ABSTRACT

The United States appliance industry must meet the U.S. Department of Energy (DOE) requirement for energy reductions of approximately 30% (from the 1993 standard) by July 2001. In addition, the manufacture of HCFC-141b, the predominant blowing agent used for rigid foam in North America will be discontinued in the United States on January 1, 2003. These two directives have driven appliance OEM’s to evaluate new cabinet designs in conjunction with alternative blowing agents. In response to these needs, The Dow Chemical Company began a study to evaluate the effects of shorter de-mold times and increased foam thickness, as OEM′s are driven to improve productivity by decreasing cycle time, while driving towards reducing energy consumption.

It was hypothesized that pressure and temperature gradients observed throughout the foam, could lead to unbalanced stresses across the part promoting de-mold splits in a typical Polyurethanes appliance formulation. It was further suggested that increased green strength would be necessary at the time of de-mold, so the foam could resist greater expanding forces.

First, modeling of the temperature profile across the foam thickness was performed. The corresponding gas pressures and pressure drops as a function of position across the foam thickness were calculated using the assumption that all gases were ideal. The second model estimated the critical gelation point (conversion), as a function of both the isocyanate and polyol functionalities. Thirdly, from calorimetric measurement, the time-conversion profile for the foaming polymerization reaction was obtained.

Examination of the modeling and experimental data reinforces the hypothesis that pressure differences within the foam coupled with inadequate green strength caused by de-molding at conversions near or below the critical gelation point, would lead to de-mold splits. From the analysis, it was also observed that at a two inch foam thickness, the temperature and consequently pressure gradients, were too small to cause de-mold splits in the foam.

The authors’ recommended increased green strength at the same de-mold time and percent overpack. To achieve this, it was suggested that a higher functionality polyol would lead to a foam with higher green strength at the time of de-mold.