Effect of Polymer Structure on the Long-Term Aging of Rigid Polyurethane Foam

Abstract

The effect of polymer structure on both initial and aged thermal conductivity ( K-factor) or thermal resistivity ( R-value) was explored by using a new procedure to estimate the long-term thermal resistance of gas-filled cellular plastics proposed by Norton [1], Edgecombe [2] and Bomberg [3]. This method uses a semi-logarithmic plot of thermal resistivity versus time that produces two distinct stages in the data, thermal drift and plateau with a break point separating the two stages. The plateau stage was fit with a straight line in order to estimate the long-term thermal resistance or K-factor of the foam. This concept was employed on the fourteen CFC-11 blown foams [4] in this study. The effect of me two major types of isocyanates, Specialty TDI (toluene diisocyanate) and PMDI (polymeric diphenylmethane diisocyanate), was isolated and compared. The significance of seven different types of polyol initiators was also evaluated with respect to K-factor and K-factor aging. In the case of the PMDI foams, the data correlated well with the model and the 20-year K-factor predictions appear to be reasonable when compared to the raw data curves. In the case of the TDI foams, however, it was more difficult to find a break point which would define the plateau region in the data. Most of these foams did contain break points, but the break point occurs at a slightly longer time. The 20-year K-factors of these foams could be predicted with reasonable confidence when there was a break point in the resistivity versus log (T) curves.