Design of a PHB-HV/clay nanocomposite foam by using sc-CO2 assisted extrusion

International audiencePoly(3-hydroxybutyrate-3-hydroxyvalerate) (PHB-HV) is a biopolymer that has recently caught the attention, due to its biodegradability, biocompatibility and natural origin. However, some drawbacks have prevented its introduction on the market as an effective alternative to the currently widespread non-degradable oil-based thermoplastics. In consequence, improving the properties of PHB-HV by the addition of a small quantity of an environmentally innocuous material such as clay, may allow this class of polymers to extend their potential uses to many fields. In this context, a great deal of effort has been made to design nanocomposite systems with the aid of supercritical carbon dioxide (scCO2).Our laboratory has developed a scCO2-assisted extrusion process. Injection of scCO2 into a single-screw extruder modifies the rheological properties of the polymer in the barrel and scCO2 acts as a blowing agent during the depressurization when flowing through the die [1]. In the present work, this process has been used to prepare a PHB-HV/clay composite. At first, the mixing of the polymer and the clay was achieved by two methods: physical mixture and twin screw extrusion. Then, the foaming process with scCO2-assisted extrusion has been applied to the PHB-HV and clay/PHB-HV mixtures obtained. The designed nanocomposite foams showed porosity up to 50 % was achieved, while having a relatively poor homogeneity. Growth and coalescence of the pores were limited by the decrease of the die temperature and the increase of the CO2 mass fraction. In spite of a favourable kinetics of triboelectrification, the dispersion of the clay was not homogeneous in foams prepared with physical mixture. In the case of extruded mixture, DRX spectra showed that intercalation and possible exfoliation occurred and was not affected by the foaming step. The presence and the dispersion of the clay particles seemed to favour heterogeneous nucleation of the CO2, but it was not sufficient to obtain a homogeneous and highly porous structure