Sustainable polycondensation of multifunctional fatty acids from tomato pomace agro-waste catalyzed by tin (II) 2-ethylhexanoate

Abstract

Bioplastics were prepared from the fatty fraction (i.e., unsaturated and polyhydroxylated fatty acids) of tomato pomace agro-wastes. Aliphatic polyesters were synthesized at different temperatures (125, 150, and 175 °C), reaction times (0.25, 0.5, 0.75, 1, 3, 5, and 7 h), and amounts of tin (II) 2-ethylhexanoate (0, 0.02, 0.05, and 0.10 mmol) used as a catalyst. The rate constants and activation energies were calculated from infrared spectra. The right combination of reaction temperature and amount of catalyst improved the reaction kinetics (apparent k from ∼1 to ∼8.5 h−1), whereas the activation energy was reduced from ∼39 without catalyst to ∼28 kJ/mol when tin (II) 2-ethylhexanoate was present. Glass transitions between ca. −25 and ∼0 °C were measured by differential scanning calorimetry, strictly depending on the degree of polymerization. The amorphous character of the samples was confirmed by X-ray diffraction. Young's modulus and hardness were calculated from indentation tests and were typical of soft materials, although increased as the polycondensation reaction progressed. High water-contact angles (maximum value ∼109°) and low water uptakes (minimum value ∼2.1%) were determined. Physical properties were compared with those of common man-made plastics and polymers, finding that these tomato pomace bioplastics could be their realistic alternatives.