The influence of pulp swelling degree during pre-treatment on the stability of lyocell solution

Abstract

Pulp pre-treatment acts as a preliminary treatment in the lyocell process, allowing aqueous NMMO solvent molecules to penetrate the wood fiber structure. The present paper first determines cellulose’s pre-swelling, swelling, and dissolution phenomena in NMMO aqueous solutions of different concentrations and temperatures. It was found that instead of “pre-swelling” and “dissolution”, the “swelling” action during pre-treatment develops the most suitable interaction between wood pulp fiber and solvent. Higher concentrations of NMMO aqueous solutions (70–80%) do not contain enough water and that limits the uniform wetting of the pulp, while a lower NMMO concentration (50%) is not able to fully establish the hydrogen bonding interaction between cellulose and NMMO. The pre-treatment of cellulose pulp using 60% NMMO aqueous solution at 65 °C can ensure that NMMO itself does not degrade and provides sufficient solvent diffusivity for pulp based on the swelling effect. This significantly reduces heterogeneous microstructure aggregation and effectively promotes subsequent dissolution processes. The dissolved cellulose has more uniformity at the molecular level, showing lower dope relaxation time and gel temperature, which offers better alternatives for stable, high-viscosity lyocell spinning, and constructs stronger fiber compared to that of un-treated pulp.