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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.
(Published on December 2007 – JEC Magazine #37)
BO MADSEN , SCIENTIST
HANS LILHOLT, SENIOR SCIENTIST MATERIALS RESEARCH DEPARTMENT, RISØ NATIONAL LABORATORY TECHNICAL UNIVERSITY OF DENMARK
Biocomposites 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 . 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.
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.
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 , with a density of 1.64 g/cm3 . 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 , and density of about 1.50-1.60 g/cm3 .
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 . Likewise, another study is addressing the influence of porosity on mechanical properties of biocomposites by presenting a modified rule-ofmixtures relationship .
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 impregnation), which restrains the increase in stiffness. For fibre weight fractions above the transition value, a component of structural porosity is developed because the available matrix volume is insufficient to fill the free space between fibres. It can be observed in Figure 2 that the experimental data are well predicted by the model lines.
Using a similar model approach as above, Figure 3 shows overall guidelines for mechanical properties (stiffness) of biocomposites with different fibre orientations: 1) unidirectional, 2) 2D random, and 3) 3D random.
For each fibre orientation, three curves are shown where the plant fibres are set to have effective stiffnesses of 60, 50 or 40 GPa, which are believed to be characteristic for plant fibres. The matrix in the composites is assumed to have a stiffness of 1.5 GPa and a density of 1.0 g/cm3, i.e. typical properties of a thermoplastic polymer like polypropylene. The predictions in Figure 3 should be considered to reflect the optimal performance of the composites in the situation of no processing-related porosity and perfect fibre/matrix interface. The guidelines in Figure 3 show that with the given setting of the model parameters, the maximum obtainable stiffness for biocomposites with the three fibre orientations is in the ranges of 20-30 GPa for unidirectional, 7-11 GPa for 2D random and 3-5 GPa for 3D random. In the case of a specific biocomposite material, the model predictions should obviously be adjusted to reflect the actual material, as shown by the examples in Figure 2.
In comparison to glass fibres, plant fibres typically have lower stiffness (about 50 GPa vs. 70 GPa), and the related composites therefore also have lower maximum obtainable stiffness. This is shown in Figure 4A, which shows model predictions for plant- and glass-fibre composites with a 2D random fibre orientation. The figure demonstrates a commonly made observation that glass-fibre composites are stiffer than plant-fibre composites.
Weight of biocomposites
A key technical advantage of plant fibres is their relatively low density of about 1.50 g/cm3. The combination of low density and good mechanical properties of the fibres means that biocomposites can be made with excellent weight-based mechanical properties (also denoted specific properties). The density of glass fibres is about 2.60 g/cm3. Figure 4B shows predictions of stiffness per weight for plant-fibre and glass-fibre composites. It is demonstrated that even though stiffness per volume is lower for plantfibre composites (Figure 4A), stiffness per weight is generally higher (Figure 4B).
Cost of biocomposites
Material cost is of critical importance in most industrial products. Even though plant fibres are derived from low-cost biomass resources, the manufacturing steps required 1) to grow the plants, 2) to extract fibres from plants, and 3) to process the fibres into suitable preforms for composites all add costs to the use of plant fibres as reinforcement in composites. However, for composites with a 2D random-fibre orientation, the cost for plant-fibre preforms is relatively low. The cost of non-woven mats of plant fibres is about €1.8/kg. The related cost of glass-fibre strand mats is about €2.6/kg. (The stated costs are overall costs, not taking into account the influence of quality and quantity of purchased products).
Figure 4C shows predictions of stiffness per cost for plant-fibre and glass-fibre composites, and it is demonstrated that the low cost of non-woven plant fibre preforms promotes a high stiffness per cost for the composites.
Design with biocomposites
As shown in the above analysis of the technical performance of biocomposites, these new generations of composites are realistic potential alternatives to traditional composites in selected products and applications.
Figure 5 shows an example of a chair designed with biocomposites and produced at Risø National Laboratory .