The indigo dye used in the dyeing of denim fabrics is not resistant to oxidation and yellowing. Under the action of ozone, indigo can be decomposed into isatin, tannic acid and a combination of the two, causing the fabric to turn yellow. Isatin is a water-soluble substance and can be easily removed by washing with warm water.
With the acceleration of industrialization and the rapid increase in the number of urban cars, the amount of ozone in the air has increased significantly, even reaching the hazardous level (0.5mg/kg). Ozone (O3) in the atmosphere is an extremely strong oxidant. Under UV light irradiation, it will decompose dyes, for example, turning the indigo dye on denim into yellow products.
Some denim fabrics use finer yarn counts than traditional denim fabrics, so that the surface area of the unit dye exposed to the air is larger, which increases the contact area of the indigo dye and the air, and accelerates the degree of yellowing. If jeans are left statically for too long, the indigo oxidation decomposition products will accumulate on the fabric to a certain extent, and yellowing will occur.