Problems in Dyeing With Direct Dyes
Direct dyes represent an extensive range of colorants that are easy to apply and also are very economical. There are three common ways to classify direct dyes, namely, according to their chemical structure, according to their dyeing properties, and according to their fastness properties.
Of these three possible ways of classifying direct dyes, the first is of least importance to the dyer, although of considerable importance to those interested in dye chemistry. According to the Society of Dyers and Colourists' classification, which is essentially based upon the compatibility of different groups of direct dyes with one another under certain conditions of batch dyeing, there are three classes of direct dyes: A, Band C.
Class A consists of self-leveling direct dyes. Dyes in this group have good levelling characteristics and are capable of dyeing uniformly even when the electrolyte is added at the beginning of the dyeing operation. They may require relatively large amounts of salt to exhaust well.
Class B consists of salt-controllable dyes. These dyes have relatively poor levelling or migration characteristics. They can be batch dyed uniformly by controlled addition of electrolyte, usually after the dye bath has reached the dyeing temperature.
Class C consists of salt- and temperature-controllable dyes. These dyes show relatively poor leveling or migration and their substantivity increases rapidly with increasing temperature. Their rate of dyeing is controlled by controlling the rate of rise of temperature, as well as controlling the salt addition.
Important dye bath variables that influence the dyeing behaviour of direct dyes include temperature, time of dyeing, liquor ratio, dye solubility, and presence of electrolyte and other auxiliaries.
Direct dyes can be applied by batch dyeing methods (on jigs, jet or package dyeing machines), by semi-continuous methods (such as pad-batch or pad roll) and by continuous methods (such as pad-steam). Many direct dyes are suitable for application by combined scouring and dyeing. In this process the usual practice is to employ soda ash and non-ionic detergent. However, dyes containing amide groups are avoided because of the risk of alkaline hydrolysis.
Direct dyes vary widely in their fastness properties, and staining effects on various fibres. Most direct dyes, however, have limited wet fastness in medium to full shades unless they are after-treated.
The fastness of selected direct dyes can be improved in several ways, such as the following:
- Treatment with cationic fixing agents
- Treatment with formaldehyde
- Treatment with copper salts such as copper sulphate