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What are the common quality problems in pre-processing?

Views: 1000     Author: Site Editor     Publish Time: 2022-02-02      Origin: Site


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The quality of pre-treatment (liming and bleaching) is not only related to the quality of the finished bleached product, but also directly affects the quality of dyeing, printing, and finishing as a semi-finished product. Some experts say that 70% of the quality problems of dyeing and finishing products are caused by poor pretreatment. The quality of dyeing and finishing is related to grey fabrics, equipment conditions and operations, process technology and execution, dyes, and auxiliaries. We focus on the influence of auxiliaries.

1. Poor whiteness and gross effect

Poor whiteness includes impure, non-uniform whiteness and less than 8-10cm/30min wool effect, which is related to the quality of knitted fabrics. The key is the process formulation and the selection of additives.

In process formulation:

① The amount of H2O2 and NaOH must be sufficient

H2O2: After pretreatment, it not only plays the role of bleaching pigments, but also has the effect of removing impurities (including removing cottonseed husks). The amount of H2O2 is insufficient (including decomposition too fast), and the whiteness cannot be guaranteed. NaOH not only provides the required amount of H2O2 bleaching Alkalinity (PH), and in combination with the scouring agent, it has the greatest effect of removing impurities. If the amount of NaOH is insufficient, the removal of impurities will be poor. First of all, the gross effect cannot meet the requirements, and of course the whiteness cannot meet the requirements. Therefore, the amount of H2O2 and NaOH It can be said to be the primary factor to ensure whiteness and hair efficiency.

② Select high-quality scouring agent

The comprehensive effect of scouring agent such as cleaning, emulsification, dispersion and penetration must be excellent to ensure the removal of impurities and ensure whiteness and gross effect. It is not only to measure permeability and solid content, but permeability is only one aspect to be considered.

③ Oxygen bleach stabilizer and chelating dispersant

These two additives are mainly used to improve water quality, adsorb and complex metal ions such as Fe3+ in water, and prevent the rapid and ineffective decomposition of H2O2 catalyzed by metal ions. If H2O2 is decomposed quickly and ineffectively, its whiteness and gross effect will not reach At the same time, it will cause brittleness or even holes in the fabric, so to achieve whiteness and wool efficiency, the oxygen bleach stabilizer should be selected with excellent varieties, and the decomposition rate of H2O2 should be considered in the selection. The addition of the chelating dispersant not only chelates iron, but also can chelate various impurities in the dispersed water, so that these impurities will not re-contaminate the fabric and cause the whiteness to turn dark and gray. Therefore, the chelating dispersant not only Its chelating power for metal ions and its dispersing power should also be considered.

④ Process conditions: It takes a certain amount of time and temperature to react the auxiliaries with impurities. If the conditions are not met, the pigments and impurities will not be removed cleanly, and the whiteness and gross effect will also not be achieved.

⑤ Washing: Washing after training and bleaching is very important. It is necessary to clean and remove impurities to ensure whiteness and hair effect.

2. The cottonseed husk is not cleanly removed

The removal of cottonseed hulls mainly depends on the expansion and swelling of the cottonseed hulls by NaOH under certain conditions, and then the cottonseed hulls are removed by a certain mechanical cleaning force. It takes time to completely remove the cottonseed hulls. H2O2 and scouring agents can help to remove the cottonseed hulls. Therefore, there must be a certain temperature and time in the pretreatment to ensure the removal of the cottonseed hulls. At the same time, sodium hydrogen sulfite (NaHSO3) or anthraquinone can be added. A substance that aids in the removal of cottonseed husks.

3. Holes and brittle damage

In addition to singeing holes, hook breaks during processing, and original holes in grey fabrics, the main reason for holes and brittle damage is the presence of Fe3+, Cu2+ and other metal ions in the H2O2 bleaching bath or on the cloth surface. Metal ions catalyze the decomposition of H2O2 to rapidly decompose, resulting in fiber brittleness.

If the metal ions (such as rust, etc.) are too concentrated in one place, the fibers and yarns will become brittle and form holes. Therefore, high-quality oxygen bleach stabilizers and chelating dispersants must be selected for H2O2 bleaching to complex and adsorb fabrics and bleaching baths. Metal ions in it to prevent H2O2 from decomposing too quickly and causing holes or brittle damage. If mercerizing and recycling alkali is used for bleaching, it must be ensured that there are no impurities such as rust in the alkali.

Of course, the cloth surface with alkali (unclean to alkali) or acid (unclean to acid) will also cause brittle damage. At the same time, in the case of high temperature and alkali, the fabric will be brittle in the presence of air, which can be prevented by adding weak reducing agents such as NaHS03. In addition, damp and moldy fabrics are also one of the reasons for brittleness and holes.

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