Impact of hot water aging on the mechanical performance and shape memory behavior of 4D-printed flax fiber-reinforced PLA/PETG composites

This work investigates how hot-water aging affects the mechanical and shape-memory properties of flax fibre-reinforced PLA/PETG composites for 4D printing. While flax reinforcement increases stiffness (Young’s modulus from 3.6 GPa to ≈ 4.0 GPa at 15 wt%), it also markedly increases water uptake because of fibre hygroscopicity: the neat PLA/PETG absorbed < 2 % at saturation, whereas the 15 wt% composite reached ≈ 20 % after 67 days. Aging at 45 °C caused pronounced stiffness and strength losses (up to 44 % and 69 %, respectively), consistent with SEM evidence of fibre swelling, interfacial debonding, matrix fragmentation and increased porosity. FTIR revealed intensified O–H bands and XRD revealed structural reorganization, including secondary recrystallization in PLA, corroborating hydrolytic degradation. Despite these degradation phenomena, the shape-memory functionality remained largely preserved, with high initial performance (fixity ≈ 100 % and recovery ≈ 100 % for the neat blend) and only a moderate reduction in shape fixity (S f ≈ 90 % at 15 wt%), accompanied by a slight decrease in shape recovery (S r ) after aging. These results demonstrate that shape-memory performance can be maintained even under severe hydrothermal exposure, addressing a critical knowledge gap in the functional durability of 4D-printed natural fiber–reinforced composites and providing a foundation for the development of more robust structures operating in humid environments. • First study linking hydrothermal aging to shape memory in flax/PLA-PETG 4D prints. • Novel FFF fabrication of bio-based 4D composites with 5–15 wt% flax reinforcement. • Hydrothermal aging (45 °C) caused up to 69 % strength loss, 44 % modulus loss. • Shape fixity stayed ≈ 90 % after aging; recovery ≈ 97 % for 15 wt% flax composites. • FTIR/XRD + SEM revealed hydrolysis, recrystallization and fiber–matrix debonding.