230 HIGH PERFORMANCE POLYMERS
poly(isocyanate), whose NCO groups are partially or
completely masked with a blocking agent, so that
the polyaddition reaction is inhibited at temperatures
below 140
◦
C.
A common blocking agent is ε-caprolactam. In this
way, blocked isocyanates require curing temperatures
in the range of 170–200
◦
C. Other blocking agents
include malonic acid diethyl ester, 2,4-dimethyl-3-
pentanone oxime or 2,6-dimethyl-4-heptanone oxime,
which cure at lower temperatures. However, the coat-
ings tend toward yellowing.
An alternative blocking agent is 1,2,4-triazole [64].
Mixtures of diisopropylamine a nd 1,2,4-triazole may
also be applied for blocking.
The crosslinking temperature of aliphatic poly-
(isocyanate)s, based on 1,6-hexane diisocyanate, ...