Use of supercritical CO2 as a foaming agent for Poly(butylen adipate-co-terephthalate) nanocomposites
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Melt intercalation technique was used to produce bionanocomposites based on poly(butylene adipate-co-terephthalate) (PBAT) and layered silicates; both organically modified and unmodified. Four different ionic liquids were chosen as modifying agents, three of them were phosphonium based and the last one was an imidazolium based. Physical and thermic characterization of the nanocomposites was made revealing no significant changes of the properties except for an augmentation of the cristalization temperature in the case of PBAT/MMT-I/IL-I. In the other hand, tensile tests showed that addition of nanoclays leads to higher tensile modules, going from E = 47 MPa for pristine PBAT to E = 59 MPa when adding 5wt% of MMT-I; without greatly compromising the characteristic good deformation of the polymer. Moreover, a synergy effect was found when PBAT/MMT-I/IL-I nanocomposites were made, increasing significantly both tensile module and elongation to rupture by approximately 50%. Overall improvement on mechanical properties of nanocomposites can be related to a good dispersion of OMMT within the polymer matrix, achieving partially exfoliated structures as confirmed by TEM images. To obtain the nanocomposites respective foams, a batch foaming process was carried out using carbon dioxide in its supercritical state (CO2 sc). All formulations increased the cell density comparing with pure PBAT (0,8x104), hence proving that nanoclays indeed act as nucleating agents. Also it was confirmed that the grade of dispersion of the charge within the polymer matrix and the chemical nature of the ionic liquid used for modification plays a very important role in the process of nucleation, since there exists physico-chemical nucleation sites. Once again PBAT/MMT-I/IL-I formulations showed the best results with a cell density of 22,3x104 due to its better dispersion and the great affinity between the nitrogen of the imidazolium and the CO2. Finally, the pressure effect over the last formulation was studied and as expected it increases notably the cell density to 61,9 x104. Furthermore a control of the cell size distribution was achieved.