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Why does jet overflow dyeing of cotton knitwear produce unstable color and poor reproducibility? How to deal with it?

Why does jet overflow dyeing of cotton knitwear produce unstable color and poor reproducibility? How to deal with it?

Conventional jet overflow dyeing of cotton knitwear often has some problems, such as unstable color (dark color, color light), poor color difference and inconsistent size. The crux of the problem is the defect of conventional jet overflow dyeing process of cotton knitted fabric.

Jet overflow dyeing of all cotton knitted fabric is different from that of all cotton woven fabric. Jet overflow dyeing of cotton woven fabric is the direct dyeing of scouring and bleaching semi-finished products. The jet overflow dyeing of cotton knitwear is the same process of scouring, bleaching and dyeing of grey fabric. The process is: knitting grey fabric into VAT → scouring and bleaching → washing → reactive dye dyeing.

The problem here is that after scouring and bleaching grey knitted fabric with caustic soda (or soda ash) and hydrogen peroxide, it is often washed with rope like water, and it is difficult to remove the alkali and hydrogen peroxide from the fabric. However, the stability of using reactive dyes to resist hydrogen peroxide is poor. Even if there is a small amount of hydrogen peroxide left in the dye solution, under normal dyeing conditions, the reactive group or chromophore group of some reactive dyes will be destroyed, thus losing the dyeing ability.

The stability order of the commonly used reactive dyes resistant to hydrogen peroxide is as follows:

Thermosetting (100 ℃ reactive dyes > high temperature (80 ℃) reactive dyes > medium temperature (60 ℃) reactive dyes and low temperature (40 ℃) reactive dyes.

The middle temperature reactive dyes and low temperature reactive dyes which are commonly used to dye cotton knitwear have the worst stability to hydrogen peroxide. Therefore, once the washing water is not clean after scouring and bleaching, there is hydrogen peroxide residue in the dye solution. When dyeing, some dyes will be damaged, resulting in color loss and discoloration. If the residual amount of hydrogen peroxide is different or uneven, it will inevitably cause color difference, cylinder difference or color flower.

To eliminate the negative effects of hydrogen peroxide on the dyeing results (depth, shade, levelness) of reactive dyes, there are two effective measures:

① Washing with water. After scouring and bleaching, the fabric is washed with hot water, warm water and cold water for many times to remove the residual alkali and hydrogen peroxide. This traditional washing method obviously has the disadvantages of high energy consumption, more pollution discharge and high cost, which is not in line with the current concept of "energy saving, emission reduction and efficiency improvement".

② Biological enzyme method. After scouring and bleaching, the residual hydrogen peroxide is removed by enzyme peroxidase.

Catalase is produced by deep fermentation of non pathogenic microorganisms. It has specificity. It can decompose hydrogen peroxide into water and oxygen quickly only when it works with hydrogen peroxide.

Even if the concentration is excessive, it will not have a negative impact on the fabric and dyeing results. Because of its good compatibility with dyes and auxiliaries, only one wash is needed after scouring and bleaching. Then neutralization, deoxidation and dyeing can be carried out in the same bath. Obviously, biological enzyme deoxidization has outstanding advantages of "energy saving, emission reduction and efficiency improvement".

Process mode:

Operation flow: after scouring and bleaching or dyeing, let off the bleach. Wash with cold water for about 10min, and drain the washing water. Add clear water, add appropriate acetic acid while running, adjust the pH value to 6.5 ~ 7.5, and increase the temperature to 40 ~ 50 ℃ (as required). Add biological enzyme (0.25-0.5g / L), run for deoxygenation for 10-20min, without draining. Add auxiliaries and dyestuffs for dyeing, adjust the temperature to the temperature required by the dyestuff, and then dye, wash, soap and water according to the routine.