Page 274 - Handbook of Plastics Technologies

ELASTOMERS

4.66 CHAPTER 4

The high-viscosity (high-molecular-weight) EPDMs are generally sold with added ex-
tender oil. The amount of oil can be 100, 200, or more parts per 100 parts of rubber (phr).
Properties of Ethylene-Propylene Rubbers. Elastic properties of EPR or EPDM are
better than those of many synthetic rubbers, but hysteresis is not as good (low) as in the
case of NR and BR. Resistance to compression set for EPR and EPDM vulcanizates is ex-
cellent. For EPDM, this is especially true in grades containing larger amounts of ter-
monomer units derived from ENB. Fatigue resistance for EPDM vulcanizates is very
good, comparable to that of SBR vulcanizates. The resistance to heat and aging of EP rub-
ber vulcanizates is better that for NR SBR and NBR vulcanizates. Peroxide vulcanizates
are notably resistant to oxidative heat aging. Also, these largely saturated polymers are
very resistant to attack by ozone. Low-temperature flexibility and oil swelling properties
of these elastomers are similar to those of NR.
Electrical properties (insulating, dielectric breakdown, corona resistance, and so on) of
ethylene-propylene rubbers are excellent. This is especially true for peroxide-cured EPR.
Uses of Ethylene-Propylene Rubbers. EPDM and EPR vulcanizates are used in ex-
truded profiles, cable insulation and jacketing, and roofing membranes. There are many
automotive uses: radiator hose, door and trunk seals, insulation, jacketing, and others.
These elastomers are also used in applications such as window and architectural profiles,
dock fenders, and washing-machine hoses. In short, their applications are extensive and
diverse. Ethylene-propylene rubbers may be the most versatile of general-purpose rubbers.
In addition, EP rubbers are added to polyolefin plastics as impact modifiers and as compo-
nents of certain thermoplastic elastomer compositions (e.g., thermoplastic vulcanizates,
which are discussed later in this chapter).
4.5.3.7 Chloroprene or Neoprene Rubber (CR). Chloroprene rubber (poly-2-chlorob-
utadiene) is produced by the emulsion polymerization of 2-chlorobutadiene, in the pres-
ence of a free-radical initiator. Its general chemical structure can be represented as
follows:

There are a number of commercially available grades of CR. They differ, for exam-
ple, with respect to processability, mercaptan modification, polymerization temperature
(effects tendency to crystallize), viscosity (processability), stabilizer, copolymerization
with other monomers (crystallizability), gel content (effects processing), reactive
groups (important for lattices), and so on. With increasing polymerization temperatures,
there is less uniformity in chain structure due to incorporation of 1,2- and 3,4-structural
units and different isomers in the monomer sequences. This reduces the rate of crystalli-
zation of the resulting polymers. If the structure is too regular, too much crystallization
occurs for the production of rubber products, because they tend to harden very rapidly
with loss of elasticity. Nevertheless, the extensively crystallizable CRs are useful in ad-
hesive compositions. For general-purpose rubber applications, there are three grades: G,
W, and T, with selected features to offer a range of processing, curing, and performance
properties.
CR compounds are vulcanized by metal oxides, i.e., ZnO. MgO is also present in the
compound to somewhat retard the action of ZnO. Thioureas are used as accelerators. For
low-water-absorption vulcanizates, PbO or Pb O can be used as the vulcanizing agent.
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