Design and Evaluation of Recycling-Driven Polymer–Matrix Composites for Lightweight Automotive Parts

Thesis Abstract

Recycling-driven polymer–matrix composites (PMCs) hold promise for reducing vehicle weight and environmental impact, yet their adoption hinges on achieving reliable mechanical performance, processability, and lifecycle sustainability comparable to conventional materials. This study investigates the design, fabrication, and evaluation of PMCs reinforced with recycled carbon and glass fibers and reinforced by recycled polymer matrices to produce lightweight automotive parts with robust performance. The aim is to develop a design framework and validated material system that integrates waste streams from composite and polymer industries into high-performance automotive components. Specific objectives are (1) to characterize the morphology, particle size distribution, and interfacial adhesion of recycled fibers and recycled thermoplastic matrices; (2) to optimise composite processing parameters for scale-efficient, defect-minimised fabrication using extrusion-compounding and compression moulding; (3) to quantify mechanical properties (tensile, flexural, impact, and fatigue) across temperature ranges representative of automotive service conditions; (4) to model the influence of recycled constituent content on stiffness, strength, and damping via regression, ANOVA, and multi-factor DOE; (5) to evaluate environmental and economic performance through life-cycle assessment (LCA) and cost analysis; and (6) to propose a design toolkit and material selection guidelines for automotive parts such as bumper beams, dashboards, and interior trims. The methodology adopts a mixed-methods design anchored in materials science and engineering experimentation complemented by lifecycle assessment. The population comprises commercial recycled carbon and glass fibers and recycled thermoplastic matrices sourced from automotive dismantling streams and industrial waste processing facilities. A stratified sampling approach selects three fiber/matrix combinations with varying recycled content (20%, 40%, 60% by volume) for fabrication of coupon specimens and representative component-scale panels. Data collection employs differential scanning calorimetry (DSC) and thermomechanical analysis (TMA) to determine cure and thermal stability; scanning electron microscopy (SEM) for interfacial morphology; dynamic mechanical analysis (DMA) for viscoelastic properties; universal testing machine (UTM) for tensile and flexural tests; impact testing (Charpy and Izod) and fatigue testing under variable amplitude loading. Processing parameters are optimised via design of experiments (DOE) with response surfaces to identify optimal extrusion temperatures, screw speed, and moulding pressures that minimise porosity and fibre misalignment. Validity and reliability are established through repeat measurements (n=5 per condition) and calibration protocols for instruments. Data analysis employs regression models to relate recycled content and processing variables to mechanical responses, ANOVA to assess significance, and multi-objective optimization to balance strength, weight, and cost. An analytical framework links microstructure findings to macroscopic properties, incorporating the Halpin–Tsai model for predictive stiffness and a generalized Rule of Mixtures for strength. The study also conducts a cradle-to-grave LCA using ISO 14040/14044 standards and a simple payback period model to evaluate economic viability. Ethical considerations address responsible sourcing of recycled materials and compliance with safety and environmental regulations. Expected findings indicate that recycled PMCs can achieve 15–25% weight reduction relative to conventional polypropylene-based composites while delivering comparable modulus (10–25 GPa) and strengths suitable for non-structural automotive components with acceptable impact resistance. It is anticipated that increasing recycled content will modestly reduce fatigue life but can be compensated through optimised fibre–matrix interfacial coupling via coupling agents and surface treatments. The regression and DOE analyses are expected to reveal statistically significant interactions between recycled content, processing temperature, and fibre orientation on mechanical performance, with the optimal balance found around 40–50% recycled content. The LCA is projected to show lower global warming potential and comparable energy consumption for certain supply chains, though end-of-life recovery remains a key consideration. The study contributes to knowledge by providing a transparent design framework, validated performance data for recycling-based PMCs, and a decision-support toolkit linking material composition, processing, and component-level performance for automotive applications. The main conclusion will be that recycling-driven PMCs can meet targeted performance criteria for lightweight automotive parts when combined with optimized processing, effective interfacial compatibility, and rigorous quality control. Recommendations include scalable manufacturing guidelines, targeted surface treatment strategies to enhance fibre–matrix bonding, expansion of recycled content ranges to meet specific component requirements, and policy-informed recommendations to improve recycling streams and end-of-life processing to sustain material value.