| dc.description.abstract | Polyolefins are versatile and widely used synthetic polymers but also significantly contribute to the accumulation of plastic waste. Two of the most used polyolefins, polypropylene (PP) and polyethylene (PE) have similar densities, which makes it difficult to separate them, and they often exist together as mixed plastic waste. Developing effective strategies for recycling and reusing these polyolefins is essential for mitigating pollution and promoting sustainable plastic management. Polymer blending is a cost-effective method for recycling, but the immiscibility between different polyolefins limits the development of blends with high performance. In the case of multilayer plastic packaging (MPP), a representative form of mixed plastic waste, recycling becomes even more challenging because its complex structure inherently contains many dissimilar polymers. This dissertation is concerned with reclaiming polyolefins from MPP structure and improving the performance of immiscible PP/PE blends by employing microcrystalline cellulose (MCC) as a bio-derived compatibilizer.
Chapter 1 provides an overview of recent advances in compatibilization strategies aimed at enhancing the PP/PE blend performance, focusing on using bio-derived compatibilizers. The mechanical properties of PP/PE blends compatibilized by various approaches, including non-reactive, reactive, and bio-derived methods, are summarized and evaluated in terms of their respective advantages and limitations. In addition, the rheological and crystallization behaviors of these compatibilized blends are reviewed to elucidate the processing-structure-property relationships. Following this comprehensive literature review, several specific research objectives are outlined, aiming to provide sustainable solutions for recycling polyolefins from MPP and enhancing PP/PE blend performance.
Chapter 2 examined the feasibility of recycling and reusing MPP by direct polymer thermal blending. The effects of multiple cycles of thermomechanical reprocessing on the properties of a low-density polyethylene (LDPE)-based MPP blend containing ethylene vinyl alcohol (EVOH) as the barrier layer were evaluated. The MPP blend underwent six cycles of thermomechanical reprocessing, including thermal compounding, grinding, and injection molding. The mechanical, morphological, thermal, and rheological properties were characterized to track property changes with increasing reprocessing cycles. The results indicate that direct polymer blending can effectively recycle MPP composed of LDPE and EVOH as structural and barrier layers for up to four cycles without significant loss of mechanical performance. Beyond four cycles, marked declines in properties were observed, primarily resulting from the degradation of EVOH barrier material in the reprocessed blend, which caused gelling and color change issues.
In Chapter 3, a solvent-based selective dissolution process was investigated to remove EVOH before recycling the polyolefins through thermal compounding. A five-layer MPP film consisting of LDPE structural layers, maleic anhydride-grafted linear LDPE (MA-g-LLDPE) tie layers, and an EVOH barrier layer was treated with formic acid to selectively remove the EVOH. The polyolefins reclaimed after EVOH removal were reprocessed into a blend through thermal compounding. Mechanical testing revealed that the reclaimed polyolefin blend exhibited improved performance compared to the MPP blend obtained by direct reprocessing, and its properties were comparable to those of virgin LDPE. Analysis of the chemical structures and rheological behaviors of the reclaimed LDPE indicated that chemical changes in the MA-g-LLDPE tie layer played a key role in governing the properties of the reclaimed blend. These findings demonstrate that solvent-based selective dissolution is a promising approach for removing EVOH and producing reclaimed polyolefins with satisfactory properties.
Considering the difficulty of separating PP and PE in mixed polyolefin waste, Chapter 4 investigated the properties of PP/HDPE blends with different weight ratios through direct thermal compounding. Nanometer scale HDPE phases dispersed within PP, co-continuous phases, and micrometer scale PP phases dispersed within HDPE were formed at the PP/HDPE weight ratios of 75/25, 50/50, and 25/75, respectively. Mechanical, morphological, and rheological properties were systematically characterized, along with the study of non-isothermal crystallization kinetics. The results indicate that phase morphology played a dominant role in determining the properties of the blend. In particular, the nanoscale dispersion of HDPE within the PP matrix significantly influenced the viscoelasticity and crystallization rate of the PP/HDPE blend.
In Chapter 5, a sustainable strategy was explored to enhance the performance of the PP/HDPE blend. A bio-derived compatibilizer, MCC, was incorporated into the PP/HDPE blend in combination with MAPE to improve its performance. The results showed that the addition of MCC improved the mechanical performance of the PP/HDPE blend, and further enhancement was achieved with the addition of MAPE. This improvement was attributed to the formation of core–shell structured particles, where MCC served as the core and MAPE entangled HDPE formed the shell at the interface.
In summary, this dissertation investigated strategies for recycling polyolefins from complex MPP structures and demonstrated the potential of the bio-derived compatibilizer MCC to enhance the performance of polyolefin blends. Future work could focus on conducting life cycle assessments to evaluate the sustainability of the methods developed in this dissertation. | en_US |
