Water absorption of hemp fiber/unsaturated polyester composites

The water absorption of hemp fiber/unsaturated polyester composites was determined by immersing the samples in water or by exposing them to air with a relative humidity of 94%. The water absorption increased with increasing fiber content. By using images obtained with a magnetic resonance imaging (MRI) system, the moisture absorption process was shown to follow a diffusion mechanism and to be more important in the longitudinal than in the transverse direction. The longitudinal diffusion coefficient was computed to be about 3 x 10(-11) m(2)/s. Composite samples immersed in water reached saturation levels after about eight months and showed no signs of cracking due to swelling. Fibers reached the same saturation limit whether submerged in water or exposed to saturated air when fiber content was less than 21 vol%. Various fiber treatments were tested but none resulted in a substantial increase of the resistance to water absorption. The most effective technique to enhance moisture resistance was to properly seal all the fibers within the matrix.