Styrene-Butadiene Rubber Adhesives: Midgley
Styrene-Butadiene Rubber Adhesives: Midgley
Styrene-Butadiene Rubber Adhesives: Midgley
and abrasion resistance, although the short sty- vent system of an appropriate polarity or by
rene blocks characteristic of these polymers ad- complexing the catalyst with ether.
versely affected resilience in tire applications. A random solution SBR will have almost
equal amounts of cis and trans configuration,
Manufacture of SBR but this can be varied. For example, a change
in catalyst composition can increase the trans
The process of manufacturing styrene-buta-
content to 70 %.
diene rubber consists of three distinct steps:
The double bond in butadiene-derived mac-
polymerization, monomer recovery, and
romolecules can be subject to attack by the free
finishing. The basic characteristics of the poly-
radicals in the emulsion polymerization reac-
mer, which are outlined below, are determined
tion. Growing chain radicals can propagate
in the polymerization step, and the product
through the double bond of an already formed
form, whether latex or dry rubber, oil extended
chain so that two chains become joined or
or not, is a function of the finishing process.
crosslinked. If this process is allowed to con-
tinue, more and more chains become linked to-
Basic Chemistry of SBR
gether until all molecules become linked to all
SBRs are produced by addition copolymeriza- other molecules and the molecular weight ap-
tion of styrene and butadiene monomers in proaches infinity. Such structures are called
either an emulsion or a solution polymerization gels. The amount of crosslinking and the gel
process. The ratio of styrene to butadiene con- content of the polymer can be controlled in
trols the glass transition temperature (Tg) of the emulsion polymers by the polymerization tem-
copolymer and thus the stiffness of the elasto- perature, the extent of conversion of monomer
mer. The higher the styrene content, the higher to polymer, and the use of chain transfer agents
the Tg and stiffness. Polymers can be made over (so-called modifiers). High reaction tempera-
the whole range of Tg from about -80°C for tures enhance the crosslinking reaction.
polybutadiene to about + lOO°C for polysty- Initiation of the solution SBR reaction is very
rene. rapid in comparison with chain propagation,
Because butadiene has two double bonds, two and because there is little termination or chain
modes of addition can take place: 1,2 and 1,4. transfer, a very narrow molecular weight dis-
The 1,2 addition yields a pendent vinyl group tribution may be produced. Broadening can be
on the growing chain, which increases Tg • The achieved by use of polar additives and contin-
1,4 addition can occur with the polymer chains uous rather than batch polymerization pro-
cis or trans to the residual in-chain double cesses. The molecular weight can be influenced
bond. The more regular trans configuration by the monomer-to-catalyst ratio. Branching
leads to better cured tensile properties. and crosslinking can be encouraged by the ad-
The microstructure of the elastomers can be dition of a third monomer or branching agent,
controlled by the manufacturing process. In such as divinyl benzene or tin tetrachloride.
emulsion polymerization, which is a free radi-
cal process, the proportion of cis and trans SBR LATEXES IN ADHESIVES
structures can be varied by changing the tem-
General
perature of polymerization. High trans (about
70% of polybutadiene) results at low tempera- An important feature of latexes is that they are
tures (about -lO°C) and higher cis at high sold and used as such. This means that in most
temperatures, where the trans content can drop applications the consumer does not normally
to about 50% (at about lOO°C). The pendent have the opportunity to modify latex properties
vinyl content remains fairly constant at about before use. In dry rubber systems the molecular
20%. The presence of styrene in the system has weight of the polymer is modified by the con-
little effect on the relative proportions of the sumer to his specific needs by milling, masti-
different butadiene structures in the polymer. cation, etc. Hence a few grades cover a
In solution SBR the pendent vinyl content can multitude of applications. This flexibility gen-
be varied from 10 to 90% by choosing a sol- erally is not possible with latex. The desired