Correlation between process parameters and the degradation of native potato starch particles/poly(lactic acid) composites during the compounding process

Poly(lactic acid) (PLA) is one of the most important bioplastics with good mechanical properties and reduced environmental impact compared to petro-based plastics. To enhance sustainability and cost-efficiency, PLA can be filled with low-cost particles, such as native potato starch. However, during processing in a twin-screw extruder, PLA and starch are prone to thermal, hydrolytic, and mechanical degradation, which can impair composite properties. This study systematically examined how key compounding parameters (screw speed and configuration, temperature profile and PLA pre-drying time) influence the properties of PLA-starch composites containing 50 wt.% native potato starch. Various analytical methods (Differential Scanning Calorimetry (DSC), Thermogravimetric Analysis (TGA), Melt Volume Rate (MVR), Scanning Electron Microscope (SEM), color measurements) were applied. The results showed that thermal properties (glass transition, crystallinity) remain largely unaffected by the compounding parameters. In contrast, significant changes were observed in thermal stability, viscosity, color, and morphology. Higher screw speeds and temperatures increased thermomechanical stress, leading to a 93.3 % higher MVR value, indicating lower molecular weight. Color analysis revealed significant discoloration due to starch degradation, which correlated with increasing yellow-blue index (121.6 %) and red-green index (1884.4 %) and a decrease in lightness (23.6 %) at higher thermomechanical stress due to elevated screw speed and temperatures. Overall, the study emphasizes the complex interactions between process parameters, in particular screw speed and temperature profile as well as the physicochemical properties of native potato starch particles/PLA composites, providing valuable insights for the design of the compounding process in order to prevent pre-damage to the composites.