flamgard, thermally loaded for 45 minutes, the average thickness of charred layer was 20.04 mm, and the maximum thickness of the layer was 31.10 mm (Table 1). When comparing the determination of the thickness of the charred layer using the critical temperature 300°C and manual measurement (in nine points on the surface of the sample), there is a significant deviation among determined values.

Similar conclusions can be find in scientific papers by WHITE and TRAN (1996), who tested different tree species at heat flows 15, 25, 35 and 50 kW∙m2. Their conclusions showed, that the charring rate of wood exposed to a constant external heat flow can be regarded as a linear function of time, however at higher levels of heat flows its behaviour can be non-linear with longer time interval needed to reach a given depth of charring. The charring rate is directly proportional to the ratio of exposure to external heat flow and density. Testing of Nordic spruce samples according to HADVIG (1981) reached charred layer 2427 mm (at thermal loading for 30 minutes) and 3032 mm (at thermal loading for 40 minutes), which is comparable with our results in case of untreated samples.

Chemical analysis of wood after thermal loading The main components of wood include holocellulose (HOL), i.e. cellulose (CEL) and hemicelluloses (HEMI), lignin (L) and extractives (EL).

Table 2 shows chemical composition of original wood sample and wood sample after thermal loading.

Tab. 2 Chemical analysis of wood before and after thermal loading.

Treatment Original wood sample Control sample Treated sample

Time of thermal loading (min)

Layer

EL (%)

L (%)

HOL (%)

CEL (%)

HEMI (%)

-

-

2.12±0.02

29.12±0.27

78.32±0.73

41.63±0.06

36.69±0.79

1 2 1 2 1 2

3.55±0.13 3.24±0.09 2.13±0.09 1.92±0.05 3.20±0.11 2.13±0.12

25.55±0.06 26.69±0.04 27.29±0.02 28.04±0.24 27.24±0.04 28.57±0.25

65.11±1.5 69.33±0.02 77.25±0.46 78.75±0.44 75.08±0.08 77.89±0.32

35.85±0.45 37.62±0.01 45.34±0.86 44.16±0.29 44.38±0.57 43.81±0.95

29.26±1.06 31.71±0.02 31.63±0.40 34.59±0.73 30.70±0.65 34.08±0.63

30 30 45

During thermal loading of spruce wood occurs to the changes of chemical composition. It is not just about surface changes such as colour or weight loss but also changes in chemical composition of the main components of wood and extractives (KAČÍK et al. 2006). It results from chemical analysis (Table 2) that thermal loading increases the percentages of extractives in samples. The amount of extractives is increased by products of macromolecule decomposition of lignin, cellulose and hemicellulose (BOONSTRA et al.

2007b, WINDEISEN et al. 2009). Lower values can be observed in the second layer of sample, and it can be assumed that there was such considerable degradation compared to the first layer.

Lignin belongs to the most thermally stable components of wood. There were no notable changes during radiant heat of samples. A slight decrease of the proportion of lignin can be observed in all samples compared to the original sample (without thermal loading).

We can conclude that the use of retardant Flamgard provided some protection against lignin degradation (decrease of lignin content of treated samples was from 1.89 to 6.46%).

Decrease of the carbohydrates component of wood (holocellulose) is minimum (from 0 to 4.14%) in case of samples treated by retardant. A slight decrease of values can be seen 82

in a layer immediately below carbonized layer (layer 1). The highest decrease of HOL was in case of control sample, i.e. by 16.87 % (layer 1), and by 11.48% (layer 2). The decrease in the amount of holocellulose relates mainly to degradation of the thermally least stable components of wood i.e. hemicellulose. Similar results for thermal degradation of wood by various means are described by WINDEISEN, WEGENER (2009), and HRČKA et al. (2018).

According to ZHANG et al. (2013), the amount of holocellulose and α-celullose decreases significantly with increasing temperature and duration of thermal loading at temperatures above 160°C, which closely relates to the loss of material weight.

The amount of cellulose in samples treated by retardant was increased. KUČEROVÁ et al. (2012) described this phenomenon as a process of charring and crosslinking of cellulose macromolecules. Decrease of this wood component was recorded only in control sample where the proportion of cellulose was degraded by 13.88% (layer 1) and by 9.63% (layer 2).

Hemicelluloses are the most thermally weak components and their degradation begins at relatively low temperatures (BOONSTRA et al. 2007 b, KO et al. 2015, HRČKA et al. 2018, JEBRANE et al. 2018). The amount of hemicelluloses in the control sample decreased by 20.25% (layer 1) and by 13.57% (layer 2) compared to the original wood. A decrease in the hemicelluloses content in the samples treated with the preservative is evident, especially in the upper layer, but the degradation of these components did not occur to the same extent as in the untreated sample. Changes in the hemicelluloses content have a significant effect on the strength properties of wood (SWEET, WINANDY 1999, KAČÍKOVÁ et al. 2013).

Depending on the type of thermal loading of wood (temperature, time, atmosphere, etc.), there are significant changes in its chemical composition, which is directly related to mechanical (BONSTRA et al. 2007a, b, ESTEVES et al. 2008, KAČÍKOVÁ et al. 2013) and the physical properties of wood (KUČEROVÁ et al. 2019).

Accrding to the changes in the proportion of carbohydrates, it can be stated that the flame retarding treatment slowed down the degradation of the main components of wood and its effect lasted also after a long time of exposure to radiant heat source.

CONCLUSIONS In this study, we investigated the effect of thermal loading by radiant heat source on spruce wood samples treated with a Flamgard retardant. During and after the thermal loading, we evaluated the degree of its charring (extent and depth). We found that once a continuous carbon layer was formed, the degradation of the wood slowed down. This is due to the well-known fact that the carbonized layer itself acts as a flame retardant for specific time. As the temperature on the surface of the sample rose, the coating foamed and increased its volume, creating a kind of protective barrier that slowed down the degradation of the wood.

We compared the results related to the thickness of charring layer by reaching a critical temperature of 300 °C, specified also in Eurocode 5, which was determined by thermocouples placed in the cross section of the sample and further manually measured (at nine places on the sample surface). We found a significant deviation between those values.

Retarder Flamgard has a demonstrable wood protective function due to its prohibitive capability to reach a critical temperature of 300 ° C, in the depth of 10 mm below the surface of the sample. However, when manually measuring the thickness of the charred layer, it reached a value of 20.04 mm, which means that the degradation of the wood nevertheless occurred at lower temperatures.

This was manifested precisely by monitoring the changes in the chemical components of wood. We performed chemical analyses from two layers of wood. The first was taken to 83

a depth of 20 mm and the second from 20 to 40 mm below the carbonized layer. The results of chemical analyses showed that by thermal loading of spruce wood: - There increases the percentage of extractives in the samples. The content of extractives is increased by the decomposition products of the lignin macromolecule and by the carbohydrate components of the wood.

- There were no considerable changes of lignin content (decrease from 1.89 to 6.46%) in the samples treated with the flame retardant. We can therefore conclude that the use of Flamgard retardant provided some protection against lignin degradation.

- The decrease in the proportion of the carbohydrate component (holocellulose) were from 0 to 4.14% in both layers for the samples treated with the flame retardant. The decrease in carbohydrate content was the highest in the control sample (16.87%).

- The content of celluloses in the samples treated with the flame retardant probably increased due to the cross-linking of the cellulose macromolecules caused by thermal loading. A decrease in the cellulose we recorded only in the control sample.

- The amount of hemicelluloses in the control sample was reduced more compared to the original wood. A decrease in these components is also evident in the samples treated with the flame retardant, especially in the upper layer, but not to the same extent as in the control sample.

- Changes in the proportion of carbohydrates showed that the flame retardant slowed down the degradation process of these components of wood and its effect lasted even after a long time of exposure to radiant heat source.

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ACKNOWLEDGEMENT This work was supported by the Slovak Scientific Grand Agency VEGA under the contract No. 1/0397/20 (50%) and No. 1/0387/18 (10%), and the Slovak Research and Development Agency APVV under the contract No. APVV-16-0326 (20%) and No. 17-0005 (20%).

ADDRESSES OF THE AUTHORS Assoc. prof. Ing. Iveta Čabalová, PhD. Ing. Michal Bélik Ing. Kristína Majerská Technical University in Zvolen Faculty of Wood Sciences and Technology Department of Chemistry and Chemical Technologies T. G. Masaryka 24 960 53 Zvolen Slovakia cabalova@tuzvo.sk xbelik@is.tuzvo.sk xmajerska@is.tuzvo.sk

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Ing. Martin Zachar, PhD Technical University in Zvolen Faculty of Wood Sciences and Technology Department of Fire Protection and Safety T. G. Masaryka 24 960 53 Zvolen Slovakia zachar@tuzvo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 89−98, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.08

ACOUSTIC DEFECTOSCOPY OF VENEER LAYERED COMPOSITE MATERIALS Olena Pinchevska – Ján Sedliačik – Olha Baranova – Valentyn Golovach – Mykola Vasylenko – Konstantin Shevchenko – Yuriy Lakyda ABSTRACT In the production of wood-based layered composites, many defects arise due to none or lack of glue and their presence in the finished product is unacceptable. The causes of internal defects are mainly due to a violation of the technological process during the plywood production. Manual intervention in the production process is eliminated by the automation of the quality control of plywood materials and the quality of the final products is improved.

The progress of the possibility of correcting the pressing mode with the application of acoustic defectoscopy allows the effect on production operations and to adjust the modes.

The defects inside the plywood materials are located by the resonance peaks of obtained oscillograms. The elimination of defective wood-based panels from the grinding process results in saving the energy and keeps the quality of production.

Key words: plywood, defectoscopy, shock acoustic method, non-destructive testing, quality.

INTRODUCTION Wood-based layered materials, such as plywood or laminated veneer lumber (LVL) materials become more widely used in different areas of modern life. Composite materials are widely used, so problems of quality increase, production technology improvement, more effective use of raw materials and equipment automation are actual (BEKHTA et al. 2009).

Main investigations of composite materials properties using acoustic and ultrasonic methods were the following: BEALL and BIERNACKI (1991) studied glued laminated bars using acoustic ultrasonic method. Method appeared to be sensitive at the defects detection.

ILLMAN et al. (2002) studied the use of acoustic parameters to monitor the wearing out of commonly used structural composites for oriented strand boards. DILL-LANGER et al. (2005) examined the conglutination defects between the layers of glued laminated bar using multiple ultrasonic parameters. BOBADILLA et al. (2009) artificially aged particleboard and fibreboards, and studied them using different methods including ultrasonic. They got correlation between the speed of ultrasonic wave and mechanical properties of studied material. SANABRIA et al. (2009) identified bundle defect between two solid spruce boards connected with polyurethane glue by studying them with ultrasonic method. They also studied structural integrity of multi-layer glued laminated bars with air-coupled sensors (SANABRIA et al. 2011). DIVOS et al. (2009) measured the depth of the cracks inside of the

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glued laminated timber with ultrasonic method. BUCUR (2011) carried out the research of wooden composites using both low and high frequency methods.

The task of significant composite materials quality increment can be solved by improvement of product quality testing methods. Main requirements for non-destructive testing are following: ability to perform effective testing on the different production stages; ability to control as many parameters as possible; coordination of quality control time with technological equipment operation time; high certainty of testing results; ability to automate technological processes control, using signals from testing equipment; high reliability of defectoscopy equipment and ability to use it in different conditions; easy testing method, equipment accessibility in production conditions. Such requirements are matched by the shock acoustic non-destructive testing equipment (STROBEL et al. 2018, WEN et al. 2020).

The purpose of this study is to create a rheological model including mass, elasticity and strength of plywood panels based on the free oscillations method.

MATERIAL AND METHODS The experiments were carried out using plywood produced by PJSC «Plywood and Slabs» (Kyiv) from hardwoods of Kyiv region such as birch (Betula) and alder (Alnus).

Bundle defect was artificially modelled by pasting of two plywood samples with planned defect area from S1 = 0.0079 m2 to S2 = 0.042 m2 and depth from 0.004 m to 0.02 m. The measurements were made at the temperature of 20 ± 2 °С and humidity of 60%. The samples were prepared with the use of PVA adhesive grade D-54P. The research is based on the free oscillations method. Veneer layered composite materials are solid patchy material that consists of two or more components with clear border between them (BEKHTA et al. 2009). In the case of bundle defect presence, the area with defect can be presented as two layers, fixed on the edges with air bubble between them.

Figure 1 shows formalized image of such defect. Considering upper and lower areas of a defect as a package of absolutely solid bodies with linear springy and viscous elements of the panel, the model of bundle defect can be obtained.

Fig. 1 Section of composite materials region with defect (air layer).

Figure 2 shows rheological model of veneer layered composite materials area. Each area of composite materials defect with mass m is characterized by elasticity: 𝐸⋅𝑆 (1) 𝑙 where: С is elasticity of composite materials rheological model (CMRM) element [N/m], Е is elasticity modulus of CMRM element (Pa), S is area of CMRM element section (m2), l is thickness of CMRM element (m).

Mass of the panel is equal to: 𝐶=

𝑚 =𝜌⋅𝑆⋅𝑙 90

(2)

where: ρ is CMRM element density (kg/m3). 𝜂 = 𝑙𝑛

𝐴1 𝐴2

(3)

where: 𝜂 is a coefficient, characterizing the viscosity of the panel, s-1, A1 and A2 are amplitudes of CMRM element region oscillations [mm].

Such representation of defect model allows to suggest that shock influence on the composite materials will cause the acoustic oscillations with form and amplitude depending on the composite materials structure (BEKHTA et al. 2015, BEKHTA et al. 2016). Analysis of such oscillations spectrum at different areas of composite materials will allow to locate the defect.

Fig. 2 Veneer layered composite material rheological model.

RESULTS AND DISCUSSION Plywood defect model allows to calculate the resonance properties of the defect and parameters of measuring device parameters to control such defects. For bundle defect and

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shock acoustic method such parameters are resonance frequencies of defect elements oscillations amplitude and damping of oscillations.

It is also necessary to calculate the impact force of measuring device hammer (Fig. 3) on the controlled region. Hammer must not damage the controlled material and have sufficient energy to cause oscillations inside the plywood which can be measured by the sensor with needed accuracy. For example, the strength of undamaged pine tree wood while compressing along the fibres is equal to 40 MPa. With the force directed across the fibres the strength does not exceed 6.5 MPa. Inhomogeneity of wood structure and presence of defects significantly (up to 30%) reduces compressive strength of wood. According to this, the impact force on the composite materials surface must not exceed about 4 MPa (LAGAŇA and ROHANOVÁ 2014).

Fig. 3 The scheme of non-destructive testing device.

Metal hammer with mass m and rigidly connected with piezo is held at the height h over the plywood surface by the electromagnet. After turning the electromagnet off, it fell with acceleration a on the tested region of plywood. After the impact electrical charges proportional to the collision impulse appeared on the piezo. Hammer movement can be described by the following equation: −𝑐𝑥 = 𝑚ℎ 𝑎

(4)

where: 𝑐 is elasticity of plywood, (N/m), x is value of plywood crimp (mm), mh is mass of hammer (kg), a is acceleration of hammer (m/s2).

Solution of this equation (4) with initial conditions x(0) = 𝑣0 = 0 presented in (PANOVKO, 1991) can be written, as: 𝑥0 (5) 𝑥 = ⋅ 𝑠𝑖𝑛𝜌𝑡 𝜌 where 𝜌 is coefficient that can be calculated: 𝑐 𝜌=√ (6) 𝑚 According to this maximum, the maximum force of plywood compression will be equal to: 𝐹𝑚𝑎𝑥 = 𝐶 ⋅ 𝑥𝑚𝑎𝑥 = 𝑣0 √𝑚 ⋅ 𝐶 (7) where 𝑣0 is oscillating speed, (m/s).

Placing the hammer at the altitude h = 0.1 m its speed in the moment of contact with the surface will be: 92

𝑣 = √2𝑔ℎ = √2 ∙ 9.8 ∙ 0.1 = 1.4(𝑚/𝑠)

(8)

For approximate calculation of maximum impact strength, following values were applied: mass of the hammer m = 0.05 kg, wood elasticity modulus E = 10 000 MPa, defect section area S = 0.0001 m2, defect length l = 0.2 m.

Substituting these values into equation (7) we will obtain: 𝐹𝑚𝑎𝑥 = 𝑣 ∙ √𝑚 ∙ E ∙

S 0.0001 = 1.4 ∙ √0.05 ∙ 10000 ∙ 106 ∙ = 990 (Pa) l 0.1

(9)

Calculated value of plywood compression is much less than the strength of wood (0.000990MPa << 4MPa) that guarantees the non-destructive testing of plywood by the selected method when using hammer with parameters described above.

After the impact the controlled region, presented as the thin plate, this will compress and transfer the speed to the neighbouring region (BABAKOV 1968). Elasticity forces that occur in the plate will deform the next layer. Elasticity forces of the second layer will stop the first layer and the second layer will get the speed. In such case the first layer will stop and the second layer will start to move and compress. Layer deformation will move across the plywood. This phenomenon is called the elastic wave that moves the initial impulse through the plywood panel.

There are different types of elastic waves: longitudinal (compression and stretching) lateral (offset waves), flexural, surface longitudinal (Rayleigh waves), surface lateral (Lyav waves) according to BABAKOV 1968. Inside the plywood flexural waves will prevail because the thickness of material is much smaller than the length of elastic wave that spreads inside.

Measurement device hammer transfers the kinetic energy to the regions of controlled material and changes their inner energy. Material areas kinetic energy density inside the elastic wave is equal to: 1 1 (10) 𝐸𝑘 = ∙ 𝜌 ∙ 𝑣 2 = ∙ 𝜌 ∙ 𝜔2 ∙ 𝐴2 (J) 2 2 where: ω is circular frequency of oscillations[1/s].

For entire panel it will be equal: 1 1 (11) ∙ 𝜌 ∙ 𝑣2 ∙ 𝑉 = ∙ 𝑚 ∙ 𝑣2 2 2 where: V is panel volume [m3].

Kinetic energy obtained by the plywood is proportional to the kinetic energy of the hammer and depends on its mass and speed. Measuring the oscillatory speed v of plywood panel, there is possible to make decision about its mechanical properties and presence of inner defects.

Oscillatory speed can be obtained from the differential equation of oscillations: 𝐸𝑘 ′ =

𝐷(

𝜕 4𝜔 𝜕 4𝜔 𝜕 4𝜔 𝛶ℎ𝑓02 𝜔 + 2 + ) − =0 𝜕𝑥 4 𝜕𝑥 2 𝜕𝑦 2 𝜕𝑦 4 𝑔

(12)

𝐸ℎ3

where: 𝐷 = 12(1−𝜈) is cylindrical rigidity of the panel in bending (kg/m), x, y are coordinate axes, f0 is own oscillations frequency (Hz), g is free fall acceleration, 𝛶 is specific weight of plywood, (kg/m3).

Longitudinal and transverse oscillations are related both through motion equations and through boundary conditions. For low-frequency oscillations, transverse movements exceed longitudinal ones. Transverse and longitudinal oscillations can be considered 93

separately, ignoring the relationship between them. In this case, an anisotropic beam can be considered as isotropic.

In addition, the task was to find the approximate frequency domain of the sample oscillations. The exact values of oscillations were found experimentally. Therefore, this formula was used in the work (TOVSTIK 2014).

Solution of equation (12) defines the oscillations form that depends on boundary conditions. During the oscillations the surface of panel is divided by the nodal lines ωi (x, y) = 0 into the regions where the oscillations will be in the same or in the opposite phases.

Uniformity of composite materials surface properties will be ensured in case the node lines cross the surface only at the edges and nowhere else (AYDIN et al. 2017). Therefore, ωi(x, y) = 0 only along the lines x = 0, x = a, y = 0, y = b. Here, а and b are dimensions of the panel along the x and y axis accordingly. These conditions can be fulfilled in the case when the plate is fixed on all four edges that corresponds to the case when the plate is located over the defect. Forms of oscillations for plates with equal areas in the defect position with the given border conditions can be written down as: ∞

𝜔(𝑥, 𝑦) = ∑ 𝐴𝑖𝑗 ∙ 𝑠𝑖𝑛 𝑖𝑗

𝑖𝜋𝑥 𝑖𝜋𝑦 ∙ 𝑠𝑖𝑛 𝑎 𝑏

(13)

where: Ai j is oscillations amplitude [m].

The first part of equation 13 never becomes equal to zero inside the plate and only on the edged ω11(x, y) = 0. Therefore, the nodal lines do not cross the plywood region surface and all elements will oscillate according to the following equation: 𝜋𝑥 𝜋𝑦 (14) 𝜔(𝑥, 𝑦, 𝑡) = 𝜔11 ∙ 𝑠𝑖𝑛(𝑥, 𝑦) ∙ 𝑠 ∈ (𝑝𝑡 + 𝛼) = 𝛼11 ∙ 𝑠𝑖𝑛 ∙ 𝑠𝑖𝑛 ∙ 𝑠𝑖𝑛(𝑝𝑡 + 𝛼) 𝑎 𝑏 Plate regions will simultaneously displace into the same direction reaching the maximum displacement values and simultaneously pass through the equilibrium position.

This is the first own oscillation of the panel. Its frequency can be determined from the equation (BARANOVA 2015): 1 1 𝐷𝑔 𝑓01 = 𝜋 2 ∙ ( 2 + 2 ) ∙ √ 𝑎 𝑏 𝛶ℎ

(15)

Such oscillations will fade during some period of time. The dissipative forces related to the fastening points and the environment resistance are considered to be the reason of damping. The oscillations energy is spent to overcome this resistance that results in decrease of oscillations amplitude peak values (BARANOVA, 2015). Peak values can be obtained from equations: (16) 𝑎1 = 𝑎𝑒 −𝜂𝑡 𝑎2 = 𝑎𝑒 −𝜂(𝑡1 +𝑇)

(17)

where: t1 is the time of the first biggest displacement (s), η is plate viscosity coefficient, 2𝜋 𝑇 = 2 2 is period (s). √𝑝 +𝑛

Coefficient η can be obtained from the logarithmic oscillations decrement equation: 𝑎𝑖 𝛿 = 𝜂𝑇 = 𝑙𝑛 (18) 𝑎𝑖+1 Considering the supports inelastic the equation of forced oscillations after the influence of outer force F(τ) can be written as follows: 94

𝑑2𝜔 𝑑𝜔 𝐹(𝜏) + 2𝑛 + 𝑓02 𝜔 = 2 𝑑𝑡 𝑑𝑡 𝑚 where: F (τ) is force that acts on the panel (N), τ is impact duration (s).

Common solution for equation (19) is: 𝜔=

(19)

𝑡 1 ∫ 𝐹(𝜏)𝑒 −𝑛(𝑡−𝜏) ∙ 𝑠𝑖𝑛𝑓0 (𝑡 − 𝜏)𝑑𝜏 𝑚𝑓0 0

(20)

where 𝑓0 = √𝑓0 2 − 𝜂2 .

Experiments showed significant difference of spectrum parameters of piezo output signals in the case of presence and absence of defects in the controlled area. Experiments were performed on the samples of five-layer plywood with the thickness of 7 mm (Fig. 4).

Fig. 4 Approbation device for defect control.

As the sensor was used a piezo-element with diameter 10 mm and thickness 1 mm, rigidly fixed on the hammer and electrically connected to the digital oscilloscope OSCILL (PINCHEVSKA, 2019). Oscillograms in Figure 5 show the output signal from piezo sensor in the region without defect. Oscillograms in Figure 6 show the output signal from piezo sensor in the region with defect. Both oscillograms allow to draw following conclusions:  in the case of hammer impact on the surface of non-defective area resonates the whole panel, oscillogram has one resonance peak (Fig. 5, b) and oscillations have the form of damping sine (Fig 5, a);

a)

b) Fig. 5 Oscillograms from non-defective region.

 in the case of hammer impact on the surface of non-defective area different regions resonates so additional resonance peaks occur (Fig. 6, b); 95

a)

b) Fig. 6 Oscillograms from defective region.

 controlling the number of resonance peaks it is possible to locate the defects inside the plywood.

The given mathematical model explains the occurrence of acoustic oscillations of different shapes under the influence of the shock method. The compression index of plywood (990 Pa) is calculated, which is much less than the strength limit of wood (4 MPa), which will guarantee a non-destructive testing. Mathematically substantiated the magnitude of the output signal of the piezoelectric element, which is affected by: the mass of the plywood, which interacts with the impact of the drummer; the force F with which the drummer acts on the surface of the plywood; oscillation frequency and viscosity.

Factors influencing the output signal are the depth of the defect, its location in the sample and the area of the defect. The research was carried out on defect-free and defective areas of plywood. Regression analysis of the influence of the defect characteristics on the parameters of the output signal of the shock sensor showed that the number of pulsations is affected by all factors almost equally; the frequency of free oscillations is most influenced by the factor of the location of the defect, which is positive for control, but is not sensitive to determine the area of the defect; the coefficient of harmonic distortion is most affected by the area of the defect, while other factors have little effect, which makes this parameter the most accurate for control. So, controlling additional parameters will allow further increase the accuracy of defect detection.

CONCLUSIONS Proposed rheological model includes mass, elasticity and viscosity of each veneer layered composite materials component: veneer, glue and air. Such model explains the appearance of oscillations with different form after the shock influence. The analysis of the hammer sensor oscillograms shows that when the hammer of the measuring instrument is exposed to the plywood, if there is no defect, the whole area of the sample begins to resonate.

There is one resonance and the oscillogram has the form of a sinusoid falling in amplitude.

When the hammer is operated as a measuring device on area of the defective plywood, different areas of the sample begin to resonate. Moreover, resonances appear in the range of the piezoelectric sensor. The number of resonances, the character of the layering and its place on the sheet of plywood can be assessed. Therefore, that in the case of a stratification defect, the integrity of the material is broken and two or more elements appear in the defect area, which have their spectrum of resonance frequencies. The analysis of the reaction of the section of the controlled material to the impacts was carried out to determine the characteristics of electrical signals on the piezoelectric sensor during the detection of defects.

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REFERENCES AYDIN, I., DEMIRKIR, C., COLAK, S., COLAKOGLU, G. 2017. Utilization of bark flours as additive in plywood manufacturing. In European Journal of Wood and Wood Products 75(1), 2017, p. 63–69.

BABAKOV, N.M. 1968. Teoriya kolebaniy. Nauka, Moscow, 1968.

TOVSTIK, P.E., TOVSTIK, T.P. 2014. Svobodnye kolebaniya anizatropnoj balki. In Vestnik SPbGU, Ser.1. T.1 (59), 2014, p. 599–607.

BARANOVA, O.S. 2015. Defektoskopiya kompozytnyh materialiv z zastosuvannyam udarnoakustychnogo metodu neruynivnogo kontrolyu. In Vasnyk KNUTD, No 6(92), 2015.

PINCHEVSKA, O., BARANOVA, O., GORBACHOVA, O., GANDZUK, V. 2019. Research of plywood quality by acoustic methods. IOP Conference Series: Materials Science and Engineering, 2019.

BEALL, F.C., BIERNACKI, J.M. 1991. An approach to the evaluation of glulam beams through acousto-ultrasonics. In 8th International Nondestructive Testing and Evaluation of Wood Symposium. Vancouver, 1991, p. 73–88.

BEKHTA, P., HIZIROGLU, S., SHEPELYUK, O. 2009. Properties of plywood manufactured from compressed veneer as building material. In Materials & Design 30(4), 2009, p. 947–953.

BEKHTA, P., ORTYNSKA, G., SEDLIAČIK, J. 2015. Properties of modified phenol-formaldehyde adhesive for plywood panels manufactured from high moisture content veneer. In Drvna Industrija 65(4), 2015, p. 293–301.

BEKHTA, P., SEDLIAČIK, J., SALDAN, R., NOVÁK, I. 2016. Effect of different hardeners for ureaformaldehyde resin on properties of birch plywood. In Acta Facultatis Xylologiae Zvolen, 58(2), 2016, p. 65–72.

PANOVKO Y.G. Vvedenie v teoriyu mekhanicheskih kolebanij. Nauka, Moscow,1991.

BOBADILLA, I., DE HIJAS, M.M., ESTEBAN, M., ÍÑIGUEZ, G., ARRIAGA, F. 2009. Nondestructive methods to estimate physical and biological aging of particle and fibre boards. In Proceedings of the 16th International Symposium on Nondestructive Testing of Wood. Beijing, 2009, p. 222–228.

BUCUR, V. 2011. Delamination in wood, wood products and wood based composites. Springer Netherlands. 2011, 401 p. eBook ISBN 978-90-481-9550-3.

DILL-LANGER, G., BERNAUER, W., AICHER, S. 2005. Inspection of glue-lines of glued-laminated timber by means of ultrasonic testing. In Proceedings of the 14th International Symposium on Nondestructive Testing of Wood. Eberswalde, 2005, p. 49–60.

DIVOS, F., SZALAI, J., GARAB, J., TOTH, A. 2009. Glued-laminated timber evaluation. In Proceedings of the 16th International Nondestructive Testing and Evaluation of Wood Symposium. Beijing, China, 2009. p. 287–293.

ILLMAN, B.L., YANG, V.W., ROSS, R.J., NELSON, W.J. 2002. Nondestructive evaluation of oriented strand board exposed to decay fungi. In Proceedings of the 33rd Annual Meeting of the International Research Group on Wood Preservation, Cardiff, 2002, 6 p.

LAGAŇA, R., ROHANOVÁ, A. 2014. Prediction of modulus of elasticity using acoustic characteristics.

In Acta Facultatis Xylologiae Zvolen, 56(1), 2014, p. 5–12.

ROSS, R.J. 2015. Nondestructive evaluation of wood: second edition. Madison, WI, Forest Products Laboratory, 2015, 169 p.

SANABRIA, S.J., FURRER, R., NEUENSCHWANDER, J., NIEMZ, P., SENNHAUSER, U. 2011. Monitored assessment of structural integrity of multilayered glued laminated timber beams with air-coupled ultrasound and contact ultrasound imaging. In: Proceedings of the 17th Symposium Nondestructive Testing of Wood, Sopron, 2011, p. 359–366.

SANABRIA, S.J., MUELLER, C., NEUENSCHWANDER, J., NIEMZ, P., SENNHAUSER, U. 2009. Aircoupled ultrasound inspection of glued solid wood boards. In Proceedings of the 16th International Nondestructive Testing and Evaluation of Wood Symposium. Beijing, China, 2009. p. 281–286.

STROBEL, J.R.A., DE CARVALHO, M.A.G., GONÇALVES, R., PEDROSO, C.B., DOS REIS, M.N., MARTINS, P.S. 2018. Quantitative image analysis of acoustic tomography in woods. In European Journal of Wood and Wood Products 76(5), 2018, 1379–1389.

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ACKNOWLEDGEMENT This work was supported by the Slovak Research and Development Agency under the contracts No. APVV-16-0177, APVV-17-0583, APVV-18-0378 and APVV-19-0269. The authors thank VEGA project No. 1/0556/19.

AUTHOR’S ADDRESS Prof. Ing. Olena Pinchevska, Dr.Sc. Olha Baranova, Ph.D. Valentyn Golovach, Ph.D. Yuriy Lakyda, Ph.D. National University of Life and Environmental Sciences of Ukraine Department of Wood Processing vul. Geroiv Oborony 15 Kyiv 03041 Ukraine Opinchewska@gmail.com olhabaranova@nubip.edu.ua vale_go@mail.ru yuriy.lakyda@gmail.com Prof. Ing. Ján Sedliačik, PhD. Technical University in Zvolen Department of Furniture and Wood Products T.G. Masaryka 24 960 01 Zvolen Slovakia jan.sedliacik@tuzvo.sk Mykola Vasylenko, Ph.D. National Aviation University Department of Aviation computer-integrated complexes Liubomyra Huzara ave. 1 Kyiv 03058 Ukraine m.p.vasylenko@nau.edu.ua Prof. Ing. Konstantin Shevchenko, Dr.Sc. The National Technical University of Ukraine "Igor Sikorsky Kyiv Polytechnic Institute Faculty of Instrumentation Engineering Department of automation of experimental researches Borschagovskaya street 126 Kyiv 03056 Ukraine autom1@meta.ua

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 99−107, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.09

THE INFLUENCE OF SOME FACTORS ON THE VIBRATIONS GENERATED BY WOODWORKING SPINDLE MOULDER MACHINE WHEN PROCESSING SPECIMENS FROM BEECH WOOD Pavlin Vitchev – Zhivko Gochev – Georgi Vukov ABSTRACT The general dynamic behavior of a woodworking spindle moulder machine, determined by the mean square value of the vibration speed (vrms, mm·s-1) measured on the shaft bearings is investigated in the study. Among the measured factors, the cutting speed (Vc) has the greatest influence on the vibration intensity, followed by the feed rate (Vf) and the depth of milling (h). The results show that the vibration velocity varies in the range from 2 to 8 mm·s-1 at the different values of the variable factors. On the basis of this results, with regard to the magnitude of the overall vibrations generated by the used milling machine in processing specimens of beech wood (Fagus sylvatica L.), the following optimum values are recommended: cutting speed Vc ϵ (34÷40) m·s1; feed speed Vf up to 4 m·min1; cutting depth h up to 8 mm. In cases when h>8 mm, the recommended feed speed is Vf < 3.5 m·min1. Based on the presented graphical relationships, the optimum values of the studied factors can be determined in order to reduce the overall vibration of the machine, which is an important prerequisite for the good work of the cutting tool and for improving the quality of the machined surfaces during processing the specimens from beech wood.

Key words: woodworking milling machine, milling, vibration severity, vibration speed.

INTRODUCTION The dynamic behaviour of each woodworking milling machine is of great importance for its operational reliability as well as for reliable operation and for maintaining the geometrical accuracy of the linear and angular parameters of the woodworking tools.

The magnitude of vibrations in woodworking machines affects the accuracy of specimens’ processing and the roughness of their surfaces (KAVALOV et al. 2014, KAVALOV et al. 2015). Increased vibrations are a prerequisite for reducing the quality of processing.

The dynamic behaviour is characterized by the magnitude of the machine’s general vibrations, the change of which can be the result of various structural and technological parameters, as well as depending on the characteristics of the materials being processed.

To estimate the vibration of a machine, it is agreed to use the maximum vibration value recorded at one of the measuring points where the measurements were taken. According to the regulatory guidelines this value is determined as vibration severity (ISO 10816-1) The increased vibration intensity of a machine may result from poor balancing of the rotating elements as well as misalignment of the belt pulleys in the belt drive (VUKOV 2008).

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The use of cutting tools balanced with insufficient precision also causes a change in the overall dynamic behaviour of the entire machine (DINKOV et al. 1990, YAITSKOV et al.

2019). Another prerequisite for increasing the overall vibration of the machine is based on the fact that modern woodworking machines operate at high rotational speeds of their cutting tools (BRUEL & KJAER VIBRO 2011, VUKOV et al. 2014, VAN et al.2018). Further, in the cutting process there are interactions between the cutting tool and the workpiece which, depending on the cutting speed and the feed rate, lead to formation of cutting forces. When the above mentioned factors are increased, an increase in the vibration energy is observed as well (ISKRA et al. 2005, KETURAKIS et al. 2007, ZHANG et al. 2011, GOCHEV et al. 2017a, GOCHEV et al. 2017b, YAITSKOV et al. 2019, GORSKI et al. 2019, VAN et al. 2020, CHUNMEI et al. 2020). Based on one of our previous studies (GOCHEV et al. 2017A) we showed that the surface quality of specimens from white pine (Pinus Sylvestris L.) wood was higher at cutting speed from 40 m·s1 to 45 m·s1, and at feed rates up to 5 m·min1 with a thickness of the specimens of up to 8 mm (GOCHEV et al. 2017a).

Another factor that may influence the vibrations’ intensity is the type of workpiece material. For example, processing of specimens from oak and fir wood is characterized with uneven intensity of the vibrations, while the vibrations generated during processing of beech wood specimens are with a constant amplitude. Those differences are explained by the presence of early and late wood in the oak and fir trees and the homogenous structure of the beech wood (LUSTUN and LUCACI 2010).

The aim of the current study was to investigate the changes in the magnitude of the general vibrations generated by the used woodworking spindle moulder machine under cutting mode with different cutting speed (Vc), feed speed (Vf) and depth of cut (h) while processing specimens from beech (Fagus sylvatica L.) wood. In addition, based on the evaluated vibration levels under this experiment, optical milling parameters are recommended for this particular machine.

METHODOLOGY The experiments were carried out using woodworking spindle moulder machine, type T1002S (ZMM “Stomana” GmbH, Bulgaria) (Fig. 1). The machine was equipped with a two-speed three-phase electric motor with power 3.2/4.0 kW, which provides the following rotating speed of the spindle: 3000, 4000, 5000, 6000, 8000 and 10000 rpm through a belt drive.

Fig. 1 Woodworking spindle moulder machine, type T1002S – general view.

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The machine is also equipped with a roll feeder, driven by self-powered electric motor which could ensure feeding speed of the processed material from 3.5 to 32 m·min1. Under the cutting mode of the machine, a monolithic cutting tool was used with technical characteristics presented in Table 1, where D is the diameter of the cutter head, d – diameter of the hole, B – milling width,  – sharpness angle,  – hook angle, z – number of teeth. The cutting tool was dynamically balanced with the necessary permissible residual unbalances allowed at rotational speed up to n1 = 8000 min1.

Fig. 2 General view of the used cutter head.

Tab. 1 Technical characteristics of the used cutter head.

D mm 140

d mm 30

B mm 12

β  58

  20

z No 6

n1 min-1

Material of the teeth

8000

Sintered tungsten carbide – cobalt, (HM-M40)

The workpiece is from beech wood (Fagus sylvatica L.) with density ρ = 690 kg·m1, moisture W = 11 % and dimensions 1000 × 50 × 50 mm.

The experiments have been carried out at the three possible rotational speeds of the cutting tool (n), namely 4000, 6000 and 8000 rpm (min-1). The cutting speed (Vc) varies depending on the speed of rotation in accordance with the following equation (GOCHEV 2018) Vc = π.D.n, [m.s1],

(1)

where: D – diameter of the cutting tool, m; n – rotation frequency of the cutting tool, s1.

In the current study, the overall dynamic behaviour of the milling machine was monitored based on the variation of the vibration magnitude measured on the non-rotating parts of the machine under cutting mode. The effect of the cutting speed (Vc) or the rotational frequency of the cutting tool, the feed rate (Vf) and the thickness of the cutting depth (h) were monitored.

In the course of the study, the three variables were measured at three levels which are presented in explicit and coded form in Table 2.

The measurements were performed in accordance with a preliminary designed matrix B3 for three factorial experiment plan of G. Box of second order. In addition to the experiments, according to the requirements of the B3 matrix for each individual experiment, five additional experiments are carried out under conditions corresponding to the middle of the factor space, i.e. х1 = 0, x2 = 0 and x3 = 0. On the basis of these measurements the error variations Sg2 have been determined. For the statistical analysis an average value from three independent measurements for each combination of factors in the experimental matrix have been used. The data were statistically analysed by a specialized software Q-StatLab.5.

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Tab. 2 Values of the variable factors Vc, Vf and h.

Minimal value Expl.

Coded 29 1 3.5 1 4 1

Variables Cutting speed Vc = x1 [m·s1] Feed speed Vf = х2 [m·min1] Cutting depth h = х3 [mm]

Medium value Expl.

Coded 44 0 7 0 8 0

Maximal value Expl.

Coded 59 1 10.5 1 12 1

The variations in the magnitude of the vibrations, generated by the tested machine and their relationship to the evaluated variables were assessed by measuring the root mean square value of vibration velocity (vrms, mm.s1) at different working modes of the machine.

The measurements were performed at four measuring points located on two bearing housings of the spindle of the machine (two measurement points on each bearing housing). The measurement points on each bearing housing are located mutually perpendicular – 2 of them radial (y and x) and 1 axial (z) to the axis of the spindle of the machine (Fig. 3). y

y

z

x

Fig. 3 Measurement points on one bearing housing.

In the current study, the measurement points are defined as follows:  For the bearing housing located in proximity to the driven belt pulley, hereinafter referred to as “lower bearing housing”, the measurement points are indicated by Dx – in the direction parallel to the feed direction and Dy – in direction perpendicular to the feed direction;  For the bearing housing located in proximity to the working top of the machine and the cutting tool, hereinafter referred to as “upper bearing housing”, the measurement points are indicated by Gx – in direction parallel to the feed direction and Gy – in direction perpendicular to the feed direction.

The requirements given in BDS ISO 10816-1 and ISO 2041 were strictly followed throughout the experiments. For the measurement of the vibration velocity a vibration meter, model Vibrotest 60 (Schenck, Germany) has been used. The vibration meter is equipped with an acceleration sensor, model AS-065 (Bruel & Kjaer Vibro) (Fig. 4).

Fig. 4 General view of the vibration meter, model Vibrotest 60, equipped with acceleration sensor.

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A magnet is used for fixing the sensor to the bearing housings of the predetermined measurement points. To ensure the good fixation, the bearing housings have been cleaned out of paint, dust and other contaminants.

RESULTS AND DISCUSSION The obtained results prove the effect of the investigated factors on the dynamic behaviour of the machine, which is confirmed by the results of other authors studying a similar type of machine (KOVACHEV et al. 2018a, KOVACHEV et al. 2018b, VAN et al. 2018, CHUNMEI et al. 2020), who observe a similar trend in the variation of the magnitude of the vibrations as a result of the change of the variable factors characterizing the cutting process.

The results obtained from the experimental study clearly show significantly higher vibration values at measurement points Gx and Gy which are located on the bearing housing in proximity of the cutting tool (upper bearing housing) compared to the vibrations measured at points Dx and Dy (lower bearing housing). Therefore, under the cutting mode conditions the dynamic behaviour of the machine was investigated and analysed based on the vibrations measured at points Gx and Gy. After applying the method of regression analysis and statistical analysis of the data (by specialized software QStatLab.5) the regression equations (2) and (3) have been derived.

These equations are used to predict the vibration magnitude in the range of the investigated factors at the measurement points Gx and Gy, as follows: Regression equation for measurement point Gx; y ̂=2,211+2,488x1+0,178x2+0,271x3+3,424x12-0,124x22-0,049x32-0,351x1.x2+ 0,169x2.x3-0,161x1.x3 (2) Regression equation for measurement point Gy; y ̂=2,231+1,441x1+0,086x2+0,045x3+1,328x12-0,083x22-0,018x32-0,163x1.x2+ 0,233x2.x3-0,373x1.x3 (3) where: x1 – cutting speed, coded; x2 – feed speed, coded; x3 – cutting depth, coded.

The calculated correlation coefficients for the two derived regression equations are: for equation (2) – R2 = 0.99; for equation (3) – R2 = 0.98.

From the values of the F-distribution and the abulated coefficient FT it was found that for the three regression equations the Fisher criteria for the adequacy of the model, namely F≤FT are fulfilled, therefore, the results could be further analysed.

From the values of the regression coefficients in front of the variable factors, for both regression equations, it can be concluded that the greatest influence on the magnitude of the vibrations had the cutting speed Vc (with regression coefficients 2.488 and 1.441), respectively the rotational frequency of the cutting tool.

From equation (1) it is visible that at one and the same cutting diameter D, the cutting speed increases with an increase of the rotational speed of the cutting tool n. The increase in the cutting speed Vc results in an increase vibration velocity v. The other two measured factors exerted almost equal influence on the vibration velocity. However, in the measurement point Gx a slightly higher influence had the thickness of the output layer (with 103

Vibration velosity in Gx point, v, mm.s-1 (r.m.s.)

regression coefficient 0.271), while in measurement point Gy the feed speed (with regression coefficient 0.086) exerted a bit higher influence.

The changes in the vibration speed at measurement points Gx and Gy in relation to the cutting speed are presented in Figures 5 and 6.

From the results in the Fig. 5 is visible that with an increase in the cutting speed Vc from 29 m·s-1 to 36 m·s-1 the vibrations magnitude decreased at all three assessed feed speeds Vf. However, when the cutting speed Vc increased above 44 m·s-1, the vibration’s magnitude increased. A distinct difference in the values of the vibration velocity for the three feed speed was observed at cutting speed ranged from 25 m·s1 to 45 m·s1.

At the lowest speed feed Vf = 3.5 m·min-1 the vibrations’ velocity is higher when compared to the higher feed speed of Vf = 7 and Vf = 10.5 m·min1.

It is worth mentioning that when the cutting speed is above 50 m·s1, the vibrations’ magnitude is equal at the three feed speeds and increased significantly by increasing the feed speed. The maximal value of the vibration velocity of vrms = 7.74 mm·s1 was reached at V = 59 m·s1.

Regarding the vibrations’ magnitude, measured at points Gx (Fig. 5) and Gy (Fig. 6), it could be concluded that the measured values were lower at point Gy, where the maximal value of vrms = 5.16 mm·s1 was measured at maximal tested cutting speed of Vc = 59 m·s1.

8

x₂=Vf=3.5 m.min⁻¹ x₂=Vf=7 m.min⁻¹ x₂=Vf=10.5 m.min⁻¹

7 6 5 4 3 2 1 0

29

32.75

36.5

40.25 44 47.75 Cutting speed, Vc, m.s-1 (x1)

51.5

55.25

59

Vibration velosity in Gy point, v, mm.s-1 (r.m.s.)

Fig. 5 Assessment of vibration velocity v at measurement point Gx in relation to the cutting speed Vc at different feed speed Vf. 5.5

x₂=Vf=3.5 m.min⁻¹

5

x₂=Vf=7 m.min⁻¹

4.5 4 3.5 3 2.5 2 1.5 1

29

32.75

36.5

40.25 44 47.75 Cutting speed, Vc, m.s-1 (x1)

51.5

55.25

59

Fig. 6 Assessment of vibration velocity v at measurement point Gy in relation to the cutting speed Vc at different feed speed Vf.

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Vibration velosity in Gy point , v, mm.s-1 (r.m.s.)

The changes in the vibration velocity, evaluated at the measurement point Gy, in relation to the cutting speed at three different values of the cutting thickness h are presented in Fig 7. This graph, as well as the previous ones, shows a slight decrease in the magnitude of the vibrations in the lower limit of the investigated cutting speed range Vc. A distinct difference in the value of vibration velocity depending on the thickness of the out-cut layer is observed in the cutting speed interval from 29 m.s-1 to 40 m·s1. With an increase of the cutting speed above 45 m.s-1, the intensity of the vibrations increases as well.

5.5 x₃=h=4 mm x₃=h=8 mm x₃=h=12 mm

5 4.5 4 3.5 3 2.5 2 1.5 1

29

32.75

36.5

40.25 44 47.75 Cutting speed, Vc, m.s-1 (x1)

51.5

55.25

59

Fig. 7 Changes in the vibration velocity v at measurement point Gy in relation to the cutting speed Vc at different out-cut layers h.

For the graphical representation of the changes in vibrations’ magnitude in relation to the feed speed Vf, at three different thicknesses of the layer h, the values of the vibration velocity v, measured at point Gx have been used (Fig. 8).

Vibration velosity in Gx point , v, mm.s-1 (r.m.s.)

3 2.75 2.5

x₃=h=4 mm x₃=h=8 mm x₃=h=12 mm

2.25 2 1.75 1.5 2.00

3.00

4.00

5.00 6.00 7.00 Feed rate Vf, m.min-1 (x2)

8.00

9.00

10.00

Fig. 8 Changes in vibration velocity v at measurement point Gx in relation to the feed speed Vf at different thicknesses of the out-cut layer h.

It can be seen from the graph that with an increase of the thickness of the out-cut layer h, the magnitude of the vibrations increase at one and the same speed feed Vf. A slight

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difference in the vibration velocity between three thicknesses of the out-cut layer h was observed at feed speed ranging from 2 to 4 m·min1. With an increase of the feed speed Vf above 6 m·min1, however, the difference increases and it is more visible between h = 4 and h = 8 mm, when compared to h = 8 mm and h = 12 mm.

CONCLUSIONS Based on the result of our study could be concluded that for this type of milling machines, higher vibration velocity v was observed at the upper bearing housing of the main shaft of the machine which could be explained with the overhanging shaft. Mounting of the cutting tool additionally changes the weight of the spindle in its upper edge, which could also be regarded as a reason for the increased vibrations, measure at the upper bearing housing of the machine in comparison to those measured at the lower bearing housing.

The results obtained under the conditions of this study confirmed the role of the evaluated factors on the overall vibrations, generated by the used milling machine. The highest influence on the increased magnitude of the vibrations exerted the cutting speed Vc, followed by the feed speed Vf and the cutting depth h.

On the basis of this results with regard to the magnitude of the overall vibrations, generated by the used milling machine in processing specimens of beech wood (Fagus sylvatica L.), the following optimal values are recommended: cutting speed Vc ϵ (34÷40) m·s1; feed speed Vf up to 4 m·min1; cutting depth h up to 8 mm. In cases when h>8 mm, the recommended feed speed is Vf < 3.5 m·min1.

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VAN, T., NGUYEN, H.L. 2018. Investigation on influence of cutting parameters on spindle vibration of CNC wood milling machine. MATEC Web of Conferences Volume 213, 15 October 2018, Article number 010072018 6th Asia Conference on Mechanical and Materials Engineering, ACMME 2018; Seoul; South Korea; 1518 June 2018; Code 145026.

VUKOV, G., MARINOV, B. 2008. Identification of the typical defects of the driving mechanism of carved veneer machines using vibrodiagnostics. In Proceedings of Scientific-technical conference “Innovation in Woodworking Industry and Engineering Design” Yundola, 14-16 November 2008, pp. 166169, ISBN 978-954-323-538-4.

VUKOV G., SLAVOV, V., KOVATCHEV, G. 2014. Investigations of the forced torsional vibrations in the saw unit of a kind of wood shapers, used in the wood production. In Innovation in woodworking industry and engineering design, 5(1): 6269, Intel Entrance, ISSN 1314-6149.

YAITSKOV, I., CHUKARIN, A., MOTRENKO, D. 2019. Theoretical research of the vibroacoustic dynamics of the cutting tools for milling recessing and chain mortise woodworking machines. In Akustika, Vol. 34: 79–84, DOI: 10.36336/akustika20193479.

ZHANG, S., HUA, J., XU, W. 2011. Study on dynamic characteristics and dynamic response of woodworking four-side planer. In International Conference on Advanced Engineering Materials and Technology, AEMT 2011; Sanya; China; 29-31 July, Code 85968 (Conference Advanced Materials Research, Vol. 291-294, pp. 1970–1976, ISSN: 10226680, ISBN: 978-303785193-7.

ACKNOWLEDGEMENTS This paper was supported by the Scientific Research Sector at the University of Forestry – Sofia, Bulgaria, under contract № НИС-Б-1012/27.03.2019.

AUTHORS’ ADDRESSES Chief Assist. Prof. Pavlin Vitchev, PhD Prof. Zhivko Gochev, PhD Prof. Georgi Vukov, PhD University of Forestry Faculty of Forest Industry 10 Kliment Ohridski Blvd.

1797 Sofia, Bulgaria p_vitchev@ltu.bg zhivko.g@ltu.bg givukov@ltu.bg

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 109−114, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.10

DETERMINATION OF PERFORMANCE INDICATORS OF PCD ABRASIVE WHEELS FOR SHARPENING TUNGSTEN CARBIDE WOOD CUTTING TOOLS Zhivko Gochev  Pavlin Vichev ABSTRACT Some results of the research on the performance of various diamond abrasive wheels for grinding of carbide knives are presented in the paper. The diamond wheels designed with metal coated and aggregated grains with organic bonds were used. The studies were performed at two levels of intensified, multi-pass sharpening TC (tungsten carbide) knives type K20, the part of a cutter head for longitudinal flat milling of solid wood and woodbased materials. The obtained results were analysed and relevant conclusions and recommendations were made. The process of sharpening carbide tools and its optimization can be better understood this way.

Key words: diamond abrasive wheel, sharpening, carbide tipped knives, wood, wood-based materials.

INTRODUCTION The diamond sharpening TC tools provides a significant increase in the productivity, accuracy, quality of surfaces, reliability, and durability of their cutting elements. The cost of maintaining them is significantly reduced. Due to their high hardness, diamond grains penetrate the hard alloy relatively lightly, deforming the surface layer slightly and causing no high stresses.

The performance of diamond wheels is an indicator that characterizes both the quality of the abrasive tool itself and the results of its impact on the sharpened TC tools (ZAHARENKO 1981).

This article aims to study the performance of diamond wheels under various sharpening conditions.

MATERIAL AND METHODS For the research, a cutter head with replaceable knives and TC edges was used for the preliminary and fine longitudinal planing of solid wood and wood-based materials (Fig. 1) (https://www.zmm-sm.com/zmmsm/english/wood.htm).

The basic parameters of the cutter head and the replaceable knives are given in Table 1. The tool body is made of aluminum and planer knives are with TC edges type K20 and heat-treated to hardness HRA 92.

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Tab. 1 Basic parameters of the cutter head and knives.

D, mm 120

d, mm 30

L, mm 120

B, mm 30

s, mm 3

z, mm 4

, 0

Type TCT – К20

45

Fig. 1 Assembled cutter head with insert knives and TC cutting edges, type K20.

The indications in Table 1 correspond to: D – Diameter of the cutter head; d – Bore size; L – Length of the knife; B – Width of the knife; s – Thickness of the knife; z – Number of knives; β – Angle of sharpening; TCT – Tungsten carbide teeth.

The cutter head is designed for shaper machines and four-side processing machines.

The TC edges type K20 (ISO grade classifications) consists of 94% tungsten carbide (WC) and 6% cobalt (Co) with a tungsten grain size of 1.02.0 μm (http://carbide.ultramet.com/viewitems/iso-grades/iso-grade-classifications-tungsten-carbide).

The abrasive PCD grinding wheel (Fig. 2) has 12A2-45 shapes (conical cup - CC) and works with its front surface (manufacturing of Russia).

The characteristics of the experimental disks according to the FEPA (Federation of European Producers of Abrasives) are given in Table 2 and Fig. 3.

Fig. 2 Abrasive grinding wheel shape 12A2-45 (conical cup).

Tab. 2 Characteristics of experimental diamond abrasive wheels.

Shape and dimensions, mm 12A2-45 125 × 5 × 3 × 32 12A2-45 125 × 5 × 3 × 32 12A2-45 125 × 5 × 3 × 32 12A2-45 125 × 5 × 3 × 32

Abrasive type

Mesh Size, μm

Bond Type

Concentration, %

Hardness

Work conditions

SDC 2

D126

B2-01

K100

R

s

SDC 2

D126

B1-13

K100

R

s

SDC 4

D126

B2-01

K100

R

s

SDC 4

D126

B1-13

K100

R

s

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Fig. 4 Abrasive wheel indication.

The indications of Fig. 4 correspond to: SDC 2 – Metal-coated synthetic diamond with aggregated grains and ordinary strength; SDC 4 – Metal-coated synthetic diamond with aggregated grains and increased strength; D126 – Mesh size 125/100; К100 – Concentration of diamond grains 100%; R – Hard bond; B2-01 – Phenol-formaldehyde-based organic bond with boron carbide filler enhancing selfsharpening process B1-13 - Phenol-formaldehyde-based organic bond with barium sulphate filler and talc for clean sharpening and smoothing Metal-coated diamond grains are better retained by the organic bond due to the presence of a metal film on their surface. This film protects the grains from shavings and ruptures, increases their strength and improves the conditions of heat removal from the sharpening zone. This results in a reduction in the specific diamond consumption and increases sharpening performance.

Aggregated grains (up to 10 pieces in one aggregate) have a significantly larger unfolded surface (regardless of the initial abrasive mesh size). Such diamond grain aggregates are better retained by the organic bond and withstand much higher loads.

Abrasive wheels have better cutting abilities. (KURDYUKOV 2014).

The investigations were carried out with the intensified multi-pass sharpening of a sharpening machine model HMS 700 of HOLZMANN - Austria, under the following conditions: - Cutting speed (V) – 18 m/s; - Longitudinal feed speed (Vl) – 2.0; 2.5 m/min; - Cross feed speed (Vdm) – 0.03; 0.05 mm/double motion.

The following indicators for evaluating the performance of diamond wheels in a multipass sharpening of TC tools have been determined (OSTROVSKII 1981, ZAHARENKO 1981, GOCHEV 2008, 2019): - Relative consumption of SDC determined by the weight method - Qr, mg/g; - Coefficient of cutting capacity - Cc; ⃗ 𝑒 , W; - Effective sharpening power of direct motion - 𝑁 - Relative power consumption of sharpening – Er.e, kWh/kg; - Complex performance indicator – Cc.i, mm3.g/min.kg.

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The first four indicators were determined using the methodology set out in the Ohrid and Zagreb publications (GOCHEV 2019).

Some researchers determine the performance of diamond wheels using a Grinding Ratio (ZAHARENKO 1981, ROWE 2009, KOROTOVSKIKH 2012, 2017).

The complex performance indicator takes into account the diamond wheel's durability, productivity, conditions, and sharpening mode and can be defined as: 𝐶𝑐.𝑖 = 𝑐

𝑃𝑎𝑐.

(1)

𝑄𝑟

Where c is a coefficient dependent on the magnitude of the cross feed speed (c = 0.2 at W = 5 mm и Vdm < mm/double motion): Pac. – Actual sharpening performance, mm3/min;

RESULTS AND DISCUSSION Table 3 shows the summary results of studies of synthetic diamond wheels with metalcoated and aggregated grains with organic bond with intensified multi-pass sharpening without cooling of TC edges type K20.

The study was conducted at two levels of sharpening modes. In mode 2, the theoretical productivity (Pth.) was increased to the maximum possible - 1050 mm3/min.

The comparison of the results shows that the performance indicators of the studied diamond wheels are comparable and practically are in the same confidence intervals.

Diamond wheels with metal-coated and aggregated abrasive grains have high cutting properties.

Tab. 3 Performance indicators for intensified multi-pass sharpening without cooling of TC edges type K20.

Diamond wheels

Mode №

Vl, m/min

Vdm, mm/dm

Pth., mm3/ min

SDC 2 D126 B2-01 K100 s SDC 2 D126 B1-13 K100 s SDC 4 D126 B2-01 K100 s SDC 4 D126 B1-13 K100 s

1 2 1 2 1 2 1 2

2.0 2.5 2.0 2.5 2.0 2.5 2.0 2.5

0.03 0.05 0.03 0.05 0.03 0.05 0.03 0.05

504 1050 504 1050 504 1050 504 1050

Cc 0.91 0.88 0.92 0.89 0.92 0.89 0.93 0.90

Performance indicators Cc.i, ⃗​⃗ Q r, Er.e, mm3.g/ 𝑵𝒆 , mg/g kWh/kg W min.kg 0.24 382 295 41 1.60 116 610 40 0.21 442 260 39 1.19 157 560 29 0.23 403 230 36 1.27 147 490 32 0.19 439 250 40 0.72 263 430 32

Organic bond, type B2-01 provides more efficient operation of diamond grains when sharpening under mode 1. At the same time, this type of bonding does not allow us to take advantage of metal-coated diamond grains (especially for the SDC 4 brand) as it does not provide reliable grains retention under heavier mode 2.

The use of diamond wheels with bond type B1-13 results in lower relative consumption of diamond, effective power of sharpening of direct motion and relative power consumption. Diamond wheels with this type of bond can operate in cross-feeding up to 0.03 mm/double motion, below 1 mg/g relative consumption of SDC and large values of the complex performance indicator.

It is difficult to compare the results with those of other authors. For example, various studies have been conducted such as: Sharpening modes of deep diamond grinding of hard 112

materials tools have been studied. Grinding wheels of different granularities and diameters were examined to sharpen YT and YG cemented carbide cutting tools. The influence of sintering temperature to the relative density (R.D.), hardness and service life of diamond grinding wheels with AlSnTi, AlSnTiNiCo, AlSnTiNiand AlSnNiCo bonding agent was studied and others. Some factors, which have significant effects, like the radial wear of the diamond grinding wheel, the components of the grinding forces, the normal and the tangential grinding force, and the surface quality of the tools are studied. Various variables such as the cutting and feed speed and the coolant supply method were varied to investigate the effect on grinding of different tool materials, the brittle silicon nitride, and the ductile cemented carbide material and others (BIERMAN et al. 2009, SHANG-XI et al. 2008, VOYACHEK et al. 2013, ZHANG et al. 2004).

CONCLUSIONS The analysis shows that when sharpening TC knives type K20, the SDC wheels with organic bond type B1-13 have a high coefficient of cutting capacity and the main work is done in the direct motion of the longitudinal feeding. Diamond grains retain well from the bond and do not fall out before they become blunt. This is confirmed by the relatively low relative consumption of SDC. Analysis of the results also shows that: - Sharpening processes by multi-pass grinding without cooling can be intensified to a productivity greater than 500 mm/min using diamond discs with metal-coated and aggregated grains type SDC 2 D126 B1-13 K100 s and SDC 4 D126 B1-13 K100 s.

- The sharpening performance, the type of bonding, the brand of diamond grains and their coverage all have a significant impact on the productivity of diamond wheels when sharpening TC tools. These indicators characterize the quantitative side of the process.

- Knowledge of the quality side of the process is also required, i.e., what phenomena occur in diamond grains and bond when interacting with the surface layer of the polished hard alloy. What is the reason for the higher or lower consumption of diamond, the higher or lower the sharpening resistance, etc.?

- Joint analysis of the quantitative and qualitative sides of the process will allow the optimization of the sharpening process of tungsten carbide tools.

REFERENCES BIERMAN, D., E. WÜRZ 2009. A study of grinding silicon nitride and cemented carbide materials with diamond grinding wheels. In Production Engineering 3(4):411416, DOI: 10.1007/s11740-0090183-z.

GOCHEV, ZH. 2008. Investigation on the grinding quality of planing knives made of high-speed steel (HSS) type M2 and specific consumption of cubic boron nitride (CBN). In Chip and Chipless Woodworking Processes, Zvolen: Technical University in Zvolen, pp. 8997, ISBN 978-80-2281913-8.

GOCHEV, ZH., P. VITCHEV, G., VUKOV 2019. Determination of performance indicators and quality of TCT knives when sharpened with PCD grinding wheels. In Wood Technology & Product Design, Skopje : S.S Cyril and Methodius University, pp. 119126, ISBN 978-608-4723-03-5.

GOCHEV, ZH., P. VITCHEV, G., VUKOV 2019. Determination of Performance Index and Effective Power for Sharpening of TC Planer Knives with PCD Abrasive Wheels. In ICWST 2019, Impementation of Wood Science in Woodworking Sector and 70th anniversary of Drvna industrija Journal, Zagreb, pp. 53-60, ISBN 978-953-292-062-8.

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KOROTOVSKIKH, V. 2012. Determination of diamond wheels working efficiency in order to optimize their composition. In Bulletin of Kurgan State University, Technical Science Series, Issue 7 № 2 (24), pp. 122124, ISSN 2222-3347 (in Russian).

KOROTOVSKIKH, V. 2017. Efficiency of hard-face tool grinding with diamond circles on organic bonds. In Bulletin of Kurgan State University, Technical Science Series, Issue 12 № 2 (45), pp.

7173, ISSN 2222-3347 (in Russian).

KURDYUKOV, V. 2014. Fundamentals of abrasive machining (Основы абразивной обработки).

Textbook, Kurgan State University, pp. 195, ISBN 978-5-4217-0254-2 (in Russian).

OSTROVSKII, V. 1981. Theoretical foundations of the grinding process, Leningrad: Leningrad University Press, p. 142 (in Russian).

ROWE, W. B. 2009. Principles of Modern Grinding Technology. Linacre House, Jordan Hill, Oxford OX2 8DP, UK, p. 480, ISBN 978-0-323-24271-4.

SHANG-XI, W., L., GENG, X., LIU, B., GENG, SH., NIU 2008. Manufacture of a new kind diamond grinding wheelwith Al-based bonding agent, Journal of materials processing technology 209(2009)1871–1876, DOI: 10.1016/j.jmatprotec.2008.04.045.

VOYACHEK, I., Y., KOSHELEVA, A., MUYXEMNEK 2013. Selecting the modes of deep diamond grinding of churlish materials for retaining circle’s cutting capacity, Technical science, Engineering and mechanical engineering, University proceedings, Volga region, № 1 (25), pp. 94101 (in Russian).

ZAHARENKO, I. 1981. Fundamentals of diamond sharpening of tungsten carbide tools. Kiev: Naukova dumka, p. 299 (in Russian).

ZHANG, C., G., WANG, H., PEI 2004. Effect of characteristics of the grinding wheel on cemented carbide cutting tool sharpening, Key Engineering Materials, Vols. 259260, pp. 5054, ISSN 16629795. https://www.zmm-sm.com/zmmsm/english/wood.htm http://carbide.ultra-met.com/viewitems/iso-grades/iso-grade-classifications-tungsten-carbide ACKNOWLEDGEMENT This paper is supported by the Scientific Research Sector at the University of Forestry – Sofia, Bulgaria, under contract № НИС-Б-1012/27.03.2019.

AUTHORS' ADDRESS Prof. Zhivko Gochev Ph.D. University of Forestry Faculty of Forest Industry Department of Woodworking machines 10 Kliment Ohridski Blvd.

1797 Sofia, Bulgaria zhivkog@ltu.bg, zhivkog@yahoo.com Chief Ass. Prof. Pavlin Vichev Ph.D. University of Forestry Faculty of Forest Industry Department of Woodworking machines 10 Kliment Ohridski Blvd.

1797 Sofia, Bulgaria p_vitchev@ltu.bg

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 115−120, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.11

COMBINATION OF WOOD AND GLASS IN THE SET OF DECORATIVE ITEMS FOR INTERIOR DESIGN Anna Mukanova – Yury Lozhkin – Mikhail Chernych – Vladimir Stollmann ABSTRACT Aesthetic capabilities of some materials are exhausted to a certain extent. The combination of materials in design objects renews the perception of traditional products providing a new decorative effect. The approach to the development of decorative functional sets for interior design, consisting of several different-purpose items produced from different materials, united graphically and stylistically using several techniques is presented in the paper. Each material and technology are unique and their combination and graphic-stylistic unity multiply the aesthetic perception both of each product and the whole set. The marinestyle set of three items is created – wall lamp, clock and mirror – with different types of wood in intarsia technique and fused art glass of different colors are combined.

Key words: wood, interior, fusing, colored glass, intarsia, pine, linden and hazel wood.

INTRODUCTION Interior is the architectural artistically designed internal space of a building providing comfortable conditions of a human’s life activities. The interior filling with objects is one of its components. The interior elements are divided based on several features: - activity degree (passive, active); - location (desktop, floor-standing, wall, overhead); - material (wooden, plastic, metal, glass, gypsum, textile, etc.).

Currently, the use of natural materials is getting more and more popular. In this regard, there is an interest in wood as the material that can combine ergonomics, economy and aesthetics in the product (SMORODINA 2017). Wood esthetic properties are defined by the richness of its color shades and variety of texture (CHERNYCH et al. 2013). Modification and natural finish significantly extend the color-texture palette of wood species (MAMONOVA 2009, CHERNYCH et al. 2013, SLABEJOVA et al. 2016). Wood is used both as a mono material and in combination with other materials, such as glass. For example, the artists Scott SLAGERMAN and Jim FISHMAN (2020) create unusual sculptures from the molten glass and wood fallen from trees. The authors named the collection “Wood&Glass”. The works represent hollow vessels inside the U-cuts of the tree and serve as vases (Figure 1).

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Fig. 1 Items from the collection “Wood&Glass” (Scott Slagerman, Jim Fishman).

The unusual combination of wood and glass can be also seen in the collection “River” by the American fine art restorer Greg KLASSEN (2020) (Figure 2).

Fig. 2 Dining table “River” (Greg Klassen).

The development of functional sets is relevant as well as the development of single items.

The aim of the work is to develop decorative functional sets for interior design. This set makes a favorable impression due to the combination of aesthetic properties of transparent and non-transparent materials and the overall image and style solutions.

MATERIAL AND METHODS The set was composed of different-purpose items, it was made of wood and glass united graphically and stylistically and combines decorativeness and functionality. The clock, mirror and wall lamp in marine style were chosen as items. The distinctive features of marine style in the interior design are as follows: - blue and white color palette reflecting sea depths; - use of natural materials (wood, linen cloth, glass); - use of souvenirs in the form of sea fruits (shells, corals, stones); - aged furniture from the natural wood.

Wood in the marine-style interior embodies the ship elements, and glass – sea depths.

The items contain the base and decorative insert produced using intarsia and fusing.

Each technology is unique and their combination increases the set esthetic value.

Intarsia is the technology of producing 3D mosaic pictures obtained by the combination of different wood species, change of tones and textured patterns. Items produced by this technique convey the material beauty and have attractive appearance, but, as many centuries ago, are produced manually and serve as attributes of expensive furniture (BARSUKOV et al. 2017).

Glass fusing is the technology of producing art items from glass, during which the glass elements are heated up till softening and joined together to form a single unit. The 116

items produced by fusing provide special exclusivity to the interior adding bright emphases (ZELINSKAYA, SEDOV 2019).

The image of blue whale – a unique sea animal – was chosen for the decorative insert.

A blue whale has peculiar longitudinal folds in the front part of the belly and belongs to the fin whale family. The sketches of the items were made based on this image (Figure 3).

а

b

c

Fig. 3 Sketches of the items: а – wall lamp; b – clock; c – mirror.

Types of the items – the circle resembles a ship steering wheel, an illuminator, as well as smooth shapes of shells.

RESULTS AND DISCUSSION Structure The decorative inserts in the lamp and mirror were produced of wood of different species, in the clock – of the glass. The lamp and mirror bases were produced of glass, and the clock one – of the wood pieces of different species glued to plywood.

Pine, linden and hazel wood, different in color and surface finish, were used in the set.

Spectrum glass (Mexico) of different colors was used to highlight certain areas of the whale body and give volume to the whale and waves due to the contrast, color and textures. Linden and hazel wood are readily cut, and pine wood has a vivid striped texture that allows imitating longitudinal folds on the whale belly (Figure 4).

Fig. 4 Scheme of placing wood plates of different species: 1 – linden, 2 – light hazel wood, 3 – dark hazel wood, 4 – pine wood.

The glass decorative elements were produced by fusion.

The lamp was equipped by the lighting element consisting of LED strip, switch and body for placing three AA 3.7 V accumulator batteries. The clock contained the quartz mechanism of smooth motion with the set of hands.

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Technology and equipment The wood elements of the items were produced in the following sequence. Cutting by the template on the band saw JWBS – 16X (Taiwan). Grinding of the elements along the outline for fitting on the machine OSTERMANN KSM-2600 (European Union). Gluing with the glue Moment Stolyar PVA (GOST 18992-80) (Russia). The final polishing and applying of protective and decorative coating – acrylic glossy varnish for artistic works “Aqua Color” (Russia). Assembling of the decorative insert with the base, lighting element and clock.

The technology of part production from glass contained the following operations.

Glass cutting with oil glas-cutter TOYO TC-10 (Japan), grinding the edges using the grinding machine Inland Contour GT (Germany), cleaning and degreasing the elements with the glass cleaner Grass Clean Glass (Russia), gluing the elements to be fused with UV-curing glue Loxeal UV 30-20 (Italy), fusing in the software-controlled furnace DF-10-072 (Russia).

The fusion temperature and time mode:  heating with the rate of three degrees per minute up to 750°С;  curing at a temperature of 750°С within ten minutes;  fast cooling to the annealing upper temperature (570°С);  annealing at a temperature of 570°С within ten minutes;  cooling with the rate of two degrees per minute to the annealing lower temperature (470°С);  cooling together with the furniture to the room temperature.

The items produced are demonstrated in Figures 5 and 6.

Fig. 5 Mirror (overall dimensions: 223 × 220 × 23 mm).

а

b

Fig. 6 Wall lamp (overall dimensions: 267 × 220 × 22 mm): а – at daylight; b – during nighttime.

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CONCLUSIONS The combination of wood and glass allows creating not only single items, but also different-purpose functional sets of items for interiors, united graphically and stylistically.

The combination of different wood species in intarsia technique allows creating 3D mosaic images due to the color and finish surface contrast of the mosaic elements.

The fused elements of the colored glass complement esthetic properties of the wood adding bright emphases to the composition and provide special exclusivity to the items.

The proposed concept of producing interior decorative sets based on the combination of transparent and non-transparent materials in items, and design of the items in one graphical and stylistic solution allows strengthening the esthetic perception, contributes to creating the comfortable environment for a person.

REFERENCES SMORODINA, E. 2017. Modern tendencies in wood application in architecture and design and Russia.

Science today: history and modernity: Proceedings of International Scientific-Practical Conference (Russia, Vologda, 25.10.2017) in 2 volumes. V. 2 – Vologda: LLC “Marker”, 2017. 152 p., p. 138– 139. ISBN 978-5-906850-78-2.

CHERNYKH, M., KARGASHINA, E., STOLLMANN, V. 2013. Assessing the impact of aesthetic properties characteristics on wood decorativeness. In Acta facultatis xylologiae Zvolen, 2013, 55(1): 21–26. ISSN 1336-3824.

CHERNYKH, M., KARGASHINA, E., STOLLMANN, V. 2013. Wood refining by the impregnation under the electric current influence. In Acta facultatis xylologiae Zvolen, 2013, 55(2): 13–19. ISSN 13363824.

MAMONOVA, M. 2009. Selection of proper coating system for stump wood. In Acta Facultatis Xylologiae Zvolen, 2009, 51(2): 39–48. ISSN 1336-3824. In slovak language.

SLABEJOVA, G., SMIDRIAKOVA M., FEKIAC, J. 2016. Gloss of transparent coating on beech wood surface. In Acta Facultatis Xylologiae Zvolen, 2016, 58(2): 37–44. ISSN 1336-3824.

Scott SLAGERMAN Glass [Electronic source] - // Internet/ - 2020 - Available at: https://www.scottslagerman.com.

Greg KLASSEN [Electronic source] - // Internet/ - 2020 - Available at: http://www.gregklassen.com (reference date: 26.02.2020).

BARSUKOV, V., GORSHKOVA, T., KOSTYLEVA, E. 2017. Technology of artistic material processing: students’ aid, 2017, 513 с. ISBN 978-5-94211-783-2.

ZELINSKAYA, M., SEDOV, E. 2015. Stained-glass art and works with glass, 2015, 103 p. ISBN 22278397.

Spectrum Glass Company [Electronic source] - // Internet/ - 2020 - Available at: http://spectrumglass.com/.

Official webpage of JET representatives. Wood-processing equipment [Electronic source] - // Internet/ - 2020 - Available at: https://www.jettools.ru/catalog/.

Kami Association. Wood-processing equipment [Electronic source] - // Internet/ - 2020 - Available at: https://www.stanki.ru/catalog/derevoobrabatyvayushhee_oborudovanie/.

GOST 18992-80: “Polyvinyl acetate homopolymeric coarsely dispersed dispersion. Technical conditions”. – М.: IPK Standard publishers, 2001. – 18 p.

LLC “AQUA-COLOR”. Product catalogue [Electronic source] - // Internet/ - 2020 - Available at: https://aquapaint.ru/c/.

Professional oil glass-cutter TOYO TC – 17B/ TC – 10B (Japan) [Electronic source] - // Internet/ 2020 - Available at: http://steklorezprof.ru/catalog/steklorezy_dlya_obychnogo_stekla/toyo_ts_17v_yaponiya_professio nalnyy_maslyannyy_steklorez/.

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Grinding machine Inland Contour GT [Electronic source] - // Internet/ - 2020 - Available at: http://vitrage.su/20030-shlifovalnyj-stanok-inland-contour-gt/.

Glass cleaner “Grass Clean Glass” [Electronic source] - // Internet/ - 2020 - Available at: https://izhevsk.vseinstrumenti.ru/avtogarazhnoe-oborudovanie/avtohimija/ochistiteli/stekla/spreisredstva/grass/lesnye-yagody-600-ml-clean-glass-125241/.

UV glue Loxeal 30-20 [Electronic source] - // Internet/ - 2020 - Available at: http://ipak.net.ru/ufklej-loxeal-30-20/.

ACKNOWLEDGMENT The paper was written under the financial support of the Ministry of Education, Science and Sports of the Slovak Republic within the project KEGA 007TU Z-4/2019.

AUTHOR’S ADDRESS Anna Mukanova, master student Kalashnikov Izhevsk State Technical University V.A. Shumilov Institute of Construction and Architecture Department of Technology of Industrial and Artistic Processing of Materials Studencheskaya St. 7 Izhevsk, 426069 Russian Federation anikm96@mail.ru Prof. Dr. Mikhail Chernych Kalashnikov Izhevsk State Technical University V.A. Shumilov Institute of Construction and Architecture Department of Technology of Industrial and Artistic Processing of Materials Studencheskaya St. 7 Izhevsk, 426069 Russian Federation rid@istu.ru Assoc. Prof. Yuri Lozhkin Kalashnikov Izhevsk State Technical University V.A. Shumilov Institute of Construction and Architecture Department of Technology of Industrial and Artistic Processing of Materials Studencheskaya St. 7 Izhevsk, 426069 Russian Federation lyv2007@mail.ru Associate professor Vladimir Stollmann Technical university in Zvolen Faculty of Forestry Department of Forest Harvesting, Logistics and Amelioration T. G. Masaryka 24 960 53 Zvolen Slovak Republic stollmannv@tuzvo.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 121−136, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.12

CORPORATE CULTURE IN SMALL AND MEDIUM-SIZED ENTERPRISES OF FORESTRY AND FOREST-BASED INDUSTRY IS DIFFERENT Silvia Lorincová – Ľubica Bajzíková – Iveta Oborilová – Miloš Hitka ABSTRACT The development of economies, globalization and the related economic changes require a radical turn in the thinking of all employees of the enterprise. Corporate culture provides an opportunity to achieve a competitive advantage. Using an Organizational Culture Assessment Instrument, corporate culture in small and medium-sized enterprises of forestry and forest-based industry operating in Slovakia is defined. The research outcomes confirm that, in the small enterprises of forestry and forest-based industry, clan corporate culture characterized by family atmosphere and friendly working environment is applied.

On the other hand, medium-sized enterprises are characterized by a hierarchy corporate culture emphasizing compliance with core values following the regulations and order. In the strategic perspective of the following 5 to 10 years, in small and medium-sized enterprises, the clan corporate culture should be applied. As a result of the presented research, the values typical for clan corporate culture are recommended being applied to the analyzed type of enterprises. If the enterprise management can focus on internal cohesion and joint achievements of goals, the enterprise will also achieve higher performance resulting in financial success and ultimately gain a competitive advantage.

Key words: Small and medium-sized enterprises, forestry and forest-based industry, corporate culture, Organizational Culture Assessment Instrument, Tukey's HSD test.

INTRODUCTION Small and medium-sized enterprises are considered a key to each country's economy.

Not only they make a significant contribution to gross domestic product, they also increase innovation, flexibly introduce new products, help to create jobs and maintain the existing ones and significantly contribute to the overall enterprise growth (SEDLIAČKOVÁ et al. 2020, CERVENKA et al. 2016, STACHO – STACHOVA 2015). However, business activities of small and medium-sized enterprises are influenced by growing globalization putting pressure on enterprises in order to create effective strategies to succeed in a highly competitive environment (BUSSE – GREGUŠ 2020, MORESOVA et al. 2020, AL-TKHAYNEH et al. 2019, ANYAKOHA 2019; STACHO et al. 2019, ĎURIŠ et al. 2018, LOUCANOVA et al. 2018, LAŠÁKOVÁ et al. 2017, NEMEC et al. 2017, NEDELIAKOVA – PANAK 2015, STACHOVA 2012).

Therefore, in recent years, more and more attention has been paid to the fact that, to a large extent, success of an enterprise depends on its values, standards, rules, patterns of behavior and rituals, i.e. on “corporate culture”. The main reason why a strong emphasis should be 121

put on corporate culture is the fact that many researches understand it as a very important factor in the process of successful enterprise operation, and also as a significant element of corporate organization (MULLAKHMETOV et al. 2019, VLAICU et al. 2019, MATRAEVA et al.

2018, REZAEI et al. 2016, OGBONNA – HARRIS 2000).

The objective of the research is the corporate culture analyzed via the forestry and forest-based industry including the forest, wood processing, furniture manufacturing enterprises and the enterprises related to pulp and paper-processing industry. In terms of sectoral structure of enterprises in Slovakia, the aforementioned enterprises can be considered significant, because of their high potential. Their strategic importance is proved by the fact that many enterprises in the wood processing industry operating in Slovakia have experienced very dynamic growth (PALUS et al. 2019). Over the last three years, the revenues of these enterprises have increased by a third and labor productivity has increased by 12% (MARKO 2019). Forest enterprises account for 0.33% of the gross domestic product of the Slovak Republic. There are approximately 1,200 to 1,300 enterprises in forestry, with revenues ranging from 220 to 240 million € (SUJOVÁ – KOVALČÍK 2017, ZELENÁ SPRÁVA 2017, PALUŠ et al. 2011). Furniture manufacturing enterprises are financially attractive as well. In 2018, they achieved record sales. Very good indicators were also achieved by pulp and paper-processing enterpripses; their sales reached almost 1.4 billion € and a profit of 125 million € (MARKO 2019).

With regards to the aforementioned importance of small and medium-sized enterprises, the aim of the research is to define the corporate culture in small and mediumsized enterprises of forestry and forest-based industry in Slovakia and to propose recommendations for the most suitable “corporate culture” corresponding with the size of the enterprise.

LITERATURE REVIEW Corporate culture is a system of accepted values and opinions creating positive informal norms of behavior in enterprises. It is a certain (intangible) product resulting from employees' thinking and the activities performed. Corporate culture incorporates a multiplicity of shared employee beliefs, values, behaviors, and symbols; therefore, it has a significant impact on individual decisions and group actions (LIZBETINOVA et al. 2016, BELIAS et al. 2015). It represents the value of an identity and a common corporate spirit for each employee, whether we examine it in private or public sector (DRDLA – Rais 2001).

Regardless of its size, form, focus and other factors, each enterprise has its own specific, original and unique type of corporate culture distinguishing it from the corporate cultures of other enterprises (GUISO et al. 2015, KACHAŇÁKOVÁ 2010). Even an enterprise running on the market for one year has already started to create its cultural history (JAVORČÍKOVÁ – Dove 2019, JAVORČÍKOVÁ et al. 2019, FAIRFIELD-SONN 2001). Cultural history reflects the ideas of the enterprise founders (owners) about the existence, success, career, remuneration system, and so on, which are later translated into a particular type of corporate culture. The sources of these ideas create the basis of the follow-up pattern of ideas common to all employees in certain situations, which manifests itself in the general approach to managers, employees, customers, and so on.

The existing typologies define typical contents of corporate culture from various perspectives. HANDY (1985) and HARRISON (1972) defined corporate culture from the analytical point of view. PFEIFER and UMLAUFOVÁ (1993) and DEAL and KENNEDY (1982) in the context of examining corporate culture, analyzed the degree of risk of the business objects and market feedback. KACHAŇÁKOVÁ et al. (1997) on the other hand, defined the 122

corporate culture in relation to the dominant orientation of the enterprise during changes and life-phases of the enterprise. BOWETT (2006) discussed the enterprise's possibilities for change. In addition to these typologies, there are numbers of other approaches (MCNAMARA 2006, SONNENFELD 1988, VRIES – MILLER 1984, MILES et al. 1978, and others), mapping the complex content of an enterprise's social environment and make it possible to distinguish and understand the basic characteristics differentiating the businesses. Generally, the vast majority of typologies quite often examine corporate culture from a psychological point of view. However, CAMERON and QUINN (2006) examine corporate culture from a different perspective. The authors focus on the link between the perception of corporate culture and enterprise's outputs, for example, general efficiency of the enterprise. This is the most complex typology and at the same time, one of the most influential and widespread typologies in the present-day corporate culture research (VALENCIA et al. 2016). It identifies the content of corporate culture not only in relation to the degree of flexibility and control but also to the degree of internal and external environment (CAMERON – QUINN 2006, CAMERON – QUINN 1999, QUINN – ROHRBAUGH 1983).

Assessing corporate culture based on the methodology of CAMERON and QUINN (2006, 1999) through the Organizational Culture Assessment Instrument (OCAI), makes possible to diagnose corporate culture by examining core values, shared assumptions and common approaches to work. Such approach represents a classification approach to culture (LIM 1995), designed to identify the existing and desired corporate culture. It is based on the model of competitive values of the enterprise developed to measure corporate culture (DI STEFANO – SCRIMA 2016). The model of competitive values of the enterprise was created on the basis of a research focused on the 39 most important indicators affecting the efficiency of the enterprise (CAMERON – QUINN 2006, CAMPBELL 2004). QUINN and ROHRBAUGH (1983) subjected this list to a statistical analysis that highlighted two basic dimensions containing four important groups of indicators. The first dimension distinguishes between efficiency criteria emphasizing flexibility, freedom of decisionmaking and dynamism, and criteria emphasizing stability, order and control. The second dimension distinguishes between efficiency criteria emphasizing internal orientation, integration and compliance with criteria emphasizing external orientation, differentiation and competition. By combining the two dimensions, four quadrants were generated, each of which integrates a different set of enterprise's effectivity indicators representing employees' values related to enterprise's efficiency. For example, an externally oriented enterprise is mostly market-driven, oriented towards new customers and competition. On the other hand, an internally focused enterprise deals with the attitude of employees and the ways in which work is to be performed. The OCAI allows to diagnose the dominant direction of the enterprise and at the same time determines the type, strength and congruence of the prevailing culture, namely clan corporate culture, adhocracy corporate culture, market corporate culture and hierarchy corporate culture.

EXPERIMENTAL PART Employees working in small and medium-sized enterprises of the forestry and forestbased industry over the period 2016 to 2019 were contacted by the random sampling method.

For setting the minimal scope of the sample, Cochran's formula (setting the ideal sample size related to the required level of precision, reliability and estimated proportion of the attribute present in the population) was used as follows:

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𝑛0 =

𝑍 2 𝑝𝑞 𝑒2

(1)

where: z – critical value corresponding to the selected reliability of the estimate, p – preliminary estimate of the relative abundance, q – represents 1 – p, e – selected error of the estimate.

At the selected 95% reliability, accuracy of at least 5% and a critical value corresponding to the chosen reliability of the estimate at the level of 1.96, minimum sample size of 385 respondents represents the sampling unit. 𝑛0 =

𝑍 2 𝑝𝑞 𝑒2

=

(1.96)2 (0.5)(0.5) (0.05)2

= 385

(2)

Consequently, for the aimed reliability (95%) and accuracy (5%) of the research results evaluation, answers from 385 respondents were sufficient to generalize the results.

A total of 3,402 employees working in the small and medium-sized enterprises in the forestry and forest-based industry (composition is shown in Table 1) were involved in the research, which, given the conventions used in our research, met the criterion of the minimum size of the sampling unit.

Tab. 1 Sampling unit composition.

Size of enterprises Small-sized

Medium-sized

Sum

Multiplicity Multiplicity Line multiplicity Total multiplicity Multiplicity Line multiplicity Total multiplicity Multiplicity Line multiplicity Total multiplicity

2016 417 24.97% 2.97% 420 24.25% 3.00% 837 24.60% 5.97%

2017 400 23.95% 2.85% 461 26.62% 3.29% 861 25.31% 6.14%

2018 403 24.13% 2.87% 441 25.46% 3.15% 844 24.81% 6.02%

2019 450 26.95% 3.21% 410 23.67% 2.92% 860 25.28% 6.13%

Total 1,670 24.64% 11.91% 1,732 23.91% 12.35% 3,402 100.00% 24.26%

The questionnaire as a social research method was used as the main research method.

The questionnaire consisted of two parts. In the first part, the respondents provided information about the enterprise identification data. The second part of the questionnaire was based on the model of competitive values of the enterprise and the resulting OCAI methodology, where the two basic dimensions of the model represent flexibility versus control and the degree of internal and versus external focus. Their combination generated four types of cultures: clan corporate culture, adhocracy corporate culture, market corporate culture and hierarchy corporate culture. The content of the corporate culture was defined in a total of six dimensions (such as dominant characteristics, organizational leadership, management of employees, organization glue, strategic emphases and criteria of success).

Each of the six dimensions was examined by four alternatives: alternative A, alternative B, alternative C and alternative D. Alternative A corresponds to the clan corporate culture.

Alternative B corresponds to an adhocracy corporate culture. Alternative C corresponds to a market corporate culture and alternative D corresponds to a hierarchy corporate culture.

The respondents' task was to divide 100 points between the four alternatives based on the extent to which individual statements described the enterprise they work for (referring to the enterprise's current level). Subsequently, the task of the respondents was to distribute

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100 points depending on what the enterprise should look like in 5 to 10 years (referring to the enterprise's required level) (CAMERON – QUINN 2006).

The methodology of CAMERON and QUINN (2006, 1999) further assumes that in the final phase, the average values of the individual alternatives from all six dimensions are added and the overall average is defined. The final output is a type of corporate culture providing an overview of the basic assumptions and values applied in the enterprise that characterize it (BREMER 2017, CAMERON – QUINN 2006).

The results of the research were further processed by mathematical-statistical methods using statistical software RStudio. The current and required level of corporate culture in small and medium-sized enterprises of the forestry and forest-based industry was defined by means of an estimated average using the Beta regression method. The significance of differences in corporate culture was tested by inductive statistical methods. Interval estimates and Tukey's HSD test allowing multiple comparisons were used. The common 5% level of significance of the test was used. Differences were interpreted as statistically significant if the p-value<0.05. The following hypotheses were tested: H1: Are there differences in the current level of individual dimensions of corporate culture in small and medium-sized enterprises in the forestry and forest-based industry?

H2: Are there differences in the current level of corporate culture in small and medium-sized enterprises in the forestry and forest-based industry?

H3: Are there differences in the required level of individual dimensions of corporate culture in small and medium-sized enterprises in the forestry and forest-based industry?

H4: Are there differences in the required level of corporate culture in small and medium-sized enterprises in the forestry and forest-based industry?

RESULTS AND DISCUSSION The results in the first analyzed area in the current level of individual dimensions of corporate culture in small and medium-sized enterprises in the forestry and forest-based industry are presented in Table 2.

The results of dominant characteristics show that, in small enterprises in the forestry and forest-based industry, the highest rating was achieved by alternative A (𝑋̂=0.331). Small enterprises are characterized by their friendly work environment whereas alternative D (𝑋̂=0.348) is characteristic for medium-sized enterprises. Medium-sized enterprises are characterized as controlled and structured places. When testing the alternatives with the highest evaluation in the dimension of dominant characteristics in small and medium-sized enterprises of the forestry and forest-based industry, statistically significant differences were confirmed (alternative A, p-value<0.0001; alternative D; p-value<0.0001).

The organizational leadership of small and medium-sized enterprises is characterized by alternative D (small enterprises 𝑋̂=0.318; medium-sized enterprises 𝑋̂=0.340).

Enterprises focus on ensuring the smooth running of business organization based on cooperation. The differences were not confirmed (p-value=0.0736).

In the dimension of management of employees, alternative A achieved the highest rating (small enterprises 𝑋̂=0.403; medium-sized enterprises 𝑋̂=0.337). Management of employees is focused primarily on teamwork and cooperation. Although the two groups of respondents agreed, the test confirmed that there are differences in the management of employees of small and medium-sized enterprises of the forestry and forest-based industry in alternative A (p-value<0.0001).

Employees of small enterprises in the forestry and forest-based industry are united by loyalty, mutual trust and dedication to the enterprise. Alternative A currently prevails in the 125

dimension of organization glue (𝑋̂=0.328). Formal rules and policies are crucial for mediumsized enterprises, which will ensure the smooth running of business processes. Alternative D (𝑋̂=0.328) was applied. The test confirmed the existence of differences in alternative A (p-value<0.0001) and alternative D (p-value<0.0001).

The values typical for alternative A (𝑋̂=0.342) are applied in the strategies of small enterprises in the forestry and forest-based industry. Human development, trust, openness and loyalty in cooperation are emphasized in these enterprises. Medium-sized enterprises are characterized by alternative D (𝑋̂=0.348). The strategies of these enterprises are based on stability, performance, control and operability. When testing both alternatives, statistically significant differences (p-value<0.0001) were confirmed.

Tab. 2 The current level of individual dimensions of corporate culture in small and medium-sized enterprises in the forestry and forest-based industry.

Dimension

Alternative

Dominant Characteristics

A B C D

Organizational Leadership

A B C D

Management of Employees

A B C

Criteria of Success

Strategic Emphases

Organization Glue

D A B C D A B C D A B C D

Size of enterprise

emmean

SE

Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized

0.331 0.274 0.215 0.192 0.258 0.260 0.284 0.348 0.291 0.258 0.279 0.231 0.214 0.222 0.318 0.340 0.403 0.337 0.220 0.201 0.189 0.202 0.278 0.300 0.328 0.259 0.236 0.200 0.259 0.270 0.273 0.348 0.342 0.261 0.244 0.213 0.241 0.248 0.274 0.352 0.381 0.315 0.248 0.207 0.228 0.222 0.268 0.353

0.00554 0.00496 0.00437 0.00398 0.00489 0.00482 0.00515 0.00555 0.00526 0.00484 0.00514 0.00453 0.00439 0.00442 0.00549 0.00555 0.00584 0.00540 0.00438 0.00407 0.00396 0.00407 0.00502 0.00512 0.00549 0.00479 0.00461 0.00408 0.00487 0.00490 0.00501 0.00552 0.00559 0.00481 0.00472 0.00426 0.00467 0.00468 0.00503 0.00555 0.00605 0.00550 0.00491 0.00429 0.00466 0.00449 0.00514 0.00577

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asymp.

LCL 0.320 0.264 0.207 0.184 0.249 0.251 0.274 0.337 0.281 0.248 0.269 0.222 0.206 0.213 0.307 0.329 0.392 0.327 0.211 0.193 0.181 0.194 0.268 0.290 0.317 0.250 0.227 0.192 0.249 0.260 0.263 0.337 0.331 0.252 0.235 0.205 0.231 0.239 0.264 0.341 0.369 0.304 0.238 0.198 0.219 0.213 0.258 0.341

asymp.

UCL 0.342 0.284 0.224 0.199 0.268 0.270 0.295 0.359 0.302 0.267 0.289 0.240 0.223 0.231 0.329 0.351 0.415 0.348 0.228 0.209 0.197 0.210 0.288 0.310 0.339 0.268 0.245 0.208 0.268 0.280 0.283 0.358 0.353 0.270 0.254 0.222 0.250 0.257 0.284 0.363 0.392 0.326 0.257 0.215 0.237 0.230 0.278 0.364

Estimate

SE

z-ratio

p-value

0.05706

0.00737

7.745

<0.0001

0.02360

0.00577

4.089

0.0011

-0.00214

0.00677

-0.316

1.0000

-0.06321

0.00751

-8.416

<0.0001

0.03348

0.00706

4.745

0.0001

0.04742

0.00675

7.023

<0.0001

-0.00785

0.00610

-1.288

0.9036

-0.02245

0.00775

-2.895

0.0736

0.065899

0.00792

8.317

<0.0001

0.018569

0.00585

3.175

0.0323

-0.012820

0.00554

-2.312

0.2865

-0.021535

0.00710

-3.034

0.0496

0.069249

0.00721

9.604

<0.0001

0.035735

0.00603

5.924

<0.0001

-0.011412

0.00681

-1.675

0.7037

-0.074843

0.00740

-10.120

<0.0001

0.08069

0.00731

11.038

<0.0001

0.03103

0.00624

4.972

<0.0001

-0.00779

0.00651

-1.198

0.9328

-0.07808

0.00743

-10.505

<0.0001

0.06565

0.00813

8.080

<0.0001

0.04094

0.00639

6.411

<0.0001

0.00632

0.00633

0.998

0.9749

-0.08464

0.00766

-11.045

<0.0001

The success of small businesses in the forestry and forest-based industry is associated with the development of human resources and teamwork. Alternative A (𝑋̂=0.381) achieved the highest rating. On the other hand, in medium-sized enterprises, performance and lowcost production are the criteria for success. Alternative D (𝑋̂=0.353) is predominant. When testing alternative A (p-value<0.0001) and alternative D (p-value<0.0001), the test confirmed the existence of differences in small and medium-sized enterprises in the forestry and forest-based industry in both analyzed areas.

Based on the results presented in Table 3, it can be stated that in small enterprises of the forestry and forest-based industry, a clan corporate culture (𝑋̂=0.327) is currently applied, the enterprise is perceived as a family. For medium-sized enterprises, a hierarchy corporate culture is typical (𝑋̂=0.331). It is typical for its formalized and structured work environment. The existence of differences between the two types of corporate culture examined (p-value<0.0001) was confirmed.

Tab. 3 The current level of corporate culture in small and medium-sized enterprises in the forestry and forest-based industry.

Type of corporate culture Clan Adhocracy Market Hierarchy

Size of enterprise

emmean

SE

asymp.

LCL

asymp.

UCL

Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized

0.327 0.267 0.213 0.176 0.214 0.227 0.270 0.331

0.00325 0.00297 0.00275 0.00247 0.00276 0.00279 0.00305 0.00320

0.320 0.261 0.207 0.171 0.209 0.222 0.264 0.324

0.333 0.272 0.218 0.180 0.220 0.233 0.276 0.337

Estimate

SE

z-ratio

p-value

0.06009

0.00440

13.672

<0.0001

0.03701

0.00367

10.079

<0.0001

-0.01317

0.00390

-3.373

0.0170

-0.06020

0.00441

-13.651

<0.0001

In the next step, the required level of individual dimensions of corporate culture in small and medium-sized enterprises in the forestry and forest-based industry in the strategic level of 5 to 10 years was examined. The results presented in Table 4.

The results presented in Table 4 how that in the dimension of the dominant characteristics, alternative A is preferred, which understands the enterprise as a multimember family (small enterprises, 𝑋̂=0.364; medium-sized enterprises, 𝑋̂=0.337). The existence of differences in small and medium-sized enterprises of the forestry and forestbased industry (p-value=0.0018) was confirmed.

In the dimension of organizational leadership, alternative A (𝑋̂=0.325) should be applied in small enterprises. At the strategic level of 5 to 10 years, managers should be perceived as mentors. Alternative D (𝑋̂=0.360) considering management as a demonstration of cooperative, organized and smooth performance should be applied in medium-sized enterprises, at the strategic level of 5 to 10 years. When examining the differences in the alternatives with the highest score achieved, the differences in alternative A (pvalue=0.0278) and alternative D (p-value<0.0001) were confirmed by the test.

Another analyzed dimension was the required level of management of employees in small and medium-sized enterprises of the forestry and forest-based industry. The most preferred alternative was alternative A. It was proven by the analysis that management of employees should focus on teamwork and collaboration (small enterprises, 𝑋̂=0.450; medium-sized enterprises, 𝑋̂=0.439). The existence of differences in alternative A (pvalue=0.8674) were not confirmed.

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Tab. 4 The required level of individual dimensions of corporate culture in small and medium-sized enterprises in the forestry and forest-based industry.

Dimension

Alternative

Dominant Characteristics

A B C D

Organizational Leadership

A B C D

Management of Employees

A B C

Organization Glue

D A B C

Criteria of Success

Strategic Emphases

D A B C D A B C D

Size of enterprise

emmean

SE

Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized

0.385 0.353 0.218 0.209 0.253 0.267 0.241 0.249 0.325 0.300 0.288 0.250 0.178 0.167 0.305 0.360 0.450 0.439 0.214 0.189 0.160 0.149 0.281 0.303 0.392 0.348 0.231 0.228 0.246 0.253 0.229 0.256 0.394 0.335 0.245 0.239 0.219 0.226 0.233 0.268 0.428 0.402 0.236 0.216 0.212 0.200 0.231 0.284

0.00587 0.00560 0.00442 0.00422 0.00485 0.00490 0.00471 0.00471 0.00560 0.00530 0.00528 0.00479 0.00388 0.00364 0.00544 0.00574 0.00602 0.00589 0.00433 0.00393 0.00354 0.00330 0.00509 0.00519 0.00579 0.00546 0.00451 0.00441 0.00469 0.00468 0.00449 0.00472 0.00572 0.00531 0.00463 0.00448 0.00433 0.00434 0.00449 0.00478 0.00609 0.00589 0.00468 0.00434 0.00436 0.00413 0.00462 0.00511

asymp.

LCL 0.373 0.342 0.210 0.200 0.244 0.257 0.232 0.239 0.314 0.290 0.278 0.240 0.170 0.159 0.295 0.349 0.439 0.427 0.205 0.182 0.153 0.142 0.272 0.293 0.380 0.337 0.222 0.220 0.236 0.243 0.220 0.247 0.383 0.324 0.236 0.230 0.211 0.218 0.224 0.258 0.416 0.390 0.227 0.208 0.203 0.192 0.222 0.274

asymp.

UCL 0.396 0.364 0.227 0.217 0.263 0.276 0.251 0.258 0.336 0.311 0.298 0.259 0.185 0.174 0.316 0.371 0.462 0.450 0.222 0.197 0.167 0.155 0.291 0.313 0.403 0.359 0.240 0.237 0.255 0.262 0.238 0.266 0.406 0.345 0.254 0.248 0.228 0.235 0.241 0.277 0.440 0.413 0.245 0.225 0.220 0.208 0.240 0.294

Estimate

SE

z-ratio

p-value

0.03214

0.00808

3.978

0.0018

0.00972

0.00598

1.625

0.7354

-0.01312

0.00679

-1.932

0.5289

-0.00715

0.00655

-1.092

0.9587

0.02463

0.00764

3.223

0.0278

0.03854

0.00703

5.482

<0.0001

0.01097

0.00515

2.129

0.3961

-0.05485

0.00785

-6.983

<0.0001

0.0116

0.00842

1.378

0.8674

0.0243

0.00571

4.266

0.0005

0.0113

0.00467

2.417

0.2327

-0.0217

0.00719

-3.011

0.0530

0.043908

0.00793

5.537

<0.0001

0.002455

0.00619

0.396

0.9999

-0.006940

0.00652

-1.064

0.9641

-0.027551

0.00641

-4.298

0.0005

0.05959

0.00778

7.660

<0.0001

0.00583

0.00634

0.920

0.9843

-0.00701

0.00602

-1.164

0.9420

-0.03517

0.00646

-5.443

<0.0001

0.02623

0.00846

3.101

0.0406

0.02003

0.00625

3.203

0.0296

0.01165

0.00586

1.987

0.4905

-0.05266

0.00678

-7.767

<0.0001

The employees of small and medium-sized enterprises of the forestry and forest-based industry both request that alternative A should be applied in the dimension of organizational glue (small enterprises, 𝑋̂=0.392; medium-sized enterprises, 𝑋̂=0.348). According to alternative A, employees should be united by loyalty and mutual trust. The existence of differences in organizational glue in small and medium-sized enterprises of the forestry and forest-based industry in alternative A (p-value<0.0001) was confirmed.

128

Strategy of small and medium-sized enterprises of the forestry and forest-based industry should move towards human development, high trust, openness and inertia in cooperation. The results presented in Table 4 show that, at the strategic level of 5 to 10 years, alternative A is preferred (small enterprises, 𝑋̂=0.394; medium-sized enterprises, 𝑋̂=0.335).

The test confirmed the existence of differences in alternative A (p-value<0.0001).

In small and medium-sized enterprises, at the strategic level of 5 to 10 years, alternative A (small enterprises, 𝑋̂=0.428; medium-sized enterprises, 𝑋̂=0.402) should be applied. The success of enterprises should be defined on the basis of human resources development and teamwork. The test confirmed the existence of differences in the criteria of success in small and medium-sized enterprises of the forestry and forest-based industry in alternative A (p-value=0.0406).

The results presented in Table 5 show that in small and medium-sized enterprises of the forestry and forest-based industry, a clan corporate culture should be applied at the strategic level of 5 to 10 years, where the work environment resembles an extended family (small enterprises, 𝑋̂=0.381; medium-sized enterprises, 𝑋̂=0.350). The results further show that the existence of differences in corporate culture in small and medium-sized enterprises of the forestry and forest-based industry (clan corporate culture, p-value<0.0001) was confirmed.

Tab. 5 The required level of corporate culture in small and medium-sized enterprises in the forestry and forest-based industry.

Type of corporate culture Clan Adhocracy Market

Hierarchy

Size of enterprise

emmean

SE

Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Small-sized Medium-sized Medium-sized

0.381 0.350 0.214 0.195 0.189 0.189 0.239 0.276 0.381

0.00338 0.00325 0.00276 0.00260 0.00260 0.00256 0.00290 0.00301 0.00338

asymp.

LCL 0.374 0.344 0.209 0.190 0.184 0.184 0.234 0.271 0.374

asymp.

UCL 0.387 0.357 0.219 0.200 0.194 0.194 0.245 0.282 0.387

Estimate

SE

z-ratio

p-value

0.03056

0.00469

6.521

<0.0001

0.01874

0.00376

4.981

<0.0001

-0.00028

0.00362

-0.077

1.0000

-0.03702

0.00416

-8.894

<0.0001

In developed economies as well as in a transforming economy, in small and mediumsized enterprises of the forestry and forest-based industry, at a strategic level of 5 to 10 years, small and medium-sized enterprises have an irreplaceable place (STACHOVA et al. 2018, MURA – GAŠPARÍKOVÁ 2010). They are considered the driving force of the economy because they contribute significantly to improving innovation, to the flexible introduction of new products, they create jobs, are adaptable and are very sensitive to change (MATUSZEWSKAPIERZYNKA 2018, CURREN – BLACKBURN 2001). Through their business activities, they also significantly influence the region and its development. In addition, they bring a number of social benefits. However, economic development, globalization and related economic changes are putting pressure on businesses to develop even more effective strategies to succeed in a highly competitive environment (PAROBEK et al. 2019, POTKÁNY et al. 2019, SEDLIAČIKOVÁ et al. 2019, DOMINGUEZ 2018, KORAUŠ et al. 2018, GRAA – ABDELHAK 2016, MIKLOSIK – DANO 2016, KOSTRUB – ŠIPOŠOVÁ 2015, STACHO – STACHOVÁ 2015). In this context, innovations are reflected in the corporate culture, which symbolizes an important factor on the basis of which enterprises can achieve a competitive advantage, and thus overall success in the market (ALMUSLAMINI – DAUD 2018, HAAPANEN et al. 2018, PARK et al. 2017).

The presented research was carried out in Slovakia. Its aim was to define the corporate culture in small and medium-sized enterprises of the forestry and forest-based industry. The 129

research results show that the current level of individual dimensions of corporate culture as well as the overall type of applied corporate culture in small and medium-sized enterprises of the forestry and forest-based industry differs. Small business in individual dimensions of corporate culture is dominated by alternative A, which means a family atmosphere prevails.

In medium-sized enterprises, alternative D is used in the vast majority of dimensions. It is a fact that enterprises put an emphasis on thorough following of business practices. Based on the results of the analysis of the overall type of corporate culture, it can be stated that a clan corporate culture is applied in small enterprises of the forestry and forest-based industry.

Enterprises with a clan corporate culture have similarities with family-type enterprises.

Members share common views and see themselves as part of one large family that is active and engaged. The work environment resembles the one of an extended family with equal opportunities for individuals as well as diversity in the workplace provided. Leadership takes the form of mentoring. Leaders play the role of teachers, advisors and parents. Core values are teamwork, participation, communication and consensus (JAEGER – ADAIR 2017, DEMSKI et al. 2016). For medium-sized enterrprises, a hierarchy corporate culture corresponding to alternative D is typical. It is characterized by its formalized and structured work environment emphasizing procedures and regulations (CAMERON – QUINN 2005). Internal sustainability is emphasized along with the need for stability and control (ANDRONICEANU – TVARONAVIČIENĖ 2019). Leadership is based on organized coordination and monitoring.

Emphasis is placed on the efficiency of smooth running, predictability, efficiency and accuracy of management procedures (HERITAGE et al. 2014). Values like consistency and uniformity are included (JAEGER et al. 2017, DEMSKI et al. 2016). Top-down communication predominates. Employee management is focused primarily on ensuring job security.

At the strategic level of 5 to 10 years, individual dimensions of corporate culture and the overall type of corporate culture in small and medium-sized enterprises of the forestry and forest-based industry do not differ. The results show that at the strategic level of 5 to 10 years, in small and medium-sized enterprises of the forestry and forest-based industry in the individual dimensions of corporate culture, alternative A should be applied when leaders are perceived as advisors. In terms of the overall type of corporate culture, the small and medium-sized enterprises of the forestry and forest-based industry should be controlled by a clan corporate culture emphasizing the development of human resources. The focuse is on family relationships within the enterprise. It works as a culture with an inner orientation, common values and goals, cohesion and participation. Individual goals are in line with corporate goals based on their trust in the enterprise (JONES – MADEY 2014). Businesses are further held together by loyalty and tradition. Commitment to the enterprise is high and the long-term benefits of each person's development are emphasized. Great importance is given to cohesion, morality and the working environment. Success is understood in relation to the internal environment and caring for people (ÜBIUS – ALAS 2009).

A similar research in the field of wood processing enterprises was conducted in 2016 (LORINCOVÁ et al. 2016). Its aim was to define the level of corporate culture in wood processing enterprises in Slovakia from the point of view of the job category of employees (managers, workers). According to the managers, a market corporate culture was applied in Slovak wood processing enterprises. The views of workers on corporate culture in wood processing enterprises in Slovakia are identical with the results of the research (MATRAEVA et al. 2016, JAEGER – ADAIR 2013, BALOGH et al. 2011) proving that a hierarchy corporate culture is applied in enterprises. Both analyzed groups of respondents demand that a clan corporate culture ought to be applied in Slovak wood processing enterprises at the strategic level of 5 to 10 years, which confirms not only our research but also earlier research of HITKA et al. (2015). Following the results, it can be seen that, it is the main role of managers to support employees, their cooperation, commitment and responsibility to the enterprise.

130

Employees should share the same values and traditions. They should be loyal. Moreover, the long-term benefits of each person's development must be emphasized as it is the employees who are the “motor” initiating other sources into move and determines their use.

Furthermore, employees are a strategic tool for managing many enterprises. They are considered invaluable and irreplaceable capital in terms of achieving long-term goals of a successful enterprise (KOT-RADOJEWSKA – TIMENKO 2018, FEJFAROVÁ – URBANCOVÁ 2016, KROPIVŠEK et al. 2011). Following the previous research (COPUŠ et al. 2019, SALAMA – OLÁH 2019, KUCHARČÍKOVÁ – MIČIAK 2018, GAUTAM – Ghimire 2017, GRAVES 2017, KUCHARČÍKOVÁ et al. 2016, LIM et al. 2016; WEBEROVÁ – LIŽBETINOVÁ 2016, AHMAD et al. 2012), the technology can be bought, a new management system in the enterprise can be introduced, funds can be borrowed but it will not help the enterprise if there is no capital in the form of high quality workforce, which is the bearer of new knowledge, ideas, experience and skills.

CONCLUSION A corporate culture is at first sight an inconspicuous but very effective tool significantly and unmistakably distinguishing one enterprise from another. The corporate culture in small and medium-sized enterprises of the forestry and forest-based industry is a key factor of financial performance and at the same time a limiting factor influencing the management processes of the enterprise.

The result of the research is the finding that a clan corporate culture is applied in small enterprises of the forestry and forest-based industry characterized by its family atmosphere and friendly work environment. We consider the clan corporate culture to be the most suitable corporate culture for small businesses in the forestry and forest-based industry, because it provides space for employee development, emphasizes communication, and success is measured in relation to the internal environment and employee care.

The medium-sized enterprise of the forestry and forest-based industry is currently dominated by a hierarchy corporate culture emphasizing compliance with regulations and order as basic values. Internal sustainability is emphasized, together with the need for stability and control. Leadership is based on organized coordination and monitoring.

Emphasis is placed on the efficiency of smooth running, predictability, efficiency and accuracy of management procedures. The values of the enterprise include consistency and uniformity. Top-down communication predominates.

At the strategic level of 5 to 10 years, a clan corporate culture should be applied in small and medium-sized enterprises of the forestry and forest-based industry. It can be achieved through collaborative employee management, where employees share the same values and are often in touch with each other. Leaders should play the role of advisors or mentors. The enterprise should unite loyalty and tradition. Dedication to the enterprise should be high. The long-term benefits of each person's development should be emphasized.

Great importance should be attached to cohesion, morality and the work environment.

Success should be understood in terms of the internal environment, care for employees and long-term investment in human resources. The method of defining the corporate culture can be applied in other industries (transport, construction, agriculture, etc.)

The benefit for managerial practice is the knowledge of the required level of corporate culture in small and medium-sized enterprises of forestry and forest-based industry and the opportunity to use the acquired knowledge in business practice. Knowledge of the corporate culture in small and medium-sized enterprises of forestry and forest-based industry can help to achieve a competitive advantage with an inconspicuous but very effective tool that 131

significantly and beyond doubt distinguishes one enterprise from another. Moreover, it has a direct impact on performance and the resulting financial success of the enterprise.

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ACKNOWLEDGEMENT This research was supported by projects KEGA 005TU Z-4/2020 “Economics, Management and Enterprising in Wood Industry Companies – university textbook with visualization support in virtual space”.

ADDRESSES OF THE AUTHORS Ing. Silvia Lorincová, PhD. doc. Ing. Miloš Hitka, PhD. Technical University in Zvolen T. G. Masaryka 24 960 53 Zvolen silvia.lorincova@tuzvo.sk hitka@tuzvo.sk prof. Ing. Ľubica Bajzíková, CSc. Comenius University in Bratislava Odbojárov 10 P.O.BOX 95 820 05 Bratislava 25 lubica.bajzikova@fm.uniba.sk Ing. Iveta Oborilová University of Pardubice Studentská 95 532 10 Pardubice 2 iveta.oborilova@seznam.cz

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 137−148, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.13

PSYCHOLOGICAL ASPECTS AND EMOTIONS EVOKED BY IMPLEMENTING THE CONTROLLING SYSTEM IN WOODPROCESSING ENTREPRISES IN SLOVAKIA Mariana Sedliačiková – Zuzana Stroková – Denisa Malá – Dana Benčiková – Marcel Behún ABSTRACT In business practice, modern business management tools have become established, including controlling. Small enterprises are characteristic of the absence of implementing the controlling system and utilizing modern tools of management for different reasons.

Controlling is implemented at a much larger scale in medium and large enterprises.

Psychological aspects of controlling enable understanding of the relation, feelings, or an opinion about controlling, while thus creating foundations and a base for establishing the real form of this tool. The main objective of the paper is to identify the key psychological aspects and emotions evoked by implementing and establishing a controlling system in wood-prcessing entreprises in Slovakia. The empirical research into the given issue was conducted by the questionnaires. In order to evaluate the research results, the descriptive, graphical, and mathematic-statistical methods were used. Based on the research results, the key psychological aspects, i.e. changes in the organizational structure, employee workload and emotions, i.e. motivation, uncertainty were identified, which have a significant on establishing and accepting controlling in wood-processing entreprises in Slovakia, with regard to their size. The achieved results led to the recommendations (e.g. efficient communication with employees, active listening, ability of self-control in stressful situations) the implementation of which could greatly contribute to eliminate stereotypes, and negative feelings and attitudes in implementing controlling into the business practice.

Key words: controlling, psychological aspects, wood-processing entreprises, size of the enterprise.

INTRODUCTION Controlling is an efficient tool for managing the future of an enterprise, while combining several management and information subsystems, the role of which is to create the basis for determining the objectives of an enterprise, its planning, checking how the plans have been fulfilled, foreseeing potential deviations, analyzing the reasons for their occurrence, and proposing measures that lead to their elimination (ŠATANOVÁ and POTKÁNY 2004; VUKO and OJVAN 2013; HAVLÍČEK 2016; PESALJ et al. 2018; SEDLIAČIKOVÁ et al.

2019). Controlling should participate in the establishment and administration of an economic information system in an enterprise (DOLINAYOVÁ and ĽOCH 2015). According to SEDLIAČIKOVÁ (2018), psychological aspects of controlling define relations, feelings, 137

opinions, or an imagination of people about controlling, while thus creating the base and foundations for establishing the real form of this tool. Realizing these factors enables more effective activity of the controller and understanding the behaviors and feelings of the people involved. There exist six psychological rules (aspects) between the controller, managers, and employees who are the recipients of the controller’s information and recommendations, which must be accepted and applied in the enterprise with regard to the effectiveness of its implementation and enforcement within the enterprise (ESCHENBACH 2004). Among these belong: motivation, feedback, communication, building trust, enforcing, and change.

The growing (decreasing) necessity (importance) of controlling leads to an increase in tendency to indicate the positive (negative) emotions that are associated with its implementation and establishment, which manifests in the increasing (decreasing) size of the enterprise, as well as more appropriate (inappropriate) understanding of controlling (KLEMENTOVÁ and SEDLIAČIKOVÁ 2017). Fear, aversion, disappointment, lack of interest, and uncertainty, belong to the negative emotions evoked by implementing and establishing controlling in an enterprise. Curiosity, happiness, enthusiasm, satisfaction, and motivation are ranked among the positive ones.

According to GAUTAM and KHURANA (2019), when organisations are working in a fiercely competitive environment, it is imperative that the challenges of diversity management as age, gender, diversity, educational qualification, work experience should be managed effectively. Academic excellence and technical expertise can no longer ensure success for an individual or an organisation. Competencies like managing one’s emotions and managing other’s emotions plays a very crucial role. The process of change involves emotions, as it is a fact that no one wishes to surrender the comfort associated with the status quo or make concessions on what this person is worth. Despite that, leaders will have to remain successful while being subjected to these challenges, among which are the emotions of those that are affected by the change. Therefore, there is an increase in focus on emotional intelligence (EI) in leadership when it comes to managing the process of change (ISSAH 2018). Leaders with a high level of EI can evoke and elicit enthusiasm, excitement and optimism among subordinates, and promote atmosphere of cooperation, through which they may subsequently develop positive interpersonal relationship with them (MINÁROVÁ et al.

2015; BASHIR 2017; EDELMAN and VAN KNIPPENBERG 2018; CUÉLLAR-MOLINA et al. 2019).

Positive interpersonal relations between the leaders and the subordinates may bring along many benefits to the enterprise, e.g. the increase in its performance. When leaders understand and are able to influence the feelings of subordinates, they are able to make them reassess the emotions they experience and the ways these emotions are expressed. In general, EI includes those skills or abilities related to emotions which underpin the ability of a leader to make major changes in an organization. Neglecting to consider the subordinates’ emotional responses to changes may in fact result in a declining trend within the enterprise (JIMÉNEZ 2018). According to MAAMARI and MAJDALANI (2017), first, the higher the EI of the leaders and employees, the better their respective communication, performance, stability and tenure, and thereby the lower the turnover. Second, the higher the EI of leaders and employees, the better the social relationships within the work-setting, higher empathy and higher levels of norming. Third, the high levels of EI generate higher levels of feelings of responsibility as well as warmth and support, affecting both employees and leaders’ effectiveness in decisionmaking, commitment and efficiency.

According to TAVAKOLI (2010), the basic idea underlying positive organizational change is that if the employees are taken seriously and if they are respected, they will blossom and their power will be oriented toward success of change plans, as well as toward an enjoyable work life. Managers who understand the psychological aspects of organizational change can better plan what methods be used, when they be used, how they 138

be used, and under what specific conditions they may lead to more positive results. Yet, positive organizational changes, in many situations, rely on managers’ creativity, enthusiasm, improvisation, exploration, and enterprise. Finding creative ways of implementing organizational changes that motivate positive responses of employees is a worthwhile challenge. HITKA et al. (2020) emphasize that the impact of motivation and meeting employees´needs on improving the performance and delivering it to required standard is fundamental. According to LORINCOVÁ et al. (2016), the emphasis should be put on friendly atmosphere in the workplace similar to extended family as well as on long-term benefit of employees’ development, morale, coherence and work environment.

Slovak Republic is relatively independent of importing the natural resources inputs, being built on a domestic resource base of sustainable character, and therefore it is able to permanently show active balance of foreign trade. In relation to the positive situation related to natural resources, their suitable geographic location, and their acceptable energetic demands for processing wood, woodworking industry represents an important field of industry for the Slovak national economy, while thus enabling further development of small and medium enterprises (HAJDÚCHOVÁ et al. 2016; MALÁ et al. 2018). Woodworking industry is composed of the wood, furniture, and cellulose-paper industries. These are based on processing wood, i.e. domestic ecological resource (POTKÁNY et al. 2018). According to HALAJ et al. (2018), the potential of the related sector depends mainly on the availability of raw material and the demand reported for wood and wood-based products.

The objective of the paper is to identify the key psychological aspects and emotions evoked by implementing and establishing a controlling system in wood-processing entreprises in Slovakia.

METHODOLOGY The research was divided into three key phases. In the first phase, it was necessary to perform a survey and analysis of the secondary literary sources with the goal to define the theoretical foundations to the problem. In the second phase, attention was given to conducting an empirical research in the given field, and identifying the key psychological factors, and emotions evoked by implementing and establishing a controlling system in wood-processing enterprises. The empirical research was performed on a sample of micro, small, medium, and large enterprises. A questioning method was used to collect the primary research data, while the total number of the processed questionnaires was 412. The data obtained in the empirical research were analyzed and processed by descriptive, graphical, and mathematic-statistical methods.

The whole sample consisted of all organizations and entreprises operating in the Slovak Republic, i.e. 559,841 active economic subjects (SLOVAK BUSINESS AGENCY 2019).

The random and purposive sampling was used for the selection of respondents into the selected sample. The purposive sampling was used for the selection of wood-processing enterprises. Respondents were addressed through electronic forms (questionnaires) sent directly to their addresses. Subsequently, the sample size was defined using a mathematical relationship to calculate the minimum number of respondents to be involved in the survey (KOZEL et al. 2006):

𝑛≥

(𝑧 2 ×𝑝×𝑞) ∆2

→≥

(1.962 ×0.5×0.5) 0.052

→ 𝑛 = 384

n – minimum number of respondents; z – coefficient of reliability (z =1.96 =>the reliability of the research reaches 95.0%); 139

(1)

p and q - the percentage of questioned respondents (the extent of knowledge of respondents with regard to the problem is unknown, the whole sample is divided in half, i.e. p and q = 50%); ∆ - maximum acceptable error (the value of maximum acceptable error was determined at 5%).

Out of the total number of 4,935 respondents, 412 respondents participated in the questionnaire survey. In order to keep the contextual framework of the paper, the evaluation of the survey results focused on the 412 wood-processing enterprises. Figure 1 presents the percentange of respondents according to the size of the enterprise. Out of the total number of respondents, 76.0% were micro enterprises, 20.4% small enteprises, 2.6% medium enterprises and 1.0% large enterprises.

2.6%

1.0%

20.4% Micro Small Medium Large

76.0%

Fig. 1 Proportion of respondents according to the size of the enteprise.

Two research questions (RQ) were formulated within the research area: RQ1 – Which psychological factors influence the implementation and establishment of controlling in wood-processing enterprises?

RQ2 – Which emotions are evoked by implementation and establishment of controlling in wood-processing enterprises?

Based on the research questions and the available literary sources (SAYERS and SMOLLAN 2009; KLEMENTOVÁ and SEDLIAČIKOVÁ 2017; KLEMENTOVÁ et al. 2017) four hypothesis were formulated as follows: H1 = It is assumed that there is a dependence between the size of enterprise and the influence of implementing controlling on the organizational structure of the enterprise.

H2 = It is assumed that there is a dependence between the size of enterprise and the influence of implementing controlling on the working time (employee workload).

H3 = It is assumed that there is a dependence between the size of enterprise and motivation (emotion) evoked by implementation and establishment of controlling in an enterprise.

H4 = It is assumed that there is a dependence between the size of enterprise and uncertainty (emotion) evoked by implementation and establishment of controlling in an enterprise.

The data obtained from the questionnaire were filtered and logically divided according to the size of enterprise, while examining the contingency and correlation between the individually selected items by means of the statistical software STATISTICA 10. For the purposes of statistical analysis, Pearson Chi-square test of goodness of fit was used, with the level of significance being 5%. Further, Cramer’s V test and Pearson correlation coefficient test were used to examine the individual dependencies.

140

RESULTS AND DISCUSSION It terms of the enterprise size, the structure of the research sample consisted mainly of micro enterprises (76.0%), small enterprises (20.4%) and medium enterprises (2.6%). Large enterprises represented the lowest proportion (1.0%).

The goal of the statistical analysis was to point out the discovered statistically significant dependencies between the size of enterprise and the psychological aspects and emotions evoked by implementing and establishing controlling in wood-processing enterprises.

When examining the dependencies between the size of enterprise and the influence of psychological aspects, the enterprises were divided into four categories according to size, as follows: 1 – micro, 2 – small, 3 – medium, and 4 – large enterprises. Among the tested psychological aspects, the responses were scaled from D11 to D115 1. The intensity of influence of the psychological aspects was scaled as follows: 1 – positive influence, 2 – no influence, and 3 – negative influence. Attention was given only to the statistically verifiable dependencies between the size of enterprise and the examined psychological signs (responses D19 and D111).

With the risk of a 5% deviation, the null hypothesis related to the existence of independence between the influence of implementing controlling on changes in the organizational structure (D19) and the size of enterprise (A1), was rejected with regard to the alternative hypothesis. The value of Cramer’s V at the level 0.1266, showed weak dependence between the size of enterprise and the influence of controlling on changes in the organizational structure in the enterprise. Based on this finding, it can be concluded that with the growing size of an enterprise, there is also a growing tendency in the influence of controlling on changes in the organization structure, even though the dependence between the variables is weak. When fulfilling the condition of data applicability and taking account of the expected and the observed frequencies, at the p-value for the Pearson Chi-square being 0.0487, at the 5% level of significance, the null hypothesis related to non-existence of dependence between implementing controlling on working time, i.e. employee workload (D111) and the size of enterprise (A1) was rejected. Values of the contingency coefficient (0.1727) and Cramer’s V (0.1240) show weak dependence between the examined signs. The results regarding statistical dependence are presented in Table 1 and Figure 2.

Tab. 1 Dependence between A1/D19 and A1/D11.

Statistics Pearson-chi square test M-V Chi-square Contingency coefficient Cramer´s V

Chisquare

Statistics A1 x D19 Degrees of freedom

pvalue

Chisquare

Statistics A1 x D111 Degrees of freedom

pvalue

13.1251

Df=6

0.0399

12.6615

Df=6

0.0487

12.7722

Df=6

0.0468

13.7427

Df=6

0.0327

0.1762

0.1727

0.1266

0.1240

1

D11-working relations, D12-employee motivation, D13-communication between employees, D14-employee behavior in the enterprise, D15-relevance of information, D16-pressure at the workplace, D17-performing tasks by employees, D18-promotion, D19-changes in the organizational structure, D110-evaluation of employee performance, D111-working time (employee workload), D112-employee satisfaction, D113-customer satisfaction, D114-number of working positions in the enteprise, D115-power in the workplace (power distribution)

141

Fig. 2 Influence of the size of enterprise on changes in the organizational structure.

By analyzing the examined signs in more detail, it can be observed that the most intensive, as well as the most negative evaluation appears to be in the category of employee workload (D111), this being reported mainly by micro enterprises. With regard to the size of enterprise, medium (3.07767) enterprises have evaluated employee workload more positively than the other enterprise types. The more positive perception of the given phenomenon was expected among large (5.7379) enterprises. Table 2 presents residual differences.

Tab. 2 Residual differences A1/D11.

A1 1 2 3 4

D111 (1) 3.86408 0.58252 3.07767 7.5242

Observed minus expected frequencies D111 (2) D111 (3) 7.20388 11.0680 0.2913 0.87379 5.0388 8.11650 1.78641 5.7379

Sum 0.00 0.00 0.00 0.00

When examining the dependencies between the size of enterprise and the emotions evoked by establishing a controlling system in an enterprise, the size of enterprise was divided in a similar way as in the previous case (dependencies between the size of enterprise and the observed psychological signs). Emotions and feelings evoked by establishing a controlling system in an enterprise were summarized in questions D21 to D2112, while attention was given to those emotions, where the dependencies between the examined phenomena proved to be statistically verifiable (answers D29 and D210).

The categories for motivation as emotion (D29) were scaled into two levels: 1 – feels motivation, 2 – does not feel motivation when controlling is being established in an enterprise. As the 5 % level of significance and at fulfilling the condition of good approximation, at the p-level of Pearson Chi-square being 0.0141, hypothesis H0 was rejected, as there is no dependence between motivation as an emotion (D29) that would be evoked by implementing and establishing controlling, and the size of enterprise (A1). It can 2

D21-fear, D22-aversion, D23-disappointment, D24-lack of interest, D25-curiosity, D26-enthusiasm, D27-happiness, D28-satisfaction, D29-motivation, D210-uncertainty, D211-other

142

be stated that the differences between the expected and the observed frequencies are not random and based on the values of the contingence coefficient (0.1584) and Cramer’s V (0.1604), weak statistical dependence was proved between the examined signs. Categories for uncertainty (D210) were identified in the same way as it was with motivation (D29). By means of statistical methods, it was discovered that the difference between the expected and the observed frequencies is not random. The value of Pearson Chi-square is 0.0198, which means that there is an existing dependence between uncertainty (D210) and the size of enterprise (A1). H0 at the 5% level of significance was rejected, and by Cramer’s V achieving the value 0.1547, weak statistical dependence between the examined signs was proved. The overall results of the statistical analysis are presented in Table 3.

Tab. 3 Dependence between A1/D29 and A1/D210.

Statistics Pearson-chi square test M-V Chi-square Contingency coefficient Cramer´s V

Chisquare

Statistics A1 x D29 Degrees of freedom

p-value

Chisquare

Statistics A1 x D210 Degrees of p-value freedom

10.6028

Df=3

0.01408

9.8561

Df=3

0.01983

10.4610

Df=3

0.01503

9.5266

Df=3

0.02305

0.1584

0.1529

0.1604

0.1547

As to the internal dependencies between the size of enterprise (A1) and motivation as an emotion (D29), the strongest tendency to indicate a positive feeling of being motivated was shown in large enterprises, which is documented in Figure 3.

Fig. 3 Influence of motivation as an emotion at implementing controlling on the size of enterprise.

With regard to dependence between uncertainty (D210) as a consequence of establishing controlling and the size of enterprise (A1), strong presence of feeling of uncertainty was identified in micro (3.5364) and small (8.0583) wood-processing enterprises. On the other hand, the absence of uncertainty was proved in medium (6.5922) and large (5.0024) wood-processing enterprises. This dependence was expected, mainly due to the absence of controlling in micro and small wood-processing enterprises. The residual differences are presented in Table 4.

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Tab. 4 Residual differences A1/D210.

A1 1 2 3 4 Total

D210 (1) 3.53641 8.05825 6.59223 5.00243 0.00000

Observed minus expected frequencies D210 (2) 3.53641 8.05825 6.59223 5.00243 0.00000

Sum 0.00 0.00 0.00 0.00 0.00

The findings of the conducted research are rather similar to the results of a Polish empirical study which showed that the most frequently mentioned factor influencing the decision to implement controlling is the size of company (DOBROSZEK 2015). Another research, which focused on the application of controlling instruments in SMEs in NorthWestern Croatia, confirmed consistency with the previous findings. Unlike large companies that use very complex instruments of controlling, SMEs focus on the use of controlling tools that increase transparency of business, while at the same time assuring that the tools are not excessively costly or demanding to use (ŠESTANJ-PERIĆ and KUKEC 2013). This research showed that operative tools are used at a lower level in Croatia in comparison with Germanspeaking countries, and that strategic tools are used very rarely (BECKER et al. 2011).

The research has proved a statistically significant dependence between the size of enterprise and the influence of implementing controlling on the organizational structure of the enterprise. Based on the findings, the hypothesis H1 was confirmed. It can be claimed that with the growing size of enterprise, the influence of controlling on changes in the organizational structure has a growing tendency as well, even though the dependence between variables is weak in relation to values of contingency coefficient (0.1762) and the Cramer´s V (0.1266). The growing size of enterprise requires a more complex organizational structure than it is in small enterprises. In an established enterprise with a stable organizational structure, it is inevitable to reserve an independent place, position, and competence for controlling when implementing it as a tool that provides relevant information for management enabling it to make the right decisions.

Statistical dependence was proved between the size of enterprise and the influence of implementing controlling on the working time (employee workload). Based on the statistical data evaluation, it was possible to confirm the hypothesis H2. Values of the contingency coefficient (0.1727) and Cramer´s V (0.1240) showed weak dependence between examined signs. Micro and small enterprises with a small number of employees perceive increase in workload with regard to implementing controlling more intensively in comparison with larger enterprises, where the given workload is categorized and allocated among a larger number of employees. However, enterprises which have already implemented controlling (most frequently medium-sized), perceive this phenomenon positively, as controlling enables them to coordinate their working time more effectively.

The influence of controlling on employee motivation that is evoked by its implementation and establishment in an enterprise showed a statistically significant dependence in relation to the size of enterprise. This led to the confirmation of the hypothesis H3. The results have thus revealed that with the growing size of enterprise, the feeling of motivation evoked by its implementation grows as well, even though the dependence between variables is weak due to values of contingency coefficient (0.1584) and Cramer´s V (0.1604).

The influence of controlling on uncertainty which is evoked by its implementation and establishment in an enterprise showed a statistically significant dependence in relation to the size of enterprise, i.e. the hypothesis H4 was confirmed. Similar to the above reported 144

findings, it was proved that the feeling of uncertainty is more frequent in micro and small enterprises, while is it absent in medium and large ones, even though contingency coefficient (0.1529) and Cramer´s V (0.1547) proved weak dependence between examined signs. It can thus be stated that with the growing size of enterprise, the feeling of uncertainty as a consequence of implementing controlling in an enterprise is increasingly eliminated. This finding leads to a conclusion that in larger enterprises, employees feel more trust towards the decisions made by the management, or, as an already implemented tool of management, controlling in these enterprises provides no reasons to fell uncertain.

SMOLLAN (2017) confirm that during the organizational change, the main source of stress is uncertainty, often about changing job roles, potential redeployment and redundancies.

In the aftermath, some employees faced heavier workloads, accompanied by even more inadequate resources, together with poorer relationships and concerns about further change.

Forms of support how employees can cope with organizational change focus on emotional, instrumental, informational and appraisal support. AL SAMAAN et al. (2018) add that one of the best ways to manage the change is to have great internal marketing procedures, and to adopt efficient communication systems that would assure the high degree of employees’ commitment. By so doing, employees will be able to identify the promising opportunities accompanying the change and will have a more decreased level of resistance to it, which leads to better performance, whether on the individual level, or the organizational one.

CONCLUSION Implementing and establishing controlling is a long-term, difficult and complex process and change that is unique and inimitable for each enterprise. Potential benefits of implementing controlling in wood-processing enterprises include clarity of information for management purposes, orientation on company´s goals, focus on company´s strengths and weaknesses, effective management of profit, contribution margin and cash-flow, and responsibility management. The implementation of this tool also brings risk such as is nonacceptance by employees, conflicts of manager vs. controller, implementation inefficiency, excessive control and conflicts with controller.

Mathematical-statistical analysis of the dependence between the size of enterprise and the influence of implementing controlling on psychological aspects, and emotions has confirmed the hypothesis H1, H2, H3 and H4. Micro and small wood-processing enterprises with small number of employees with simple organizational structure perceive increase in workload with regard to implementing controlling more intensively in comparison with larger enterprises. This involves a high level of involvement of enterprise owners and managers. They must to understand and respect psychological aspects (changes in the organizational structure, employee workload) and emotions (motivation, uncertainty) evoked by implementing and establishing controlling in an enterprise. It is essential to be able to communicate, feel empathetically, listen, be tolerant to others and have the ability of self-control at coping with stressful situations. On this basis, there exists a precondition for finding creative ways of successful implementation and establishment of controlling into business practice that motivate positive responses of employees.

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ACKNOWLEDGEMENT The paper has been written as a partial result of the projects APVV-18-0520, APVV-18-0378, APVV-17-0456 and APVV-17-0583 and by funds of KEGA project KEGA 005TU Z-4/2020.

ADDRESSES OF THE AUTHORS doc. Ing. Mariana Sedliačiková, PhD. Ing. Zuzana Stroková, PhD. Technical University in Zvolen Department of Economics, Management and Business T. G. Masaryka 24 960 01 Zvolen Slovakia sedliacikova@tuzvo.sk strokova@tuzvo.sk doc. Ing. Denisa Malá, PhD. Matej Bel University in Banská Bystrica Department of Corporate Economics and Management Tajovského 10 975 90 Banská Bystrica Slovakia denisa.mala@umb.sk

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PhDr. Dana Benčiková, PhD. Matej Bel University in Banská Bystrica Department of Language Communication in Business Tajovského 10 975 90 Banská Bystrica Slovakia dana.bencikova@tuzvo.sk Ing. Marcel Behún, PhD. Technical University in Košice Vice-President for Economics Nemcovej 5 042 00 Košice kvestor@tuke.sk

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ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 149−164, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.14

EVALUATING THE INTERNATIONAL COMPETITIVENESS OF POLISH FURNITURE MANUFACTURING INDUSTRY IN COMPARISON TO THE SELECTED EU COUNTRIES Emilia Grzegorzewska – Mariana Sedliačiková – Josef Drábek – Marcel Behún ABSTRACT The international competitiveness in the foreign trade of the Polish furniture manufacturing industry products in comparison to other EU countries is evaluated in the paper. Poland is the fourth largest furniture exporter in the world and the third in the EU. The source of the research material was the statistical database of the International Trade Centre (ITC). The years 2010-2017 were analysed in the research. The selected resultoriented indicators enabling the ex post determination of competitiveness were utilized. The Polish furniture exporters show a relatively large competitive advantage in the foreign trade.

A high level of the conducted export specialization and a relatively favourable level of the relative import penetration index were noted. The lack of a relative advantage of the Polish furniture exports was observed in the cases of Estonia and Lithuania, i.e. in the countries where the furniture manufacturing industry is important for the national economy. The highest competitive advantage in terms of the furniture exports was obtained in relation to small countries, ones with a small share in the value generation of the EU furniture manufacturing. Poland has also demonstrated competitiveness in relation to its largest competitors from the EU, Germany and Italy.

Key words: intenational competitiveness, foreign trade, furniture manufacturing, EU countries.

INTRODUCTION International competitiveness and comparative advantage are important concepts in economic theory. While investigating the phenomenon of national competitiveness, the focus should not be on economy as a whole, but on certain industries and industrial segments (MILIĆEVIĆ et al. 2017). In that context, the competitiveness of industry means the ability to create value added in comparison to the same productive sectors in other countries, the ability to attract the factors of production in relation to other industries within the same country or other countries, and the ability to adjust industry to social-economic conditions (TOMING 2011).

Competitiveness of countries and industries on the world markets is the basis for the theory of international trade and economic growth. In many empirical studies, a positive relationship between trade openness and economic growth has been noticed (DOLLAR and KRAAY 2004; FREUND and BOLAKY 2008; JENKINS and KATIRCIOGLU 2010; KATIRCIOGLU 149

2010; KLASRA 2011; SBIA and ALROUSAN 2016).

The concept of international competitiveness is indeed a complex one, and it is closely related to a number of different aspects (FAGERBERG 1996; CANTWELL 2005). There are many definitions of competitiveness that cover various areas of activity and different levels of aggregation (KRUGMAN 1994; WILLOUGHBY 2000; AJITABH and MOMAYA 2004; SHAFAEI et al. 2009). For example, PORTER (1990) defined the competitiveness of a nation as the productivity with which a nation utilizes its human, capital and natural resources.

ALTOMONTE (2012) defined external or international competitiveness as the ability to exchange the goods in which a country is abundant for the goods and services that in the same country are scarce.

The complexity of the issues relating to competitiveness requires a variety of methods for assessment. The indicators of competitiveness are classified in two basic groups: the indicators directed towards results and indicators directed towards determinants (DIETER and ENGLERT 2007). One popular measure for international trade competitiveness is Balassa (1979) index termed “Revealed Comparative Advantage” (RCA). RCA is an index that compares the export share of a given commodity or sector in a country with the export share of that commodity or sector in the world market. However, this index has been modified by many authors (VOLLRATH 1991; HADZHIEV 2014; LAURSEN 2015). The ever-evolving Balassa effect currently represents a cannon of research studies on competitiveness potential in the sphere of international trade as well as in the more broadly understood international exchange (MISALA 2011). This article used result-oriented indicators, which enable the expost detection of a competitive position.

The evaluation of international competitiveness applies also to forest-based industries, such as furniture manufacturing (HAN et al. 2009; RATAJCZAK 2009; ZHANG et al. 2012; HAJDÚCHOVÁ and HLAVÁČKOVÁ 2014; RATAJCZAK-MROZEK and HERBEĆ 2014; HAJDÚCHOVÁ et al. 2016; PAROBEK et al. 2016; MALÁ et al. 2017; MILIĆEVIĆ et al. 2017).

The level of competitiveness of enterprises, as well as the entire wood and furniture industries, is determined by external and internal factors. One of the most important factors affecting the growth and development of these industries is the limited availability of wood, which still remains one of the basic production raw materials. According to MYDLARZ et al.

(2013) and HALAJ et al. (2018) the potential of the related sector depends mainly on the availability of raw material and the demand reported for wood and wood-based products. In addition, the research concerning wood-based industries, including the furniture manufacturing, highlights the importance of work efficiency for the growth and development of enterprises (MERKOVA et al. 2019, GRZEGORZEWSKA et al. 2019) and the impact of motivation and meeting employees’ needs on improving the performance and delivering it to required standard is fundamental (HITKA et al. 2020). Moreover, the knowledge, competences and decisions of managers in individual companies that determine implemented internationalization strategies are not without significance. This can influence the situation of the entire furniture industry, especially when medium and large enterprises are of great importance for the furniture market. DRÁBEK and HALAJ (2008) as well as SEDLIAČIKOVÁ et al. (2016) emphasized the increase in the efficiency of enterprises by making optimal decisions in the field of financial management, which are a prerequisite for the growth of enterprise results. As a consequence of the growing importance of environmental issues, including the deficit of raw wood materials, there is increased interest in sustainable development concepts. Wood processing should strive to increase its competitiveness by implementing modern management methods, utilising new technologies, or concentrating on large-scale production. There is also a need for optimal solutions, which reflect the principles of sustainable development (HAJDÚCHOVÁ et al. 2016, MALÁ et al.

2018). In this context, MALÁ et al. (2017) and WIĘCKOWSKA and GRZEGORZEWSKA (2019) 150

pointed out that new, more ecological products, open up new market opportunities for enterprises, and thus contribute to higher profits.

Poland is now one of the world’s top furniture manufacturers and the fourth biggest furniture exporter in the world besides China, Italy, and Germany. The Polish furniture industry has one of the highest shares among all the sectors of the economy (GRZEGORZEWSKA and STASIAK-BETLEJEWSKA 2014; GRZEGORZEWSKA and WIĘCKOWSKA 2016a,b). Because the furniture industry plays a significant role in trade in Poland and Europe, the international competitiveness potential of the domestic furniture manufacturers on the foreign market is particularly important. Thus, in the present article a set of resultoriented indices was used to analyze and assess the changes in competitiveness of Polish furniture industry on international market.

METHODOLOGY The study evaluated international competitivness of Poland in the foreign trade of the furniture industry products in comparison to other EU member states. The study was preceded by a discussion of results of foreign trade. The primary source of the research material was the International Trade Center database. The years 2010-2017 were analysed in the research.

Assessing the competitiveness of an industry is a complex process, and it can be analyzed from several perspectives. Among the methods for measuring the international competitiveness position ex post, quantitative indicators, which reflect the effects of utilizing factors of production, can be distinguished. These outcomes are a result of quantity, structure quality, and productivity of resources. They allow for the assessment of the general economic condition of the economy or a specific sector, especially when the situation in the foreign trade is taken into account (PAWLAK 2013). As emphasized SIRGMETS et al. (2019), competitiveness should be analyzed as a combination of indicators to provide an assessment that is as complete as possible. For this reason, a system of most popular indices evaluating the trade competitiveness of the sector and its commodities was adopted, as described below.

The Import Penetration Rate The Import Penetration Rate (MP) is the ratio between the value of imports to the supply on the internal market. Low values of the MP index are considered desirable and are recorded as follows (OECD 2011; FRONCZEK 2017), 𝑀

𝑀𝑃 = 𝑄−𝑋+𝑀

(1)

where M is import, X is export, and Q is production.

The Specialization Indicator The Specialisation Indicator (SI) compares the share of a product in the export of country k with the share of this product in the world export. High values of the SI index indicate the existence of competitiveness of a given national economy or a selected sector. This indicator takes the following form (PAWLAK 2013), 𝑆𝐼𝑘 =

𝑋𝑖𝑘 𝑋𝑖𝑤 : 𝑋𝑘 𝑋𝑤

(2)

where Xik is export of product i in country k, Xk is total export of goods in country k, Xiw is export product i worldwide, and Xw is total export of goods worldwide.

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The Trade Coverage Ratio The Trade Coverage Ratio (TC), also referred to Export / Import Ratio, is the relation of the value of export of a given product (group of products) to the value of its import. The values of the TC indicator above 100 indicate export specialization of a given country in the considered area, which may lead to the conclusion that it has an advantage over its partners in this respect. This indicator takes the following form (LUBIŃSKI et al. 1995; PALUŠ et al.

2015; KUBALA and FIRLEJ 2019), 𝑋

𝑇𝐶𝑖𝑘 = 𝑀𝑖𝑘 ∙ 100%

(3)

𝑖𝑘

where Xik is export of product i in country k, and Mik is import of product i in country k.

The Relative Revealed Comparative Export Advantage Index One of the mostly used indicators for competitiveness of exports of a given country or sector is the Revealed Comparative Advantage Index (RCA). Although it should be mentioned that since then many researchers have attempted to refine this index. One of them is the Relative Revealed Comparative Export Advantage Index (XRCA) described by following equation (PAWLAK 2013; FROHBERG and HARTMAN1997), 𝑋𝑅𝐶𝐴𝑖𝑘 =

𝑋𝑖𝑘 𝑋𝑖𝑚

∑𝑗,𝑗≠𝑖 𝑋𝑗𝑘

∶

(4)

∑𝑗,𝑗≠𝑖 𝑋𝑗𝑚

where X is export, i, j are product categories, and k, m are countries.

The index measures the competitiveness of a given product export from one country to another and is defined as the ratio of two quotients - the ratio of export of a given product in the country k to the export of this product in the country m and relation of general export of goods in both countries (excluding the analyzed product). Values above 1 suggest that the country has a competitive advantage in the considered product category, whereas values below 1 point to a competitive disadvantage (PAWLAK 2013).

The Relative Import Penetration Index The Relative Import Penetration Index (MRCA) is defined as the ratio of two quotients: the ratio of import of a given product in the country k to the imports of this product in the country m and the relation of general import of goods in both countries (excluding the analyzed product) (PAWLAK 2013; FROHBERG and HARTMAN1997): 𝑀𝑅𝐶𝐴𝑖𝑘 =

𝑀𝑖𝑘 𝑀𝑖𝑚

∶

∑𝑗,𝑗≠𝑖 𝑀𝑗𝑘

(5)

∑𝑗,𝑗≠𝑖 𝑀𝑗𝑚

where M is import, i, j are product categories, and k, m are countries.

A value above 1 is a sign of competitive disadvantage, and values below that are an indication of competitive advantages.

The Relative Trade Advantage Index The Relative Trade Advantage Index (RTA) is more complex than the other two factors (PAWLAK 2013; FROHBERG and HARTMAN1997). This index gives the difference between the XRCA and the MRCA, 𝑅𝑇𝐴𝑖𝑘 = 𝑋𝑅𝐶𝐴𝑖𝑘 − 𝑀𝑅𝐶𝐴𝑖𝑘

(6)

The indicator RTA is interpreted as follows: RTA < 0 means comparative disadvantages in the industry (commodity group); RTA > 0 means comparative advantages in the country for export commodities for that industry (or commodity group); and RTA > 1 identifies the industry (commodity) as internationally competitive (PAWLAK 2013; FROHBERG and HARTMAN 1997).

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The Trade Balance Index The Trade Balance Index (TBI) analyzes whether a country has specialization in export (as net-exporter) or in import (as net-importer) for a specific group of products (ECHEVARRIA 2008). This ratio is calculated as follows (WIDODO 2009), 𝑋 −𝑀

𝑇𝐵𝐼𝑖𝑗 = 𝑋𝑖𝑘 +𝑀𝑖𝑘 𝑖𝑘

(7)

𝑖𝑘

where Xik and Mik represent exports and imports of product i in country k, respectively.

Values of the index range from -1 to +1. Extremely, the TBI equals -1 if a country only imports, in contrast to the TBI equals +1 if a country only exports (WIDODO 2009). The values between 0 and 1 mean comparative advantage. In turn, values between 0 and -1 indicate that foreign trade is not advantageous. This indices shows the country's degree of specialization in exporting a particular product (PRASAD 2004; PAROBEK et al. 2016).

Moreover, the coefficient of variation (V) was employed, which allows for the determination of the differentiation level of the examined traits in a specified time. This coefficient is a relative measure of dispersion, the value of which is determined as the ratio of standard deviation to the arithmetic mean (ABDI 2010).

RESULTS AND DISCUSSION According to data from the International Trade Center, in 2010, the value of the global furniture exports reached EUR 104.7 billion. The export of the EU member states accounts for nearly half of this amount. The leader on the list of the EU furniture industry exporters was Germany. The value of the products of this industry, which arrived at the foreign market from Germany amounted to EUR 10.9 billion, which represented 21.4% of the EU export and 10.4% of the world export. Italy (EUR 9.3 billion) ranked second on the list of the EU furniture exporters. In 2010, in addition to Germany and Italy, Poland was an important exporter of furniture. The exports of the Polish furniture in value terms which was provided on the market, in the range of EUR 5.6 billion, representing 5.3% and 10.9% of the global and EU exports of these products, respectively (Fig. 1). Between 2010 and 2017, the global furniture industry export more than doubled and amounted to EUR 210.8 billion at the end of the analyzed period. Of this amount, EUR 77.8 billion was generated in the EU member states (52.8% more than in 2010), which constituted 36.9% of the world furniture export. This indicates that the importance of the EU countries in creating the value of global exports has decreased. Among the countries at the top of the ranking of exporters, Poland recorded the highest growth dynamics of the export value (191.6% corresponding to EUR 10.6 billion). On the other hand, the value dynamics of the furniture exported from the Italian and German markets was lower and amounted to 139.4 and 127.0% for Italy and Germany, respectively. The share of these countries in the value of the EU furniture export increased by 2.6%; therefore, together with Poland, they remained at the forefront of the largest furniture exporters.

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10.0 8.0 6.0 4.0

11.4

10.9 6.3

5.6

11.0

7.1

12.0

10.6

9.8

9.0

7.9

7.2

13.7 16.0

13.2

13.1

12.3

11.6

8.0

1.3

1.3

0.0 2010

2.8

2.5

2.6

2.0

2011

Export

2.6

1.6

2012 Import

2.7

1.5

2013

3.1

2.2

2.0

1.6

2014

3.1

2015

3.2

[%]

[billion EUR]

12.0

4.0

2.4

0.0 2016

Share in export EU

2017 Share in import EU

Fig. 2 Export and import of the Polish furniture industry in the years 2010-2017.

Moreover, in 2010, the world furniture import was at the level of EUR 106.8 billion, marginally higher than the export, which resulted in a trade deficit in this area of products.

Nearly half of this amount was generated by the EU member states. In the EU, the largest importers of the furniture industry products were Germany (EUR 11.2 billion), France (EUR 6.8 billion) and the United Kingdom (EUR 6.3 billion). The value of the furniture imports to these countries accounted for nearly 50% of the total worth of the EU import. In the Polish furniture industry, a noticeably lower import value of furniture was observed – EUR 1.3 billion, which accounted for 2.6% of the EU import.

In the years 2010 to 2017, the global furniture import increased by 86.4% to a level of EUR 199.1 billion. The export growth rate was significantly higher, which resulted in a positive furniture trade balance. In the EU member states, the import value dynamics of the furniture industry products was similar to the export. Thus, the trade surplus was at a similar level as at the beginning of the period. The value of the furniture imports in Poland increased more than twice in the considered period, i.e. to EUR 3.2 billion. However, in 2017, the country's share in EU furniture import remained at a low level of 2%. Furthermore, it is worth emphasizing that the Polish furniture industry has been showing a positive trade balance for many years, and in the studied period, its level increased twice, which also confirms the higher growth rate of the furniture export value in comparison to the import value.

The conducted analyses of the international competitiveness ability demonstrate that in 2010, Polish manufacturers and exporters showed a relatively high comparative advantage in the foreign trade, both in comparison to other EU countries and the world (Table 1).

During this period, the value of export revenue in the Polish furniture industry exceeded the amount of the incurred import expenses four times, and the TC ratio equaled 443.06%. A high level of the implemented export specialization was also noted, which was measured by the SI index (SI = 4.78). Moreover, a positive value of the trade balance index (TBI = 0.63) and a favourable relative import penetration index (MRCA = 0.55) were noted.

Tab. 1 Selected result-oriented indices for Polish furniture industry in the years 20102017.

Indicator MP SI TC (%) TBI

2010 0.55 4.78 443.06 0.63

2011 0.93 4.79 467.38 0.65

2012 1.00 5.01 452.43 0.64

2013 1.56 4.48 486.70 0.66

2014 1.64 4.27 505.23 0.67

2015 1.59 4.31 453.51 0.64

2016 1.58 3.87 438.51 0.63

2017 1.94 4.09 443.35 0.63

V 34.54 8.73 5.13 2.27

In 2010, the relative revealed comparative export advantage index (XRCA) and the relative import penetration index (MRCA) confirmed the high competitiveness capacity of Poland in relation to the world and the EU (Tables 2 and 3). The positive, but also relatively 154

high level of the RTA index is also noteworthy. Denoting the difference between the XRCA and MRCA indexes, the RTA index also takes into account the export and import situation of the country. It should be emphasized, however, that the situation concerning the individual EU member states varied (Table 4).

Tab. 2 Relative Revealed Comparative Export Advantage Index for Polish Furniture Industry in the Years 20102017.

Country World EU28 Austria Belgium Bulgaria Croatia Cyprus Czech Republic Denmark Estonia Finland France Germany Greece Hungary Ireland Italy Latvia Lithuania Luxembourg Malta Netherlands Portugal Romania Slovakia Slovenia Spain Sweden United Kingdom max min mean st. deviation

2010 5.01 3.76 2.80 6.70 3.72 1.83 5.85 2.47 2.14 0.88 6.43 6.55 4.38 12.12 3.82 37.44 1.83 2.32 0.96 11.92 20.90 11.49 1.80 1.56 2.52 1.41 5.11 2.90 9.23 37.4 0.9 6.0 7.4

2011 5.00 3.85 3.01 7.29 3.64 1.83 4.35 2.38 2.16 0.86 6.79 7.37 4.44 13.38 3.85 34.93 1.87 2.41 1.00 13.62 70.03 10.89 1.82 1.62 2.72 1.45 5.54 3.09 9.42 70.0 0.9 7.7 13.5

2012 5.23 3.90 3.18 8.28 3.90 1.76 9.02 2.52 2.26 0.96 7.00 7.61 4.45 16.18 3.58 35.18 2.02 2.66 1.06 16.52 90.32 8.48 1.93 1.70 2.95 1.51 6.14 3.19 9.51 90.3 1.0 8.8 16.8

2013 4.67 3.82 3.08 8.75 3.03 1.72 4.60 2.55 2.13 0.95 6.97 7.62 4.45 13.04 3.22 35.48 2.02 2.46 0.96 12.44 75.63 8.73 1.86 1.51 3.06 1.20 5.85 2.95 8.71 75.6 0.9 7.8 14.4

2014 4.45 3.76 3.09 9.27 2.98 1.49 9.99 2.24 2.18 0.86 7.20 7.51 4.54 17.93 3.07 26.27 1.96 2.20 0.93 11.36 66.53 8.37 1.82 1.44 2.83 1.26 5.42 2.90 9.44 66.5 0.9 7.5 12.5

2015 4.50 3.85 3.22 10.41 2.67 1.46 8.06 2.30 2.16 0.88 7.61 8.03 4.82 16.80 3.30 26.34 2.11 2.35 0.86 11.84 118.21 8.73 1.77 1.46 2.88 1.47 5.64 3.19 8.19 118.2 0.9 9.2 21.3

2016 4.04 3.74 3.43 10.18 2.52 1.42 8.16 2.13 2.04 0.74 7.75 7.80 4.79 14.03 3.33 26.52 2.06 2.32 0.78 11.32 79.68 7.29 1.68 1.42 2.89 1.57 5.42 3.14 7.65 79.7 0.7 7.7 14.6

2017 4.29 3.81 3.98 10.40 2.51 1.52 11.06 2.16 2.08 0.75 7.88 8.15 5.00 12.82 3.54 26.68 2.20 2.11 0.76 12.46 115.13 7.08 1.70 1.49 3.08 1.48 5.21 3.47 7.25 115.1 0.8 9.0 20.7

V 8.75 1.44 10.97 15.98 18.00 10.69 32.39 6.89 3.11 9.44 7.01 6.47 5.00 14.69 8.12 16.17 5.99 7.07 11.42 13.53 38.57 17.57 4.63 6.28 6.35 8.94 6.08 6.10 10.20 -

During this period, the comparative advantage index of Poland in relation to other EU countries was typically above 1, and in the case of as many as 19 countries, it was above 2.5, which confirms the strong competitiveness of the Polish furniture industry exports. The exceptions were Lithuania and Estonia, in relation to which, the relative advantage index of the Polish furniture exports was below 1 and equaled 0.96 and 0.88, respectively. In addition, the highest XRCA index was observed in Ireland, Malta and Greece. This means that in relation to these countries, Poland showed the highest competitiveness in the area of furniture export. These are small countries, which show a relatively low share of the furniture industry production value in the total industry value and a relatively low economic labour productivity. This is manifested in the lower average furniture production value per employee than it is recorded in other EU countries.

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Tab. 3 Relative Import Penetration Index for Polish Furniture Industry in the Years 20102017.

Country World EU28 Austria Belgium Bulgaria Croatia Cyprus Czech Republic Denmark Estonia Finland France Germany Greece Hungary Ireland Italy Latvia Lithuania Luxembourg Malta Netherlands Portugal Romania Slovakia Slovenia Spain Sweden United Kingdom max min mean st. deviation

2010 0.99 0.80 0.49 1.02 1.07 0.50 0.38 0.78 0.53 0.75 0.80 0.66 0.70 0.82 1.37 1.12 1.47 0.75 1.08 0.53 0.65 1.14 0.74 1.04 0.77 0.58 0.92 0.62 0.73 1.5 0.4 0.8 0.3

2011 0.91 0.74 0.49 1.00 1.06 0.51 0.40 0.82 0.45 0.75 0.72 0.59 0.63 0.84 1.41 1.04 1.37 0.71 1.26 0.53 0.80 1.08 0.74 1.13 0.75 0.56 0.78 0.58 0.66 1.4 0.4 0.8 0.3

2012 1.02 0.82 0.52 1.10 1.35 0.53 0.45 0.87 0.49 0.97 0.81 0.66 0.69 1.11 1.33 1.28 1.51 0.83 1.48 0.64 1.04 1.01 0.77 1.18 0.76 0.62 0.96 0.62 0.77 1.5 0.5 0.9 0.3

2013 0.94 0.77 0.45 1.07 1.36 0.52 0.51 0.78 0.48 0.85 0.70 0.62 0.64 1.53 1.16 1.20 1.47 0.65 1.27 0.61 1.11 1.05 0.84 1.09 0.68 0.40 1.02 0.58 0.74 1.5 0.4 0.9 0.3

2014 0.93 0.77 0.46 1.02 1.27 0.56 0.59 0.71 0.48 0.75 0.73 0.67 0.63 1.59 1.03 1.17 1.44 0.66 1.12 0.63 1.11 1.07 0.92 1.03 0.66 0.43 1.03 0.55 0.67 1.6 0.4 0.9 0.3

2015 1.03 0.85 0.53 1.28 1.34 0.65 0.72 0.78 0.53 0.87 0.85 0.75 0.69 1.64 1.04 1.29 1.51 0.80 0.95 0.70 1.34 1.21 0.97 1.16 0.68 0.54 1.11 0.61 0.74 1.6 0.5 0.9 0.3

2016 1.00 0.85 0.58 1.42 1.35 0.67 0.75 0.75 0.53 0.79 0.79 0.77 0.71 1.54 1.06 1.47 1.54 0.83 0.91 0.75 1.16 1.09 0.92 1.17 0.64 0.59 0.98 0.61 0.72 1.5 0.5 0.9 0.3

2017 1.03 0.87 0.65 1.46 1.32 0.70 0.87 0.71 0.56 0.86 0.81 0.77 0.73 1.46 1.03 1.57 1.55 0.88 0.88 0.79 1.42 1.01 0.88 1.16 0.58 0.53 1.02 0.65 0.80 1.6 0.5 1.0 0.3

V 4.93 5.42 12.50 16.03 9.87 14.01 30.62 7.02 7.33 9.55 6.55 10.43 5.69 25.86 13.88 14.00 4.04 10.96 18.56 14.52 23.69 6.04 10.44 5.30 9.73 14.49 9.86 5.12 6.41 -

It is also worth emphasizing that in relation to the largest furniture manufacturers and exporters, i.e. Germany and Italy, the XRCA index for Poland was above 1. Moreover, the comparative advantage of our country was significantly higher in relation to Germany (XRCA = 4.38 versus XRCA = 1.83). In addition, it should be noted that the Polish furniture industry demonstrated a lack of competitiveness in the area of import in relation to five EU countries, which confirms the value of the MRCA index above 1. The highest values of the aforementioned index were observed in Italy (MRCA = 1.47) and Bulgaria (MRCA = 1.38).

The high competitiveness of the Polish furniture industry in the international exchange area at the beginning of the studied period was also confirmed by the positive values of the relative trade advantage index in comparison to all countries, except for Lithuania, where a negative index value was obtained (RTA = –0.12). In this respect, a relatively low advantage of Poland was noted in the cases of Estonia, Italy and Romania, where the RTA values were estimated at 0.14, 0.36 and 0.52, respectively. However, it should be emphasized that these values were positive. Moreover, the greatest competitiveness of the Polish furniture industry was found in relation to Ireland, Malta, the Netherlands, Luxembourg and Great Britain, which was evidenced by the highest values of the relative trade advantage index. It is 156

noteworthy that in this group, in addition to small countries of little importance for the activity of the EU furniture industry, there are also countries generating significant furniture sold production value, i.e. Great Britain or the Netherlands. In 2010, these countries produced furniture worth EUR 3.2 and 7.2 billion, respectively. Correspondingly, this accounted for 4 and 8% of the total EU furniture production.

Tab. 4 Relative Trade Advantage Index for Polish Furniture Industry in the Years 20102017.

Country World EU28 Austria Belgium Bulgaria Croatia Cyprus Czech Republic Denmark Estonia Finland France Germany Greece Hungary Ireland Italy Latvia Lithuania Luxembourg Malta Netherlands Portugal Romania Slovakia Slovenia Spain Sweden United Kingdom max min mean st. deviation

2010 4.02 2.96 2.31 5.67 2.65 1.33 5.46 1.70 1.61 0.14 5.63 5.89 3.68 11.30 2.45 36.33 0.36 1.57 -0.12 11.40 20.25 10.35 1.06 0.52 1.76 0.83 4.19 2.29 8.50 36.3 -0.1 5.2 7.4

2011 4.08 3.10 2.52 6.28 2.58 1.32 3.95 1.56 1.72 0.11 6.07 6.78 3.81 12.54 2.45 33.89 0.50 1.70 -0.26 13.08 69.23 9.81 1.08 0.49 1.96 0.89 4.76 2.51 8.76 69.2 -0.3 6.9 13.5

2012 4.22 3.09 2.66 7.19 2.55 1.23 8.57 1.65 1.77 0.00 6.18 6.96 3.76 15.07 2.26 33.90 0.51 1.83 -0.42 15.88 89.27 7.47 1.16 0.51 2.19 0.90 5.17 2.57 8.74 89.3 -0.4 7.9 16.8

2013 3.73 3.04 2.62 7.68 1.68 1.20 4.09 1.77 1.65 0.09 6.26 7.00 3.81 11.51 2.06 34.28 0.54 1.81 -0.31 11.83 74.52 7.68 1.03 0.42 2.38 0.80 4.83 2.37 7.97 74.5 -0.3 6.9 14.3

2014 3.52 2.99 2.63 8.25 1.72 0.93 9.40 1.53 1.70 0.11 6.46 6.84 3.90 16.35 2.04 25.10 0.52 1.54 -0.19 10.72 65.42 7.30 0.91 0.41 2.16 0.82 4.40 2.35 8.76 65.4 -0.2 6.6 12.4

2015 3.47 3.00 2.69 9.13 1.33 0.81 7.34 1.52 1.63 0.01 6.76 7.28 4.13 15.17 2.26 25.05 0.60 1.56 -0.09 11.14 116.88 7.52 0.81 0.30 2.19 0.93 4.53 2.58 7.45 116.9 -0.1 8.3 21.2

2016 3.03 2.89 2.85 8.76 1.17 0.74 7.41 1.39 1.51 -0.05 6.96 7.04 4.08 12.50 2.27 25.06 0.52 1.49 -0.13 10.57 78.52 6.21 0.76 0.25 2.26 0.97 4.43 2.54 6.93 78.5 -0.1 6.8 14.5

2017 3.26 2.94 3.33 8.94 1.19 0.82 10.19 1.46 1.52 -0.11 7.07 7.38 4.27 11.36 2.50 25.11 0.65 1.24 -0.12 11.67 113.71 6.07 0.82 0.33 2.50 0.95 4.19 2.82 6.44 113.7 -0.1 8.1 20.6

V 11.43 2.40 11.00 16.47 34.54 23.61 33.38 8.09 5.55 39.16 7.55 6.56 5.29 15.13 7.59 17.24 15.68 12.08 -56.51 14.42 38.81 19.69 15.76 25.21 10.62 7.06 7.47 6.70 11.47 -

From 2010 to 2017, the relative revealed comparative export advantage index (XRCA) below 1 was observed only in Estonia and Lithuania, demonstrating Poland's lack of competitiveness in this area. These countries were characterized by the largest average share of the furniture production value in relation to the value of the domestic industry production, which confirmed the important role of the furniture industry in the total industry in these states. The importance of furniture export in relation to the total value of the exported goods was also significant. At the same time, it is worth emphasizing that these countries were characterized by a lower value of furniture production per employee.

In 2017, Poland once again obtained the largest competitiveness advantage in the export of furniture in relation to Malta and Greece, i.e. small countries with a minor share in the generated EU furniture production value. The Polish furniture industry demonstrated a significant comparative advantage in the area of export in comparison to countries belonging 157

to the group of the largest exporters, i.e. France (XRCA = 8.15), UK (XRCA = 7.25), or the Netherlands (XRCA = 7.08). The value of this index was very high, which proves the strong competitiveness of the Polish furniture on the foreign market. Poland improved its competitive position in the furniture industry export field in relation to the first of these countries, whereas a negative trend was noted with respect to the other countries.

Furthermore, the international competitiveness of the export of Polish furniture in relation to the largest competitors, i.e. Germany and Italy, increased, as confirmed by the increase of the XRCA index during the studied period (from 4.38 to 5.00 and 1.83 to 2.20, respectively). During the considered period, the competitiveness of the Polish furniture industry decreased in the area of import. Since 2015, no comparative advantage in relation to the global furniture importers was observed, which was confirmed by the average MRCA index value above 1. Moreover, there was an increase in the number of countries, in relation to which the relative import penetration level of the Polish furniture industry decreased.

Notably, in 2010, the lack of this advantage was observed in relation to 8 countries. Seven years later, these countries were joined by Greece, Malta, and Spain. A positive downward trend of the MRCA index calculated for Poland was found in relation to countries such as the Czech Republic, Hungary, Lithuania, the Netherlands, Slovakia, and Slovenia; however, these changes were only marginal.

The analysis of international competitiveness demonstrated that as early as 2010, Poland showed a comparative trade advantage in relation to the whole world (RTA = 4.02) and the EU countries (RTA = 2.96). Considering the EU member states, the Polish furniture industry was highly competitive. With respect to the furniture trade, Poland obtained the highest comparative advantage in relation to countries with the highest values of the relative comparative export advantage index, i.e. Ireland, Greece, Malta, and the Netherlands. This situation applied to the entire analyzed period, and it should be stated that the international competitiveness in this area increased in relation to the first two countries, while different trends were noted for the remaining two. In years 2010 to 2017, the changes in the level of international competitivness of Poland in the furniture trade in relation to the analyzed EU countries were varied. Relative to the largest furniture exporters among the EU member states, i.e. Germany and Italy, the comparative advantage in the furniture industry trade has increased (from 3.68 to 4.27 and 0.36 to 0.65, respectively). Positive trends were also observed when comparing the competitiveness of the Polish furniture industry with the French one (RTA increase from 5.89 to 7.38). The analysis of the competitiveness of the German forestry sector against the background of international wood markets also confirmed the greater comparative advantage of Poland with respect to finished wood products (DIETER and ENGLERT 2007). The comparison of Poland with the leading European countries in furniture trading reveals that they all display relatively similar value added characteristics; however, the domestic value added content of the Polish exports is generally lower. A greater openness to trade is particularly revealed in the case of Germany. In Italy, the data on the ownership and size of exporting companies indicate greater possibilities of appropriating created value (AUGUSTYNIAK and MIŃSKA-STRUZIK 2018).

In contrast, the competitive position of the country in this area has decreased relative to ten countries, as evidenced by the decrease in the values of the RTA index. This group includes the countries included in EU-13, which show a positive furniture trade balance, namely Bulgaria, Croatia, the Czech Republic, Estonia, Latvia and Romania. The decrease in the comparative advantage in the area of furniture trade in relation to the Czech Republic, which is the fourth largest exporter of the furniture industry products among the EU countries, is noteworthy. An important competitor of Poland among the European countries is also Romania, where in recent years, the furniture production and the furniture export have

158

increased significantly, primarily due to industry restructuring and large investments in new technologies (BURJA and MĂRGINEAN 2013).

In addition, the highest significance of the furniture industry in the total industry was observed for Lithuania and Estonia, which were characterized by a high growth rate in the area of furniture production and export. In these countries, one in ten people who worked in the industry field worked in the furniture industry. Research demonstrates that the other EU13 countries also exhibit a comparative advantage in the wood-based industries (ZHELEV 2013); however, it should be noted that the competitiveness indicators for those countries are lower than in Poland. According to PAROBEK et al. (2014), with the exception of the pulp and paper industry, the Slovak forest industry producing final higher added value wood commodities such as furniture, construction timber, etc. is often still unable to compete on the European market. The present article confirms the comparative advantage of the Polish furniture industry against Slovakia. In the considered period, the RTA increased from 1.76 to 2.50.

Despite the relatively good competitiveness of Poland in the furniture trade compared to other EU countries, there is a need to improve the production capacity and seek new ways of building competitive advantage on the external market. The existing strengths of the Polish furniture manufacturers, i.e. the high quality of the products and relatively low labour costs compared to the Western competitors, may prove insufficient in the future, particularly when taking into consideration the relatively low work efficiency in this industry in comparison to the countries included in the EU15 group. Similarly, the weakness of the competitiveness of the wood processing industries in other East and South European countries can be seen in the fact that the competitive advantage is visible in the favourable prices, not in manufacturing complex products with high value added. Business and development, which are based only on the strategy of low costs and cheap final products are increasingly less sustainable for the enterprises, which aspire towards improvement of the competitiveness (MILIĆEVIĆ 2017).

However, price, next to product quality and design, remains one of the most important factors influencing consumer decisions in the furniture industry (KOZAK et al. 2004; MOTIK et al.

2003; LOUČANOVÁ et al. 2015). In addition, PALUŠ et al. (2012) pointed out that the materials from which furniture is made are of important to consumers because of ecological properties, environmental appropriateness, renewability and naturalness. The study carried out by RATAJCZAK (2009) founded that the Polish comparative advantages change with the level of wood products processing. Thus, there should be more investment in the R&D activities, in case of the furniture industry, particularly in the design and/or marketing and sales activities, including the creation of good access routes to distribution channels (AUGUSTYNIAK and MIŃSKA-STRUZIK 2018). In turn, SMARDZEWSKI (2009) estimated that the exchange rate has an impact on the level of competitiveness of the Polish furniture industry. The importance of exchange rate fluctuations that determine foreign trade and the competitiveness on the domestic economy as well as individual economic entities on the international market have been emphasized by BOSE (2014) and BOSTAN et al. (2018). However, this situation may be short-term, so a competitive advantage should be built on a wide spectrum of both internal and external factors. The support of government agencies for the furniture industry is not without significance.

CONCLUSION The Polish furniture manufacturers and exporters exhibited a relatively large international competitiveness in the foreign trade in relation to both other EU countries and the world. The value of the export revenue in the Polish furniture industry was four times higher than the value of the incurred import expenses. A high level of implemented export 159

specialization and a relatively favourable level of the relative import penetration were also noted. The situation varied in relation to the individual EU member states. The lack of relative export advantage was observed in the cases of Estonia and Lithuania, i.e. countries for which the furniture industry is significant for international exchange and the national economy. Poland had the largest competitive advantage in the area of furniture export in relation to small countries, ones with a minor contribution to the generated EU furniture production value.

The Polish furniture industry has shown a significant comparative advantage in the area of export and foreign trade in comparison to the largest competitors, i.e. Germany and Italy, as confirmed by the increase of the XRCA and RTA indexes during the studied period.

Despite relatively high competitiveness of Poland in the furniture trade in comparison to other EU countries, there is a need to improve the production capacity and seek new ways of building competitive advantage on the external market. It is necessary to expand the current research and conduct scientific investigations which will be directed towards the optimization of productive and exporting structure of furniture industry, aimed at further increase of competitiveness on foreign market.

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ACKNOWLEDGEMENT The authors are grateful for the support of the National Science Centre Poland, Grant No. 2019/03/X/HS4/01342 and Slovak project agency APVV – projects APVV-18-0520, APVV-180378, APVV-17-0456 and APVV-17-0583.

ADDRESSES OF THE AUTHORS Emilia Grzegorzewska, D.Sc. Warsaw Univerisity of Life Sciences – SGGW Department of Technology and Enterpreneurship in Wood Industry Nowoursynowska 159 02-776 Warsaw Poland emilia_grzegorzewska@sggw.edu.pl ORCID ID 0000-0002-7532-9287 163

doc. Ing. Mariana Sedliačiková, PhD. Technical University in Zvolen Department of Economics, Management and Business T. G. Masaryka 24 960 01 Zvolen Slovakia sedliacikova@tuzvo.sk ORCID ID 0000-0002-4460-2818 doc. Ing. Josef Drábek, CSc. Technical University in Zvolen Department of Economics, Management and Business T. G. Masaryka 24 960 01 Zvolen Slovakia drabek@tuzvo.sk Ing. Marcel Behún, PhD. Technical University in Košice Vice-President for Economics Nemcovej 5 042 00 Košice Slovakia kvestor@tuke.sk

164

ACTA FACULTATIS XYLOLOGIAE ZVOLEN, 62(2): 165−174, 2020 Zvolen, Technická univerzita vo Zvolene DOI: 10.17423/afx.2020.62.2.15

CONSUMERS' PERCEPTION OF RETRO-INNOVATION OF WOOD PRODUCTS

Erika Loučanová  Miriam Olšiaková ABSTRACT Consumer satisfaction is one of the main attributes influencing the company success in the market. As consumer satisfaction is not an absolute and objectively measurable value, companies must carry out several activities to ensure its maximum level, which include innovative activities as well. In the context of innovation, we are increasingly confronted with the term of retro-innovation connecting consumers with the periods from the past that are supposed to be nostalgic, interactive and environmental. The application and implementation of retro-innovations supports a positive impact on consumers, employees, communities and the environment, and therefore their use is also appropriate in connection with wood products. The Slovak consumers’ perceiving the retro-innovations of wood products is examined in the paper. Main observed requirements were determined through the Kano model in relation to the retro-properties of wood products, which have a significant impact on consumer satisfaction. The results confirm that consumers perceive retroinnovations of wood products positively. From the consumers' point of view, retro-designs in furniture and carpentry products as well as other wood products are attractive and thus have an obvious impact on their satisfaction.

Key words: retro-innovation, eco-innovation, KANO model, wood products, consumers´ perception.

INTRODUCTION The issue of consumer’s satisfaction has a long tradition in business practice. Previously insufficient attention has been given to this issue in the furniture market. One of the oldest references can be found in literature in the 19th century, when concepts such as "targeting the offer", "perceived value" or "consumers´ expectations" are mentioned (DUCÁR et al. 2006).

This area became the subject of systematic interest of economists in the 1980s, when under the influence of internationalization of the economy, beginning of globalization and generation of innovations, new opportunities began to be sought to increase business competitiveness by satisfying consumer requirements. Consumer satisfaction with the product is not an objective or absolute value. It is important to find new possibilities and innovations, which help the product create needs that the consumers perceive as satisfactory. One of the possibilities to look for and identify variables characterizing consumers´ satisfaction is to monitor the specific properties of products, which can be described by nonlinear and asymmetrical relationship dependence between the importance and satisfaction of consumers with the various features 165

of representative products. These dependencies are analysed during the individual transaction, alternatively they are based on cumulative satisfaction with the monitored products and services (LOUČANOVÁ et al. 2015). The study of OLŠIAKOVÁ et al. (2018) describes consumers' satisfaction with wood framed houses by ordering the selected features according to the level of satisfaction.

Nowadays, we often encounter the interpretation of consumer satisfaction, following the theory of contradiction, which we can also apply in the sale of storage furniture. When monitoring the values that represent consumers´ satisfaction with products, it is appropriate to confront them with the characteristics of the product, where there is an experiential feeling of compliance or contradiction with their expectations. The issue of individual values has been studied by several authors, who take into account the theory of different perceptions of product parameters derived from two-factor motivation, while changing the conceptual apparatus for product requirements. CADOTTE and TURGEON (1988) define product requirements as requirements of dissatisfaction, satisfaction, and critical character. On the other hand, BRANDT (1988) evaluates them at several levels and defines minimum requirements, satisfaction-enhancing values and hybrid requirements. Similarly, LLOSA (1997) characterizes the requirements as basic, plus and key ones.

TOMEK et al. (2007) describes the processes of creating consumer value, representing an effort of the company to gain consumers and ensure their loyalty. Above all, the company tries to attract and keep attractive consumers that realize relevant sales or ensure a reasonable turnover. Based on the above assumptions, the company concentrates on so-called advantageous market segments and consumers. Appropriate structure of consumers and their number are a significant contribution to the company´s value, which is the future growth guarantee. Consumer satisfaction is defined by MARUCA (2000) as a measure of meeting consumer expectations in relation to product characteristics and provided values, which are not in many cases assessed accurately and objectively.

There are several reasons why it is necessary to focus on monitoring consumer satisfaction, not only at the business but also at the macroeconomic level. At the business level the main reason of the interest is the impact of consumer satisfaction on the company's financial results. At the macroeconomic level, it is a matter of creating a measure for comparing companies (consumer satisfaction indices), which is later applied as a tool of forecasting possible development trends of individual companies. This issue is based on the theory of contradiction, which considers the assumption that consumers have a certain idea of the product characteristics. They confront this idea with the characteristics of the given product, which they obtained by purchasing. At this point, there is a situation in which the consumers feel a match or discrepancy between their experience and their expectations.

PALUŠ (2010) presents that consumer preferences are reflected in consumer markets.

They are decisive when buying products and they relate to the material, its quality, appearance, functionality etc. RAMETSTEINER et al. (2007) dealt with the issue of attitudes towards wood and wood products. The study aimed at collecting and presenting the results of consumers' views and attitudes towards wood and defined categories of wood products in European countries over the last 10 to 15 years. The results and conclusions of the surveys point to very similar attitudes of consumers in European countries towards some of observed properties of wood products. The preferred features of consumers include design and quality. Consumers appreciate mainly the naturalness of wood and the pleasant atmosphere it creates in the interior design. In the case of wood used outdoors, they appreciate strength, durability and environmental friendliness most of all. Similar conclusions were reached by studies (RAMETSTEINER et al. 2007; LOUČANOVÁ et al. 2015) pointing to the importance of ecoinnovation. In an international context that sometimes involves inherent conflicts among economic progress, limited natural resources, and environmental problems and threats, eco166

innovation has become a central topic between leading researchers and policy makers and it is considered to be a key driver of long-term stable economic development (CHEN et al. 2017).

There is a strong link between economic and environmental performance (RACHISAN et al.

2015, TILINA et al. 2016) in the meaning that environmental improvements as a source of innovation can increase marketability while focusing on reducing the negative effects on the use of natural resources and on the quality of the environment through less harmful and more productive methods (ADEDE 1992).

Eco-innovation can be characterized as the creation of a new or modification of an old production process, system, practices or products to reduce or eliminate its environmental impact (ISTRATE et al. 2019).

In the case of modification of an old product, production process, system, practices or the products we signify retro-innovations. They present innovations that authentically imitate a product or experience from the past to take the user back to the past, or that use a nostalgic format to meet new needs, alternatively which use a new format to meet old needs are called retro-innovation (LEBERECHT 2013). Within the concept of retro-innovation, products are designed to connect us with ways from the past that are nostalgic, interactive and environmental.

Retro-innovations have a positive impact on consumers, employees, the community as well as the environment.

The concept of retro-innovation is often underestimated, but its importance in the context of sustainable development is indispensable from the point of view of the whole society. Therefore, the aim of this paper is to evaluate the perception of retro-innovation of wood products by Slovak consumers.

MATERIALS AND METHODS The Kano model was used as a primary method to evaluate the perception of retroinnovations of wood products. It considers theories of contradiction to identify the differentiation variables of the product by creating its unique position on the market. The analysis is primarily focused on finding the values of the product that the consumer considers to be must-be, attractive and one-dimensional. The must-be requirements are significant from the consumer point of view because in the case of their non-compliance they cause his strong dissatisfaction. On the other hand, if they are met, they have little effect on consumer satisfaction. It is a basic product criterion that the consumer requires automatically.

One-dimensional requirements are defined as claims, where we can see a linear dependence between their fulfilment and consumer satisfaction. The more requirements are met, the more satisfied the consumer is. Attractive values include requirements that lead exponentially to an increase in consumer satisfaction. Regarding the above-mentioned information these requirements have the most significant impact on consumer satisfaction. In addition to the above explained requirements, there are also identified reverse, questionable and indifferent requirements not influencing the consumers. Of course, it is not possible to strictly separate individual requirements. They overlap and influence each other at the same time.

The analysis of parameters focused on the examined problem was followed by the methodical procedure to assess the retro-innovation perception of wood products by Slovak consumers, such as:  retro-materials – the focus was on examining consumers' perceptions and preferences for materials of a traditional nature, such as wood, which is nowadays often replaced by substitute materials,  retro-technologies – presenting traditional technological processes, 167



   

retro-design – characterizing authentically imitating products or experience from the past to carry the user back to the past, or which represent a nostalgic format to meet new needs or which use a new format to meet old needs and nostalgic view of the traditional design of wood products, price – representing the amount that the consumer must waive in order to obtain a good or service, in our case to monitor the respondents´ perception of wood products price, quality – researching the compliance of consumer requirements with wood products, environmental friendliness - examining the perception of the ecological impact of wood products on the environment, retro-design of furniture, buildings, carpentry products and other wood products - we aimed at monitoring the attitudes of respondents towards various types of secondary wood processing products, such as furniture, construction and carpentry products, buildings and other wood products, including toys, musical instruments, etc.

After precisely determined parameters, a questionnaire was formed according to the KANO model needs. The questionnaire creation involved the generation and formulation of two questions for each examined parameter. In the first case, the question was formulated to detect the consumers´ responses whether their requests were met. On the contrary, in the second case, the question was formulated in such a way that the consumers’ requests were not met. In some specific cases, instead of formulating questions, the consumers´ reactions to the formulated statements concerning the given parameter were monitored. Consumers had the opportunity to express agreement or disagreement with the question or statement on the Likert scale (1 – like, 5 – dislike).

Then measures for the questionnaire implementation were determined. The questionnaire presents a versatile method for obtaining and gathering information about consumer activities and attitudes.

The sample of respondents was 1515 that fulfils the minimum number of respondents (666), with regard to the sample size calculation, with the average permanent population in Slovakia, gained from the data presented by the Statistical Office of the Slovak Republic (5,452,257 inhabitants in 2019). The sample was calculated at the 99 % confidence level and margin of error 5 %. Respondents at the age 18 years and over, of both genders (52 % women, 48 % men) participated in the survey.

After the actual implementation of the survey by means of a questionnaire, a database of obtained data was created, where the examined parameters for wood products were defined and subsequently assigned a numerical expression of consumer agreement or disagreement with the given question concerning the defined parameter.

For each parameter, the individual answers to the positively and negatively asked question (statement) were evaluated separately using the cross rule of the KANO model (Table 1). By such a determination, individual properties are specified: attractive (A), mustbe (M), reverse (R), one-dimensional (O), questionable (Q) or indifferent (I).

Tab. 1 KANO model for evaluation of consumer requirements.

Like Acceptable No Feeling Must-be Do not like Source: Grapentine, 2015; KANO et al., 1984 Answer to the Functional Question

Like Q R R R R

168

Answer to the Dysfunctional Question Acceptable No Feeling Must-be Do not like A A A O I I I M I I I M I I I M R R R Q

The identified consumer requirements are divided into groups and redistributed with regard to the proportions of respondents’ sample in percentage. The most represented group of requirements characterize the resulting perception of the examined parameter or value.

To generalize and identify the individual dependencies among the examined parameters of wood products in retro design and better knowledge of consumer requirements, the data from the database were evaluated by statistical methods.

The result of the correlation analysis is the coefficient r, which acquires values in the interval from 1 to +1. Minus 1 means an absolute indirect linear dependence, 0 means no dependence, and 1 means an absolute direct linear dependence. In other words: the closer the correlation coefficient is to 0, the weaker the relationship among the examined variables is (not existing). On the contrary, the closer it is to 1 or to 1; the relationship among the variables is stronger.

We interpret the values of the correlation coefficient according to CHRÁSKA (2000), who describes their dependence as follows: 0.9 to 1 (0.9 to -1) are considered to be very highly dependent, so there is a very strong interdependence among the variables. Values 0.7 to 0.9 (0.7 to 0,) are highly dependent, from 0.4 to 0.7 (0.7 to 0.4) are moderately dependent, from 0.2 0.4 (0.4 to 0.2) are with low dependence, from 0 to 0.2 (0.2 to 0.0) the values are considered to be weak (without dependence).

RESULTS AND DISCUSSION With regard to the above-mentioned methodological procedure the Kano model categorizes individual responses based on a cross-rule into specified categories, where the properties of the examined parameters are determined as attractive, must-be, reverse, onedimensional, questionable and indifferent (Table 2).

Tab. 2 Specific requirements for retro-innovation of wood products (%).

Parameters Retro-materials Retro-technology Retro-design Price Quality Environmental friendliness Retro-design: - Furniture - Buildings - Carpentry products - Others (toys, musical instruments, etc.)

A 16.37 6.01 66.67 33.99 6.01 15.38

O 16.37 30.17 4.09 14.19 30.17 37.10

M 4.09 21.98 12.61 1.85 38.68 4.09

I 14.79 38.68 6.60 33.99 21.98 17.29

R 37.10 1.91 9.37 4.09 1.91 14.79

Q Category* 11.29 R 1.25 I 0.66 A 11.88 A,I 1.25 M 11.35 O

41.19 2.77 33.99

13.20 0.00 15.25

2.77 4.09 1.91

30.83 50.50 33.00

6.01 39.93 4.09

6.01 2.71 11.75

A I A,I

60.66

5.94

6.01

14.79

11.88

0.73

A

*requirements - attractive (A), must-be (M), reverse (R), one-dimensional (O), questionnable (Q) or indifferent (I).

Table 2 shows that retro-innovations of wood products are perceived by consumers in Slovakia differently, but mostly positively, because only two examined parameters have no effect on them (retro-technologies and retro-design of buildings). These two parameters are insignificant for Slovak consumers and their fulfilment or non-fulfilment does not affect their satisfaction or dissatisfaction.

Slovak consumers perceive the ecological friendliness of wood products as a onedimensional requirement. It means the higher the rate of compliance with these requirements is, the more satisfied the consumers are. There is a direct linear relationship between meeting 169

these requirements and consumer satisfaction. Retro-design, retro design of furniture, construction and carpentry products, other wood products and their price are attractive for Slovak consumers, which means that these examined parameters have an obvious effect on their satisfaction, because these requirements are not expected by them. If they are not fulfilled, it will not result in consumer dissatisfaction.

Quality is a parameter that must be provided automatically (must-be requirements).

They can be marked as primary (basic) and therefore consumers deal with them only in case of non-compliance. Their identification is of fundamental importance because their fulfilment will be reflected in consumers´ satisfaction, but their deficit and non-fulfilment will be immediately realized by consumers and they are dissatisfied. Finally, such a situation will be reflected in their maximum dissatisfaction and the product will lose its competitiveness in the market.

Only retro-materials are perceived in reverse way. It means that consumers react in exactly opposite way.

Based on the procedures presented in the methodology, the specific collected requirements need to be generalized. At the same time, the individual dependencies among the identified consumer requirements for the examined parameters of retro-innovations of wood products were analysed. In order to describe the causality among the individual identified properties of investigated parameters of retro-innovations of wood products, a correlation coefficient symmetrically arranged in a correlation matrix is applied (Table 3).

Others (toys, musical instruments, etc.)

Carpentry products

Buildings

Furniture

Environmental friendliness

Quality

Price

Retro-design Retro-design

Retro-technology

Retro-materials

Tab. 3 Correlation matrix of consumer requirements for retro-innovation of wood products.

Retro-materials

1.000 Retro-technology 0.019 Retro-design 0.310

1.000

Price

0.260

0.001 0.145

1.000

0.105

0.022 0.047

0.340

1.000

0.226

0.024 0.024

0.007

0.185

1.000

0.104

0.080

0.154

0.109

1.000

Buildings 0.008 0.040 0.051 0.205 0.044 Carpentry 0.067 -0.080 0.011 0.164 0.163 products Others (toys, musical 0.084 0.090 0.027 0.026 0.101 instruments, etc.)

0.114

0.087

1.000

0.035

0.022

0.300

Quality

Retro-design :

Environmental friendliness Furniture

0.079

0.043

1.000

0.208

0.142 0.091

1.000

0.124 0.037

1.000

The correlation matrix confirmed the low causality among the monitored parameters of retro-innovation of wood products. The matrix shows a statistically significant relationship among retro-materials and retro-design, price and environmental friendliness.

At a certain price, Slovak consumers also demand a certain quality standard, while the correlation coefficient with the dependence of these two variables shows the highest value 170

of 0.340. Consumers associate retro-design mainly with furniture. There is a relationship between the design of buildings and carpentry products. The negative dependence is reflected between the price and design of buildings. There is no significant dependence among other examined parameters.

The results of this paper show that retro-innovations of wood products are perceived positively by consumers. They focus mainly on retro-design in furniture and carpentry products as well as in other wood products. These are attractive to them and have an obvious effect on consumer satisfaction. In this context, our results correlate with the results of the study by CURRAJ (2018), where he argues that retro-design in furniture is of high interest in the market economy. Local involved persons, businesses and academia should create and use furniture and its components in nostalgia and retro-design.

However, in terms of materials, consumers prefer modern design of wood materials to classic design. Preferences for furniture styles (retro style inculding) were also a part of investigation in the studies of JOST et al. (2020) and KAPUTA et al. (2018). A study by ZHU (2020) presents that wood products are an important element in people's lives, because wood products made of specific materials (their combinations and innovative forms of wood as a material) will instantly create a kind of the feeling of what can be a sense of security or other "feelings" that can calm the heart and create “material emotion”. The essence of material emotion is the specific expression of human emotion, but in the process of exploration, we need to use different materials as the media to release it, and then make the heart produce a sense of pleasure. This kind of emotion seems invisible and untraceable, but it exists in every link of life. Therefore, under the guidance of material emotion, design and specific materials in wood products can provide more wonderful details of life and make life more wonderful.

Several studies (JONSSON 2006, RAMETSTEINER et al. 2007, PALUŠ 2012, ŠUPÍN 2014, KAPUTA et al. 2010;) found out that consumers prefer wood products to its substitutes. Those products of wood have a special position in preferences of consumers, especially for its environmental friendliness, environment suitability, renewability and naturality as well as tradition and health and safety features. This is caused by the wood material itself (PALUŠ et al. 2012). In comparison with the previous period, price is no longer a relevant parameter of the consumer behaviour. Consumers focus on a product quality and in proportion to the price they require the quality (correlation coefficient 0.340) as it is also confirmed by RAMETSTEINER et al. (2007). Price as a decisive factor in the case of negative dependence was reflected in the retro-design of buildings (the higher the prices, the lower interest in this type of buildings).

Another reason rests in a fact that it represents a significantly higher investment than the other examined type of wood products and that Slovak consumers prefer brick buildings to buildings made of wood (LOUČANOVÁ, OLŠIAKOVÁ 2020), because as it is mentioned by TOIVONEN (2012), the main target group for wood buildings are people with strong environmental values and a willingness to buy and pay a higher price for such products (HANSMANN et al. 2006; O'BRIEN and TEISL 2004).

CONCLUSION The retro-innovation through knowledge and practices change the value of items from daily and old-fashioned to unique and desirable. In this value creation process, retroinnovations represent the connection of the past with the present and the dedication of the cultural heritage and traditions of the forestry and wood-processing industry to future generations, which represents a renewable wealth in Slovakia. Based on the results of the research, we can suppose that Slovak consumers perceive retro-innovations of wood 171

products as attractive and the individual examined parameters of retro-innovations of wood products lead to consumer satisfaction and they consider them to be natural. These results show that the use of retro-elements and their design in wood products are desirable part of Slovak consumers and therefore it is appropriate to incorporate them into the product portfolios of businesses offering wood products on the market. They should consider the identified requirements of consumers connected with this kind of products.

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ACKNOWLEDGEMENTS The authors would like to thank the Scientific Grant Agency of the Ministry of Education, Science, Research and Sport of the Slovak Republic and the Slovak Academy of Sciences, grant number 1/0674/19, “Proposal of a model for the eco-innovation integration into the innovation process of companies in Slovakia in order to increase their performance" and grant number 1/0666/19 “Determination of the development of a wood-based bioeconomy”.

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AUTHORS ADDRESSES Ing. Erika Loučanová, PhD. Ing. Miriam Olšiaková, PhD. Technical University in Zvolen Faculty of Wood Sciences and Technology T. G. Masaryka 24 960 01 Zvolen Slovenská republika loucanova@tuzvo.sk olsiakova@tuzvo.sk

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