Important Factors of Cold Brand Reactive Dye | Factors of Reactive Dye

The Important Factors are as Follows:

1) pH of the Dye Bath:
The optimum pH for fixing cold brand reactive dyes on cotton and viscose rayon depends on individual dyes, the temperature and time of dyeing. pH decreases with increasing temperature and time of dyeing. For most of the dyes the optimum pH is 10.8 to 11 at 20o to 25oC. Soda ash has been the best alkali for dyeing at 30oC for cotton, mercerized cotton and linen. Increased fixation (due to higher temperature) and increased dye bath stability and better reproducibility are the advantages of soda ash as the fixing agent.

For viscose rayon the optimum pH is 10.3 at 20o to 25oC.

2) Amount of Alkali:

The amount of alkali used for fixing depends on the depth of shade dyed and the liquor ratio employed. 

3) Dyeing Temperature:
Since increase in temperature affects the rate of physical and chemical processes involved in dyeing, it is important in dyeing reactive dyes also. The affinity of the dye for the fibre decreases with increases in temperature and at the same time the rate of hydrolysis of the dye increases and adversely affects the fixation of color yield. However the rate of diffusion of the dye in the fibre increases with increased temperature. At temperatures lower than 20oc, the rate of fixation is very low. Hence for most of the dyes a temperature of 20o to 25oC is the recommended temperature while for some other dyeing at 50o to 60oC with sodium bicarbonate as the alkali gives maximum color value.

4) Electrolyte Concentration:
Since reactive dyes have low affinity for cellulose exhausting the dye bath by adding common salt or Glauber’s salt prior to fixation can increase the fixation. The amount of salt required producing adequate exhaustion decreases with decreasing liquor ratio. Thus for pale shade on cotton and viscose rayon 15 and 10 g/l of common salt used. The quantities may be increased to 30 and 20 to 30 g/l for medium and deep shades on these fibres.

5) Time of Dyeing:
Generally the dye may be added in two portions. The salt may also be added in two lots. The exhaustion takes place in 20 to 30 min. There is generally no advantage in extending the period beyond 30 min. The alkali is then added and the dyeing continued for 30 to 90 min. The depth of shade and reactivity of the dye decide the time of dyeing. For deeper shades larger times are required.

6) Liquor Ratio:
With decreased liquor ratio, both exhaustion and fixation take place to increased exert. However the rate of fixation of most of the dyes is not significantly affected. As the liquor ratio is decreased, the effectiveness of increasing salt addition also decreases. Hence lower amount of salt are sufficient to get optimum exhaustion. 

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