Valorisation of waste mussel shells and harakeke fibres for enhanced performance in polypropylene composites

Abstract

This doctoral research develops high-performance, environmentally sustainable polypropylene (PP) composites by valorising low-value biogenic waste. Within a Circular Economy framework, it addresses challenges in plastic sustainability and the need for bio-based alternatives by utilising mussel shells (MS), an aquaculture by-product, and harakeke fibres (HF, Phormium tenax), derived from agricultural waste, as reinforcing materials. Through systematic characterisation, surface modification, filler hybridisation, and multi-scale evaluation, this study demonstrates the successful transformation of these biogenic low-value resources into functional reinforcements for PP. The structural and surface properties of MS-derived fillers were first investigated, focusing on functionalization with a mussel-inspired polydopamine (PDA) coating. Solid-state nuclear magnetic resonance (SS-NMR) and Fourier-transform infrared spectroscopy (FTIR) confirmed PDA formation, revealing its indole/indoline units and quinonoid groups. X-ray diffraction (XRD), SS-NMR, and FTIR showed that MS contains calcite and aragonite phases, which remain intact after coating. Thermogravimetric analysis (TGA) confirmed the thermal stability of MS, slightly improved by PDA, while X-ray photoelectron spectroscopy (XPS) verified successful coating deposition on MS fillers via nitrogen-containing groups. Surface energy analysis revealed that PDA coating increased MS filler hydrophilicity, whereas maleic anhydridegrafted polypropylene (MAPP) treatment particularly when combined with PDA enhanced filler hydrophobicity, establishing PDA/MAPP co-modification as an effective strategy to improve filler interaction with the hydrophobic PP matrix. Polypropylene (PP) composites reinforced with pristine, maleic anhydride-grafted polypropylene (MAPP)-modified, and PDA/MAPP co-modified mussel shell (MS) fillers were systematically compared with neat PP to assess thermal and mechanical performance. Thermogravimetric analysis (TGA) revealed improved thermal stability across all composites, most notably with PDA/MAPP-MS. X-ray diffraction (XRD) and differential scanning calorimetry (DSC) confirmed that MAPP and PDA/MAPP surface treatments enhanced nucleation and crystallinity, promoting -crystal formation in the PP matrix. Mechanical testing showed that unmodified MS reduced tensile and flexural strength, an effect mitigated by MAPP modification. PDA/MAPP co-modification yielded the greatest improvements, with tensile strength, flexural strength, and modulus all significantly enhanced at 40 wt.% loading. Dynamic mechanical analysis (DMA), creep recovery, and melt rheology further supported the advantages of PDA/MAPP comodification, consistent with scanning electron microscopy (SEM) observations of improved interfacial bonding. The study further explored hybrid reinforcements combining MS with HF. Crystalline structure analysis showed both fillers acted as nucleating agents, with hybrid systems producing higher crystallinity than neat PP. XRD confirmed the co-existence of - and -crystals. Composites with 10% MAPP-MS/30% HF and 10% PDA/MAPP-MS/30% HF showed the highest -phase content (17.32% and 16.71%, respectively), enhancing toughness and elongation while retaining strength and stiffness. SEM backscattered electron (BSE) analysis confirmed improved fibre matrix adhesion and polymer bridging, while energy-dispersive X-ray spectroscopy (EDS) mapping showed uniform filler distribution. These hybrid systems outperformed single-filler composites, with 10% MAPP-MS/30% HF achieving a 48% increase in tensile strength over neat PP. DMA confirmed a higher storage modulus, improved energy dissipation, and better adhesion. Creep-recovery tests demonstrated greater dimensional stability, particularly for the 10% PDA/MAPP-MS/30% HF system. Melt rheology behaviour suggested the formation of a hybrid filler network that further restricts chain mobility. Finally, composites with 5% PDA/MAPP-MS fillers demonstrated superior UV stability. After 1,000 hours of accelerated weathering, neat PP exhibited cracking, roughening, and discolouration, while PDA/MAPP-MS composites retained smooth, intact surfaces. FTIR confirmed their lowest carbonyl index increase, reflecting suppressed photo-oxidative degradation. XRD and DSC showed a stable crystalline structure, and mechanical testing revealed only a 15.6% tensile strength loss, compared to 62% for neat PP. DMA further confirmed superior viscoelastic stability. These results indicate dual protection: MS act as UV shield, while PDA scavenges free radicals to delay degradation. Overall, this research establishes mussel shell and harakeke fibre as sustainable, high-performance reinforcements for PP. PDA/MAPP co-modification and hybridisation strategies optimise filler matrix interactions, yielding composites with enhanced thermal, mechanical, rheological, and weathering properties. These findings highlight the potential of low-value biogenic fillers for sustainable, durable, and environmentally resilient composites.