On the chemical fate of propyl gallate as stabilizer in Lyocell spinning dopes

Abstract

Propyl gallate (PG, gallic acid propyl ester, Tenox®) is a very frequently used stabilizer and antioxidant in food and material applications, also used on large scale in the Lyocell process to stabilize the cellulose spinning dopes. In the present study we have investigated the fate of PG under classical Lyocell conditions (cellulose dissolved in N-methylmorpholine-N-oxide monohydrate at temperatures above 80 °C) by means of multiple analytical techniques, including NMR, GPC, and MS-hyphenated chromatography. It is demonstrated that propyl gallate is quickly hydrolyzed to gallate or, as a side reaction, aminolyzed to gallic acid morpholide. The products of the antioxidative action of gallic acid and propyl gallate, namely ellagic acid and its bis(ortho-quinone), are very easily adsorbed to cellulosic surfaces and are chiefly responsible for the discoloration of Lyocell spinning dopes and the resulting fibers. However, gallic acid morpholide and its parent compound gallic amide, while both having good antioxidant activity similar to propyl gallate itself, do neither form ellagic acid nor the bis(ortho-quinone) and are thus significantly superior to the conventionally used propyl gallate with regard to chromophore generation and resulting brightness loss. Also gallate moieties in tannins can be converted into the amide or morpholide to be further used as Lyocell stabilizers, which might open the way both to new Lyocell stabilizers and to a niche utilization for tannins.