Page 194 - Handbook of Plastics Technologies

THERMOSETS

3.64 CHAPTER 3

3.1.8.2 Polystyrene. Polystyrene is cross-linked by copolymerization with divinyl ben-
zene (Fig. 3.70). This is used to produce polystyrene beads for ion exchange beds such as
water softeners.

FIGURE 3.70 Styrene-divinyl benzene copolymerization.

3.1.8.3 Polyvinyl Chloride. In plastisol processing, PVC is dispersed in liquid plasticizer
then fused to a sol and cooled to a gel to produce rubbery products. When the processor
wants to decrease the plastisol viscosity without excessive softening of the final product, he
uses a polymerizable plasticizer such as a glycol diacrylate (Fig. 3.71) plus a little peroxide.
The plasticizer gives a more fluid plastisol, and then, during hot fusion, the peroxide cross-
links the plasticizer to prevent excessive plasticization of the finished product.

FIGURE 3.71 Polymerizable plasticizers.

3.1.8.4 Polyvinyl Alcohol and Acetals. Polyvinyl alcohol fibers are solution-spun, and
then cross-linked with formaldehyde (Fig. 3.72) to make them resistant to water. Polyvinyl
formal is cross-linked with phenolic resole to make baked coatings on electrical wire. Polyvi-
nyl butyral is cross-linked with phenolic resin to make extremely tough bullet-proof helmets.

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