Synthesis, Characterization, Application And Recycling of Bio-based Polyesters

The PhD research programme focused on the development of new formulations based on bioplastic using polymers with linear and stellar geometry, commercial and ad hoc synthetized. Specifically, these systems were studied starting from two bioplasticss, polycaprolactone (PCL) and polylactic acid (PLA), which, due to their versatility, have been considered among the best alternatives to traditional polymers. In addition, several polymers obtained by enzymatic synthesis starting from a cardanol-based monomer, which is a waste product of the food industry, have been studied. For all the systems prepared, the effects of the chemical nature of the biopolymers on the final properties of the formulations analysed have been extensively studied, and in some cases the end-of-life of these has been investigated by verifying their actual biodegradability. In this research project,two aspects of particular importance were the use of polymers with a star molecular topology, commonly referred to as ‘star-shaped’ polymers, characterised by a low molecular mass, and the study of new sustainable synthesis routes as an alternative to the traditional ones. The star-shaped molecular configuration makes it possible to modulate certain fundamental physical properties of polymers, such as crystallinity, solubility, and the viscosity of the melt and the resulting solutions. In particular, it makes it possible to obtain systems with a higher quantity of functional groups than linear polymers with the same molecular mass, and it is precisely the combination of these characteristics that makes these polymers particularly suitable for combination with other polymer matrices. Specifically, the research activities carried out during the PhD focused on the development of three different systems, two contain polymers with a star-shaped geometry, one PCL-based and one PLA-based, while the last one contains linear cardanol-based polyesters with low molecular, prepared by means of enzymatic synthesis. The first project focused on the development of cross-linked materials characterised by dynamic covalent bonds, starting with a star-shaped polycaprolactone with numerous terminal hydroxyl groups (PCL-OH) used for the development of nanostructured systems based on reduced graphite oxide (rGO). The cross-linking reaction was carried out using methylenediphenyl diisocyanate (MDI) to create systems capable of exchanging bonds through transesterification and transcarbamoylation reactions, in the presence of a tin-based catalyst. In this regard, the combination of dynamic network and the use of rGO, resulted in a material characterised by recycling and self-healing properties with interesting rheological properties at high temperatures. The second project focused on the development of an environmentally friendly synthesis of cardanol-based polyesters to be used as plasticisers for polylactic acid (PLA), starting with a cardanol-based diol and various dimethyl esters such as dimethyl succinate (DMS) and dimethyl adipate (DMA). The innovative aspect of the work involved the use of cardanol-derived diol and the development of an enzymatic polycondensation, which was carried out under milder conditions (85 °C) than those generally used in industry. Finally, the last project focused on one of the most critical aspects for the wide application of polylactic acid (PLA): its recycling. An innovative and effective approach was developed to obtain star-shaped polymers starting from a commercial PLA, using an environmentally friendly solvent-free process based on compounds derived from renewable sources. In particular, starting from a high-mass linear commercial PLA, a depolymerization route based on a bulk alcoholysis reaction in the molten state was developed. For this purpose two polyalcohols, pentaerythritol and dipentaerythritol, and an catalyst from renewable sources, i.e., zinc stearate, were utilized. The polymers thus obtained were then blended with an epoxide derived from castor oil (GE-35H) in order to obtain homogeneous systems with better mechanical properties than the starting polymers.