Page 271 - Handbook of Plastics Technologies

ELASTOMERS

ELASTOMERS 4.63

Uses. Emulsion SBRs are generally used in combination with BR in the production of
car and light truck tires. Other applications include belting, shoe soles, hose, molded rub-
ber goods, cable insulation, roll coverings, and so forth. Uses of various grades of E SBRs
are shown in Table 4.12.
Random solution SBR is used in blends with emulsion SBR to improve processability.
Blocky S-SBR is used in hard shoe soles, roll coverings, and special mechanical goods.
SBRs compete with NR, IR, and BR in many applications and as components in
blends.

4.5.3.5 Butyl, Chlorobutyl, and Bromobutyl Rubbers (IIR, CIIR, and BIIR)
Butyl Rubber (IIR). This polymer is produced by cationic copolymerization of
isobutylene mixed with minor amounts of isoprene. Its molecules are 97 to 99.5 mole per-
cent derived from isobutylene, the rest being derived from isoprene. Its structure can be
represented as follows:

It is prepared by cationic copolymerization of isobutylene with minor amount of iso-
prene generally in the solvent methylene chloride with aluminum chloride as a catalyst or
in hexane with a dialkyl aluminum chloride as catalyst at very low temperatures (–100 to
–50°C). Average molecular weights of IIR are generally between 300,000 and 500,000.
Properties of Butyl Rubber. Because it is a largely saturated polymer, it has good re-
sistance against oxidation and ozonolysis. It also has a very low gas permeability. How-
ever, because of its low amount of unsaturation, butyl rubber is rather slow to vulcanize. It
can be cured by sulfur, generally in the presence of fast accelerators (e.g., dithiocarbam-
ates). It can be cured by quinone dioxime in the presence of the oxidant PbO . Butyl rub-
2
ber is also vulcanized by using the resin curing system (phenol/formaldehyde resoles, e.g.,
the product SP1045) in the presence of a Lewis acid activator (e.g, SnCl ), added or
4
formed in situ, e.g. by the action of ZnO with a source of HCl, e.g., a halogenated polymer
of a resole containing halomethylene groups (e.g., the product SP 1056). The heat resis-
tance and resistance to weathering of IIR vulcanizates are excellent.
Uses of Butyl Rubber. Butyl rubber is used in the manufacture of inner liners of
tubeless tires, inner tubes, cable insulation, pharmaceutical stoppers, curing bags, and
bladders for tire manufacture. When tires are in the molds for vulcanization, the inside of
the tire is filled with a butyl rubber bag or bladder of steam under enough pressure to ob-
tain the vulcanization temperature. This is possible only because of the good resistance of
butyl rubber to heat and water.
Chlorobutyl Rubber and Bromobutyl Rubbers (CIIR and BIIR). The addition of
chlorine or bromine to IIR in an inert solvent (e.g., hexane) gives the facile attachment of
one halogen atom per isoprene unit in the allylic position. Compared with IIR, the haloge-
nated butyl rubbers have certain advantages. The cure reactivity is increased to give faster
vulcanization rates, greater extents of vulcanization, and reduced reversion. Also, the halo-
genation improves the compatibility of the isoprene polymer with other types of rubber
(e.g., NR) to make useful rubber-blend compositions possible.

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