3. Dyeing cotton with reactive dye (example of type II)

1) Characteristics of the rate of dyeing

In the reactive dye/cotton dyeing system, the dye is absorbed on to the surface of the fiber, then the dye gradually penetrates into the fiber and makes covalent bonds with the functional group of the fiber (OH group in the case of cotton).

Therefore, the rate of absorption curve and rate of fixation curve are measured as the rate of dyeing curve of reactive dye as shown in Figure 2.

The levelness should be considered on the basis of the absorption curve, although the fixation curve is important for evaluating the final build up. ( The absorption curve can be regarded as a overlapping of two of the curves of type Ⅱ and the fixation curve is a general type Ⅱ)

Figure 2 Rate of dyeing curve of reactive dye

The absorption curve can be divided into two parts; the primary exhaustion which proceeds with the addition of inorganic salt and the secondary exhaustion which proceeds with the addition of alkali.

The primary exhaustion of individual dye depends largely on its affinity, and commercial dye can be classified according to its degree of primary exhaustion as shown in Figure 3.

Figure 3 Classification of reactive dye by its absorption curve

The exhaustion of the dye of type A proceeds quickly after the addition of alkali, therefore, the rate of exhaustion can be controlled by adjusting the addition of alkali.

The primary exhaustion of the dye of type B after the addition of inorganic salt is almost the same as that of secondary exhaustion after the addition of alkali. Therefore, this type of dye is the easiest for obtaining level dyeing.

The affinity of the dye of type C to the fiber is as high as direct dye, which can be almost entirely absorbed by the addition of inorganic salt only. Therefore, the rate of dyeing is controlled by adjusting the addition of inorganic salt.

2) Dyeing cycle for the level dyeing

1. Control by the rate of temperature rise

There is no direct proportion between the rate of temperature rise and the rate of dyeing (exhaustion and fixation), although the rate of dyeing may partly be controlled by the rate of temperature rise. For example, 50% reduction of the rate of temperature rise gives around 30% decrease of the rate of dyeing. Then, it is difficult to control the rate of dyeing by the rate of temperature rise only, and this method is not effective because of the longer dyeing time.

However, this method is introduced for some dyes as “all-in method”, but it generally gives the problem of levelness when the time for temperature rise is 30～60min., so it is applied only in the dyeing of heavy shades.

2. Control by portion-wise addition of auxiliary

The rate of exhaustion can be controlled by adjusting the addition of inorganic salt or alkali because primary exhaustion progresses by the addition of inorganic salt and secondary exhaustion by the addition of alkali.

An example of the rate of exhaustion when the portion-wise addition of inorganic salt or alkali is carried out is shown in Figure 4.

In practical dyeing, there are two methods for the addition of auxiliaries; portion-wise addition and continuous dosing, and the former is more convenient.

Figure 4 Control of the rate of dyeing by portion-wise addition of auxiliaries

Example of practical portion-wise addition of Glauber’s salt anhyd. or soda ash is shown in Table 2. The method of portion-wise addition differs depending on the type of dye classified by its primary exhaustion shown in Figure 3, and the dyeing program should be set considering the type of primary exhaustion.

• Type A (low primary exhaustion) : In this type, primary exhaustion is low and secondary exhaustion is high, then 1～2 portion-wise addition of Glauber’s salt anhyd. and 4～5 portion-wise addition of alkali are recommended.
• Type B (medium primary exhaustion) : 2 portion-wise additions of Glauber’s salt anhyd. and 3～4 portion-wise additions of alkali are recommended.
• Type C (high primary exhaustion) : In this type most dye is exhausted in the primary exhaustion, then 3 portion-wise additions of Glauber’s salt anhyd. and 1～3 portion-wise additions of alkali are recommended.

Table 2 Optimum portion-wise additions for Glauber’s salt anhyd. or soda ash

On the other hand, as far as the dosing method is concerned, the amount of inorganic salt required for dyeing is large and the amount of its solution for dosing (30% concentration is maximal for Glauber’s salt anhyd.) becomes huge and the dosing liquor may overflow the dyeing vessel.

Glauber’s salt anhyd. (powder) is added to the auxiliary tank and dissolved with dye liquor, then gradually added to the dyebath, giving the same effect as the dosing method and being popular in dyehouses.

The role of alkali is to make the dyebath pH suitable for reaction, and strong alkali such as caustic soda may be used if the pH can be strictly controlled. In the alkali dosing method, the increase of dye liquor can be ignored by using strong alkali.

The dye of type A is suitable for alkali dosing method, where a progressively increasing method is generally employed (some examples are shown in Figure 5).

In the case of the dye of type B, the alkali dosing may be employed but the range controlled by alkali dosing is about half of the total exhaustion, then it is less effective.

In the case of the dye of type C, the range of exhaustion controlled by alkali addition is very small so that it is not effective.

Figure 5 Example of dosing curves