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RubberTech Articles
Design Guide Article Series #1 - Selecting the Right Elastomer
Article: # 3
Author: R. J. DelVecchio, Technical Consulting ServicesCommon Elastomer TypesIntroduction
In recent years the use of thermoplastic elastomers (TPEs) has grown considerably. When operating temperatures are not elevated they can fulfill many functions quite well, and have the advantage of easier, more efficient processing, plus of course being easily recycled. The earliest types of TPE were not oil/solvent tolerant at all, but recently some grades with built-in fluid resistance have become available. However, details of TPEs will not be explored here.
This brief guide to the many types of elastomers is only intended to give those not familiar with the field some idea of the variations available. As in any technical area, those not expert in the field are best off gaining enough knowledge to be able to communicate effectively with those whose expertise will allow them to solve whatever problem is being addressed. While there are some applications that require the special properties of a particular type of elastomer, very often more than one type or a blend may be used successfully. It is the compounder's task to find whichever material will serve the function while still being readily processable and cost effective.
The descriptions of elastomer types below are capsule summaries. Much more detailed descriptions are available, including tables identifying property levels in many categories. For instance, resistance of specific elastomers’ ratings are available for several classes of fluids.
For the sake of simplicity and brevity only comparative terms are used here. Differences such as the tendency of one polymer to become tough and leathery as the temperature declines, compared to other polymers which get hard and more brittle, may be glossed over. Some elastomeric polymers can be compounded into very specialized forms, such as when bowling balls were made of Natural Rubber ebonite or silicone rubber is made as a very soft gel, but those kinds of variations will not be described here either.
Also, the descriptions fit the basic properties of each polymer. In this way Natural Rubber is termed as deficient in aging and heat resistance. Yet if all the techniques available to maximize its resistance to time and high temperature are used, the resulting compound can rival a typical Neoprene compound in its properties. Urethanes are considered vulnerable to attack by water (hydrolysis) but modern optimized compounds can have quite useful lifetimes even in humid environments.There is usually some way to improve the performance of any polymer in its weakest area, but enhancing one characteristic often leads to poorer performance in some other property.
Elastomer Types
Natural Rubber (ASTM code NR) is polyisoprene from a plant source, most commonly the rubber tree. (Other plants also yield forms of NR, but none is cultivated in commercial quantities.) NR is available in different quality grades, suited for different applications. Compounds can be made from 30-95 durometer, and processing properties are generally good. Its cost is moderate, but susceptible to changes in world markets. The strong points of the polymer are its excellent fatigue resistance, high resilience, low heat build up, good tolerance of aqueous solutions, good bondability, good low temperature flexibility, excellent abrasion resistance, inherent strength and high elongation. Its disadvantages are poor oil and solvent resistance, limited heat tolerance, and susceptibility to attack by the environment, especially ozone. Part of the fatigue resistance is due to the structure of the polymer which allows it to strain crystallize. The superior fatigue resistance is exhibited in applications where the part is always under some positive strain and does not cycle through zero.
Synthetic Natural Rubber (strange name, isn't it?) is coded IR. It is also polyisoprene, but from the chemical facility, not the growing plant. Its properties are generally quite similar to the original harvested polymer, but its chemical purity, better processing, and increased consistency make it useful in some applications. Synthetic polyisoprene has narrower molecular weight distribution than natural rubber and natural rubber is 99+% cis content while synthetic polyisoprene is around 98% cis content. While this may not seem like much difference, it does reduce slightly the strain crystallization tendency and thus, synthetic polyisoprene performs more poorly in certain applications involving fatigue.
Neoprene, which is technically Chloroprene Rubber, code CR, is polychloroprene. The structure of polychloroprene is identical to that of natural rubber except that a methyl sidegroup has been replaced by a chlorine atom giving it more polarity. Like NR, it has inherent strength, elongation, abrasion resistance, resilience, fatigue, low heat build up, bondability, and low temperature flexibility. (It does not quite match up to NR in all of these properties, but is comparable overall.) However, because of the increased polarity, it is definitely superior in resistance to oils, solvents, heat, aging, and ozone. These properties make it very attractive as a general purpose elastomer. Its cost is somewhat higher than NR, and many different grades of chloroprenes are made for various purposes. Like NR, it is used in latex form in many adhesives. Sometimes chloroprene compounds are prone to premature crosslinking (called scorching) during storage, which can be a problem for the processor.
Butadiene Rubber, or BR, is polybutadiene, which can be made in several significantly different grades (by two different processes, either solution or emulsion polymerization). Its chemical structures are all similar enough to NR to give it the same poor oil and solvent resistance, plus susceptibility to weathering and ozone attack. Its strength is not inherently high, because it does not strain crystallize, so compounds must contain some form of particulate reinforcement. But its flex life is good, and its response to cold is even better than NR. In fact only silicone has better low temperature resistance than polybutadiene. Processing is more of a problem, however. This polymer is often used in blends with other polymers as the major phase. By itself it can be formulated to cover a range of 45-80 durometer.
Styrene Butadiene Rubber, very commonly called SBR, is made by copolymerizing two different monomers, styrene and butadiene. They combine in an irregular pattern that provides little inherent strength, but by use of appropriate particulate reinforcement, the polymer can be made into many usable general purpose compounds. It has few special physical properties, only moderate low temperature tolerance, and is made from 40-90 durometer. Since it is the lowest cost elastomer available and can be used in tires, it is produced in the highest volume of all the synthetics. Many grades are available, including some with black and/or oil predispersed in the polymer to improve properties or processing. For applications which cycle through zero strain, SBR can perform as well or better than natural rubber in fatigue.
Nitrile Rubber, NBR, is another copolymer using butadiene, combined with acrylonitrile (which gives high polarity) in varying proportions depending on what properties are desired. The acrylonitrile content imparts oil resistance to the elastomer, and the more acrylonitrile used in the copolymer, the higher the resistance. However, low temperature flex is impaired by increasing acrylonitrile concentration. NBR is the least expensive elastomer with really good fluid (oil and fuel) resistance (but costs a good deal more than NR), and so it is used widely in seals, O-rings, hoses, etc. Ozone resistance is not high, but may be improved by blending with some PVC.
Hydrogenated Nitrile Rubber, HNBR, is fairly new technology material. It is based on NBR, but has been chemically altered (hydrogenated) so that its main chain is much less vulnerable to attack by heat, ozone, and oxygen, while its acrylonitrile content maintains good oil resistance. It can be vulcanized by sulfur or peroxide systems, from 30 to over 90 durometer, and even made into a specialized ebonite. It has good tensile strength and abrasion resistance, plus moderate low temperature properties. It can be compounded specifically for low or high temperature applications, or to have a balance of properties inbetween. This polymer exhibits strain crystallization, which contributes to its high flex life. Cost is markedly higher than nitrile, but it is possible to make NBR/HNBR blends to maintain excellent properties and be more cost effective. Its good properties have led to penetration of markets such as synchronous timing belts, power steering systems, and numerous other demanding automotive and industrial applications.
Ethylene-propylene-diene Rubber, EPDM for short, is yet another copolymer, using all the named monomers together in varying proportions. (A variation using only the ethylene and propylene, EPM rubber, is available, and has to be vulcanized using peroxides only, but has no major contrasts in properties.) These polymers have no double bonds (chemical unsaturation) in their main chain structure, which makes them immune to many forms of chemical attack that affect most elastomers. EPDM can be made 30-90 durometer, has very good low temperature properties, tolerates high temperatures well, and has outstanding environmental resistance. It does not resist most fluids well, with the exception of Skydrol, an aviation hydraulic fluid. It is used frequently as a blending polymer with other elastomers to improve aging and ozone resistance.
Hypalon, a trade name for chlorosulfonated polyethylene (code CSM) has a slightly restricted hardness range, 50-95 duro, and is known for its resistance to weathering, ozone, aqueous solutions, etc. Colored compounds are commonly made, and retain their colors well even after long term exposure to sunlight. CSM is also flame retardant, tolerant of moderately high temperatures, and has good cold flex properties. However, many of its other physical properties are undistinguished and it resembles polychloroprene in many respects, other than being less resilient. Some oils, solvents and hydraulic fluids attack it, and it is not used in many dynamic applications. A new version of this polymer is an alkylated form designated ACSIUM, which was developed especially for dynamic applications. Its lower damping makes it more comparable to polychloroprene in resilience, and it has better heat aging and low temperature performance.
Chlorinated polyethylene (CM) is an elastomer produced by chemical modification of what is normally a thermoplastic. This material has come into substantial use in the last decade, primarily for wire, cable, and hose, where it is very cost effective. Its fluid resistance is poor and it stiffens at -30°F, but it withstands heat better than NR.
Polyacrylate Rubber, designated as ACM, is a copolymer with varying amounts of ethyl or butyl acrylate with a small proportion of another monomer that provides sites for crosslinking. Polyacrylates have outstanding resistance to the combined effects of heat and oil, and to attack by hot high pressure lubricants. Weathering, ozone, and high temperature are well tolerated by this material. However, it has poor low temperature characteristics, is degraded by water/steam, and swells badly in a number of other fluids. It is moderately expensive, and not used in high volume.
Polysulfide Rubber, code T, is unlike most conventional elastomers in that its chemistry is built around the sulfur atom instead of the carbon atom. Its well known trade name, Thiokol, is better known than its chemical name. All its compounds, which cover 20-80 duro, have a characteristic unpleasant odor, generally poor physical properties, limited temperature tolerance, and don't process very much like other elastomers. Its strong points are good weathering/aging, very low permeability to gases, and outstanding resistance to hydrocarbon solvents and fuels. It is moderately costly, and is available in liquid form (useful for making sealants and caulks) as well as conventional millable form, used to make molded elastomer parts.
Epichlorohydrins come in three types, the unmodified polyepichlorohydrin (CO), a copolymer with ethylene oxide (ECO), and a terpolymer with ethylene oxide and allyl glycidal ether (GECO). The CO type has gas permeability even lower than butyl rubber, good resistance to heat, weathering, and many fuels, and excellent ozone resistance. However, it stiffens quickly at moderately low temperatures, and is attacked by some solvents and hydraulic fluids. The copolymer is more tolerant of cold, but has higher permeability; the terpolymer's properties are similar to those of the copolymer, and it can be sulfur or peroxide cured, due to the unsaturation in the AGE side groups. One special characteristic of these polymers is their inherent low volume resistivity, which can be useful where buildup of static charges is a problem. The cost of polyepichlorohydrins is somewhat high, and processing is a bit more difficult than standard elastomers.
Urethanes as a chemical type are by far the largest group of all elastomers (codes AU, EU). In fact, there are solid (millable) urethanes, liquid urethanes that vulcanize to elastomer form, urethanes which are thermoset plastics, true thermoplastics, and thermoplastic elastomers as well. Considering just the thermoset elastomer varieties, a range of from 35 durometer on the normal scale (Shore A) to 75 durometer on the higher D scale can be covered. At the upper hardness level the elastomer will feel just like a hard plastic instead of like a piece of rubber. Urethanes are outstanding in strength and abrasion resistance, but do not withstand either high or low temperatures well. They tolerate many oils well, but not some solvents, and will usually be degraded by water/steam over time. Their cost can be moderate to appreciable, and they are used extensively in many special applications, especially for abrasion resistance.
Butyl Rubber, code IIR, is polybutylene, a non-resilient rubber known for its low gas permeability. It is available in chemically modified forms, using chlorine or bromine to significantly improve some aspects of processing and final properties. (Bromobutyl or chlorobutyl rubbers, BIIR and CIIR respectively.) They all have excellent aging/weathering resistance, good low temperature flex, and cover the 30-80 duro range. Cost is higher than NR, and processing has some special requirements. A new variation of butyl rubber is Exxpro, a copolymer of polyisobutylene and paramethylstyrene (abbreviated PMS and used between 2 and 10% to supply cure sites). Its properties are very similar to butyl except that it is more heat stable due to the change in vulcanization sites.
Ethylene acrylic rubber, most often referred to by its trade name of Vamac, is a copolymer of ethylene and methyl acrylate, plus a low percentage of a third monomer which provides the cure sites in the resulting polymer. It combines high heat tolerance with good oil resistance and reasonable low temperature characteristics, at a cost less than the more exotic but better known elastomers. It does not withstand some fuels and solvents well and presents real processing problems, but overall has an attractive set of properties. Vamac is still gaining in usage levels, and recently a new product, named ADVANTA, was introduced, which is a blend of ethylene copolymer with fluorocarbon polymer. This material is intended for use in applications which demand better fluid resistance and high temperature tolerance than ethylene acrylic rubber has, but don't necessarily require all the properties of the more expensive fluorocarbon elastomers.
Fluorocarbon Rubber, code FKM, has the same carbon atom backbone of most elastomers, but has had many of the hydrogen atoms that are normally attached to the chain replaced by fluorine atoms. The resulting rubber provides the best overall combination of resistance to fluids and heat, plus high environmental/ozone resistance. However, its low temperature tolerance is the worst overall of any elastomer, and steam and/or some fluids can still attack it. Durometer range is 60-90. Cost is quite high; processing can be a little difficult.
Aflas is the trade name for polytetrafluoroethylene-propylene, a chemical variation of fluorocarbon rubber. It has substantially improved resistance to steam and many fluids. Although its low temperature properties are even worse than conventional fluoroelastomers, it has proven very useful in some specialized applications, such as seals used in the hot, corrosive environment at the bottom of oil wells. Another subvariety of the FKM class is those few elastomers whose structure contains no hydrogen atoms at all. These are called perfluoroelastomers (trade name Kalrez or Chemraz), and they have the ultimate chemical/heat resistance. At almost $3000/lb, they are used very seldom, in situations where absolutely nothing else will do.
Silicone Rubber is a broad category, in that several subtypes exist, with codes MQ, VMQ, PMQ, PVMQ. They are all basically polydimethylsiloxane, the chemical structure of which is very unlike standard elastomers based on carbon chemistry. This special structure does not produce elastomers with high levels of strength or other basic physical properties, but all the properties are much less sensitive to temperature changes. Compounds from 25-80 duro are possible. Their operating temperature range is from -80 to 450°F, with excellent flame, weather, and ozone resistance. Biocompatibility is good as well. Fluid resistance is generally poor. Cost can be moderately high to fairly expensive, and special processing is required.
Fluorosilicone (FVMQ) is a modified form of silicone rubber, made expressly to improve fluid (fuel and oil) resistance. It has a slightly narrower temperature range than silicone rubber, lower physical properties, a 40-80 duro range, and costs a good deal more. (Up to $100+ per pound.) Its use is limited to areas in which the normal advantages of silicone compounds must be combined with good fluid resistance. Processing is worse than silicone compounds.
Polyphosphazene Rubber (code FZ), also known as PNF, was the newest of the exotic elastomers, and its employment was limited to special functions such as aircraft and space seals. It has a unique chemistry, in that the polymer backbone is made up of phosphorous and nitrogen atoms. The material had been quite expensive, but its combination of functionality at very low temperature, good heat tolerance, and excellent overall fluid resistance made it attractive. When the poor physical properties of fluorosilicone make its use inappropriate, but oil resistance, low temperature flex, etc, are required, PNF becomes the material of choice. Its hardness range is about 50-85 durometer. However, its manufacture changed hands in late 1993, and it became prohibitively expensive (~ $300/lb). It is now all but discontinued and is being replaced in most of its applications by fluorosilicone.
Vestenemer is a German trade name for the one of the newer available elastomers, which is polyoctenylene. This polymer has a very unusual molecular structure which contributes to nontypical behavior. It has barely begun to be used, and then mainly as an additive to other elastomers and some plastics.
Polypropylene oxide rubber (GPO code) was a development of the 70's, a copolymer of propylene oxide and allyl glycidyl ether. The resulting material could be compounded similarly to NR, but with some advantages. It has a 40-90 durometer range, good high and excellent low temperature performance, good physical properties, and some oil resistance. Fuels attack it and it is prone to high set. It was used in some dynamic applications such as automotive engine mounts, but volume sales never materialized and it is being discontinued.
Polynorbornene, code PNR, is a fairly recent addition to the field, with some interesting properties. Its hardness range is unusually low, 10-60 durometer, and its level of resilience can be varied from quite high to quite low by compounding techniques. Its abrasion rating is good, but it cannot withstand high temperatures, flame, and many fluids. Cost is not prohibitive. For the moment its applications are not broad, but use is growing slowly.
Royaltherm is the Uniroyal trade name for a somewhat unusual product, a graft polymer of EPDM and silicone. It was intended to fill a market niche for a less expensive and more easily processable elastomer which had some of the desirable characteristics of the more costly silicone materials. While it does have interesting properties, its use is still at a low level and it remains to be seen how it will fit into the industry.
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