Influence of surface treatments and addition of a reactive agent on the properties of PLA/flax and PLA/bamboo composites

Polylactic acid (PLA) composites reinforced with 10 wt% of flax (FF) or bamboo (BF) fibers were prepared via an internal mixer and/or twin-screw extrusion. Alkali pretreated fibers were soaked in silane to improve adhesion between fibers and matrix. 0.8 wt% of Joncryl™, a grafted copolymer acting as PLA chain extender, was also used alone or in combination with silane treatment of fibers to improve interfacial adhesion. The influence of silane treatment and/or Joncryl on the composite materials on mechanical, thermal and thermomechanical properties of materials processed through injection molding was investigated. Improved adhesion of the fibers to the matrix was shown using a scanning electron microscope. Fourier Transform Infrared Spectroscopy indicated that chemical bonds were formed between the silane coupling agent and fibers. X-ray Photo-electron Spectroscopy confirmed that fibers and silane derivatives were effectively coupled. XPS also highlighted that silane coupling agent reacted in higher amounts on bamboo than flax fibers, probably due to a higher amount of lignin in the case of bamboo fibers. Thermogravimetric analyses indicated that silane-treated flax and bamboo increased the thermal stability of the corresponding composites (PLA-SFF and PLA-SFB) compared to non-treated fiber composites. The incorporation of Joncryl alone entailed a degradation of the thermal stability of the corresponding composites (PLAJ-FF and PLAJ-FB) but enhanced the PLA/fibers interfacial adhesion. The combination of Joncryl and silane treatment resulted in strong improvements of thermal stability and interfacial adhesion for the PLAJ-SFF and PLAJ-SBF composites. Increase in tensile moduli and decrease in tensile strengths with the incorporation of the pristine fibers were noted. For silane-treated fibers, the tensile modulus and the strength of the corresponding composites were improved when adding Joncryl alone or in combination with silane. From also rheological and molar weight measurements, it could be concluded that Joncryl acts both as PLA chain extender and coupling agent.