Manufacturing of starch-based materials using ultrasonic compression moulding (UCM): toward a structural application

An experimental study of the ultrasonic compression moulding (UCM) to manufacture biobased composites made of semicrystalline starch powders and softwood fibres is described. The main objective was to assess the potential of using this fast and economical processing technique to elaborate a 100% biobased composite which might substitute more usual polymer materials for structural applications. The starch powder was chosen as raw material for the matrix while the reinforcement was made of softwood fibres. Tablets made of starch only and composite beams were processed under different conditions and characterised by several techniques. Three types of starch powders and two types of fibres were used as raw materials. A morphological and crystalline analysis was carried out by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The native semicrystalline structure of starch granules was not totally preserved so as to obtain a homogeneous material. Diametral compression tests on tablets were performed to improve the processing route and obtain the materials with the best properties. Bending tests were used on composite beams to quantify the mechanical properties and study the effects of the processing parameters. The optimum processing parameters were defined and allowed obtaining a matrix for which the flexural strength reached 21 MPa. Mechanical properties were improved when fibres were added into the matrix: three-points bending tests showed a Young's modulus of 6 GPa, a flexural strength of 75 MPa and a flexural strain at break of 6% for a bulk density of 1.25. Considering these results, UCM appears to be a promising process to design a 100% biobased composite with mechanical properties comparable to those of classical discontinuous fibre composites.