Influence of alkali treatment on flax fiber for use as reinforcements in polylactide stereocomplex composites

The flax and equivalent proportion of poly(l ‐lactic acid)/poly(d ‐lactic acid) (PLLA/PDLA) were melt compounded and injection molded to prepare flax‐reinforced polylactide stereocomplex (sc‐PLA) bio‐composite, and the effect of alkali treatment on the structure and properties of flax as well as the flax/sc‐PLA composite was investigated. SEM and FTIR results showed hemicellulose in flax was almost completely removed after alkali treatment and the treated flax (ALK‐flax) bundles were more separated with a cleaner surface than untreated flax (UN‐flax). DSC results showed homo‐crystallites (hc, T_m = 160–170°C) and stereocomplex crystallites (sc, T_m ～210°C) coexisted in sc‐PLA and flax/sc‐PLA composites. Compared with sc‐PLA, the total crystallinity and sc‐crystallinity of flax/sc‐PLA composite increased regardless of whether the flax were treated with alkali, whereas ALK‐flax/sc‐PLA composite showed a little higher crystallinity than UN‐flax/sc‐PLA composite. TGA results confirmed ALK‐flax/sc‐PLA composite had a higher thermal degradation temperature than UN‐flax/sc‐PLA composite. The mechanical tests indicated although the mechanical properties of sc‐PLA increased significantly by reinforcing with flax, the ALK‐flax/sc‐PLA composite showed little lower mechanical properties than UN‐flax/sc‐PLA composite. The alkali treatment of flax had no obvious influence on the Vicat softening temperature (VST) of flax/sc‐PLA composites, a higher heat resistance with VST at ～155°C could be obtained for flax/sc‐PLA composite. POLYM. ENG. SCI., 55:2553–2558, 2015. © 2015 Society of Plastics Engineers