Biocompósitos termoplásticos de ésteres de amido do ácido esteárico com modificação dupla incorporados com celulose microfibrilada

Abstract

The development of starch-based bioplastics is a technology that has presented many scientific and industrial advances to overcome the functional limitations that make them less competitive than fossil-based plastics. Among the alternatives, the chemical modification in starch and the reinforcement of thermoplastic matrices with cellulosic micro and nanofibers stand out. This research was divided into two phases. Initially, the objective was to evaluate the dual modification of cassava starch, through mechanical activation by ball-milling and heterogeneous esterification by in situ activation of stearic acid with p-toluenesulfonyl chloride. The physicochemical changes of the starch with the ball-milling carried out for 3 hours and 6 hours in relation to the native starch were analyzed. Then, the impact of ball-milling time on the degree of substitution (DS) achieved in the esterification was studied. With ball-milling, changes were observed in the morphology and size of the granules, water absorption properties, crystallinity, and thermal properties of the starch, as well as in the infrared absorption band characteristic of the ester groups (1740cm-1) and in the DS of the starch esters produced, which varied between 0.39-1.15 as a function of mechanical activation time. Therefore, double starch modification showed to be effective in increasing the DS of cassava starch esters. In a second phase of the research, the DS of the starch esters was optimized as a function of the reaction time and the molar ratio of stearic acid. Two esters produced with DS1=0.11 and DS2=0.14, in addition to native thermoplastic starch (AN) were reinforced with 5% and 10% micro fibrillated cellulose (MFC), to obtain six thermoplastic bio-composites that were processed by extrusion and molded into films by thermos-compression. Films were characterized by structural, thermal, mechanical, hydrophobicity and water vapor permeability (WVP) analysis as a function of the DS of the esters and the MFC content. Starch esterification was found to reduce the crystalline phase of the films and the degradation temperature as confirmed in the diffractograms and thermograms, however the addition of MFC favored thermal stability. It was observed in the bio-composites with GS2 the appearance of the glass transition at 41°C and 49°C, an increase in hydrophobicity (θ>90°), and these films were more rigid and less resistant because of the increase in GS. The modulus of elasticity increased, and the elongation was reduced proportionally with the MFC content. Tensile strength was reduced in biocomposites with starch esters. PVA did not change significantly between treatments. Thus, the double modification of starch and the dispersion of MFC during processing, observed in fracture microscopy, were decisive in the mechanical performance of bio-composites. The films produced can be useful in the composition of laminated cellulosic packaging to promote moisture barrier properties.