ANTI BACK STAINING

Denim garments are looked at as a major trend setter by our youth. The spread of denim culture all over the world brought with it a trend of fast changing fashions. One after another several washes were introduced such as stone wash, acid wash, moon wash, etc. Over the last decade, India has probably seen the most dramatic and exciting changes in the washing of denim garments. As per the denim garment export market, this high quality garment has superior aesthetics and great value for price. In terms of the quality aspect, back staining plays a vital role in improving the appearance of the denim garment. Back staining: Back staining implies soiling of the weft thread and the pocket lining by detached indigo or its reduced leuco form. Back staining mainly occurs during desizing or stone washing or enzyme stone washing.

In order to avoid such back staining, which will reduce the garment’s value in the world market, some anti back staining agents are used in the wash bath. These agents are added in the bath along with other chemicals associated with the respective washes. These agents are the ones capable of prohibiting the action wherein the removed dyes re-deposit themselves on the garment.

CHEMISTRY OF BACK STAINING

During stone washing or enzyme washing, cellulose is degraded and indigo dye is released. For instance, the cellulase enzymes are temporarily bound to the cellulose by means of an anchor. This gets split in terms of 1,4-ß glycoside composition. After this process it is made available for further hydrolysis reaction.

Enzyme-catalyzed hydrolytic dissociation of cellulose

This reaction is much more complicated in detail, as the customary cellulase consists of several components. Below you can see the dissociation of cellulose in detail:

Hydrolysis of Cellulose: • Endo Gluconases: By means of the endo gluconases the cellulose is dissociated in a statistically distributed sequence. • Exo Gluconases: The exo gluconases dissociate the cellulose chains into glucose molecules from the unreduced end. • Cellobiohydrolasis: Separation of cellobiose (double bond glucose molecule) from the unreduced end of the cellulose. • ß- Glucosidasis: Dissociation of the cellobiose into glucose molecules.

As seen from the picture, the cellulase is degraded hydrolytically by cellulases partially until it becomes glucose. The glucose is partially able to reduce the indigo, both on the fibre and in the treatment liquor. This reduced form has low affinity to cellulose fibre and thus soils the weft thread and the pocket lining. This is also assisted by alkaline conditions and the temperature of the wash bath.

The acid cellulase enzymes have a strong effect on cellulase hydrolysis and create more glucose formation, resulting in increased back staining of the denim garment. Neutral cellulase has its optimum pH at 6-8 at temperatures of 50 – 60°C. Compared to acid cellulase they have a less negative effect on the tensile strength; hence there is less back staining.

As a whole, obtaining a good contrast between the blue and white yarns is often described as minimised back staining. The contrast is the difference between the blueness of the blue yarns and the whiteness of the white yarns. But this is difficult to quantify. Therefore, back staining can be quantified by measuring the re-deposition of indigo on white denim.