Conversion of a
Polyol and Isocyanate at Gel Point
Depending on the functionality of the polyol and the
isocyanate the reaction leads to a gel at different degree of conversion. A di functional
polyol and a di functional isocyanate would not gel even at complete conversion. (1.0). A
difunctional polyol and a trifunctional isocyanate would gel at about 60 % conversion
(0.6)
Conversion
at gel point
The reaction of a polyol with an isocyanate leads to an increase in
viscosity and to gelation. After gelation. the reaction continues at a slower rate to
complete reaction. Higher functional polyols and isocyanates lead to earlier gelation at a
lower conversion of the hydroxyl groups and the isocyanate. The gel point can be estimated
using the equation.
a crit.
= 1/(f-1)
f = average functionality of reactants, a crit. = gel point.
The actual gel point is at higher
conversion due to ineffective cyclic links. Past the gel point the polymer stops to flow
and the viscosity approaches infinity.
Last edited on:
November 14, 2006
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