Page 218 - Handbook of Plastics Technologies

ELASTOMERS

4.10 CHAPTER 4

stress. This rapid recovery from a deformation is responsible for the bouncing of a rubber
ball. The opposite of resilience is damping or hysteresis, i.e., tan δ. It is a viscous effect
rather than an elastic effect. The presence of cross-links reduces the viscous effect while it
increases elasticity of these viscoelastic materials.
The presence of cross-links also helps in maintaining a desired geometry. Engineering
thermoplastics, which contain no cross-links, are subject to creep, or cold flow, under load.
They also have poor compression set resistance. Set is a permanent deformation that oc-
curs under a load. Cross-linked elastomers can vary significantly in their set resistance.
The choice of polymer, vulcanization system, and degree of cross-linking can profoundly
affect set resistance (as well as many of the other properties of the vulcanizates).

4.3 TYPES OF ELASTOMERS

Until relatively recently, all elastomers were vulcanized. However, thermoplastic elas-
tomers were first introduced in 1954 with the introduction of urethane thermoplastic elas-
tomers. Thus, the two major types of elastomers are vulcanizable (conventional) and
thermoplastic elastomers. The conventional elastomers are frequently broadly classified as
natural and synthetic rubbers.

4.3.1 Conventional Vulcanizable Elastomers
A large number of chemically different conventional, vulcanizable elastomers exist.
ASTM D 1418 describes many of these. Table 4.2 lists some elastomers and their proper-
ties.

• Natural rubber (NR) is still used in many applications. It is not one uniform product; it
varies with the nature of the plant producing the sap, the weather, the locale, the care in
producing the elastomer, and many other factors.
• Isoprene rubber (IR) is similar in chemistry to NR, but it is produced synthetically. It is
a more consistent product than NR and contains no proteins that can give allergic reac-
tions (e.g., with surgical and examination gloves).
• Butadiene elastomers (BRs) are usually blended with SBR or NR in tire stocks.
• Epichlorohydrin elastomers (CO, ECO) are flame-retardant because of the presence of
chlorine. Their electrical properties are modest, but they age well and resist many chem-
ical environments.
• Neoprene (CR) (chloroprene) was the first synthetic elastomer and is widely used in in-
dustry. It is nonflammable and has some resistance to oils.
• Chlorosulfonated polyethylene (CSM) is similar to CR, with some improvement in elec-
trical properties and better heat resistance.
• Ethylene-propylene terpolymer (EPDM) is synthesized from ethylene, propylene, and a
third monomer, a nonconjugated diene. The diene permits conventional sulfur vulcani-
zation. The elastomer is exceptionally resistant to radiation and heat. The glass transi-
tion temperature is –60°C, and electrical properties are good.
• Ethylene-propylene copolymer (EPM or EPR), which was often used in wire insulation,
has been largely replaced by EPDM.
• Butyl rubber (IIR) is highly impermeable to air and water vapor. Butyl rubber has good
aging characteristics and good flexibility at low temperatures.

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