Physical and morphological studies of polymer blends: a multiscale investigation

Abstract

Polyolefins, polypropylene (PP) and polyethylene (PE), are two commonly used plastics and account for nearly half of all plastic wastes. As the quantity of plastic wastes surpasses millions of tonnes per year, a focus has been placed on combatting polyolefin plastic wastes to achieve a circular plastic economy. Plastic waste management options include landfill, energy recovery and recycling, with the latter being the most attractive environmental option. Currently, the recycling industry is faced with challenges of poor quality and performance polyolefin recyclate. The poor physical and mechanical performance of recyclate is attributed to the difficulty of PP and PE separation during recycling resulting in the formation of immiscible blends; the presence of thermo-mechanical and thermo-oxidative degradation mechanisms; and contaminants. The quality in recyclate will vary from site to site, and for different waste streams. Studies have been carried out to improve the performance of polyolefin recyclate, for example, through the addition of compatibilisers and fillers. However, understanding performance variability of recycled blends for a range of compositions before addition of other components is just as important in order to achieve a circular plastic economy. The primary aim of this thesis was to provide a comparative study of the thermal properties, mechanical properties and phase morphology of virgin and recycled PP: high density polyethylene (HDPE) blends prepared by extrusion mixing and injection moulding. There is a lack of comparative studies over a broad range of compositions for virgin and recycled PP:HDPE blends in the literature. Therefore, a range of blend compositions were investigated from PP:HDPE 10: 90 weight percentage (wt%) up to PP:HDPE 90:10 wt%, along with pure PP and HDPE. To understand the thermal properties of the virgin and recycled blends differential scanning calorimetry (DSC) was carried out. Tensile testing and dynamic mechanical analysis (DMA) were used to determine the mechanical properties and finally, scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to determine the morphology. For the AFM study, a new mode called the Quantitative Imaging (QI) mode was implemented to determine the phase morphology of both the virgin and recycled blends. The thermal studies concluded that both the virgin and recycled blends were incompatible shown by the presence of two peaks in the melting thermograms. Recycled blends had a lower crystallinity, melting and crystallisation temperatures compared to the virgin blends. These observations were attributed to the polymer degradation during recycling. Mechanical studies began with DMA to understand the effect of recycling and blend composition on the storage modulus, along with the relaxation processes present. There was little variation in the storage modulus of recycled and virgin blends, but the alpha and beta relaxation temperatures were lower in recycled blends due to structural deterioration during the recycling process. The tensile properties of recycled blends were not substantially affected by the recycling process in comparison to the virgin blends. Interestingly, the virgin HDPE (vHDPE) up to PP:HDPE 25:75 wt% exhibited a higher yield strength, lower elongation at yield and at break than expected, due to the high chain orientation caused by the injection moulding process. The deterioration in the thermal and mechanical properties of recycled blends are caused by the presence of degradation mechanisms and contaminants, causing shorter polymeric chains and the formation of imperfect crystallites during reprocessing. AFM was used to complement initial SEM images and found nano-phase morphology that was dependent on composition in both the virgin and recycled blends. The Young’s moduli obtained through the QI mode for the virgin and recycled blends were similar in value to the Young’s moduli obtained from tensile testing. The morphology was found to be related to changes in the thermal and mechanical properties illustrating the importance of understand the processing- structure-properties relationship.