Page 273 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.65

Properties of Halobutyl Rubbers. Properties of vulcanizates are enhanced over
those of IIR. BIIR vulcanizates have lower gas permeability, offer better ozone and
weather resistance, and are faster curing than those of CIIR or IIR. The properties of CIIR
are between those of BIIR and IIR.
Uses of Chlorobutyl and Bromobutyl Rubbers. CIIR and BIIR are used in inner
liners of tubeless tires with improved (over IIR) covulcanization (in blends) and adhesion
to other components of the tires, in inner tubes for heavy-duty applications such as in truck
and bus tires, and in belts, hoses, seals, injection molded parts, and pharmaceutical stop-
pers.
4.5.3.6 Ethylene-Propylene Rubbers (EPR and EPDM). Polyethylene, though it is a
semicrystalline solid, has a very low glass transition temperature (about – 80°C). Polypro-
pylene is also a semicrystalline solid with at glass transition temperature of about –10°C.
Copolymers of similar amounts of ethylene and propylene are noncrystalline (or only
slightly crystalline) and have glass transition temperatures of roughly the weight average
between that of polyethylene and polypropylene (i.e., between –40 and –60°C). They are
rubbery materials. The chemical structure of ethylene-propylene rubber can be expressed
as follows:

Most EPR rubbers contain 40 to 80 percent by weight of monomer repeat units derived
from ethylene. The grades containing the higher concentrations of ethylene-derived mono-
mer units generally contain some crystallinity.
Since EPR rubber molecules do not contain unsaturation, they can be vulcanized only
by organic peroxide curing systems. If a third monomer is added during the polymeriza-
tion, i.e., a diene monomer (wherein only one of the two double bonds takes part in the po-
lymerization), unsaturation can be introduced into the molecule, and it can then be
vulcanized by accelerated sulfur curing systems. A chemical structure for ethylene-propy-
lene-diene-monomer (EPDM) rubbers can be expressed as follows:

The diene monomers are nonconjugated, and the double bonds are thus located in side
groups of the polymer chains. Common third monomers are dycyclopentadiene (DCP),
ethylidene norbornene (ENB), and trans-1,4-hexadiene (HX). EPDM rubbers are gener-
ally vulcanized by accelerated-sulfur systems. The amount of ter-monomer can range be-
tween about 2 and 10 percent. Higher concentrations of unsaturation give faster
vulcanization and can give higher cross-link densities. The grades derived from HX vul-
canize less rapidly than do the others.
There are many types of commercially available EPDM rubbers. They differ with re-
spect to ethylene/propylene monomer ratio, amount and selection of ter-monomer (uncon-
jugated diene), molecular weight, molecular-weight distribution, viscosity, amount and
type of extender oil (if present), processability, and other qualities.

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