The Thermal Conductivity of Isocyanate-Based Rigid Cellular Plastics: A Technique for Predicting the 25 Year Values

摘要

This paper reviews the factors determining the thermal conductivity of isocyanate-based rigid cellular plastics. Details of the requirements and the characteristics of other blowing agents being used as alternatives to CFCs are assessed. It provides data obtained from a research programme conducted at Salford University, UK, to establish the basis of a method based on an accelerated procedure for the determination of the thermal conductivity of these products which can be assumed to apply, without taking into account production variation, to a product over its 50 year life. The latter is used to correspond to the term "reasonable economic life" used in the EU Construction Products Directive. The data is limited to products principally expanded with HCFC 141b and n-pentane blowing agents but the authors believe, although proof will be necessary, that the method has a good chance to be acceptable for all future products which are expanded principally with a gas which is shown to remain, for the most part, in the cells over its reasonable economic life and for which the gaseous composition of the cell gas is changed only by dilution, to varying degrees, by the ingress of air. In essence the ingress of air is accelerated by ageing the test specimen in air at 70°C for 25 weeks. The paper also discusses the merits of various other procedures claiming to produce "realistic aged thermal conductivity values". The ageing characteristics of the new blowing agents were found to be similar to CFC11 blown products in that they conform to the same kind of thermal conductivity ageing curves. In addition, the thermal conductivities were similarly seen to satisfy the original Macintyre and Kennedy relationship which was used extensively with the CFC11 blown materials. This research brings for the first time a novel feature in that it proposes a mechanism of predicting the 25 year ageing values for all polyurethane products ranging from those which hardly age at all in 25 years to thin samples without facing materials for which the thermal conductivity increases at a fast rate and the process may be completed in 1 to 2 years.