Mechanical properties of alkali treated plant fibres and their potential as reinforcement materials. I. hemp fibres

In this study a thorough analysis of physical and fine structure of hemp fibre bundles, namely surface topography, diameter, cellulose content and crystallinity index, have been presented. The fibre bundles have been alkalised and physical and mechanical properties analysed. Alkalisation was found to change the surface topography of fibre bundles and the diameter decreased with increased concentration of caustic soda. Cellulose content increase slightly at lower NaOH concentrations and decrease at higher NaOH concentrations. The crystallinity index decrease with increase in caustic soda concentration up to 0.24% NaOH beyond which, it decreases with increase in NaOH concentration. It was also found that the tensile strength and stiffness increases with increase in the concentration of NaOH up to a limit. Tensile strength and Young’s modulus increase with decrease in cellulose content, while crystalline cellulose decreases slightly but with improved crystalline packing order resulting in increased mechanical properties. Similar observations are elucidated by the crystallinity index. Alkalised hemp fibre bundles were found to exhibit a similar specific stiffness to steel, E-glass and Kevlar 29 fibres. The results also show that crystallinity index obtained following alkalisation has a reverse correlation to the mechanical properties. Stiffer alkalised hemp fibre bundles are suitable candidates as reinforcements to replace synthetic fibres. The improvement in mechanical properties of alkali treated hemp fibre bundles confirms their use as reinforcement materials. C 2006 Springer Science + Business Media, Inc. 1. Introduction The strength of plant fibres is attributed to the rigidity and high molecular weight of cellulose chains, intermolecular and intramolecular hydrogen bonding and fibrillar and the crystalline structure of the fibres [1]. The strength and stiffness of fibres have also been shown to be dependent on the crystallinity index and micro-fibril angle. McLaughlin and Tait [2] have showed that strain is more dependent on the micro-fibril angle and that it increases with increase in the micro-fibril angle. Fibres with higher cellulose content have also been found to be stronger than those with low cellulose content as long as their micro-fibril angle is small. Table I [3–5] shows physical and mechanical properties of ∗Author