Diffusion and Permeation of Gases in Fiber-Reinforced Composites

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Abstract
The presence of filler in a matrix phase greatly alters and also complicates the diffusion process, making experimental data very difficult to handle analytically. In this study both diffusion and permeability data were obtained in unfilled cellulose acetate films and cellulose acetate films filled with cheese cloth. Cheese cloth being highly permeable, the permeabilities and diffusivities of gases in the filled films are greater than in unfilled films due to reduced effective film thickness for molecular diffusion. A mathematical model has been developed which considers molecular diffusion to occur in that part of the film having no filler, while diffusion through the low-resistance cheese cloth is approximated by convective flow controlled only by the pressure gradient across the thickness of the cheese cloth yarn. The mathematical model predicts permeability values somewhat lower than experiment at low flow rates, but agrees well with experimental permeabilities at higher rates. The poor agreement between theoretical and experimental diffusivities is attributed mainly to errors involving the measurement of time lags. Morphological studies of the composite surfaces and cheese cloth filaments show that the cellulose acetate did not penetrate into the cheese cloth yarn which essentially retains its porous structure in the composite.

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