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Abstract
It is known that, the pH of the printing paste which is prepared for printing cotton fabrics with reactive dyes should be alkaline. The importance of the alkaline medium of the printing paste is to increase the ionizing power of the hydroxyl groups of cellulose. This medium is able to give reactive dyes a chance to react with water and thus dye hydrolysis occurs.
Under the ideal printing conditions of cotton fabrics with reactive dyes, only about 60-70% of reactive dyes can be fixed on cotton fabrics. In case of printing wool fabrics with reactive dyes, it is found that 99% of reactive dyes can be fixed under the ideal printing conditions.
The aim of the present work is to study the possibility of using an acidic printing paste to print cotton fabrics in order to minimize the ability of hydrolysis of the dye during paste preparation and printing process to obtain maximum fixation of reactive dyes on cotton fabrics. Therefore, reactive dyes of high reactivity can be applied on cotton.
It is found that to obtain an acid printing paste and in the same time an alkaline medium during fixation of reactive dyes on cotton fabrics [by steaming or thermofixation], trichloro- acetic acid [TCAA] can be used as it is a very strong acid owing to its high dissociation constant. It decomposes at high temperatures [about 100°C] to give a free radical. The decomposition of [TCAA] salt at high temperatures [during the steaming or thermofixation processes] leads to changing the pH level from the acidic to alkaline range. The behavior of this acid has proved the ideal steps and requirements to obtain an acid printing paste and an alkaline medium during fixation of reactive dyes on cotton fabrics at high temperatures.