Effect of water absorption on the mechanical properties of poly(3-hydroxybutyrate)/vegetable fiber composites

The present work studies the effect of water absorption on the performance of composites of poly(3-hydroxybutyrate) (PHB) – a fully biodegradable semi-crystalline thermoplastic obtained from renewable resources through low-impact biotechnological process, biocompatible and non-toxic – and vegetable fiber from the fruit (coconut) of babassu palm tree.Water resistance is an important characteristic of structural composites, that may exposed to rain and humid environments. Both water absorption capacity (water solubility in the material) and the rate of water absorption (controlled by the diffusivity of water in the material) are important parameters. However, water absorption per se may not be the most important characteristic, insofar as the performance and applications of the compounds. It is the effect of the water content on the ultimate properties that determine the suitability of the material for applications that involve prolonged exposure to water.PHB/babassu composites with 0–20% load were prepared in an internal mixer. Two different types of babassu fibers having two different article size ranges were compounded with PHB and test specimens molded by compression. The water absorption capacity and the kinetic constant of water absorption were measured in triplicate. Mechanical properties under tension were measured for dry and moist specimens with different amounts of absorbed water.Results indicate that the performance of the composites is comparable to that of the pure matrix. Water absorption capacity increases from 0.7% (pure PHB) to 4% (PHB/20% babassu), but the water diffusivity (4·10□8 cm2/s) was found to be virtually independent of the water absorption level. Water absorption results in moderate drop in elastic modulus (10–30% at saturation, according to fiber content) but has little effect on tensile strength and elongation at break. Fiber type and initial particle size do not have a significant effect on water absorption or mechanical properties.