Recent Advances in the Development of Rigid Polyurethane Foams of Improved Thermal Insulation Efficiency

Abstract

The fundamental mechanisms of heat transfer through rigid polyure thane (PUR) foam are reviewed in terms of three distinct elements, λ g, λ s and λ r, which represent heat transfer by thermal conductivity through the gas phase, solid phase and by radiation respectively. It is demonstrated that values for λ g and λ s can be estimated, and from the measured λ-value of the foam, an approximation of λ r ob tained by difference. This shows that there is considerable scope for improving the thermal insulation efficiency of these materials. Small reductions in λ-value can be obtained by op timisation of λ g and λ s elements, but the main scope for improvement is by reduction of λ r. This can be partially achieved by the production of foam with very fine cells. However there are strong indications that the cell windows are transparent to radiative heat transfer and several other factors need to be taken into account in re ducing λ r to its minimum practical value. Combining all these factors, laboratory foams can be produced consistently with λ-values 20-30% lower than typical com mercial material. λ-value ageing, which results from air ingress into the cells of the foam, can be substantially eliminated by covering the foam with air impermeable barriers. How ever recent developments have shown that by optimisation of foam formulations, the rate of ageing and the aged λ-value can also be significantly reduced in uncovered foams. The excellent insulation properties of PUR foams compared with competitive ma terials is a consequence of the low value of λ g which results from the encapsulation of the Refrigerant 11 blowing agent inside the foam cells. Supplies of Refrigerant 11 may become limited in the future, with potential adverse effects on the insulation efficiency of PUR foam. In such a situation the indications are that any losses in in sulation efficiency can be minimised and be more than compensated for by further optimisation of the approaches discussed in this paper.