Elastomers and Rubbers

Families and grades of both thermoset and thermoplastic rubber are now available to serve in almost any environment.

Elastomers and rubber are differentiated from polymers by the mechanical property of returning to their original shape after being stretched to several times their length. The rubber industry differentiates between the terms "elastomer" and "rubber" on the bases of how long a deformed material sample requires to return to its approximate original size after a deforming force is removed, and of its extent of recovery. ASTM D1566 defines an elastomer as "a macromolecular material which, at room temperature, is capable of recovering substantially in shape and size after removal of a deforming force." The same standard is more specific and quantitative in defining rubber as "a material that is capable of recovering from large deformations quickly and forcibly . . . (and which), in its modified state, free of diluents, retracts within one minute to less than 1.5 times its original length after being stretched at room temperature to twice its length and held for one minute before release."

Thus, by these definitions, all rubbers are elastomers, but not all elastomers are rubbers, since no return time or deformation hold time is specified in the elastomer definition. Also, some plastics qualify as elastomers, according to the rather loose definition of that category.

Chapters on Elastomers and Rubbers:

Thermoset Rubber, Structure of Rubbers Material and Process Selection, Controlling the Strech, Industry Standards Non-oil Resistant Rubbers Oil Resistant Rubbers Thermoplastic Elastomers

Thermoset rubber

Originally, rubber meant the material obtained from the rubber tree Hevea Brasiliensis. Today, the term rubber means any materials capable of extreme deformability, with more or less complete recovery upon removal of the deforming force. Synthetic materials such as neoprene, nitrile, styrene butadiene (SBR), and butadiene rubber are now grouped with natural rubber. These materials serve engineering needs in fields dealing with shock absorption, noise and vibration control, sealing, corrosion protection, abrasion protection, friction production, electrical and thermal insulation, waterproofing, confining other materials, and load bearing.

Of all the available choices, SBR dominates the field, accounting for approximately one-half of all rubber -- natural and synthetic -- used in the U.S. The demand for SBR has been responsible for the building of a massive production capability for this material. More than half of SBR production goes into passenger-car tires in the U.S. Natural rubber is used almost exclusively in more demanding areas such as truck, bus, aircraft, and off-highway tires.

Structure of rubbers

In contrast with the ordered and rigid crystalline arrangement of atoms in metals, rubber atoms are arranged in long, chainlike configurations, which are in constant, thermally induced motion. The result is a tangled mass of kinked, twisted, and intertwined elements similar to a snarled fishing line. Along the chain, the atoms remain substantially the same distance apart, but the spatial distance from one point on a chain to another is always less than that measured along the chain's length.

Statistically, at a specific temperature, there is one most probable spatial distance between any two points on a given chain. When an applied force changes this distance, the thermal movement of the system sets up a force to restore the distance to what it was originally. This action accounts for the elasticity, or recovery, of a deformed rubber component. It also explains why the modulus of an elastomer, when heated, increases in the direction of strain.

Within the elastic solid, the tangled chain segments are relatively free to move with respect to one another, except to the extent that they encounter mechanical entanglement or, upon being vulcanized, are "hooked" together at chemically reactive sites on the chains and attain structural integrity.

As with almost any material, selecting a rubber for an application requires consideration of many factors, including mechanical or physical service requirements, operating environment, a reasonable life cycle, manufacturability of the part, and cost. Further, within the framework of each family of rubbers exists a wide range of available properties. These are created by compounding; that is, incorporating additives that improve a weak property, make the compound easier to process, or reduce cost without significantly affecting properties. In addition to the varieties of rubbers available, almost any physical or chemical property can be altered. Thus, selecting the best material for an application can involve considerable investigation and almost always involves a compromise.

Manufacturing rubber parts is accomplished in one of three ways: transfer molding, compression molding, or injection molding. The choice of process depends on a number of factors, including the size, shape, and function of the part, as well as anticipated quantity, type, and cost of the raw material.

The three methods, however, share certain basic characteristics. The custom-molding process begins with the design and construction of a precision machined steel mold. This mold, or tool, consists of two or more steel plates into which the rubber is placed or injected. These plates are exposed to heat and pressure to cure the part. The exact mix of time, temperature and pressure depends on the molding process and material.

A rubber mold consists of two or more custom-tooled steel plates carefully registered to ensure consistent close tolerances and appropriate surface finish. The line where the two molds meet is called the parting line.

Sometimes the presence of a parting line is objectionable to the designer for functional or aesthetic reasons. This condition can be prevented by shifting the parting line to another location. Keep in mind that the location of the parting line can have a dramatic impact on mold and finished part costs.

The excess amount of material that forms at the parting line after pressure has been applied to the filled cavity is called "flash." This thin ridge of material usually is removed in a secondary operation. If a cured part is too delicate to remove by hand from a two-plate mold, three or more plates are sometimes used. In other cases, the finished part is removed by compressed air.

Of the three basic molding processes -- transfer, compression, injection, selection is determined by:

The size and shape of the part. The material's hardness, flow, and cost. The number of parts to be produced.

Generally, the simpler the mold, the easier it is to form a part. Corners, holes, sharp edges, deep undercuts, and other special requirements make the molding process more challenging. For example, a part with a deep undercut may require a mold that splits horizontally as well as vertically to permit removal of the part.

Controlling the stretch

The response pattern of rubber to a deforming force is a function of the degree of ease with which the chainlike segments of molecules can move relative to one another. This motion can be hindered, for example, by any filler substance put into the mass of tangled, twisting chains of molecules; the result is a stiffer rubber compound. Conversely, anything put in to lubricate the system makes the compound softer because a lubricant increases the ease of chain movement. The structure of the molecule itself also affects stiffness. The smaller and fewer the chemical attachments along the chains, the less hindrance there is to relative movement, and the greater the resiliency and elasticity.

Industry standards

Among the available standards on various aspects of rubber, the following are recommended as aids to identification and selection:

ASTM D1418 -- Rubber and Rubber Lattices -- Nomenclature. This standard describes all available rubbers in terms of their chemical compositions. ASTM D1566 -- Standard Definitions of Terms Relating to Rubber. This reference helps to ensure unambiguous communication among producers, molders, and designers. ASTM D2000 -- Standard Classification System for Rubber Parts in Automotive Applications. This standard -- despite its title -- is not limited to automotive parts, and is probably the most important document of all.

The D2000 classification system, also available as SAE J200, is based on the premise that properties of all rubber compounds can be arranged into characteristic material designations. These designations are determined by "Type," based on resistance to heat aging, and "Class," based on resistance to swelling in oil. Basic levels are thus established which, together with values describing additional requirements, permit complete characterization of all rubber materials.

ASTM D2000 and SAE J200 standards designate rubber materials according to their performance in thermal and oil-immersion tests. The Type designation is determined by a thermal test, which establishes a maximum service temperature; letters A through J indicate the range from 70 to 275°C. Class designations, based on maximum volume swell with immersion in a prescribed (ASTM #3) oil test, are also letters; letters A through K represent the 10 classes in these specifications.

Type and Class designations are written together. For example, AK defines a requirement for a rubber that can be used at 70°C continuously and that will not swell more than 10% when immersed in an ASTM reference oil.

The table lists the rubber materials that are most often used in meeting typical requirements as spelled out by ASTM D2000 and SAE J200. This list is not limiting; other polymers may meet the same specification.

Specification D2000 can also be used to describe, in these same terms, a material not yet available, but preferred. The standard allows the rubber technologist and the design engineer to discuss materials in a mutually understood fashion without the rubber technologist's divulging the chemical makeup of his material, which would be of little value to the engineer anyway.

The material descriptions that follow are designed to help make a quick preliminary selection of a base material. The accompanying Rubber Selection and Service Guide can further aid the selection. ASTM Standard D2000 can then be used to "name" or specify an available material (or one that should be developed) for the application. These brief descriptions are grouped into two categories: those listed as having no requirement for oil resistance, and those that do. The headings include the common name, ASTM D1418 designation (chemical composition), and material designation Type and Class, according to ASTM D2000.

This group includes those materials in the Class A designation (no requirement regarding volume swell due to oil) and Types A and B designations (for continuous use not exceeding 70 and 100°C).

Natural rubber (NR, AA): The commercial base for natural rubber is latex, a milklike serum, generated by the tropical tree, Hevea Brasiliensis. The latex is collected in much the same fashion as maple sap. However, latex should not be confused with the sap of the tree. Latex is secreted in the inner bark of the tree, and a tree can be severely harmed if a tapping cut is deep enough to draw sap as well as latex. Naturally occurring latex is a dispersion of rubber in an aqueous serum containing various inorganic and organic substances. The rubber precipitated out of this solution can be characterized as a coherent elastic solid.

All other rubbers should be measured against natural rubber. For centuries it was the only rubber available, and it was used extensively, even before the discovery of vulcanization in 1839.

Synthetic rubbers have been developed either by accident or as the result of pressures of political upheaval or wartime restrictions and consequent unavailability of the natural product. However, no synthetic material has yet equaled the overall engineering characteristics and consequent wide latitude of application available with NR.

As with other rubbers, many grades and types of NR are available, produced by varying impurity levels, collection methods, and processing techniques. Natural rubber is generally considered to be the best of the general-purpose rubbers -- those having properties and characteristics suitable for broad engineering applications. Compounds can be produced over a wider stiffness range with natural rubber than with any other material. Natural rubber is often the best choice for most applications except where an extreme performance or exposure requirement dictates the use of a special-purpose rubber, often at some sacrifice of other, less-critical characteristics.

Natural rubber has a large deformability capacity. This, coupled with its ability to strain crystallize, gives it added strength while deformed. Its high resilience, which is responsible for a very low heat buildup in flexing, makes NR a prime candidate for shock and severe dynamic loads. Thus, in applications where properties such as flexure, cut resistance, abrasion resistance, and general endurance would be adversely affected by heat in less-resilient rubbers, NR is recommended because of its low heat buildup.

NR also has low compression set and stress relaxation; these characteristics favor its application in sealing devices where maintenance of sealing forces and the surface conformability of high-quality soft stocks are important. Further advantages are excellent green (uncured) strength, building tack, and general processing characteristics.

Natural rubber does have some shortcomings. The useful service temperature of NR ranges generally from -65 to (in special cases) 250°F. Other drawbacks of NR such as poor oil, oxidation, and ozone resistance can be minimized, either by proper design accommodation and/or by compounding. Degradation from such environments are essentially surface effects that can be tolerated or minimized by using thicker cross sections, by shielding, or by adding antioxidants and antiozonants.

Natural rubber can often be the first choice for many high-performance applications if it can be made to survive in the service environment. It remains the best choice for tires, shock mounts and other energy absorbers, seals, isolators, couplings, bearings, springs, and dynamic applications.

Synthetic natural rubber (IR; AA): The synthetic rubber that is closest to duplicating the chemical composition of natural rubber is synthetic polyisoprene. It shares with natural rubber the properties of good uncured tack, high unreinforced strength, good abrasion resistance, and those characteristics that provide good performance in dynamic applications. However, because of some of the inherent impurities in the natural product that affect vulcanization characteristics in a positive fashion, natural rubber scores somewhat better on overall ratings.

A significant disadvantage of IR is its lack of green strength. IR can be used interchangeably for natural rubber in all but the most demanding applications. Specific product applications are about the same as those for natural rubber.

Styrene butadiene (SBR; AA, BA): This material emerged as a high-volume substitute for NR during World War II because of its suitability for use in tires. Despite the fact that the basic feedstock for SBR is crude oil, it has remained competitive in cost because of the extensive production capacity for SBR in the U.S.

SBR continues to be used in many applications where it replaced NR, even though it does not have the overall versatility of natural rubber and the other general-purpose materials. For most applications, SBR must be reinforced (hence, stock are stiffer) to have acceptable tensile strength, tear resistance, and general durability. SBR is significantly less resilient than NR, so it has higher heat buildup on flexing. Further, it does not have the processing and fabricating qualities of NR, lacking both green strength and building tack.

An important reason for the continued high volume use of SBR is that it did a creditable job in passenger car tires, having good abrasion resistance and general durability. Recently that picture has changed, however, because of the greater need for the green strength and building tack of natural rubber in radial tires and for the better low-temperature flexibility of natural rubber for snow tires. High-performance tires, such as for trucks and aircraft, have always been made from natural rubber if it was available.

Polybutadiene (BR; AA): This general-purpose, crude-oil-based rubber is even more resilient than natural rubber. It was the material that made the solid golf ball possible. It is also superior to natural rubber in low-temperature flexibility and in having less dynamic heat buildup. However, it lacks the toughness, durability, and cut-growth resistance of NR. It can be used as a blend in natural rubber or SBR to improve their low-temperature flexibility. Silicones have superior low-temperature flexibility, but this is achieved at a much higher price and at a sacrifice in other properties such as tensile strength, tear resistance, and general durability.

A large volume of polybutadiene is used in blends with other polymers to enhance their resilience and reduce heat buildup. It is also used in products requiring high resiliency over a broad temperature range such as industrial tires and vibration mounts.

Butyl (IIR, CIIR, AA, BA): The two types of rubber in this category are both based on crude oil. The first is polyisobutylene with an occasional isoprene unit inserted in the polymer chain to enhance vulcanization characteristics. The second is the same, except that chlorine is added (approximately 1.2% by weight), resulting in greater vulcanization flexibility and cure compatibility with general-purpose rubbers.

Butyl rubbers have outstanding impermeability to gases and excellent oxidation and ozone resistance. The chemical inertness is further reflected in lack of molecular-weight breakdown during processing, thus permitting the use of hot-mixing techniques for better polymer/filler interaction.

Flex, tear, and abrasion resistance approach those of natural rubber, and moderate-strength (2,000 psi) unreinforced compounds can be made at a competitive cost. Butyls lack the toughness and durability, however, of some of the general-purpose rubbers.

The attribute responsible for the high-volume use of butyl rubber in automotive inner tubes and tubeless tire interliners is its excellent impermeability to air. Butyls are also used in belting, steam hose, curing bladders, O-rings, shock and vibration products, structural caulks and sealants, water-barrier applications, roof coatings, and gas-metering diaphragms.

Ethylene propylene (EPR, EPDM; AA, BA, CA): Like the butyls, the EP rubbers are of two types. One is a fully saturated (chemically inert) copolymer of ethylene and propylene (EPR); the other (EPDM) is the same as this plus a third polymer building block (diene monomer) attached to the side of the chain. EPDM is chemically reactive and is capable of sulfur vulcanization. The copolymer must be cured with peroxide.

Physical properties of EPR and EPDM are not as good as those obtainable with NR. However, property retention is better than that of NR on exposure to heat, oxidation, or ozone. Bonding is somewhat more difficult, especially with EPR. These materials have broad resistance to chemicals but not to oils and other hydrocarbon fluids. Electrical properties are good.

Typical applications are automotive hose; body mounts and pads; O-rings; conveyor belting; wire and cable insulation and jacketing; window channeling; and other products requiring resistance to weathering. EPDM sheeting, either unsupported or reinforced, is used in roofing and as liners for water conservation and pollution-control systems.

These rubbers include grades suitable for service at temperatures to 250°C and having maximum resistance to oils and greases. Some are considered specialty materials and are quite expensive.

Neoprene (CR; BC, BE): Except for polybutadiene and polyisoprene, neoprene is perhaps the most rubberlike of all, particularly with regard to dynamic response. Neoprenes are a large family of rubbers that have a property profile approaching that of natural rubber, and with better resistance to oils, ozone, oxidation, and flame. They age better and do not soften on heat exposure, although high-temperature tensile strength may be lower than that of NR.

These materials, like NR, can be used to make soft, high-strength compounds. A significant difference is that, in addition to neoprene being more costly than NR by the pound, its density is about 25% greater than that of natural rubber. Neoprenes do not have the low-temperature flexibility of natural rubber, which detracts from their use in low-temperature shock or impact applications.

General-purpose neoprenes are used in hose, belting, wire and cable, footwear, coated fabrics, tires, mountings, bearing pads, pump impellers, adhesives, seals for windows and curtain-wall panels, and flashing and roofing. Neoprene latex is used for adhesives, dip-coated goods, and cellular cushioning jackets.

Chlorinated polyethylene (CM; DE): This family of elastomers is produced by the random chlorination of high-density polyethylene. Because of the high degree of chemical saturation of the polymer chain, the most desirable properties are obtained by crosslinking with the use of peroxides or by radiation. Sulfur donor cure systems are available that produce vulcanizates with only minor performance losses compared to that of peroxide cures. However, the free radical crosslinking by means of peroxides is most commonly used and permits easy and safe processing, with outstanding shelf stability and optimum cured properties.

Chlorinated polyethylene elastomers, sold by the Dow Chemical Co. under the trade name Tyrin, are used in automotive hose applications, premium hydraulic hose, chemical hose, tubing, belting, sheet packing, foams, wire and cable, and in a variety of molded products. Properties include excellent ozone and weather resistance, heat resistance to 300°F (to 350°F in many types of oil), dynamic flexing resistance and good abrasion resistance.

Chlorosulfonated polyethylene (CSM; DE): This material, more commonly known as Hypalon (Du Pont), can be compounded to have an excellent combination of properties including virtually total resistance to ozone and excellent resistance to abrasion, weather, heat, flame, oxidizing chemicals, and crack growth. In addition, CSM has low moisture absorption, good dielectric properties, and can be made in a wide range of colors because it does not require carbon black for reinforcement. Resistance to oil is similar to that of neoprene. Low-temperature flexibility is fair at -40°F.

Hypalon is a special-purpose rubber, not particularly recommended for dynamic applications. It is used generally where its outstanding environmental resistance is needed. Typical applications include coated fabrics, maintenance coatings, tank liners, protective boots for spark plugs and electrical connectors, cable jacketing, and sheeting for pond liners and roofing.

Nitrile (NBR; BF, BG, BK, CH): The nitriles are copolymers of butadiene and acrylonitrile, used primarily for applications requiring resistance to petroleum oils and gasoline. Resistance to aromatic hydrocarbons is better than that of neoprene but not as good as that of polysulfide. NBR has excellent resistance to mineral and vegetable oils, but relatively poor resistance to the swelling action of oxygenated solvents such as acetone, methyl ethyl ketone, and other ketones. It has good resistance to acids and bases except those having strong oxidizing effects. Resistance to heat aging is good, often a key advantage over NR.

With higher acrylonitrile content, the solvent resistance of an NBR compound is increased but low-temperature flexibility is decreased. Low-temperature resistance is inferior to that of natural rubber, and although NBR can be compounded to give improved performance in this area, the gain is usually at the expense of oil and solvent resistance. As with SBR, this material does not crystallize on stretching, and reinforcing materials are required to obtain high strength. With compounding, nitrile rubbers can provide a good balance of low creep, good resilience, low permanent set, and good abrasion resistance.

Tear resistance is inferior to that of natural rubber, and electrical insulation is lower. NBR is used instead of natural rubber where increased resistance to petroleum oils, gasoline, or aromatic hydrocarbons is required. Uses of NBR include carburetor and fuel-pump diaphragms and aircraft hoses and gaskets. In many of these applications, the nitriles compete with polysulfides and neoprenes.

Epichlorohydrin (CO, ECO; CH): Epichlorohydrin rubber is available as a homopolymer (CO) and a copolymer (ECO) of epichlorohydrin. Reinforced, these rubbers have moderate tensile strength and elongation properties, plus an unusual combination of other characteristics. One of these is low heat buildup, which makes them suitable for applications involving cyclic shock or vibration.

The homopolymer has outstanding resistance to ozone, good resistance to swelling by oils, intermediate heat resistance, extremely low permeability to gases, and excellent weathering properties. This rubber also has low resilience characteristics and low-temperature flexibility only to 5°F -- characteristics that may be unsuitable for some applications.

The copolymer is more resilient and has low-temperature flexibility to -40°F, but it is more permeable to gases. Oil resistance of both compounds is about the same. Typical applications include bladders, diaphragms, vibration-control equipment, mounts, vibration dampers, seals, gaskets, fuel hose, rollers, and belting.

Ethylene/acrylic: This family of rubbers is sold by Du Pont, under the trade name of Vamac. Introduced in 1975 in masterbatch form, the family was expanded in 1983 by the addition of a gum polymer. Vamac materials provide, at a moderate price, heat and fluid resistance surpassed by only the more expensive, specialty polymers such as fluorocarbons and fluorosilicones. The material has very good resistance to hot oils, hydrocarbon-based or glycol-based proprietary lubricants, transmission and power-steering fluids. It is not recommended for use with esters, ketones, highly aromatic fluids or high-pressure steam. A special feature of Vamac is its nearly constant damping characteristic over broad ranges of temperature, frequency, and amplitude.

The polymer is recommended for applications requiring a durable, set-resistant rubber with good low-temperature properties and resistance to the combined deteriorating influences of heat, oil, and weather. It is used in various automotive components such as mounts, gaskets, seals, boots, and ignition-wire jackets. Electrical applications include oil-well platform cable jackets, plenum cable, transit-wire jackets, and marine cable.

Perfluoroelastomer (FFKM): Chemical resistance of perfluoroelastomer parts is similar to that of PTFE, and mechanical properties are similar to those of the fluorocarbon rubbers. This high-performance, high-priced rubber, produced by Du Pont as Kalrez, and by Greene, Tweed & Co. as Chemraz, is essentially unaffected by all fluids, including aliphatic and aromatic hydrocarbons, esters, ethers, ketones, oils, lubricants, and most acids. However, some fully halogenated fluids and strong oxidizing acids may cause swelling. The parts are suitable for continuous service to 290°C and intermittent service to 316°C. Resistance to ozone, weather, and flame is exceptional. Radiation resistance is good and high-vacuum performance excellent.

Perfluoroelastomer parts are used primarily in demanding fluid-sealing applications in the chemical-processing, oil-production, aerospace, and aircraft industries.

Acrylate (ACM, ANM; DF, DH): These are specialty rubbers based on polymers of methyl, ethyl, or other alkyl acrylates. They are highly resistant to oxygen and ozone, and their heat resistance is superior to that of all other commercial rubbers except the silicones and the fluorine-containing rubbers. Water resistance is poor, however, so the acrylates are not recommended for use with steam or water-soluble materials such as methanol or ethylene glycol. However, flex life is excellent as is permeability resistance. Resistance to oil swell and deterioration is also excellent at high temperatures.

Low-temperature flexibility is not good, and these rubbers decompose in alkaline solutions and are swelled by acids. Low-temperature flexibility and water resistance can be improved, but only with a marked decrease in heat and oil resistance. These materials are used extensively for bearing seals in transmissions, and for O-rings and gaskets.

Polysulfide (PTR; AK, BK): These polymers have outstanding resistance to oils, greases, and solvents, but they have an unpleasant odor, resilience is poor, and heat resistance is only fair. Abrasion resistance is half that of natural rubber, and tensile strength ranges from 1,200 to 1,400 psi. However, these values are retained after extended immersion in oil.

Basic properties of polysulfide polymers are determined by the type of chain structure and the number of sulfur atoms in the polysulfide groups. Increased sulfur concentration improves solvent and oil resistance, and also reduces permeability to gases. These materials are used in gasoline hose, printing rolls, caulking, adhesives, and binders.

Silicone (VMPQ, PVMQ; MQ, PMQ, FC, FE, GE): Silicone rubber comprises a versatile family of semiorganic synthetics that look and feel like organic rubber, yet have a completely different type of structure from other rubbers. The backbone of the rubber is not a chain of carbon atoms but an arrangement of silicone and oxygen atoms. This structure gives a very flexible chain with weak interchain forces, which provides a remarkably small change in dynamic characteristics over a wide temperature range.

Silicone rubbers have no molecular orientation or crystallization or stretching and must be strengthened by reinforcing materials. The cost of silicone rubbers is not as dependent on petroleum cost as are costs of the synthetic organic rubbers. Although silicones are at the high end of the cost range for rubbers, they can be made to withstand temperatures as high as 600°F without deterioration. At the other end of the scale, silicones retain useful flexibility at -150°F.

While the strength of silicone rubbers is lower than that of other rubbers, these materials have outstanding fatigue and flex resistance. They do not require high tensile strength to serve in dynamic applications. Fall-off in tensile properties with extended exposure to high temperature is much less than for other rubbers. Resistance to chemical deterioration, oils, oxygen, and ozone is also retained under these conditions. Chemical inertness makes these materials well suited for surgical and food-processing equipment. One and two-part silicone sealants are used as structural adhesives and weatherseals in commercial buildings.

Fluorosilicone (FVMQ; FK): This type of silicone provides most of the useful qualities of the regular silicones plus improved resistance to many hydrocarbon fluids such as fuels. Exceptions are ketones and phosphate esters; however, FVMQ rubbers can be blended with conventional dimethyl silicones, which have good resistance to these fluids at temperatures to 300°F. The FVMQ rubbers are most useful where the best in low-temperature flexibility is required in addition to fluid resistance, although resistance to fluids (especially those containing aromatics) is poorer than that of the FKM-type fluorocarbon rubbers.

Fluorosilicone rubbers have moderate dielectric properties, low compression set, and excellent resistance to ozone and weathering. They are expensive and definitely special purpose. Typical applications include seals, tank linings, diaphragms, O-rings, and protective boots in electrical equipment.

Fluorocarbon (FKM; HK): Generally produced as a copolymer of vinylidene fluoride and hexafluoropropylene, the fluorocarbons are high-performance, high-cost rubbers known generally as Viton (Du Pont) and Fluorel (3M). These rubbers have outstanding resistance to heat and to many chemicals, oils, and solvents compared to any other commercial rubber. In air, fluorocarbon rubber parts retain at least half of their original properties after 16-hr exposure at 600°F. These same compounds offer low-temperature stability to -40°F.

In the reinforced state, these rubbers offer moderate tensile strength but relatively low elongation properties. They resist oxidation and ozone, and they do not support combustion. Several versions are available, and conventional compounding produces formulations within a hardness range of 65 to 95 Shore A. Fluorocarbon rubbers are severely attacked by highly polar fluids such as ketones, hydrazine, anhydrous ammonia, and Skydrol (phosphate ester) hydraulic fluids. Postcuring is required to develop optimum properties. Typical applications are seals, gaskets, diaphragms, pump impellers, tubing, and vacuum and radiation equipment.

Urethane (AU, EU; BG): These rubbers, combinations of polyesters or polyethers and diisocyanates, are unusual in that physical properties do not depend on compounding materials. Urethanes crosslink and undergo chain extension to produce a wide variety of compounds. They are available as castable or liquid materials and as solids or millable gums.

Urethane polymers have outstanding abrasion resistance, excellent tensile strength and load-bearing capacity, and elongation potential, accompanied by high hardness. Other properties include low-temperature resistance, high tear strength, either high or low coefficient of friction, good radiation resistance, and good elasticity and resilience, even in very hard stocks.

Typical applications include seals, bumpers, metal-forming dies, valve seats, liners, coupling elements, rollers, wheels, and conveyor belts, especially where abrasive conditions are present.

Thermoplastic elastomers (TPEs) have two big advantages over the conventional thermoset (vulcanized) types -- processing ease and speed. Other compelling reasons for considering the TPEs are recyclability of scrap, lower energy costs for processing, and the availability of standard, uniform grades (not available in thermosets). This last advantage is particularly important to multinational corporations.

The TPEs are molded or extruded on standard plastics-processing equipment in considerably shorter cycle times than those required for compression or transfer molding of conventional rubbers. They are made by copolymerizing two or more monomers, using either block or graft polymerization techniques. One of the monomers develops the hard, or crystalline, segment that functions as a thermally stable component (which softens and flows under shear, as opposed to the chemical crosslinks between polymeric chains in a conventional, thermosetting rubber); the other monomer develops the soft, or amorphous segment, which contributes the rubbery characteristic.

Properties can be controlled by varying the ratio of the monomers and the lengths of the hard and soft segments. Block techniques create long-chain molecules that have various sequences, or blocks, of hard and soft segments; graft methods involve grafting one polymer chain to another as branches. Graft techniques offer more possibilities to vary the copolymer because both the backbone monomer and the grafted branches can be rubbery, glassy hard, or somewhere between. In general, environmental and fluid resistance are totally predictable.

The four oldest thermoplastic elastomer types are polyurethanes, polyester copolymers, styrene copolymers, and the olefinics. Mechanical properties of the first two types are generally higher than those of the last two. Dynamic properties, such as flex life are also generally better. Newest TPEs are three classes of high-performance materials. One is based on polyamide (nylon) chemistry; another, called elastomeric alloys, consists of polymer alloys of an olefinic resin and rubber. The third group, melt-processible rubbers, are sold by Du Pont under the Alcryn tradename.

The polyamide TPEs are low-density, high-elongation materials with good solvent and abrasion resistance. They are expected to fill specialty needs in automotive, sports, medical, and electric-electronic equipment. The elastomeric alloys are based on olefins but their proprietary manufacturing methods give them higher properties than the conventional thermoplastic olefins. They are designed to replace thermoset rubbers such as EPDM, nitrile, and neoprene.

Polyurethanes: The first major elastomers that could be processed by thermoplastic methods were the urethanes. Thermoplastic urethanes do not have quite the heat resistance and compression-set resistance of the thermoset types (see chapters on Thermoset rubber and Polyurethane, but most other properties are similar. They are available in a wide range of hardness grades and in a number of forms, from several manufacturers.

Urethanes are a reaction product of a diisocyanate and long and short chain polyether, polyester, or caprolactone glycols. The polyether types are slightly more expensive and have better hydrolytic stability and low-temperature flexibility than the polyester types.

Mechanical properties of the polyester types are generally higher, however. Caprolactones offer a good compromise between the polyether and polyester types. Abrasion resistance of the urethanes is outstanding among elastomers, low-temperature flexibility is good, oil resistance is excellent to 180°F, and load-bearing capability ranks with the best of the elastomers. Additives can improve dimensional stability or heat resistance, reduce friction, or increase flame retardancy, fungus resistance, or weatherability. Resistance of the polyester types to strong acids, organophosphorous esters, and steam is poor.

Urethane tubing is used for fuel lines, fluid devices, and parts requiring oxygen and ozone resistance. The excellent abrasion resistance of urethanes qualifies them for use in bumpers, gears, rollers, sprockets, cable jackets, chute linings, textile-machinery parts, casters, and solid tires. Other applications include gaskets, diaphragms, shaft couplings, vibration-damping components, conveyor belts, sheeting, bladders, keyboard covers, and films for packaging.

The most recently introduced commercial thermoplastic polyurethanes are polyether aliphatic diisocyanates based on 1,4-butane diol, HMDI, and polytetramethyl-ethylene diol. These lower molecular-weight materials have better color stability to UV radiation and hydrolysis than the conventional grades. The softer grades are used in medical applications (with suitable antioxidants) and as adhesives in security glazing for armored vehicles, prisons, banks, and in aircraft glazing. Other new grades are stabilized for use as wear layers for aircraft wings.

Copolyesters: These thermoplastic elastomers are generally tougher over a broader temperature range than the urethanes. Also, they are easier and more forgiving in processing. Several grades are produced by Du Pont (Hytrel), Hoechst-Celanese (Riteflex), and Eastman Chemical (Ecdel), ranging in hardness from 35 to 72 Shore D. These materials can be processed by injection molding, extrusion, rotational molding, flow molding, thermoforming, and melt casting. Powders are also available.

Copolyesters, which along with the urethanes, are high-priced elastoplastics, have excellent dynamic properties, high modulus, good elongation and tear strength, and good resistance to flex fatigue at both low and high temperatures. Brittle temperature is below -90°F, and modulus at -40°F is only slightly higher than at room temperature. Heat resistance to 300°F is good.

Resistance of the copolyesters to nonoxidizing acids, some aliphatic hydrocarbons, aromatic fuels, sour gases, alkaline solutions, hydraulic fluids, and hot oils is good to excellent. Thus, they compete with rubbers such as nitriles, epichlorohydrins, and polyacrylates. However, hot polar materials, strong mineral acids and bases, chlorinated solvents, phenols, and cresols degrade the polyesters. Weathering resistance is low but can be improved considerably by compounding UV stabilizers or carbon blacks with the resin.

Copolyester elastomers are not direct substitutes for rubber in existing designs. Rather, such parts must be redesigned to use the higher strength and modulus, and to operate within the elastic limit. Thinner sections can usually be used -- typically one-half to one-sixth that of a rubber part.

Applications of copolyester elastomers include hydraulic hose, fire hose, power-transmission belts, flexible couplings, diaphragms, gears, protective boots, seals, oil-field parts, sports-shoe soles, wire and cable insulation, fiber-optic jacketing, electrical connectors, fasteners, knobs, and bushings.

A copolyester-based thermoplastic elastomer, trademarked Lomod, was introduced by General Electric Plastics in 1985. In addition to general-purpose, flame-retardant and high-heat grades, specific grades have been developed for airdams, fascias, and filler panels with excellent impact resistance down to -40°F and capable of withstanding on-line painting. Lomod thermoplastic elastomers are also used in connectors, wire, cable, hose, tubing, and other applications.

Styrene copolymers: The styrenics are the lowest priced thermoplastic elastomers. They are block copolymers, produced with hard polystyrene segments interconnected with soft segments of a matrix such as polybutadiene, polyisoprene, ethylene-propylene, or ethylene-butylene. These elastomers are available from Shell (Kraton) in several molding and extrusion grades ranging in hardness from 28 to 95 Shore A.

Tensile strength of these materials is lower and elongation is higher than SBR or natural rubber, weather resistance is about the same. Other resistance characteristics can be improved by the addition of resins such as polypropylene or ethylene-vinyl acetate. The styrenic elastoplastics resist water, alcohols, and dilute alkalies and acids. They are soluble in, or are swelled by, strong acids, chlorinated solvents, esters, and ketones. One type has a service temperature limit of 150°F; another grade can be used to 250°F. Both have excellent low-temperature flexibility to -120°F.

Applications for the styrene-butadiene block copolymers include disposable medical products, food packaging, tubing, sheet, belting, mallet heads, and shoe soles. These materials are also used as sealants, hot-melt adhesives, coatings, and for wire and cable insulation.

Olefins: Thermoplastic olefin (TPO) elastomers are available in several grades, having room-temperature hardnesses ranging from 60 Shore A to 60 Shore D. These materials, being based on polyolefins, have the lowest specific gravities of all thermoplastic elastomers. They are uncured or have low levels of crosslinking. Material cost is mid-range among the elastoplastics.

These elastomers remain flexible down to -60°F and are not brittle at -90°F. They are autoclavable and can be used at service temperatures as high as 275°F in air. The TPOs have good resistance to some acids, most bases, many organic materials, butyl alcohol, ethyl acetate, formaldehyde, and nitrobenzene. They are attacked by chlorinated hydrocarbon solvents. Compounds rated V-0 by UL 94 methods are available.

Elastomeric alloys: This class of thermoplastic elastomers consists of mixtures of two or more polymers that have received a proprietary treatment to give them properties significantly superior to those of simple blends of the same constituents. The two types of commercial elastomeric alloys are melt-processible rubbers (MPRs) and thermoplastic vulcanizates (TPVs). MPRs have a single-phase; TPVs have two phases.

Thermoplastic vulcanizates are essentially a fine dispersion of highly vulcanized rubber in a continuous phase of a polyolefin. Critical to the properties of a TPV are the degree of vulcanization of the rubber and the fineness of its dispersion. The crosslinking and fine dispersion of the rubber phase gives a TPV high tensile strength (1,100 to 3,900 psi), high elongation (375 to 600%), resistance to compression and tension set, oil resistance, and resistance to flex fatigue. TPVs have excellent resistance to attack by polar fluids and fair-to-good resistance to hydrocarbon fluids. Maximum service temperature is 275°F.

Elastomeric alloys are available in the 55A to 50D hardness range, with ultimate tensile strengths ranging from 800 to 4,000 psi. Specific gravity of MPRs is 1.2 to 1.3; TPV's range is 0.9 to 1.0.

In 1981, Monsanto Chemical Co. commercialized a line of TPVs, called Santoprene, based on EPDM rubber and polypropylene, designed to compete with thermoset rubbers in the middle performance range. In 1985, the company introduced a second TPV, Geolast, based on polypropylene and nitrile rubber. This TPV alloy was designed to provide greater oil resistance than that of the EPDM-based material. The nitrile-based TPV provides a thermoplastic replacement for thermoset nitrile and neoprene because oil resistance of the materials is comparable.

The MPR product line, called Alcryn, was introduced in 1985 by Du Pont Co. It is a single-phase material, which gives it a stress-strain behavior similar to that of conventional thermoset rubbers. MPRs are plasticized alloys of partially crosslinked ethylene interpolymers and chlorinated polyolefins. These materials have excellent oil and weather resistance. Maximum recommended service temperature is 275°F.

Alcryn is available in black and colorable grades, in hardnesses from 55A to 80A. Unlike other TPEs, it can be processed on rubber equipment as well as on conventional thermoplastic equipment. Several injection-molding grades are now available. Commercial applications of elastomeric alloys include automotive protective boots, hose covering, electrical insulation, seals, gaskets, medical tubing and syringe plungers, architectural glazing seals, and roofing sheet.