Analysis of causes of yarn yellowing after bleaching and whitening

Analysis of causes of yarn yellowing after bleaching and whitening

After the bleached and whitened yarn is left for a certain period of time, yellow spots will appear, and the whiteness will decrease as time goes by.

1. Analysis of the main reasons

1. Analyze the reason for the occurrence of yellow spots. It is caused by the poor quality of bleaching water and high content of divalent iron ions. The divalent iron ions are oxidized or trivalent iron ions over time, thus forming yellow spots on the yarn. This kind of yellow spots not only affects the appearance quality, but also makes the fibers brittle and damaged.

The way to overcome it is:

When the water quality is poor, pickle the yarn with 1.5-2.0g/L oxalic acid at 80-85℃ before bleaching. Because oxalic acid can form complex ions with iron ions and dissolve in water, the iron in the water can be removed to achieve the purpose of non-yellowing.

2. The main reason why the whiteness of bleached and whitened yarns decreases over time is due to insufficient scouring. If the yarn is not scoured enough and the impurities in the cotton fiber are not completely removed, the yarn will be bleached and whitened immediately. Over time, the impurities and natural pigments inside the fiber will be exposed, causing the whiteness to decrease.

The way to overcome it is:

It is necessary to ensure that the wool effect of bleached yarn reaches 13-15cm/30min, and the wool effect must be uniform to ensure that the bleached and whitened yarn does not turn yellow.

3. In addition, when dechlorinating after bleaching with sodium hypochlorite, too much sodium hypochlorite or insufficient washing can also cause the yarn to turn yellow.

4. When drying yarn, the temperature of the drying room is too high and the drying time is too long, which will also cause the yarn to turn yellow.

2. Other reasons:

1. It contains acid and alkali, hydrogen peroxide cannot be washed clean, softener, etc. will cause yellowing.

2. Different processes will produce different yellowing. Such as oxygen bleaching and hydrogen peroxide has not been washed, the pH value is too high or too low, and there are silicone oil-type softeners, etc.

3. Excessive dosage of whitening agent will cause the fabric to turn yellow, especially when the fabric has been left for a period of time, the greater the moisture content of the fabric, the more obvious the yellowing will become. You cannot blindly increase the dosage of whitening agent to achieve the whiteness requirement.

Nowadays it is common to use DT whitening agent. The analysis is as follows: From the principle of fluorescent whitening agents, it can be known that the whitening effect of whitening agents on fibers or fabrics mainly depends on the content of ultraviolet rays in sunlight and the concentration of fluorescent whitening agents on the fabrics. When the concentration of the whitening agent reaches an appropriate concentration, the whitening effect is good. If the concentration of the whitening agent exceeds a certain range, it will exceed the yellowing point of the whitening agent. Not only the whitening effect of fibers or fabrics cannot be improved, but it is also reduced, which is commonly referred to as yellowing of fibers or fabrics.

The optimal concentration of optical brightener DT is 0.8%, which means that when the dosage exceeds 0.8%, the whiteness of the fiber will decrease and lead to yellowing.