Extrusion processing of wood-based biocomposites

Abstract: Interest in biocomposite materials and their use in various applications has been growing steadily over the past 10-15 years. Increasing environmental awareness and lower material costs are the main driving forces for using renewable materials, such as wood and cellulose fibers, as reinforcement in polymer composites. In addition to the materials used in the composite preparation, the processing has a large impact on the final properties of the composite. Therefore, the main thrust of this work has been on processing of wood-based biocomposite materials using twin-screw extrusion. In the first part of the work (Papers I, II and III), wood-plastic composites were manufactured using twin-screw extrusion. Currently, wood flour consisting of small wood particles with low aspect ratio is used as the main wood raw material in commercial woodplastic composites. A better reinforcement is achieved by using wood fibers with a higher aspect ratio, but individual fibers are seldom used in composite manufacturing due to processing problems and higher cost. Therefore, the objective of the first part of this work was to study the use of wood chips as raw material in wood-plastic composites and the possibility to separate individual fibers from the wood chips during the composite manufacturing process. The effect of extrusion parameters and raw materials on the aspect ratio of the wood particles/fibers and on the mechanical properties of the composites was evaluated. The study showed that wood chips can be used as raw material in a one-step process for manufacturing wood-plastic composites, and that it is possible to separate individual fibers with a higher aspect ratio than wood flour from the wood chips under suitable processing conditions. The second part of the work (Papers IV and V) focused on the twin-screw extrusion of cellulose nanocomposites. The use of nanosized cellulose fibers to reinforce polymer matrices has many benefits over the macrosized fibers, such as the high surface area and large aspect ratio. However, the preparation of cellulose nanocomposites is more complicated due to the high hydrophilicity and aggregation tendency of nanocellulose, meaning that drying of the nanofibers is not recommended when good dispersion of nanofibers is needed. Therefore, the aim was to study the processing of green cellulose nanocomposites with twin-screw extrusion using thermoplastic starch as the matrix polymer and cellulose nanofibers with high water content as the reinforcement (Paper IV). In addition, the effect of twin-screw extrusion on separating micro/nanoscale fibers from cellulose fibers during the compounding of biocomposites was studied (Paper V). The fibrillation of nanocellulose is a highly energy intensive process; therefore, it would be very beneficial if it could be done at the same step as the compounding of the composites. The preparation of thermoplastic starch and composite compounding was performed in one step, and the effects of extrusion compounding on the dispersion of the cellulose nanofibers, on the micro/nanofibrillation of cellulose fibers, and on the composites’ mechanical, optical and moisture absorption properties were studied. The results showed that some aggregation of cellulose nanofibers occurred during the extrusion process, but that the addition of cellulose nanofibers had a positive effect on the properties of the prepared bionanocomposites. Nanofibrillation of cellulose was not accomplished using the selected processing conditions; however, dispersion of the fibers was enhanced.