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.