Chemistry and Applications
e dited by Brendan rodgers
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Sudhin Datta and Syamal S. Tallury
Wesley A. Wampler, Leszek Nikiel, and Erika N. Evans
Anke Blume, Louis Gatti, Hans-Detlef Luginsland, Dominik Maschke, Ralph Moser, J.C. Nian, Caren Röben, and André Wehmeier
Harry G. Moneypenny, Karl-Hans Menting, and F. Michael Gragg
James E. Duddey
Preface
Rubber compounding describes the science of elastomer chemistry and the modification of elastomers and elastomer blends by addition of other materials to meet a set of required mechanical properties. It is therefore among the most complex disciplines in that the materials scientist requires a thorough understanding of materials physics, organic and polymer chemistry, inorganic chemistry, thermodynamics, and reaction kinetics.
The rubber industry has changed since the publication of the book’s last edition. Asia and particularly China and India have emerged as major tire and industrial rubber products manufacturing centers. Other developing countries are actively pursuing development of rubber industries because of the technology and skills the industry brings. Every investor in the industry can acquire the same equipment, design the same types of factories, and build similar tire constructions and designs. However, materials science and technology cannot be replicated and, along with uniformity, has become the most important factor in differentiating the quality of tires and industrial rubber products from different manufacturers. Tire uniformity, tire and industrial rubber products durability, and performance parameters such as tire rolling resistance or conveyor belt energy efficiency are all dependent on materials quality and compound technologies. The new global regulatory environment has set new standards in tire performance, which, again, depend to a very large extent on materials technology.
The tire industry has evolved from production of bias to tubeless radial constructions and now ultralow-profile designs. The service lives of tires and industrial products such as automobile engine hoses have improved dramatically. None of these improvements would have been possible without an emphasis on the understanding of the chemistry of raw materials and compounds. As was mentioned in the first edition of this book, the advances in materials technologies over the last number of years have included
1. Commercialization of functionalized and coupled, solution-polymerized polymers, leading to improvements in tire wear resistance and traction performance
2. Development of the silica tread compound for high-performance tires, leading to a significant improvement or reduction in tire rolling resistance and in turn vehicle fuel economy
3. Thermoplastic elastomers
4. Hybrid filler systems and nanocomposite technologies, allowing improvement in tire air pressure retention
5. Reversion-resistant vulcanization systems, leading to improved tire durability
6. Halobutyl polymers which were the foundation for the development of the tubeless radial tire
7. A new emphasis on recycling and renewable sources for raw materials
As was the case in the first edition, the philosophy behind this work continues to emphasize more the chemistry of the materials used in building a compound formulation for a tire or engineered product and also now elaborates on product technologies. The depth to which subjects are presented is not at an introductory level nor is it an advanced treatise. Rather, it is intended as a tool for the industrial compounder, teacher, or other academic scientist to provide basic information on materials used in the rubber industry. In addition, it continues to address a gap in the body of literature available to the chemists in industry and academia. The text has been redesigned to add sections on recycling, expanded discussion on tire technology and industrial rubber products adding more information on hydraulic hose and conveyor belting.
Fred Barlow mentioned in the first edition that no comprehensive review of a subject such as this could be written by one individual. The compilation of this work thus depended on many contributors, and I express my thanks to the authors who participated in the project. All are recognized authorities in their field, and this is reflected in the quality of their contribution. I also express my thanks to the ExxonMobil Chemical Company for permission to pursue this project; to Dr. James P Stokes, Polymers Technology Manager at ExxonMobil Chemical Company, for his support; and most importantly to my wife, Elizabeth, for her encouragement.
Brendan Rodgers
ExxonMobil Chemical Company
ExxonMobil Baytown, Texas
Editor
Brendan Rodgers was senior engineer at ExxonMobil’s Baytown Technology and Engineering Center in Baytown, Texas, and is now technology advisor at the ExxonMobil Chemical Company, Shanghai Technical Center, China. Previously, he was as an engineer with the Goodyear Tire and Rubber Company with assignments in Europe and the United States. He earned his BS from the University of Ulster, Northern Ireland; his MS in polymer technology; and PhD in chemical engineering from Queen’s University, Belfast, Northern Ireland.
Contributors
Tonson Abraham
ExxonMobil Chemical Company Akron, Ohio
Anke Blume
Evonik Industries AG Wesseling, Germany
George Burrowes
Veyance Technologies, Inc. Fairlawn, Ohio
Howard Colvin Cooper Tire & Rubber Company Findlay, Ohio
Sudhin Datta
ExxonMobil Chemical Company Baytown, Texas
Bernard D’Cruz
ExxonMobil Chemical Company Baytown, Texas
James E. Duddey (Retired) Akron, Ohio
Erika N. Evans
Sid Richardson Carbon & Energy Company Fort Worth, Texas
Louis Gatti
Evonik Industries AG Wesseling, Germany
F. Michael Gragg
ExxonMobil Lubricants & Petroleum Specialties Company Fairfax, Virginia
Sung W. Hong Crompton Corporation Uniroyal Chemical Naugatuck, Connecticut
Frederick Ignatz-Hoover Eastman Chemical Company Akron, Ohio
Hans-Detlef Luginsland
Evonik Industries AG Wesseling, Germany
Dominik Maschke
Evonik Industries AG Wesseling, Germany
Karl-Hans Menting
Schill + Seilacher “Struktol” GmbH Hamburg, Germany
Harry G. Moneypenny Moneypenny Tire & Rubber Consultants
Den Haag, the Netherlands
Ralph Moser
Evonik Industries AG Wesseling, Germany
J.C. Nian
Evonik Industries AG Wesseling, Germany
Leszek Nikiel
Sid Richardson Carbon & Energy Company
Fort Worth, Texas
Wolfgang Pille-Wolf
Arizona Chemical B.V. Almere, the Netherlands
Caren Röben
Evonik Industries AG Wesseling, Germany
Brendan Rodgers
ExxonMobil Chemical Company Baytown, Texas
Syamal S. Tallury
ExxonMobil Chemical Company Baytown, Texas
Byron H. To (Retired)
Flexsys America L.P. Baltimore, Maryland
Andy H. Tsou
ExxonMobil Chemical Company Baytown, Texas
Walter H. Waddell
ExxonMobil Chemical Company Baytown, Texas
Wesley A. Wampler
Sid Richardson Carbon & Energy Company
Fort Worth, Texas
André Wehmeier
Evonik Industries AG Wesseling, Germany
1 Natural Rubber and Other Naturally Occurring Compounding Materials
Brendan Rodgers
I. INTRODUCTION
The nature of the tire and rubber industry has changed over the last 20–30 years in that, like all other industries, it has come to recognize the value of using renewable sources of raw materials, recycling materials whenever possible, and examining the potential of reclaiming used materials for fresh applications. Renewable raw materials range from natural rubber (NR),
of which is used than any other elastomer,
naturally occurring process aids such as pine tars and resins, and novel biological materials such as silica derived from the ash of burned rice husks. Naturally occurring materials include inorganic fillers such as calcium carbonate, which is distinct from naturally occurring organic material, whose total supply may be restricted. However, NR is by far the most important of these materials both in terms of the quantity used and also in meeting the performance target of tires and other industrial rubber products.
II. NATURAL RUBBER
Of the range of elastomers available to technologists, NR is among the most important, because it is the building block of most rubber compounds used in products today. In the previous edition of this text, Barlow [1] presented a good introductory discussion of this strategic raw material. Roberts [2] edited a very thorough review of NR covering topics ranging from basic chemistry and physics to production and applications. NR, which is a truly renewable resource, comes primarily from Indonesia, Malaysia, India, and the Philippines, though many more additional sources of good quality rubber are becoming available. It is a material that is capable of rapid deformation and recovery, and it is insoluble in a range of solvents, though it will swell when immersed in organic solvents at elevated temperatures. Its many attributes include abrasion resistance, good hysteretic properties, high tear strength, high tensile strength, and high green strength. However, it may also display poor fatigue resistance. It is difficult to process in factories, and it can show poor tire performance in areas such as traction or wet skid compared to selected synthetic elastomers. Given the importance of this material, this section discusses
1. The biosynthesis and chemical composition of NR
2. Industry classification, descriptions, and specifications
3. Typical applications of NR
A. Chemistry of NAtur Al rubber
NR is a polymer of isoprene (methylbuta-1,3-diene). It is a polyterpene synthesized in vivo via the enzymatic polymerization of isopentenyl pyrophosphate. Isopentenyl pyrophosphate undergoes repeated condensation to yield cis-polyisoprene via the enzyme rubber transferase. Though bound to the rubber particle, this enzyme is also found in the latex serum. Structurally, cis-polyisoprene is a highly stereoregular polymer with an –OH group at the alpha terminal and three to four trans units at the omega end of the molecule ( Figure 1.1). The molecular-weight distribution of Hevea brasiliensis rubber shows considerable variation from clone to clone, ranging from 100,000 to over 1,000,000. NR has a broad bimodal molecular-weight distribution. The polydispersity or ratio of weight-average molecular weight to number-average molecular weight, Mw/Mn, can be as high as 9.0 for some varieties of NR [3,4]. This tends to be of considerable significance in that the lower-molecular-weight fraction will facilitate the ease of processing in end product manufacturing, while the higher-molecular-weight fraction contributes to high tensile strength, tear strength,
trans-Polyisoprene (repeat units, n = 100; Mw = 7000)
cis-Polyisoprene (repeat units, n = 1,500 –15,000; Mw = 100,000–1,000,000)
FIGURE 1.1 cis- and trans-Isomers of natural rubber.
and abrasion resistance. The biosynthesis or polymerization to yield polyisoprene, illustrated in Figure 1.2 , occurs on the surface of the rubber particle(s) [5].
The isopentenyl pyrophosphate starting material is also used in the formation of farnesyl pyrophosphate. Subsequent condensation of trans-farnesyl pyrophosphate yields trans-polyisoprene or gutta-percha. Gutta-percha is an isomeric polymer in which the double bonds have a trans configuration. It is obtained from trees of the genus Dichopsis found in Southeast Asia. This polymer is synthesized from isopentenyl pyrophosphate via a pathway similar to that for the biosynthesis of terpenes such as geraniol and farnesol. Gutta-percha is more crystalline in its relaxed state, much harder, and less elastic.
NR is obtained by “tapping” the tree H. brasiliensis. Tapping starts when the tree is 5–7 years old and continues until it reaches around 20–25 years of age. A knife is used to make a downward cut from left to right and at about a 20°–30° angle to the horizontal plane, to a depth approximately 1.0 mm from the cambium. Latex then exudes from the cut and can flow from the incision into a collecting cup. Rubber occurs in the trees in the form of particles suspended in a protein-containing serum, the whole constituting latex, which in turn is contained in specific latex vessels in the tree or other plants. Latex constitutes the protoplasm of the latex vessel. Tapping or cutting of the latex vessel creates a hydrostatic pressure gradient along the vessel, with consequent flow of latex through the cut. In this way, a portion of the contents of the interconnected latex vessel system can be drained from the tree. Eventually, the flow ceases, turgor is reestablished in the vessel, and the rubber content of the latex is restored to its initial level in about 48 h.
The tapped latex consists of 30%–35% rubber, 60% aqueous serum, and 5%–10% other constituents such as fatty acids, amino acids and proteins, starches, sterols, esters, and salts. Some of the nonrubber substances such as lipids, carotenoid pigments, sterols, triglycerides, glycolipids, and phospholipids can influence the final properties of rubber such as its compounded vulcanization characteristics and classical mechanical properties. Hasma and Subramanian [6] conducted a comprehensive
CO2 H2O
Carbohydrates
Glucose Fructose
Fructose diphosphate
Fructose-6-phosphate
Phosphoenol pyruvat e COO H CO CH2 P
Hydroxy-3-methyl glutaryl-CoA
Acetoacetyl CoA
Mevalonic acid C CH3 OH CH2 CH2 OH HOOC
Mevalonate-5-PP
Mevalonite-5-P C CH3 OH CH2 CH2 O HOOC P C CH3 OH CH2 CH2 O HOOC PO P
Isopentyl pyrophosphate
Rubber cis-polyprenyl cis-transferase
Dimethylallyl pyrophosphate
C10 Prenyl OPP
C15 Prenyl OPP OPP
FIGURE 1.2 Simplified schematic of the biosynthesis of natural rubber.
TABLE 1.1
Definition of Natural Rubber Terms
Latex
Serum
Whole field latex
Cup-lump
Lace
Fluid in the tree obtained by tapping or cutting the tree at a 20°–30° angle to allow the latex to flow into a collecting cup
Aqueous component of latex that consists of lower-molecular-weight materials such as terpenes, fatty acids, proteins, and sterols
Fresh latex collected from trees
Bacterially coagulated polymer in the collection cup
Trim from the edge of collecting vessels and cut on tree
Earth scrap Collecting vessel overflow material collected from the tree base
RSS Sheets produced from whole field latex
LRP Large rubber particles
NSR
SIR
SLR
SMR
SRP
SSR
TSR
TTR
Nigerian Standard Rubber
Standard Indonesian Rubber
Standard Lanka Rubber
Standard Malaysian Rubber
Serum rubber particles
Standard Singapore Rubber
Technically specified Rubber
Thai Tested Rubber
study characterizing these materials to which further reference should be made. Lipids can also affect the mechanical stability of the latex while it is in storage, because lipids are a major component of the membrane formed around the rubber particle [7]. NR latex is typically coagulated, washed, and then dried in either the open air or a “smokehouse.” The processed material consists of 93% rubber hydrocarbon; 0.5% moisture; 3% acetone-extractable materials such as sterols, esters, and fatty acids; 3% proteins; and 0.5% ash. Raw NR gel can range from 5% to as high as 30%, which in turn can create processing problems in tire or industrial products factories. Nitrogen content is typically in the range of 0.3%–0.6%. For clarity, a number of definitions are given in Table 1.1.
The rubber from a tapped tree is collected in three forms: latex, cup-lump, and lace. It is collected as follows:
1. Latex collected in cups is coagulated with formic acid, crumbed, or sheeted. The sheeted coagulum can be immediately crumbed, aged and then crumbed, or smoke-dried at around 60°C to produce typically ribbed smoked sheet (RSS) rubber.
2. Cup-lump is produced when the latex is left uncollected and allowed to coagulate, due to bacterial action, on the side of the collecting cup. Field coagulum or cup-lump is eventually collected, cut, cleaned, creped, and crumbed. Crumb rubber can be dried at temperatures up to 100°C.
3. Lace is the coagulated residue left around the bark of the tree where the cut has been made for tapping. The formation of lace reseals the latex vessels and stops the flow of rubber latex. It is normally processed with cup-lump.
The processing factories receive NR in one of two forms: field coagula or field latex. Field coagula consist of cup-lump and tree lace ( Table 1.1). The lower grades of material are prepared from cup-lump, partially dried small holders of rubber, rubber tree lace, and earth scrap after cleaning. Iron-free water is necessary to minimize rubber oxidation. Field coagula and latex are the base raw materials for the broad range of natural grades described in this review. Fresh Hevea latex has a pH of 6.5–7.0 and a density of 0.98 [3,4]. The traditional preservative is ammonia, which in concentrated solution is added in small quantities to the latex collected from the cup. Tetramethylthiuram disulfide and zinc oxide are also used as preservatives because of their greater effectiveness as bactericides. Most latex concentrates are produced to meet the International Organization for Standardization (ISO) 2004 [8]. This standard defines the minimum content for total solids, dry rubber content, nonrubber solids, and alkalinity (as ammonia, NH3).
b. ProduCtioN of NAtur Al rubber
Total global rubber consumption in 2014 was approximately 27.3 million metric tons of which 11.8 million tons (43%) was NR and the remaining was synthetic rubber [9]. The world production of NR was up by nearly 14% from the same period in 2010 as economies improve their performance from the low points in 2008 and 2009. The major regional consumers of NR have undergone a major shift from the period 2000 to 2004 with China being the largest followed by the European Union, North America, India, and Japan. It is also anticipated that western European and Japanese consumption will increase due to economic growth in both areas, with sustained economic expansion in the United States; the net impact will be further growth in consumption toward 14.0 million tons/year. Further, NR consumption will then increase slowly, this being dependent on global economic conditions ( Figure 1.3). Globally, NR consumption is split—tires consuming around 75%, automotive mechanical goods at 5%, nonautomotive mechanical goods at 5%–10%, and miscellaneous applications such as medical and health-related products consuming the remaining 5%–10% [10].
There are around 25 million acres planted with rubber trees, and production employs nearly 3 million workers, with the majority coming from smallholdings in order: Thailand, Indonesia, Malaysia, India, Vietnam, China, and West Africa. Many times, the dominance of smallholdings has raised issues regarding quality and consistency, which will be discussed later. Smallholdings produce mainly cup-lump, which is used in block rubber. Sheet rubber is generally regarded to be of higher quality, typically displaying higher tensile and tear strength.
In 1964, the ISO published a set of draft technical specifications that defined contamination, wrapping, and bale weights and dimensions, with the objectives of improving rubber quality, uniformity, and consistency and developing additional uses for contaminated material [11,12].
The three sources leading to crumb rubber (i.e., unsmoked sheet rubber, aged sheet rubber, and field cup-lump) typically provide different grades of TSRs. For example, one grade of TSR (L) is produced from coagulated field latex, TSR 5
Tons × 000
is produced from unsmoked sheets, and lower grades such as TSR 10 and 20 are produced from field coagulum. A simplified schematic of the production process is presented in Figure 1.4
C. NAtur Al rubber ProduCts ANd Gr Ades
NR is available in the following six basic forms:
1. Sheets
2. Crepes
3. Sheet rubber, technically specified
4. Block rubber, technically specified
5. Preserved latex concentrates
6. Specialty rubbers that have been mechanically or chemically modified
Among these six types, the first four are in a dry form and represent over 90% of the total NR produced in the world. In the commercial market, these four types of dry NR are available in over 40 grades, consisting of RSS; air-dried sheets (ADS); crepes, which include latex-based and field coagulum-derived estate brown crepes and remilled crepes; and TSR in block form. Among the four major types, crepes are now of minor significance in the world market, accounting for less than 75,000 tons/year. Field coagulum grade block rubbers have essentially replaced brown crepes except in India. Only Sri Lanka and India continue to produce latex crepes. Figure 1.4 presents a simplified schematic of the process followed in the production of NR.
FIGURE 1.3 Global natural rubber productions (millions of tons).
Cup lump and tree lace Brown crepes
Natural coagulation Field grades
SMR 10, SMR 20
Baling
Transportation
Coagulation
1. Latex
2. Cup lump and lace
3. Blends
1. Sheet Rubber
Baling
75 lb bales
NR in sheet form is the oldest and most popular type. Being the simplest and easiest to produce on a small scale, smallholders’ rubber in most countries is processed and marketed as sheet rubber. From the end user’s perspective, two types of sheet rubbers are produced for the commercial market: RSS and ADS. Of the two, RSS is the most popular due to its mechanical properties and high tensile strength. RSS rubbers are made from intentionally coagulated whole field latex. They are classified by visual evaluation. To establish acceptable grades for commercial purposes, the International Rubber Quality and Packing Conference prepared a description for grading, and the details are given in The Green Book [13]. Whole field latex used to produce RSS is first diluted to 15% solids and then coagulated for around 16 h with dilute formic acid. The coagulated material is then milled, the water is removed, and the material is sheeted with a rough surface to facilitate drying. Sheets are then suspended on poles for drying in a smokehouse for 2–4 days. Only deliberately coagulated rubber latex processed into rubber sheets, properly dried, and smoked can be used in making RSS. A number of prohibitions are also applicable to the RSS grades. Wet, bleached, undercured, and original rubber and rubber that is not completely visually dry at the time of the buyer’s inspection are not acceptable (except slightly undercured rubber as specified for RSS 5). Skim rubber made
Lace Washing
FIGURE 1.4 Simplified schematic of the natural rubber production process.
TABLE 1.2
Grade Classification of Ribbed Smoked Sheet Rubber
1X
1
2 Slight Slight No No No No No sand or foreign matter
3 Slight Slight Slight No No No No sand or foreign matter
4 Slight Slight Slight Slight No No No sand or foreign matter
5 Slight Slight Slight Slight N/A
of skim latex cannot be used in whole or in part in patches as required under packing specifications defined in The Green Book. Prior to grading RSS, the sheets are separated and inspected, and any blemishes are removed by manually cutting and removing defective material. Table 1.2 provides a summary of the criteria followed by inspectors in grading RSS. The darker the rubber, the lower the grade. The premium grade is RSS 1, and the lower quality grade is typically RSS 4. ADS are prepared under conditions very similar to those for smoked sheets but are dried in a shed without smoke or additives, with the exception of sodium bisulfate. Such rubber therefore lacks the antioxidation protection afforded by drying the rubber in a smokehouse. This material can be substituted for RSS 1 or RSS 2 grades in various applications.
2. Crepe Rubber
Crepe is a crinkled lace rubber obtained when coagulated latex is selected from clones that have a low carotene content. Sodium bisulfite is also added to maintain color and prevent darkening. After straining, the latex is passed several times through heavy rolls called creepers, and the resultant material is air-dried at ambient temperature. There are different types of crepe rubber depending upon the type of starting materials from which they are produced. Sri Lanka is the largest producer of pale crepes and the sole producer of thick pale crepe.
The specifications for the different types of crepe rubbers for which grade descriptions are given in The Green Book are as follows:
1. Pale latex crepes: Pale crepe is used for light-colored products and therefore commands a premium price. Trees or clones from which the grade is obtained typically have low yellow pigment levels (carotenes) and greater resistance to oxidation and discoloration. There are eight grades
in this category. All these grades must be produced from the fresh coagula of natural liquid latex under conditions where all processes are quality controlled. The rubber is milled to produce both thin and thick crepes. Pale crepes are used in pharmaceutical appliances such as stoppers and adhesives (Table 1.3).
2. Estate brown crepes: There are six grades in this category. All six grades are made from cup-lump and other higher grade rubber scrap (field coagulum) generated on the rubber estates. Tree bark scrap, if used, must be precleaned to separate the rubber from the bark. Powerwash mills are to be used in milling these grades into both thick and thin brown crepes (Table 1.4).
3. Thin brown crepes (remills): There are four grades in this class or category. These grades are manufactured on powerwash mills from wet slab unsmoked sheets at the estates or smallholdings. Tree bark scrap, if used, must be precleaned to separate the rubber from the bark. Inclusion of earth scrap and smoked scrap is not permissible in these grades ( Table 1.5).
4. Thick blanket crepes (ambers): The three grades in this category are also produced on powerwash mills from wet slab unsmoked sheets, lump, and other high-grade scrap ( Table 1.5).
TABLE 1.3 White and Pale Crepes
TABLE 1.4
Estate Brown Crepes
1X
1X
2X
2X
3X
3X
brown crepe
brown crepe
crepe
brown crepe
crepe
crepe
Discoloration
TABLE 1.5 Compo, Thin Brown, Thick Blanket, Flat Bark, Pure Smoked Blanket Crepe
Discoloration
5. Flat bark crepes: The two grades of rubber in this category are produced on powerwash mills out of all types of scrap NR in uncompounded form, including earth scrap ( Table 1.5).
6. Pure smoked blanket crepe: This grade is made by milling on powerwash mills smoked rubber derived from RSS (including block sheets) or RSS cuttings. No other type of rubber can be used. Rubber of this type must be dry, clean, firm, and tough and must also retain an easily detectable smoked sheet odor. Sludge, oil spots, heat spots, sand, dirty packing, and foreign matter are not permissible. Color variation from brown to very-dark brown is permissible (Table 1.5).
3. Technical Classification of Visually Inspected Rubbers
The Malaysian Rubber Producers’ Research Association has published a technical information sheet describing sheet rubbers that have been further tested and classified with respect to cure characteristics [14]. The cure or vulcanization classes are distinguished by a color coding (i.e., blue for fast cure, yellow for medium cure, and red for slow cure) (Table 1.6) when the rubber is compounded using the American Society for Testing and Materials (ASTM) No. 1A formulation [15]. This color coding is limited to RSS 1 and ADS. Upon cure classification, the rubbers are further tested, and at 0.49 MPa, the strain on the sample is measured after 1 min. This classification scheme has not received wide acceptance, which is clearly unfortunate, for a more quantitative classification scheme is required for visually inspected grades of NR. For example, rubber meeting a specific visually determined grade or classification might display poor mechanical properties when compounded with carbon black and vulcanizing agents owing to a broad or lower-molecular-weight distribution. This may in turn create factory processing difficulties and product performance deficiencies.
4. TSR
The ISO first published a technical specification (ISO 2000) for NR in 1964 [11]. Based on these specifications, Malaysia introduced a National Standard Malaysian Rubber (SMR) scheme in 1965, and since then, all the NR-producing countries have started production and marketing of TSRs based on the ISO 2000 scheme. TSRs are shipped in “blocks,” which are generally 33.3 kg bales in the international market and 25.0 kg in India. All the block rubbers are also guaranteed to conform to certain technical specifications, as defined by the national schemes or by ISO 2000 (Table 1.7).
TABLE 1.6
Technical Certification of Sheet Rubber
TABLE 1.7
TSRs Defined in ISO 2000
The nomenclature describing technically specified rubbers consists of a three- or four-letter country code followed by a numeral indicating the maximum permissible dirt content for that grade expressed as hundredths of 1%. In Malaysia, the TSR is designated as SMR. In Indonesia, the designation given is Standard Indonesian Rubber (SIR). In Thailand, the TSRs are called Standard Thai Rubber (STR, sometimes denoted as TTR). In India, the TSRs are designated as Indian standard NR. Grading is based on the dirt content measured as a weight percent. Dirt is considered to be the residue remaining when the rubber is dissolved in a solvent, washed through a 45 μm sieve, and dried.
TSR accounts for approximately 60% of the NR produced worldwide. The advantages claimed for the TSRs over the conventional sheet and crepe grades of rubbers are the following:
1. They are available in a limited number of well-defined grades, intended to ensure a uniform, defined quality.
2. Data on the content of foreign and volatile matter can be provided, again to ensure better uniformity.
3. They are shipped as compact, polyethylene-wrapped bales of standard weight.
4. They can be prepared to prevent degradation of the rubber during storage, handling, and transportation.
5. They have a standard bale size to enable ease of transport through mechanized handling and containerization.
ISO has specified six grades of TSR. The detailed characteristics of the different grades of TSR are discussed next.
TSR CV: TSR CV, the CV designating “constant viscosity,” is produced from field latex and is stabilized to a specified Mooney viscosity. The storage hardening of this grade of rubber must be within 8 hardness units. It is shipped in a 1.2 ton pallet, which facilitates handling, transportation, and storage space utilization. Each pallet
consists of 36 bales of 33.3 kg net weight, and each bale is wrapped in a polyethylene bag that is dispersible and compatible with rubber when mixed in an internal mixer at temperatures exceeding 110°C, which are, of course, typical in any rubber-mixing facility. TSR CV rubber is generally softer than conventional technically specified grades. Coupled with its CV feature, it can provide a cost advantage by eliminating premastication. When used in open mills, the rubber forms a coherent band almost instantaneously, thus potentially improving milling throughput. Additional claimed benefits of TSR CV include
1. Reduction of mixing times, giving higher throughput
2. Reduction of scraps and rejects due to better material uniformity
3. Better resistance to chipping and chunking for off-the-road (OTR) tires
4. Better green strength
TSR CV rubber is available in different viscosities, with 50 and 60 being the more common. This material can be used for high-quality products such as mechanical mountings for engines and machinery, railway buffers, bridge bearings, vehicle suspension systems and general automotive components, large-truck tire treads, conveyor belt covers, cushion gum for retreading, masking tapes, injection-molded products including rubber–metal bonded components, industrial rolls, inner tubes, and cement.
TSR L: TSR L is a light-colored rubber produced from high-quality latex; it has low ash and dirt content and is packed and presented in the same way as TSR CV. The advantage of TSR L is its light color together with its cleanliness and better heataging resistance. Technologically, TSR L shows high tensile strength, modulus, and ultimate elongation at break for both black and nonblack mix.
This material can be used for light-colored and transparent products such as surgical or pressure-sensitive tape, textiles, rubber bands, hot-water bottles, surgical and pharmaceutical products, large industrial rollers for the paper printing industry, sportswear, bicycle tubes, chewing gum, cable sheaths, gaskets, and adhesive solutions and tapes.
TSR 5: TSR 5 is produced from fresh coagulum, RSS, or ADS. It is packed and shipped to the same specifications as TSR CV and TSR L. TSR 5 is typically used for general-purpose (GP) friction and extruded products; small components in passenger vehicles such as mountings, sealing rings, cushion gum, and brake seals; bridge bearings; ebonite battery plates; separators; adhesives; and certain components in tires.
TSR 10: TSR 10 is produced from clean and fresh field coagulum or from unsmoked sheets. It is packed and shipped in the same way as TSR CV, TSR L, and TSR 5. TSR 10 has good technological properties similar to those of RSS 2 and RSS 3, but has an advantage over RSS because of its
1. Lower viscosity
2. Easier mixing characteristics (more rapid breakdown)
3. Technical specifications and packaging in 33.3 kg bales
It can be used for tires, inner tubes, cushion gum stocks, joint rings by injection molding, raincoats, microcellular sheets, upholstery and packing, conveyor belts, and footwear.
TSR 20: This is a large-volume grade of technically specified NR. It is produced mostly from field coagulum, lower grades of RSS, and unsmoked sheets. It is packed and shipped to the same specifications as TSR CV, TSR L, TSR 5, and TSR 10. TSR 20 has good processing characteristics and physical properties. Its low viscosity and easier mixing characteristics (compared with the RSS grades) can reduce the mastication and mixing period considerably. It is used mostly for tires, cushion gum stock, bicycle tires, raincoats, microcellular sheet for upholstery and packing, conveyor belts, footwear, and other general products.
TSR 50: This is the lowest grade of TSR and is produced from old, dry field coagulum or partly degraded rubber. It is packed and shipped in the same way as other grades of TSR. It should be noted that these specifications will continue to be improved as production methods improve. For example, in 1991, the Rubber Research Institute of Malaysia revised the dirt levels of SMR CV60, CV50, and L from 0.05 to 0.025, that of SMR 10 from 0.10 to 0.08, and that of SMR 20 to 0.016.
In addition, Malaysia has produced grades of rubber outside the specific scope of ISO 2000. SMR GP is a standard GP rubber made from a 60:40 mixture of latex-grade sheet rubber and field coagulum. It is viscosity stabilized at 65 Mooney units using hydroxylamine neutral sulfate (HNS). It is similar to SMR 10 in specification.
To illustrate the distribution and consumption of these various grades, shipments of SMR from Malaysia are typically SMR 20, 60%; SMR 10, 27%; SMR CV and SMR L, 5%; SMR GP, 7%; and SMR 5, 1.0%.
VisCosity ANd VisCosity stAbilizAtioN of NAtur Al rubber
The properties of NR that are most important regarding its use in the manufacture of tires or other products include viscosity, fatty acid bloom, and compliance with the technical specifications. Of these three parameters, viscosity is probably the most important. This property relates to the molecular weight, molecular-weight distribution, and amounts of other materials present in the polymer such as low-molecularweight resins, fatty acids, and other natural products. It affects the initial mixing of the rubber with other compounding ingredients and subsequent processing of the compounded materials to form the final manufactured product.
NR viscosity is a function of two major factors: viscosity of the rubber produced by the specific clone and the viscosity stabilization method. A range of methods are available to characterize the viscosity of NR. The most popular is Mooney viscosity (Vr), which is obtained by measuring the torque that is required to rotate a disk embedded in rubber or a compounded sample. This procedure is defined in ASTM D 1646, “Standard Test Methods for Rubber—Viscosity, Stress Relaxation, and Prevulcanization Characteristics (Mooney Viscometer)” [16]. The viscosity will typically range from 45 to over 100. The information obtained from a Mooney viscometer can include the following:
1. Prevulcanization properties or scorch resistance for the compounded polymer, a test that is conducted at temperatures ranging from 120°C to 135°C ( Figure 1.5).
d.
2. Mooney peak, which is the initial peak viscosity at the start of the test and a function of the green strength and can be a measure of compound factory shelf life.
3. Viscosity (Vr), typically measured at 100°C, provides a measure of the ease with which the material can be processed (Figure 1.6). It depends on molecular weight and molecular-weight distribution, molecular structure such as stereochemistry and polymer chain branching, and nonrubber constituents. Caution is always required when attempting to establish relationships between Mooney viscosity and molecular weight. Mooney viscosity can be expressed as ML 1 + 4 (i.e., Mooney large rotor, with 1 min pause and 4 min test duration).
4. Stress relaxation, which can provide information on gel (T-95), is defined as the response to a cessation of sudden deformation when the rotor of the Mooney viscometer stops. The stress relaxation of rubber is a combination
1.6 Mooney viscosity and stress relaxation.
FIGURE 1.5 Mooney scorch typically conducted at 121°C and 135°C.
FIGURE
of both elastic and viscous response. A slow rate of relaxation indicates a higher elastic component in the overall response, whereas a rapid rate of relaxation indicates a more highly viscous component. The rate of stress relaxation can correlate with molecular structural characteristics such as molecular-weight distribution, chain branching, and gel content. It can be used to give an indication of polydispersity or Mn /Mw. It is determined by measuring the time for a 95% (T-95) decay of the torque at the conclusion of the viscosity test.
5. Delta Mooney, typically run at 100°C, is the final viscosity after 15 min. This provides another measure of the processing characteristics of the rubber. It indicates the ease of processing compounds that are milled before being extruded or calendered (e.g., hot feed extrusion systems).
Much work has been done to establish a relationship between the Mooney viscosity (ML) and molecular weight of NR as well as the molecular-weight distribution. Bonfils et al. [17] measured the molecular weight and molecular-weight distribution of a number of samples of rubber from a variety of clones of H. brasiliensis and noted the following trend:
where P0 is initial Wallace plasticity, ML 1 + 4 is Mooney viscosity after 4 min, and Mw is molecular weight.
Though clearly not linear, there is an empirical relationship between Mooney viscosity and molecular weight. Nair [18,19] explored this, established a relationship between intrinsic viscosity and Mooney viscosity, and determined a correlation coefficient of 0.87. This correlation can be improved by mastication of the test samples, which improves the homogeneity. Mastication or milling also narrows the molecular-weight distribution, which is an important factor in this respect [20].
The cure characteristics of NR are highly variable due to such factors as maturation of the specific trees from which the material was extracted, method of coagulation, pH of the coagulant, preservatives used, dry rubber content, and viscosity stabilization agent.
A standardized formulation has been developed to enable a comparative assessment of different NRs; it is known as the American Chemical Society No. 1. The formulation consists of NR (100 phr), stearic acid (0.5 phr), zinc oxide (6.0 phr), sulfur (3.5 phr), and 2-mercaptobenzothiazole (0.5 phr). This formulation is very sensitive to the presence of contaminants or other materials such as fatty acids, amines, and amino acids, which may influence the vulcanization rate.
NR is susceptible to oxidation. This can affect both the processing qualities of the rubber and the mechanical properties of the final compounded rubber. Natural antioxidants will offer protection from the degradation of NR, which can
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