* The role of sequestering agents in reactive dyeing

Is the use of a sequestering agent required when dyeing with reactive dyes?

When the water of the dyebath contains metallic ions, there is the danger that uneven dyeing, such as specking, or reduction of the concentration of the dye will be caused by the dyestuff's coagulation and reduced solubility. Aside from in the water, metallic ions can also be introduced by impurities in Glauber's salt or common salt, so even when soft water is used in the dyeing, the presence of metallic ions can lead to problems. In Japan, the water quality is generally very good and the chemicals used contain very few impurities, so sequestering agents are often not used. Overseas, however, the use of sequestering agents is very important.
In the past, traditional thinking has been that "if the chromophore of a dyestuff is a metallic complex salt, then sequestering agents should not be used." Recently, however, it has been ascertained that sequestering agents added to the dyebath have practically no effect on the metal in the chromophore of the dye.
Sequestering agents effective in neutral to alkaline conditions are preferable, and agents combining the effects of sequestering agents with those of anionic surfactants and marketed for use with reactive dyes are now being developed.

* The importance of surfactants in reactive dyeing

The standard recipes for reactive dyestuffs given in the examples provided by dyestuff manufacturers often do not include the addition of surfactants such as dyebath lubricants. Doe this mean that they are not required?
In the dyeing of cotton with reactive dyes, because hydrophilic fibers are dyed with water-soluble dyestuffs, the dyeing itself does not require the use of surfactants. However, in the dyeing of cloth, it is difficult to spread the cloth out in the dyebath, and so it is often dyed in rope-form, but because it is sometimes difficult to circulate the liquid over the cloth, rope marks can easily form. The use of a dyebath lubricant is recommended as a method for dealing with this problem. The use of cold and warm dyeing dyestuffs when the temperature of the dyeing is below 55°C means the fiber is dyed while it is in a hard state, making a dyebath lubricant especially important for improving the quality of the dyeing. When dyeing is conducted at a temperature above 60°C with warm and hot dyeing reactive dyestuffs, however, the fiber itself softens and spreads easily, so the use of a dyebath lubricant is not as important.
However, when the solubility of a reactive dyestuff is reduced due to factors such as association, the use of a surface-active dyebath lubricant can result in a dispersion effect. Furthermore, when a small amount of wax still remains on the substrate due to insufficient scouring, the use of a dyebath lubricant can promote the permeation of the dye liquid. Thus to lower the degree of risk in dyeing, the use of an appropriate dyebath lubricant is recommended.

* The role of alkali in reactive dyeing

When dyeing with reactive dyestuffs, soda ash is often used as the alkali in dyestuff manufacturers' recipes. Recently, however, a synthetic alkali has appeared. What is the role of alkalis in reactive dyeing and what should be considered when choosing one?

In the dyeing of cellulose with reactive dyes, alkali is necessary because it acts as a catalyst in the reaction between the dyestuff and the fiber. The important point is not the type or amount of alkali but rather the pH of the dyebath, which must be closely supervised.
The most suitable pH for dyeing varies with the temperature, being approx. 11.5 for common warm dyeing (dyeing at approx. 60°C), 10-11 for hot dyeing (80°C) and 12.5 for cold dyeing (40°C). The relationship between temperature and the optimum pH is shown in the following graph.

Soda ash is often used because 20g/l usually produces a pH of around 11.5. However, it can be used with sodium phosphate or caustic soda when a higher pH is required, or with sodium bicarbonate (baking soda) when a lower pH is required. Caustic soda is not often used because it is a strong alkali and achieving a pH in the range of 10-12 is difficult. Nothing prevents it from being used, however, if the pH can be adjusted accurately with automatic equipment.
Recently introduced alkaline agents specifically for use with reactive dyes adjust the pH of the dyebath to a level suitable for dyeing with reactive dyestuffs and are easy to use because they come in liquid form, but the alkaline agent to be used should be chosen in consideration of the optimum pH for the temperature at which the dyestuff is to be used. The pH of the first bath is often checked to determine the pH for the dyeing, but attention should be paid to the pH after dyeing because if alkali is not added in sufficient quantity, the pH at the conclusion of dyeing may be considerably lower.