Aging of cellular plastic insulations

This report summarizes our studies on cellular plastic insulations and outlines the future research in the project of " Innovative Low Energy Building Structure" in the Finnish energy research program RAKET - Energy Use in Buildings. Heat and mass transfer within porous media is a classical topic. However, the problems become extremely complicated if the rigid close-cell cellular plastic insulation is considered. The mass transfer regimes may include molecular diffusion, Knudsen flow, viscous flow as well as blowing agent(s) evaporation and condensation. Whereas, the heat transfer may include conduction, convection, radiant heat exchange and phase-change-caused heat absorption and release. All these factors are foam structure dependant. In addition, the introduction of facing the foam arose from more technical questions which have not been solved yet in controlling the efficiency. To phase out the tradition blowing agent, CFC-11, also bring us with some basic works which include the re-generation of a database for the new products. Therefore, to prepare an apparatus for measuring gaseous transport properties within foam insulations, to establish a more general ageing model, to analyze the effect of facing strategy, and to predict the ageing performance of some newly developed cellular plastics then became the main objectives of our last year's study. For the purpose of getting gaseous transport properties of a cellular plastic, a diffusion chamber system has been designed. The critical parameters related to the system have been figured out theoretically. A new method for accelerating the measurement was recommended. It is believed that the new method could allow us to shorten the required-test-time in comparison to the traditional method. In order to fulfil the requirement in predicting the thermal property of CFC-free foam insulations, an ageing model, called ACP, was updated, which originally can be used for calculating the ageing of CFC-based foams. The new version of ACP program right now can handle the heat and mass transfer of oxygen, nitrogen, carbon dioxide, CFC-11, CFC-12, CFC-22, n-Pentane, iso-Pentane, neo-Pentane and cyclo-Pentane. In comparison to the short-term ageing measurements, the program has shown the ability in evaluating the long-term ageing of CFC-based, pentane-based and CO2/pentane-based foams. To reduce the emission of blowing agent(s) and then slow down the ageing, facing foil has been used to encapsulate the foam. As an example, n-pentane-based foam insulation was selected as the samples to evaluate the possible improvement in thermal performance by using this strategy. It has been found that the average thermal conductivity increase after 10 years is only about 1mW/m.K for a foam with perfect facing on upper and lower surfaces, whereas the corresponding values is about 4.5 mW/m.K for a commercially available foam with so called "diffusion tight" facing. It has been found from the measurements that the ageing process is still going on within the foam with diffusion-tight facing metal foil. Thus, gaseous mass transfer through metal-foil was examined. It has been found that any metal-foil could be used as facing material of close-cell cellular plastics if there is no any defect on the foil. The defects of the facing foil and the poor contact between facing foil and the foam body are the reason of ageing of faced foams. Two measurement methods were recommended. The benchmark tests have shown that the methods work properly.