Coupling effect in compacted panels based on micronized nanocellular polymers: Modeling of the thermal conductivity

Abstract

Compacted panels based on micronized nanocellular polymers show reduced thermal conductivity in comparison with bulk nanocellular polymers, especially under vacuum, so they are promising materials to be used as vacuum insulation panels (VIP). The discontinuous structure formed by micrometric particles allows for decreasing the conduction through the solid phase since the contact points between the particles act as additional thermal resistances to the heat transmission. However, the discontinuous structure also leads to the appearance of the coupling effect, which cannot be modeled using the typical equations for cellular polymers. In this work, a semi-empirical model able to predict the thermal conductivity of compacted panels based on nanocellular poly(methyl-methacrylate) (PMMA) is developed. The model allows quantifying each heat transfer mechanism contribution (conduction through the solid phase, conduction through the gas phase, radiation, and coupling effect). The model shows that the contribution of the coupling effect in the compacted panels is higher than 50 % of the total thermal conductivity for pressures higher than 5 mbar, supporting the need for the model to correctly predict the insulation performance of these materials. The model predicts minimum thermal conductivities of 32.5 mW/(m·K) at ambient pressure and of 10 mW/(m·K) at maximum vacuum.