Desarrollo de biocompuestos a partir de mezclas de almidón termoplástico/ácido poliláctico compatibilizadas con ácido cítrico

Abstract

This work aims to determine the morphological, structural, and thermal properties of starches from two tuber species, Ipomoea batatas (SP) and Dioscorea rotundata (DY), as well as the influence of glycerol as a plasticizer and the development of a polymeric composite from thermoplastic starch (TPS)/polylactic acid (PLA) blends. The results of the characterization of native and plasticized starches evidenced an amylose/amylopectin content is 25.59/74.4 in DY; the granules are round and elliptical in shape with an average size of 25.5µm, and the degradation temperature is 311.01 °C. The amylose/amylopectin content is 45.9/54.1 in SP; the granules are polygonal, oval, and semi-oval in shape with an average size of 9.5µm, and a degradation temperature of 312.89 °C. The morphology of granules in the different starch/glycerol formulations showed no structural damage in both species. However, an increase in degradation temperature was observed in plasticized starches and in-band shape and intensity. The results indicate that the addition of glycerol improves the properties of native starches for processing and their potential use as bioplastics. TPS/PLA blends have been studied due to the desired mechanical properties of PLA and the low processing cost of TPS. However, the TPS/PLA combination is thermodynamically immiscible due to the poor interfacial interaction between hydrophilic starch granules and hydrophobic PLA. The objective of this study was to improve the compatibility between thermoplastic starch and polylactic acid by incorporating citric acid grafted starch at 1% and 5% (CA-g-Starch) with a degree of substitution (DS) of 0.04 and 0.06 for sweet potato and yam starch, respectively. Biocomposites were fabricated with a constant TPS: PLA ratio of 60:40 by weight for the two types of starches with and without compatibilizer, the mixtures were obtained in a torque rheometer at conditions of (1) 50 rpm, 150 °C, and 10 min, (2) 60 rpm, 180 °C, and 15 min and (3) 60 rpm, 130°C, and 16 min. The results indicated that at the last processing condition no melting of the materials is achieved. Regarding the results of conditions 1 and 2, it is evident that the incorporation of citric acid decreases the particle size in the TPS/PLA/ CA-g-Starch blends, as shown by their SEM with a less rough surface and better interaction. However, the TPS/PLA blends without compatibilizer for the two starches in processing conditions 1 and 2, confirmed the droplet/matrix morphology characteristic of the immiscible blends. Structural properties were determined by Fourier-transform infrared spectroscopy, the blends showed signals characteristic of thermoplastic starch and PLA, as well as some overlapping peaks, and signals corresponding to citric acid. Thermal analyses were also performed using thermogravimetric analysis and the results suggest that higher incorporation of citric acid loading to the blends allowed better thermal stability. In conclusion, the use of citric acid grafted starch (CA-g-Starch) for TPS/PLA blends allows for obtaining a better interfacial adhesion.