UBC Theses and Dissertations
NMR imaging and spectroscopic investigations of blowing gases in polymer insulating foams Mei, Zhiming
Rigid polystyrene and polyurethane plastic foams are extensively used as insulating materials in many important energy saving applications due to their extremely efficient thermal insulating and good mechanical properties. A critical foam component responsible for this superior performance is the chlorofluorocarbon (CFC) blowing agent, which has a much lower thermal conductivity than air and when trapped within the foam cells serves as the insulating gas. Over time, the CFC gas slowly diffuses out of the cells and causes a gradual decrease in the foam's thermal resistance. There has long been a need for a reliable and fast method for estimating the long-term thermal performance of foams, especially since recent environmental concerns require the fast development of new foam products with CFC replacements. The 1 9F NMR microscopic imaging technique described in this thesis is demonstrated to be a non-destructive, efficient and reliable method for the investigation of fluorinated blowing agents in various foams. Detailed studies of polystyrene foams made with CFC, HCFC and HFC blowing agents show that 1 9F NMR microscopic imaging technique is ideally suited for monitoring the spatial distribution of gaseous blowing agents in foams and that it can yield a quantitative description of the distribution of cell gas in foams as a function of time and temperature as well as other factors. There is no other technique available which is able to directly measure and monitor the spatial distribution of the concentrations of blowing gases inside foams. A detailed investigation on the measurement of the diffusion coefficients of the blowing gases in polystyrene foams shows that a one-dimensional imaging experiment can be used to obtain the quantitative spatial distribution of blowing gases in foams as functions of time and temperature. The diffusion coefficients for different sample geometries can be simply determined by fitting theoretical model calculations to the experimental data and results obtained from both ambient temperature and elevated temperatures. The technique makes it possible to predict long-term foam insulation performance and will provide valuable information for improving foam formulation and manufacturing methods. Fast MAS high resolution 1 9F solid-state NMR spectroscopy has been used to investigate the effects of the dissolved blowing gas components in the polymer matrix. It is demonstrated that it is possible to detect and quantify the blowing gas dissolved in the solid-phase in polystyrene foam systems. The results are presented for different blowing gases in various PS foams and the effect of the dissolved gas on the diffusion process is also discussed. Both the imaging and solid-state NMR experiments are extended to various polyurethane foam systems to investigate the diffusion process of blowing gases as well as the effect of the dissolved gas components. The results show a significant contribution from the dissolved component to the diffusion of the gaseous component in PU foams in the accelerated aging process. The theoretical models were therefore modified in order to address such an effect. Satisfactory results are obtained and the diffusion coefficients determined. Foam systems which had been post-treated with a second blowing gas were also investigated to probe the outward and inward diffusion processes using the above techniques. The results are presented as functions of time and temperature. The diffusion coefficients determined for each gas in the opposing diffusion processes are presented.
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