Improved Low Thermal Conductivity Fine Cell HCFC-141b Appliance Foam

Abstract

The issue of global warming is receiving more attention on the world stage. Carbon dioxide emission from the burning of fossil fuels is suspected of being a major contributor in the phenomena of global warming. In the U.S.A., the National Appliance Energy Conservation Act of 1987 (NAECA) was passed into law, in part to reduce the demand for electrical power, thereby decreasing carbon dioxide emissions from power generating plants. Under the provisions of NAECA, the U.S. Dept. of Energy (DOE) has received authority to further restrict the amount of electricity used by household refrigerators and freezers sold in the U.S.A. The U.S. DOE will probably exercise that authority at some future date. Consequently, the continued development of better insulating HCFC-141b blown polyurethane foams is necessary to meet the future needs of the domestic refrigeration industry.

Initial fine-cell HCFC blown foam systems having low thermal conductivity were first reported by Mobay Corporation (now Bayer Corporation) at the SPI in 1990. These early systems had high densities and long demolding times. Then, in 1991, at the SPI, Mobay Corporation reported a foam system with reduced demolding times comparable to commercial CFC-11 blown foams. But the high density of this foam still impeded it from becoming a commercially viable system. In 1992, at the SPI, Miles Inc. (now Bayer Corporation) reported a polymeric MDI HCFC-141b foam system with an initial core k-factor of 0.115 Btu-in./hr. ft²°F, a freeze stable density of 2.23 pcf, and an improvement in demolding properties over commercial CFC-11 blown foam systems. This system, in spite of its good k-factor and density, did not pass a cabinet freeze stability test due to foam cracks. Further improvements led to commercial HCFC-141b foam systems in 1994 which had initial core k-factors of approximately 0.117 Btuin./hr. ft²°F and densities of approximately 2.15 pcf. Cell diameters were around 194 microns.

This paper will describe a new system of finer cell polymeric MDI-based HCFC-141b blown foam. It has been tested commercially on cabinets and doors, showing a 3.4% reduction in initial core k-factor and a 2.8% reduction in reverse heat leak as compared with a standard MDI/HCFC-141b blown foam system. The system has an initial core k-factor of 0.113 Btu-in./hr. ft²°F and a freeze stable density of 2.25 pcf. The cell diameter was reduced by 15%, which lowered the radiative heat transfer. Also, lower water levels produced less carbon dioxide, further reducing the thermal conductivity due to less heat transfer by cell gas conduction.

This development is of particular interest for appliance manufacturers who may need an extra boost to lower refrigerator energy consumption to be in compliance with the energy consumption regulations established for the products sold in the U.S.A. It can also help manufacturers' products qualify for energy efficiency rebate programs sponsored by the utility companies.