Soybean hulls: Optimization of the pulping and bleaching processes and carboxymethyl cellulose synthesis

Soybean hulls, a co-product generated in high volumes, were used to obtain pulp and CMC. The pulping process was optimized with the aid of 1%, 2%, and 2.5% NaOH solutions at 90 °C for 2 h. A 2² central composite design was used in order to optimize the bleaching process and the CMC synthesis. Volumes of bleaching solution (V_S) of between 55 and 65 mL/g at temperatures between 85 and 95 °C and V_S of 70 and 75 mL/g at 95 °C were applied in the pulp bleaching process. The factors considered in the carboxymethylation were the chloroacetic acid mass (1.2-2.1 g/g) and the reaction time (192-228 min), at 63 °C. The soybean hulls contain 40.62% of cellulose and have a low lignin content. The pulping process was optimized when 1% NaOH was used at 90 °C/2 h and bleaching process applying V_S = 75 mL at 95 °C/4 h. The pulps showed low lignin content (<6%) and the cellulose had a high degree of crystallinity. The SEM, ¹H NMR, XRD, FTIR and TGA/DTG analysis results demonstrated that it is possible to synthesize CMC (DS = 1.45) by acetylating the bleached pulp with 2.1 g of chloroacetic acid for 192 min, at 63 °C.