Moisture sorption and desorption in epoxy composites reinforced with aligned vinyl ester triangular bars: determination of diffusion coefficient and effect of varied factors

Sorption and desorption of moisture in epoxy composites reinforced with vinyl ester triangular bars were studied. Composites were fabricated according to 4 varied factors: bar orientation relative to diffusion direction (pointed or base side), bar alignment (parallel or staggered), spacing between bars (1 or 5 mm), and bar cross-sectional shape (triangular or V-shaped). Unlike previous studies, the bars were initially coated with small amount of epoxy resin to improve wetting during moulding of composites. Weight change of composites during one-side exposure to water vapor (50% relative humidity at 60^oC) and hot air (60^oC) was monitored with time. Experimental results show weight change of composites during moisture sorption and desorption varies linearly with square root of exposure time. Diffusion coefficients of composites were determined by assuming the material to be semi-infinite and fitting the mathematical solution to Fick’s second law of diffusion to weight change data. Data analysis shows diffusion coefficient of composites during moisture sorption is increased when bars are oriented pointed and parallel relative to diffusion direction, spacing is at 1 mm, and bar cross-sectional shape is triangular. However, during desorption, the diffusion coefficient is increased when base side of bars are exposed and when the bars are aligned staggered. The observed effects of factors on moisture diffusion coefficients of epoxy composites during sorption and desorption are discussed in relation to path length, available diffusion area, and status of epoxy/vinyl ester interface before and after moisture sorption in composites.