Thermal esterification of soybean husk polysaccharides with citric acid

The study was aimed at analyzing the efficiency of thermal esterification of soybean husk polysaccharides with citric acid and its effect on the mechanical characteristics of compressed husks. The thermal treatment of a mixture of husks and citric acid was studied within the temperature range of 110–170 °С without the use of solvents and catalysts. The duration of thermal treatment varied from 30 to 180 min. This process was found to be accompanied by the hydrolysis of husk polysaccharides and their esterification within the analyzed temperature range. The main products of thermal treatment are modified husks and bio-oils. The bio-oils contain predominantly the products of citric acid transformation (55 to 82%). The low-molecular-weight transformation products of husk polysaccharides are represented by furan compounds. The process of esterification was confirmed by the results of studying modified husks via infrared spectroscopy and conductometric titration. The accumulation dynamics of ester linkages in husk biomass were also analyzed in relation to temperature and thermal treatment duration. The cross-linking degree of cellulose polymer chains was found to increase with increasing temperature and thermal treatment duration. Comparative tests of the original compressed husk samples and the modified husks revealed an improvement in their mechanical properties following thermal treatment in the presence of citric acid. A 1.2-fold strength increase and a 2.5-fold stiffness increase were observed. The presented results experimentally confirm the potential of esterification of soybean husks with citric acid, with their use as a filler in the production of composite materials.