Chemistry Reference
In-Depth Information
though beneficial, is mostly confined to United States. Orientation is limited to sheets and filaments,
and copolymerization usually lowers the softening point too much.
Addition of rubbery materials, however, does improve the impact resistance of polystyrene. This is
done, therefore, extensively. The most common rubbers used for this purpose are butadiene-styrene
copolymers. Some butadiene homopolymers are also used but to a lesser extent. The high-impact
polystyrene is presently prepared by dissolving the rubber in a styrene monomer and then
polymerizing the styrene. This polymerization is either done in bulk or in suspension. The product
contains styrene-butadiene rubber, styrene homopolymer, and a considerable portion of styrene-graft
copolymer that forms when polystyrene radicals attack the rubber molecules. The product has very
enhanced impact resistance.
Past practices, however, consisted simply in blending a mixture of polystyrene and rubber on a
two-roll mill, or in a high shear internal mixer, or passing through an extruder. The impact strength
of the product was only moderately better than that of the unmodified polymer. Another procedure
was to blend polystyrene emulsion latex with a styrene-butadiene rubber emulsion latex and then to
coagulate the two together. The product is also only marginally better in impact strength than styrene
homopolymer. This practice, however, may still be in existence in some places.
In high-impact polystyrene, the rubber exists in discrete droplets, less than 50
m in diameter.
In effect the polymerization serves to form an oil in oil emulsion [
199
] where the polystyrene is in the
continuous phase and the rubber is in a dispersed phase. The graft copolymer that forms serves to
“emulsify” this heterogeneous polymer solution [
200
].
Commercial high-impact polystyrene usually contains 5-20% styrene-butadiene rubber. The
particle size ranges from 1 to 10
m
m. High-impact polystyrene may have as much as seven times
the impact strength of polystyrene, but it has only half its tensile strength, lower hardness, and lower
softening point.
m
6.13.2 ABS Resins
Styrene-acrylonitrile copolymers are produced commercially for use as structural plastics.
The typical acrylonitrile content in such resins is between 20 and 30%. These materials have better
solvent and oil resistance than polystyrene and a higher softening point. In addition, they
exhibit better resistance to cracking and crazing and an enhanced impact strength. Although the
acrylonitrile copolymers have enhanced properties over polystyrene, they are still inadequate for
many applications. Acrylonitrile-butadiene-styrene polymers, known as ABS resins, were therefore
developed.
Although ABS resins can potentially be produced in a variety of ways, there are only two main
processes. In one of them acrylonitrile-styrene copolymer is blended with a butadiene-acrylonitrile
rubber. In the other one, interpolymers are formed of polybutadiene with styrene and acrylonitrile.
In the first one, the two materials are blended on a rubber mill or in an internal mixer. Blending
of the two materials can also be achieved by combining emulsion latexes of the two materials together
and then coagulating the mixture. Peroxide must be added to the blends in order to achieve some -
cross-linking of the elastomer to attain optimum properties. A wide range of blends are made by this
technique with various properties [
201
]. Most common commercial blends of ABS resins
may contain 70 parts of styrene-acrylonitrile copolymer (70/30) and 40 parts of butadiene-nitrile
rubber (65/35).
In the second process, styrene and acrylonitrile are copolymerized in the presence of polybutadi-
ene latex. The product is a mixture of butadiene homopolymer and a graft copolymer.
