Poly(lactic acid)/flax composites: effect of surface modification and thermal treatment on interfacial adhesion, crystallization, microstructure, and mechanical properties

In this study, unidirectional flax fabrics were used to reinforce poly(lactic acid) (PLA). Flax/PLA composites were produced by thermo-compression using as-received flax fibers and titanium dioxide (TiO2) coated flax fibers. The aim was to investigate the effect of annealing temperature and time, under quiescent or mechanical stress conditions, on the microstructure, interfacial adhesion, crystallization, and mechanical properties of composites. Results showed significant differences between oxidized-TiO2-coated flax fibers/PLA composites (oxi-TiO2-C) and as-received flax fibers/PLA composites (AR-C). Indeed, above the glass transition temperature (Tg) of PLA, the storage modulus (E’) of both composites significantly increased due to PLA crystallization. However, once E’ reached a maximum value, composites with treated flax fibers leveled off while the E’ of composites with untreated flax fibers dropped drastically. This peculiar result was explained by a partial destruction or damage of AR-C interface during PLA crystal growth. This phenomenon was not observed with oxi-TiO2-C, suggesting a good interfacial adhesion. Despite the presence of a trans-crystalline layer for both samples, it was shown that TiO2 coating of flax fibers strengthened the adhesion between flax fibers and the trans-crystalline structure. Interlaminar shear strength measurements revealed a 15% increase in the interfacial strength.