Assessment of the thermal, mechanical and morphological properties of polypropylene composites with endogenous bio-based additives

Abstract

Synthetic polymers have become indispensable, driving industry, science and economic development for over a century. However, the unrestricted exponential growth of synthetic polymer production and consumption led to seri-ous problems related to their disposal, mainly since its main routes, till recent times, were landfills. As a result, soil’s and oceans’ plastic waste contamination became a significant hazard for wildlife well-being and human health. Over the recent years, several measures have been implemented to lessen some of the impacts of polymer-based pollution, including more efficient recycling methods and developing degradable and bio-based polymers and com-posites. The latter has gained significant interest and underwent extensive research, especially in what concerns the incorporation of endogenous vegetal fibre waste into the polymer matrix aiming at fulfilling the quality requisites of polymer components and, at the same time, reducing the amount of the synthetic polymer, and so contributing to a more sustainable product. This goal is aligned with the objectives of the project “OLIpush – Redesign for greater circularity and a smaller environmental footprint”. The main objective of the OLIpush project is to redesign the flush toilet system components recurring to environmentally friendly polymer materials and optimized design. The bio-based alternatives must have similar performance and lower cost to be considered a worthy alternative to synthetic polymers. The latter criterion is easily accomplished in the case of endogenous vegetal fibre waste, which is a rela-tively affordable material. However, to guarantee the performance required for the intended application, the proper-ties of the bio-based composites should be thoroughly quantified. To address the challenges mentioned above, the thermal, rheologic, mechanical (tension and flexural modes) and morphological properties of the polypropylene (PP) based composites with 20 and 30% of rice husk (PPrh) and olive pits (PPop) fillers were evaluated. It was conclud-ed that the higher content of the rice husk and olive pits led to a similar reduction of the Melt Flow Index (MFI), about 21% and 38%, for 20% and 30% of the additive content, respectively. Compared to the virgin PP (vPP), the inclusion of the vegetable fillers into the polymer matrix does not alter the melting and crystallization temperatures of the composites in a statistically significant way, independently of the type and content of the fillers. Elastic modu-lus in tensile and flexural deformation modes was improved up to 49% for all tested PPrh and PPop composites compared to vPP. PPrh demonstrated more significant improvement in the flexural modulus and Young’s modulus. For both types of PP composites, the flexural modulus has increased with the fillers’ concentration. The Young’s modulus, however, demonstrated the inverse trend decreasing slightly with increased rice husk and olive pits con-tent. PPrh and PPop composites underwent some reduction in the yield stress and a significant decrease in ductili-ty, rupturing at a much lower strain than vPP. The flexural strength of the PPrh was similar to vPP, especially for 30% of rice husk contents. While in PPop composites, the flexural strength decreased by 7-9% regarding vPP. The morphology analysis has corroborated the mechanical testing data concerning the higher stiffness of the PPrh and PPop composites. The scanning electron micrograph at the fracture location showed the gaps in the polymer matrix caused by pushing the rice husk and olive pits particles, indicating a weak interfacial adhesion between the PP ma-trix and the fillers. The obtained results are quite promising; nevertheless, more research is required to improve further the compatibilization of PP bio-based composites and their mechanical performance.