Utilization of hydroxyl-compatible fillers in citric acid-crosslinked thermoplastic starch foam: Communication of strengths and limitations

Abstract

This work aims to disclose the strengths and limitations of utilizing hydroxyl-compatible fillers, rice husks and silica (SiO₂), in citric acid-crosslinked thermoplastic starch foams. FTIR characterization demonstrated that citric acid could establish ester linkages with starch molecules for both neat and composite foams, with the C = O peak shifting from 1724 cm⁻¹ (neat foam) to 1730–1732 cm⁻¹ (composite foams). The key role of hydroxyl-compatible fillers was the strong starch-filler interactions that enabled more thermally stable composite foams, which elevated thermal decomposition temperatures to 300–302°C with higher residue weights of 22.73–25.33%. These strong starch-filler interactions were also responsible for retarding cell foam expansion. The composite foams showed 16.61–92.03% improved flexural strength, 2.82–10.67% increased densities and lower moisture absorption. However, incorporation of hydroxyl-compatible fillers in composite foams increased water solubility to 16.84–20.73%, compared to 12.14% for a neat foam. These experimental results indicate that the employed fillers might interfere with the crosslinking process, leading to decreased crosslink formation in composite foams. Consequently, possible interruption mechanisms of crosslink formation by the fillers were suggested and confirmed by qualitative analysis. The findings of the strengths and limitations of hydroxyl-compatible fillers illustrate crucial trade-offs in using starch-based composite foams as sustainable food packaging materials.