Physical blowing agents such as n-pentane and methyl formate and, for comparison,
chemical blowing agents such as water were used to prepare structural polyurethane
rigid foams of different densities by reaction injection molding. Experimental
runs were carried out with formulations based on oligomeric isocyanate and
a mixture of polyether polyols. The constitutive equations for the vaporization rate of
the two blowing agents and the polymerization kinetics data are reported. Experimental
results were compared with the prediction of a simplified theoretical model,
and they showed a satisfactory agreement in terms of temperatures and density
profile. All the specimens were characterized by physical-mechanical properties
such as hardness, impact strength, flexural strength and elastic modulus and the
results were reported in function of the densities. The best mechanical performance
were obtained with the physical blowing agents, due to a better density distribution
profile and a thicker skin layer.