Strategies for Utilizing Biobased and Recycled Resources for Polylactide Plasticization

Abstract: There are many aspects that should be considered to achieve truly "environmental friendly" and "sustainable" materials. These include the raw material resource, the energy consumption, the chemicals generated and used, the emissions during production, processing, transportation, disposal and so on. The work presented in this thesis focused on development of strategies for valorization of biopolymers and further utilization of the derived chemicals for plasticization of polylactide (PLA). This would retain the material value of the biopolymers, the degradability and biobased nature of PLA, while simultaneously broadening the application areas and properties of PLA.Two green, and efficient methodologies for chemical recycling of poly(3-hydroxybutyrate) (PHB), a natural biodegradable biopolymer produced in microorganisms as an energy storage material, were demonstrated. Full degradation to monomeric degradation products: crotonic acid (CA), 3-hydroxybutanoic acid (3HB) and 3-methoxybutanoic acid (3MB) was obtained in only 20 minutes through microwave assisted degradation in green solvents. Degradation parameters, such as solvent and basicity, were carefully evaluated for achieving high yield and fast degradation and the constituent ratio of degradation products could be tuned by adjusting the degradation time under the optimized condition. Meanwhile PHB oligomers with crotonic end groups (dPHB) were formed by thermal degradation of PHB during extrusion, which is an industrially viable process. The effects of degradation temperature and degradation time on the formation of degradation products were carefully adjusted, and the degradation kinetics at different temperatures were evaluated. Glucose, a depolymerization product of cellulose and starch, was modified by hexanoic acid to star shaped glucose esters (GHx) with different varying degrees of substitution by adjusting the reaction time.Two methodologies to plasticize PLA by utilizing the products from PHB recycling or the synthesized glucose esters were evaluated aiming at plasticizers with enhanced migration resistance compared to the physical blends or commercial analogues: 1) anchoring linear oligomeric plasticizers onto PLA and 2) blending low molar mass star shaped molecules into the PLA matrix. Specifically, PLA-CA-PBSA was prepared by coupling poly(butylene succinate-co-adipate) (PBSA) to CA functionalized PLA, PLA-g-dPHB was obtained by coupling dPHB onto the PLA main chain via reactive extrusion. PLA blends with star shaped GHx were also prepared. Decrease of Tg was observed for all the plasticized PLA samples. Blending GHx reduced the Tg of PLA by about 30 ºC and a decrease in Tg for approximately 25 ºC and 20 ºC was realized by coupling PLA with dPHB or PBSA, respectively. All plasticized PLA materials showed increased elongation at break. For PLA-g-dPHB a 66-fold increased elongation at break was achieved. The crystallization capacity of both PBSA and dPHB coupled PLA was improved at the same times as the PLA was plasticized. This is explained by the grafting points acting as nucleating sites and is further facilitated by the improved chain mobility of the PLA chains.The proposed approaches for PHB recycling, modification of glucose and the coupling or blending strategies demonstrated great promise for the concept of utilizing recycled biopolymers/biomass for plasticization of PLA.