Influence of alkaline delignification on moisture uptake behavior and bonding enthalpies of hemp

Abstract

Raw hemp fibers are alkaline delignified to evaluate the effect of lignin on themoisture uptake behavior of hemp. Samples are subjected to moisture absorp-tion/desorption isotherms from 5% to 95% relative humidity at three differenttemperatures (15, 25, and 35 C). The sorption ratio between delignified anduntreated hemp is lowered by 25%. Hysteresis makes evident the greater influ-ence of lignin on moisture binding at low relative humidity while at highrelative humidity, cellulose plays the most important role. The GAB,Hailwood-Horrobin and Dent models are fitted to absorption/desorptionisotherms. Delignification and temperature decrease the size of the monolayer.The energy constants of the monolayer differ according to the model used.Lignin causes great differences between absorption and desorption. The greaterthe temperature is, the lower the monolayer energy constant. The energy con-stants of the multilayer are identical for the three models, showing a greaterrange of variation in delignified samples than in the untreated samples. Basedon primary and secondary energy constants, bonding enthalpies are determined.Estimations in desorption show higher correlation coefficients with the energyconstants, which enables to conclude that measures in desorption better explainthe effect of delignification on the water uptake behavior of hemp