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Elastomer Engineering Guide Elastomer Engineering Guide Contents Elastomer Engineering Guide Elastomer Engineering Guide Contents Introduction to elastomer engineering 4 Elastomer types 7 Elastomer compounding 11 Manufacturing techniques 16 Material & product testing 20 Material selection 25 Designing with elastomers 30 Elastomer failure modes 33 Glossary of terms 39 About James Walker 46 General information 47 3 Introduction to elastomer engineering This guide has been produced by A wide variety of synthetic rubbers have James Walker to provide engineers since been developed, and in the early with a reference source to a wide 1960s production of natural rubber range of essential information on was surpassed by that of synthetic elastomers and their applications. The elastomers. By 1990, two-thirds of world aim is to bring together in one place rubber production consisted of synthetic the core information on elastomer varieties. engineering that might otherwise be time-consuming to obtain. Fundamental properties It may also be a useful educational Figure 1: Uncured sheets being sulphur resource for those looking for an coated and sun dried using the old of elastomers solarisation method of vulcanisation. introduction to elastomer engineering Elastomers are based on polymers which in practice. For more information on any have the property of elasticity. They are topic on this guide, please contact your made up of long chains of atoms, mainly local James Walker company via the out of the Amazon basin, began to carbon, hydrogen and oxygen, which contact details on the back page. compete with traditional sources. have a degree of cross-linking with their The period between World Wars I and II neighbouring chains. It is these cross- History of elastomers witnessed the first development of a true linking bonds that pull the elastomer back synthetic substitute for natural rubber, ie, into shape when the deforming force is removed. Rubber was first brought to Europe in sodium polymerised butadiene, which 1493 from the Americas by Columbus, but was produced in Germany as Buna The chains can typically consist of it remained little more than a novelty for rubber and in the USSR as SK rubber. Introduction to elastomer engineering 300,000 or more monomer units. They over 200 years. Interest eventually began In the 1930s, Germany developed the can be composed of repeated units of to grow, and in 1770 Joseph Priestley emulsion copolymerisation of butadiene- the same monomer, or made up of two or noted its ability to rub out pencil marks, styrene (Buna S), whereas sodium more different monomers. Polymers made hence the name 'rubber'. polybutadiene continued as the principal general purpose synthetic rubber in the up of two types of monomer are known as copolymers or dipolymers, while those This was followed by a rapid growth in Soviet Union. made from three are called terpolymers. technical developments and applications in the 19th century. Rubber began to The advent of World War II highlighted the be used as containers, flexible tubing, importance of rubber as a raw material. elastic bands and waterproofing, When the Axis powers gained control of spurred by developments from Charles nearly all the world’s supplies of natural Macintosh and Thomas Hancock. Charles rubber, this led to an urgent stepping up Goodyear’s discovery of vulcanisation in the development of synthetic rubbers, using sulphur increased the natural particularly in the USA. Production of strength and durability of rubber by styrene-butadiene rubber (SBR), then cross-linking the molecules of the soft called GR-S, began in a US government Figure 3: Single monomer units polymerised gum rubber into a tougher material. plant in 1942. Over the next three years, government-financed construction of 15 to form a polymer. Other technological advances included SBR plants brought annual production to improved compounding techniques more than 700,000 tonnes. which enabled the use of anti-oxidants and accelerators, and the incorporation of carbon black to improve strength. This led to a vast increase in the number of applications, which included seals, belts, Figure 4: Two different monomers form a copolymer (or dipolymer). flooring, electrical insulators, springs, and pneumatic tyres. As the number of applications increased, demand for the raw material grew rapidly. South America, particularly Brazil, was the prime source of natural rubber until the Figure 5: Three different monomers early 1900s. Then, British Asian colonies, Figure 2: Elastomer moulding after form a terpolymer. using rubber trees from seeds smuggled World War II at James Walker. 4 Introduction to elastomer engineering Elastomers are arguably the most returns to its original configuration when occurs when the compound is subjected versatile of engineering materials. They the stress is removed. As a result of to pressure and heat. Thermoplastic Introduction to elastomer engineering behave very differently from plastics and this extreme flexibility, elastomers can elastomers, on the other hand, have metals, particularly in the way they deform reversibly extend by approximately weaker cross-linking and can be and recover under load. 200 – 1000%, depending on the specific moulded, extruded and reused like plastic material. Without the cross-linkages or materials, while still having the typical They are complex materials that exhibit with short, uneasily reconfigured chains, elastic properties of elastomers. unique combinations of useful properties, the applied stress would result in a the most important being elasticity permanent deformation. and resilience. All elastomers have the ability to deform substantially by stretching, compression or torsion and Resilience then return almost to their original shape after removal of the force causing the Resilience as applied to elastomers is deformation. essentially their ability to return quickly to their original shape after temporary Their resilience enables them to return deflection. In other words, it indicates the quickly to their original shape, enabling speed of recovery, unlike compression for example dynamic seals to follow set, which indicates the degree of Figure 7: Before curing, the long molecular variations in the sealing surface. recovery. chains can slide past each other, exhibiting little elasticity. When an elastomer is deformed, an energy input is involved, part of which is not returned when it regains its original shape. That part of the energy which is not returned is dissipated as heat in the elastomer. The ratio of energy returned to energy applied to produce the deformation is defined as the material’s resilience. Most elastomers possess a number of other useful properties, such as: Figure 8: After curing the chains are cross- • Low permeability to air, gases, water linked, which ensures they return to position when the deforming force is removed. and steam • Good electrical and thermal insulation • Good mechanical properties Figure 6: Elastomer sample undergoing • The ability to adhere to various fibres, tensile testing. metals and rigid plastics. Also, by proper selection of compounding Elasticity ingredients, products with improved or specific properties can be designed to meet a wide variety of service conditions. Elasticity is the ability of a material to return to its original shape and size after This remarkable combination of properties being stretched, compressed, twisted is the reason elastomers serve a vast or bent. Elastic deformation (change number of engineering needs in fields of shape or size) lasts only as long dealing with sealing, shock absorbing, as a deforming force is applied, and vibration damping, and electrical and disappears once the force is removed. thermal insulation. The elasticity of elastomers arises from Most types of elastomers are thermosets, the ability of their long polymer chains to which gain most of their strength after reconfigure themselves under an applied vulcanisation – an irreversible cross- stress. The cross-linkages between linking of their polymer chains that the chains ensure that the elastomer 5 Introduction to elastomer engineering Elastomer products The inherent elastic properties of vibration or as a conduit to pass other and applications elastomers make them a natural choice hoses, pipes or wires. for sealing applications. They are designed in geometry and formulation to resist the Elastomers are also used in personal The beneficial properties of elastomers pressure, motion and environment to protection and diving products. These have led them to be used in a vast range which they are exposed in service. include face masks, nasal units, fixing of applications, from hydraulic and In some cases, where the elastomer straps, neck seals, ankle and wrist seals, pneumatic seals in industrial machinery material is not inherently strong enough regulator valves and mouthpieces. through to precision pharmaceutical to withstand the environment in which it mouldings. Their applications can be is exposed, additional components can Liquid silicone elastomers often find divided into two broad categories, be bonded or included in the seal design application in products that require high ‘Sealing’ and ‘Non-sealing’. to increase the elastomer’s performance precision, such as electrical connectors, envelope, such as engineering plastics or multi-pin connectors, infant products metallic components. where smooth surfaces are desired such Seals are used in applications such as as bottle nipples, medical applications instrument stems, rods, shafts, flanges, as well as kitchen goods
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