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Efficient stabilization of cellulose solutions in NMMO(1) against side reactions and their harmful effects meansprevention of both homolytic and heterolytic side reactions, which is mainlyaccomplished by trapping radicals, formaldehyde, andN-(methylene)iminium ions (5). Whileradical trapping is commonly reflected by the antioxidativeefficiency, the effectivity against heterolyticdegradationin the Lyocell dope can be expressed by the newly introduced term‘formaldehyde trapping capacity’ (FTC). Propyl gallate (PG,4), the most widely applied Lyocell stabilizer nowadays, actsas a phenolic antioxidant, and is finally oxidized to a deeply colored, highlyconjugated chromophore (11) via ellagicacid (10). It was demonstrated that 4 is alsoa quencher of formaldehyde and N-(methylene)iminium ions,both in organic solutions of NMMO and in Lyocell dope. The processes of radicaltrapping and scavenging of HCHO/5 are competitive in the caseof propyl gallate. A novel oxa-chromanol derivative, PBD (14),was designed as stabilizer for Lyocell solutions. In analogy to propyl gallate,PBD acts as a scavenger of all three dangerous species, namely HCHO,5 and radicals. Upon oxidation by radical species, PBDreleasesacetaldehyde which acts as a very efficient HCHO trap. Thus, in contrast topropyl gallate, radical trapping and HCHO trapping are not competitive. Boththeantioxidative efficiency and the capacity to trap HCHO and 5are higher for PBD as compared to propyl gallate. In preliminary stabilizertesting, mixtures of PBD and PG proved to be especially effective.