Page 190 - Handbook of Plastics Technologies

THERMOSETS

3.60 CHAPTER 3

FIGURE 3.63 Bisphenol A dicyanate.

TABLE 3.57 Polycyanate Cured Properties

Density 1.19
T g 265°C
Heat deflection temperature 230°C
–6
Coefficient of thermal expansion 44 × 10 /°C
Dielectric constant 2.7
Loss tangent 0.003
Moisture absorption at 100°C 1.3%

There is very little shrinkage (<1 percent) during cure, and the cured products have high
adhesion, high heat and electrical resistance, and low moisture absorption, making them
desirable for electronic applications such as printed circuit boards. There is also consider-
able interest in blending them with engineering thermoplastics, forming semi-interpene-
trating polymer networks with polysulfones, polycarbonate, polyesters, polyarylate, and
nitrile rubber, to combine optimum properties in the cured products. And there is interest
in blending or copolymerizing them with other thermosetting resins such as epoxy or
polyimide, and end-capping heat-resistant oligomers with aryl cyanate end-groups.
3.1.7.2.2 Isocyanate/Isocyanurate. Isocyanates react with polyols to form rigid poly-
urethane foams, a major type of thermoset plastics. While these are very useful in thermal
insulation, they are limited by failure at high temperature and by flammability. One way to
solve these problems is to convert part of the isocyanate to isocyanurate by cyclotrimeriza-
tion (Fig. 3.64). Whereas the isocyanate-polyol reaction forms polyurethane rapidly at
room temperature, the cyclotrimerization of isocyanate to isocyanurate requires strong al-
kaline catalysis and heat to compete successfully. The resulting isocyanurate rings build
considerable heat resistance (150 to 250°C, short-term ≤800°C) and flame-retardance into
the polyurethane foam. They are useful for insulating pipelines and boilers.
Pure polyisocyanurate would have the highest heat and flame resistance, but the foam
is too brittle to be useful. Copolymerization with polyurethane gives a more useful balance

FIGURE 3.64 Cyclotrimerization of isocyanate
to isocyanurate.

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