Compression Creep and Long-Term Dimensional Stability in Appliance Rigid Foam

Abstract

Diffusion of gases and condensation of blowing agents can cause the formation of lower-than-ambient pressures inside the cells of closed-cell rigid foam. The net result is a shrinkage force which can cause partial collapse of the cells and dimensional instability of the foam. Computer modeling of the cell gas pressures inside the cells of a typical appliance foam formulation shows that a loss of 1-3 psi of pressure can be due to the loss of carbon dioxide through diffusion out of the cells. An additional 6-9 psi loss of pressure can be due to the condensation of the blowing agent (141b in the present example). This total loss of pressure of 7-12 psi can represent 30-40% of the compression yield strength of the foam, enough to cause dramatic shrinkage in the foam over a period of time. This deformation at low loads and long times is known as creep and is a phenomena common to viscoelastic materials such as polymers.

In the present paper, compression creep experiments were used to study the effects of varying external loads on the deformation behavior of samples of rigid polyurethane foam. A model similar to that used by other researchers to predict the creep behavior of polyurethane foams and thermoplastics was found to agree very closely to the creep data that was obtained. Using this model, the creep behavior could be extrapolated to longer times than those used in the actual creep experiments. HCFC 141b foams showed poorer creep performance than CFC 11 foams when all important parameters, such as density, index and water content, were held constant. This result would agree with evidence from other researchers that HCFC 141b is soluble in the foam polymer causing a loss in mechanical properties.