The earliest synthetic polymers were derived from naturally occurring substances such as casein, gelatin, shellac, and natural rubber. These materials were inherently biobased, as petroleum-derived chemicals were not yet in use. From the mid-20th century onward, most of these natural-origin plastics, aside from a few like cellulose- and rubber-based types, were largely replaced by petrochemical alternatives. In recent years, the growing environmental awareness as well as the fact of the finite nature of fossil resources have sparked a renewed interest in biobased and biodegradable materials. As a result, these materials are once again gaining attention from the public, policymakers, industry, and researchers alike. The growing demand for sustainable alternatives to conventional plastics has intensified research into biobased and biodegradable polyhydroxyalkanoates (PHA), among which isotactic poly(3-hydroxybutyrate) (i-P3HB) stands out due to its favorable mechanical and chemical properties. However, i-P3HB suffers from brittleness due to its high crystallinity of around 70 % and a narrow processing window, as its thermal degradation onset begins at temperature range of 180 - 190 °C, close to its melting point (175 °C), which significantly limits its processability and application. This thesis investigates extending the processing window of i-P3HB by developing i-P3HB based materials that can be processed at reduced temperature, thereby increasing the temperature gap between processing temperature and onset of thermal degradation and thus avoiding thermal degradation during processing. This goal is achieved by disrupting the crystallization of i-P3HB through blending with atactic poly(3-hydroxybutyrate) (a-P3HB) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate) (P34HB), resulting in materials with an extended processing window, along with tunable crystallinity and enhanced elongation at break, thereby significantly broadening the application range of i-P3HB. The a-P3HB was synthesized via self-polycondensation of racemic ethyl-3-hydroxybutyrate, a new synthesis route for this polymer. The polymerization process was optimized through catalyst screening and design of statistical experiments (DOE). In this way, a-P3HB with different average molecular weights could be obtained with reduced environmental impact, compared to conventional polymers such as polyethylene and polypropylene as well as to the previously known ring-opening polymerization, used for the production of a-P3HB. These advantages were confirmed by a life cycle assessment (LCA). In the next step, binary and ternary blends of i-P3HB with a-P3HB and P34HB were prepared via solution casting. The thermal analysis of the obtained blends revealed a significant depression of the melting temperature and crystallinity in blends containing a-P3HB, facilitating processing at reduced temperatures without risking degradation. Furthermore, the impact of a-P3HB and P34HB on the crystallization behavior of i-P3HB was analyzed using polarized light microscopy (PLM) and temperature modulated differential scanning calorimetry (TMDSC). These studies confirmed that a-P3HB strongly disrupts the crystallization of i-P3HB, leading to the formation of smaller, less stable crystals, whereas P34HB showed only a limited impact. Since the solution casting process is not feasible to industrial upscaling, solvent-free processing of i-P3HB blends via extrusion was developed. The resulting blends showed almost identical properties to those obtained by solution-casting and were processable at temperatures between 25 and 40 °C below the onset of thermal decomposition of i-P3HB, thereby avoiding thermal degradation. Analysis of the thermal and mechanical properties of the obtained blends confirmed that a-P3HB improves the processability of i-P3HB by disrupting its crystallization, leading to reduced processing temperature. Furthermore, the obtained blends also showed stable thermal and mechanical properties even after one month, indicating hindered post crystallization. By comparing the properties of the obtained blends, ternary blends of all three polymers provided a balanced compromise, combining improved processability, tunable crystallinity, and enhanced elongation with long-term stability. These findings offer a viable path toward sustainable, scalable bioplastic materials for advanced manufacturing applications. The obtained i-P3HB blends in this work were also successfully further processed within the European research project “Waste2bioComp” by industrial partners to produce packaging materials via blown extrusion, foams for footwear, and rigid packaging via injection molding, demonstrating the industrial scalability and transferability of the developed i-P3HB blends.
Wael Almustafa (Thu,) studied this question.