Fused filament fabrication (FFF) notoriously suffers from weak interlayer adhesion, leading to anisotropic mechanical properties in the fabricated parts. The present study addresses this challenge by developing thermoplastic-based vitrimers via reactive extrusion of maleic anhydride-grafted polypropylene (PP-g-MAH) with an epoxy crosslinker and a transesterification catalyst. The vitrimers were further compounded with carbon fibres (CF), processed into filaments, and used for FFF to fabricate specimens for testing interlayer adhesion. Spectroscopic and thermomechanical analyses revealed the formation of a crosslinked network, characterised by β-hydroxyester linkages and a pronounced rubbery plateau. Vitrimers exhibited enhanced mechanical properties, with notched Charpy impact strength increasing by up to 155%. As-printed vitrimeric FFF specimens demonstrated enhanced flexural toughness, indicating that dynamic transesterification reactions during layer deposition promote more consistent interlayer bonding. Vitrimeric specimens exhibited slightly increased flexural strength and preserved toughness upon post-printing thermal annealing at 150 °C, while non-vitrimeric specimens exhibited systematic embrittlement. The results demonstrate that vitrimerisation of thermoplastic polymers is a viable and effective strategy for improving interlayer adhesion in FFF fabricated parts.

This study investigates the non-isothermal crystallization behavior of isotactic polypropylene (iPP) reinforced with two types of talc fillers: micrometric talc (μ-talc) and standard talc (S-talc). Composites containing 20 wt% filler were analyzed using differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and scanning electron microscopy (SEM). Non-isothermal crystallization at cooling rates β = 5–20°C/min was evaluated using the Ozawa, Kissinger, Coats-Redfern, and Criado methods. Both μ-talc and S-talc act as efficient nucleating agents compared with neat iPP, but they operate through distinct mechanisms. μ-talc, owing to its finer particle size and higher surface area, increases the number of nucleation sites, thereby reducing the half-crystallization time (t_1/2) and accelerating the crystallization kinetics. By contrast, S-talc, despite its lower surface area, enhances crystallization by virtue of its lamellar morphology, which promotes selective chain orientation, raises the crystallization temperature (T_c), and stabilizes secondary crystallization. Kinetic analysis confirmed this distinction: the activation energy (E_a) increased from neat iPP (≈185 kJ/mol) to μ-talc/iPP (≈215 kJ/mol) and further to S-talc/iPP (≈274 kJ/mol). The Coats-Redfern and Criado analyses consistently identified a second-order (F2) mechanism for the S-talc composites across all cooling rates, underscoring their stabilizing role in crystallization.

With the increasing use of e-mobility, the demand for high-quality materials to produce polymer gears is growing. Due to the tendency towards using natural materials and reducing carbon footprint, basalt fibers (BF) are tested as a substitute for glass fibers (GF). For this purpose, composites of polyamide 6 (PA6) with GF and BF, with and without compatibilizer and polytetrafluoroethylene (PTFE) are produced, tested, and properties compared. The mechanical, thermomechanical, thermal, and tribological properties of the composites, as well as the size of the fibers after the production of the composites and after injection molding, are determined. The compatibilizer improves the impact strength, while the glass fibers have a better reinforcing effect. The fibers increase crystallinity, but the effect is minimal. The thermal conductivity increases approximately the same for both fibers and is highest at composites that, in addition to 30% fibers, also contain PTFE. The tribological properties are comparable but slightly better for glass fibers. The fiber length is greatly reduced during the production of the composites and is around 200–300 μm in both cases. SEM imaging and mapping analysis show good dispersion of the fibers in the polymer and relatively poor compatibility with PA6.

The study investigates the use of repurposed milled carbon fibre (mCF) as reinforcement for polyamide 66 (PA66) in gear applications, addressing environmental and cost concerns of virgin carbon fibres. Neat PA66 and PA66 composites reinforced with mCF, glass fibres (GF), and carbon fibres (CF), with and without polytetrafluoroethylene (PTFE), were injection moulded and evaluated for microstructure (fibre length), thermal, mechanical, surface, and tribological properties, as well as gear performance under VDI 2736 guidelines. CF reinforced composites showed the highest modulus and tensile strength, followed by mCF and GF. PTFE reduced modulus and strength in binary composites. All reinforced composites significantly lowered the coefficient of friction (COF) and wear rate compared to neat PA66, with mCF showing the most notable improvements. PTFE slightly improved tribological performance only for GF (wear) and CF (COF) composites. In gear testing, binary composites outperformed neat PA66, with CF performing best, followed by mCF and GF. Ternary composites had slightly lower performance than their binary equivalents. Correlation analysis showed that gear performance is closely linked to structural integrity. Failure analysis revealed higher crack susceptibility in mCF reinforced gears due to shorter fibre length. The findings highlight mCF reinforced PA66 as a sustainable, cost-effective material for durable polymer gears.

Crude oil, a natural hydrocarbon mixture, is a key energy source and the petrochemical industry’s primary feedstock. Its large-scale processing produces heavy oil fly ash (HOFA), a solid waste that demands effective management. This study explores HOFA’s valorization as a low-cost, carbon-based filler in isotactic polypropylene (iPP) composites. Composites containing 1–20 wt% HOFA were prepared by extrusion and injection molding, then subjected to tensile, impact, and structural analyses (differential scanning calorimetry (DSC), wide angle X-ray scattering (WAXS), microscopy). Mechanical testing revealed that even small HOFA additions raised Young’s modulus proportionately to filler content, consistent with typical filled-polymer behavior. Impact strength increased by roughly 25% at both 1 and 20 wt% loadings, while tensile strength and elongation at break remained comparable to neat iPP. DSC and WAXS demonstrated that HOFA acts as a nucleating agent, promoting the β-crystalline phase, known to enhance toughness, without significantly altering the melting or crystallization temperatures.
These findings confirm that HOFA, a waste by-product, can be an effective filler for iPP, improving stiffness and impact resistance without compromising other key properties. Its use offers both economic advantages, through reduced material costs, and environmental benefits by diverting industrial waste from disposal. This approach holds promise for developing sustainable, high-performance polymer composites.