Page 211 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.3

In this chapter, we discuss both vulcanizable elastomers and thermoplastic elastomers.
There is another, albeit narrow-niche type (i.e., liquid rubbers), which we will not discuss
here. Our focus will be on the various types of elastomers, their properties, applications,
and processing.

4.2 DIFFERENCES AND SIMILARITIES BETWEEN
ELASTOMERS AND PLASTICS

Elastomers and plastics are generally considered to be significantly different types of ma-
terials. Mechanical and functional requirements of parts made from these materials are
quite different. They cannot be readily used interchangeably. Also, methods for the fabri-
cation of plastics and conventional elastomers into usable parts are generally very differ-
ent. The preponderant (vulcanizable) rubbers and elastomers are thermoset materials,
requiring heat for the vulcanization process, which develops a molecular network of cross-
links between polymer backbone chains. The predominant plastics (the thermoplastic
types) require heat only for flow in processing. The language and thinking of the elastomer
and plastic industries are quite different. In addition, plastic materials are frequently used
“out of the bag” or with relatively few ingredients added (or compounded) by the fabrica-
tor of the end-use parts. On the other hand, elastomer materials technology, the technology
of making rubber parts, requires extensive compounding. A typical recipe contains elasto-
meric polymer, filler, antidegradants, plasticizer, processing or flow agent, vulcanizing
agent, vulcanization accelerator, vulcanization activators, and so on. In addition to the se-
lection of the polymer (or blend of polymers), the properties of the vulcanized rubber de-
pend very strongly on the specific compound formulation. Complicated rubber recipes
existed long before plastics were developed, having been first devised in the mid 1800s.
Frequently, one prejudicially thinks that plastics engineering is modern technology, but
that and rubber-product development is a black art.
Rigid plastics are used because they can support stress without major strain or defor-
mation, whereas soft elastomeric materials are used because they can be easily deformed
by the application of a small amounts of stress. These differences are illustrated by the
stress-strain curves of Fig. 4.2.
Nevertheless, many elastomers and plastics are fundamentally very similar. Most plas-
tics and elastomers comprise long chains of one or more types of linked monomer units. In
fact, many of the same monomers are found in both thermoplastic and elastomeric poly-
mers—e.g., styrene, acrylonitrile, ethylene, propylene, and acrylate esters. Because of the
chemical similarities between elastomers and plastics, these materials are susceptible to
many of the same types of chemical attack. Therefore, many of the same material-selec-
tion principles come into play for both plastics and elastomers.
Unvulcanized conventional elastomers comprise linear, long-chain molecules, exactly
as do thermoplastic plastics. However, an engineering thermoplastic is a rigid solid at
room temperature, whereas an uncured elastomer is a soft, pliable gum. A rigid thermo-
plastic has a high elastic modulus at room temperature, whereas an elastomer has a low
modulus at room temperature. These differences between the moduli of elastomers and
plastics are due to differences in mobility of long molecular segments. Rubber molecules,
at room temperature, are far more mobile than those of rigid plastics. Segmental molecular
mobility depends on the ability of main-chain atoms to rotate with respect to one another
at single chemical bonds in the molecular main chains. Since the angle between adjacent
bonds is less than 180° (see Fig. 4.3, wherein poly-cis-1,4-butadiene is used as an exam-
ple), rotations between linked atoms enable bending of the chain and thus the motion of

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