Several Factors of Color Shading in Reactive Dyeing

Due to the natural structure of cotton fibre, it must undergo rigorous scouring to ensure relatively uniform water absorption and dye uptake during the dyeing process. A successful pretreatment is half the battle. The pretreatment must ensure sufficient scouring agent and alkali, as well as adequate treatment time and temperature. The auxiliaries should be added in batches and stepwise. Hydrogen peroxide is best added at 70 °C. If problems such as fabric entanglement or clogging of the dyeing machine occur, the holding time must be prolonged. It is essential to ensure that the fabric has sufficient and uniform wicking ability to guarantee even dyeing.

After pretreatment, washing must be thorough. An acid rinse can be used to ensure that the fabric surface has a neutral pH. Otherwise, under alkaline conditions – especially when the alkalinity is uneven – the dyeing rate can become too fast, leading to colour shading.

Reactive dyes are susceptible to oxidation. Residual hydrogen peroxide, especially when unevenly distributed, can attack and destroy the attached reactive dyes, causing them to change colour. This results in colour shading and off‑shade tones. Nowadays, catalase (deoxygenase) is commonly used to remove residual hydrogen peroxide. Ensuring sufficient deoxygenation time and performing oxygen‑content tests after deoxygenation can help avoid this risk.

Stir thoroughly and adjust the dissolving temperature appropriately. Different types of reactive dyes have different solubilities and temperature stabilities. In general, the dissolving temperature for reactive dyes should not exceed 60 °C; otherwise, high temperatures can cause rapid hydrolysis of the dye. For dyes such as Turquoise Blue and Brilliant Blue, which have larger molecular structures, the dissolving temperature can be appropriately increased. In addition, it is advisable to enhance filtration by placing a high‑density filter screen at the feed inlet to prevent undissolved dye particles from entering the dyeing machine and coming into contact with the fabric.

Salt and alkali are key to controlling the rate of dye exhaustion and fixation. Adding them too quickly causes pH differences within the dye liquor, leading to uneven dye uptake and resulting in colour shading. Therefore, during the dyeing process – especially when dyeing light or sensitive shades – salt and alkali must be added slowly and stepwise. After each addition, an interval of 10–15 minutes should be allowed for thorough mixing before proceeding to the next step. When dyeing rayon or light colours, it is also possible to pre‑add 1/10 of the total alkali in advance to avoid drastic pH changes later and to prevent colour shading.

Due to energy‑saving and emission‑reduction constraints, many factories now reduce the liquor ratio to lower costs and reduce the pressure on wastewater treatment. In particular, with air‑jet dyeing machines, the very low liquor ratio imposes stricter requirements on the dyeing process. It is essential to operate meticulously and not to be careless.

If washing after dyeing is insufficient, or if the pH during soaping is too high and the temperature is raised too quickly, colour shading can also occur. To avoid this, it is necessary to rinse out residual alkali as much as possible before soaping, control the heating rate, and ensure adequate soaping time. Alternatively, acidic soaping agents can be used to neutralise some of the alkali during the soaping process.

If the residual bath after soaping is not washed away, when the fabric is taken out, the concentration of residual liquor on the surface and inner layers may differ. If the fabric is not dried promptly, this can cause colour patches – which is commonly known as wind marks (or air‑drying stains). Therefore, at least one additional water wash is required after soaping to remove the residual liquor.

Adding colour is a common practice in dyehouses, but the amount of dye used is often very small – essentially a light‑shade dyeing. Moreover, since the original dyebath already contains a large amount of salt and alkali, the dyeing rate increases. Therefore, controlling the addition speed is even more important. When necessary, and provided that the required depth of shade can still be achieved, it may be useful to drain part of the dye liquor and add water to reduce the salt/alkali concentration. In some cases, when the colour difference is not large, using reactive dyes for topping‑up carries too much risk; disperse dyes can be used for fine adjustment instead. Because disperse dyes exhaust more slowly, they cause only slight staining on cotton fabrics. Thus, when the shade is slightly off and the colour is not too bright, disperse dyes can be used for fine adjustment.