The quality of pre-treatment agent is not only related to the quality of bleached products, but also as a semi-finished product, it will directly affect the quality of dyeing, printing, finishing, etc. Some experts say that 70% of the quality problems of dyeing and finishing products are caused by poor pre-treatment, which makes sense. The quality of dyeing and finishing is related to the grey fabric, the equipment and operation, the process technology and execution, the dye, and the auxiliaries. We focus on the impact of auxiliaries.

Common quality problems in pre-treatment and selection of auxiliaries

1.1 Poor whiteness and hair effect

Poor whiteness includes impure and uneven whiteness and hair effect that does not reach 8cm or more in 30min. This is related to the quality of the knitted fabric. For example, if the content of low-grade cotton is high, whiteness and hair effect are often difficult to achieve. Of course, the most critical issue is process formulation and auxiliaries selection. In process formulation:

(l) The amount of H2O2 and NaOH must be sufficient:

H2O2 not only plays the role of bleaching pigments in pre-treatment, but also has the effect of removing impurities (including removing cottonseed shells). If the amount of H2O2 is insufficient (including decomposition too quickly), whiteness cannot be guaranteed; NaOH not only provides the alkali (pH) required for H2O2 bleaching, but also cooperates with the scouring agent to play the greatest role in removing impurities. If the amount of NaOH is insufficient, impurity removal is poor, and hair effect cannot meet the requirements first. Of course, whiteness cannot meet the requirements either, so the amount of H2O2 and NaOH can be said to be the primary factor in ensuring whiteness and hair effect.

(2) Select high-quality scouring agents:

The comprehensive effects of scouring agents such as cleaning, emulsification, dispersion, and penetration must be excellent to ensure that impurities are completely removed and whiteness and hair effect are guaranteed. However, the quality of scouring agents on the market varies. We must choose products with excellent comprehensive effects instead of just measuring penetration and solid content. Penetration is only one aspect to consider.

(3) Oxygen bleaching stabilizers and chelating dispersants:

These two additives are mainly used to improve the nature, adsorb and complex metal ions such as Fe3+ in water, and prevent metal ions from catalyzing the rapid and ineffective decomposition of H2O2. If H2O2 decomposes rapidly and ineffectively, its whiteness and hair effect will not meet the requirements, and it will cause brittle damage or even holes in the fabric. Therefore, in order to achieve whiteness and hair effect, excellent varieties of oxygen bleaching stabilizers should be selected, and the decomposition rate of H2O2 should be considered in the selection. The addition of chelating dispersants can not only chelate iron, etc., but also chelate various impurities in the dispersed water, so that these impurities will no longer stain the fabric again and cause the whiteness to darken and gray. Therefore, the chelating dispersant should not only consider its chelating power for metal ions but also its dispersing power.

(4) Process conditions:

It takes a certain amount of time and temperature for the additives to react with impurities. If the conditions are not met, the pigments and impurities cannot be completely removed, and the whiteness and hair effect cannot be achieved.

(5) Washing:

Washing after scouring and bleaching is very important. The impurities removed must be cleaned to ensure whiteness and hair effect.

Incomplete removal of cottonseed hulls

The removal of cottonseed hulls mainly relies on the expansion and swelling of cottonseed hulls by NaOH under certain conditions, and then the cottonseed hulls are removed by a certain mechanical cleaning force. Incomplete removal is mainly caused by insufficient temperature and time, so sufficient time must be allowed at a certain temperature to completely remove the cottonseed hulls. H2O2 and scouring agents have a certain effect on the removal of cottonseed hulls, so a certain temperature and time must be maintained in the pretreatment to ensure the removal of cottonseed hulls. At the same time, NaHSO3 or anthraquinone substances can be added to help remove cottonseed hulls.

Holes and brittleness

In addition to singeing holes, hooking during processing, and original holes in grey cloth, the main reason for holes and brittleness is the presence of metal ions such as Fe3+ and Cu2+ in the H2O2 bleaching bath or on the cloth surface; these metal ions catalyze the rapid decomposition of H2O2. causing fiber brittleness. If metal ions (such as rust, etc.) are too concentrated in one place, the fibers and yarns will become brittle and form holes. Therefore, in H2O2 bleaching, high-quality oxygen bleaching stabilizers and chelating dispersants must be selected to complex and adsorb metal ions on the fabric and in the bleaching bath to prevent H2O2 from decomposing too quickly and causing holes or brittle damage. If bleaching is used

Of course, the fabric surface with alkali (not completely alkali-removed) or acid (not completely acid-removed) will also cause brittle damage. At the same time, under high temperature and alkali conditions, the fabric will also become brittle when there is air. In this case, weak reducing agents such as NaHSO3 can be added to prevent this.