The influence of pulp swelling degree during pre-mixing on the stability of Lyocell solution

Abstract

Pulp pre-mixing acts as a preliminary treatment in the lyocell process which allows aqueous NMMO solvent molecules to penetrate wood fiber structure. The present paper first determines the pre-swelling, swelling, and dissolution phenomena of cellulose in NMMO aqueous solutions of different concentrations and temperatures. It was found that instead of “pre-swelling” and “dissolution”, the “swelling” action during pre-mixing develops the most suitable interaction between wood pulp fiber and solvent. Higher concentrations of NMMO aqueous solutions (70–80%) contain a small amount of water, which limits the uniformly wetting of the pulp, while lower NMMO concentration (50%) are not able to fully establish the hydrogen bonding interaction between cellulose and NMMO. The pre-mixing of cellulose pulp using 60% NMMO aqueous solution at 65^oC can ensure that NMMO does not degrade and provide sufficient solvent diffusivity for pulp based on the swelling effect, significantly reducing the formation of heterogeneous microstructure aggregations and effectively promoting 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 to the need for stable, high-viscosity lyocell spinning, and constructs stronger fiber compared to that of untreated pulp.