Study of biocomposites based on PLA and natural fibers : influence of fibers and fibers treatment

Composites interest the scientific community because of their interesting properties but also because of their versatility. However, most composites manufactured today are difficult to recycle because they are based on thermoset matrices. It is why research into thermoplastic matrix composites and especially biocomposites is emerging. The development of more environmentally friendly composites is a major challenge for scientists. The objective of this work is to study the influence of the fiber loading and the NaOH treatment of the fibers on the properties of biocomposites based on PLA and natural fibers. For this purpose, composites based on semicrystalline polylactic acid (PLA) with different treated poplar and banana fiber loadings were investigated. In addition, composites based on PLA and 10 wt% of non-treated poplar and banana fibers were also studied. Different analyses were used to characterize the physical, thermal, mechanical and morphological properties of poplar fibers, banana fibers and their composites including fourier transform infrared spectroscopy (FTIR), dynamic vapor sorption (DVS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), tensile test, dynamic mechanical analysis (DMA), scanning electron microscopy (SEM) and polarized optical microscopy (POM). Poplar and banana fibers are promising reinforcements for PLA. Indeed, tensile tests and DMA reveal that both the stiffness and the storage modulus significantly increase with the addition of treated fibers. Though, the glass transition temperature decreases with the addition of natural fibers. The same result is observed concerning the thermal stability of the composites. In addition, the DVS analysis reveals that the absorption kinetic of the composites is higher when the fiber loading increases. SEM analyses show that the addition of natural fibers induces an important roughness to the fracture surface after tensile test. Furthermore, trans-crystallization is observed on the fiber surfaces thanks to the POM analysis. Finally, the results evidence the positive impact of the NaOH treatment on the properties of the composites. Indeed, the composites based on treated fibers show slower kinetic of water absorption, better thermal stability and higher storage modulus compared to the ones based on non-treated fibers. In addition, SEM analyses show that the fiber-matrix interface is improved thanks to the NaOH treatment of the fibers.