Chromophores in spinning dopes of cellulose and imidazolium ionic liquids

Spinning of cellulose dissolved in 3-alkyl-1-methylimidazolium ionic liquids is a current approach to manmade cellulosic fibers. To curb cellulose degradation in the dope, stabilizers have to be used. Generation of chromophores in the dope and the resulting fibers is unavoidable, with most of the discoloration resulting from the employed antioxidants. In the present work, the chemical structures of the chromophores derived from the most common “green” stabilizers currently used, namely propyl gallate (a gallic acid derivative), hydroxytyrosol (and antioxidant from olives), and tocopheramines (vitamin E derivatives), were elucidated by a combination of analytical techniques and final confirmation by comparison with independently synthesized samples. While most compounds are oxidation and rearrangement products of the antioxidants themselves, coupling products with the ionic liquid´s imidazolium cations were also found. Identification of the colored compounds is a critical issue for both the optimization of fiber bleaching steps and solvent purification/recovery in the spinning process. Now that the nature of the generated chromophores is known, milder yet more effective bleaching treatments can be developed and solvent purification/recycling can be specifically optimized.