By: Naresh M. Saraf and Deepak V. Alat

Jack 'n' Jill, after returning from the hill, have grown into adolescent teens. Like any other member of the community, they had taken up to jeans as a symbol of youth.

Things went well for quite some time, as they started with basic denim. Then the new fashion came, where pre-bleached, faded denims were in vogue. So they bought new pairs. Jack went to town in search of a job while Jill went to the beach with her friends to enjoy her freedom. After one week, when both returned to their village and, to their dismay, both their bleached jeans had turned yellow!

Jill tried to wash them, though she is not supposed to wash jeans at least for one lunar month, but the dirty yellow tinge remained. She discussed the problem with her friends, teachers and learned men and women in village but could not find satisfactory answer.

A visiting consultant told her it was phenolic yellowing, which he had heard about from some other consultant.

Finally she approached a professor of textile chemistry. He asked the background details and concluded that this is nothing but Ozone Fading!

He narrated his experience and findings, which were fascinating and enlightening to both Jack 'n' Jill.

Indigo dyeing is carried out on continuous ranges, either in sheet form or in rope form.

Unlike conventional dyeing, this kind of yarn dyeing is in ring-dyed form, where dyestuff is not allowed to diffuse inside.

The ring-dyed yarn leads to a faded look on usage, because of loss of colour due to abrasion or wear a tear. Fading can also be enhanced artificially in garment form, by either mechanical, chemical or enzymatic assistants.

 

Unfortunately these fashion garments are prone to yellowing as a result of decomposition of indigo (Isatin and Anthranilinic acid) due to environmental influences. This yellowing is an irreversible phenomenon. The destruction of indigo is due to NOx and oxidants, particularly atmospheric ozone, which is, major cause of yellowing. Ozone is present naturally in air near the seashore, where there are fewer pollutants. Apart from giving a feeling of freshness to the holidaymakers, ozone also silently fades the denims. In industrial cities, chemical smog along with ozone is the main culprit in the yellowing of denims.

Two types of smog have been reported in literature. 'London smog' is caused by soot particles am sulphur dioxide. These particles absorb part of sunlight, making a visible' smog. Sulphur dioxide in this smog is capable of destroying indigo. The second type of smog is called 'LA smog' and is invisible. It consists of nitrogen oxides from automobile exhausts. This is also called a 'photochemical smog' and is present usually in the summer months and in populated areas with heavy automobile pollution.

Due to the depleting layer of natural ozone, the sun's UV radiation, mainly shorter wavelengths, increasingly reaches the earth's surface in the summer months. The UV light reacts with nitrogen oxides as we as oxygen in the air to form an ozone molecule, which is unstable. The ozone thus formed destroys indigo. Since ozone generation is continuous process, yellowing continues.

Based on the molecular weight of Ozone and Indigo, theoretically approx 1 gm ozone can destroy 10.9 gm of Indigo. Apart from ozone, nitrogen oxides present in air also destroy indigo.