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Abstract
Changing the blowing agent from CFC-11 to pentane in the formation of Al-faced polyurethane foam insulation results in a lower adhesion to the Al facing. This decrease in bond strength is due to a change in the OH number of the polyether polyol used. In order to establish the relationship between OH number and the adhesion of the polymer to the Al, the bond strengths of rigid polyurethane films to Al were measured, by indentation debonding, as a function of polymer crosslink density and of the number density of the spherulites which formed at the interface. The interphase between the two materials, formed by transcrystallization, crucially affected the failure mode. Cohesive fracture occurred between the spherulites and the amorphous portion of the bulk polymer. In the regions where no spherulites existed, the failure took place by separation of the amorphous polymer from the Al surface. The bond strength appeared to be proportional to the crosslink density calculated using the elastic modulus of the polymer. Upon aging the samples at 170°C, the number density of spherulites was found to be the decisive factor in bond strength instead of spherulite size. In addition, there was an optimum OH number for the highest bond strength.