A STUDY OF SURFACE MODIFICATION EFFECT OF HEMP FIBERS ON THE BULK PROPERTIES OF HEMP-POLY (LACTIC ACID) COMPOSITES: THERMAL STABILITY, MECHANICAL, THERMO-MECHANICAL AND BIODEGRADABILITY

Abstract

Biocomposites made with, natural fiber and bio-based polymers, have many advantages over their synthetic counterparts including low cost, low density, high strength and biodegradability. However, some biocomposites can present problems due to high moisture absorption, low thermal stability during processing, and poor adhesion between the fiber and polymer matrix. Recent studies have shown that surface modification of the fiber can improve its adhesion to the polymer matrix and enhance the bulk material properties. Nevertheless, the mechanisms by which such surface modifications exert their effects on bulk material properties have not been systematically studied. Therefore, the main goal of this study is to investigate the impact of surface modifications of hemp on the thermal stability, mechanical, thermo-mechanical, and biodegradability of biocomposites comprised of hemp and poly (lactic acid) (PLA). This pairing was selected because it offers superior mechanical properties. The three surface treatments tested were: alkali (mechanical interlocking), silane (coupling) and acetic anhydride (grafting). The latter was most effective at improving thermal stability, mechanical, and thermo-mechanical properties of hemp-PLA biocomposites, and all treatments improved these properties relative to untreated hemp-PLA controls. The thermal stability of the composites increased with an increase in fiber content up to 30% by fiber volume fraction for both silane and acetic anhydride modified hemp. However, thermal stability decreased with fiber content for alkali and untreated composites due to hydrogen bonding and inferior fiber-matrix adhesion, respectively. The activation energy of thermal degradation was assessed by applying Flynn-Wall-Osawa kinetic modeling to understand the fiber-matrix interface. The model predictions were consistent with experimental results and suggested that the mechanism by which, acetic anhydride treatment yielded superior thermal properties was related to high energy bond formation (C=O) between the fiber and polymer matrix. When tensile and flexural properties of composites were assessed, 30% fiber volume fraction was optimal, and this ratio also improved stiffness and damping properties of the composites during thermo-mechanical study. A biodegradability study of the treated and untreated hemp-PLA biocomposites was undertaken. ASTM standard 5511-11 was modified to stimulate landfill disposal conditions. Degradation of all treatments as well as untreated biocomposites was negligible over 50 d, although visual inspection of SEM images showed greater evidence of cracking in the composite samples than in pure PLA controls. From this study it can be concluded that higher bond energy at the fiber-matrix interface due to surface modification of natural fiber results in higher activation energy of thermal degradation resulting in enhanced bulk material properties of the biocomposites