Improving properties of poly(lactic acid) biopolymer for use in food packaging

Sammanfattning: The most popular polymers used in the food packaging industry are polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and polystyrene (PS), which together account for more than 90% of the total volume. These polymers are non-biodegradable; thus, poor waste management of these polymers results in high amounts of microplastic waste in both land and marine ecosystem. In this context, polylactic acid (PLA) biopolymer is a potential candidate to replace petroleum-based polymers due to its biodegradability, and eco-friendly behavior. In addition, it has excellent mechanical properties transparency and economic viability in comparison to many other biopolymers. However, despite the benefits, PLA has disadvantages that limit its use in food packaging applications, such as low crystallinity, inherent brittleness, poor melt strength and moderate gas barrier performance. To address these issues, chitin nanocrystals (ChNCs) produced from shrimp shell waste and another biopolymer poly hydroxybutyrate (PHB) obtained from microbial fermentation were used as PLA additives, along with various plasticizers.  PHB was used to improve the crystallinity of PLA and ChNCs were used to improve the mechanical, and thermal properties, while plasticizers were used to improve the flexibility (ductility) as well as dispersion of ChNCs in PLA matrix. PLA-based bionanocomposites were prepared by adding ChNCs, PHB and plasticizers in different combinations via liquid assisted melt extrusion process. In addition, the extruded nanocomposites were also subjected to film blowing and calendering processes in order to orient the PLA-ChNCs nanocomposite films to improve the mechanical properties of the resultant material. In this thesis, 5 different studies were carried out. The first study demonstrated the effectiveness of triethyl citrate (TEC) plasticizer in combination with ChNCs in improving the properties of PLA. It was found that the dispersion of ChNCs into the PLA matrix was improved with the addition of TEC, which attributed to the higher toughness of the resultant material. In addition, the nanocomposite exhibited antimicrobial properties due to the presence of ChNCs. In the second study, it was reported that the crystallinity of the PLA was significantly improved by the addition of homogenously dispersed ChNCs with the help of TEC resulting in improved thermal and oxygen barrier properties. In the third study, the effect of polymer chain orientation (achieved by calendering and solid-state drawings) on the mechanical properties of PLA-ChNC nanocomposites was investigated. It is observed that the oriented film from melt state drawing (calendering) exhibited superior mechanical properties as well as enhancement in crystallinity. The fourth study discussed the impact of PHB and ChNCs addition on PLA crystallinity in the presence of glycerol triacetate (GTA) plasticizer. Observations revealed that the PLA blend containing 25 wt% PHB and 1 wt% ChNCs significantly improved the degree of crystallinity and crystallization rate, thereby decreasing oxygen (O2) and carbon dioxide (CO2) permeability and increasing overall mechanical properties. The fifth study investigates the influence of ChNCs and oligomer lactic acid (OLA) plasticizer on the blowing process of PLA-PHB based nanocomposites for food packaging applications.  It was observed that the PLA-PHB-OLA-ChNC nanocomposite exhibited easier processability during the film-blowing process and exhibited smooth and homogenous surface film with higher crystallinity, excellent mechanical properties, and lower O2 permeability. Furthermore, the nanocomposite film was disintegrated within 45 days under composting conditions, which are favorable in packaging applications.In conclusion, this thesis shows that the addition of ChNCs and PHB together with a plasticizer and further processing can result in PLA nanocomposites with varied properties that can be used for packaging applications. It was also demonstrated that the processing technique in this study can be a step forward for the large-scale production of bionanocomposites.