ELASTOMERS-Synthetic rubbers-natural rubber-crude rubberSynthetic rubbers, often referred to as rubbers, are hydrocarbon polymeric materials similar in structure to plastic resins. The difference between plastics and elastomers is largely one of definition based on the property of extensibility, or stretching. The American Society for Testing and Materials defines an elastomer as “a polymeric material which at room temperature can be stretched to at least twice its original length and upon immediate release of the stress will return quickly to approximately its original length.” Some grades of plastics approach this rubberlike state, for example, certain of the polyethylenes. Also, a number of plastics have elastomer grades, such as the olefins, styrenes, fluoroplastics, and silicones. As indicated above, the major distinguishing characteristic of elastomers is their great extensibility and high-energy storing capacity. Unlike many metals, for example, which cannot be strained more than a fraction of 1% without exceeding their elastic limit, elastomers have usable elongations up to several hundred percent. Also, because of their capacity for storing energy, even after they are strained several hundred percent, virtually complete recovery is achieved once the stress is removed.
Up until World War II, almost all rubber was natural. During the war, synthetic rubbers began to replace the scarce natural rubber, and since that time, production of synthetics has increased until now their use far surpasses that of natural rubber. There are thousands of different elastomer compounds. Not only are there many different classes of elastomers, but also individual types can be modified with a variety of additives, fillers, and reinforcements. In addition, curing temperatures, pressures, and processing methods can be varied to produce elastomers tailored to the needs of specific applications.
In the raw-material or crude stage, elastomers are thermoplastic. Thus crude rubber has little resiliency and practically no strength. By a vulcanization process in which sulfur and/or other additives are added to the heated crude rubber, the polymers are cross-linked by means of covalent bonds to one another, producing a thermosetlike material. The amount of cross-linking which occurs between the sulfur (or other additive) and the carbon atoms determines many of the elastomer’s properties. As cross-linking increases, resistance to slippage of the polymers over one another increases, resiliency and extensibility decrease, and the elastomer approaches the nature of a thermosetting plastic. For example, hard rubbers, which have the
highest cross-linking of the elastomers, in many respects are similar to phenolics. In the unstretched state, elastomers are essentially amorphous because the polymers are randomly entangled and there is no special preferred geometric pattern present. However, when stretched, the polymer chains tend to straighten and become aligned, thus increasing in crystallinity. This tendency to crystallize when stretched is related to an elastomer’s strength. Thus, as crystallinity increases, strength also tends to increase.
There are roughly 20 major classes of elastomers; we cannot do much more here than identify them and highlight the major characteristics of each group. Two basic specifications provide a standard nomenclature and classification system for these classes. The ASTM standard D1418 categorizes elastomers into compositional classes. A joint ASTM-SAE specification (ASTM D2000/SAE J200) provides a classification system based on material properties. The first letter indicates specific resistance to heat aging, and the second letter denotes resistance to swelling in oil.
Styrene-butadiene elastomers, sometimes also called Buna S, SBR, and GR-S, are copolymers of butadiene and styrene. They are similar in many ways to the natural rubbers, and were the first widely used synthetics. They top all elastomers in volume of use, chiefly because of their low cost and use in auto tires. A wide range of property grades are produced by varying the relative amounts of styrene and butadiene. For example, styrene content varies from as low as 9% in low-temperature resistant rubbers to 44% in rubbers with excellent flow characteristics. Those grades with styrene content above 50% are by definition considered plastics. Carbon black is sometimes added also as it substantially improves processing and abrasion resistance. SBR elastomers are similar in properties to natural rubber. They are non-oil-resistant and are generally poor in chemical resistance. Although they have excellent impact and abrasion resistance, they are somewhat below natural rubber in tensile strength, resilience, hysteresis, and some other mechanical properties. The largest single use is in tires. Other applications are similar to those of natural rubber. Styrene-butadiene latex, typically a 70% SBR emulsion in water, is used mainly for coatings and adhesives. Carbon tetra-chloride, CCl4, long used as a weight modifier in some of the latex, is being phased out for environmental reasons. Japan Synthetic Rubber, that country’s largest producer of latex, has developed a non-halogen hydrocarbon-based substitute for all latex grades formerly using CCl4. New solution polymers of Goodyear Chemical Co., based on styrene-isoprene-butadiene rubber, improve wet traction of tires over standard emulsion SBR. This is also true of Li polymers, so called because of the lithium-based catalyst used during polymerization in hexane or other organic solvent.
Neoprene, also known as chloroprene, was developed in the 1930s, and it has the distinction of being the first commercial synthetic rubber. It is chemically and structurally similar to natural rubber, and its mechanical properties are also similar. Its resistance to oils, chemicals, sunlight, weathering, aging, and ozone is outstanding. Also, it retains its properties at temperatures up to 250°F (121°C), and it is one of the few elastomers that does not support combustion, although it is consumed by fire. In addition, it has excellent resistance to permeability by gases, having about one-fourth to one-tenth the permeability of natural rubber, depending on the gas. Although it is slightly inferior to natural rubber in most mechanical properties, neoprene has superior resistance to compression set, particularly at elevated temperatures. It can be used for low-voltage insulation, but is relatively low in dielectric strength. Typical products made of chloro-prene elastomers are heavy-duty conveyor belts, V belts, hose covers, footwear, brake diaphragms, motor mounts, rolls, and gaskets. Butyl rubbers, also referred to as isobutylene-isoprene elastomers, are copolymers of isobutylene and about 1 to 3% isoprene. They are similar in many ways to natural rubber and are one of the lowest-priced synthetics. They have excellent resistance to abrasion, tearing, and flexing. They are noted for low gas and air permeability (about 10 times better than natural rubber), and for this reason they make a good material for tire inner tubes, hose, tubing, and diaphragms. Although butyls are non-oil-resistant, they have excellent resistance to sunlight and weathering and generally have good chemical resistance. They also have good low-temperature flexibility and heat resistance up to around 300°F (149°C); however, they are not flame-resistant. They generally have lower mechanical properties, such as tensile strength, resilience, abrasion resistance, and compression set, than the other elastomers. Because of their excellent dielectric strength, they are widely used for cable insulation, encapsulating compounds, and a variety of electrical applications. Other typical uses include weather stripping, coated fabrics, curtain wall gaskets, high-pressure steam hoses, machinery mounts, and seals for food jars and medicine bottles.
Isoprene is synthetic natural rubber. It is processed as natural rubber, and its properties are quite similar, although isoprene has somewhat higher extensibility. Like natural rubber, its notable characteristics are very low hysteresis, low heat buildup, and high tear resistance. It also has excellent flow characteristics and is easily injection-molded. Its uses complement those of natural rubber. And its good electrical properties plus low moisture absorption make it suitable for electrical insulation. Polyacrylate elastomers are based on polymers of butyl or ethyl acrylate. They are low-volume-use, specialty elastomers,
chiefly used in parts involving oils (especially sulfur-bearing) at elevated temperatures up to 300°F (149°C) and even as high as 400°F (204°C). A major use is for automobile transmission seals. Other oil-resistant uses are gaskets and O rings. Mechanical properties such as tensile strength and resilience are low. And, except for recent new formulations, they lose much of their flexibility below — 10°F (—23°C). The new grades extend low-temperature service to —40°F ( — 40°C). Polyacrylates have only fair dielectric strength, which improves, however, at elevated temperatures.
Nitrile elastomers, or NBR rubbers, known originally as Buna N, are copolymers of acrylonitrile and butadiene. They are principally known for their outstanding resistance to oil and fuels at both normal and elevated temperatures. Their properties can be altered by varying the ratio of the two monomers. In general, as the acrylonitrile content increases, oil resistance, tensile strength, and processability improve while resilience, compression set, low-temperature flexibility, and hysteresis characteristics deteriorate. Most commercial grades range from 20 to 50% acrylonitrile. Those at the high end of the range are used where maximum resistance to fuels and oils is required, such as in oil-well parts and fuel hose. Low-acrylonitrile grades are used where good flexibility at low temperatures is of primary importance. Medium-range types, which are the most widely used, find applications between these extremes. Typical products are flexible couplings, printing blankets, rubber rollers, and washing-machine parts. Nitriles as a group are low in most mechanical properties. Because they do not crystallize appreciably when stretched, their tensile strength is low, and resilience is roughly one-third to one-half that of natural rubber. Depending on acrylonitrile content, low-temperature brittleness occurs at from —15 to —75°F (—26 to —60°C). Their electrical insulation quality varies from fair to poor.
Hydrogenated nitrile rubber, of Bayer AG of Germany, has good heat stability, abrasion resistance, and dynamic-load capacity. It is used for synchronous belts in auto applications. Zeptol elastomers, of Zeon Chemicals, Inc., are hydrogenated nitrile-butadiene rubbers for service temperatures of 30 to 302°F (—1 to 150°C). They have good tensile strength and resistance to lubricants. Phosphonitrile elastomers have high elasticity and high-temperature resistance. They are derived from chlorophosphonitrile, or phosphonitrilic chloride, PNCl,
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which has a hexagonal ring of alternating atoms of phosphorus and nitrogen with chlorine atoms attached. (In phosphonitrile plastics, the chlorine atoms are replaced by other groups.)
Polybutadiene elastomers are notable for their low-temperature performance. With the exception of silicone, they have the lowest brittle
ELASTOMERS
or glass transition temperature, — 100°F ( — 73°C), of all the elastomers. They are also one of the most resilient, and have excellent abrasion resistance. However, resistance to chemicals, sunlight, weathering, and permeability by gases is poor. Some uses are shoe heels, soles, gaskets, and belting. They are also often used in blends with other rubbers to provide improvements in resilience, abrasion resistance, and low-temperature flexibility.
Polysulfide elastomer is rated highest in resistance to oil and gasoline. It also has excellent solvent resistance, extremely low gas permeability, and good aging characteristics. Thus, it is used for such products as oil and gasoline hoses, gaskets, washers, and diaphragms. Its major use is for equipment and parts in the coating production and application field. It is also widely applied in liquid form in sealants for the aircraft and marine industries. Its mechanical properties, including strength, compression set, and resilience, are poor. Although it is poor in flame resistance, it can be used in temperatures up to 250°F (121°C). Ethylene-propylene elastomers, or EPR rubber, are available as copolymers and terpolymers. They offer good resilience, flexing characteristics, compression-set resistance, and hysteresis resistance, along with excellent resistance to weathering, oxidation, and sunlight. Although they are fair to poor in oil resistance, their resistance to chemicals is good. Their maximum continuous service temperature is around 350°F (177°C). Typical applications are electrical insulation, footwear, auto hose, and belts. The terpolymer, ethylene propylene diene monomer (EPDM), has recently been produced with metallocene and other single-site catalysts used in polyethylene and polypropylene production.
Urethane elastomers are copolymers of diisocyanate with polyester or polyether. Both are produced in solid gum form and viscous liquid. With tensile strengths above 5,000 lb/in2 (34 MPa) and some grades approaching 7,000 lb/in2 (48 MPa), urethanes are the strongest available elastomers. They are also the hardest, and have extremely good abrasion resistance. Other notable properties are low compression set, and good aging characteristics and oil and fuel resistance. The maximum temperature for continuous use is under 200°F (93°C), and their brittle point ranges from —60 to —90°F ( — 51 to —68°C). Their largest field of application is for parts requiring high wear resistance and/or strength. Typical products are forklift truck wheels, airplane tail wheels, shoe heels, bumpers on earth-moving machinery, typewriter damping pads, seals and flexible linings for sewage-treatment and chemical-storage tanks. For seal applications, maximum recommended deflection limits are 50% for Shore A hardness 40 to 60, 30% for 70, 20% for 80, 10% for 95, and 5% for Shore D 70.
Sorbothane is a polyether urethane from Sorbothane Inc., in Shore 00 hardness 25 to 80. Intended for shock, vibration, and noise control, it is used for stud mounts, grommets, bushings, pads, and other isolation-damping products. At Shore 50, the specific gravity is 1.3, density 0.047 lb/in3 (1300 kg/m3), tensile strength 125 lb/in2 (0.86 MPa), tear strength 23.5 lb/in2 (0.16 MPa), and compression set 6.2%. Master Bond Inc.’s UV15X-5 urethane is a one-component, nonyellowing, ultraviolet-curing, transparent elastomer of excellent flexibility and abrasion resistance for bonding to metals, plastics, elastomers, and glass and for sealing, coating, or casting. Totally reactive, it emits no volatiles on curing and has a service temperature range of —80 to 250°F ( — 62 to 121°C). Shore D hardness exceeds 30 and the tensile strength is 1800 lb/in2 (12.4 MPa). Isoloss is a series of specially formulated urethanes, of E-A-R Specialty Composites Div of Cabot Safety Corp., for damping noise, vibration, and shock.
Polyethylene elastomers are rubberlike materials made by cross-linking with chlorine and sulfur, or they are ethylene copolymers. Chlorosulfonated polyethylene elastomer, commonly known as Hypalon, contains about one-third chlorine and 1 to 2% sulfur. It can be used by itself or blended with other elastomers. It is noted for its excellent resistance to oxidation, sunlight, weathering, ozone, and many chemicals. Some grades are satisfactory for continuous service at temperatures up to 350°F (177°C). It has moderate oil resistance. It also has unlimited colorability Its mechanical properties are good but not outstanding, although abrasion resistance is excellent. Hypalon is frequently used in blends to improve oxidation and ozone resistance. Typical uses are tank linings, high-temperature conveyor belts, shoe soles and heels, seals, gaskets, and spark plug boots.
Ethylene-propylene elastomer, produced by various companies, is a chemically resistant rubber of high tear strength. Ethylene butadiene can be vulcanized with sulfur to give high hardness and wide temperature range. For greater elongation a terpolymer with butene can be made. Epichlorohydrin elastomers are noted for their good resistance to oils and excellent resistance to ozone, weathering, and intermediate heat. The homopolymer has extremely low permeability to gases. The copolymer has excellent resilience at low temperatures. Both have low heat buildup, making them attractive for parts subjected to repeated shocks and vibrations.
Fluorocarbon elastomers, like their plastic counterparts, excel in resistance to oxidation, chemicals, oils, solvents, and heat. They are also quite costly. Many have continuous-use service temperatures as high as 400°F (204°C), some can withstand higher temperatures, and they will not support combustion. Most are brittle, however, at — 10°F (—23°C), and their mechanical properties are only moderate. Viton, of Du Pont Dow, comes in families designated A, B, F, and ETP. Viton A’s are dipoly-mers of vinylidene fluoride (VF2) and hexafluoropropylene (HFP). The B’s and F’s are terpolymers of VF2, HFP, and tetraflurorethylene (TFE). ETPs are peroxide-cured terpolymers of ethylene, TFE, and perfluo-romethylvinylether (PMVE) with a small amount of cure-site monomer to permit peroxide cross-linking. Resistance to fluids generally rises with increasing fluorine content—66% in Type A, 67 in ETP, 68 in B, and 70 in F—but low-temperature flexibility tends to decline with that increase.
3M’s Fluorel elastomers, with 65 to 71% fluorine, are either dipoly-mers of VF2 and HFP or terpolymers of VF2, HFP, and TFE. Depending on the grade, their density is 0.065 to 0.069 lb/in3 (1799 to 1910 kg/m3), tensile strength 1,460 to 2,560 lb/in2 (10 to 18 MPa), elongation 180 to 330%, Shore A hardness 70 to 84, compression set for 70 h at 392°F (200°C) 9 to 45%, and the continuous-use temperature 0 to 392°F (—17.8 to 200°C). The properties of Fluorel II, a VF2-TFE-propylene elastomer, are within these ranges except for density [0.058 lb/in3 (1605 kg/m3)]. Aflas, a 3M dipolymer of TFE and propylene with 57% fluorine, has a density of 0.056 lb/in3 (1550 kg/m3) and a service temperature range of 35 to 392°F (2 to 200°C). Depending on the grade, tensile strength is 1,690 to 2,440 lb/in2 (11.7 to 17 MPa), elongation 220 to 325%, Shore A hardness 72 to 73, and compression set for 70 h at 392°F (200°C) of 42 to 50%. Except for its higher low-temperature service temperature and the greater acid resistance of peroxide-cured Fluorel, the overall environmental resistance of Alfas is superior to that of Fluorel and Fluorel II. A phosphonitrilic fluorocarbon elastomer, developed by Firestone Tire and Rubber Co., retains flexibility at temperatures as low as — 70°F (—57°C), sustains temperatures as high as 350°F (177°C), and is especially resistant to oils and solvents over this temperature range. Kalrez perfluorocarbonelas-tomer, of Du Pont Dow, has the highest continuous-use service temperature of any fluorocarbon elastomer: 550°F (288°C). This most costly of elastomers can withstand short-term temperatures to 650°F (343°C) and is also resistant to a variety of solvents, bases, and fuels.
Chlorinated polyethylene elastomers are produced by substitution of chlorine for hydrogen on a high-density polyethylene chain, resulting in a fully saturated structure with no double or triple bonds. The elastomer requires the catalytic reaction of a peroxide for curing. Thus, most molded parts are black. Five grades of CPE polymers are produced, differing principally in chlorine content. The higher-chlorine-content grades have best oil and fuel resistance, tear resistance, gas impermeability, and hardness. Those with lower chlorine content have lower viscosities, better low-temperature properties, and improved resistance to heat and compression set.
Silicone elastomers are polymers composed basically of silicone and oxygen atoms. There are four major elastomer composition groups. In terms of application, silicone elastomers can be divided roughly into the following types: general-purpose, low-temperature, high-temperture, low-compression-set, high-tensile–high-tear, fluid-resistant, and room-temperature vulcanizing. All silicone elastomers are high-performance, high-price materials. The general-purpose grades, however, are competitive with some of the other specialty rubbers and are less costly than the fluorocarbon elastomers. Silicon elastomers are the most stable group of all the elastomers. They are outstanding in resistance to high and low temperatures, oils, and chemicals. High-temperature grades have maximum continuous service temperatures up to 600°F (316°C); low-temperature grades have glass transition temperatures of — 180°F (—118°C). Electrical properties, which are comparable to the best of the other elastomers, are maintained over a temperature range from — 100°F (—73°C) to over 500°F (260°C). However, most grades have relatively poor mechanical properties. Tensile strength runs only around 1,200 lb/in2 (8 MPa). However, grades have been developed with much improved strength, tear resistance, and compression set. Liquid silicone elastomers are more costly than conventional solid silicones, especially in terms of mold cost, because of the greater precision required. Production cost may be less, however, because of much faster cure time. Molding, at temperatures of 250 to 400°F (121 to 204°C), is performed in injection-molding machines similar to those for injection-molding plastics. Applications include gaskets integrally molded onto their respective plastic or metal component, spark plug covers, bellows, and various seals. Fluorosilicone elastomers have been developed which combine the outstanding characteristics of the fluo-rocarbons and silicones. However, they are expensive and require special precautions during processing. A unique characteristic of one of these elastomers is its relatively uniform modulus of elasticity over a wide temperature range and under a variety of conditions. Silicone elastomers are used extensively in products and components where high performance is required. Typical uses are seals, gaskets, O rings, insulation for wire and cable, and encapsulation of electronic components.
ELECTRICAL-CONTACT MATERIALS. These are materials used to make and break electrical contact, thus make-and-break electric circuits, or to provide sliding or constant contact. Both require high electrical conductivity to ease current flow, high thermal conductivity to dissipate heat, high melting point or range to inhibit arc erosion and prevent sticking,corrosion and oxidation resistance to prevent formation of films that impede current flow, high hardness for wear resistance, and amenability to welding, brazing, or other means of joining. The sliding-contact types also require low friction, and a lubricant is always required between the sliding materials to prevent seizing and galling.
The materials used range from pure metals and alloys to composites, including those made by powder-metallurgy methods. Copper is widely used but requires protection from oxidation and corrosion, such as by immersion in oil, coating, or vacuum sealing. Copper-tungsten alloys or mixtures of copper-graphite increase resistance to arcing and sticking, and some copper alloys provide greater hardness, thus greater wear resistance, and better spring characteristics. Silver is more oxidation-resistant in air and, pure or alloyed, is the most widely used metal for make-and-break contacts for application at currents to 600 A. Silver-copper alloys provide greater hardness but less conductivity and oxidation resistance; silver-cadmium alloys increase resistance to arc erosion and sticking; and silver-platinum alloys, silver-palladium alloys, and silver-gold alloys increase hardness, wear resistance, and oxidation resistance. All alloying elements, however, decrease conductivity. Gold has outstanding oxidation and sulfidation resistance but, being soft and prone to wear and arc erosion, is limited to low-current (0.5-A maximum) applications. To enhance these properties, gold alloys, such as gold-silver, gold-copper, gold-silver-platinum, gold-silver-nickel, and gold-copper-platinum-silver, are more commonly used. Platinum and palladium are also used for contacts but, again, in alloy form more than as pure metals. Among the most common ones are platinum-iridium, platinu ruthenium, platinum palladium-ruthenium, palladium-ruthenium, palladium-copper, and palladium-silver. A palladium-silver-platinum-gold alloy for brushes and sliding contacts is noted for its exceptional modulus of elasticity and high proportional limit. Aluminum, tungsten, and molybdenum are also used for electrical contacts but mainly in composite form. Aluminum used for contacts provides an electrical conductivity of about 60% that of copper, but is prone to oxidation and thus clad or plated with silver, tin, or copper. The refractory metals, though providing excellent resistance to wear and arc erosion, are poor conductors and oxidize readily.
The principal metals made in composite form by PM methods are the refractory metals, including those in carbide form, and copper-base and silver-base metals. The refractory metals, notably tungsten and molybdenum or their carbides, usually serve as a base for infiltrating with copper or silver, thus combining electrical conductivity and resistance to wear and arc erosion in a single material. Many such composites are common, including tungsten-copper, tungsten-silver, tungsten carbide-silver, tungsten carbide-copper, tungsten-graphite-silver, and molybdenum-silver. The
amount of conductive metal may exceed or be less than that of the refractory metal or refractory-metal carbide. A common silver composite is silver-cadmium oxide, which, for a given amount of silver, provides greater conductivity than a silver-cadmium alloy as well as greater hardness and resistance to sticking. Others include silver-graphite, silver-nickel, and silver-iron. The silver-graphite composites are used mainly for sliding or brush contacts.
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