The chemistry of the lyocell process: a critical review of recent developments

Abstract The lyocell process, which utilizes N -methylmorpholine N -oxide monohydrate (NMMO) as a direct, non-derivatizing solvent for cellulose, currently represents the most environmentally benign technology for producing man-made cellulosic fibers. While dissolution and spinning are often perceived as physical processes, the system is, in fact, governed by a complex network of chemical side reactions. This review provides a critical examination of the “chemistry” of the lyocell process, with a focus on developments from the past two decades. The discussion encompasses the homolytic (radical) and heterolytic (ionic) degradation pathways of NMMO, which lead to solvent consumption and the formation of byproducts, such as N -methylmorpholine and morpholine. These reactions are linked to the autocatalytic degradation of NMMO and related “exothermic events”, but also to oxidative degradation of cellulose, resulting in chain scission and the generation of carbonyl functionalities. A significant consequence of these processes is the formation of chromophores, which cause discoloration of the spinning dope and the final fibers; their identification and formation mechanisms are also discussed. Given the unavoidable nature of these side reactions, a substantial part of this account is dedicated to the chemistry of the stabilizers, with propyl gallate serving as a key example, which are introduced to minimize those side reactions and their effects. The review analyzes the dual antioxidant and intermediate-scavenging roles of stabilizers, their consumption mechanisms, and their unintended involvement in chromophore generation. Finally, the influence of critical process factors, such as temperature, cellulose feedstock quality, and the impact of metal ions, is evaluated, providing a comprehensive overview of the chemical challenges and advancements in modern lyocell technology.