Compósitos poliméricos obtidos pela combinação de estireno e lignina

Abstract

Lignin is a lignocellulosic materials-derived natural polymer with high availability, produced in large-scale, presenting a huge potential for production of new polymeric composites. Due to its complex macromolecular structure, and its low compatibility with styrene, eucalyptus wood-extracted lignin through modified Kraft method was esterified with methacrylic anhydrideexhibiting a yield of 64 %in order to ensure homogeneity in the organic phase into the reaction medium. The mass-suspension sequential polymerization process was employed in order to ensure adequate dispersion of lignin in the reaction medium. The evaluation of both the natural and esterified lignin through infrared (IR) spectroscopy showed a decrease of the hydroxyl band, characteristic of natural lignin (3200-3400 cm-1) and an increase of the characteristic ester band (1720 to 1740 cm-1). According to nuclear magnetic resonance (1 H NMR), intense peaks were observed in the range from 1.7 to 2.05 ppm (-CH 3) and 5.4 ppm to 6.2 ppm (=CH 2), related to methacrylic anhydride. According to the thermogravimetric analysis (TGA), esterified lignin showed a decrease in its thermal stability when compared to natural lignin, exhibiting two main weight losses between 200 °C and 300 °C and in the interval from 550 °C to 800 °C. Comparatively, the esterified lignin also displayed an increase in its glass transition temperature (Tg) for para 98 °C, related to natural lignin, whose Tg was determined to be equal to 91 °C. The polymer composites obtained by the combination of styrene and natural or esterified lignin, in a proportion of 5%, 10% and 20% of lignin, were successfully synthesized, presenting regular morphology. The incorporation of lignin (natural and modified) into the thermoplastic matrix of polystyrene (PS) led to a significant increase in the viscosity of polymer composites, in comparison to the one observed for the pure PS. Additionally, polymeric materials from viscosity essays were analyzed once more by thermal analysis (TGA and DSC), showing that the good thermal stability is kept, displaying weight losses lying in the interval from 350 °C e 480 °C. Particularly for the polymer composites containing modified lignin, the Tg was increased in comparison to pure PS, as a result of copolymerization between styrene and esterified lignin.