Starch/PCL blend-based food packaging materials mechanochemically obtained via organocatalytic esterification of salicylic acid: Effect of l(+)-tartaric acid as an organocatalyst

Abstract

The objective of this work was to analyze films based on poly(ε-caprolactone) (PCL)/thermoplastic starch (TPS) blends obtained mechanochemically via organocatalytic esterification of salicylic acid (SalAc – esterifying agent) using tartaric acid (TAc) as an organocatalyst for their potential application as an active and sustainable food packaging material. For this purpose, six formulations were fabricated and labeled as follows: 1) TPS, 2) PCL, 3) a 50:50 (w/w) blend of TPS and PCL (PCL/TPS), 4) PCL/TPS containing SalAc at 2% w/w with respect to the weight of the polymeric matrix/plasticizer mixture (maximum allowable limit) (PCL/TPS +SalAc), 5) PCL/TPS containing TAc (same molar ratio as SalAc) (PCL/TPS+TAc), and 6) PCL/TPS containing a 1:1 molar blend of SalAc and TAc (PCL/TPS+SalAc+TAc). These materials were then analyzed in structural, thermal, crystalline, physicochemical, morphological, mechanical, rheological and functional terms. The results obtained demonstrated that the presence of SalAc produced the esterification of TPS, thereby improving its compatibility with PCL, while TAc, in the absence of an esterifying agent, showed a hydrolyzing effect on TPS and caused decarboxylation of PCL. SalAc acted as an antioxidant compound, thus partially inhibiting PCL decarboxylation. Nonetheless, this reaction was not decisive in the overall physicochemical and mechanical performance of the developed materials. The addition of SalAc and TAc, separately and mixed, in the PCL/TPS-based materials also led to a decrease in viscosity compared to the PCL/TPS material. Regarding antioxidant activity, values were low in all formulations, without reaching significant levels. However, the antimicrobial properties of the materials containing mainly SalAc were outstanding, mainly against Pseudomona aeruginosa (Gram-negative bacteria). Nevertheless, an ecotoxic effect on lettuce (Lactuca sativa) seedlings was mainly observed due to the use of SalAc, which could be related to its dose-dependent effect, thus limiting the potential compostability of the materials. With regard to the scaling up of such materials, none achieved to improve the mechanical properties and reduce the hydrophilic nature of TPS.