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
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
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
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.