Japanese consumers have strict criteria for colorfastness of cellulose fiber dyed with reactive dyestuff. Deterioration in colorfastness is often caused by insufficient soaping after dyeing. There is a method, however, to solve the problem economically.

1. Preface

Reactive dyestuff belongs to the group of dyestuffs widely used for dyeing/printing of cellulosic fiber. A number of reactive dyestuffs are sold and their product lines are diversified, from conventional ones to high-tech products by discharge and resist printing processes. In the case of reactive dyestuffs, new production technology has been established and reports have been made on various studies; basic technology, complex application technology, production control, control engineering, waste water treatment. The soaping process, as far as reactive dyes are concerned, is one part of the dyeing process which contains more complex factors than the soaping process used with dyes belonging to any other group of dyestuffs. Reactive dyestuff has many different types of reactive groups that range from mono-functional to poly-functional groups, and its chemical structure also varies in many ways. Equally, fixing rates also differ. What is common in all cases though is that they do not react with cellulose fiber completely. After dyeing or post-printing steaming, the fiber will contain the kinds of dyestuff shown below:

(a) Reactive dyestuff fixed on fiber (b) Inactive, unfixed dyestuff (c) Hydrolyzed dyestuff produced in the presence of water and alkaline or acid

Although the amount of such reactive dyestuff remaining on the fiber may even increase depending on the condition of pH level, temperature and processing time, it can all be removed completely by soaping. During high-temperature alkali dyeing and washing, hydrolyzed dyestuff is produced as the bond between the dyestuff and fiber breaks down. This hydrolyzed dyestuff holds so strong an affinity that it causes redeposition and adsorption, therefore, it is very difficult to remove the compounds (b) and (c). Deficient soaping often leads to bleeding, staining of white ground and redeposition, resulting in insufficient wet and washing fastness. What is required in soaping after reactive dyeing, therefore, is powerful soaping properties that can strike a balance between the removal of unfixed dyestuff and prevention of redeposition.