Abstract
HCFC-141b blown rigid foams have now been used successfully for a couple of years in North America as insulation in refrigerated home appliances. Manufacturers have had time to adjust to the differences in processing between these systems and the CFC-11 systems that they replaced. Many appliance manufacturers have now focused their interest on foams that will provide lower cost without sacrificing properties or processing characteristics.
Although there are a number of ways to reduce cost, one of the best ways is to reduce the density of the foam. Bayer Corporation has previously reported on low density HCFC-141b appliance foam which had similar densities to CFC-11 foams of around 1.9 to 2.0 pounds per cubic foot (pcf). These foams had somewhat higher k-factors but still were successful in replacing some of the CFC-11 foams commercially. Other HCFC-141b foam systems were developed that gave equivalent or lower k-factor foams but at somewhat higher densities. These systems replaced most of the commercial CFC-11 systems.
Thermal conductivity of rigid polyurethane foams is to a large extent affected by the blowing agent, the closed cell content, cell size and the density. It has been reported that there exists a minimum thermal conductivity in the density range of around 2 to 3 pcf for CFC-11 blown foams. This appears to be true also for HCFC-141b foams, although the density range may be slightly different or narrower. It is not easy to reduce the density below 2 pcf without increasing the k-factor of the foam. Around or below 2 pcf, the increase in k-factor is caused not only by an increase in the radiation contribution to thermal conductivity due to coarser cells but to higher open cell content and possible convection currents within the cells. Since strength properties are also affected by density. There is a need by developers and manufacturers to evaluate long term dimensional stability of lower density foams.
This paper will discuss the development of new low density HCFC-141b appliance foams which have similar k-factors compared to the majority of commercial appliance foam systems in the 2.1 to 2.2 pcf range. These new systems will also be shown to have substantially lower k-factors with only a slight increase in density compared to a commercial system in the 1.9 to 2.0 pcf range. In addition, physical properties and processing characteristics of these foams will be compared to commercial foams.
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