The Effect of Polymer Structure on the Gas Permeability of Model Polyurethanes

In the face of impending CFC elimination, a large variety of low-boiling alternate blowing agents have come under scrutiny for use in rigid foam insulation applications. The permeability of both the alternate blowing agent and atmospheric gases through the rigid foam thermoset polymer contributes to the rate at which the k-factor of the resulting foam deteriorates. In this study, a compression moulding method for synthesizing polymer films from polyurethane formulation components (a variety of diols with pure and polymeric methylenediphenyl diisocyanate) was developed and utilised to produce films containing distinct structural features. The permeabilities of the films to oxygen and carbon dioxide were then determined using a diffusion cell combined with either an infrared or coulombic detector. The effects on permeability of structural features such as diol molecular weight, alkylene oxide repeat unit, and isocyanate functionality are presented. Structural features determined to promote improved barrier performance in polymer films have been incorporated into a fully formulated foam system. Foam permeability and k-factor ageing comparisons were then made which demonstrate the improved foam barrier performance. The resulting structure/permeability correlations derived from these experiments will be useful in designing rigid foam polymers for use in alternate blowing agent applications.