What are the reasons for fabric yellowing?

There are four main reasons for yellowing:

1 Yellowing caused by the raw material (fabric) itself

Generally wool, viscose, nylon, spandex, etc. are easy to cause yellowing. For example, nylon and spandex are all derived from the same raw material TDI (toluene diisocyanate), and BHT (2,6-di-tert-butyl-p-cresol) is often added to TDI to prevent light aging during storage and use. The BHT in these raw materials chemically reacts with nitrogen oxides (NOx) in the air, resulting in yellowing, which is called BHT yellowing.

2 Yellowing caused by the production process

During the production process, the treatment process, the use of additives and the pH value during the treatment can all contribute to yellowing of the fabric. For example, in fabric finishing, due to the amphoteric nature of hydrogen peroxide, the residual hydrogen peroxide on the fabric will make the fabric yellow under suitable reaction conditions.

The concentration of optical brighteners also has an effect on textile yellowing. Usually, in order to obtain satisfactory whiteness of the fabric, fluorescent whitening agent finishing is sometimes required in the finishing process. When the concentration of fluorescent whitening agent reaches a certain value, the whiteness will reach the highest value; but when it exceeds this concentration, the whiteness will decrease with the increase of the concentration, resulting in yellowing of the fabric. In the process of processing, steam ironing or uneven drying will also cause the whitening agent to migrate, resulting in high local concentration and local yellowing.

Auxiliaries used in the post-dyeing process may also cause yellowing of the fabric. For example, the cationic softener has a strong bond with the fiber, which can make the fabric obtain excellent softening effect, but the disadvantage is that it is more prone to yellowing than the fabric treated with anionic softener; when adding an antibacterial and deodorizing finishing agent, because the finishing agent contains nitrogen atoms, Fabrics can also yellow. In addition, many direct dyes can form complexes with heavy metal ions, resulting in a decrease in solubility and a change in dyeing color. Even a small amount of copper or iron ions can inhibit fluorescence, reduce the whiteness of the optically whitened fiber material, and cause yellowing.

3 Yellowing caused by packaging and storage

The packaging materials for storing various textiles are polyethylene and other materials. In most cases, these materials use sterically hindered phenolic compounds as antioxidants. Among them, 2,6-bis(2,6-bis) is widely used and inexpensive. Tert-butyl-4-methyl (abbreviated as BHT). Because the solubility of BHT in polyethylene (or polypropylene) is very small, and it is easy to volatilize.

Therefore, the BHT in the polyethylene film will diffuse to the surface of the packaging material, and the polyamide-based fabric or clothing is packaged with the polyethylene film containing BHT. During storage, the BHT can be transferred to the fabric or clothing and interact with the air and packaging materials. Nitrogen oxides (mainly NO2) absorbed or stored in (such as cardboard, etc.) undergo complex chemical reactions to form yellow substances.

4 Yellowing during the washing process

Sunlight exposure, changes in air temperature, changes in dryness and humidity, and movement and contact may cause yellowing. During the washing process, it is necessary to pay attention to whether the washing powder is removed cleanly, whether the bleaching agent, the detergent with unbalanced pH value, etc. are improperly applied in the cleaning agent. In the presence of nitrogen oxides, any trace amounts of phenolic antioxidants are transferred to the fabric surface by volatilization or friction, all of which will cause yellowing over time