New Functionalization - Reinforcement Strategies for Cork Plastics Composites: Opening A Wide Range of Innovative Applications for Cork Based Product

Advances on forest-based composites, shows a growing trend in the use of lignocellulosic materials as filler and/or reinforcement in plastic composites. Cork-Polymer Composites (CPC) is one of the most promising fields in cork technology to produce new materials based on sustainable development. Cork combined with polymer matrices, has the prospective of lead to composite materials with better properties. In this context, new fields of application where cork cannot compete alone might be reached. The present thesis focuses on the investigation of engineering properties of the thermoplastic matrices, such as High Density Polyethylene (HDPE) and Polypropylene (PP), combined with cork trough melt based technologies, by taking advantage of their intrinsic properties to create cork-based composites. Towards to reach more bio-based composite materials, cork was combined with biodegradable aliphatic polyesters mainly, PolyhydroxyButyrate-co-HydroxyValerate (PHBV); Poly(L-Lactic Acid) (PLLA); Poly-epsilon- CaproLactone (PCL) and Starch-Poly-epsilon-CaproLactone (SPCL). Cork and its by-products obtained from finishing industrial operations and end-of-life products were compounded, promoting added-value to cork based composites. The process methods used to create CPC were either pultrusion or twin-screw extrusion processes. Whereas, compression moulding and injection moulding were used to obtain the final composite product. During this process, the compatibility and adhesion between the polar cork and the non-polar polymer is one of the key challenges. Reinforcement strategies show that the lignocellulosic--matrix compatibility was improved by (i) via the structure of matrix, by employing coupling agents (CA) based on maleic anhydride, (ii) the use of natural fibre, or by modifying the fibre surface (i.e. hybrid composites). Indeed, all of them lead to cork based composites with considerably better mechanical properties. In addition, the interfacial adhesion of cork-polymer was also improved either using suberin or lignin isolated from cork byproducts as bio-based coupling agents with significant benefits for the environment. The findings presented in this thesis show that the general properties of CPC materials reveals the required: (i) dimension stability with reduced water absorption, (ii) homogeneous distribution and dispersion of the cork particles in the polymer matrix, (iii) improved fire resistance to the matix, good thermal and acoustic insulation properties and (iv) an interesting range of mechanical properties. As for as bio-based composite materials, it was engineering a class of cork biocomposites more sustainable with acceptable in-service performance and tendency for rapid out-of-service biodegradation. Lignocellulosic materials such as natural fibres and cork also offer economic and environmental advantages over traditional inorganic reinforcements and fillers. The work described in this thesis brings new knowledge and contribute to a deeper understanding in the promising cork-polymer composites (CPC) materials field. Overall, the findings presented in this thesis make a significant contribution to better understand the CPC materials. Therefore, the combination of cork with polymeric matrices reveals to be a significant added-value to cork based materials, with high potential for a wide range of innovative applications.