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Narrative
A viscous liquid containing a polyether polyol, a blowing agent, a silicone surfactant, and a catalyst is mixed with a second liquid contains a polyfunctional isocyanate. The polyol and the polyfunctional isocyanate react to form polyurethane. The blowing agent adds a gas to the mixture to produce a foam. Because the reactants are polyfunctional, there is a high degree of cross-linking in the polyurethane. This causes the foam to become rigid after cooling.