JEC COMPOSITES MAGAZINE - Issue #37 - December 2007 - 37

Materials Guidelines for mechanical design with biocomposites: properties, weight and cost Biocomposites are being used increasingly as alternative materials to traditional composites, due to their sustainable profile and their good technical performance. This paper presents guidelines on what to expect in terms of biocomposite mechanical properties, weight and cost. BO MADSEN , SCIENTIST HANS LILHOLT, SENIOR SCIENTIST MATERIALS RESEARCH DEPARTMENT, RISØ NATIONAL LABORATORY TECHNICAL UNIVERSITY OF DENMARK iocomposites are new generations of composite materials where at least one of the constituent fibre and matrix parts are derived from a natural and renewable resource, such as plant-based biomass. As such, biocomposites provide environmentfriendly and sustainable alternatives to traditional composite materials. Based on the knowledge and experience accumulated from research and development over the last few decades, biocomposites are gradually gaining impact in various industries, particularly the automotive industry [1]. Biocomposites being new types of materials in the composite market, some overall guidelines on what to expect in terms of performance would be welcome. Designers and manufacturers in the process of materials selection during product development are in need of such guidelines. This article gives an overview of the range of mechanical properties, weights and costs that are typical for biocomposites, in comparison to traditional glass-fibre-reinforced composites. The article also presents a practical example of a chair designed with biocomposites. Based on calculations of inter-atomic bond energies and distances in cellulose molecules, the theoretical stiffness and strength of crystalline cellulose have been estimated to be about 120 GPa and 15,000 MPa, respectively [2], with a density of 1.64 g/cm3 [3]. These values represent the upper limit for tensile performance of cellulose fibres. However, the structural configuration of cellulose in the cell wall of plant fibres (e.g. degree of crystallinity, microfibril angle, content) considerably restrains the attainable tensile properties of plant fibres. The properties depend on the type of plant fibre. For flax and hemp fibres, you can expect stiffness of about 30-70 GPa, strength of about 300-900 MPa [2], and density of about 1.50-1.60 g/cm3 [3]. Material models for biocomposites In principle, by having knowledge of composite parameters such as fibre content, fibre orientation, and fibre and matrix properties, you can calculate the mechanical performance of biocomposites using the existing material models normally applied for traditional composites. However, in some cases, it has been found that porosity (or void content) takes up a non-negligible part of the total volume of a biocomposite material, and the models therefore need to be modified to account for this. The influence of porosity on the conversion between fibre weight fraction and fibre volume fraction is presented in a recent study by the authors [4]. Likewise, another study is addressing the influence of porosity on mechanical properties of biocomposites by presenting a modified rule-ofmixtures relationship [5]. Structure and properties of plant fibres Plant fibres such as flax and hemp (Figure 1) are the typical choice for reinforcement in biocomposites. Plant fibres themselves are composite materials, with a cell wall consisting of stiff, strong cellulose microfibrils embedded in a matrix of hemicellulose and lignin polymers. Mechanical properties of biocomposites Figure 2 shows examples of experimental data and model predictions for two biocomposites: (i) unidirectional hemp fibre/polyethyleneterephthalate (PET) and (ii) 2D random-oriented flax fibre/polypropylene (PP). The figure shows stiffness of the composites versus fibre weight fraction. The model predicts that stiffness increases non-linearly up to a transition fibre weight fraction that is correlated with the maximum obtainable fibre volume fraction, and stiffness decreases thereafter. For fibre weight fractions below the transition value, the composites contain processing-related porosity (e.g. due to incomplete fibre Fig. 1: Hemp fibres in a twisted yarn configuration No37 December 2007 / JEC Composites Magazine

JEC COMPOSITES MAGAZINE - Issue #37 - December 2007

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #37 - December 2007