Influence of the time of alkaline delignification on moisture uptake behaviour of hemp

Abstract

Raw hemp fibres are alkaline delignified at different times in order to evaluate the effect of time of delignification on moisture uptake behaviour of hemp. Lignin content of samples is measured. Samples of original and delignified hemp are subjected to moisture absorption/desorption isotherms from 5% to 95% of relative humidity at 25ºC and moisture uptake behaviour is assessed through measuring the sorption ratio of delignified vs. original hemp. Hysteresis makes evident the greater influence of lignin in moisture binding at lower relative humidities, while at higher humidities cellulose plays the most important role. GAB, Hailwood-Horrobin and Dent models are fitted to absorption/desorption isotherms. Delignification decreases the size of the monolayer that depends on the way it has been measured: Size measured in desorption is always greater than that in absorption. Lignin greatly contributes to the growth of the size of the monolayer. The energy constants of the monolayer differ according to the used model: results of delignified fibre in absorption are higher than those of the original ones while the contrary occurs in desorption. Lignin causes great differences between absorption and desorption. The energy constants of the multilayer are identical for the three models showing greater range of variation in delignified samples than in the original ones. Results in absorption are higher than those in desorption. The level of relative humidity at which primary moisture uptake ties with the secondary one is determined and it has been observed that decreases with the energy constant of the multilayer, while primary and secondary regain at this level show a strong relationship with the size of the monolayer and growths with them