Page 209 - Handbook of Plastics Technologies

Source: Handbook of Plastics Technologies

CHAPTER 4

ELASTOMERS

Aubert Y. Coran
Longboat Key, Florida

4.1 INTRODUCTION

The term elastomer is often used interchangeably with the term rubber. Elastomers (or
rubbers) are amorphous polymers. Their normal-use temperatures are above their glass
transition temperatures, so considerable molecular segmental motion is possible. Hard
plastics normally either exist below their glass transition temperatures, or they are semic-
rystalline solids at room temperature.
Elastomers are different from other polymers because of their special properties. Ac-
cording to ASTM (D 1566), a rubber is a material that is capable of recovering from large
deformations quickly and forcibly and can be, or already is, modified (i.e., vulcanized) to a
state in which it is essentially insoluble (but can swell) in boiling solvent. A rubber, in its
vulcanized state, retracts within 1 min to less than 1.5 times its original length after being
stretched at room temperature to twice its length and held for 1 min before release. The
term elastomer is often used designate polymers that have properties similar to those of a
rubber. At ambient temperatures, rubbers are thus soft and deformable.
Elastomers are used in a wide variety of applications because of their unusual physical
properties (flexibility, extensibility, resiliency, and durability), which are unmatched by
other types of materials. Other useful properties include abrasion resistance, resistance to
aqueous and other polar fluids, weathering resistance, and high frictional coefficients for
traction. Elastomeric materials are frequently tailor-made for specific applications, be-
cause they can be significantly modified by compounding.
Vulcanized elastomers are thermosets (having required vulcanization). The long poly-
mer chains are cross-linked during curing (i.e., vulcanization). Vulcanization, which gener-
ally requires some time at an elevated temperature, must take place after the elastomer is in
its final shape or form (e.g., in a heated mold) because, after significant cross-linking, the
polymer cannot flow (e.g., in the mold). Thermoplastic elastomers (TPEs) are elastomeric
without being vulcanized. They are processed in the same way as rigid thermoplastics, e.g.,
polystyrene and polyethylene, without the need for time-consuming vulcanization.
The first known elastomer was natural rubber. (It was vulcanizable.) Precolumbian
peoples of South and Central America used it, however, without vulcanization, to make
balls, containers, and shoes and for waterproofing fabrics. Mentioned by Spanish and Por-
tuguese writers in the 16th century, natural rubber did not attract the interest of Europeans

4.1
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