28/8/2011
This general-purpose synthetic rubber, a polymer of butadiene and styrene,
exceeds all others in consumption. Its symbol, SBR, covers a range of materials
in which proportions of the two ingredients vary.
Styrene, a liquid, boils at 145 C (293 F), but butadiene, a gas at normal
temperatures, boils at -4 C (25 F); butadiene may be stored in liquid
form under pressure. Both liquids are pumped continuously into reactors and
mixed with water, soap, and catalyst by constant agitation at controlled
temperature. After the short stop is added, the unconverted butadiene and
styrene are recovered and re-used. Then the antioxidant is added and the
coagulant to deposit the rubber.
Varieties of SBR, each with its own properties, can be produced either by
altering the ratio of the two ingredients; by using hot or cold polymerization;
or by varying other elements in the manufacturing process. For example, with
23.5 percent SBR, fatty acid-soap emulsifying agents give a faster curing
rubber, while the rosin acid-soap types yield a less tacky final product. Oils
of different grades added to the reaction mixture of styrene and butadiene
determine the final product's toughness.
A convenient feature of styrene-butadiene rubber is its final packaging in
34-kilogram (75-pound) polyethylene plastic bags, shaped for easy handling and
shipping. If the minimum temperature in the user's mixing machine approximates
140 C (284 F), the rubber, bag and all, can be
tossed into the mixer where the polyethylene will soften and disappear.
For many purposes SBR directly replaces natural rubber, the choice depending
simply on economics. Its particular qualities include abrasion and crack
resistance and generally better aging properties. Its limitations are poor
strength (without such reinforcing fillers as carbon black), low resilience,
low-tear strength (particularly at high temperatures), and poor tack (i.e., it
is not tacky or sticky to the touch).
These characteristics determine its use in tire treads; its proportions
decrease as need for heat resistance increases, until 100 percent natural is
reached in the heaviest and most severe uses, such as tires for buses and
aircraft. SBR, however, is used in great quantities for tire carcasses;
generally some natural rubber is admixed to produce the necessary tack in
assembly.
