Studies in the stability of bonds between fibre-reactive dyes and cellulose
A breakdown of the bond between dyes containing a 2,4-fluoro-5-chloropyrimidinyl reactive group and cellulose occurs when the dyeing is treated at room temperature in a detergent containing sodium perborate or an equivalent amount of hydrogen peroxide and is subsequently exposed to a source of heat and light. Studies of the breakdown of the dyeings have shown that a nucleophilic substitution reaction takes place at the 5 position of the pyrimidinyl ring when the dyeing is treated with hydrogen peroxide, during which the chlorine substituent is replaced by a hydroperoxide group. It has been shown also that the dyeing has to be in its hydrolysed form (Type III) before it can form the hydroperoxide. Other dyeings have been shown to form hydroperoxides but not all show dye fibre bond breakdown. Dichlorotriazinyl dyeings are in this last category. Dichloroquinoxalinyl dyeings, on the other hand, showed as much breakdown as the 2,4-fluoro-5-chloropyrimidinyl. Three pyrimidinyl dyes were prepared with different substituents in the 5 position of the ring: H, Cl and CN. The 2,4-dichloropyrimidinyl dyeing (with H at the 5 position) did not show any significant breakdown, even at a high pH in the hydrogen peroxide solution, whereas with 2,4,5-trichloropyrimidinyl dyeings the breakdown at pH 12 was significant. The 2,4-chloro-5-cyano pyrimidinyl dyeing showed the highest breakdown of all, which confirms that the substituent at the 5-position of the pyrimidinyl ring is a determining factor. The reaction which causes the breakdown of the dye-cellulose bond has been shown to be a radical reaction initiated by the decomposition of the hydroperoxide. The presence of some antioxidants when the hydroperoxide of the dyeing was exposed to the heat and light source was found to reduce the breakdown. One of the aspects of the breakdown of the dyeings was that it was accompanied by a deterioration in brightness of the colour, and this was very marked with the yellow and orange colours. Both the deterioration in brightness and the dye-cellulose bond breakdown were reduced by previous alkaline hydrolysis at the 5-position of the pyrimidine ring, thus confirming the relevance of the formation of a hydroperoxide at this position to the breakdown of the dyeings. One pyrimidinyl dye which showed bond breakdown with hydrogen peroxide but could not fit the above theory was the 2-methylsulphonyl-4-methyl-5-chloropyrimidinyl dye. This dye does not produce the hydrolysed forms of the dyeing and consequently it is not easily understood why the apparently unreactive 5 position of the pyrimidinyl ring should form a hydroperoxide via a nucleophilic reaction.