Surface photo-crosslinking of plasticized thermoplastic starch films

• UV irradiation improved strength, and decreased degree of swelling.
• Crystallinity and retrogradation correlated with strength and moisture absorption.
• Malic acid and citric acid act as a strong retrogradation agents.
• Photosensitizer increased the mechanical strength and decreased the solubility.
• UV irradiation had no effect on the degree of crystallinity.

The study aims to explore ultraviolet (UV) cross-linking as a tool to steer the mechanical properties of malic and citric acid plasticized thermoplastic starch (TPS) based films. TPS films were prepared through compression molding of spray dried amorphous powder. Sodium benzoate (SB) was used as photosensitizer. Under dry conditions (RH0%) all samples maintained their amorphous nature while at RH50% and RH100% retro-gradation was suppressed for all films. The rate of moisture uptake, and as a consequence, the rate of retro-gradation, depends on the concentration of the glycerol in combination with citric and malic acid. UV irradiation had no effect on the degree of crystallinity, improved the mechanical properties and decreased the solubility and degree of swelling. Citric acid plasticized and co-plasticized films showed slightly better mechanical properties before and after UV irradiation treatment compared to malic acid, but citric acid formulations appeared to be very moisture sensitive prior to cross-linking.

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