The Ozone fading of indigo dyed garments can be termed as the oxidation of indigo. The oxidation by-products of indigo formed are isatin, anthranillic acid and complexes of the two products. These two factors together give a yellow colour (generally related as yellowing of denim garments) due to ozone fading.

Fading rate of Indigo dyestuff and yellowing of the garments is dependent on several factors:

• Position of the dyestuff in the fibre


• Absorption and diffusion of moisture into the fibre


• Humidity


• Protective chemicals on the fibre


• Ozone Concentration

The yellowing due to ozone is directly proportional to the amount of backstaining generated in the wash process. Backstaining is the re-deposition of indigo dye released during various wash processes. Re-deposited dye has more loose sites for oxidation resulting in the yellowing of denim garments. The use of anti-redeposition chemicals in the desize and abrasion baths can prevent redeposition of the released dye by suspending the dye particles in the bath and not allowing the redeposition to happen.

The increase in humidity accelerates ozone fading of indigo dyestuff as high humidity gives rise to a surface water film that increases the ozone absorption rate. This is due to the increased surface concentration and also the increased rate of diffusion in the fibre. Moreover, the swollen fibres are in a receptive condition. Thus in Indian conditions. ozone fading will be maximum during the monsoons i.e. in the months of July, August and September. The problem of ozone fading will be more prominent in high humidity regions such as Mumbai (than in Delhi).

Anti-Ozonate Softeners

The conventional antioxidants such as sodium bisulfite, ethylene diamine derivatives, etc provide only short-term protection against ozone fading. These type of antioxidants undergo very rapid chemical decomposition and do not provide the protection required when garments are stored for extended periods of time or in areas of high ozone concentration.

Since most of the softeners are made on amine chemistry, it was discovered that select amine based softeners would provide the same protection. Since the softeners are applied in substantially large quantities as compared to antioxidants, they provide protection over an extended period of time. The usage rate of these antiozonate softeners is highly dependent on the activity of the product. For optimum exhaustion of these softeners onto the garments, the temperature should be 50ºC to 52ºC and the recommended pH for the same is between 5.0 to 5.5.

The use of anti-ozonate softener to prevent the fading due to ozone is one of the aspects; it is also important to check out other reasons for fading of indigo dyed denims for complete control.

Ozone Fading in Denim Finishing : Preventive Solution (Apr-Jun'03 Issue)

One of the key noxious by-products of urban photochemistry is ozone and this can reach dangerously high levels of 0.5 ppm. In the presence of UV light, there is an interaction between the hydrocarbons, oxides of nitrogen and oxygen that causes release of ozone (in addition to other compounds). The release is during the daytime due to presence of sunlight.

Indigo dyestuff tends to fade / turn yellow due to ozone reaction. Ozone is present in atmosphere in most industrially active and urban dwellings and is formed in the presence of sunlight or UV light. The levels of ozone can reach dangerous levels of 0.5 ppm and the deteriorating effect it has on denim apparel (particularly that stored in retail shelves) is practically irreversible.

The fading rate of indigo dyestuff is dependent on

(a) The position of the dye in the fibre

(b) Solubility and diffusion of ozone within the fibre

(c) Absorption of ozone on the surface of the fibre

Further, high humidity raises the absorption rate since the fibre is more receptive due to swelling. Studies have shown that with 0.2 ppm ozone level, the time to fail (a 2.5 rating on the AATCC scale) increases substantially as the relative humidity increases. At RH of 80, the time to fail would be 34 hours and this would fall to 16 hours as the RH goes to 85.

Based on molecular weights, one gram of ozone will destroy 10.9 grams of indigo. This factor becomes increasingly more important as the overall shade gets lighter.

As a result of the above, one realizes that with ozone fading of denims, one is treading a path, which can hardly be cured; prevention would be the best mechanism.

Early on, it was believed that conventional antioxidants such as sodium bisulphite, ethylene diamine derivatives provide indigo dyestuffs good protection against ozone fading. However, this type of antioxidant undergoes very rapid chemical decomposition and does not provide the protection required when garments are stored for extended periods of time or in areas of high ozone concentration.

Thereafter, it was discovered that selected amine based softeners would provide the same protection against ozone fading. Since softeners are applied in substantially large quantities as compared to anti-oxidants, they provide protection over an extended period of time.

The rate at which a cationic softener exhausts onto cellulose is related to the pH of the softener bath. As with other cationic softener, antiozonates are solubilized by lowering the pH of the product. Once in solution, an increase in pH reduces the solubility of the softener and the exhaust rate increases significantly. As the pH of the bath reaches a certain point (pH 8 or above), the softener exhausts so rapidly that it plates out on the surface of the fabric. The excess softener on the surface of the fabric has a clammy, tacky feel and will yellow severely when dried. A close control of the pH of the softener bath at 5.5 to 6.5 will eliminate this problem.

By employing an anti-ozonate softener, one can ensure that goods returned due to ozone fading (characterized often by the formation of a yellow band at garment folds) are substantially minimized or eliminated and thus offer a decent savings in goods returned. The slight higher cost of employing this finish is more than recovered with the improvement of performance characteristics of the denim garment. Furthermore, anti-ozonates could be a good “marketing” tool too. Jeans brands could capitalize on the “ozone resistant” tag both in terms of offering a better technological finish and relevant functional finish.

— Team S&A