<<

Elastomer Engineering Guide 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 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 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 which in practice. For more information on any have the property of . 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 , and , 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 is removed. Rubber was first brought to Europe in sodium polymerised , 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 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, (Buna S), whereas sodium more different . Polymers made hence the name 'rubber'. continued as the principal general purpose in the up of two types of monomer are known as 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. 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 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 .

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 (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 is removed. As a result of to and heat. engineering elastomer to Introduction behave very differently from and this extreme flexibility, elastomers can elastomers, on the other hand, have , 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 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 . and resilience. All elastomers have the ability to deform substantially by stretching, compression or 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 . 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 , 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, , 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 to pass other and applications elastomers make them a natural choice hoses, pipes or . 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 , 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 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- connectors, infant products metallic components. where smooth surfaces are desired such Seals are used in applications such as as nipples, medical applications instrument stems, rods, shafts, flanges, as well as kitchen goods such as baking cylinders and pump pistons. They are pans and spatulas. is widely used in the food industry, including frequently overmoulded onto other parts in bottle lifters, keg seals, manway joints, made of different plastics. hopper seals, cones and pipe seals. Another use is for expansion joints, Seals are constructed using virtually all which are flexible connectors typically types of elastomers with a wide range consisting of a fabric-reinforced of reinforcements, fillers, additives and elastomer construction, often with Introduction to elastomer engineering cross-linking technology. The versatility of reinforcements. Industrial gaiters elastomer systems offers value in the 'fine and bellows are another significant tuning' of elastomer properties to specific application for the protection of moving service conditions. However, there may components from the elements and Sealing applications be complex trade-offs in properties of other contamination. elastomers, such as high compression set Seals are precisely formed, moulded resistance but poor dynamic performance Elastomers also have many uses or machined shapes that seal or high heat resistance but low chemical in civil engineering, for example for or gases by conforming to the sealing resistance to specific fluids. mounting structures to reduce effect surfaces and supplying adequate sealing of external noise, vibration or seismic force to prevent passage of the sealed ; accommodating thermal medium. Initial sealing interference with Non-sealing applications movement (bridge bearings, the surfaces increases as the system expansion joints, pipe pressure grows, since the pressure is Elastomers are also widely employed in couplings, etc); transmitted omnidirectionally in the seal. non-sealing applications. Elastomeric acting as a barrier belts, for example, are used in a great to water (water This helps to ensure effective sealing number of applications including drive stops, plant linings, performance. systems and power transmission. They tunnel ) and a wide variety of are usually composite products that rely others including roofing membranes, upon reinforcing structures and specific rubberised asphalt, rail pads, inflatable construction techniques to perform in formers and concrete texturing. their intended applications. As in many a reinforced rubber products, fabrics and The automotive sector is another large other materials such as glass fibre are user of elastomers, with tyres being by often used to bear much of the load. far the largest application by . Another key use is in suspension Flexible hoses are also typically systems, where the components constructed of reinforced elastomers can be designed with very specific b and may be multi-layered by design. dynamic properties. Other areas where They are most commonly used to transfer elastomers make an appearance include Figure 9: Cross-section of an ‘O’ ring fluids from one point to another or to exhaust mounts, anti-impact devices, showing (a) initial sealing force and (b) the increase in sealing force due to transmit energy, such as in hydraulic weather strips, windscreen wipers, seat system pressure. applications. They can also be used as foam and interior trims. a connector to help absorb surges and

6 Elastomer types

Over the last century or so, a large Butadiene (BR) (polybutadiene) Chlorosulfonyl (CSM) number of basic and specialty types Elastomer elastomers have been developed to CH CH CH CH 2 2 CH CH CH CH CH CH meet a wide range of applications n 2 2 2 2 and operating environments. Their x y z CI SOCI properties vary widely in terms of their Widely used in blends with natural elasticity, range, strength, rubber and styrene butadiene rubber Good resistance to oxygen, and , compatibility, environmental for tyres, where it reduces heat build- light. Oil resistant and with low resistance, etc, and there is also a large up and improves abrasion resistance. permeability. Its excellent UV stability range in their costs. Low , good flexibility at low makes it useful as roof sheeting and for , high abrasion resistance pond liners, as well as and cable The principal types of elastomers are in severe conditions. Also used in , applications, coated fabrics and hoses. outlined here. It should be noted that conveyor and transmission belts. their basic properties can be substantially modified by compound design Epichlorhydrin (CO/ECO) (see section on Compounding on p11). Butyl (IIR) (- copolymer) CH O CH2 CH n 3 CH CI Thermoset elastomers 2 CH C CH CH CH CH 2 2 2 High resistance to ageing, oxidation, Thermoset elastomers are vulcanised x y CH ozone and hot oil. Good resistance to (cured) to produce a degree of cross- 3 linking between the polymer chains. The hydro­carbon solvents, moderate low cross-linking is irreversible, unlike with Low permeability to gases and hence temperature flexibility, poor electrical thermoplastic elastomers which will soften used for inner tubes. High damping properties and abrasion resistance. and flow above a given temperature. at ambient temperatures. Good Attacked by strong mineral and oxidising ozone, weathering, heat and chemical agents and chlorine. Main use is in the resistance but not oil resistant. Other automotive sector for seals, hoses, Acrylic (ACM) uses include wire and cable applications, gaskets and ‘O’ rings. (alkyl acrylate copolymer) pharmaceutical closures and vibration isolation. CH CH2 CH CH2 propylene (EPM/EPDM) x y O = C O Chlorinated polyethylene (CPE) + monomer CH CH CH CH O CH2 2 2 2 in EPDM CH x y z CH2 CH2 CH2 Ethyl acrylate CH2 CH2 Chloroethyl CH (95%) vinyl ether (5%) x y 3 CH Cl CI 3 Excellent ozone/weathering resistance; Outstanding resistance at normal and Good chemical resistance to hydrocarbon excellent hot water and steam resistance; high temperatures to oil and oxygen. fluids and elevated tem­peratures. good resistance to inorganic and polar Good weathering and ozone resistance. Poor mechanical strength: mechanical organic chemicals. Low resistance to Poor resistance to moisture, acids and properties may deteriorate above 100°C. hydrocarbons. Typical temperature range: bases. Commonly used in automotive Uses in the wire and cable industry, as -45°C to +150°C (-49°F to +302°F), transmission seals and hoses. Also used well as for pond liners. up to +180°C (+356°F) in steam. in formulations. Mineral oil/grease should not be used to aid assembly. Chlorobutyl (CIIR) Bromobutyl (BIIR) modified by the introduction Butyl rubber modified by the introduction of a small amount of chlorine, giving of a small amount of bromine, giving improved ozone and environmental improved ozone and environmental resistance, stability at high temperatures resistance, stability at high temperatures and compatibility with other diene rubbers and compatibility with other diene rubbers in blends. Also increased adhesion in blends. Also increased adhesion to other rubbers and metals. Similar to other rubbers and metals. Similar properties to bromobutyl. properties to chlorobutyl.

7 Elastomer types

Fluorocarbon (FKM) Common Natural Common SBR EPDM CSM Acrylic Vamac® Epichlorohydrin Butyl Silicone HNBR Fluorocarbon Fluorosilicone Kalrez® CH3 Name Rubber Name

CF +CF CF +CSM CF2 CH2 CF2 2 2 in Ter- in Tetra- Ethylene CF Styrene Propylene Chlorosulfonyl Ethylene Hydrogenated 3 Chemical Polychloroprene Polyacryclic Epichlorohydrin Polyisobutylene Fluorocarbon Fluorosilicone Perfluorocarbon Chemical Polyisoprene Butadiene Diene Polyethylene Butadiene Acrylic Polysiloxane Nitrile Name Rubber Rubber Rubber Rubber Rubber Rubber Rubber Name Rubber Monomer Rubber Rubber Rubber Rubber Rubber Excellent ozone/weathering resistance; good heat resistance. Limited resistance to steam, hot water and other polar fluids Nomenclature NR SBR EPDM CR CSM NBR ACM AEM ECO IIR Q HNBR FKM FQ FFKM Nomenclature Elastomer types (except Tetra-) although new peroxide cured grades with no metal oxides Relative Relative 1 1 1.5 1.5 1.5 1.5 3.5 4 4 4 11 20 30 40 1000 are better. Attacked by , limited Cost Cost low temperature capabilities (except specialised grades). Typical temperature Hardness Hardness 30-95 40-95 30-85 30-90 40-85 40-100 50-85 45-90 40-85 40-85 40-80 50-95 50-95 40-80 65-90 range: -20°C to +230°C (-4°F to +446°F). Range (IRHD) Range (IRHD) Properties vary significantly with type. Full Full Limited Full Full Limited Limited Limited Limited Full Limited Limited Limited Limited Colours Black Colours Range Range Range Range Range Range Range Range Range Range Range Range Range Range

Heat Heat Resistance Resistance Maximum 75°C 85°C 130°C 95°C 130°C 100°C 150°C 150°C 140°C 120°C 205°C 150°C 205°C 180°C Maximum Continuous Continuous Maximum Maximum Intermittent 105°C 115°C 150°C 125°C 160°C 130°C 180°C 180°C 160°C 135°C 300°C 180°C 250°C 200°C 325°C Intermittent

Minimum -60°C 0°C Minimum -60°C -55°C -50°C -40°C -25°C -50°C to -5°C -20°C -40°C -30°C -50°C (special grades -30°C -40°C to 0°C -60°C (special grades Temperature Temperature Hydrogenated nitrile (HNBR) -80°C) -25°C) Table 1: Basic data for main elastomer types.

CH2 CH2 CH2 CH2 CH2 CH x y Isoprene (IR) Natural (NR) CN (synthetic cis-polyisoprene)

Derived from conventional nitrile by hydrogenation of the unsaturated bonds CH2 CH= C CH2 CH2 CH= C CH2 in the butadiene unit of the polymer. Good CH n CH n oil/ and chemical resistance and good 3 3 weathering resistance. Excellent mechanical properties including tensile strength, tear, Similar chemical structure to natural High resilience and tensile strength; good modulus, elongation at break and abrasion. rubber, but less easy to process and can abrasion resistance; low cost. Poor oil Wide temperature range and can be have lower tensile and tear strength. Its resistance and weathering resistance. compounded for excellent resistance to relative purity provides better performance Typical temperature range: -50°C to rapid gas decompression. Disadvantages at lower temperatures. Can be used +100°C (-58°F include cost and limited resistance to interchangeably with natural rubber in all to +212°F). aromatics. Typical temperature range: but the most demanding applications. Can be used -40°C to +160°C (-40°F to +320°F). Special with some grades can be sulphur cured for dynamic dilute applications but then the maximum inorganic temperature falls. As with nitrile, many chemicals properties can be influenced by varying its and polar acrylonitrile to butadiene ratio. organics.

8 Elastomer types

Chloroprene (CR) Common Natural Common types Elastomer SBR EPDM Neoprene CSM Nitrile Acrylic Vamac® Epichlorohydrin Butyl Silicone HNBR Fluorocarbon Fluorosilicone Kalrez® Name Rubber Name

CH2 CH C CH2 Ethylene Cl n Styrene Propylene Chlorosulfonyl Acrylonitrile Ethylene Hydrogenated Chemical Polychloroprene Polyacryclic Epichlorohydrin Polyisobutylene Fluorocarbon Fluorosilicone Perfluorocarbon Chemical Polyisoprene Butadiene Diene Polyethylene Butadiene Acrylic Polysiloxane Nitrile Name Rubber Rubber Rubber Rubber Rubber Rubber Rubber Name Rubber Monomer Rubber Rubber Rubber Rubber Good weather and ozone resistance, Rubber and fair resistance to inorganics. Resistant to many chlorofluorocarbons. Nomenclature NR SBR EPDM CR CSM NBR ACM AEM ECO IIR Q HNBR FKM FQ FFKM Nomenclature Low cost. Moderate oil resistance and limited temperature resistance. Typical Relative Relative temperature range: -40°C to +120°C 1 1 1.5 1.5 1.5 1.5 3.5 4 4 4 11 20 30 40 1000 Cost Cost (-40°F to +248°F). Useful in pneumatic applications. Hardness Hardness 30-95 40-95 30-85 30-90 40-85 40-100 50-85 45-90 40-85 40-85 40-80 50-95 50-95 40-80 65-90 Range (IRHD) Range (IRHD)

Full Full Limited Full Full Limited Limited Limited Limited Full Limited Limited Limited Limited Colours Black Colours Range Range Range Range Range Range Range Range Range Range Range Range Range Range Polysulphide OT / EOT (condensates of sodium polysulphides Heat Heat with organic dihalides). Resistance Resistance Maximum 75°C 85°C 130°C 95°C 130°C 100°C 150°C 150°C 140°C 120°C 205°C 150°C 205°C 180°C Maximum Continuous Continuous Very good resistance to oils, , Maximum Maximum solvents, oxygen and ozone. Intermittent 105°C 115°C 150°C 125°C 160°C 130°C 180°C 180°C 160°C 135°C 300°C 180°C 250°C 200°C 325°C Intermittent Impermeable to gases. Poor mechanical properties and poor heat resistance. Minimum -60°C 0°C Minimum -60°C -55°C -50°C -40°C -25°C -50°C to -5°C -20°C -40°C -30°C -50°C (special grades -30°C -40°C to 0°C -60°C (special grades Temperature -80°C) -25°C) Temperature

Polyurethane (AU, EU) Nitrile (NBR) Perfluorocarbon (FFKM)

O H O H H

C N C N C O C C O H H H H H n CH CH CH CH CH2 CH CF CF CF 2 2 2 2 CF2 CSM x y CN O Butadiene Acrylonitrile Very versatile, with good abrasion CF resistance, high tensile and tear strength, 3 good resistance to aliphatic solvents and Good aliphatic hydrocarbon oil/fuel Ultimate in performance regarding heat mineral oils, oxygen and ozone. Can be resistance and resilience. Limited and chemical resistance. Very expensive. formulated to have high modulus with a weathering resistance and only Some grades are suitable for continuous high content. Poor heat resistance modest temperature resistance. Typical use at 327°C (620°F), with chemical and can have poor resistance, temperature range: -30°C to +120°C resistance being almost universal. particularly in moist conditions. Uses in (-22°F to +248°F). Widely used in sealing However, their moderate mechanical seals, metal forming dies, liners, rollers, applications. Low temperature grades properties deteriorate rapidly at elevated wheels, conveyor belts, etc. available down to -50°C (-58°F). As with temperatures, and at temperatures hydrogenated nitrile, many properties can below 0°C. be influenced by varying its acrylonitrile to butadiene ratio.

9 Elastomer types

Tetrafluoroethylene propylene (FEPM) are not good enough (eg, Aflas®) for use as primary insulation, but their general toughness leads to their use in cable jacketing. Other uses include fabric CF CH CH CF2 2 2 coatings, bellows and automotive body components.

CH3 Excellent ozone/weathering resistance; good heat resistance; excellent resistance Styrenic block copolymers to steam and radiation; good overall (SBS, SIS, SEBS) Elastomer types Silicone (Q) chemical resistance. Disadvantages include high compression set and Styrenic block copolymers are the largest high temperature. volume and lowest priced member of CH 3 Difficult to process and has poor the family. They Si O extrusion resistance especially at high are readily mixed with other polymers, n temperatures. Typical temperature range: oil and fillers, enabling versatile tuning CH 3 0°C to 200°C (32°F to +392°F) or +260°C of product properties. They are used in (+500°F) in steam. enhancing the performance of bitumen Only moderate physical properties but in road paving and roofing applications, capable of retaining them over a very particularly under extreme weather wide temperature range. Some types Thermoplastic elastomers (TPE) conditions. They are also widely applied are affected by moisture. Good electrical in , , coatings and in resistance properties. Readily available in Thermoplastic elastomers have many footwear. liquid form (LSR). Widely used in sectors of the physical properties of vulcanised such as pharmaceutical, medical, wire rubbers but can be processed as and cable, automotive and aerospace. . Since their commercial introduction in the 1960s, they have Copolyether ester elastomers become a significant part of the elastomer industry, and are used in applications as These materials are strong, tough and oil Styrene butadiene (SBR) diverse as adhesives, footwear, medical resistant, but are only available in a limited devices, automobile parts and asphalt hardness range. They are also resistant to oxygen and ozone. CH CH CH CH modification. CH2 2 CH2 x y They require little or no compounding, They are used in moulded goods with no need to add reinforcing agents, applications requiring exceptional stabilisers or cure systems. Their toughness and flex resistance together Needs reinforcing fillers for high strength, disadvantages are the relatively high cost with moderate heat and chemical when it has similar chemical and physical of raw materials, poor chemical and heat resistance. Applications include cable properties to natural rubber, with generally resistance, high compression set and low jackets, tubing, automotive bellows, gear better abrasion resistance but poorer thermal stability. wheels and business machine parts. resistance. Widely used in car and light vehicle tyres. Also conveyor belts, moulded rubber goods, soles and roll coverings. Thermoplastic urethane elastomers amide elastomers (TPAU, TPEU, TPU) Similar properties to copolyether ester Thermoplastic polyurethanes are available elastomers, except service temperatures in a more limited hardness range than are lower. Good strength and toughness the styrenics, and are characterised as well as being oil resistant. Also by excellent strength and toughness, resistant to oxygen and ozone. Limited and oil resistance. Of the two major hardness range and hydrolytic stability. types, polyester and polyether, the latter has better hydrolytic stability and low temperature performance.

The electrical properties of the

10 Elastomer compounding

Compounding Ingredients Diluent, or non-reinforcing, fillers have a large particle size and do not 'bond' to the compounding Elastomer The basic properties of elastomers Polymers polymer in the same way as reinforcing are highly dependent on the polymers fillers. They are mainly added to reduce used in their manufacture. These The polymer, or blend of polymers, is the cost. Examples include soft clay, calcium properties can be modified, however, fundamental component in determining carbonate, and talc. through the appropriate addition of the properties of the compound. It is compounding ingredients. Some are selected to optimise service performance Fine-ground natural silica is used to added to accelerate cross-linking, and processing requirements while provide dimensional stability, improved others improve processability, while also taking cost into account. Very thermal conductivity, and good electrical others improve the properties of the high molecular weight polymers can insulation properties at low cost. finished product. for example produce extremely tough materials. They can however lead to Some compounds are required to deliver problems with poor flow, poor joins and Accelerators the highest levels of performance in the particularly backrinding. end product, with cost being a secondary These speed up the cure. Modifications issue. In this case compounding requires in their levels can control the cure speed the use of materials selected to give and elastomer properties. It is common the required characteristics without the to use more than one accelerator in a inclusion of non-essential ingredients formulation. Peroxide cured materials which could compromise performance. often use what is known as a co-agent Fillers along with the peroxide which can act In other applications, compounds may be like an accelerator or modify the physical designed to minimise cost, with extenders Fillers are added primarily to provide properties. and diluents being added to reduce the reinforcement and secondly to reduce proportion of high priced components cost. They fall into two basic categories: in the mix. This inevitably leads to reinforcing or semi-reinforcing, and compromises in the mechanical and diluent (non-reinforcing, generally for other properties, of course, but for certain cheapening). applications this may be acceptable. The most popular reinforcing and semi- Activators Other important factors that affect the reinforcing fillers are carbon blacks, which quality of the resulting elastomer include are categorised primarily by means of In most sulphur-cured rubbers, zinc the quality of the raw ingredients, the particle size. Carbon blacks and non- oxide and stearic acid are added to help style of mixer and the quality control black fillers become more reinforcing as initiate the cure. In other rubbers, different in mixing. End product properties can particle size decreases. Highly reinforcing materials are added which assist the cure also be influenced by the processing of fillers can make a compound tough, in an indirect way. the compounded material into the final which can result in poor flow. Carbon product shape. blacks are alkaline in nature and tend to Very fast cure systems can give problems accelerate cure. with scorch/orange peel and backrinding. Of the thermoplastic elastomers, styrenic Peroxide cures can give sticky flashlines block copolymers are the only type that Non-black fillers tend to be acidic and can due to their inability to cure in the is fully compoundable in a similar way retard cure as well as absorb moisture, presence of air. Very high levels of some to conventional thermoset elastomers. which can result in blistering problems curatives can lead to fissuring/blistering Although fillers such as carbon black can during the processing stage. Glycols help either in mould or during post cure. be added to cheapen the material, they to overcome this retardation effect with do not have a reinforcement effect in acidic fillers. thermoplastic elastomers. Blowing agents Nano-fillers such as super-fine clays The description of ingredients that follows have a high surface area compared to Blowing agents are used in the gives a basic guide to their uses and their volume and can produce better manufacture of sponge rubber. Sodium properties. mechanical performance. Although they bicarbonate was the first commercially are more expensive than conventional used blowing agent, which reacts with fillers, the same weight of material goes stearic acid to produce carbon dioxide further because the particles are so at vulcanisation temperatures. Today’s much finer. commonly used blowing agents rely on the formation of nitrogen as the expansion agent.

11 Elastomer compounding

Bonding promoters Desiccants Peptisers

Most large volume elastomer products For a number of applications, it is Peptisers are substances that act as chain are bonded composites, such as tyres, necessary to add a desiccant to remove terminating agents during mastication hoses and belts. Bonding agents are traces of water introduced in fillers or of rubber. While natural rubber is usually often added to the compound to increase produced from chemical reactions during masticated and can be peptised, this is the bond strength between the different vulcanisation. Failure to remove this water unnecessary for most synthetic rubbers. components. Agents include cobalt-based can result in uncontrolled porosity in the Peptisers may also act as pro-oxidants. salts such as cobalt naphthenate or cobalt product and problems where the cure is They significantly reduce the time stearate as well as proprietary materials sensitive to moisture. required to lower the of the based on cobalt and boron complexes. rubber to a workable level, thereby cutting The usual agent used for this purpose mixing time and energy. is calcium oxide (quicklime). It is difficult Co-agents to disperse in its dry powder form so it Although many materials, such as some is usually offered in a variety of carrying accelerators, are known to possess Co-agents are reactive substances media to ensure adequate dispersion and chemical peptising activity, in practice Elastomer compounding which improve the effectiveness of help prevent moisture take up prior to the choice is limited due to additional peroxide cross-linking. Most of them are incorporation in the compound. considerations such as health and safety, methacrylates or derivatives containing effect on vulcanisation characteristics

allyls (H2C=CH-CH2-), but polymeric and price. materials with a high content of vinyl Extenders groups also react in a similar way. Sulphur and sulphur donors can also be used. Extenders are compounding ingredients Pigments Their effects may be explained by the that are added to the elastomer to reduce co-agent suppressing undesirable side the cost of the compound. Although most rubber compounds are reactions between the polymer radicals. black, due to the widespread use of carbon black as a filler, coloured rubber Flame retardants compounds are frequently required to add Coupling agents appeal to consumer items. Colour coding Most elastomers support combustion, of products is also often desirable. Coupling agents provide a stable bond and the resulting by-products can be between two otherwise poorly bonding extremely hazardous. To improve their Inorganic or organic pigments are surfaces, for example, silanes added to flame resistance a number of products available. Inorganic pigments are often mineral fillers to bond to polymers. may be added to the compound, either dull and in some cases too opaque inorganic or organic. They include to provide the desired colour. They antimony trioxide, zinc borate, aluminium are insoluble and thus cannot bloom. Curatives hydroxide and chlorinated paraffins. Organic pigments generally give brighter shades but are more sensitive to heat Added to form cross-links, these vary and chemicals and are also relatively according to the type of elastomer. In Odorants and deodorants expensive. They can also fade badly in sulphur-cured rubbers, sulphur donors long-term exposure to sunlight. as opposed to sulphur give better heat This class of compounding ingredient stability as they tend to give single was more common in the days when sulphur cross-links. Peroxide cures give natural rubber was the main rubber for good thermal stability due to the short production. The early forms of natural Plasticisers/process aids length of the cross-links between polymer rubber gave products with a distinct chains. Fluorocarbons, along with some aroma and to overcome this a wide range Plasticisers need to be compatible with other polymer types, can have their own of odorants was offered. Many of the the polymer. They reduce hardness with specialised cure systems. synthetic rubbers have their own distinct a given level of filler, and can help with aroma and often this has to be masked filler incorporation and dispersion. to make the final product acceptable to Special types of plasticiser can improve the user. the low temperature flexibility of some rubber types (eg, nitrile and neoprene). Process aids can also assist with filler dispersion, although they are normally added to improve processability downstream.

12 Elastomer compounding

High levels of plasticiser/process aid can Protectants/antidegradants Tackifiers bloom to the surface of make-up and give compounding Elastomer knitting problems (flow marks and poor These materials are added to inhibit attack Tackifiers are compounding ingredients joins) in the manufactured product. They by oxidation and ozone. Antidegradants introduced to enhance the surface of can also create difficulties when bonding fall into two broad groups – amines uncured elastomers. They are usually to metal. Excessive softening of the (staining) and phenolics (non-staining), low-molecular weight compounds. compound can lead to air trapping in the with the latter able to be used in non- mould. black compounds. Some anti-ozonants (such as waxes) can bloom to the Other ingredients Plasticisers can also cause problems surface and give similar problems to when a product is subjected to thermal plasticisers/process aids. There are a range of other ingredients cycling and/or certain solvents, as they that are sometimes added to compounds can leach out at high temperatures and to achieve specific properties. These adversely affect the low temperature Retarders range from the inclusion of iron powder properties. to enable seal fragments in food to Retarders are used to prevent premature be detected with a metal detector, curing, or scorching, of compounds to radiation shielding for use in x-ray Pre-dispersed ingredients during processing and storing. During environments. The effects on moulding mixing and further processing in a vary enormously depending on the Some types of ingredient that are calender, extruder or moulding press, ingredient. difficult to disperse, for example, certain the elastomer is continuously subjected accelerators and anti-oxidants can be to heat which can result in premature The selected ingredients are combined to obtained pre-mixed in an inert polymer. curing, or pre-curing. To prevent this, produce a formulation for the mix. Table The concentrations are normally 75 to retarders are mixed with the compound. 2 shows an example of a simple formula 80%. When in this form, the ingredients Excessive use of retarders results in designed to produce a 90 IRHD nitrile for are more readily dispersed during a porosity in compounds and they are rarely an application involving contact with a mix. Some powders such as peroxides used today. hydrocarbon fuel. are supplied absorbed onto an inert filler or dampened with oil which helps dispersion, and health and safety. Occasionally, are used absorbed Table 2: Example formulation for a 90 IRHD nitrile compound for fuel resistance. onto a powder which aids handling, and can give a faster incorporation into the 41% ACN 100 High ACN content for fuel resistance mix. In some cases these ingredients can be dispersed into the polymer. Zinc oxide 5 Cure activator

Stearic acid 1 Cure activator Process aids

A process aid is an ingredient that is SRF carbon black 120 Semi-reinforcing filler to obtain hardness added in a small dosage to an elastomer compound to influence the performance Adipate plasticiser 6 To aid processing of the compound in factory processes or to enhance physical properties by Sulphur 2 Curative aiding filler dispersion. Examples include physical peptisers, lubricants, silicone Accelerator 1 To control cure rate modified processing additives and anti- stick agents. Anti-oxidant 1 To reduce the effects of ageing

Quantities relate to 100 phr (parts per hundred rubber).

13 Elastomer compounding

Compound design decreasing its concentration improves Mixing process low temperature flexibility. This is due Elastomer compounds can be designed to the influence of the ACN as a plastic The mixing cycle is crucial in dispersing for specific purposes by modifying their modifying the rubber influence of the the ingredients sufficiently so that the characteristics through varying the butadiene. elastomer’s physical and resistance quantities of their constituents. This can properties can be optimised. range from compounding using diluent Mixing fillers and basic ingredients to keep costs In a conventional cycle the polymer is down, through to the use of specific Three types of processes are used for added first and mixed for a short time additives to produce properties such as mixing the compound ingredients. to ensure homogeneity and to soften high tensile strength or wear resistance. sufficiently to accommodate the fillers. Open mill. Here the rubber is banded The fillers are added in one or more Varying quantities and the selection of around the front roll and the ingredients stages depending on their levels, with the ingredients can heavily influence the end incorporated in the nip. ram being lowered after each addition to properties of the compound, as illustrated ensure the material is fully compressed in the following examples. into the chamber. Elastomer compounding

These days most fillers are automatically

Tensile strength weighed and fed directly into the mixer for accuracy and to avoid contamination. These records are automatically stored in the mixing computer system. If Cost plasticisers are used they are usually Diluent filler added with the fillers to aid dispersion. Reinforcing filler Figure 12: Two-roll mill. It is important to optimise the chamber Level of filler Internal mixer. The internal mixer has the volume fill so that the shear on the advantage of being totally enclosed. It compound is maximised. Figure 10: Effects of varying concentration of reinforcing and diluent fillers. mixes a batch of material in about 4 to 6 minutes as opposed to up to 30 minutes The curatives are added late in the cycle to minimise their residence time since Figure 10 shows the effect on price and on an open mill. In most cases the the mixer heats up due to . In performance of varying the concentration compound exits the mixer onto a two-roll some cases the curatives will be added of reinforcing and diluent fillers. The mill where it is cooled and compressed in a second stage, either in the mixer or reduction in cost by increasing the levels into sheet form ready to be supplied to mill, to avoid starting the cure process or of diluent filler content when compared to the manufacturing process. reducing its efficiency. the reinforcing filler content needs to be balanced against the lower performance. Feed Some polymers such as EPDM do not need the initial softening and can be Ram mixed 'upside down', with some of the Oil swell, % Casing - cored for fillers and oils added first, before the cycle heating/cooling continues as normal. Rotor The material is dumped from the mixer at a pre-set temperature and/or energy Mixing Rotor chamber value to ensure consistency of the Minimum operating temperature final compound. Again this full cycle is recorded in the mixing computer. Discharge door ACN% The compound is then milled for initial Figure 13: Internal mixer. Figure 11: Effects of varying ACN content. cooling and to ensure homogeneity with soft compounds. A secondary The effect of varying the acrylonitrile Continuous mixer. This machine is cooling takes place in cooling racks or a (ACN) content in a nitrile elastomer similar to a long extruder, with the specialised take-off unit which can apply is shown in Figure 11. Increasing ingredients added via hoppers along the anti-tack as needed. the concentration of ACN can be barrel. It is mainly used where only a few seen to improve oil resistance, while ingredients are added.

14 Elastomer compounding

Quality checking Hardness. Testing the hardness of the compound using an indentor provides a compounding Elastomer The initial stage of quality checking to check that the correct levels of filler have ensure the material meets the required been incorporated. The hardness should standards normally includes the following normally fall within +5 and -4 IRHD of the tests. specified value. The strict process and quality control regimes in place at James Walker manufacturing sites however, Cure characteristics. How an elastomer ensures any variation in compound cures over time is measured on a hardness is minimal and far below the . As the compound cures industry standard range outlined here. between the hot platens it becomes stiffer. This is measured via a strain Details on the test procedure are given on gauge connected to an oscillating rotor in page 20. contact with the elastomer. The resistance of the material to the oscillating motion is plotted on a graph against time, known as Density. This is a measure of the weight a rheograph, which enables the moulding per unit volume. It gives an indication characteristics to be predicted. of whether the correct quantities of ingredients have been added. See also the Materials & product testing section on page 20. Weight. This test checks the weight of the compound after leaving the mixer and compares it to the input weight of the mix to confirm all the constituents have been added. 10.00 8.00 6.00 4.00 (in.lb) T orque 2.00 0 0 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00 TIME (min)

Figure 14: Typical rheometer trace showing progress of cure against time.

15 Manufacturing techniques

Manufacturing The calender process allows a high Feed degree of control on the thickness of the rubber sheet. This sheet is generally The manufacturing of products in Hopper Barrel elastomers involves a number of often then used either to stamp a shape for Exit complex operations to turn the raw placing into a mould in the next process, material sheets into a finished product or to manufacture cross-linked elastomer Die suitable for use. The operation can be sheeting from which gaskets or other Screw spilt into three distinct areas of activity: finished products can be . Figure 16: Screw extruder.

Material preparation This includes all the operations up to the point of moulding.

Moulding This includes turning the material into a cross-linked product.

Post moulding operations This involves

Manufacturing techniques finishing the product and ensuring that it meets all the necessary quality requirements.

Material preparation Figure 17: Screw extruder producing Elastomers extrusion for the manufacture of 'O' rings.

The elastomer from the mixer is Figure 15: Typical calender. normally available either as a sheet of Ram extruders. For a ram extruder the predetermined thickness or split into elastomer needs to be rolled and warmed, rolls of material of known thickness and Strip usually by placing it in a bath of hot water width. The latter option is often used or taking it directly from the mill/calender. where the material is fed directly into an If the elastomer is to be fed directly into This roll is then placed into the cylinder machine. A number of an injection moulding machine, the sheet housing the ram. The head of the extruder options are available in the development from the mixing stage can be slit to create containing the die is then locked in place of the pre-form (blank) to be used in strips of elastomer. These are then fed at the front of the extruder and the ram the moulding process. These include directly into the screw feed of the injection traversed forward, forcing the material calendered sheet, strip form material moulding machine. out of the die orifice. When the material and extrusion. exits the die it can either be pulled off in lengths or cut to length/weight by a Extrusion rotating blade affixed to the front of the Calendering machine. For most materials (silicone There are two main types of extruders: being an exception) the extruder cylinder A calender is similar to a mill and has screw and ram. and head are heated. two or more rollers (known as bowls) that can be adjusted to change the size Screw extruders. Screw extruders have Extruder head of the nip which controls the thickness a screw housed within a barrel, with the of the elastomer sheet. These bowls can screw turned by mechanical means. The Rolled rubber Die be mounted horizontally or vertically and elastomer is first fed into the barrel via a hopper and then forced down the barrel Cylinder range in size from small laboratory devices Ram to devices weighing several tonnes. by the screw whilst heat is added (created by the action and via the heated The material from the mixer is fed between barrel and screw). At the end of the barrel, the nips on the calender and pulled away in the extruder head, is a die through from the bowl by a manual or mechanical which the material is forced out. The die is device. The desired sheet thickness is profiled to produce elastomer shaped for Figure 18: Ram extruders. achieved by adjusting the nips. the next stage of processing.

16 Manufacturing techniques

In some cases, the extrusion profile created is the finished product and needs techniques Manufacturing to be cross-linked to retain its shape. This, for example, is the process used for the manufacture of windscreen wiper blades. For products manufactured using this technique, the extrusion is cured either as it exits the machine through a hot box, or by other means following the extrusion process, such as autoclaving.

Non-elastomeric components

If non-elastomeric components are required to be added to the elastomer when the product is formed in the moulding process, for example metal parts for reinforcement, these need to be prepared to ensure that they bond to the Moulding The advantages of compression moulding elastomer. Depending on the component are the lower cost of the moulds, the material, different bonding techniques are Three principal moulding techniques are large sizes of mouldings possible and required. In all cases involving bonding used to manufacture elastomer products: the relatively quick changeover between agents, the surface of the component compression moulding, injection different moulds. needs to be treated to remove any grease moulding and transfer moulding. or oils. Where possible a key is created The main disadvantage is output, as on the surface of the component to they are generally loaded and unloaded which any bonding agents are applied. Compression moulding manually and the elastomer is often Once the component is treated it should placed into the cavity ‘cold’ so cure times be appropriately stored so that the Compression moulding describes the are longer. Some difficulties that can preparation is not affected by moisture or forming process in which an elastomer occur are positioning the blank in the other elements in the atmosphere. profile is placed directly in a heated cavity and the ‘flash’ that results from the mould, then softened by the heat, and additional material placed in the cavity to forced to conform to the shape of the ensure compression in the cavity when mould as the press closes the mould. the mould shuts. Another disadvantage of this type of moulding is the care and time The presses are mostly hydraulically required to manufacture the blank (weight driven and can be either upstroking, and profile) to place into the cavity. where the lower platen moves up and the upper platen is fixed, or downstroking, where the upper platen is driven downwards and the lower platen remains fixed.

Top heated platen

Upper mould half

Rubber blank Lower mould half

Bottom heated platen

Figure 19: Compression moulding press.

Figure 20: Open mould (left) with elastomer blank placed in cavity, and (right) mould closed forming the finished product profile.

1717 Manufacturing techniques

Tie Bar Feed Nozzle Clamping Unit Hydraulic Heaters Mould Motor Ejector & Gears

Stationary Rear Platen Platen Moveable Platen

Manufacturing techniques Figure 21: Horizontal injection moulding machine. Figure 22: Injection moulding machine.

Injection moulding In the case of liquid silicone rubber (LSR), grooves and a mat of material left in the the injection machine can also be used transfer pot. Injection moulding machines are either to mix the two LSR constituents before vertically or horizontally run. An example of injection into the mould. The advantage of this process over a horizontal machine is shown in Figure 21. conventional compression moulding is the ability to form delicate parts and to mould Injection moulding is a process where Transfer moulding parts having inserts requiring specific heated elastomer is injected into a closed positioning within the product. It also has cavity via a runner system. Uncured Transfer moulding is a combination of simpler blank requirements and faster elastomer is fed into the injection cylinder injection moulding and compression cure times, since the elastomer heats up where it is preheated and accurately moulding and takes place on a quickly as it is transferred from the pot to metered into the mould. This is done by compression press. Elastomer of set the cavity. controlling the pressure, injection time weight is placed in the transfer pot, and, and temperature. as with compression moulding, the pot is The main disadvantages are the additional closed by the press forcing the elastomer cost of the tooling, the additional waste The advantages of injection moulding down the sprues and into the cavity. material due to the pot and sprue, and are its suitability for moulding delicate A small amount of excess material difficulties that can be experienced parts, shorter cycle times compared with flows out of the cavity through vents, with when transferring high hardness or high compression moulding, the high levels of other excess material lying in the sprue molecular weight materials. automation that can be introduced in the process and lower levels of flash since the mould is shut when the material is Top heated platen injected. Upper mould half The main disadvantages of injection moulding are the costs of the tool, the longer changeover times resulting from the more complex tooling and the waste Rubber blank of material in the runner system where Transfer pot a hot runner system is employed (for Sprue thermoset materials). Material waste also Lower mould half occurs when jobs are run sequentially with either differing materials or different colours, which requires extensive purging Bottom heated platen of the machine. Figure 23: Transfer moulding process showing mould open (left) with the elastomer blank in the transfer pot, and mould shut (right) with the elastomer injected into the cavity.

18 Manufacturing techniques

Post-moulding operations With some materials a post-cure is Inspection required in an autoclave. This is a device techniques Manufacturing Following the moulding process a which generally uses steam to post-cure One of the final steps in the number of operations may need to the components under pressure. An manufacturing process is finished part be performed to finish the product. additional benefit of this type of post- inspection. This can be carried out by These include post-curing, removal curing is that it provides a comprehensive hand, with an inspector visually examining of flash and injection sprues, other ‘wash’ of the products and is often and measuring the products. Alternatively, trimming requirements, testing, etching, used to finish products for food or for reasonably simple components, inspecting and packaging the product. pharmaceutical applications. inspection can be performed by machine, using a contact or non-contact system, in Another use of autoclaves is to cure some cases working fully autonomously. Trimming products (not to be confused with Autonomous measurement is particularly post-curing, as in this instance it is the suited to high volume production runs for Various trimming techniques are primary curing process) that are too products such as ‘O’ rings. available depending on the size and big or unsuitable to be moulded. shape of the component and the type of Examples of such products are extrusions elastomer used. They include cryogenic, and sheetings. where the part is cooled below its glass transition temperature and tumbled and blasted with beads to remove the flash, Product testing buffing using abrasive wheels and belts, cutting the finished shape using cutting In some cases the products manufactured dies, formers, knives and in some need to be non-destructively tested cases , using lathes to and to ensure they meet the required chamfer components, and rota finishing specifications. This normally takes the where components are rotated amongst form of a hardness test. abrasive stones or other abrasive media. Destructive testing of representative samples is also often carried out (compression set and immersion testing).

Figure 25: Non-contact inspection.

Packaging

The finished products need to be packaged appropriately before shipping, xsprotection from dirt and dust, etc, some components may need to be sealed against moisture or contamination by other fluids, or protected against UV light.

Figure 24: Component (left) before and (right) after trimming.

Post-curing Etching

For many heat resistant elastomers, For some applications, the elastomer such as fluorocarbon and silicone product requires to be etched to provide materials, it is necessary to supplement identification of its origin for branding the press cure with an oven post-cure to purposes or other customer requirements. eliminate residues from peroxides and complete the curing process.

1919 Material & product testing

Material & product testing Hardness – BS ISO 48 Tensile strength – BS ISO 37

The test methods and terminology used The hardness quoted for an elastomer This is a measure of the stress required to characterise the physical properties usually refers to the result obtained from to rupture a standard test piece. Tensile of elastomers can differ from those a “standard” test piece, see BS ISO 48. strength is a useful quality control tool of other materials. The following is an Results obtained from a non-standard to monitor inter-batch consistency. It overview of terms and methods. test piece e.g. seal, are normally referred does not however give any indication of to as “apparent hardness”. The results extrusion resistance for example. obtained from measuring the hardness of a material batch, using a standard test Temperature has a marked effect on Cure characteristics piece, may differ from the results obtained the strength properties of elastomers, from testing product manufactured from whether tensile, tear or compressive. How a rubber cures over time is the same batch. Room temperature testing rarely gives measured on a rheometer. As the an accurate indication of their strength compound cures between the hot platens at elevated temperatures: for example, it becomes stiffer. This is measured via a at 100°C (212°F) some elastomers retain strain gauge connected to an oscillating only 10 per cent of their room temperature Material & product testing rotor in contact with the rubber. The strength. To create a more meaningful (resistance to ) of the result, tensile testing can be performed at material is plotted on a graph against elevated temperatures. time, known as a rheograph (Figure 26). This information predicts the moulding characteristics, since the rheograph shows the time available to load the press (red box), the time of cure (blue box) and the final state of cure (green box). Figure 27: Hardness machine.

The final state of cure shown in the green box is not always a flat line (plateau). For some rubbers the cure continues, as shown by the upper dashed line. This is known as a marching modulus. In time however this line would plateau. For other materials, such as natural rubbers, a reversion of the cure can occur, as shown by the lower dashed line. This is due to the heat breaking the actual polymer chains rather than the cross links formed during the curing stage. Figure 28: Tensile testing machine. Marching modulus

Elongation at break – BS ISO 37

This refers to the elongation (percentage strain) measured at the point of rupture. A high value is important if substantial stretching is required during fitting, and also in applications where seals are

Torque energised across relatively large gaps.

Reversion

Time

Figure 26: A rheograph enables moulding characteristics to be predicted.

20 Material & product testing

Modulus – BS ISO 37 set is highest at the extremes of an 100 elastomer’s operating capability: 90 Material & product testing Reduction in sealing force, % In elastomer terminology this is defined irreversible at high temperatures because 80 as the stress at a particular strain or of chemical degradation, and reversible 70 elongation (whereas in metals it is the at low temperatures because of physical 60 ratio of stress to strain as this is a linear stiffening and ‘freezing’. 50 40 relationship). Modulus tends to increase 30 Compression set, % with hardness, with higher modulus 20 materials, in the main, being more 10 resistant to deformation and extrusion 0 (see Figure 29). 0 20 40 60 80 100 120 140 160 180 Time/Hours

Figure 32: Compression set and Slope = corresponding reduction in sealing force Young’s Modulus Stress, σ for a sample nitrile elastomer at 100°C. (psi, MPa) Metal

Elastomer Modulus = stress Fluid resistance – BS ISO 1817 at any given strain Figure 30: Example of compression set apparatus. Immersion of samples in various fluids at Strain, ε (ratio or %) differing temperatures, followed by testing Figure 29: Comparison of modulus for Compression stress relaxation for volume change, tensile strength elastomers and metals. (CSR) testing change, hardness change, etc, will give a reliable indication of how well the Figure 1 – Volume expansion vs. temperature A compression stress relaxometer measures material will perform under similar service Compressionfor set various – BS ISOElast-O-Lions 815-1 & -2 the residual sealing force of elastomer conditions. Complications arising from (reference volume of 1 at 73°F/23°C) 1.04 samples as they are held between parallel solubility parameters mean that this is an In any seal, at a constant temperature, a 1.02 plates under a constant strain. important test, see page 27. mechanically loaded elastomer will exhibit 1 time dependent relaxation. If the seal is Compression jigs can be placed in e, relative 0.98

subsequentlyum unloaded, the elastomer l 0.96 various environments, for example an will Vo recover towards its original shape oven. The jig is placed in the relaxometer, 0.94 to an extent defined by chemical and and a reading taken. After this the jig is physical0.92 degradation. Such relaxation –200 –150 –100 –50 0 50 100replaced in the conditioning environment, and recovery phenomenaTemperature, are °determinedC allowing changes in sealing force to be primarily by the viscoelastic101 nature201 of985 plotted over time. elastomers and by the chemical reactions that occur between the material and the environment.

Compression set is widely used for assessing recovery. Standard methods require a compressedFigure 2 – L.T sample. Torsion to modulus be exposed600 for a fixed time, at a fixed temperature,500 and then allowed to FR58/90 recover400 (generally for 30 minutes) at room temperature. Compression set odulus, MPa 300 is expressed as the percentage of the 200

sion m originalr deformation not recovered after

To this recovery100 period: 0 per cent indicating full recovery,0 100 per cent indicating no –25 –20 –15 –10 –5 0 recovery. Temperature, °C

As many types of elastomer recover more quickly at elevated temperatures, the test is used primarily as a quality control tool; high compressionFigure set 3 –is L.T not. Retraction conducive Figure 31: Compression stress relaxometer. to long100 term sealability. Compression 90 80 21 70 60 50 40

Retraction, % 30 20 10 0 –50 –40 –30 –20 –10 0 10 Temperature, °C Material & product testing

over the same abrasive surface. This procedure avoids loss of cutting power and clogging of the abrasive media with detritus. • Pico Machine (ASTM D2228) which abrades by means of knives of controlled geometry and sharpness.

It is sometimes wrongly believed that tensile strength is related to abrasion resistance, and while a high tensile strength compound can have good abrasion resistance the converse can also be true. Abrasion resistance is related more to polymer type and the nature/level of compounding ingredient used. High

Material & product testing modulus and high tear strength can be Figure 34: Temperature retraction test Figure 33: Example of Gehman test better correlated to abrasion resistance equipment. equipment. but the relationships are not definitive. Low temperature testing Various types of test pieces can be used, Abrasion resistant elastomers must there- and depending on the method employed fore be specifically developed. Torsion modulus – BS 903 Pt. A13/ the maximum or median force achieved is ISO 1432. Also known as the Gehman used to calculate the tear strength. test, this is used to measure the torsion Air ageing – BS ISO 188 modulus by twisting a strip test piece, at room temperature and several reduced Abrasion resistance Exposure to air or oxygen-rich temperatures, to give a temperature- environments at elevated temperatures modulus curve. The result is often Abrasion damage can occur when can cause gradual loss of mechanical quoted as the temperatures at which the there is dynamic motion against an properties. Many of these changes occur modulus is two, five, ten or 50 times the abrasive counterface, or when the sealed at a molecular level and are irreversible. value at room temperature. However, a environment is intrinsically abrasive and They include chain and/or crosslink more useful measure is the temperature either passes across or impinges upon scission, crosslink formation and crosslink at which the modulus increases to a the seal. translocation. Samples (tensile, hardness, predetermined value, normally 70MPa etc) are placed in ovens at a controlled (10,153psi), which corresponds to the Standard abrasion tests depend on elevated temperature for a pre-determined limit of technically useful flexibility. producing relative motion between a time, removed, allowed to cool and rubber sample and an abrasive surface, then tested and compared against the Temperature retraction – BS ISO 2921. pressed together by a predetermined original properties for the material at room This test is carried out by elongating a test force. Unfortunately, such tests do not temperature. specimen and freezing it in the elongated correlate particularly well with application position. The specimen is then allowed experience, or with each other! Machines to retract freely whilst the temperature in national standards include: Ozone testing, weathering and UV is slowly raised at a uniform rate. The • Akron Machine (BS ISO 4649), where percentage retraction can be calculated a rubber disc is rotated so as to drive, Deterioration in physical properties can at any temperature from the data by its edge, an abrasive wheel, the two occur when elastomers are exposed to obtained. In practice, the temperature being pressed together by a constant weather. This includes cracking, peeling, corresponding to 30% retraction (TR30) force. The abrasive action is produced chalking, colour changes and other roughly correlates to the limit of useful by tilting the plane of the disc relative to surface defects that ultimately may lead flexibility. Often however a figure of 10% the wheel. retraction (TR10) is quoted. to failure. The most important causes of • National Bureau of Standards Machine deterioration are ozone and sunlight. (ASTM D1630), where a rubber test Tear Strength – BS ISO 34-1 & -2 block is pressed, by constant force, Ozone resistance is determined by the against a rotating cylinder. appearance and magnitude of cracks Tear strength is a measure of the • Conti Machine (DIN ISO 4649) which is formed on the elastomer surface when resistance of an elastomer to tearing. It similar to the above, but the test block is subject to surface tensile strain in an is measured using a tensile test machine traversed slowly along the length of the atmosphere containing specific levels operating at a constant rate of traverse cylinder so as not to pass repeatedly of ozone. until the test piece breaks. 22 Material & product testing

Radiation testing measured by increasing the voltage until Infrared spectroscopy electrical breakdown occurs. Patterned Infrared spectroscopy (FTIR) involves testing product & Material Radiation interacts with elastomers in surfaces should be wetted beforehand passing infrared radiation onto or through two ways: chain scission, which results in with a conductive solution. a sample. The of peaks and reduced tensile strength and elongation; troughs in the spectra produced then and cross-linking, which increases tensile enables the components in the elastomer strength but reduces elongation and Material analysis to be identified. This technique is valuable finally leads to embrittlement. in identifying materials, failure analysis Differential scanning calorimetry and compound development. Radiation testing is usually carried out Differential scanning calorimetry (DSC) with sequential exposure to radiation and is the most frequently used thermal heat. Combined radiation and thermal analysis technique. It compares the Chemical compatibility ageing can be performed to act as exothermic and endothermic reactions of spot checks. Testing consists of visual samples with a reference while subject Many chemical species cause inspection, followed by compression set to controlled heating. The technique degradation to elastomeric compounds, measurements and hardness tests. enables the accurate determination of either by attacking the polymer or some of cure characteristics, glass transition its compounding ingredients. Degradation temperature, crystallisation and melting caused, for example, by water and amines Permeation testing point. The technique is valuable for failure is irreversible. It is often seen as elastomer analysis and compound development. hardening or softening, increased Permeability tests are carried out in a test compression set, cracking, and in the cell maintained at constant temperature most extreme cases, dissolution. and divided by a disc test piece into high pressure and low pressure sides. The After immersion in the test fluid under high pressure is held constant defined conditions of time, concentration and the volume of gas and temperature, samples can be tested permeating into the for hardness and swell. This may be low pressure side can followed by elongation at break testing. be measured by a capillary tube. A table showing the chemical compatibility of the main elastomers is given in the Material selection section on page 25.

Thermogravimetric analysis Thermogravimetric analysis (TGA) is another thermal analysis technique and provides information complementary Electrical testing to DSC. TGA continuously weighs the sample to high accuracy as it is heated. Elastomers are used extensively in During the heating, different components electrical applications because they of the elastomer burn off at different provide an excellent combination of temperatures and the loss in weight flexibility and electrical properties. provides a precise indication of the components present in the formulation. The anti-static and conductive properties The technique is valuable for compound of elastomers are determined by development, process control and failure measuring their electrical resistance. analysis. Measurements are obtained by placing a disc of elastomer of known thickness between two electrodes and measuring the current flow. Electrical strength is

23 Material & product testing

Product testing RGD testing test vessel. After purging the vessel with Although rapid gas decompression test gas to remove any air, the vessel is Environmental testing (RGD), also known as heated and pressurised with test gas to Environmental testing is used to predict decompression (ED), is generally found the chosen pressure and temperature how products will behave in actual in the oil and gas industry, it can be and held for an exposure period. conditions of use. Test pieces are placed experienced in any application where in a controlled environment, such as there is a rapid drop in gas pressure. Decompression is then performed at a low or high temperatures or , RGD damage has been noted in sealing predetermined rate while maintaining the and can be tested for properties such as applications ranging from paint guns and test temperature as constant as possible. sealing performance. extinguishers to systems containing After a hold period at ambient pressure refrigerants. the pressure cycling is repeated for typically five to ten cycles. RGD damage consists of structural failure in the form of blistering, internal cracking The seals are then externally examined and splits caused when the gas pressure, for visual appearance and cut into four to which the seal is exposed, is rapidly radial sections. Magnification is used to reduced. examine the cross-sections for internal Material & product testing cracks, and the samples are rated for The elastomeric components of a RGD damage according to the number system are, to a greater or lesser extent, and lengths of any cracks found. susceptible to the permeation and diffusion of gases dissolving in their surface. With time, these components will become saturated with whatever gases are in the system. Under these conditions – as long as the internal gas pressure of the elastomer remains at equilibrium with Figure 35: Low temperature environment chamber for testing sealing on static the ambient pressure – there is minimal 'O' rings. damage, if any, and no deterioration in performance of the elastomeric Specific application testing component occurs (unless caused by It is important to replicate operating other factors such as chemical or thermal conditions. One of the best ways to do degradation or by extrusion damage). this in a controlled environment is to replicate these conditions on specific test When the external gas pressure is equipment. For example if the application removed or pressure fluctuations occur, is rotary, testing can be conducted using large pressure gradients are created different shaft speeds, pressures, shaft between the interior and the surface of finishes, media, temperature, etc. the elastomeric component. This pressure differential may be balanced by the gas simply diffusing/permeating out of the elastomer, especially if any external constraints are not removed. Figure 37: RGD test rigs. However, if the physical properties of the elastomeric compound cannot resist crack and blister growth during the Load deflection testing permeation process, then structural failure is the inevitable result. The results of this test are highly dependent upon sample dimensions, RGD damage can manifest itself in due to the ‘shape factor’ effect when Figure 36: Example of bespoke test various ways from internal splits that are testing elastomers. The ‘shape factor’ is equipment for evaluating a rotary not visible on the surface of the seal to the ratio of the area of the test sample cartridge assembly. surface blisters, fractures and complete compared to the area of the sample that fragmentation. is ‘free-to-bulge’.

Testing is usually carried out on constrained 'O' rings which are placed in a

24 Material selection

Material selection Temperature Low temperature applications Material selection Material

When selecting elastomers for specific The important considerations when When elastomers are cooled to applications, a number of criteria need selecting appropriate materials for sufficiently low temperatures they exhibit to be considered, including the expected applications involving temperature are the characteristics of glass, including service conditions, chemical compatibility to know the maximum and minimum hardness, stiffness and brittleness, and with service fluids, physical factors such continuous operating temperatures, do not behave in the readily deformable as temperature, life prediction and design intermittent maximum and minimum manner usually associated with considerations. exposure temperatures and times, system elastomers. As temperatures are raised, pressure for low temperature applications, the segments of the polymer chain gain The main considerations can be broken whether there will be thermal cycling sufficient energy to rotate and vibrate. At down as: and also environmental factors involving high enough temperatures full segmental rotation is possible and the material Static or dynamic: If there is movement, knowledge of the media to which the behaves in the characteristic rubbery way. is it rotary, linear, due to thermal elastomer will be exposed. The usefulness of an elastomer at low expansion or pressure cycling? temperatures is dependent on whether Temperature: Continuous, minimum, High temperature applications the material is above its glass transition maximum, thermal cycling, glass temperature (Tg), where it will still behave transition temperature shift. The limit to the upper temperature at elastically, or below its Tg, where the Application: Clearance gaps, surface which an elastomer can be used is material will be hard and relatively brittle. finish. generally determined by its chemical Media: Chemical compatibility, solubility stability, and will vary for different Elastomers between their useful low parameter. elastomers. Elastomers can be attacked temperature flexibility and brittle point go through a stage which is referred to as Pressure: Continuous, maximum, by oxygen or other chemical species, and 'leathery' (see Figure 38). fluctuations, rate of decompression. because the attack results in a chemical reaction, their potency will increase with Aesthetics: Colour, surface finish, Brittleness temperature. avoidance of split lines, etc. ) e l “Leathery” a Cost: Primary consideration, trade off in Degradative chemical reactions are c region o g s performance, cost of failure, total cost of l

generally of two types. The first are those a ownership (TCO). that cause breakage of the molecular

y o n Limit of full l Approvals and specifications: chains or cross-links, softening the rubber l flexibility u s a

International standards, such as ISO, because they weaken the network. The ( industry standards or customer specific. second are those that result in additional Torsion modules, MPa or psi cross-linking, hardening the rubber, and Temperature often characterised by a hard, cracked or Figure 38: Variation of torsion modulus Static or dynamic degraded forming on the elastomer with temperature. component. When considering a material for an The low temperature performance of a application, it is important to understand It is also important to understand the full material can be categorised in a number whether the material will be subjected to application details so the right material of ways and specific laboratory tests are static or dynamic conditions. For example, selection can be made. Elastomers performed to give low temperature limits if the material is subjected to dynamic significantly weaken at elevated based on sample flexibility and retraction forces then it may require enhanced temperatures and in the case of seals, properties. These tests are described abrasion resistance and excellent thermal can result in a significant reduction in in greater detail in the Material testing conductivity properties. The application extrusion resistance. For applications section on page 20. details required for a dynamic application involving elevated temperatures, would include whether it is a rotary, especially at high pressures, anti- reciprocating or vibrating environment. It extrusion elements may also need to would also be important to understand be used, either incorporated into the whether the application would be seal design or added as an additional subjected to thermal cycling, as this small component when fitting the seal. amount of dynamic movement may also need to be considered in selecting the correct elastomer.

25 Material selection

Tg shift. According to conventional • However, if pressure is applied Media theory the free volume of an elastomer is before the temperature is reduced, constant at any particular temperature. the elastomer can often operate Fluids can affect elastomers in two ways: This is why elastomers are generally at temperatures far below its physical interaction, such as swelling, and considered incompressible: ie, their recommended minimum operating chemical interaction. The first is generally volume will not change regardless of any temperature. reversible, while the other is not. deforming force, although the shape will • Reducing the temperature of an alter. It is here that conventional theory elastomer causes it to harden and lose breaks down when considering high flexibility. This occurs gradually and Physical interaction applied pressures, because the free the compound changes from flexible volume can be reduced. to brittle, becoming progressively The degree and type of physical

Material selection more leathery. The leathery phase for interaction depend on a number of This manifests itself as a Tg shift. Applying some compounds can span a broad factors, including the cross-link density pressure to an elastomer in a hydraulic temperature range. and type, the filler level and type, the or gas system results in an increase in Tg • Reducing the temperature of an polymer type, the type and viscosity of the at a rate of approximately 1°C per 5.2MPa elastomer to its brittle point or below media and the solubility parameters of the (1.8°F per 750psi). Hence a system has no detrimental effects on its polymer and media. pressure of 103MPa (15,000 psi) will chemical resistance and its physical reduce the low temperature limit of the properties will return to original values The effects of the physical interaction of compound by approximately 20°C (36°F), once the temperature is returned to fluids (elastomers are fluids) are normally ie, if the elastomer had a low temperature ambient. observed as the swelling of an elastomer limit of -40°C (-40°F) at atmospheric • Subjecting an elastomer component to through fluid absorption from its pressure, at 103MPa (15,000 psi) it will changes in applied stress and strain, environment. This is generally reversible. have a low temperature limit of -20°C (-4°F). shock loading or impact, when at or The magnitude of the effect depends on The Tg shift can be seen in Figure 39. below the brittle point, can result in the environmental fluid, the elastomer =Tg shift damage and fracture of the elastomer. and the temperature, and reflects the Approx. 1°C higher readiness with which the elastomer and per 5.2MPa pressure its surroundings mix, ie, the relative Thermal cycling. It is also important to magnitudes of the solubility parameters of understand whether the elastomer will the two components. be subjected to thermal cycling. Even for elastomers that cycle within their upper Solubility parameter (δ) is a

(usually on a log scale) and lower temperature limits, some thermodynamic property related to the Torsion modulus, MPa or psi modulus, MPa Torsion Temperature problems can result where elastomers energy of attraction between molecules. have been heavily plasticised or had Thus if a fluid has a solubility parameter Figure 39: Effect of pressure on torsion modulus. other additives included to gain low close to that of an elastomer then temperature flexibility or to reduce cost. In attraction (and mixing potential) will be In dynamic applications, Tg can also these cases the additives can leach out at high, and high volume swell will result. rise with an increase in frequency. For elevated temperatures and reduce the low The level of volume swell will decrease example, between 1 and 50Hz, Tg can temperature capability of the elastomer as the difference in solubility parameters rise by about 10°C. when it is next cycled. increases between an elastomer and its environment. Fluid viscosity also has a significant effect. Other important considerations for low temperature applications.There Application are some applications where acceptable The effect of high volume swell is to elastomer performance can be achieved When considering elastomeric materials, degrade physical properties (such at temperatures far lower than the there are certain application conditions as tensile strength, modulus and tear recommended minimum temperature. which need to be known ahead of strength) and to reduce the elastomer’s In these instances the following must be selecting an appropriate material, hardness. These actions can give rise taken into consideration: which are in addition to knowing the to seal damage due to extrusion between • If an elastomer is cooled below its temperature, pressures, media and metal parts, amongst other things. In minimum recommended operating whether the conditions are static or general, volume swells greater than 10 temperature prior to applying system dynamic. These include the surface per cent have a negative effect whilst pressure (gas or liquid), bypass leakage finish of the materials in contact with lower levels of swell can benefit by can occur. the elastomer, any extrusion gaps increasing or maintaining sealing which may exist in the application and contact stress.

housing design.

2626 Material selection

Occasionally elastomer shrinkage is Chemical interaction observed, which occurs as a result selection Material of constituents within the elastomer Many chemical species cause such as process aids, plasticisers and degradation to elastomeric compounds, protective systems being leached out. either by attacking the polymer or This is rare but can cause loss of seal some of its compounding ingredients. interference, increase of hardness, system Degradation caused (for example) by contamination, reduced low temperature water and amines is irreversible. It often flexibility and a reduction in the ageing results in seal hardening or softening, characteristics of the material. increased compression set, cracking, and in extreme cases, dissolution. Such degradation is often highly dependent on exposure temperature, in terms of both the reaction initiating in the first place, and then the rate at which the reaction proceeds. Laboratory tests may Case history not provide a true indication of reaction potential or elastomer compatibility, An example of the dangers of unexpected solubility effects concerns a solubility particularly if performed at reduced parameter analysis of a 70:30 mixture of iso-octane:methanol with low ACN nitrile. temperatures or exposure times. A weighted average of the solubility factors for the mixture indicates an expected volume swell of 20%. However, as Figure 40 shows, the volume swell in practice Examples of agents that can cause was almost 60%. It can be seen from Table 3 that the solubility parameter for a chemical degradation are acids, bases, low nitrile is 9.3, which matches the solubility parameter of the 70:30 mix of iso- water, hydrogen sulphide, zinc bromide, octane:methanol. oxygen, ozone, mercaptans, free radicals and biocides as well as and 70 ionising radiation. 60

50

40 ∆Ⅴ,% 30

20

10

0 I so-octane 100% 70:30 Methanol 100%

δ= 6.9 9.3 14.7 Figure 40: Variation of volume swell for low ACN nitrile in blends of iso-octane:methanol.

Fluids Polymers • Iso-octane 6.9 • EPDM 8.2 • Hexane 7.3 • FEPM 9.0 • Di-ethyl ether 7.7 • NBR (low) 9.3 • Xylene 8.8 • NBR (high) 11.0 • Toluene 9.0 • HNBR - as NBR • MEK 9.6 • FKM 10.7 to 11.5 •  13.0 • Methanol 14.7 • Water 23.2

Table 3: Solubility parameter, δ, for selected fluids and polymers at 23°C.

27 Material selection

Chemical compatibility chart

Hydraulic fluids fire resistent

MATERIAL

Material selection TYPE Air orWater oxygenWater – up Diluteto– 80°C80°CDilute acids andLower alkalis above AldehydesalcoholsAminesChlorinatedEthersKetones solventsHydrocarbonsHydrocarbonsLeaded – Kerosenealiphatic petrol – Animalaromatic ()Fuel oils oils Lubricatingand and fatsLubricating dieselSilicone oils oils – Vegetable oilsmineral oils –Hydraulic syntheticand oilsChlorinatedgrease fluidsOil in – Waterwater mineralWater emulsionsin oilbased Phosphateemulsions– glycolPhosphate based esters esters – aliphatic – aromatic

Acrylic ACM 2 4 4 4 4 4 4 4 3 3 4 1 3 1 1 1 1 1 2 1 1 1 4 4 4 4 4 4

Tetrafluoroethylene propylene FEPM 1 1 1 1 1 1 1 1 3 4 4 1 3 2 1 1 1 1 2 1 1 1 2 1 1 1 1 2

Butyl IIR 1 1 2 1 1 1 1 1 4 4 1 4 4 4 4 2 4 4 4 1 3 4 4 4 4 1 2 2

Chlorosulphonyl polyethylene CSM 2 1 3 4 1 1 3 4 4 4 4 3 4 4 4 3 3 4 4 1 2 2 4 4 3 1 4 4

Epichlorohydrin CO/ECO 2 1 2 3 2 2 4 1 4 4 4 3 4 1 1 1 1 1 4 1 1 1 4 2 2 2 4 4

Ethylene-propylene EPM/EPDM 1 1 1 2 1 1 1 2 4 3 1 4 4 4 4 2 4 4 4 1 3 4 4 4 4 1 1 2

Fluorocarbon FKM 1 1 3 1 2 4* 4* 4* 1 3* 4* 1 1 1 1 1 1 1 2 1 1 1 2 1 1 2 1 1

Fluorosilicone FQ 1 1 2 3 2 1 4 4 2 3 4 1 1 2 1 1 2 1 2 1 1 2 2 2 2 2 3 3

Hydrogenated nitrile HNBR 1 1 1 1 2 1 2 1 2 4 4 1 3 2 1 1 2 1 1 1 1 1 4 2 2 2 3 4

Perfluorocarbon FFKM 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1

Natural NR 3 1 2 3 2 2 3 2 4 4 4 4 4 4 4 4 4 4 4 1 4 4 4 4 4 3 4 4

Neoprene CR 1 1 2 3 1 1 3 2 4 4 4 2 4 3 2 2 3 2 3 1 3 3 4 4 4 3 4 4

Nitrile NBR 2 1 2 3 2 1 3 2 3 4 4 1 3 2 1 1 2 1 2 1 1 1 4 3 3 1 4 4

Polyurethane AU/EU/PU 1 4 4 4 4 4 4 4 3 2 4 2 4 2 2 2 3 2 4 1 2 1 4 4 4 4 4 4

Silicone Q 1 1 2 3 2 1 2 2 4 4 3 3 4 4 4 2 4 3 3 4 1 4 3 3 4 2 2 3

Key 1 Very good; 2 Good; 3 Fair; 4 Not recommended. * Fluoroelastomer grades are available that offer resistance to these chemicals. Note: These figures are for guidance only. Service life will depend on type of application, whether static or dynamic specific pressure medium, temperature cycle, time of exposure, etc.

2828 Material selection

Pressure However, when the external gas pressure Aesthetics is removed or pressure fluctuations occur, selection Material The effect pressure can have on the low large pressure gradients are created Aesthetics need to be taken into account temperature behaviour of elastomers between the interior and the surface of in applications where appearance is and problems it can create with the elastomeric component. This pressure important, for example in consumer extrusion, have already been discussed differential may be balanced by the gas products. in this section. Another important simply diffusing/permeating out of the consideration when discussing elastomer, especially if any external Thermoplastic elastomers lend pressure is to understand the direction constraints are not removed. But if the themselves to producing products with or directions it is applied from, its physical properties of the elastomeric high gloss and are readily available in a magnitude and whether it fluctuates. compound cannot resist crack and blister variety of different colours. growth during the permeation process, Air orWater oxygenWater – up Diluteto– 80°C80°CDilute acids andLower alkalis above AldehydesalcoholsAminesChlorinatedEthersKetones solventsHydrocarbonsHydrocarbonsLeaded – Kerosenealiphatic petrol – Animalaromatic (gasoline)Fuel oils oils Lubricatingand and fatsLubricating dieselSilicone oils oils – Vegetable oilsmineral oils –Hydraulic syntheticand oilsChlorinatedgrease fluidsOil in – Waterwater mineralWater emulsionsin oilbased Phosphateemulsions– glycolPhosphate based esters esters – aliphatic – aromatic The latter consideration can lead to significant elastomer damage if the structural failure is the inevitable result. pressure is reduced rapidly. This is An example of an elastomer failing due referred to as rapid gas decompression to RGD can be seen on page 37 in the Cost (RGD), formerly known as explosive failure modes section. decompression. Cost is almost invariably a primary consideration. It usually needs to be balanced against performance requirements, taking into account the Rapid gas decompression total cost of ownership. A small reduction in price can lead to a disproportionate Although rapid gas decompression deterioration in quality. (RGD) is generally found in the oil and gas industry, it can be considered a possibility in any application where there is a rapid drop in gas pressure above Approvals and specifications 50 bar. A variety of national and international RGD damage is structural failure in the standards organisations are responsible form of blistering, internal cracking and for preparing and issuing standards splits caused when the gas pressure relating to elastomers. These include to which the elastomer is exposed is the International Organisation for rapidly reduced. Figure 41: Cross-section of 'O' ring showing Standardisation (ISO), the British RGD damage. Standards Institution (BSI), the American The elastomeric components of a Society for Testing and Materials (ASTM), system are, to a greater or lesser extent, the Deutsches Institut für Normung (DIN), susceptible to the permeation and the European Association of Aerospace diffusion of gases dissolving in their Factors that influence RGD resistance are: Industries (AECMA) and the Association surface. With time, these components • Polymer type Français de Normalisation (AFNOR). will become saturated with whatever • Compounding, mixing and processing gases are in the system. Under these • Gas type/mix The work of the international bodies conditions, as long as the internal gas is supported by national standards pressure of the elastomer remains at • Temperature organisations, who are in turn supported equilibrium with the ambient pressure, • Pressure by trade associations, companies, there is minimal damage (if any) and • Rate of decompression government departments and local no deterioration in performance of • Levels of and groove fill authorities. the elastomeric component occurs – unless caused by other factors such as • Number of pressure cycles chemical or thermal degradation or by • Seal cross-section. extrusion damage.

29 Designing with elastomers

Designing with elastomers Finite element analysis (FEA) Material selection

Designing elastomeric components Finite element analysis (FEA) is a Selection of an appropriate elastomer used to be referred to as a black art, with computer-aided engineering technique material is obviously a crucial factor in the unpredictable nature of elastomers that provides an indication of the satisfying the design requirements for a often being blamed for difficulties in strength and/or deformation of a product product. A wide range of basic elastomer assessing how the product would behave under loading that might typically be types are available and within these there in service. With many modern tools now experienced in its operating environment. are an almost infinite number of variations at the disposal of engineers and with The technique is commonly used at the of formulations that can be produced. advances in machining capability and design stage for components but can manufacturing techniques, the design of also be used to help determine why parts A number of criteria need to be engineering solutions using elastomers is have failed. considered when selecting elastomers now more of a predictable science. This for specific applications, including the section explains the role of these tools FEA simulates the behaviour of a real expected service conditions, chemical and the considerations required to ensure component with an idealised mathematical compatibility with service media, the right design. model that includes the physical conditions physical factors such as temperature, life in which it operates. The finite element prediction and design considerations. Designing with elastomers model is then analysed by a finite element analysis solver, which calculates data Descriptions of the main elastomer types Advanced product quality reflecting the design behaviour to the are given on page 7. planning (APQP) applied boundary conditions, and can help to identify weaknesses or potential failures Prototyping Advanced product quality planning in the design. (APQP) is a structured method of After the initial design stage of product defining and establishing the steps Finite element analysis of elastomer development it is useful to produce a necessary to develop products that meet components is more complex than when prototype for assessment purposes. This customer requirements. It originated in compared to thermoplastic or metallic assessment may consist of visual inspection the automotive industry, but can equally parts, for example, since the composition as well as a variety of tests. be applied to product design across all of elastomers makes their behaviour more sectors. difficult to model.

APQP focuses on up-front quality planning, and subsequently determining if customers are satisfied by evaluating the output and supporting continual improvement. It consists of five phases: • Plan and define program • Product design and development verification • Process design and development verification • Product and process validation • Launch, feedback, assessment and corrective action.

There are five major activities: planning, product design and development, process design and development, product and process validation, and production. The aim is to ensure effective communication both within the manufacturer and between manufacturer and customer.

Figure 42: Example of finite element analysis.

30 Designing with elastomers

Prototyping is particularly appropriate close temperature control and timing of When making decisions on product with elastomeric materials, since moulded cycles may also be necessary. design, the total cost of ownership elastomers with Designing their complicated nature means their (TCO) should be taken into account. performance is generally harder to As well as the cost of raw materials and predict than for plastics or metal manufacture, TCO estimation should components, for example. Surface finish include life expectancy, maintenance costs and the possibly massive costs of The surface finish of elastomeric component failure. components can be important in certain Specification applications. For example, it can affect the coefficient of friction in dynamic Specifying elastomer components should applications such as seals. It can also be Tool life expectancy cover a wide range of parameters such as important where aesthetic considerations the required performance of the product, need to be taken into account, such as in Tool life expectancy depends on many its quality, the conditions under which it consumer products. factors, including the mould material, the needs to operate, the standards it needs complexity of the mould, the required to adhere to, etc. The relevant standards tolerance, and the quantity of parts to be may be international, such as the ISO, or produced. they can be industry specific or defined by Cost issues the customer. The choice of material is obviously a prime factor in the cost of the finished part. The raw materials for specialised Tolerances perfluoroelastomers, for example, can cost over a thousand times more than a Being flexible, elastomers do not basic natural elastomer. lend themselves to the same level of tolerancing as rigid materials. Tolerancing A guide to selecting the appropriate needs to take into account shrinkage, elastomer is given in the Material selection which varies with the type of elastomer, section on page 25. and particularly hardness. Soft elastomers generally shrink more than harder The manufacturing process to be varieties. Shrinkage is also affected by employed is another factor affecting cure time, temperature, pressure, inserts, costs. Factors include the moulding post-cure, etc. technique, usually compression, injection or transfer moulding, with compression As shrinkage in elastomers is a volume generally having the lowest mould costs effect, complex shapes in the moulded and injection moulding the highest. product or the presence of inserts may have the effect of restricting the Manufacturing processes are discussed in shrinkage in one dimension and the Manufacturing techniques section on increasing it in another. page 16.

Most insert materials such as metals, plastics or fabric have their own standard tolerances. However, when designing inserts for moulding to elastomers, other factors need to be considered, such as fit in the mould cavities, the location of the inserts with respect to other dimensions, proper hole spacing to match with mould , and the fact that inserts at room temperature must fit into a heated mould.

Precision moulding requires additional preparations for tooling, which may require extra features, cavity finishes or cavity flow provisions. In processing, very

31 Designing with elastomers

CAD/CAM

The use of CAD/CAM systems for the design and manufacture of tooling for elastomer production increases productivity and can help to improve the quality of the design. It is particularly useful in the design of tooling for complex 3D parts. Designing with elastomers

Before moving to full production of tooling on a machining centre, CAD/ CAM can be used to produce a rapid In addition to enabling drafting from prototype of the component. The sketches on existing components, CAD/ prototype, which can be made in a CAM systems accept design data in a variety of materials, mimics the function variety of proprietary file formats. The data of the elastomer part. It can also be can be supplied in the form of 2D designs economic to produce production quality or as 3D models. 2D drawings are built up parts in relatively small numbers. into 3D models by extruding, revolving or sweeping the 2D representation in The production of complex 3D shapes to create the base feature of the on machining centres is made easier by design. the use of five-axis milling machines. By enabling the workpiece to be rotated, the required complexity and accuracy of the part can be produced in one setting, avoiding the need to use spark erosion in many cases.

Figure 43: CAD/CAM images of mould When built up, the model can be design, tool path simulation and finished part. visualised in three dimensions under simulated operating conditions and with the addition of any mating components.

Unlike traditional CAD/CAM modelling systems, parametric 3D modelling uses parameters to define the model's features, such as its length or radius, and geometric relationships between constituent parts such as relative position and tangency. This makes it easy to modify the model to allow for elastomer shrinkage for example, while ensuring the desired relationships remain as specified.

32 Elastomer failure modes

Elastomer failure modes In an ideal world, elastomer failure analysis would be performed with full modes failure Elastomer Recent advances in life prediction knowledge of the system variables, such techniques for elastomer components as material design, product and housing such as seals have made it possible for design, precise environmental conditions, a) b) c) d) cycling conditions and length of service. scheduled maintenance to be carried Figure 45: Effect of low and high out at a pre­defined level of elastomer In practice, however, failure analysis is compression set on an elastomer. deterioration so that costly failures can largely based on experience combined a) Original unconstrained sample usually be avoided. with certain analytical techniques. An additional complication is that elastomer b) Original sample compressed However, because elastomer life failure is often the result of a combination c) Sample when compressive force of factors. has been removed showing low predictions are usually based on compression set Arrhenius principles, which consider only d) Sample when compressive force time/temperature dependent chemical has been removed showing high (and occasionally physical) effects, Time/temperature dependent compression set. unpredicted failures still occur, since there physical & chemical degradation are many modes of failure that are not time/temperature/chemical dependent. Volume swell Most component failures occur in this category, as it encompasses the This section outlines the various modes Volume swell is a combination of interaction of the elastomer with its and causes of degradation and failure that chemical and physical interaction. environment. Modern techniques for can occur in elastomeric components, All fluids (including polymers) interact, life prediction have enabled end users and the ways in which they are analysed. to an extent that is dictated by a and manufacturers to understand better multitude of factors. and plan for this type of degradation. There are many factors that can affect The following describes the main failure the incidence of failures, including choice Volume swell of less than 10 per cent modes that they measure. of material, processing techniques, is not usually a problem, particularly component and housing design, storage in static applications where it can be conditions, inspection techniques, beneficial in increasing or maintaining Compression set (and stress relaxation) methods of fitting, system changes or seal interference and countering such system definition and human errors. effects as compression set. However, Compression set can be described as higher levels of volume swell (see Figure the ability of an elastomer to recover from Failure in seals is generally identified 46) may cause failure because of loss an imposed strain. Stress relaxation is a through excessive fluid leakage. This of physical properties, groove overfill, measure of the ability of the elastomer to is caused either by a loss of seal product extrusion or even metalwork maintain contact stress. interference (seal contact stress), or loss fracture. of seal integrity (generally some form of The magnitude of these effects is strongly physical damage). The causes of these affected by temperature and the fluid may be classified under the following environment. Low values are essential headings: to maintain effective sealing, whereas high values may mean a loss of seal • Time/temperature dependent physical interference resulting in bypass leakage. and chemical degradation • Housing effects • Application effects • Rapid gas decompression Figure 46. • Storage and handling effects • Manufacturing defects Figure 44. changes that are frequently associated with volume swell, • Wear and fatigue The effects of compression set are clear particularly at high levels, are softening • Thermal cycling effects. in an ‘O’ ring, as can be seen in Figure 44. and reduction of mechanical properties However, in elastomeric products of more such as tensile and tear strengths. complicated profile, knowledge of the original dimensions is often required.

33 Elastomer failure modes

Elastomer shrinkage Chemical degradation Housing effects

Many elastomer compounds contain The effects of chemical degradation These effects are often the most obvious ingredients that are designed to leach (unlike physical swell) are irreversible when analysing seal failure, and are out over time, or that under certain and range from hardening or softening, usually the easiest to remedy. It is conditions may be extracted or volatilised. surface crazing and large property important however not to confuse these If this is not compensated for by volume change, to fracture, complete with rapid gas decompression damage, swell, the resultant shrinkage may, for fragmentation or even dissolution. In as frequently happens. example, cause a reduction or total loss of some instances, very low levels of a seal interference. Failures caused this way chemical (fractions of a percent) can are rare. cause gross chemical degradation. It is Extrusion damage therefore important that full environmental data are provided prior to material This occurs when housing clearances /contraction selection, regardless of the presumed are too large or when a seal that has no insignificance of some components. or inadequate anti-extrusion elements Like all materials, elastomers expand and

Elastomer failure modes is forced into or through a clearance. It contract when exposed to temperature Radiation, free radicals and many manifests itself in various forms and is changes to an extent governed by their chemical species can cause material normally evident on the low-pressure coefficient of thermal expansion. This degradation. Common chemical species side unless swell, thermal expansion or effect must be compensated for at the that cause elastomer degradation are: pressure trapping has occurred. design stage. It should be noted that • Water volumetric thermal expansion coefficients Classical extrusion into a small clearance • Oxygen for elastomers are at least an order of occurs over medium to long periods of magnitude higher than those for . • Ozone time and results in lace-like debris – see Figure 50. Extrusion may also happen • Sour gases (H S, CO ) 2 2 catastrophically over a localised portion of Compression fracture • Acids the seal due to sudden failure of portions of any anti-extrusion device – see Figure 51. If elastomers are over-compressed, due • Bases to either poor product/housing design •  inhibitors (eg, amines) or excessive volume swell/thermal expansion, compressive fracture may • Mercaptans occur in the plane parallel to the applied • Aromatic hydrocarbons force. On occasions this failure mode may be confused with certain types of rapid • Brines (especially heavy brines). gas decompression failure. However, compression fracture, as shown in Figures 48 & 49 illustrate the visible Figure 47, is usually very rare. characteristics of some forms of chemical degradation/attack. Other forms may not be so obvious and require identification by analytical Figure 50. techniques such as infrared spectroscopy (eg, hydrolysis of NBR or HNBR).

Figure 48. Figure 47.

Figure 51.

Figure 49.

34 Elastomer failure modes

It can also be due to housing dilation at This can have serious effects on double Port damage high pressures causing the clearance to acting seals, especially if dynamic. On a modes failure Elastomer increase – see Figure 52. In this instance pressure reversal, the nibbled fragments This occurs during installation or the rigid gland ring was unable to deform may be forced across the seal/housing application, when the product passes in order to close the clearance and interface causing a leakage path. over a hole or port, especially if these exacerbated the situation by forming a have sharp edges – see Figures 57 & 58. knife-edge. Shaving effect

This is normally associated with a continuous application of pressure, and occurs most often with ‘O’ rings and other designs that may rotate in a housing. Here the seal is forced into a clearance and, with time, unwinds into that clearance: hence the shaving effect. The result of this Figure 52. is shown in Figure 55 and schematically in Figure 56. Figure 57. Nibbling

This type of damage, shown in Figure 53 and diagrammatically in Figure 54, is normally observed when pressure cycling occurs. When system pressure is applied the housing lifts or dilates, causing the clearance to increase. A nub of rubber extrudes into this clearance and is subsequently ‘nibbled’ off when the pressure is dropped and the clearance Figure 58. reduced.

Other housing effects causing seal damage Figure 55. The relationship between seal and housing size must be considered with account taken of thermal expansion and equilibrium volume swell. Surface finish, eccentricity of housing components and lead-in chamfers should also be considered as factors that may lead, directly or indirectly, to seal failure. Figure 53.

Figure 56.

The thickness of the shaving correlates directly to the dimensions of the housing clearance under pressure conditions. Any sharp housing edges at the clearance will cause extrusion damage to be initiated more easily, and thereafter cause the Figure 54. rate of extrusion to be higher than when acceptable radii are present.

35 Elastomer failure modes

Application effects wears, abraded particles are dragged towards the forward sealing edge where The modes of failure described under they may act as a grinding paste or this heading are often a combination create an uneven contact area ultimately of component design and mode of resulting in leakage. application. The resultant effects may be extrusion damage, abrasive wear or others. Figure 62 shows the effects of lubrication While these may be the eventual causes of starvation in an ‘O’ ring. Figure 63 shows the failure, it is the reasons behind them that Figure 60. short-stroke failure of a rubber/fabric seal. need to be identified and remedied.

Air entrainment/dieseling Figure 62. A small amount of air may become

Elastomer failure modes trapped in any closed system, although this is most often encountered in reciprocating applications. Severe damage can occur when the air is Figure 61. entrained near an elastomer. On a lesser scale, air entrainment may Additionally, air entrained in a cause delamination of the fabric plies in hydrocarbon fluid in a rapidly cycling a double acting piston seal, as shown in dynamic application may become highly Figure 61. dangerous if no automatic venting is Figure 63. provided or if compression is rapid. Short-stroke failure Seal failure can occur in two ways: by For short-stroke applications, it is rapid gas (air) decompression or by It is essential that contact faces of important to use shallow seals, even dieseling. Dieseling is when a dynamic seals are suitably lubricated. single element (unit) seals, as multi-lip of air/oil mist self ignites when the rise In multi-lip reciprocating applications, packings can reduce seal life and cause in pressure is fast enough to cause a this lubrication is effected by the relative premature failure. significant increase in temperature and movement of the rod or cylinder across hence create ignition. This can cause the seal. A short-stroke in this context severe localised damage to a seal, and refers to the distance travelled as a ratio of Pressure trapping can melt any plastic components. overall stack depth. This can develop in the annular clearance Figure 59 shows a schematic of a typical Practice has shown that a minimum between two double acting seals, where case in an inclined cylinder where air may stroke length of 2 ½ times the stack depth inter-seal pressures many times system become trapped adjacent to the piston is required to provide lubrication to all pressure may build up. It can cause seal seal (and gland seal). Figure 60 shows contact lips, although the failure, system lock-up or metalwork the catastrophic effect that dieseling underlying this are unclear. fracture. Most modern squeeze seals and may have on seals. It is important to ‘O’ rings can perform as double acting eliminate this problem by ensuring that It is ironic that for more arduous, high seals. Extrusion of such seals into the the minimum of air is present and that, if pressure applications the tendency has applied pressure can be seen in typical possible, adequate venting is provided. been to increase the number of sealing cases as shown schematically in Figure 64. rings and therefore the depth of the stack. This is often seen in PBR seal stacks that may be metres in length, and can result in the stroke length being insufficient to transport to the entire contact area of the seal.

In this situation, the sealing elements towards the low pressure side can run dry and abrasive wear occurs. This is worsened at elevated pressures because Figure 59. contact stresses are higher. As the seal Figure 64.

36 Elastomer failure modes

During multiple reversals, this type of Bunching Although there is no fixed rule, RGD sealing arrangement can act as a pump, conditions, and therefore potential modes failure Elastomer causing inter-seal cavity pressure to Bunching causes both circumferential damage, should be considered a build up due to viscous drag past the compression and in different possibility at pressures above 5MPa unenergised seal. The effect is that both areas of a seal simultaneously. It is (725psi) in gas or dissolved gas systems seals become permanently energised, encountered on slow rotary applications, with decompression rates greater than resulting in higher friction and wear, an especially if subject to reversals such as in 1MPa (145psi) per hour. increased tendency for extrusion damage, swivels, or where seal assembly requires piston seizure, catastrophic seal failure the use of threaded gland nuts. There are various features of RGD and potential fracture of metalwork. damage that may be related to operating With such relative motion there is a high conditions, type of gas or, more often, Figures 65 & 66 show seal damage level of linear drag which, if accompanied to the type of seal material. It is also caused by pressure trapping. The most by uneven squeeze, causes part of worth noting that, while there may be no important characteristic is damage on the the seal to be pushed or bunched up, evidence of RGD damage on the surface high-pressure side. To avoid this type of causing other parts to be stretched. This of the seal, there may be internal damage failure, two double-acting seals should again causes reduction in seal section which could impair the performance and never be used on the same component. If with the potential for seal leakage. At its life of re-used seals. opposed seals are necessary then at least worst it may result in tensile fracture of the Fracture surfaces in constrained seals one of them should be a true single acting seal or seal components. are generally in the plane perpendicular seal, so that any inter-seal pressure build- to the applied pressure. There is often up is automatically vented. Rapid gas decompression an identifiable nucleation site for each flaw and, on occasions, contamination or Rapid gas decompression (RGD) damage undispersed particulate matter may be is the name given to structural failures in observed at this site. the form of blistering, internal cracking and splits caused when the gas or Irreversible blistering is often seen condensate pressure to which the seal is in materials with poor filler/polymer exposed falls from high to low. interactions such as high fluorine- containing elastomers. Most failure Figure 65. The elastomeric components of a however is through cracking and fracture system are, to a greater or lesser extent, as seen in Figures 67 and 68. susceptible to the permeation and diffusion of gases dissolved in their surface. With time, these components will become saturated with whatever gases are in the system. As long as the internal gas pressure of the elastomer remains at equilibrium with the environment, there is minimal damage, and no deterioration in Figure 66. performance occurs.

However, when the external gas pressure Figure 67. Spiral twist is removed, or pressure fluctuations occur, large pressure gradients are This type of failure is rarely seen in seals created between the interior and the other than ‘O’ rings. It generally occurs surface of the seal and the dissolved fluid when ‘O’ rings are used in reciprocating may actually go through a phase (and applications as rod or piston seals, or therefore volume) change. This pressure when an installation requires stab-in, differential may be balanced by the gas particularly when seal compression levels simply diffusing/permeating out (the are high. reverse of the uptake process) especially if any external mechanical constraints The mechanism is that the ‘O’ ring (eg, housings) are not removed. Figure 68. becomes fixed at one or more points However if the physical and chemical around its circumference, preventing even characteristics of the elastomer compound roll around its circumferential axis. This cannot resist crack and blister growth may be caused by housing eccentricity, during the outgassing process then ‘O’ ring pinching or uneven lubrication. structural failure is the inevitable outcome.

37 Elastomer failure modes

Storage and handling effects Wear and fatigue Thermal cycling

Vulcanised elastomers will degrade if Wear or abrasion damage can be caused While thermal cycling can induce stored under unsuitable conditions of by dynamic motion, or when the sealed mechanical damage, it may also cause temperature, humidity, light and oxygen/ environment is intrinsically abrasive by-pass leakage with no damage to the ozone. Such effects as hardening, and either passes across or impinges seal whatsoever. This generally happens softening, cracking and crazing may upon the seal. Wear patterns created when seals are cycled from high to low render the product unsuitable for use or by dynamic motion are generally in the temperatures, especially if pressure is significantly reduce its service life. direction of the motion (ie, axial wear in cycled simultaneously. reciprocating seals and circumferential Careful choice of storage conditions wear in rotary seals). Figure 70 shows More often, by-pass leakage occurs in should minimise these effects. Additionally, abrasive wear. seals made from thermoplastics and components should be stored in a relaxed those with a tendency to high temperature condition free from tension, compression flow. Such materials (eg, Aflas®) should or other deformation. not be subjected to large thermal

Elastomer failure modes transients or rapid thermal cycling. Care should be taken to ensure that good Similar effects are noted if elastomers handling and fitting practice is observed, are used below or approaching their limit using correct tools and following Figure 70. of elastomeric flexibility (glass transition manufacturer’s instructions whenever temperature, Tg) as they cannot respond, possible. Figure 69 shows the level of seal The exception to this is lip cracking or at least only very slowly, to any system damage that may result simply from poor in rotary applications. Damage here, change, such as the application of fitting procedures. as shown in Figures 71 & 72, is pressure. perpendicular to the direction of motion. The damage frequency is a function of material design, particularly modulus and Bonding failure frictional characteristics. It happens due to a being forced ahead of the motion, In rubber to metal bonding, the bond which produces contact peaks and non- strength is generally greater than the contact troughs (‘slip-stick’). It causes strength of the elastomer, so the failure heat build-up at the contact bands which mode is usually within the elastomer. ultimately leads to cracking. This effect is known as a Schallamach wave. Bond failures are relatively rare, and may be due, for example, to insufficient degreasing of the metal or premature curing of the bonding agent. Figure 69.

Figure 71. Aesthetics

Where aesthetic appearance is important, such as in consumer products, a deterioration in visual appearance can be considered a failure mode. An example is colour fading due to exposure to Figure 72. ultraviolet radiation in sunlight. Lower friction, higher modulus materials (Figure 71) will exhibit an increased number of shallower cracks compared with higher friction, lower modulus materials (Figure 72). This is because heat build-up is less and ‘stick’ is reduced. Optimised materials for this type of application do not exhibit lip cracking as they enable a stable hydrodynamic film to be established.

38 Glossary of terms

A Ageing: The irreversible change of Anti-ozonant: A material added to a material properties during exposure to rubber compound to reduce damage terms of Glossary Abrasion: The surface loss of a material a deteriorating environment. This can resulting from the effects of ozone. due to frictional forces applied to it. include environments such as UV, oxygen Anti-tack: Substance applied to the Abrasion resistance: The ability to resist and those containing ozone gases. surface of an elastomer to stop it adhering mechanical wear. A material with a high Ageing can also refer to the controlled either to itself or other elastomers. abrasion resistance helps to maintain exposure of rubber samples to a variety the material’s original appearance and of deteriorating influences to allow for Arrhenius principle: An empirical structure. the evaluation of anti-oxidants and anti- relationship stating that the rate of a ozonants. chemical reaction increases exponentially Abrasion resistance index: A measure with temperature. of the abrasion resistance of a rubber Agglomerate: A cluster of particles of relative to that of a standard rubber under one or more compounding materials ASTM: Abbreviation for American Society the same specified conditions, expressed loosely held together. One of the primary for Testing and Materials. as a percentage. roles of the mixing process is to break down agglomerates and promote good Atmospheric cracking: Cracks on Accelerated life test: The testing of a dispersion. the surface of a material as a result of material by subjecting it to conditions in exposure to atmospheric conditions. excess of its normal service parameters in Air traps: A rubber moulding defect This is usually as a result of sunlight an effort to approximate, in a short time, that can occur either at the surface of and/or ozone. the deteriorating effect of normal long- the moulding due to air being trapped AU: Abbreviation for polyester type term service conditions. between the mould and the material, or within the moulding. The use of vacuum polyurethane rubbers. Accelerator: A chemical which speeds up technology both at the extrusion and Autoclave: A vessel that vulcanises the vulcanisation reaction. This allows the press stage can greatly reduce the risk of rubber products in a pressurised steam rubber to cure in a shorter timeframe, at a air traps. environment. lower temperature or both. Amorphous: Having no definite shape; Axial squeeze: Compression applied to Acid acceptors: Mostly used in non-crystalline. the top and bottom of a seal’s surface. fluoroelastomers to absorb the acid produced by the chemical reactions that Aniline point: The lowest temperature take place during curing and to some at which equal parts of aniline and a test degree during service. Acid acceptors are liquid (usually oil) will mix or blend. In B usually metal oxides. general, the lower the aniline point of an Back-up ring: See Anti-extrusion ring. oil, the more a given rubber compound Acid resistance: The ability of a will swell, therefore the test indicates if an Backrinding: Tearing or distortion of a material to resist attack as a result of oil is likely to damage an elastomer with moulded rubber product at the line of exposure to acids. The degree of attack which it has come into contact. separation of the mould pieces. Factors is both temperature and concentration which can have an influence on back- dependant. ANSI: American National Standards rinding are blank weight, blank shape, Institute. ACM: Abbreviation for acrylic rubbers. temperature, moulding pressure and Antidegradants: These are materials breathe cycles. ACN: Abbreviation for acrylonitrile. added to a rubber compound to reduce Banbury mixer: The type of internal mixer Activator: A substance, which by the effect of deterioration caused designed by FH Banbury. chemical interaction promotes a chemical by oxidation, ozone, light and/or action of a second substance. Most combinations of these. Bank: This term can relate to the amount commonly used in elastomers to activate of rubber adjacent to the nip of the rolls Anti-extrusion ring: A ring installed on on both mills and calenders. accelerators. the low pressure side of a seal to stop the Aftercure: The amount of cure received sealing material being extruded into the Barrel: That part of an extruder in which after the termination of the cure clearance gap. The requirement for the the screw rotates or the ram moves. proper. The term is also applied to the fitting of such devices is dependent on the Bias: The angle at which the continuation of the curing effect that pressures, sealing materials and extrusion material is cut with respect to the running results from exposure of the article to heat gap. edge of the fabric. in use, or from accelerated ageing. See Antiflex cracking agent: A material Post cure. Blank: A measured weight or dimension added to a rubber compound to reduce of a rubber compound suitable to fill the Age resistance: The ability of a given cracking caused by cyclic deformations. cavity of a compression or transfer mould. material to resist deterioration of its Anti-oxidant: A material added to a Usually the blank weight/volume is slightly properties caused by ageing. rubber compound to reduce deterioration higher than the finished component to caused by oxidation. allow for full compression in the cavity.

39 Glossary of terms

Bleeding: The flow of a compounding Buna N: A general term for the copolymer Checking: Shallow, short cracks on the material, often oils or lubricants, from the butadiene and acrylonitrile, also referred surface of a rubber, which is usually as a surface of vulcanised or unvulcanised to as NBR or nitrile. result of environmental attack. rubber. Can also be referred to as leaching. Buna S: Butadiene and acrylonitrile CO: Epichlorohydrin homopolymer. Blemish: An unwanted imperfection on copolymer. This material is also referred Coagent: An ingredient added to a rubber the surface of a moulded product. to as SBR. compound, usually in small amounts to Blister: A surface or internal imperfection, Butyl: A copolymer of isobutylene and increase the cross-linking efficiency of produced by entrapped air, gases or isoprene. non-sulphur vulcanising systems, such as other volatiles normally as a result of the organic peroxides. manufacture process. Coated fabric: A product constructed by C

Glossary of terms Bloom: A solid or liquid material that coating a fabric with a rubber, resulting in has migrated to the surface of a rubber CAD: Abbreviation for computer-aided a flexible material which can be moulded material normally leaving a waxy or milky design. into products or used in conjunction with deposit. rubbers to provide higher rigidity and Calender: A machine with two, three or improved extrusion resistance. Blow: The volume expansion that occurs four parallel, counter-rotating rolls. Often in the production of cellular or sponge used to form rubber sheeting where Coefficient of thermal expansion: The rubber resulting from the action of a the thickness needs to be accurately average expansion per degree over a blowing agent incorporated into the controlled. stated temperature range, expressed as a compound. fraction of the initial dimension. CAM: Abbreviation for computer-aided Blowing agent: A compounding manufacture. Cold flexibility: Flexibility of the elastomer ingredient introduced into an elastomer following exposure to a specified low which produces a gas by chemical or Carbon black: A black pigment and temperature for a specified period of time. physical action during the processing reinforcement filler used in rubbers. Cold flow (also called Creep): A slow stage. Used in the manufacture of sponge Carbon black is a form of amorphous deformation, under gravitational force for rubbers. carbon that has a high surface-area-to- volume ratio. The degree of reinforcement example, at or below room temperature. Bonding agent: A material used to increases with decreasing particle size. Cold resistant: The ability of an promote the bonding of rubber to other elastomer to function at low temperatures. materials during the processing stage. Catalyst: A chemical, usually added to a mix in small quantities relative to the Compound: A term applied to a mixture BR: Abbreviation for rubber based on reactants, that modifies and increases the of polymers, reinforcements, curatives butadiene. rate of a reaction without being consumed and other ingredients to produce a rubber in the process. Breakdown: The plasticising of raw material. The compound is prepared rubber prior to the incorporation of Cavitation: A condition in which vapour according to a prescribed formula and compounding ingredients. This is or gas bubbles occur, normally in an area mixing process. normally the first stage of the mixing where there is a temperature change or Compression moulding: A moulding process. reduction in pressure, which results in process in which an uncured rubber blank a collapse of the bubble and high local Breakout friction: The force required to is placed directly in the mould cavity and impact pressures. This can lead to both initiate sliding between a rubber seal and compressed to its final shape by closing equipment wear and reduced seal life. the surface in which it is in contact. the mould. This process normally results Cellular rubber: A generic term for in excess material in the form of flash. Brittle point: The highest temperature at rubbers containing either open, closed or which a rubber specimen will break under Compression set: The amount a rubber both types of cells dispersed throughout a measured sudden impact. This is one specimen fails to return to its original the material. These cells are formed by indication of low temperature flexibility. shape after being released from a blowing agents during the processing of constant compressive load. This testing Brittleness: The tendency of an the rubber. normally takes place at an elevated elastomer to crack when deformed or Cement: An adhesive that is either a temperature and helps to develop an impacted. liquid dispersion or a solution of raw or understanding of the reduction in sealing BS: Abbreviation for British Standard. compounded rubber, or both, usually force which may be encountered in dissolved in solvent, and used to bond service. BSI: Abbreviation for British Standards rubbers to other rubber or non-rubber Institution. Conductive rubber: A rubber which has products. been produced such that it is capable of Bumping: The operation of opening and Chalking: The formation of a residue conducting electricity. closing the press rapidly in the first stages on the surface of a rubber which is of the cure. This action is designed to commonly as a result of UV damaging the drive out any trapped air in the mould surface of the material. cavity. Also referred to as breathing.

40 Glossary of terms

Copolymer: A polymer composed from Cure time: The required amount of time Differential pressure: The difference in two different monomers, for example an needed to complete the curing process pressure between the high-pressure and terms of Glossary NBR composed of polybutadiene and to a pre-determined level. The time taken low-pressure side of a sealing system. acrylonitrile. to cure is dependent on the temperature, Dimensional stability: The ability material type and section of the rubber CR: Abbreviation for rubbers. of the elastomer to retain its original profile. shape and size having been exposed Corrosion: Progressive wearing away of Curing temperature: The temperature at to a combination of stresses and a surface because of a chemical reaction. which vulcanisation takes place. temperatures. Cracking: Axial cracks on elastomeric DIN: Abbreviation for Deutsches Institut seals on the lip contact surface. für Normung – German Institute for D Crazing: The formation of shallow cracks standardisation. on the surface of a rubber. This can be as Damping: The property of a material Dipping: A method of manufacturing a result of exposure to UV light or certain or system that causes it to convert rubber articles by dipping a former of the chemicals. Although they look similar, mechanical energy to heat when shape required into a rubber solution. crazing differs from , as it subjected to deflection. In rubber, the does not depend on the presence of an property is caused by hysteresis. Dispersion: The distribution of particles externally applied strain. throughout a medium. For rubbers Deflashing: The process of removing this often refers to the distribution of Creep: The time-dependent part of a excess material from the flash-line compounding ingredients in rubber mix. strain resulting from stress. resulting from the moulding process. Also see Cold flow. Various methods exist, including buffing Dough: Rubber compounded and swollen and cryogenic trimming. in solvent and worked on a ‘wet’ mill until Cross-section: A section formed by a it reaches the consistency of dough. It is plane cutting through an object, usually at Degassing: The passing of a gas out of a then applied behind the doctor blade of right angles to an axis. rubber, normally generated by the volatile a spreading machine through which the ingredients in the rubber mix which are Cross-linking (see also vulcanising): fabric to be coated is passing. Self-curing activated at elevated temperatures. The formation of chemical bonds doughs are used in the repair of rubber between polymer chains to give a three- Delamination: The separation of layers of products (tyre tread cut-filling) and the dimensional network structure. rubber (normally in a plied format) or the splicing of belting. rubber separating from a surface to which Cross-link density: A measure for the Durometer: An instrument for measuring it is bonded. relative number of cross-links in a given the relative hardness of rubber. volume of elastomer. Demoulding: The operation of removing Dynamic properties: The response in an a vulcanised rubber product from the Crumb rubber: Vulcanised waste or elastomer to forces applied to them. mould in which it has been cured. This scrap rubber which has been ground can be done carefully by hand, but in Dynamic seal: A seal used in an down to a known mesh size and can then some cases pins or brushes can be environment that is subjected to any type be added to new compound as a filler. incorporated into the mould or press to of movement relative to its position and Crystallinity: The orientation of the perform this function automatically. that of the contact / sealing surface. disordered long-chain molecules of a Density: The weight per unit volume of a polymer into repeating ordered patterns. substance. Many rubber materials have a degree E of crystallinity, and some will tend to Desiccant: A rubber compounding Ebonite: ‘Hard rubbers’ which are formed crystallise under certain conditions. The ingredient used to absorb moisture when they are cured with high levels of degree of crystallinity effects stiffness, irreversibly, particularly for the purpose vulcanising agents. hardness, low temperature flexibility and of minimising the risk of porosity and/or heat resistance. blisters during vulcanisation. ECO: Epichlorohydrin copolymer with ethylene oxide. Curatives: The collective term for the Diametral clearance gap: The difference chemicals involved in curing the rubber in diameters between two mating Efficient vulcanisation: A term applied to material. These include, for example, surfaces. vulcanisation systems in which sulphur or accelerators, vulcanising chemicals such a sulphur donor is used very efficiently for Die: The shaped plate fitted in the head as sulphur, and activators. cross-linking the rubber. of an extruder designed to create a profile Cure: Another term for 'vulcanisation'. suitable for the moulding process. Elasticity: The rapid recovery of This process results in the cross-linking of a material to its initial shape after Die swell: The change in dimensions of polymer chains. deformation and release of an applied an extruded rubber section as it exits the force. die. This swell is mainly due to the elastic recovery of the material.

41 Glossary of terms

Elastomer (also known as rubber): FDA: Food and Drug Administration (USA). G A general term used to describe FEA: Abbreviation for finite element both natural and synthetic polymers Gate: The point through which a rubber analysis. possessing the ability to return to their is injected into the moulding cavity in both transfer and injection moulding original shape after the deforming force is FEPM: Abbreviation for techniques. removed. tetrafluoroethylene/propylene dipolymers. Gate mark: A witness mark left on the Elongation, per cent: The extension of a FFKM: Abbreviation for moulding as a result of injecting rubber specimen as a result of an applied tensile perfluoroelastomers. stress, expressed as a percentage of the through the gate. This can be either a original length. Filler: A compounding ingredient which is raised or sunken mark on the surface of added to a rubber usually in finely divided the moulding. Elongation at break: The elongation form. There are into two main categories

Glossary of terms Glass transition temperature (Tg): measured at the point of rupture. A high of filler: reinforcing which adds strength The point at which the material loses its value is important if substantial stretching to the elastomer (see Reinforcing fillers) flexibility at low temperature. This point is is required during fitting of the product. and extending, which has the function of affected by system pressure and varies for cheapening the elastomer (see Extender). EPM: Abbreviation for ethylene-propylene different polymers. rubber. Finite element analysis: A mathematical Green strength: The strength of a rubber technique developed to predict the stress- EPR: Abbreviation for ethylene-propylene in the uncured state. strain behaviour of objects which do not rubber. lend themselves to simple analysis. Groove: The machined glandular recess EU: Abbreviation for polyether urethane. into which an ‘O’ ring is fitted. Fire retardant: An additive used in rubber Explosive decompression: compounding to reduce the fire hazard. Gough-Joule effect: When rubber is See Rapid gas decompression. stretched adiabatically (without heat FKM: Abbreviation for fluorocarbon entering or leaving the system) heat is Extender: A material added to a rubber rubber. generated by the material. The effect was compound which is designed to reduce Flame resistance: The resistance to originally discovered by Gough in 1805 the cost of the compound without burning of a rubber material. and re-discovered by Joule in 1859. imparting any enhanced physical properties. Flash: The excess material resulting from the moulding operation found at the Extensometer: A device used to H mould split lines. determine the elongation of a specimen Hardness: Measurement of the resistance as it is strained under testing conditions. Flex cracking: Repeated flexing of a to indentation. The most common units Often these machines can also record the rubber resulting in the material cracking. are Shore A and IRHD. See and tensile strength and modulus values of a IRHD . given material. Flex life: The number of cycles required Shore A to produce a specified state of failure Heat ageing: A test for the reduction in Extrudate (also referred to as extrusion): in a rubber specimen. The test uses a physical properties of an elastomer as a The profiled material which results from prescribed method of flexing, such as result of exposure to temperature. the extrusion process. shear or . Heat history: The total heat which has Extruder: A machine designed to create Flexural strength: Ability of an elastomer been received by the rubber compound a profiled rubber shape by forcing the to flex without permanent distortion or (mixing, milling, extruding, calendering), rubber through a die which has a shape damage. particularly the temperatures reached by similar to that of the required profile. the rubber and the time it has been held The two most common types are screw Flow marks: Marks present on the at these temperatures. and ram. surface of a moulding caused by insufficient or improper flow of the Heat resistance: A rubber’s ability to Extrusion (seal): The distortion, under material in the moulding cavity. undergo exposure to some specified level pressure, of some of the sealing element of elevated temperature and retain a high into the clearance between mating parts. FMQ: Abbreviation for fluoro methyl silicone. level of its original properties. Formula: A list of the ingredients and Heteropolymer: A polymer composed of F their amounts used in the preparation of a differing monomers. compound. Fatigue: The weakening of an elastomer HNBR: Abbreviation for hydrogenated during repeated deformation, strain or FPM: Abbreviation for fluorocarbon nitrile rubber. compression. rubber. Homopolymer: A polymer formed from a Fatigue life: The number of deformations FSA: Food Standards Agency (UK). single monomer. required to produce a specified state of fatigue in a test specimen. FVMQ: Abbreviation for fluoro vinyl methyl silicone.

42 Glossary of terms

Hooke’s law: Within the limits of elasticity K Mineral oils: and other of a material, tension is proportional to hydrocarbons oils obtained from mineral terms of Glossary elongation, or strain is proportional to the Knit line (also known as weld line): A sources. In rubber compounds they act stress producing it. line present in a moulding as a result of as softeners and extenders. opposing flow fronts during the forming of Hysteresis: The difference between the the rubber material in a mould not knitting Mixer: A machine with a closed chamber energy input and output under elastic together. A knit line is an area weakness in which specially shaped rotors masticate deformation in a rubber is known as in the moulding. the rubber and incorporate compounding hysteresis. The loss of energy results in materials through the action of heat build-up. mechanical work (shear) with the aim of creating a homogenised finished material. Hysteresis loss: The loss of mechanical L energy due to hysteresis. Latex: A stable dispersion of a polymeric Modulus: In elastomer technology this is substance in an aqueous medium. defined as the stress at a particular strain or elongation. Modulus tends to increase I Leaching: See Bleeding. with hardness, with higher modulus materials, in the main, being more ID: Inside diameter. Leakage rate: The rate at which a fluid passes through or around a seal. resistant to deformation and extrusion. Immediate set: The deformation found by Life test: A test of the amount and Modulus of elasticity: The ratio of stress measuring immediately after removal of to strain in an elastic material. the force causing deformation. duration of a product’s resistance to destructive forces. Molecular weight: The weight of a Impact resistance: The resistance to of a substance. fracture under a quickly applied load. Liquid silicone rubber: High purity cured silicone with low Mould cavity: Profiled shape cut into a Impact strength: A measure of the compression set, great stability and ability mould within which the rubber is cured to toughness of the material to rapidly to resist extreme temperatures of heat and produce the product. applied loads. It is often represented as cold. the energy required to break a specimen Mould marks: An imperfection transferred with a single swinging blow. Low temperature flexibility: The ability of to a moulded product from corresponding an elastomeric product to be flexed at low marks present on the mould surface. Inhibitor: A compounding ingredient temperatures without cracking. which is added to a mix to suppress a Mould release: A substance applied to chemical reaction such as the curing of a Litharge: Lead monoxide, PbO, formerly the surface of a mould cavity to aid the rubber material. used as an inorganic accelerator but now release of the rubber product after curing. mainly used as a vulcanising agent in Injection moulding: Moulding process some polychloroprene rubbers. Mould shrinkage: Dimensional loss in where preheated rubber is injected under a moulded rubber product that occurs pressure through a series of runners and during cooling after it has been removed into a closed mould cavity. M from the mould. Insert: Normally a metal or plastic Make-up: Uncured elastomer that is cut component to which rubber is chemically to a profile, weight and/or length prior to N and/or physically bonded during the placing in a mould. moulding process. NBR: Abbreviation for nitrile-butadiene Master batch: A homogeneous mixture rubber. Internally lubricated rubber: A rubber of polymer and one or more materials in containing lubricating additives designed known proportions. Nerve: The toughness and elasticity of to reduce the material’s coefficient of unvulcanised, unmasticated rubber. friction. Mastication: The breakdown or softening of raw rubber by the combined action Nibbling: Normally observed when IRHD: Abbreviation for International of mechanical work (shear). This can be pressure cycling occurs. When system Rubber Hardness Degrees. This is a accelerated by the use of a peptiser. pressure is applied the housing lifts or dilates, method of measuring rubber hardness. causing the clearance to increase. A nub of IRHD is similar to Shore A durometer Memory: Ability of a rubber to return to its rubber extrudes into this clearance and is units, but uses different test method and original shape after deformation. subsequently ‘nibbled’ off when the pressure apparatus. Microwave curing: Vulcanisation of is dropped and the clearance is reduced. ISO: International Organisation for rubbers by heat produced by high Nip: The radial distance between the rolls Standardisation. frequency radiation. on a mill or calender, measured at the line Mill: A machine with two counter-rotating of centres. rolls used for rubber mastication, mixing Non-fill: A defect in a rubber product or sheeting. caused by the rubber failing to completely fill the mould.

43 Glossary of terms

Non-sulphur vulcanisation: The process Permanent set: Amount of deformation in the rubber to rapidly expand. If the force of vulcanisation without the use of sulphur. a rubber after the distorting load has been of the expanding gas is greater than removed. the strength of the material then cracks, Abbreviation for natural rubber. NR: blisters and catastrophic material failures Permeability: Measure of the with can occur. which a liquid or gas can pass through a O rubber material. Recovery: The degree to which a rubber product returns to its original dimensions. OD: Outside diameter Peroxide: One of the ingredients which can be used for vulcanising rubbers. Reinforcement: The act of increasing the Oil resistant: The ability of vulcanised mechanical strength of a rubber. rubber to resist swelling and other effects Pig: Roll of rubber cut from a mill. which reduce the performance of the Reinforcing filler: A compounding Pigment: A material used to impart colour

Glossary of terms material whilst exposed to oils. ingredient added to the rubber to to a rubber compound. increase the resistance of the material to Oil swell: The change in volume of a Plasticiser: A substance, usually a mechanical forces. rubber due to the absorption of oil. heavy liquid or oil, which is added to an Resilience: The ratio of energy output to Optimum cure: The state of vulcanisation elastomer to decrease stiffness, improve energy input in a rapid full recovery of a at which a desired property value or low temperature properties, reduce cost deformed rubber specimen. combination of property values is and/or improve processing. obtained. In some materials this may Retarder: A compounding ingredient Poisson’s ratio: The measure of the require post-curing or autoclaving to which is added to the mix and is designed simultaneous change in elongation and produce this desired level of cure. to reduce the tendency of a rubber in cross-sectional area within the elastic compound to vulcanise prematurely. Orange peel: Pitted or uneven surface on range during a tensile or compressive a moulded part, resembling the surface of test. For thermosetting elastomers, typical Reversion: The deterioration of an orange. values of 0.48 to 0.50 are achieved which vulcanisate properties that may occur is why elastomers can be successfully when vulcanisation time is extended ‘O’ ring: Solid elastomer seal of circular used for sealing applications. beyond the optimum. Usually shown by cross-section. reduced tensile strength and modulus, Polymer: Literally means ‘many units’ increased elongation at break and Outgassing: The release of vapours or and is a large molecule constructed from gases from a rubber compound. tackiness. This is a particular problem in many smaller monomers. natural rubbers. Overcure: A degree of cure greater than Post cure: The application of heat to the optimum. In some cases this can lead : The science of the deformation a thermosetting rubber after curing to and flow of matter. to a loss of elongation and an increase enhance one or more properties. in hardness. In the case of natural rubber Rheometer: An instrument for the this can lead to reversion. Pre-form: See Blank. study of the rheological properties of Oxidation: The reaction between Processability: The relative ease with elastomers. oxygen and a rubber which can lead to a which raw or compounded rubber can be RTV: Abbreviation for room temperature detrimental change in physical properties. processed. This can relate to all aspects vulcanisation. of manufacturing. Ozone resistance: The ability to withstand the deteriorating effect of Processing aid: A compounding ozone. ingredient that is added to the mix with S the aim of improving the material’s ability Saturation: Saturated chemical to be processed. compounds are those whose constituent P PU: Abbreviation for polyurethane. molecules contain no double or triple Parting line: The line on the surface of a valency bonds; such compounds do not moulded part where the separate mould form addition compounds. parts meet and create a small clearance R SBR: Abbreviation for styrene-butadiene gap. Radial seal: Seal having compression rubber. applied to its outside and inside PB: Abbreviation for polybutadiene. Scission: Breaking of chemical bonds. diameters. PCP: Abbreviation for polychloroprene. Scorching: The premature vulcanisation Compression on a seal’s Radial squeeze: of a rubber compound. Peptiser: A compounding material used outside and inside diameters. to accelerate, by chemical action, the Secondary accelerator: An accelerator

softening of rubber under the influence of Rapid gas decompression (RGD): used in smaller concentrations, when Also known as explosive decompression mechanical action or heat (or both). compared to the primary accelerator, to (ED). The rapid release of applied system achieve a faster rate of vulcanisation. pressure, causing dissolved gases in

44 Glossary of terms

Shelf-life: Length of time a moulded Tensile strength: A measure of the stress V compound can be stored without suffering required to rupture a standard test piece. terms of Glossary significant loss of physical properties. Visco-elasticity: A combination of Terpolymer: A polymer formed from three viscous and elastic properties in a Shore hardness: The relative hardness monomer species. material. of an elastomer measured on a Shore Thermal degradation: An irreversible durometer instrument. Viscosity: Resistance to flow. change in the properties of a material due Shrinkage: The reduction in size upon to exposure to heat. VMQ: Abbreviation for vinyl methyl cooling of a moulded rubber part. silicone. Thermal expansion: Linear or volumetric Solubility: The degree to which one expansion caused by temperature Vulcanisation: Heat induced process substance will dissolve in another. increase. whereby the long chains of the rubber molecules become cross-linked by Sponge rubber: One type of cellular Thermoplastic: Applied to high polymers a vulcanising agent to form three rubber. Conventional sponge rubber has which soften by the application of heat dimensional elastic structures. a porous structure, the cells being open and which may be resoftened by heating, and intercommunicating; it shows very provided chemical decomposition does Vulcanising agent: A compounding high absorption of water. not take place. material that produces cross-linking in rubbers. Spreading: Coating, normally a fabric, Thermoplastic elastomer (TPE): A with rubber. diverse family of rubber-like materials that, unlike conventional vulcanised rubbers, Sprue: Channel through which the W can be processed and recycled like elastomer enters the mould cavity; also thermoplastic materials. Water absorption: The amount of water the cured elastomer remaining in this absorbed by a material under specified channel. Transfer moulding: The process of test conditions. moulding a material by forcing rubber Squeeze: The amount of radial or axial from a reservoir chamber through a Weathering: The tendency of rubbers compression of a seal between two gate into the moulding cavity of a closed to surface crack on exposure to surfaces when installed. mould. atmospheres containing ozone and other Stabiliser: A substance added to a pollutants. Thermoset: Materials that undergo rubber to maintain properties at or near chemical cross-linking of their molecules Weld line: See Knit line. their initial values during its production, when processed, and cannot be softened processing and storage. and reshaped following further application Staining: Change of colour of a rubber of heat. X when exposed to light or change of colour TR-10: A measure of the low-temperature XNBR: Abbreviation for carboxylated in a material in contact with, or adjacent capability of an elastomer. It is the nitrile rubber. to, a rubber. temperature at which a stretched and State of cure: The degree of frozen specimen has retracted 10% of vulcanisation of a rubber compound. the stretched amount. TR stands for 'temperature retraction'. Static seal: A seal between parts that have no relative motion. Sulphur: An agent responsible for the U vulcanisation of some rubbers. Ultimate elongation: The percentage a specimen was stretched at the point of rupture. T Under-cure: A condition where rubber Tack: The property that causes contacting has not been cured to its optimum state surfaces of unvulcanised rubbers to stick and will exhibit a reduction in its physical to each other. properties. Tackifier: A compounding material that Unsaturation: In organic compounds enhances the ability of vulcanised rubber the linking of some of the atoms of the to adhere to itself or another material. molecule by more than one valency bond, Tear resistance: Resistance to the growth ie, double or triple bonds. of a nick or cut in a rubber specimen UV absorber: A compounding material when tension is applied. that retards the deterioration caused by Tear strength: The maximum force sunlight and other UV light sources. required to tear a specified test specimen.

45 About James Walker

About James Walker Industry leading research

James Walker works at the forefront • Rotary and reciprocal application of materials science and fluid sealing test rigs technology to create engineered solutions • World-leading rapid gas decompression for virtually every industrial sector. We are testing facility constantly reviewing material performance and seeking to develop new compounds • Chemical compatibility testing and variants that will address the •  plant allows batch compounding operational problems faced by our clients for product and manufacturing tests and the industry sectors we serve. Across • Environmental chambers offers industries as diverse as aerospace, power testing of components across a wide generation and bioprocessing, James temperature range. The company has a long history of About James Walker Walker technical ability and expertise has innovation and customer support helped create what are now recognised from product design and process as class-leading, best practice products improvement, problem solving, adapting and solutions. or modifying existing equipment through to manufacture. Our expertise includes the capability to re-engineer existing High performance elastomers applications. Early involvement in the design process helps us advise on value • Over 300 grades formulated and engineering aspects of both the elastomer compounded in-house product and tool design, optimising • Elastomers developed to meet optimum the cost and efficiency of the product requirements development and production process. • State-of-the-art facilities for precision compounding • Rigorous testing and control with full Custom design & manufacture traceability. • Development of precision elastomeric With our own, in-house laboratories, components testing facilities and research production • Prototyping, testing and full-scale unit at the James Walker Technology Elastomer manufacturing manufacture Centre, all processes are under one roof • In-house tool design and CAD/CAM – from compound formulation through to • High precision injection moulding, machining. product design, manufacture and testing. compression moulding and moulding This allows us to provide the flexibility under vacuum of service required to find and produce • From complex miniature components to those bespoke solutions. giant seals • Plastics or metal inserts securely moulded in place • Extrusion of simple or complex profiles • Bonding to metals and plastics, plus production of special composites • Complex, thin-walled 3D mouldings • Liquid silicone rubber and thermoplastic elastomer moulding expertise.

46 GeneralGlossary information of terms

Free copies of these technical brochures and white papers can be requested from your local James walker company or downloaded from the website at; www.jameswalker.biz/pdf_docs

Elastomeric seals & components for Walkersele® Radial Lip Seals Hydraulic Sealing Guide the Oil & Gas industry Issue 9 Issue 28.2

• Rod/gland seals ● High-efficiency lip seals for • Piston seals rotary duties • Wipers & scrapers ● Proven long-term bearing • Bearing strips protection • ‘O’ rings ● Many standard sizes ex-stock ● Unlimited diameters to order ● Custom-designed specials ● Also V-ring and metal cased lip seals

• World-leading sealing technology • High integrity products for safety, environmental & revenue protection • RGD-resistant elastomers qualified to Norsok M-710 • Global stockholding & support • Proven track record

High Performance Sealing Technology High Performance Sealing Technology High Performance Sealing Technology

Elastomeric components Walkersele radial lip seals Hydraulic sealing guide for the oil & gas industry

Sealing products for the Pharmaceutical ‘O’ Ring Guide & Bioprocessing industries Precision elastomer engineering Issue 6 Issue 2.1

The comprehensive guide to ‘O’ ring • Range of FDA compliant materials sealing systems including Tested & certificated to USP Class VI • ‘O’ ring selection • • General & high performance materials • Seals, gaskets, clamps & packings • Housing design & tolerances • Custom-moulded parts • Cords, kits & lubricants

Innovation – from product design and process engineering to prototype and full production

High Performance Sealing Technology High Performance Sealing Technology 'O' ring guide Sealing products for the Precision elastomer pharmaceutical industry engineering

EPDM materials for the pharmaceutical Low temperature sealing capability Engineering elastomers by optimising RGD resistance and bioprocessing industry of o-rings and low temperature sealing performance

A white paper presented by A white paper presented by A white paper presented by James Walker James Walker James Walker

High Performance Sealing Technology High Performance Sealing Technology High Performance Sealing Technology EPDM materials for the Low temperature sealing RGD resistance and pharmaceutical and capability of o-rings bioprocessing industry low temperature sealing performance

Trademarks

The following trademarks are acknowledged: Aflas® Asahi Glass Kalrez® EI Du Pont de Nemours & Company or its affiliates Tecnoflon® Solvay Solexis TOSO-CSM® Tosoh Corporation Vamac® EI Du Pont de Nemours & Company or its affiliates Viton® EI Du Pont de Nemours & Company or its affiliates

4747 James Walker worldwide sales and customer support

James Walker Asia Pacific James Walker Deutschland James Walker Mfg (USA) Tel: +65 6777 9896 Tel: +49 (0)40 386 0810 Tel: +1 708 754 4020 Fax: +65 6777 6102 Fax: +49 (0)40 389 3230 Fax: +1 708 754 4058 Email: [email protected] Email: [email protected] Email: [email protected]

James Walker Australia James Walker France James Walker New Zealand Tel: +61 (0)2 9721 9500 Tel: +33 (0)437 497 480 Tel: +64 (0)9 272 1599 Fax: +61 (0)2 9721 9580 Fax: +33 (0)437 497 483 Fax: +64 (0)9 272 3061 Email: [email protected] Email: [email protected] Email: [email protected]

James Walker Benelux James Walker Iberica James Walker Norge (Belgium) Tel: +34 94 447 0099 Tel: +47 22 706800 Tel: +32 3 820 7900 Fax: +34 94 447 1077 Fax: +47 22 706801 Fax: +32 3 828 5484 Email: [email protected] Email: [email protected] Email: [email protected] (Netherlands) James Walker Inmarco (India) James Walker Oil & Gas (USA) Tel: +31 (0)186 633111 Tel: +91 (0)22 4080 8080 Tel: +1 281 875 0002 Fax: +31 (0)186 633110 Fax: +91 (0)22 2859 6220 Fax: +1 281 875 0188 Email: [email protected] Email: [email protected] Email: [email protected]

James Walker Brasil James Walker Ireland James Walker South Africa Tel: +55 11 4392 7360 Tel: +353 (0)21 432 3626 Tel: +27 (0)31 304 0770 Fax: +55 11 4392 5976 Fax: +353 (0)21 432 3623 Fax: +27 (0)31 304 0791 Email: [email protected] Email: [email protected] Email: [email protected]

James Walker China James Walker Italiana James Walker UK Tel: +86 21 6876 9351 Tel: +39 02 257 8308 Tel: +44 (0)1270 536000 Fax: +86 21 6876 9352 Fax: +39 02 263 00487 Fax: +44 (0)1270 536100 Email: [email protected] Email: [email protected] Email: [email protected]

Health warning: If PTFE or fluoroelastomer (eg, FKM, FFKM, FEPM) products are heated to elevated temperatures, fumes will be produced which may give unpleasant effects, if inhaled. Whilst some fumes are emitted below 250°C from fluoroelastomers or below 300°C from PTFE, the effect at these temperatures is negligible. Care should be taken to avoid contaminating tobacco with particles of PTFE or fluoroelastomer, or with PTFE dispersion, which may remain on hands or clothing. Material Safety Data Sheets (MSDS) are available on request. Information in this publication and otherwise supplied to users is based on our general experience and is given in good faith, but because of factors which are outside our knowledge and control and affect the use of products, no warranty is given or is to be implied with respect to such information. Unless governed by type approval or contract, specifications are subject to change without notice. Statements of operating limits quoted in this publication are not an indication that these values can be applied simultaneously. To ensure you are working with the very latest product specifications, please consult the relevant section of the James Walker website: www.jameswalker.biz. Environmental statement: This brochure is manufactured using advanced environmentally friendly technologies and follows the strict environmental standard BS EN ISO 14001. Made from chlorine-free pulp (ECF) with post-consumer recycled fibre obtained from sustainable wood forests, and printed using vegetable-based inks, by Binfield Printers Ltd. For those who wish to reduce further their impact on the environment, this publication is also available as a PDF from: www.jameswalker.biz

BP4400 0317/pdf Registered Office: James Walker Sealing Products and Services Ltd, Lion House, Oriental Road, Woking, Surrey GU22 8AP, United Kingdom. Reg No. 00264191 England PIIL2766263 © James Walker 2017