Technology | Why does the small sample not match the large sample when dyed?

2022-12-13 | Textile Fabric Knowledge

When polyester knitted fabric is dyed with disperse dyes, why does the small sample not match the large sample?

Dyeing factories usually make a sample in the laboratory, and then enlarge the sample in the workshop according to the sample. The inconsistent shade and color difference between the sample and the large sample may be caused by the following aspects.


01 different grey fabrics

The grey fabric should be refined or degreasing treatment before dyeing, and the sample may not be pre-treated, or the sample treatment method is different from the large sample production in the workshop. The moisture content of the grey cloth is different, and the different moisture content of the sample has a greater impact, and the weighing is different due to the different moisture content. For this reason, it is required that the grey fabric to be sampled must be exactly the same as the grey fabric produced in the workshop.

In addition, is the pre-treatment of the grey fabric pre-shaped? If the large-scale gray fabric has been pre-shaped, the small-scale gray fabric has not been pre-shaped, even the large and small samples have been pre-shaped, and the different setting temperature can also cause different absorption colors.


02 The difference of dyes

Although the dyes used for the small samples and the large samples are of the same variety and strength, different batch numbers or insufficient precision when weighing the small samples may cause differences between the small samples and the large samples. It is also possible that the dyes used in large-scale production have agglomerated and become damp, and some dyes are unstable, resulting in a decrease in power.

03 Different ph value of dye bath

Generally, it is more accurate to control the pH value of the dye bath for small samples, but the pH value is unstable during the production of large samples or no acid-base buffer is added. Due to the alkalinity of the steam during dyeing, the pH value rises during the production of large samples. Some disperse dyes such as Containing ester groups, amido groups, cyano groups, etc. are hydrolyzed under high-temperature alkaline conditions. There are some dyes whose carboxyl groups can be ionized under alkaline conditions, increasing water solubility and decreasing dye uptake. When the pH value of most disperse dyes is 5.5-6, the shade is normal and stable, and the dye uptake rate is also higher.

But when the ph value increases, the color light changes. Such as disperse black s2bl, disperse dark blue hgl, disperse gray m and other dyes when the pH value>7, the shade changes significantly. Sometimes the grey fabric is not fully washed after pre-treatment and it is alkaline. The pH value of the dye bath increases during dyeing, which affects the shade.


04 The influence of bath ratio

In the small sample test, the bath ratio is generally larger [1: (25~40)], while the bath ratio of the large sample varies according to the equipment, generally 1: (8~15). Some disperse dyes are less dependent on the bath ratio, and some are highly dependent, so that the color difference is caused by the difference between the bath ratio of the small sample and the large sample.

05 The impact of post-processing

Post-processing is one of the reasons that affect chromatic aberration. Especially for medium and dark colors, if the reduction cleaning is not carried out, or the cleaning is not clean, in addition to the floating color, it can also affect the color light and produce a certain color difference. Therefore, the reduction cleaning must be the same as the sample.


06 The effect of heat setting

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

The same type of dyes should be selected for color matching. In case of high temperature and low temperature color matching, the setting temperature should not be too high during heat setting to avoid excessive temperature, which may cause some dyes to sublime and affect the shade and produce color difference. The requirements for the shaping conditions of the small sample and the large sample are basically the same.

Because the pre-treatment and setting conditions (temperature) have a great influence on the color absorption of polyester (the greater the degree of setting, the lower the dye uptake), so the small sample fabric must be the same as the large sample (that is, before production Resample with workshop semi-products), this is one of the keys.

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