How to do Cold pad-batch dyeing of knitted fabrics

Cold pad-batch dyeing, as a semi-continuous dyeing method, realizes environmentally friendly dyeing of cotton knitted fabrics through processes such as semi-product pretreatment, pad dyeing solution, coiling, stacking, washing, soaping, washing and drying.

1. Pre-processing preparation

The cold pad-batch dyeing process uses low-temperature dyeing. In order to allow the dye to fully penetrate into the fiber, the impurities, sizing agent, spinning oil and machine oil stains on the fabric to be dyed must be thoroughly removed during the treatment, and the treated fabric must be Uniform and good wettability.

2. Dyeing process dye selection

 The selection of dyes should be comprehensively considered in terms of directness, reactivity and fixation rate.

The directness of the dye is low, which can reduce the color difference between the head and the tail, but the directness is too low and it will affect the adsorption and fixation of the reactive dye during dyeing. Therefore, it is more appropriate to choose a dye containing a vinylsulfone group and a s-triazine multi-reactive group with medium directness. 

Dyes have low reactivity and low fixation rate at room temperature, but highly reactive dichloro-s-triazine (X-type) reactive dyes have poor diffusibility on the fiber and are greatly affected by pH, which is easy to appear Floating color, poor wet fastness, and color difference is not easy to control.

When mixing and dyeing, the dyeing speed of each component dye should be balanced to reduce the color difference between the head and the tail, the color difference between batches, etc. In addition, taking into account the existence of slotting, roll changing, temporary parking, etc. in large-scale production, in order to maintain The uniformity of shade also has certain requirements for the alkali resistance and hydrolysis stability of the dye. The dye needs to have the characteristics of medium directness, good diffusibility, good permeability, good alkali resistance, excellent detergency, stable color and light, good reproducibility and high fixation rate.

3. Preparation of dye solution

In order to allow the dye to fully diffuse into the fiber before fixing and improve the color fastness, a suitable alkali agent must be selected. Someone once used a lye system composed of sodium silicate and caustic soda. Although sodium silicate can stabilize the pH value of the dye solution, it is easy to scale on the fabric and rollers, causing stains or fabric scratches, and the fabric after dyeing The hand feels hard. In view of the above reasons, a lye system composed of soda ash and caustic soda was used to adjust the appropriate pH value for different dye dosages.

4. Pad dyeing equipment 

For the cold pad-batch dyeing process of knitted fabrics, the dyeing paddle is the core of the whole device.

Because knitted fabrics are easy to curl, and when the warp tension is too large, the cloth surface is easy to stretch, causing weft shrinkage, weft skew, and even a decrease in fabric surface density, so the cloth feeding system is equipped with an automatic central unwinding device and tension adjustment Device to ensure that the fabric enters the rolling car evenly and steadily. At the same time, the fan-shaped expansion device is used when the cloth is fed, which can center and expand the fabric on the one hand, and on the other hand, ensure that the tension of the fabric fed into the rolling car is low.

In addition, in order to ensure the same pressure on the upper and lower sides of the fabric, some equipment companies use vertical roller arrangements for dyeing paddles for elastic fabrics, and the clamping guide rollers have also changed from a conventional horizontal arrangement to an optimized 70° arrangement. That is, a set of rollers are arranged obliquely according to the cloth travel path. The fabric is fed into the nip point synchronously at the same angle up and down to prevent the difference in rolling pressure on both sides of the fabric. The diagonally placed guide rollers reduce the tension on the fabric during the pulling process.

5. Rolling and stacking

After the fabric passes through the rolling car, it is sent to the winding device through the guide roller. In order to avoid the color difference caused by the tension change of the knitted fabric during the winding process, the A-frame needs to be automatically driven to rewind, that is, as the cloth roll increases, it passes Under the control of the tension detection device, the A frame moves forward from the initial position at a constant speed away from the roll, achieving constant tension and constant line speed to ensure that the dyed fabric is smoothly wound up.

6. Washing

In order to fully remove the unreacted dyes and hydrolyzed dyes on the fabric, the rolls are stacked for a certain period of time and then the rolls are rolled back, and washed in the open-width drum washing machine to remove floating colors to improve the wet fastness of the dyed knitted fabrics. You can wash it with hot water first, then cold water. If the washing fastness is very high, it can be soaped and washed first, and then fixed with a fixing agent.