Abstract: [Objective] The effects of isothermal crystallization on the crystallinity, physical and mechanical properties of wood fiber (WF)/poly(lactic acid) (PLA) composites were investigated in this study, which provides the basis for improving the processing and application of PLA composites.[Method] The pure PLA panels and WF/PLA composites were prepared by hot-pressing and followed by a cooling process for isothermal crystallization at different temperatures (90, 100, 110, 120 and 130℃). The crystallinity and melting characteristics of pure PLA panels and WF/PLA composites were characterized by differential scanning calorimeter(DSC), and the isothermal crystallization process was observed by polarizing microscope(POM), then the effects of isothermal crystallization process on the crystallization properties of PLA were analyzed by combining with crystalline morphology. Moreover, the effects of microstructures and molecular weights on the mechanical properties were also investigated by scanning electron microscope(SEM) and gel chromatography(GPC).[Result] The results showed that the crystallinity of pure PLA panels and composites increased from 4.6% and 15.1% to 24.8% and 40.5%, respectively. Compared with the fracture surfaces of virgin WF/PLA composite, it was obvious to seen that wood fibers were pulled out from the matrix of WF/PLA after isothermal crystallization at 90, 100 and 110℃, indicating that the isothermal crystallization make a poor effect on interfacial bonding. The molecular weight of PLA decreased after the isothermal crystallization. According to the result of POM, it could be seen that the crystal size increased with increasing isothermal crystallization temperatures. After isothermal crystallization, the bending properties and tensile strength of pure PLA panels were improved. Moreover, the flexural modulus of WF/PLA also increased with isothermal temperatures, however, the flexural and tensile strength reduced to some extent.[Conclusion] The crystallinities of pure PLA panels and WF/PLA composites increased gradually after the isothermal crystallization, moreover, the crystallinity of the WF/composites showed a larger improvement due to the ability of promoting nucleation of wood fiber. The lower of the isothermal crystallization temperature, the smaller of the crystal size was. The higher of the isothermal crystallization temperature, the larger of the crystal size was. The bending and tensile strength of pure PLA panels were improved after isothermal crystallization. However, the mechanical properties of WF/PLA showed irregular rules. Isothermal crystallization exhibited different influences on the mechanical properties of pure PLA panels and its composites, which maybe relate to the poor interfacial bonding.

Key words: isothermal crystallization, poly(lactic acid), WF/PLA composites, crystallization properties, mechanical properties