Moisture Resistance, Dimensional Stability, and Biodegradation Behavior of Pennisetum purpureum Fiber-reinforced Thermoplastic Cassava Starch/Candelilla Wax Composite

Abstract

The practical use of starch-based biodegradable composites is often limited by high moisture sensitivity and dimensional instability in humid environments. This study investigates the effect of Pennisetum purpureum fiber (PPF) loading on the physical properties, moisture behaviour, and biodegradation performance of thermoplastic cassava starch/candelilla wax (TPCS/CW) composites prepared by thermo-compression moulding with fiber contents ranging from 0 to 60 wt%. Increasing fiber loading reduced density and significantly improved resistance to moisture-related deterioration. Moisture content, water absorption, thickness swelling, and water solubility decreased progressively with higher PPF content, indicating enhanced dimensional stability and reduced water permeability. After seven days of exposure, equilibrium moisture absorption decreased from 8.3% in neat TPCS/CW to 3.8% at 60 wt% PPF, while water solubility decreased from 29.2% to 13.1%. Soil burial testing confirmed that all composites remained biodegradable, although higher fiber loading moderated the degradation rate, with weight loss reduced from 67.4% in the neat matrix to 43.4% at 60 wt% after four weeks. Slightly higher degradation at intermediate fiber contents was attributed to interfacial voids that facilitated moisture ingress. Overall, PPF incorporation improved moisture resistance and structural stability while preserving biodegradability, supporting the potential of this fully bio-based composite for biodegradable packaging films, disposable packaging liners, and paperboard coating applications requiring moderate moisture resistance under humid conditions.