Melt Behavior of Soil-Buried Polypropylene/Polylactic Acid Blends Reinforced with Kenaf Particles

Abstract

The increasing demand for sustainable materials has intensified research into the flow behavior of biodegradable polymer blends reinforced with natural fillers such as kenaf particles. Understanding their rheological performance, especially after environmental exposure like soil burial, is crucial for optimizing polymer processing and for recycling purposes. Thus, this study was set up to investigate the flow behavior of polypropylene (PP)/polylactic acid (PLA) blends filled with kenaf particles before and after the soil burial testing. The polymer blends were formulated with PP/PLA ratios of 90/10 and 70/30, incorporating 5 and 10 phr of kenaf particles. The compounding process was carried out using a heated two-roll mill at 190 °C. Meanwhile, the rheological behavior was evaluated using the melt flow index (MFI) and a capillary rheometer to examine the effects of soil degradation and aging on the prepared blends and composites. After 3 months of soil burial, the MFI of the 90/10 blend decreased from 2.23 to 2.01 g/min, suggesting the formation of crosslinked structures, possibly due to the presence of ethylene copolymer segments in the PP chains. In contrast, the 70/30/10 composite exhibited a substantial increase in MFI, from 3.34 to 5.50 g/10 min, indicating significant PLA degradation caused by polymer chain scission, which led to an increased fraction of low molecular weight PLA. A similar trend was observed in the apparent viscosity (napp) of the 70/30/10 composite that showed a consistent decline ofter 3 months of soil degradation period. This was further supported by a reduction in extrudate die swell, confirming molecular weight loss due to chain scission during soil burial. Overall, the results suggest that a higher content of kenaf particles in PP/PLA blends accelerates the composites degradation under soil burial conditions. This is attributed to the water retention capability of incorporated kenaf particle, which promotes hydrolysis and consequently causing molecular weight reduction for the PLA chain during the degradation process.