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Adhesion and Adhesives: Fundamental and Applied Aspects
The topics to be covered include, but not limited to, basic and theoretical aspects of adhesion; modeling of adhesion phenomena; mechanisms of adhesion; surface and interfacial analysis and characterization; unraveling of events at interfaces; characterization of interphases; adhesion of thin films and coatings; adhesion aspects in reinforced composites; formation, characterization and durability of adhesive joints; surface preparation methods; polymer surface modification; biological adhesion; particle adhesion; adhesion of metallized plastics; adhesion of diamond-like films; adhesion promoters; contact angle, wettability and adhesion; superhydrophobicity and superhydrophilicity. With regards to adhesives, the Series will include, but not limited to, green adhesives; novel and high-performance adhesives; and medical adhesive applications.
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Library of Congress Cataloging-in-Publication Data
ISBN 978-1-119-76889-0
Cover image: Pixabay.Com
Cover design by Russell Richardson
Set in size of 11pt and Minion Pro by Manila Typesetting Company, Makati, Philippines
To
K. Malathi and N. Kotnees
Asmit Bhowmick
Our students and mentors
Kundakali Bhowmick
2.8
2.5.3
2.6.3
2.7.2.1
2.10
2.9.3
2.10.4.4
2.11.6.1
2.12
3.2.3
3.2.5
3.2.3.1
3.2.8
3.2.7.2
3.2.8.1
5
4.5
6.5
7.2
7.3
Foreword
My expertise is in the area of Civil engineering and administration. Rubber is used in many Civil engineering applications including seismic and vibration isolation, bridge bearing pads, airport runways, anti-cracking roads, expansion joints in buildings, etc. The importance of tire and many rubber products is overwhelmingly accepted in modern civilization. Across various streams of science and engineering, rubbers are usually acknowledged as strategic materials which can be customised by adding different compounding ingredients to address the needs of various industrial applications. The ability of joining or bonding different layers of rubbers is a pre-requisite for the successful performance of many composite engineering products like tyres, belting, hoses etc. Unsuccessful adhesion of rubbers in contact presents a number of problems. This also means that overcoming a particular problem related to uncured rubber adhesion cannot be regarded as a complete solution for all the problems associated with adhesion between cured rubber layers.
These fascinating problems have prompted many scientists and technologists to explore this subject and come up with a clear understanding and new findings. I have been told that a number of theories and mechanisms have been proposed for understanding adhesion strength between rubber layers. Owing to their vast experience and expertise in the areas of adhesion science and technology, the authors of this book have made strenuous efforts to carefully review and analyse all the traditional and modern aspects of rubber to rubber adhesion. This would also help the readers to achieve a deeper understanding of the subject and bring all the necessary concepts together in a coherent manner.
The College of Engineering, University of Houston honours the magnitude and diversity of various disciplines and subjects, which provide both depth and breadth of learning to our students and a platform for exchange of knowledge and communication. Diversity makes us all stronger and better. Particularly, the Department of Chemical and Biomolecular Engineering is unique in our system with current expansion of the polymer program.
Houston is a powerhouse of many industries, and provides many opportunities for innovation and start-ups. Our mission is to align our work and dream in line with the local as well as global needs. Our future depends on innovation, collaboration and farsightedness. An adequate adhesion is required between our thoughts and execution, present and future, current work and needs of the society. This book very well fits the objectives.
This book is indeed a one-stop shop which covers all the theoretical and practical aspects of rubber to rubber adhesion. I hope that this book will be of immense value and suffice to the needs of both, academicians and industry experts. The content of this book is not only comprehensive and coherent but also introduces the readers to some of the contemporary approaches (like self-healing elastomers), nascent ideas, results, and many open and complex questions in this ever growing and fascinating field of rubber to rubber adhesion.
Prof. Joseph W. Tedesco, Ph.D., P.E. Dean of the Cullen College of Engineering Professor of Civil and Environmental Engineering
UH Department of Civil and Environmental Engineering Houston, TX
Preface
The world is moving very fast. Big phones in our residences and offices have been gradually replaced by small smart phones. All the information is on our personal computer. Yet, a few things continue with their legacy. Tire is one such item, first developed in 1920, is still an important item for modern civilization with a lot of changes in its technology from the original concepts. Rubber to rubber adhesion which is used in tires and a host of other rubber products holds its importance ever than before because of critical and stringent requirements, and the advent of modern technology. Many things are yet to be understood. Lots of research are going on, especially in companies. Academicians contribute to the development of theories and understanding. Unfortunately, literature is not available in one single cover. This is the justification of bringing out this book.
This book starts with an introduction on rubber, then characterization of rubber, rubber surface and joints, and finally covers other chapters on rubber to rubber adhesion. The following topics are included in the book: Introduction to Rubber, Important Physical Properties for Understanding Rubber Adhesion and Measurements of Rubber Adhesion, Adhesion between Unvulcanized Elastomers, Self-healing of Elastomers, Adhesion between Compounded Elastomers by co-crosslinking, Adhesion between partially Vulcanized Compounded Rubber and partially Vulcanized Compounded Rubber, Adhesion between Vulcanized Rubber and Unvulcanized Rubber- or partially Vulcanized Rubber, and Adhesion between Vulcanized Rubber and Vulcanized Rubber.
Over the last fifty years, many researchers including Nobel Laureate Professor Pierre-Gilles de Gennes have contributed to the topic of rubber to rubber adhesion. We would like to record our indebtedness to their seminal work. Students all over the world have also worked incessantly to strengthen the knowledge on the subject. This is an opportunity to thank our mentors, especially Professor Alan N. Gent, at the University of Akron, Akron Ohio, and Professor Derek Aubrey, London School of Polymer Technology, London, UK, who taught one of us (AKB) the subject of
rubber adhesion and exposed the horizon. At the last stage of the work, our students Subhabrata, Pranabesh, Preetom, Shib Shankar, Surya, Amrita, Riya, Saikat, Michael, Kapil, Harekrishna, and Sreenath helped us with some Figures and Tables. Special thanks go to Dr. Ganesh Basak and Dr. Satyanarayana. The authors also gratefully acknowledge the cooperation from various publishers for giving permission to reproduce the Figures and the Tables from the published literature. Our student, Miss Surya Parameswaran helped us in getting permission from different publishers.
Professor Bhowmick is grateful to the University of Houston, especially to Professor Renu Khator, Chancellor and President, Professor Joe Tedesco, Dean, Cullen College of Engineering, Professor Michael P. Harold, Former Department Chair, Professor T.J.(Lakis) Mountziaris, Present Department Chair, Department of Chemical & Biomolecular Engineering, Professor A. Karim, Professor R. Krishnamoorti and Professor M. Robertson, all from the Department of Chemical & Biomolecular Engineering for their support and cooperation.
Last but not the least, we would like to thank Ms. K. Malathi, Mr. N. Kotnees, Dr. Kundakali Bhowmick, and Dr. Asmit Bhowmick for their sacrifice as well as Mr. Martin Scrivener of Scrivener Publishing, MA for publication of the book and our earlier work in the area of rubber adhesion.
Dinesh K. Kotnees
Indian Institute of Technology Patna, India
Anil K. Bhowmick
The University of Houston, USA Formerly with IIT Kharagpur, India
Introduction to Rubber
1.1 History
Rubber is a soft and unique engineering material, which is used in a large number of critical applications. It is a constituent of tire that drives the modern civilization. However, the rubber obtained from nature is an old material. Its history dates back to 1493, when rubber was known only to the natives of South and Central America. The latex exuded by cutting the trunks was used to make balls for games, dolls, boots, containers, and many other items. During the period, 1493–1785, there was a scientific curiosity about this material [1, 2]. Rubber sample was brought to Europe for study and its properties became known for its possible end-use. Columbus reported after the return from his voyage to the Caribbean that the natives played with elastic balls. It was Joseph Priestley, who christened the material with the designation “rubber”, as it wiped the marks of a black lead pencil. In around 1751, De la Condamine reported rubber trees in French Guiana and the French called the new substance, “caoutchouc,” meaning “weeping tree”. The beginning of the rubber industry started during 1785-1845. The first rubber factory was started by Thomas Hancock in England in 1820. By this time, the general properties and uses of rubber were known. Raincoats, rubber balloons, thread, erasers, inflatable goods, etc. were manufactured during this period. Few rubber companies were founded in Europe. The period, 1830-1910, saw the growth of the usage of rubber. By this time, in 1839, the process of vulcanization was discovered by Charles Goodyear [3], which improved the properties significantly and reduced degradation. The shape also could be retained. The discovery was of great value to the industry. By 1920, the plantation sector met approximately 90% of the world’s demand. Pneumatic tires - various types and designs, were developed during the period, 1910 till date. A very rapid expansion of the industry took place. The pneumatic tire was invented independently by J.B. Dunlop in 1888 and the present day car tire was established by 1920. However,
natural rubber could not be grown in Europe and North America. There was a constant effort to develop synthetic rubber, especially after the World war 1. A large number of synthetic rubbers were developed since 1930 [4]. Gradually their nature, quality, and specification improved. Although various plants exude rubbery substance, Hevea brasiliensis (“Hheve tree” as called by the natives) is the only tree which is tapped extensively to obtain natural rubber commercially. The rubber hydrocarbon appears as a milky white fluid, commonly known as NR latex, in the trunk of Hevea Brasiliensis tree. It contains typically 30-35 % solid content in the field latex which varies from clone to clone. Concentration of the latex increases the rubber hydrocarbon content to 60 % in the latex. Apart from the rubber hydrocarbon, NR latex contains various non-rubber constituents like proteins, lipids, phospholipids, fatty acids, amines, and some inorganic constituents. In recent times, due to the shortage of natural rubber, other plants are also being tried. Unlike Hevea, the extraction technology is more complicated for guayule (Parthenium argentatum). Russian dandelion (Taraxacum kok-saghyz) is a species of dandelion native to
Figure 1.1 Various natural rubber plants (a) Hevea (b) Russian dandelion (c) Jelutong (d) Goldenrod (e) Guayule (Reprinted from [5] with the kind permission of Wiley).
Kazakhstan of the former Soviet Union. Figure 1.1 shows the photograph of the plants that have been exploited by human mankind [5].
1.2 What is a Rubber?
Rubbers belong to a class of materials known as polymers, which also include plastics, resins, and fibers. Polymers consist of molecules of high individual molecular weights which may vary from a few thousand up to the tens of millions. The molecules of these substances are very large compared to water, salt, ammonia, etc. The molecules of each of these consist of a sequence of one or more basic units (called monomers) linked together in chains or networks of covalent bonds. The structure of some polymers is given in Figure 1.2.
What is the difference between rubber and other polymers? There are three characteristic properties of rubber: The rubber has very high molecular weight, like many polymers. This can be as high as 106 Dalton unit. Rubber has very low interchain interaction, much lower than fibers and plastics. The most important point is that a rubber has a glass transition temperature which is lower than the ambient temperature. The glass transition temperature is a temperature below which a rubber becomes glassy, whereas above this temperature, it becomes rubbery in nature. Table 1.1 shows the glass transition temperature of a few representative polymers which demonstrate the lowest T g that could be obtained with a rubber [6]. The glass transition temperature of natural rubber is -70 °C, that of styrene
1.2
Figure
Structure of some common polymers (a) Polyethylene (b) Polyvinyl chloride (c) Nylon 66 (d) cis 1, 4 Polyisoprene and (e) Polyurethane.
4 Rubber to Rubber Adhesion
Table 1.1 Glass transition temperature of various polymers [6].
Polymer
Polyethylene
Polypropylene isotactic
Poly[cis-isoprene]/Natural rubber
Styrene-butadiene rubber
Poly-1-octene, isotactic
Poly-1-pentene, isotactic
Poly-1-butene, isotactic
Poly(4-methyl-1-pentene)
Polymethyl methacrylate, isotactic
Polymethyl methacrylate, syndiotactic
butadiene rubber is -50 °C, etc., whereas the glass transition temperature of a plastic, polyvinyl chloride is 80 °C. This also indicates that a rubber does have useful application in the temperature range. Rubbers at room temperature without stretching are also mostly amorphous in nature. In the case of nylon fiber, the polyamide chains can exert powerful attractive forces towards each other. During cold drawing, these chains orient and are strong enough to cause the oriented chains to elongated crystals containing rigid chains, thus raising their glass transition temperature. The glass transition temperature or the interchain forces could be changed by
the incorporation of special ingredients – or by chemical modification. For example, polyvinyl chloride, a plastic, could be made rubbery by the addition of a plasticizer, which reduces its Tg below the ambient temperature.
1.3 What is the Structure of Rubber?
To ordinary people, a rubber means the material, which can deform and stretch and snap back to its original length on releasing the force. However, to specialists, there are more than hundred different rubbers varying in structure, molecular weight, sequence of monomer chain, etc. Some are polar and some are non-polar. Some are stronger than others. Some are more thermally stable than the others. Table 1.2 gives the structure of some common rubbers. One can see that the rubbers can be made from similar monomers (homopolymer) or two or three monomers (copolymer or terpolymer). For a few structures, polar monomers are used making the rubber polar in nature and resistant to oil. Higher the polarity, higher will be oil resistance and higher will be Tg. Although we write the structure in a simple way, it is much more complicated than that shown in the Table. Any structure has to accommodate bond angle, bond distance, and associated groups of atom on the side chain. Recently, molecular dynamics simulation has been done on various rubber chains [7]. Figure 1.3 shows the results for natural rubber. It has been demonstrated how chain entanglements take place beyond certain chains.
In the case of natural rubber, the connection of repeat units is shown in Figure 1.4a and the peak 1 is attributed to the C=C double bond and the peak 2 is for the C-C single bond (Figure 1.4b). The peak 3 represents the distance of the second adjacent pair, C1-C3 distance. As the isoprene repeat units are cis in nature here, the third adjacent pair, is C1-C4 of the main chains as depicted by Peak 4. Side methyl groups in trans position give the peak 5 at a higher distance than cis C1-C4. The bond length of different adjacent pairs is given in Table 1.3. In order to reduce the steric hindrance of the bulky methyl group, the angle C1-C2-Cmethyl increased to 125 degrees as compared to the angle for conventional sp2 hybridization. The chains of natural rubber are shown inside a box and individual atoms could be visualized (Figure 1.5). The glass transition temperature could be calculated from this simulation and the theoretical value (intersection point, Figure 1.5b) is shown. The configuration and arrangement of atoms (given by different colors) for nitrile rubber (NBR) and fluorocarbon rubber (FKM) are also shown in Figure 1.6. Table 1.3 gives the calculated bond length of various adjacent pairs of different rubbers.
6 Rubber to Rubber Adhesion
Table 1.2 Structure of some common rubbers.
Polybutadiene rubber (BR)
Natural rubber (NR)
Acrylonitrile butadiene rubber or Nitrile rubber (NBR)
Styrene butadiene rubber (SBR)
Isobutylene-isoprene rubber (IIR) or Butyl rubber
Ethylene propylene diene monomer rubber (EPDM)
Polychloroprene rubber or Neoprene rubber
Thiokol rubber
Silicone rubber
Figure 1.3 Spatial distribution of NR Chains in free state at 300K with the number of repeat units (40, 100, 500, 1000, 1500, and 2000). Designation of atoms: Carbon: gray and Hydrogen: white (Reproduced from Reference [7] with the kind permission of Wiley).
Figure 1.4 Structure of the repeat units of NR at 300 K (a) connection of repeat units; (b) radial distribution of intramolecular carbon atoms; (c, c′) spatial disposition of the atoms in the repeat units. Designation of atoms—carbon: gray and hydrogen: white (Reproduced from Reference [7] with the kind permission of Wiley).
Bond length of different adjacent pairs in various rubbers [7].
Figure 1.5 (a) Amorphous unit cell of NR at 300 K and (b) change in specific volume of the amorphous cell with temperature. Designation of atoms—carbon: gray and hydrogen: white (Reproduced from Reference [7] with the kind permission of Wiley).
1.6 Amorphous unit cell of (a) Nitrile rubber and (b) Fluorocarbon rubber at 300 K (Designation of atoms- carbon: gray, hydrogen: white, nitrogen: deep blue, fluorine: sky blue) (Reproduced from Reference [7] with kind permission of Wiley).
1.4 Why is Rubber Chosen Over Other Materials?
The choice of rubber for a particular application over other materials frequently results from one or more of the many benefits as given below:
1. Low density
2. Low prices
3. Less energy-intensive fabrication
4. Amenable to high-speed production
Figure
5. Resistance to corrosion and related electrochemical action
6. Low thermal and electrical conductivity
7. Design flexibility
8. Wide range of properties
For example, the density of natural rubber is 920 kg/m3 as compared to 2600 kg/m3 of glass and 7860 kg/m3 of mild steel. No other material can match the elongation properties of rubber, often more than a thousand percent. It has a very high elongation at break. A filled rubber has superior abrasion resistance and hence is used in a tire. All rubbers have very high elasticity and are highly flexible. Rubbers give very good damping and isolation properties, just to name a few. In addition, it has the processing advantage, lower temperature processing compared to metals. It can be given any shape, has low energy-intensive fabrication and high-speed production.
1.5 Brief Outline of Preparation of Rubber
Although natural rubber can be isolated from plants, like Hevea Brasiliensis, other varieties of rubber-like styrene-butadiene rubber, nitrile rubbers, etc. are made synthetically. For example, SBR is made by emulsion polymerization as well as by solution polymerization. Polybutadiene is prepared by polymerization in emulsion or solution. Segmented polyester-polyether block copolymers are prepared by melt trans-esterification. A typical flowchart for polymer formation by solution polymerization is given below (Figure 1.7) [8]. There are, of course, many methods for polymerization:
Addition polymerization, condensation polymerization, emulsion polymerization, anionic polymerization, Zeigler-Natta polymerization, cationic polymerization, atom transfer radical polymerization, reversible addition−fragmentation chain-transfer polymerization, etc. Polymerization reaction can be homogenous or heterogeneous. It can be a gas, a liquid or a solid phase polymerization. All the techniques are not used for rubber synthesis. Each may be specific to a type of monomer. For example, isobutylene is polymerized to polyisobutylene by cationic polymerization only. Figure 1.8 shows the synthetic route for a new rubber by emulsion
RAW MATERIAL PREPARATION
SOLVENT CATALYST MONOMER
POLYMERIZATION VESSEL
CATALYST DEACTIVATOR
SURGE VESSEL
Introduction to Rubber 11
DEWATERING UNIT
SOLVENT (RECYCLED)
STABILIZER
SOLVENT STRIPPER STEAM
Figure 1.7 Flow diagram of Solution polymerization (Reference [8] with kind permission of the Rubber Division ACS).
polymerization technique. These are dependent on temperature, emulsifier, water to monomer ratio, nature of free radical generator, etc. [9].
A typical mechanism of block co-polymer formation using styrene and butadiene by anionic polymerization is given below (Figure 1.9):
12 Rubber to Rubber Adhesion
Persulfate initiated emulsion polymerization:
Emulsifier + DI water + buffer
250 rpm for 20 min
Stable micelle
β-myrcene added dropwise, 250 rpm for 20 min
Acidified ethanol
Vacuum dr y @ 50 °C, 24 h
Nitrogen, 70 °C
Stable emulsion
Aq. Solution of APS/20h
latex
Figure 1.8 (a) Flow diagram of emulsion polymerization and (b) reactant and products of emulsion polymerization of β-myrcene [9].
Polymyrcene
Figure 1.9 Anionic polymerization of Styrene –Butadiene –Styrene block copolymer.
1.6 Types of Rubber
Various rubbers are classified and represented by symbols as follows [10]:
M: Rubbers having a saturated chain of the polymethylene type, i.e. Terpolymer of ethylene, propylene, and a diene (EPDM), Polyacrylate (ACM), Chloropolyethylene (CM), Fluorocarbon Rubber (FKM), Chlorosulfonated polyethylene (CSM), etc.
N: Rubber having nitrogen, but not oxygen or phosphorus, in the polymer chain (Acrylonitrile butadiene rubber).
O: Rubber having oxygen in the polymer chain. Epichlorohydrin Rubber (CO/ECO).
