Factors Affecting the Field Performance of SPF

The aging process can be explained using the distributed parameters continuum (DIPAC) model developed and verified experimentally by the National Research Council of Canada (Bomberg and Kumaran, 1995). The DIPAC model illustrates the relative significance of different aging mechanisms. Figure 3 shows thermal resistivity (inverse of thermal conductivity coefficient) versus aging time. Four curves are shown in Figure 3. Curve 1 shows the aging of a 25 mm(1 inch) thick SPF specimen, fully encapsulated on all sides. The encapsulation prevents entry of air into the foam, but has no effect on the redistribution of the BA (BA) within the encapsulated foam. Part ofthe BA enters and saturates the polymer matrix, reducing the concentration of the BA in cell gas. This, however, does not change the thermal performance of the fully encapsulated specimen. The thermal conductivity of the cell gas does not depend on the pressure of the gas [as long as the gas pressure does not fall below 0.01 atmosphere (Tsederberg, 1965)]. The thermal conductivity (k-factor) does not change as long as air has not entered the cells of the foam, despite the change in pressure caused by the cell gas redistribution. For example, foams with impermeable sheet metal facings demonstrate high thermal performance for extended periods (Baumann, 1982). This is true even if the thermal efficiency of the BA is low, e.g., carbon dioxide. Curve 2 shown in Figure 3 relates to the hypothetical aging of the same specimen when only air is allowed to enter the foam. In this computer simulation, the BA redistribution is eliminated by using zero values for the effective diffusion and solubility coefficients of the BA; the effective diffu-