The Effect of Blowing Agent Solubility on the Long Term Dimensional Stability of Rigid Polyurethane Foams

Abstract

More environmentally acceptable blowing agents, such as hydrochlorofluoro-carbons (HCFCs), hydrofluorocarbons (HFCs), hydrocarbons, and carbon dioxide have now replaced chlorofluorocarbon (CFC) blowing agents in many rigid polyurethane (PUR) and polyisocyanurate-modified polyurethane (PIR) foam applications. However, these new blowing agents were not simply "drop-in" replacements for CFCs. In most cases, the structural matrix of the foam had to be adapted in order to accommodate the new blowing agent. Concerns about the long term performance of these new technologies have led to extensive programs of research, not only to address thermal performance, but also to understand the effects of these blowing agent changes on mechanical properties. While a great deal of progress has been made, the polyurethane industry is still in need of a means to fully assess long term mechanical and dimensional stability performance. One particular problem related to the long term dimensional stability of rigid polyurethane foams is the effect of blowing agent dissolution into the polymer matrix. Such dissolution may cause a softening of the matrix known as a "plasticization effect." In order to reduce the risk of mechanical failure, foam density may be increased, in order to strengthen the foam. However, this raises foam cost, and makes polyurethanes less competitive against other insulation materials. Clearly, the rigid polyurethane foam industry is in need of an accurate means of predicting lowest reasonable density which provides acceptable long term performance. This paper describes the results of a new test designed to quantify the plasticization effect, which provides a fast and accurate means of assessing long term dimensional stability. The new methodology was successfully applied to a number of PUR and PIR foams, expanded with several blowing agents. The degree of blowing agent uptake into the matrix was quantified, and its effects on mechanical properties of the foam was established. In addition, a few correlations were drawn between the new test and real life long term performance data. The results indicate that the methodology presented in this paper allows for effective prediction and quantification of possible plasticizing effects associated with any blowing agent in a PUR or PIR matrix.