The effect of heat and mass transfer on the cellular plastic insulation and the long-term aging: Dissertation

To produce environmental-friendly products, foamed plastic industries are facing the challenge to replace the traditional blowing agents chlorofluorocarbons (CFCs) with zero ozone depletion potential (ODP) alternatives. Problems arose from such a change are far from manufacturing difficulties. Quality changes, control and evaluation of the new products are among the important issues that have to be considered in selecting a new production formula. Such tasks have formed the contents of our studies on the rigid closed-cell cellular plastic insulation which are composed, originally, by the low thermal conductivity cell gases (the blowing agents) and the solid polymer matrix/ or skeleton. After a plastic insulation was produced, the blowing agents are diffusing out off the foam whilst the relatively high thermal conductivity air components are diffusing into it, the aging occurs. Such a gaseous transfer process will not only change the overall thermal conductivity of the foam but also change its dimensions because the mass transfer rate of blowing agents and air could be dramatically different. After a series of studies were completed, more understandings and new findings have been achieved with respect to the rigid closed-cell cellular plastic insulations or foamed plastic insulations (FPIs). The mechanism of heat transfer within the FPIs was examined. A new formula for calculating the solid polymer matrix thermal conductivity has been deduced based on the law of energy conservation and Fourier equation of heat conduction. All the parameters involved in this formula can be easily measured. By comparing the simulation results with measurements, the Brokaw equation is recommended for the prediction of the thermal conductivity of a cell-gas mixture. The foamed plastic deformation was also discussed. A new model has been established for predicting the elastic modulus of the foamed plastics. In comparison to the published measurements, it was found that the new model gives fairly good results. A diffusion chamber has been designed and constructed for measuring the gaseous transport properties within the FPIs. To overcome the difficulties of the traditional method, a new measurement procedure and post test data treatment have been suggested. The measurement accuracy is equivalent to the traditional method with an exception of much short time being required. The diffusion coefficients of CO2, O2, and N2 within five n-pentane/CO2 based polyurethane (PUR) foams have been obtained from the diffusion chamber tests. Measurements showed that the relationship between the gaseous diffusion coefficients within FPIs and temperature follows the Arrhenius type. No identical relationship between diffusion coefficients and foam density was reached. To predict the long-term aging property of CFC-free foamed plastic insulations, a two dimensional aging model, called ACP, was first updated, and then a 3-D computer code was written based on the more understandings to the mass transfer process in the FPIs and on the establishment of a new solving method oriented to the 3-D parabolic partial differential equations. To retard the aging process of a foam, the PUR foams have been encapsulated by the gaseous impermeable metal foil or metal sheet. In a parallel study, the n-pentane based PUR foam insulation was selected as examples 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 1 mW/m.K for a 50 mm foam with perfect facing on upper and lower surfaces, whereas the corresponding value is about 4.5 mW/m.K for a commercially available foam with so called "diffusion tight" facing. It has been found that the long-term (25 years) thermal conductivity increase or aging of a partly faced PUR foam is a simple logarithm function of the ratio between the perimeter and area of the un-faced surfaces. Such a correlation has been generated with respect to CFC-11 and pentane based PUR foams under different facing conditions. In this study, the transient 1-D non-linear mass transfer within the un-faced heterogeneous foams have also been solved with respect to seven different diffusion coefficient distributions. Results showed that the traditional slicing aging test normally under-evaluates the long-term aging of an un-faced inhomogeneous foam. A weighing test method has been developed, by which the foam aging quality can be known by simply monitoring the mass changes of the foam slices selected from a foam panel in consideration along with time elapse.