Why and which opacifier for perlite based vacuum insulation panels (VIPs) in the average temperature range of 10–70 °C

Abstract

Little is known on the effect of opacifiers in perlite core vacuum insulation panels at lower temperatures in the range of 10–70 °C, which are important for applications like refrigerators, transport boxes, buildings, and domestic hot water storage tanks. This paper shows that the effect of opacifier on perlite core Vacuum Insulation Panels (VIPs) is radically different than that on fumed silica VIPs. Identifying an optimum proportion of opacifiers for perlite core VIPs is not a trivial task because of a concomitant rise in solid conductivity as the radiative conductivity decreases. Further, the effect of opacifiers on temperature dependent thermal conductivity of VIPs has been resolved. Opacifiers in perlite are shown to yield clear benefits by reducing the overall thermal conductivity for insulation exposed to ≥70 °C. This was not the case for ≤20 °C. Several VIPs were manufactured using a low-density expanded perlite and 3 different proportions of 4 opacifiers i.e., carbon black, graphite and two variants of silicon carbide (SiC I, SiC II). These VIPs were tested at various combinations of sealing pressure and four different average temperatures i.e., 10 °C, 20 °C, 40 °C and 70 °C. A novel methodology has been developed to derive the evacuated thermal conductivity of perlite core VIPs by curve fitting technique, in order to eliminate the effect of gaseous conduction. The results were also compared with fumed silica core vacuum insulation panels with different proportions of carbon black as opacifier. Among the opacifiers tested (carbon black, graphite, SiC I and SiC II) in this research, Silicon carbide I with a mean particle size of 20 μm was found out to be the best opacifier, with one of its composite resulting in a 50% lower thermal conductivity at 70 °C in comparison to pristine perlite core VIPs.