Yellow spots, oil spots, pulp spots, white spots, alkali spots, rust spots, calcium spots, and air-drying marks are all spots. What should I do?

Product defects are an important factor affecting the quality of dyeing and finishing products, and are also an important indicator of the production and management level of enterprises. Pretreatment is the front process of dyeing and finishing, and it is also a very important process. In daily production, if there are many defects in semi-finished products after pretreatment, it will not only cause waste of manpower and material resources, but also reduce the quality of finished products and affect the reputation of the factory; for this reason, it is an important task for dyeing factories to improve the quality of pre-treated semi-finished products and strive to reduce the defects of pre-treated semi-finished products. This article describes the common spot defects in the pre-treatment process according to their morphology, causes, and solutions.

1. Common spot defects in the pre-treatment process

1. Yellow spots; 2. Oil spots; 3. Pulp spots; 4. White spots; 5. Alkali spots; 6. Rust spots; 7. Calcium spots; 8. Air-drying marks

2. Causes and solutions

1 Yellow spots

①. Causes of yellow spots:

The common cause of yellow spots is the use of river water, which is more likely to occur in the rainy season. This is mainly due to the poor water quality of the river. The river water contains organic suspended matter. During the pre-treatment process, these organic suspended matter cannot be fully dispersed. During the pre-treatment process, they are re-condensed with the impurities removed from the fabric and returned to the fabric, resulting in yellow spots. During the post-finishing process, such as whitening, white spots will be produced; if dyeing, color spots will be produced.

②. Solutions:

For yellow spots caused by this reason, a chelating dispersant with better dispersibility should be selected during the pre-treatment process. The chelating dispersant has a certain chelating force, can chelate metal ions in the water, has excellent dispersing performance, is equivalent to a reactive dispersant, has strong pertinence, and has a better effect.

2 Oil spots

①. Causes of yellow spots:

Pre-treatment oil spots are mainly manifested as oily, nearly transparent dot spots produced during the pre-treatment process. The general cause is the large amount of fiber oil contained in the fabric, and the spandex oil containing dimethyl silicone oil on the spandex fabric. After these oils are washed off during the pre-treatment process, they cannot be fully emulsified and dispersed, causing condensation and staining on the fabric. Especially for low-grade chemical fiber fabrics, if inferior chemical fiber oil is used or the amount of oil is increased, oil spots are easy to occur.

②. Solution:

For chemical fiber oil and dimethyl silicone oil spandex oil, it is necessary to select an emulsified degreasing agent with excellent oil dispersibility, and use it in combination with degreasing agent. It can be used alone for fabrics with light oil.

3 Slurry spots

①. Cause analysis:

It is mainly due to the large amount of chemical slurry contained in the fabric. Generally, chemical slurries are high molecular polymers with high polymerization degree, high viscosity and easy to condense. After being washed off during the pre-treatment process, they cannot be fully emulsified and dispersed, causing condensation and staining on the fabric surface. Its shape is similar to cloud-like white or yellow patches. Some are similar to sticky substances or yellow colloid substances, which stick to the fabric to form irregular dots.

②. Solution:

For the above situations, dispersants or desizing agents with excellent dispersibility of slurry should be selected during the desizing process. Emulsifying refining agents have excellent dispersing effect on slurry, have a decontamination effect, and also have an emulsifying effect on oil agents. Desizing agents focus on desizing effect and have a good dispersing effect on slurry. They all belong to the type of surfactants, which achieve desizing and anti-return through penetration, emulsification, and dispersion. They are mostly used in continuous desizing and overflow machines.

In addition, especially PVA (polyvinyl alcohol) slurry, the solubility in water has a certain relationship with temperature. When the temperature drops suddenly, the PVA in the desizing tank will re-condense. This condensed particulate matter will re-stick to the cloth surface to produce slurry spots. Therefore, the temperature of the desizing liquid cannot fluctuate. In addition, the desizing liquid must be replaced regularly. During the desizing process, the slurry concentration in the desizing tank will gradually increase. When it reaches the saturation point, the slurry will re-condense and re-stick, so the desizing liquid should be replaced in time.

4 White spots

①. Cause analysis:

Water quality reasons, high water hardness and impurities can cause white spots during whitening;

Slurry re-sticking, white spots will appear when the place where the slurry spots appear is whitened;

Oil stains, white spots will appear when the cloth with oil is whitened.

②. Solution:

For white spots caused by water quality reasons, water treatment should be strengthened during pre-treatment, and chelating dispersants should be added to improve water quality.

If it is caused by slurry re-sticking, add emulsifying desizing agents during desizing to improve emulsification and dispersibility. After desizing, water washing should be strengthened to prevent slurry re-sticking.

If it is oil re-sticking, the emulsification of degreasing should be strengthened, and emulsifying degreasing agents should be used.

5 Alkali spots

①. Causes of occurrence:

The scouring liquid contains many impurities, which make the alkaline impurities stick to the fabric; the air-dried marks and alkaline spots are formed if the fabric is not dried in time after desizing and mercerizing.

②. Solution:

Wash and dry in time after desizing and mercerizing;

If the scouring liquid contains many impurities, add chelating dispersant to the scouring liquid to improve the water quality to prevent the occurrence of alkaline spots; repair the alkaline spots. If it is confirmed to be an alkaline spot, it can be removed by washing with dilute hot hydrochloric acid.

6 Rust spots

①. Causes of occurrence:

Rust on the grey cloth itself, rainwater dripping when the cloth is stored, and water leakage on the iron shed roof cause rust spots; rust on the cloth cart, equipment, rust and rot of the water pipes may cause rust spots.

②. Solution:

When the water supply is cut off or the production is stopped for a long time, drain the water for a while before adding water for production or wrap gauze at the water pipe mouth to filter the water before production.

If the grey cloth itself has rust, it can be washed away with warm oxalic acid solution before pre-treatment. Note that fabrics treated with oxalic acid must be cleaned with clean water to prevent the fabric from being brittle and causing a decrease in strength. Methods for cleaning rust spots,

For reference only: Oxalic acid    20g/L

Acetic acid (98%) 30ml/L

7 Calcium spots

①. Cause analysis:

It is mainly caused by the hardness of water. Due to the high hardness of water, there are more calcium and magnesium ions in the water. During scouring, insoluble calcium and magnesium substances will be precipitated and attached to the fabric to produce calcium spots; in addition, during bleaching, calcium spots are easily caused by the calcium remaining on the fabric (such as calcium hypochlorite used in the bleaching liquid, and the lime decomposed in the bleaching liquid).

②. Solution:

If the water used for printing and dyeing is hard, some water softeners, sodium hexametaphosphate, CT powder, etc. should be added to soften the water during pre-treatment; in addition, the calcium remaining on the fabric during bleaching should be washed away.

If calcium spots are found before dyeing, they can be removed by washing with dilute hot hydrochloric acid.

8 Air-drying marks

①. Cause analysis:

This air-drying mark mainly occurs after desizing and before oxygen bleaching. The defect shape is dot-shaped and irregular horizontal stripes. After this defect occurs, it cannot be covered up in the subsequent processes such as whitening and dyeing. These defects are mainly caused by the fact that the sizing material is not completely washed after desizing, the wet cloth is piled for too long, and the fabric exposed to the air surface is partially air-dried and produces spots.

②. Prevention and solution:

Preventive measures

Reasonably control the production progress, and do not pile up too much fabric after desizing and before the oxygen bleaching machine, and the stacking time should not be too long; strengthen the cleaning of fabric after desizing. If there is too much fabric after desizing, cover the fabric with plastic film to prevent the fabric from being partially blown dry by the wind and causing air-drying marks. Solution

After the air-drying mark appears, the fabric that produces the air-drying mark should be desizing again and then oxygen bleaching, and the air-drying mark can be removed.