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

Abstract

The influence of the alkaline delignification of hemp on its moisture uptake has been studied under the project "Preparation and functionalization of hemp for textile substrates" focussed on the reduction of cotton imports for the manufacturing of clothing, household textiles and medical devices. The project looks for the partial replacement of cotton by hemp on the production of natural cellulosic textiles. Hemp fibres are alkaline delignified at different times to evaluate its effect on their moisture uptake behaviour to evaluate the ability of replacing cotton in the next-to-skin goods manufacturing. The paper gives three easy tools to analyse the moisture uptake behaviour by determining the sorption ratio, the hysteresis and the parameters of the isotherms fitted using three different models (GAB, Hailwood-Horrobin and Dent) to the absorption desorption isotherms. The samples of the original and delignified hemp have been subjected to moisture absorption/desorption trials from 5% to 95% RH at 25 C. Sorption ratio and hysteresis reveal the greater influence of lignin on moisture uptake at low relative humidities, while at high, was cellulose which plays the most important role. The models fitted to the isotherms show that delignification decreases the size of the monolayer that, in desorption results always greater than in absorption. The energy constants of the monolayer for delignified fibres in absorption were higher than those for the original fibres, while the opposite occurred in desorption. Lignin caused great differences between absorption and desorption. The energy constants of the multilayer show a greater range of variation in delignified samples than in the original one, and results in absorption were higher than those in desorption. Delignification time of 3 h reached the highest cellulose content which best approached to the cellulose content of cotton causing that the moisture uptake behaviour drew near to it, especially at higher relative humidity.

The Viscoelastic and Hygroscopicity Behavior of Delignified and Densified Poplar Wood

“…According to the Hailwood-Horrobin model, the moisture sorption isotherms can be divided into monolayer (M h ) and multilayer (M s ) water absorption regimes (Table 1 and Figure S2). As moisture absorption proceeds under dynamic equilibrium, water molecules gradually penetrate to result in a water-absorbed monolayer and continue to form a water polylayer through a secondary multilayer absorption process [36]. The M h is of particular interest as it represents the water directly bound to the hydroxyl groups in the active sites.…”

Kinetics of alkaline delignification of hemp and determination of lignin content by thermogravimetry

“…The different behavior between lignin and cellulose, the main component of cotton, at different moisture levels was identified. [6] The thermal degradation of the different components of hemp [7] can be monitored by thermogravimetry, as it was demonstrated previously for wood. [8] A revision made by Beall and Eickner showed the different steps of lignin decomposition, which exhibited a maximum decomposition rate between 350 C and 450 C. [9] In a previous work, [10] raw hemp was subjected to delignification by means of the application of sodium hydroxide [1M] at different times from 5 to 480 min.…”