Valorisation of natural by-products and wastes as fillers for bio-composites

The present Ph.D. thesis is focused on the use of natural wastes or by-products from agricultural and industrial processes for the production of new materials. Two goals were simultaneously pursued: i) reducing wastes through material re-using in value applications and ii) improving some selected properties in order to achieve the standard performances required for a practical application. One of the principal methods for re-using natural sources, if they are not suitable for direct utilization, is to combine them with neat materials thus obtaining what is normally described as composite. In this field the focus is to maximize the amount of waste employed and minimize the neat materials used. The research carried out during the Ph.D. period is focused on the production of sustainable composites by using natural materials in the form of particles, short fibres, long fibres and fabrics obtained directly from wastes/by-products (e.g. hemp, alfalfa, grape wastes and rice husk ash) or employed as potential recyclable materials (e.g. cotton or hemp fabrics). In combination with the above mentioned fillers, two biodegradable polymers were selected in order to make a completely bio-based and biodegradable composite: the poly(lactic acid) (PLA) and the poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHB). In addition, bio-based but non-biodegradable polymers were also investigated: a fully renewable (PA10.10) and two partially renewable polyamides (PA6.10 and PA5.10). The properties to be improved were selected on the base of possible final applications. In particular, the use of wastes or by-products in the form of particles or short fibres were investigated in order to maximize the use of the wastes without lose excessively the mechanical properties. For this purpose, waste thermal and chemical analyses were carried out to verify their applicability in the matrices. A specific morphological characterizations were used to correlate the mechanical properties to the filler distributions and dispersions. Since one of the bio-based polymer limitations results in the use at relatively high temperatures, the thermo-mechanical property was investigated. The achieved heat deflection temperatures turned out to higher the field of applications of the PAs, especially for PA5.10. Different mathematical models were applied for predicting and verifying the mechanical properties and adhesion performances achieved by the natural fillers. Scale-up activities demonstrated how the best results can be obtained with both hemp and alfalfa by moving from a laboratory to a pilot scale. This fact is a good perspective for future developments in this field of research. In order to maximize the use of wastes or by-products while maintaining high performances, alternative processing solutions such as the impregnation with water-soluble polymers (e.g. starch) or the layer by layer deposition of polyelectrolytes have been investigated with rice husk particles and hemp fibre mats for the production of particle or fibre boards. As one of the composites application field is the building sector, the produced boards were characterized from the thermal, mechanical, aging and flammability point of view. Achieved results demonstrate the possibility of obtaining mechanically robust and flame retarded building panels using wastes and a low amount of polymer as binder. In order to achieve better mechanical properties for automotive applications, the production of composites obtained by compression moulding of stacked natural fabric (i.e. cotton and hemp) and polymer layers (PLA or PHB) was evaluated. The prepared materials yielded the best mechanical characteristics among the composites under study in this thesis with some added features, i.e. an improved impact resistance, which is hardly obtained with the other approaches. Such performances were compared to standards for building panels or automotive application, reporting satisfactory results. Finally, the use of a surface pre-treatment of the fibres exploiting either an epoxy coupling agent or the layer by layer assembly of a nanostructured coating were attempted in order to increase the compatibility between fabric and matrix and thus the mechanical performances. Both solutions improved the good surface adhesion with comparable improvements in mechanical properties.