Dyeing of polyester knitted fabrics with disperse dyes

Dyeing factories usually make small samples in the laboratory, and then produce large samples in the workshop according to the small samples. The reasons for the inconsistent color light and color difference between small samples and large samples may be in the following aspects.

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① Different grey cloth.

The grey cloth shall be refined or degreased before dyeing, and the small sample may not be pretreated, or the treatment method of the small sample is different from that of the large sample production in the workshop. The moisture content of grey cloth is different, and the moisture content of small samples has a great impact. Due to different moisture content, the weighing is also different. Therefore, it is required that the grey cloth with small sample must be completely consistent with the grey cloth produced in the workshop.

In addition, is the pre-treatment of grey cloth predetermined? If the large sample grey cloth has been shaped, the small sample grey cloth has not been shaped, and even the large sample and small sample have been shaped, and the different setting temperature can also cause different color absorption.

② Different dyes.

Although the dyes used for small samples are the same as those used for large samples in terms of variety and strength, the differences between small samples and large samples may be caused by different batch numbers or inaccurate weighing of small samples. It is also possible that the dyes used in bulk production have been caked and damped, and some dyes are unstable, resulting in a decrease in strength.

③ The pH value of dyeing bath is different.

Generally, the pH value of the dyeing bath is more accurate for small samples, while the pH value is unstable or no acid-base buffer is added in the production of large samples. Due to the alkalinity of steam during dyeing, the pH value increases in the middle of the production of large samples, and some disperse dyes such as ester group, amido group and cyano group are hydrolyzed under high-temperature alkaline conditions. In some dyes, the carboxyl group can be ionized under alkaline conditions, the water solubility increases and the dye uptake decreases. When the pH value of most disperse dyes is 5.5 ~ 6, the color light is normal and stable, and the dye uptake is also high.

However, when the pH value increases, the color light changes. For example, disperse black s2bl, disperse dark blue HGL, disperse grey m and other dyes change significantly when pH > 7. Sometimes grey cloth is alkaline without sufficient cleaning after pretreatment, and the pH value of dyeing bath increases during dyeing, which affects the color light.

④ Effect of bath ratio.

During the small sample test, the bath ratio is generally large [1: (25 ~ 40)], while the large sample bath ratio varies according to different equipment, generally 1: (8 ~ 15). Some disperse dyes are less dependent on bath ratio and some are more dependent on bath ratio, which will cause color difference due to the different bath ratio between small sample and large sample.

⑤ Effect of post-treatment.

Post treatment is one of the reasons affecting the color difference. Especially for medium and dark colors, if reduction cleaning is not carried out or the cleaning is not clean, in addition to floating color, it can also affect the color light and produce a certain color difference. Therefore, the reduction cleaning must be consistent with the small sample and the large sample.

⑥ Effect of heat setting.

Disperse dyes can be divided into high temperature type, medium temperature type and low temperature type.

The same type of dye shall be selected during color matching. In case of high-temperature and low-temperature color matching, the setting temperature shall not be too high during thermal setting, so as to avoid the influence of color light and color difference caused by the sublimation of some dyes due to too high temperature. The setting conditions of small sample and large sample are basically the same.

Because the pre-treatment setting or not and the setting conditions (temperature) have a great impact on the color absorption of polyester, the cloth for small samples must be consistent with the large samples (i.e. duplicate samples of semi-finished products in the workshop before production), which is one of the keys.