Stabilizer chemistry of the lyocell process: I—solubility, readsorption and general oxidation chemistry of ellagic acid and its quinoid oxidation products

Ellagic acid (EA) is the main transformation product of the stabilizer propyl gallate in the lyocell process and can undergo oxidation to highly chromophoric species. These compounds significantly contribute to discoloration of spinning dopes and regenerated fibers, affecting fiber quality and necessitating additional purification or bleaching steps. Despite their relevance, their behavior under lyocell conditions remains poorly understood. This study investigates the solubility, adsorption, and readsorption behavior of EA and its oxidation product, ellagiquinone (EQ), in aqueous systems relevant to the lyocell spinning bath, with particular emphasis on the influence of N -methylmorpholine N -oxide (NMMO) and its degradation products in the spinning bath and in the aqueous washings. Solubilities of EA and EQ were determined in water and in water containing defined concentrations of NMMO, N -methylmorpholine (NMM), and morpholine (M). Adsorption and readsorption experiments were conducted using regenerated cellulose and model washing systems. The distribution of the compounds between liquid and solid phases was quantified by UV/Vis spectrophotometry and chromatographic methods. EA and EQ exhibited very low solubility in pure water (≈ 10 mg L −1 and lower), but their solubility increased markedly in the presence of NMMO. Concentrations above approximately 3 wt% NMMO enhanced solubility sufficiently to prevent interaction with cellulose surfaces, while NMM and M increased only the solubility of EA (not that of EQ) by deprotonation. During cellulose regeneration, efficient removal of EA and EQ was achieved at NMMO concentrations ≥ 5 wt%, whereas lower concentrations resulted in significant retention within the cellulose matrix. Readsorption experiments demonstrated that, at low or zero NMMO concentrations, dissolved EA and EQ are transferred back onto cellulose fibers, leading to enrichment on the surface. When both compounds were present, co-adsorption often resulted in the formation of the intensely colored ellagiquinhydrone complex. This process was suppressed at higher NMMO concentrations or under alkaline conditions (presence of NMM). These findings highlight the critical role of spinning bath composition in controlling solubility and adsorption phenomena. Maintaining sufficient NMMO levels during early washing stages is essential to prevent chromophore deposition and fiber discoloration in the lyocell process.