In the previous article, we introduced three methods on how to control the color difference of dyeing samples. Next, we will continue to introduce several other methods.

1. Control of operating process

When producing large samples, choose the appropriate dyeing method according to the type and color of the fabric. Production practice has proved that proper dyeing and reasonable process formulation are the prerequisites for achieving dyeing success once. Dyes with different structures have different dyeing properties. The dyeing process must be adapted to the practical properties of the dye to obtain the best dyeing effect.

1.1 Dyeing temperature

Dyeing temperature is one of the key factors affecting the dyeing results. Whether it is making small samples or large vat production, the control of dyeing temperature must be particularly careful. Different dyes have different dyeing temperatures. For example, disperse dyes require high temperature and high pressure dyeing, while acid dyes require 100 degrees, and medium-temperature reactive dyes can be colored at only 60 degrees, so we must determine different coloring temperatures according to the different fiber components of the grey cloth. In addition, the temperature when making small samples should be consistent with the temperature of large vat production. Dyeing temperature actually refers to the temperature of the dye liquor. The temperature displayed on the temperature meter of the small sample machine and the large vat sometimes cannot correctly reflect the actual temperature of the dye liquor. Therefore, we need to regularly detect and correct the temperature of the small sample machine and the large vat to fundamentally eliminate the influence of the dyeing temperature on the dyeing results, so as to effectively reduce the color difference between the large and small samples and the difference between the large vats.

1.2 pH value of dye bath

The influence of the pH value of the dye bath on the dyeing results is sometimes greater than that of the dyeing temperature. Various dyes have a pH value range when dyeing. Only by controlling the pH value of the dye bath within the optimal range can the dye fully develop color and obtain good reproducibility. For example, the optimal fixing pH value range of medium-temperature reactive dyes is 10.5-11 (except for reactive turquoise, which is 10.5-12). When dyeing polyester with disperse dyes, it must be carried out under weak acidic conditions. The pH of the commonly used disperse dye bath is about 5-6. which is the safest, the color is the purest, and the color is the deepest. A high pH value means a low safety factor, especially for dyes with poor alkali resistance and stability, such as Runtu's 200% Deep Blue HGL and Deshida's XF Navy Blue. However, when the pH of the dye bath is too low (<4), many dyes will also tend to be lighter or darker.

On-site testing found that using the pH value of the residual liquid after dyeing as the basis for testing and controlling the pH value of the dye bath is safer and more reliable. This is because the pH value of the water bath before dyeing and the pH value of the residual liquid after dyeing are different.

There are three main factors that cause the pH value of the dye bath to move:

(1) The water used for dyeing is alkaline: River water in many areas is alkaline water, that is, the pH value before heat treatment is close to neutral, but it becomes alkaline after cooling after heat treatment above 100°C.

(2) The semi-finished product to be dyed contains alkali: Generally, grey cloth must be pre-treated with alkaline before dyeing. The alkali agent carried by the fabric is difficult to remove even after neutralization. Therefore, alkali release occurs during the dyeing process.

(3) The pH value of the dye solution is unstable: different dyes have different pH values ​​at different concentrations. For example, the pH value of acid yellow M-2R is 7.8 at 2g/L, and 8.5 at 5g/L.

When these alkaline substances introduced into the dye bath from the outside world have a weak pH buffering capacity, the pH value of the dye bath will exceed the safe range set by the process, resulting in a significant decrease in the reproducibility of dyeing and a significant increase in the rework and repair rate. Therefore, improving the buffering capacity of the pH value of the dye bath is the key to stabilizing the dyeing quality. It must be pointed out here that the practical ratio and concentration of the pH buffer system determine the pH value and buffering capacity of the dye bath, so it must be used correctly and must not be added at will. The pH adjuster used in laboratory small samples and large tank production must be unified, which is also one of the main factors in controlling the color difference between large and small samples.

1.3 Bath ratio control

The bath ratios for small samples and large vat production must be unified. For water-soluble dyes such as reactive dyes, acid dyes, and direct dyes, the bath ratio has a considerable impact because the dyes will hydrolyze in water. Even for non-water-soluble disperse dyes, the bath ratio is also more obvious when making dark samples. Therefore, before making small samples, the bath ratio for large vat production should be calculated according to the number of meters and gram weight of the grey cloth invested by the customer, and then the small samples should be made according to this bath ratio. According to the formula requirements, the amount of solution of the added dye and auxiliary agent should be added up first, and the amount of water to be added should be calculated. Then, water should be taken with a measuring cylinder and added to the dye cup to reach the formula bath ratio, and then dyeing and proofing should be carried out. Now most of our factories use automatic dripping systems. The instrument will automatically add the required water according to the amount of dye or auxiliary agent added to reach the set total bath volume. It has high precision, small error, and no operation error between different operators. When the bath ratio is designed to be small or the sample fabric area is large, the fabric is not evenly immersed in the dye solution, and it is easy to have color spots in the sample. Once the sample color spots appear, it is difficult to formulate the dyeing formula, and the success rate of dyeing in one time is reduced. Therefore, to ensure the uniformity of the sample, the sample can be cut into several small pieces and then added to the dye cup to improve the uniformity of the sample.

1.4 Heating rate and holding time

The heating rate should be 1-2℃/min. Too fast a speed may cause excessive temperature gradient in the cylinder, resulting in uneven dyeing and color spots; too low a speed may affect production efficiency. The holding time is divided according to the color depth. Light colors are generally kept warm for 30 minutes, and medium and dark colors are generally kept warm for 45-60 minutes. After the insulation is completed, the temperature is lowered to room temperature, and the first washing after dyeing is carried out to fully wash away the residual liquid and floating color remaining on the cloth. Too fast heating and cooling may cause defects such as color spots and wrinkles, which may cause repairs and directly affect the success rate in one time. When making small samples, the holding time must also be unified with the workshop. No matter how compatible the dyes are, their dyeing temperatures and times are different. Different holding times will result in slightly different colors after dyeing. For example, the three most commonly used light primary colors 2BLN blue, E-GL yellow, and 3B red, the shorter the holding time, the worse the color of 3B red, so controlling the uniformity of the heating rate and holding time is also a key point to reduce the color difference between large and small samples.

In addition to the above points, the reasons for the color difference between large and small samples include the degree of automation and accuracy of dyeing and finishing equipment, the accuracy of the number of barrels, the correctness of the proofing operation, the speed and temperature of the post-processing finalization, the drying method of the sample cloth, the color matching light source, etc., which will cause color differences. Therefore, we must also implement standardized and refined management, formulate detailed and complete operating standards, strictly check the standardized operation, and continuously introduce advanced automation equipment (computer color measurement and matching system, fully automatic feeding system, small sample automatic dripping system, etc.) to ensure that our first process is correct, so as to reduce the color difference between large and small samples and improve the one-time success rate of dyeing.

There are many reasons for the color difference between large and small samples. It not only involves different processes such as pre-treatment, dyeing, and finishing, but is also closely related to equipment, management, operating factors, and the selection of dyes and chemicals. Therefore, we must carefully control every link and detail in the production process and master the laws of change, reduce the difference between large and small samples, and improve the one-time success rate of dyeing.