Elaboration of poly(lactic acid)/halloysite nanocomposites by means of water assisted extrusion: structure, mechanical properties and fire performance

Poly(lactic acid) (PLA)/halloysite nanotube (HNT) nanocomposites were prepared using a water assisted extrusion process. Morphology, structure, thermomechanical properties and flame retardant properties of these nanocomposites were compared to those obtained in the case of nanocomposites obtained via the conventional extrusion process. Whatever the synthesis route used, mechanical tests indicate that HNTs have a reinforcing effect ascribed, at least partially, to the dispersion of the HNTs in the PLA matrix. Differential scanning calorimetry also shows that HNTs improve PLA crystallization and play the role of a nucleating agent. Moreover, cone calorimeter experiments show a drastic improvement in the reaction to fire of PLA when HNTs are added, especially at high filler content. This improvement arises from the fact that HNTs induce the formation of a protective inorganic layer at the surface of the sample. As the main results of this study, it has been shown that water assisted extrusion not only induced a better dispersion of the HNTs into the matrix but also that this process prevents polymer degradation during the extrusion. This unusual effect has been ascribed to a barrier effect of the water molecules that preferentially locate at the HNT-matrix interface. Even if not fully explained, it could be ascribed to a decrease in local shear levels (mechanochemical effect) and/or to a decrease in the catalytic effect of the HNT surface (chemical effect). Consequently this study reveals that water-assisted extrusion is a reliable route to prepare PLA/halloysite nanocomposites with enhanced properties.