Perlite based core materials for vacuum insulation panels – development and characterisation

Abstract

Vacuum insulation panels currently offer the lowest thermal conductivity, ranging from 2 mW/m/K to 7 mW/m/K, a value four to ten times lower than the thermal conductivity of conventional insulation materials such as polyurethane foam and fibreglass. The major challenges facing the uptake of vacuum insulation panels in refrigerators is their high cost and limited manufacturing know-how. Fumed silica, one of the most common and successful core materials, costs around £6-8/kg. Expanded perlites come at one-tenth of the price and recent research on perlite and perlite-fumed silica composite based vacuum insulation panels has shown promising results in terms of the thermal performance. However, the scope of such studies is limited to experimental measurements performed on only one type of perlite. The knowledge on the effect of perlite specific properties like particle size, pore size, porosity combined with vacuum insulation specific properties including gas pressure and density on the thermal conductivity could unlock further potential of perlites to develop low cost vacuum insulation panels. The aim of this work was to experimentally develop alternative (to fumed silica) core materials based on perlite for vacuum insulation panels targeting cold chain equipment such as refrigerators and freezers, and predict the materials’ thermal performance using a computer model developed specifically for this purpose. Seven different grades of perlite (P1-P7), eight different opacifiers along with fumed silica were researched to characterise their suitability for core materials in vacuum insulation panels. The study has been presented in a four phase process involving i) an extensive literature review concerning physics, material science and manufacturing of VIPs as well as modelling of cold chain equipment like refrigerators-freezers, vaccine boxes and fish boxes; ii) lab based characterisation of several powders (perlites, fumed silica, opacifiers) through various tests including Fourier Transform Infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), particle sizing, bulk density testing, thermal conductivity testing for their inherent properties like specific extinction, surface morphology and pore/particle sizes, loose powder thermal conductivity, composite powder thermal conductivity; iii) development of computer models in COMSOL and MATLAB to predict the thermal performance of VIPs developed with characterized core materials; iv) manufacturing and testing of VIPs made with characterized core materials, using a heat flow meter, and validating the developed models. Material characterisation showed that the given perlites could be mainly divided into two types: milled perlites, and expanded perlites. SEM images and pore size distribution analysis confirmed that in composites of fumed silica and perlites, small particles of fumed silica migrated into the on-particle pores of perlites, reducing the pore size. The effect of such migration was widely studied and is a novelty in this work. The computer model developed considering two different packing orders was able to calculate the bounds of conductive thermal conductivity (including solid, gaseous and coupling conductivity), and its variation with particle size, intra-particle pore size, porosity, sealing pressure and contact ratio. The radiative conductivity was obtained using the IR measurements and diffusion equation and its variation with composition and temperature was studied. The lowest kp=0 of a VIP was measured for a core composition of 85% fumed silica and 15% carbon black by mass (composition termed as FSCB). A VIP with core composition of 80% FSCB and 20% P1 returned a kp=0 of 1.2 times that of FSCB whereas a similar composite of P3 returned 1.5 times; both achieving a cost benefit of 18.5%. The measurements performed and computer model developed can be employed to produce bespoke perlites for most cost-effective thermal insulation systems.