Modification of flax fiber surface and its compatibilization in polylactic acid/flax composites

Fiber-matrix interaction plays a key role in the development of high-performance polymer/fiber composites. Flax fiber surface was modified by alkali treatment, corona discharge, maleic anhydride (MA) grafting and aminopropyltriethoxysilane treatment. Chemical structure of the treated fibers was characterized by FTIR, and the results revealed that alkali treatment removed hemicelluloses, lignin, waxes and oils. In addition, MA and silane reacted with the fiber to form -OCO-CH=CH-COOH and -(CH2)(3)-NH2 groups. Oil/water test was carried out on the treated fiber, and the results indicated that MA grafting and silane treatment changed the wettability of fiber from hydrophilic to hydrophobic. The morphology of fibers was observed by scanning electron microscope (SEM). It was obvious that the surface of alkali-treated fiber was very rough, and the surface of MA-grafted fiber and silane-treated fiber was covered by a coating layer. High toughness polylactic acid (PLA)/flax composites were fabricated by blending the modified fibers with PLA. The mechanical properties of PLA/flax composites were evaluated. The results showed that the impact strength and elongation-at-break of PLA/treated fiber composites were higher than those of PLA/untreated fiber composites. The compatibilizing mechanism between the fibers and PLA was discussed according to experimental phenomena and their corresponding results. Alkali treatment led to a rougher fiber surface, and formed more contacting points to enhance mechanical interlocking between PLA and fiber. The -OCO-CH=CH-COOH group of MA-grafted fiber and the -(CH2)(3)-NH2 group of silane-treated fiber acted as adhesive agents to hook the fiber to the matrix, and increased the fiber-matrix interaction.