Strength retention and moisture resistant properties of citric acid modified thermoplastic starch resins

Abstract

The strength retention and moisture resistant properties of thermoplastic starch (TPS) resins were significantly enhanced by modifying with proper amounts of citric acid (CA) (i.e. TPS100CAx specimens) or by melt-blending with poly(lactic acid) (PLA) (i.e. (TPS100CA0.1)xPLAy specimens). In contrast to the distinguished retrogradation effect found for all conditioned TPS specimens, one can barely find any recrystallized starch crystals in TPS100CAx and/or (TPS100CA0.1)xPLAy specimens maintained at 20 °C/50% RH for less than 42 days. The tensile/impact strength retention values of properly prepared conditioned TPS100CA0.1 and (TPS100CA0.1)30PLA70 specimens were equivalent to 1.5 MPa/0.28 KJ/m2 and 41.8 MPa/1.63 KJ/m2, respectively, which were more than 4/4 times and 105/23 times higher than those of corresponding TPS specimens maintained at 20 °C/50% RH for 70 days. In comparison with conditioned TPS specimens, significantly less and shorter drawn debris were found on the fracture surfaces of the corresponding conditioned TPS100CA0.1 and (TPS100CA0.1)30PLA70 specimens with the same amounts of conditioned time. As revealed by Fourier transform infrared spectroscopy, Solid-state 13C Nuclear Magnetic Resonance analyses, disruption of intra and interhydrogen-bondings within starch molecules did occur after addition of small amounts of CA during the modification processes of TPS100CAx specimens. The relatively unchanged in retrogradation effect, significantly less drawn debris and considerable improvement in moisture resistant and/or strength retention properties of the conditioned TPS100CAx and/or (TPS100CA0.1)xPLAy specimens is most likely due to the efficient hydrogen-bonding CA molecules with the moisture-absorbing hydroxyl (free or hydrogen-bonded) of starch molecules that prohibits moisture absorption during their conditioning processes.