3. Finishing Chemicals

Some softeners protect denim while others enhance yellowing.

A popular misconception is that silicone emulsions aggravated yellowing. However, a detailed study demonstrated that a surfactant used for emulsification of the silicone oil is the main culprit, rather than silicone oil itself. Silicone oil, irrespective of its viscosity, amine value or modified structure, does not significantly contribute to yellowing. (Figs. 4 & 5)

4. Intensity of UV & NOx in the Environment

Higher UV intensity and higher concentrations of NOx lead to higher concentration of Ozone, which in turn leads to quicker yellowing.

 

5. Duration of Exposure to UV and NOx

The longer the exposure, the greater the yellowing.

6. Packing Material of Garments

Yellowing during storage due to reaction between antioxidant in packing material & NOx.

Prevention of Yellowing

It is a known fact that certain finishing chemicals accelerate decomposition of indigo. The extent of decomposition would depend upon the concentration of the finishing chemical on the garment, its chemical nature and the form of its decomposition products. Thus, to protect indigo from destruction, the 'anti-matter', or formulations that have exactly the opposite properties from the destructive species, have been evaluated.

Various formulations were tried, to assess their effect on the destruction of indigo, and suitable formulations were identified and marketed as special finishes for denim to prevent yellowing.

 

These finishes form a protective layer on a substrate and react with ozone as well as NOx simultaneously during exposure.

The preferential reaction of softener or finish formulation with ozone and NOx protects indigo from decomposition.

A protective softener undergoes degradation, sacrificing itself with ozone, thereby preventing destruction of indigo. Primary oxidation takes place at the free amine group in the softener, whereas secondary oxidation takes place in the main structure, which will vary depending upon the chemistry of softener. However, the softener can slow down degradation of indigo but cannot prevent it completely.