Carbon fibre-reinforced polypropylene vitrimers for fused filament fabrication: Enhancing interlayer adhesion through transesterification-based dynamic covalent chemistry

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.