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. TPS₁₀₀CA_x specimens) or by melt-blending with poly(lactic acid) (PLA) (i.e. (TPS₁₀₀CA_0.1)_xPLA_y specimens). In contrast to the distinguished retrogradation effect found for all conditioned TPS specimens, one can barely find any recrystallized starch crystals in TPS₁₀₀CA_x and/or (TPS₁₀₀CA_0.1)_xPLA_y specimens maintained at 20 °C/50% RH for less than 42 days. The tensile/impact strength retention values of properly prepared conditioned TPS₁₀₀CA_0.1 and (TPS₁₀₀CA_0.1)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 TPS₁₀₀CA_0.1 and (TPS₁₀₀CA_0.1)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 TPS₁₀₀CA_x 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 TPS₁₀₀CA_x and/or (TPS₁₀₀CA_0.1)_xPLA_y 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.

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