An Introduction to Rubber Technology

Andrew Ciesielski

Rapra Technology Limited

Preface

This book is written as an introduction to the subject of rubber technology, leaving the in depth specialization to other texts. “Introduction to Rubber Technology” is aimed at those people who simply wish to gain a basic overall understanding of this field. Thus the purchasing agent, engineer, polymer chemist, student of rubber technology, shop floor manager, and indeed the president and upper management, involved in the industry will want to read this book. Customers who use rubber in their products can obtain an understanding of those technical aspects with which they are unfamiliar from this book. A knowledge of the content of the eight chapters will also provide the reader with a communication tool for discussing the subject with a rubber specialist.

Rubber technology has a fascinating history from the jungles of Brazil to its designation as a strategic material during World War II. The primary material comes in many variations, most of which are of synthetic origin. All of these variations have their own special property combination to help them find a niche in the product marketplace. The rubber technologist then formulates a blend of this primary raw gum elastomer with other chemicals to produce rubber formulations, drawing from a potentially infinite variation of material combinations. Machines mix, extrude, calender and mold the blend of materials in this formulation known as the rubber compound. At this point the rubber laboratory has an armoury of tests related to the compound to ensure that the customer is satisfied with the final product. Rubber chemistry and physics are used to explain why rubber behaves as it does and helps push the boundaries surrounding its use. With a greater understanding of the special deformation and elastic properties of this unusual engineering material the engineer can find unique solutions to some of his applications. One of the many rubbery materials, urethane, is ideally suited to be cast as a liquid into molds to produce products which exploit its high strength. This book opens the door to all of these areas. I would like to thank the Holz Rubber Company for its support in my endeavor to produce this book, especially Mr. Ted Cooper for drawing many of the graphics. I would also like to express my appreciation to Mr. Robert Klingender of Zeon Chemicals Inc., for his helpful comments on the manuscript and to Mr. Nick Williams of The JR Clarkson Company for his contribution to the section on finite element analysis, and to  Mr. Koh Murai (of the same company) for his observations relating to the engineering

chapter. I would especially like to thank Dr. Alan Gent, Professor of Polymer Physics An Introduction to Rubber Technology

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and Polymer Engineering at The University of Akron, USA, for reviewing the chapter on Engineering. Thanks go to Uniroyal Chemical for a review of the urethane chapter and Mr. Wayne Cousins of Bayer for his useful comments on some of the elastomers mentioned in Chapter 2. A final thanks to Frances Powers, my technical editor for her insightful questions and enthusiasm during the editing of the manuscript.

Andrew Ciesielski

Holz Rubber Company

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1 History

“So rubber comes from trees, or maybe it was invented by Charles Goodyear?”

…wondered Andrew.

“Well, some types we get from a tree, and good old Charles made it more useful”

…answered Lucy.

1.1 Natural rubber

There is much discussion about natural products which may yet be discovered in the fast disappearing rain forests, indeed a significant portion of pharmaceutical organic chemicals attribute their origin to the vast diversity of plant life found there. Even though alchemy has been left far behind there is a long way to go before all the elegant pathways which nature uses to synthesize her molecules are discovered. In the meantime we often rely on her to do it for us. This book will focus on materials which owe their historical origin to a single chemical called polyisoprene, found in the sap of a tree named Hevea braziliensis (see Figure 1.1) found growing originally in the jungles of Brazil, and is also found in the milky latex of the humble milkweed (Asclepias spp.) and dandelion (Taraxacum spp.).

Polyisoprene, especially when chemically modified by vulcanization, has the remarkable ability to substantially return to its original shape after being stretched considerably. Any material which fulfills this requirement, is entitled to be called rubber. ASTM D 1566 gives a more detailed definition of rubber [1]. Polyisoprene extracted from Hevea braziliensis is called natural rubber (NR). This elastic property, as Suzuki points out, eventually led to a multi-billion dollar industry, and has affected the lives of the vast majority of the people on this planet [2]. Early ‘rubber technologists’ were found among the Aztecs and Mayas of South America, who used rubber for shoe soles, coated fabrics, and playballs, well over 2,000 years ago. An MRPRA (Malaysian Rubber Producers’ Research Association) article [3] mentions that the Aztec king, Montezuma was paid tribute by the lowland tribes in the form of 16,000 rubber balls, and that ball courts have been excavated in Snaketown in the southwestern United States dating back to AD 600-900.

An Introduction to Rubber Technology

Figure 1.1 Leaf and seed from the Hevea braziliensis tree

Subramaniam, in the late Maurice Morton’s book Rubber Technology [4], attributes Christopher Columbus as the first European to discover NR, in the early 1490s, when he found natives in Haiti playing ball with an extract from a tree. The book goes on to describe how, by the 18th century, uses for NR were well established in Europe, where the English chemist J. B. Priestley gave it its name since it ‘rubbed’ out pencil marks. Stern [5] mentions the Scotsman Macintosh who in 1823 used the solvent naphtha to

dissolve rubber and applied the resulting solution to textiles to produce rainproof clothing. Rubber at that time was supplied in hard blocks. Stern [5] notes that Thomas Hancock in London in 1830 used what can be described as the first internal mixing machine (see section 4.3), which mechanically worked the rubber, making it softer and therefore more easily processable. Stern mentions that Hancock moved on to two roll mills (see section 4.2), and that it took a hundred years before the internal mixer re-appeared, becoming a key element in the industry. Buist [6] says that Hancock’s internal mixer was invented in 1820, and mentions that Hancock called it a ‘pickle’, as Buist puts it ‘to confuse his competitors’. He also mentions that ‘Hancock’s company, James Lyne Hancock Ltd., in London was the first British Rubber company, founded in 1820’. Rubber products, up to the 19th century, had a major flaw, they were sticky on hot days,

and very stiff when cold. This doesn’t seem like much of a problem until you sit down in your sticky rubber raincoat on a hot day and lift up the chair with you when you stand. This problem was solved by a major discovery attributed to Charles Goodyearof Woburn, Massachusetts, USA in 1839. Duerden [7] writes that Goodyear accidentally visited the

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rubber goods store of the Roxbury Company in New York, around 1832, and as a result became obsessed with the problems of rubber manufacture, to the extent of financial crisis, resulting in frequent visits to the pawnbrokers shop. Duerden comments that, in his search to modify rubber to make it more useful, ‘Goodyear purchased the claim of combining sulfur with India Rubber’ from Nathaniel Hayward. Goodyear was then awarded a contract from the US Government to manufacture mail bags. These bags were made from rubber containing sulfur and white lead. Before long the mail bags started to decompose. Instead of leading him to riches, Duerden mentions that it brought Goodyear and his large family to poverty. He must have been close to giving up when a momentous discovery took place. By chance he heated the raw rubber-sulfur-lead combination, and found that the material charred like leather, and vulcanized rubber, as we know it, was born. The resulting composition was a much stronger material and was no longer sticky at higher temperatures. Duerden writes that Goodyear took out a US patent for this momentous discovery in 1841 but that he profited little from it. Later, in 1843, Hancock was also combining sulfur with rubber and using heat. Stern [8] states that an artist friend of Hancock, coined the term vulcanization for this process, after Vulcan, the god of fire. In this book, the words vulcanize, cure, and cross-linking will be used synonymously. This discovery expanded significantly the number of uses for rubber, since it achieved far more than just making a non-sticky material. In fact the vast majority of rubber products today, owe their existence to vulcanization. A relative newcomer on the scene, thermoplastic elastomers, do not need curing. As time went on, the quantity of rubber consumed continuously increased. This created an intense demand from the jungles of Brazil, and the dark side of our human nature appeared in all its ugliness. Suzuki [1] mentions that the native people of the Amazon were ruthlessly exploited, and that a rubber tapper could be killed, simply for not bringing in the required quantity of rubber from the surrounding trees. Supply and transportation problems began to occur in the Amazon basin, which was the only known source of raw rubber at that time. White [9] describes how, in 1876, seeds were taken out of Brazil and grown into seedlings at Kew Gardens in England. They were then shipped to the Far East. Suzuki comments that virtually all of the supply of natural rubber today comes from millions of trees which owe their heritage to those few seedlings. This is food for thought, when the topic of preserving genetic diversity, in this case, the rainforest, arises. In 1889 John Dunlop in England invented the first commercially successful pneumatic tire, which was at that time used for bicycles [10]. Dunlop produced his first vehicle pneumatic tire in 1906 [11]. An interesting observation by Stern, is that in 1904, in England, it was found that a powder called carbon black (see section 3.3.7), blended into rubber, significantly increased a number of its mechanical properties [12]. It seems surprising that this major discovery was then ‘left on the shelf’ for about eight years. By History

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1910 the motor car truly arrived and both the use and price of natural rubber exploded. Stern mentions that, after this time, with improved fabrics, the rubber treads on tires were wearing out before the fabric reinforcement. The hunt was on to improve the wear life of the rubber. The carbon black discovery was finally taken off the shelf [12].1.2 Synthetic rubber

Around this time, chemists were actively searching for rubbery materials which could be manufactured artificially. Kuzma [13] notes that the Russians, in 1910, prepared such a rubber, known chemically as polybutadiene. In the 1930s, the Germans began commercial production of a synthetic rubber called Buna-S (styrene butadiene copolymer) [14, 15]. With the outbreak of the Second World War, both the USA and Europe were extremely vulnerable to a shortage of supply of natural rubber, which could have had a catastrophic effect on the war effort. A huge R & D project was initiated, between government and industry in the United States. Styrene butadiene rubber was improved, then manufactured on a large scale and called Government Rubber-Styrene (GR-S), later to be known generically as SBR, which today is a major material in the rubber industry.

Although SBR is the most significant synthetic rubber in terms of tonnage, other rubber materials were produced around the same time, and play an important role in today’s market. A priest synthesized a chemical building block which led to the discovery of a rubber by DuPont who marketed it as Duprene [16], in 1931, then changed the name to Neoprene. Although the generic term for this material is polychloroprene (CR, chloroprene rubber) it is still most often referred to by its DuPont name. Bryant [17] points out that in 1934 production was started in Germany, of an oil resistant rubber called Buna-N, the name later changed in 1937 to Perbunan. Its generic name is nitrile rubber (NBR, nitrile butadiene rubber). Butyl rubber (IIR, isobutylene isoprene copolymer) was developed in the 1940s. Other significant materials are Hypalon (CSM, chlorosulphonated polyethylene) and Viton (FKM, fluoroelastomer) by DuPont (now DuPont Dow Elastomers) in the 1950s and ethylene propylene terpolymer rubber (EPDM) in the 1960s. It is interesting to note that a commercially successful synthetic analogue of NR did not appear until around 1960, when it was commercialized by Shell as ‘Shell Isoprene Rubber’ and shortly after by Goodyear as Natsyn. It is chemically known as polyisoprene (IR), and while it has not in any way displaced its natural cousin, it has found a niche in the market place. An important material discovered by Bayer in the 1950s is polyurethane, which can be a coating or a rubber (and a rubbery stretch fabric), depending on its exact chemical composition (polyurethanes may have other forms such as thermoplastics and foams but these do not normally exhibit rubbery properties). A significant recent 7

addition to the armory of the rubber industry is a class of materials called thermoplastic elastomers which are gaining increasing prominence in the marketplace. They behave like rubber at room temperature but soften like plastic when heated. When cooled down, they return to their rubbery state. The rubber that started it all, NR, has survived the onslaught of the synthetic rubbers exceptionally well and today still represents nearly one-third of all rubber in the marketplace. The new awareness of our environment gives NR the added advantage of being seen as a renewable resource, because most synthetic elastomers are derived from petroleum oil based starting materials. 1.3 What exactly does the word ‘rubber’ mean? Rubber seems to be a fairly straightforward word. The French call it caoutchouc recognizing its historically South American ‘native’ origin. The word derives from a South American Indian word, meaning ‘weeping wood’ [18, 19]. The dictionary definition of rubber is, ‘a material that when stretched returns quickly to its approximate original shape’. This definition fits the vulcanized material quite well. ASTM Standard D 1566- 98 [1] has a detailed definition of rubber implying the vulcanized material. Unfortunately the rubber industry tends to be somewhat casual in the use of the term rubber. When a rubber product is made, the primary raw material is a polymer. This polymer often has some elasticity, but not always. It is then mixed with chemicals to make a rubber ‘compound’ which is subsequently vulcanized. This compound is simply a physical mixture of chemicals and indeed a number of ingredients in the vulcanizate might be present only as a physical blend. The industry often calls both the polymer and the uncured compound, ‘rubber’. Unvulcanized silicone, for example, (both polymer and uncured compound) does not fit the dictionary definition too well, since it can have the consistency of butter. The word used in this book for the primary raw polymer will be raw gum elastomer. Some people use the term rubber, to mean NR only, but there have been instances when a customer asked for rubber (expecting the vendor to choose the right type, neoprene, natural, etc.), the vendor however assumed the customer was specifically asking for natural rubber, which in that case turned out to be the worst choice. Naturally, the vulcanized material is also called rubber, as indeed it should be. The word ‘elastomer’ and ‘rubber’ are often used by the industry to mean exactly the same thing, which is a waste of such an interesting word (elastomer), which maybe, could have been reserved exclusively to describe the raw polymer. Blow [20] makes comments similar to these, about the word ‘rubber’ having several meanings. He suggests that the vulcanized material be called ‘elastomer’. ASTM Standard D 1566-98 defines ‘elastomer’ as ‘a term often used for rubber and polymers that have properties similar to those of rubber’.