Do you look at the laundry label when washing clothes? Why does the soap sometimes develop color when washing clothes?

1 What is the testing principle of color fastness to soaping?

According to GB/T3921-2008 "Color fastness test of textiles, color fastness to soaping": textile samples are sewn together with one or two specified standard backing fabrics, placed in soap solution or mixed with soap and anhydrous sodium carbonate In the liquid, mechanical agitation is carried out under the specified time and temperature conditions, and then it is washed and dried.

Using the original sample as a reference sample, use a gray sample card or an instrument to evaluate the discoloration of the sample and the staining of the lining fabric.

2 How to confirm whether the product is qualified?

It is necessary to carry out the test according to the product standard of each product and judge whether it is qualified according to its inherent quality requirements. Different product standard test methods and inherent quality requirements may be different.

For example, sample A is a qualified product of knitted casual clothing FZ/T73020-2019. According to this standard, the color fastness to soaping, discoloration and staining ≥ grade 3 are qualified.

As shown below, the right side of Figure 1 is the original sample, the left side is the sample after the test, and the discoloration gray card is used to rate the discoloration of the sample.

The right side of Figure 2 is the lining fabric before the test, and the left side is the lining fabric after the test.

It can be seen from Figure 1 and Figure 2 that after the GB/T3921-2008 A(1) test, the sample is rated as 4-5 for discoloration and 3-4 for staining, which is qualified. (The larger the series, the better the color fastness)

3 Why sometimes the soap will develop color when new clothes are washed?

The principle of soaping and decontamination mainly uses the hydrophilic group and lipophilic group of soap. The hydrophilic group destroys the attraction between water molecules and reduces the surface tension of water, making it easy for water to penetrate clothes; Oily dirt is emulsified and dispersed in water to achieve the purpose of decontamination.

In the dyeing process, the dye will combine with the fiber, and some dyes can be chemically combined with the fiber to form a covalent bond, which is not easy to fade; while some dye molecules are only unilaterally attached to the surface of the fiber to form a floating color. As soon as it is acted on by soapy water, we will see that the soapy water is dyed. After rinsing with a lot of water, the color fading phenomenon will be reduced, and the color of the soapy water will become lighter. Before wearing new clothes, it is recommended that you wash them first, so as not to affect your health.

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