Cellulose solutions in N-methylmorpholine-N-oxide (NMMO) -: degradation processes and stabilizers

Efficient stabilization of cellulose solutions in NMMO (1) against side reactions and their harmful effects means prevention of both homolytic and heterolytic side reactions, which is mainly accomplished by trapping radicals, formaldehyde, and N-( methylene) iminium ions (5). While radical trapping is commonly reflected by the antioxidative efficiency, the effectivity against heterolytic degradation in 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, acts as a phenolic antioxidant, and is finally oxidized to a deeply colored, highly conjugated chromophore (11) via ellagic acid (10). It was demonstrated that 4 is also a quencher of formaldehyde and N-( methylene) iminium ions, both in organic solutions of NMMO and in Lyocell dope. The processes of radical trapping and scavenging of HCHO/ 5 are competitive in the case of 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, PBD releases acetaldehyde which acts as a very efficient HCHO trap. Thus, in contrast to propyl gallate, radical trapping and HCHO trapping are not competitive. Both the antioxidative efficiency and the capacity to trap HCHO and 5 are higher for PBD as compared to propyl gallate. In preliminary stabilizer testing, mixtures of PBD and PG proved to be especially effective.