Influence of recycling on different polyesters and their MMT nanocomposites: Effects on morphology, mechanical properties, and rheological behaviour

This study investigates the effect of recycling and nanofiller incorporation on the morphological, rheological, and mechanical properties of various injection moulded polyesters. The research compares the behaviour of two types of bio-based (polylactic acid (PLA) and polybutylene succinate (PBS)) and two types of petroleum-based (polyethylene terephthalate (PET) and polybutylene terephthalate (PBT)) polyesters, including polymers with aliphatic and aromatic structures. Recycling was simulated by repeated extrusion for both unreinforced and 6 wt% montmorillonite (MMT) reinforced nanocomposites. Although in all cases the rheology measurements resulted in shear-thinning behaviour, the complex viscosity range and shape of the curves varied differently for each material depending on the MMT reinforcement and the number of extrusions. The melt flow index (MFI) values showed that neither reprocessing nor MMT had a significant effect on the results of PBT, PBS, and PLA. A sharp increment was observed when PET was processed with MMT, indicating the polymer’s notable degradation. Non-isothermal crystallization was used to investigate the extent of overcooling, and the results were compared at 10°C/min and at cooling rates extrapolated to 1°C/min. The undercooling of PBS and PBT barely changed with reprocessing and MMT content, but degradation in PET also modified the crystallization tendency, while in PLA MMT reinforcement inhibited molecular ordering. The wide-angle X-ray diffraction (WAXD) study showed an intercalated structure for all types of polyester nanocomposites, with minor differences in the layer-stack number. Dynamic mechanical analysis (DMA) indicated changes in the glass transition temperature and storage modulus with reprocessing and the addition of MMT. While below the glass transition temperature, the glassy amorphous phase has a more pronounced effect on the storage modulus, above this temperature the crystalline phase tends to dominate, as the contribution of the amorphous phase in the rubbery state is minor to the stiffness. The homogeneous distribution of MMT was confirmed by the shape of the Cole-Cole diagrams.