Effect of crosslinking followed by thermoplastic processing via extrusion on starch properties for sustainable packaging and coating applications

Crosslinking combined with extrusion offers an effective strategy to alter starch structure and overcome its inherent limitations for material applications. In this study, native corn starch (NS) was modified using a 50:50 (w/w) mixture of sodium trimetaphosphate (STMP) and sodium tripolyphosphate (STPP) (10% w/w, dry starch basis) to produce crosslinked starch (CS) with a degree of crosslinking of 41.6%. Furthermore, NS and CS were extruded to prepare thermoplastic starch (TPS) and crosslinked thermoplastic starch (C-TPS). The effects of crosslinking and extrusion on structural, morphological, and thermal properties were systematically investigated. SEM measurements showed that NS granules lost their elliptical and disc-shaped appearance in the CS sample, mostly in the TPS and C-TPS samples. Pasting analysis revealed a substantial reduction in peak viscosity from 41,759 cP for NS to 24,355 cP for CS, 4902 cP for TPS, and 323 cP for C-TPS, indicating restricted granule swelling and enhanced structural integrity. Rheological measurements confirmed shear-thinning behavior for all samples, with a decreased consistency index after crosslinking, an increase upon extrusion, and a decrease in the combined system. The gelatinization temperatures were similar for all samples, while the enthalpy decreased from 2.67 to 0.88 and 0.48 J/g for NS, TPS, and C-TPS. Among all, C-TPS exhibited the most balanced combination of low viscosity, enhanced thermal stability, and compact morphology, indicating superior processability and structural integrity compared to CS, TPS, and native starch. These results provide a basis for the further development and evaluation of these starch materials in film and coating formulations.