The employment of plastics in food packaging production is of paramount importance due to the multiple advantages obtained with respect to paper and glass. On the other side, limits on recyclability of plastic wastes and the continuous depletion of petroleum feedstocks for the synthesis of commodity polymers represent an issue that must be accounted. Bioplastics, that are bio- based and/or biodegradable polymers, are being researched as possible solutions. This thesis work focused on three bioplastics, namely polylactic acid (PLA), polybutylene succinate (PBS), and starch. Considering the multiple drawbacks linked to available bioplastics, a viable strategy to obtain more performant materials is the production of blends, as the synergistic effect of different contributes enables to reach upgraded properties while limiting costs. Due to chemical differences that reflect especially on their polarity, production of PLA/starch or PBS/starch binary blends requires the use of compatibilizers. They can be either small weight molecules, that might have the same problems of glycerol when used as plasticizer in thermoplastic starch, or copolymers, having two different segments with different polarities, respectively compatible with the involved materials. The aim of the project, developed in collaboration with Zürich University of Applied Sciences (ZHAW), was to synthetize starch- PLA grafted copolymers and to study their possible compatibilization effect on PBS/potato peel blends. Focus went firstly at the synthesis of starch/PLA copolymers. Employing solvent-free industrial-like conditions, two starch/gluten/PLA copolymers were synthetized and named LSG70 and LSG50. Gluten was added to resemble potato-peel high protein content [1]. According to FT-IR, PLA grafting resulted relatively poor, therefore an alternative synthetic path was evaluated, consisting in the solution polymerization of lactide in dimethyl sulfoxide (DMSO) preceded by starch gelatinization in the same solvent [2]; higher grafting efficacy was detected. Processing of PBS/potato peel films was partially developed in collaboration with ZHAW. Tensile tests on benchmark blends showed negligible glycerol influence over films’ strenght, and the lower content evaluated (5%wt with respect to potato peel) was selected as ideal. LSG70 was then introduced in the formulation of various blends. Mechanical properties were monitored every two weeks: tensile strenght statistically decreased in benchmark blends, while in presence of LSG70, even in complete absence of glycerol, tensile strenght remained statistically constant. LSG70, LS70 and LS50 were employed in blends processed through injection molding to obtain dog-bone specimens (ISO527-5A). Complex modulus was evaluated with DMA before and after accelerated ageing. Results showed that copolymers, with or without glycerol, improved blends’ complex shear modulus with respect to PBS and benchmark blends. After ageing, all samples showed degradation signs, with decreases in G*, but the presence of LSG70 and LS70 showed the higher contribute to the stability of blends against degradation, inducing a lower reduction in complex shear modulus.

References

1. A. Joshi, S. Sethi, B. Arora, A.F. Azizi, B. Thippeswamy Potato 2020, 229.
2. B.M. Trinh, D.T. Tadele, T.H. Mekonnen Mater. Adv. 2022, 3, 6208.