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An analysis of diffusivity of composites is described in which an electrical resistance analogy is used. This leads to an expression for diffusivity, which takes into account the contributions of fiber diffusivity, matrix diffusivity, and the diffusivity of the interphase, when this is different from that of the polymer matrix. The case involving the interphase requires numerical integration for its solution. However, the effect of the interphase can be very accurately simulated in a more approximate model; a closed form expression for composite diffusivity is obtained that agrees with the numerical analysis within better than 1% if an appropriate adjustment for interphase width is made. The same approach was used for including voids in the analysis, and it is shown that, in some experiments described previously with glass and carbon fiber pultrusions, the results can be explained on the basis of the voids in the composite providing an enhanced diffusion path for water. This “void path” apparently has a diffusivity of 15 times that of the polymer. Since the results obtained using the interphase diffusion model do not agree so well with the experimental data as does the void path model, it is concluded that, at least for carbon-epoxy and glass-epoxy pultrusions, the most important conducting path for water is provided by the voids.
Professor, University of Toronto, Ontario
Materials science engineer, Digital Equipment Canada Ltd., Kanata, Ontario
Stock #: CTR10271J