Page 270 - Handbook of Plastics Technologies

ELASTOMERS

4.62 CHAPTER 4

Emulsion polymerization is done hot (about 50°C) or cold (about 5°C), depending on
the selection of polymerization initiator. SBR prepared by emulsion polymerization
(emulsion SBR or E-SBR) generally contains about 23 percent styrene-derived units, ran-
domly distributed in the molecular polymer chains. SBR made by solution polymerization
(solution SBR or S-SBR) has about the same styrene-derived unit content. Both random
and block polymers can be made by solution polymerization. Both emulsion SBR and so-
lution SBR are available in oil-extended versions (OE-SBR). These have as much as 50
parts of extender oil per 100 parts by weight of polymer (phr).
E-SBR is available in Mooney viscosities (ML 1+4 100°C) ranging from about 30 to
120, corresponding to average molecular weights of about 250,000 to 800,000. It is sup-
plied as dry gum, oil-extended or carbon-black-filled polymer. In some respects, the
lower-viscosity grades are more easily processed, while the higher-viscosity grades have
better green strength, accept higher filler and oil loadings, and tend to give less porous vul-
canizates.
Cold E-SBRs (those produced at the lower temperatures) contain less long-chain
branching than do the so-called hot rubbers. An effect of this is that the cold-process rub-
bers generally can be more easily processed than the hot-process rubbers. SBRs can be
vulcanized by the same types of systems as used for NR. As with NR, accelerated sulfur
curing systems are, by far, the most used.
Properties. The mechanical properties of E-SBR vulcanizates depend on the type and
level of filler in the compound. Unfilled gum vulcanizates have very poor tensile strength
and ultimate elongation, because the rubber lacks self reinforcing of the type found NR
rubber vulcanizates, i.e., strain-induced crystallization. This inadequacy is offset by the
addition of reinforcing fillers, i.e., carbon black or chemically coupled silica. At optimum
loadings with reinforcing carbon black, mechanical properties similar to those of NR can
be achieved. However, NR compounds exceed SBR compounds in tear strength because of
NR’s strain-induced crystallization.
Emulsion SBR vulcanizates have better aging, fatigue, and heat resistance than do
those of NR. Antidegradants, however, are required for this. E-SBR vulcanizates, unlike
those of NR, are reversion resistant. By using reinforcing fillers, one can achieve better
abrasion resistance with E-SBR than with NR. In part, for these reasons, emulsion SBRs
have replaced very much of NR. However, E-SBR vulcanizates are more hysteretic than
those of NR and, thus, heat buildup during heavy duty flexing is a greater problem with E-
SBR than with NR.
E-SBR vulcanizates are resistant to many polar solvents, dilute acids and bases, and so
on. However SBR vulcanizates swell considerably in contact with oils, fats, gasoline, ker-
osene, and others.
Random-distribution solution SBR vulcanizates are less hysteretic than are comparable
vulcanizates of E-SBR. Also, solution polymers contain less nonrubber material. This is
because there is absence of emulsifier (e.g., soap) during polymerization. During coagula-
tion of the polymerized emulsion to obtain the rubber, fatty acids are formed. The pres-
ence of such fatty acid, in part, reduces the rate of vulcanization with respect to that of
solution SBR compounds. The absence of such nonrubber components also reduces the
electrical conductivity of S-SBR compounds compared to those of E-SBR. Vulcanizates of
solution SBRs, having blocky monomer distributions, have very low brittleness tempera-
tures due to the presence of relatively long polybutadiene chain segments. They have good
elastic properties, low water adsorption, low electrical conductivity, and excellent abrasion
resistance.
Oil-extended SBR (OE-SBR) grades contain polymer of very high molecular weight.
This enables the presence of high concentrations of oil with the maintenance of viscosities
similar to those of nonoil-extended SBRs for easy processing.

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