Fiber Spinning from Cellulose Solutions in Imidazolium Ionic Liquids: Effects of Natural Antioxidants on Molecular Weight, Dope Discoloration, and Yellowing Behavior

Abstract

Spinning of cellulosic fibers requires the prior dissolution of cellulose. 3-Alkyl-1-methylimidazolium ionic liquids have proven to be suitable solvents for that purpose, but the degradation of cellulose in the spinning dope can be severe. Suitable stabilizers are therefore required that prevent cellulose degradation, but do not adversely affect spinnability or the long-term yellowing behavior of the fibers. A group of twelve renewables-based antioxidants was selected for stabilizing 5% cellulose solutions in the ionic liquids and their effects on cellulose integrity, dope discoloration, and aging behavior were tested by gel permeation chromatography (GPC) and ISO brightness measurements. Propyl gallate (a gallic acid derivative), hydroxytyrosol (from olives), and tocopheramines (a vitamin E derivative) performed best in the three test categories, minimizing both cellulose degradation, chromophore formation in the spinning dope, and yellowing upon accelerating aging of the spun fibers. The use of these stabilizers for cellulose solutions in the imidazolium-based solvent system can therefore be recommended from the point of view of both performance and sustainability.

Synthesis and analytical characterization of N-methylated derivatives of α-tocopheramine and their oxidation products

“…Using the reaction conditions of previous work (350 °C for 3 h), we observed complete consumption of the starting tocopheramine (1) in the presence of basic alox, a yield of 72% of the targeted N,N-dimethyl-α-tocopheramine (13), absence of the quaternized product, and formation of byproducts as black, viscous tar. As higher reaction temperature and longer reaction times promote side reactions, in particular chromophore formation, we tried to reduce temperature and reaction time as much as possible, while in return increasing the amount of alox to keep the conversion complete.…”

“…Apparently, the initially formed N-methyl-α-tocopheramine (12) was a better nucleophile than the starting amine 1, because even with a 1.4fold molar excess of alkylating agent, some starting amine remained unmethylated. With a 1.5-fold molar excess, the starting amine was completely consumed, but half of the starting amine was converted to the N,N-dialkylated product N,N-dimethyl-α-tocopheramine (13) besides the targeted N-monomethylated 12, so that chromatographic purification was required (46% and 49%, respectively). The alternative Leuckart-Wallach approaches provided always a mixture of mono-and demethylated products when using less than 2.5 molar HCHO-equivalents.…”

“…Tocopheramines are biocompatible [2,3], have been proposed as food and feed additives [4,5] and studied for their promising anticancer and proapoptotic activities [6][7][8][9][10]. Similar to tocopherols, which made the move from medical, physiological uses and usage as major food/feed additives to all kinds of technological applications as antioxidants, also tocopheramines have been tested with good success with respect to general uses as antioxidant stabilizers in technical applications, such as surfactants [11], polymer stabilizers [12], and stabilizers for Lyocell spinning dopes and the resulting cellulosic fibers [13,14]. However, large scale application had been hampered by two facts: first, the limited availability: α-tocopheramine is usually synthesized according to a rather complex multi-step approach which has not yet left lab scale [15].…”

“…At least the obstacle of unknown degradation products has been removed recently, with the structural elucidation and analytical characterization of all monomeric N-oxidation [18] and dimeric N-N coupling products of α-tocopheramine [19]. It can also be expected that the problem of limited availability will be tackled with new vigor as recent results have shown tocopheramines to be excellent stabilizers for cellulosic fiber spinning from ionic liquid solutions [13] which should result in much greater demand to be met. α-Tocopherylquinone (3) [17,20] is the main product of the oxidation of α-tocopheramine under aqueous conditions or in the presence of water, accompanied by small amounts of monomeric N-oxidized derivatives (4-6), usually around 2-5% (Scheme 1).…”

“…Tocopheramine derivatives have recently been employed as stabilizers for cellulose solutions in N,N-dialkylimidazolium ionic liquids, to hold autoxidation reactions at bay that otherwise would oxidatively damage the polymer, decrease the molecular weight of the celluloses and cause discoloration of the spinning dope and the resulting fibers [13]. Interestingly, the N-alkyl-protected derivatives, available only in milligram amounts, performed better than the parent α-tocopheramine, both with regard to the antioxidative effect and with regard to the minimization of chromophoric compounds.…”