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.
- β- 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 60C. 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.
Anti Back Staining:
In order to avoid such back staining, which will reduce the garments 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.
Compositions and Chemistry of the Anti Back Staining Agents:
Anti back staining agents mostly comprise of three substances - dispersing agents, chelating agents, and emulsifying agents. All these must be of a particular concentration in order to inhibit the adherence of indigo dyes on the white portions of denim garments. Most of these anti back staining agents are highly efficient in soft water, but in hard water they might be less effective and so the dyes will re-deposit. This is so because hard water contains salts, called the electrolytes (exhausting agents), which help indigo dyes to soil faster. Nowadays, anti back staining agents contain sequestering agents as well, so that they can be used in hard water. Acid liberating agents are also used along with anti back staining agents to reduce the alkaline conditions and enhance the enzyme activity.
As a latest innovation we have anti back staining agents made from polyglycol ether derivatives and polymer phosphates. This is an inbuilt sequestering agent, which can be used more effectively in hard water.
One of the safest modes of anti back staining is the enzymatical way, where some enzymes, like the cutinase enzymase and other lipolytic enzymes are also used as anti back staining agents. All these efficiently reduce back staining, but are costlier too. Some anti back staining agents are used as after-washing agents, which have very high emulsification power.
Another way of avoiding back staining is by using oxidizing agents. According to the chemistry of back staining, it is caused by the formation of glucose during stone or enzyme washing of the denim garment. This glucose is the reducing agent and it helps the indigo dye to exist on the white part of the denim, thus reducing the contrast of the garment. Now these oxidizing agents will act on the glucose and decrease its reducing power and enable zero back staining. These are the bleaching agents such as hypo chlorites, permanganates, peroxides, etc. There are some mild oxidizing agents also, which are called Resist Salts.