Relationship of k-factor versus Density for Various Appliance Foam Formulations Containing Next Generation Blowing Agents

Abstract

With the eminent elimination of CFC-11 (trichlorofluoromethane) as a blowing agent in rigid polyurethane foams, a greater understanding of the effect of the new alternative blowing agents on foam physical properties is needed. This is especially true in the appliance industry, where k-factor is of supreme importance to the overall energy consumption. Stringent energy requirements, along with the market pressures to continually lower the density of the foam, gives added challenges to new system development. This paper addresses the critical balance between k-factor and density, or cell gas content. Since the thermal conductivity of the gas plays the major role in the overall k-factor equation, it could be theorized that the addition of more blowing agent should lower the overall k-factor of the foam. However, a point exists at which the addition of blowing agent is actually detrimental to the overall k-factor, and lower gas levels can actually improve the insulation properties. This point is largely controlled by the amount of gas trapped within the cell, which directly affects the density and cell size of the foam. When too much gas is trapped within the cells during their formation, the cells will expand due to higher internal pressures, thus increasing the cell size and radiative contribution to k-factor. There will be an optimal level of gas contained within the cells to achieve the lowest k-factor for any given foam formulation. To address this issue, several foam formulations containing various alternate blowing agents were studied. Systems studied include TDI and MDI based formulations, blown with HCFC-141b (1,1-dichloro-1-fluoroethane) along with two MDI based systems; one blown with HFC-134a (1,1,1,2-tetrafluoroethane) and the other with cyclopentane. All foams were co-blown with various amounts of carbon dioxide generated from the water/isocyanate reaction. After the basic components for the foam formulations were determined for each of the blowing agents studied, a systematic approach was used to vary the cell gas level in each formulation. A graphical representation of k-factor versus cell gas level was generated for each system along with other physical properties. Based on this information, a better understanding of the effects of density on k-factor for the next generation of blowing agents can be obtained.