Preparation of biocomposites based on gluten resin and unidirectional flax fibers

Gluten, a side product of the starch industry, has a long standing track record in food technology. However, recently wheat gluten is also being used in non-food applications such as bio-polymers and bio-composites (Magnuson, KM., 1985; Bietz and Lookhart, 1996; Lagrain et al., 2010). Due to very difficult melt processing of wheat gluten, many methods have been suggested to make gluten composites by using a plasticized matrix, for instance a combination of gluten with glycerol (Kunanopparat et al., 2008; Song et al., 2008; Corradini et at., 2009; Muensri et al., 2010). However, the drawback of this method is that typically low mechanical properties are obtained, with modulus, strength and strain to failure values of the order of 80 MPa, 6.5 MPa and 20 %, respectively. Moreover, in these studies, short fibers have typically been used, which does not lead to the best mechanical performance. In order to improve the mechanical properties of gluten composites, two factors should be taken into account: plasticizers and fiber length. In this work, we would like to suggest a new method for making gluten composites without the need to (semi-) permanently plasticize the gluten and the need to control fiber straightness during compression molding. Here, we combined the effect of water as processing aid with the use of unidirectional flax fibers in order to obtain the strongest properties in the fiber direction. The advantage of using water is that it will evaporate during drying and subsequent thermomolding. Since it is not evident to control the straightness of fibers during the thermo-molding process, a prepreg methodology has been developed. It consists of a dry combing step, followed by a wetting step to increase straightness. After a subsequent drying procedure at 60 °C, to keep the straight configuration, they were wetted again with enough water in order to stick gluten powder on their surface. Finally, the products were dried at room temperature to avoid the reaction sensitivity of gluten. Thus, a prepreg material was obtained. Compression molding was used to make gluten composites from the prepregs. At first, gluten powder is distributed in the mold with a sieve of mesh (1 mm), after which the aligned flax fiber prepregs were put into the mold. Subsequently, another gluten powder layer was passed through the mesh. these steps were repeated till the required fiber volume fraction and composite thickness was obtained. With the new technique, the mechanical properties of the gluten composites have been improved. A bending-modulus of 12 GPa, strengths of 117 MPa and failure strains of approximately 1.3 % at 35 % fiber volume fraction have been obtained. Afterwards, the interfacial adhesion between the flax fibers and the gluten matrix was studied through scanning electron microscopy in order to explain the properties. In addition, it was hypothesized that powder particle size affects on properties of gluten composites.status: publishe