ANTI BACK STAINING METHOD FOR DENIM

A Novel Anti Back Staining Method for Denim

Garments: A Cost-effectiveProcess

Abstract:

Denim and its various items like pants, shirts, jackets, belts, caps, etc, are the most preferred clothing for today’s youth. This study discusses compositions and methods of reducing or preventing back staining of indigo dyes on the weft yarn and pocket of denim garments. The dye, as it is removed from the denim material, post treatment with cellulase or by a conventional washing process may cause ‘back staining’ or ‘re-deposition’ on the denim material; e.g.: re-colouration of blue threads and blue colouration of white threads, resulting in less contrast between the blue and white threads.

Introduction:

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.

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-β 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. • 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 – 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.

Anti Back Staining:

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.

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 afterwashing 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.

These oxidizing agents will be added to the enzyme bath or the stone wash bath and will inhibit the reducing action of glucose during processing. As far as this study is concerned, some denim trials have been taken in which we found hydrogen peroxide to be a good substitute as an anti back staining agent. It has to be used in low concentrations to avoid imparting a more bleaching effect on the garment. All these tests were taken during the enzymatic processing techniques as they are mostly used in all garment industries. All these trials have been done on both the sample machines and a bulk trial has also been taken and the report of the same has been given. The results of the trials are as follows:

Results and Discussion:

Some of the trials have been taken to prove the efficiency of oxidizing agents as anti back staining agents, along with their recipe proposal and trial result. The bulk trial was also taken to check the effectiveness of the same in a mass quantity and whether it stays effective in larger baths also.

Discussion:

On analysing the oxidising agents used in this study we found that hydrogen peroxide proves to be the safest anti back staining agent than the rest. The hypochlorite imparts some yellowness after being kept for a long time and is also toxic in nature. The potassium permanganate also imparts a slightly yellower tone. But peroxide gives a brighter look and gives better results at very low concentrations. The working of the peroxide as an anti back staining agent is very simple. Say, when the denim garments are in the bath its pH will be slightly alkaline. So this helps the peroxide to start liberating nascent oxygen thereby helping in anti back staining activities. So, this way the denim garments are imparted with a good salt and pepper effect than with other anti back staining agents.

Epilogue:

The project results, samples and the photographs indicate that usage of oxidising agents like otassium permanganate, sodium hypochlorite, hydrogen peroxide in the enzyme bath provide good anti back staining properties. It was found that hydrogen peroxide was performing well as an anti back staining agent with dosage between 0.5 to 1.0%. This also proves to be the most cost-effective anti back staining agent. In both dosages it has imparted good anti back staining results. The results were compared with normal biofading enzymes treated with and without anti back staining agents. The dosage has to be worked out for different denim substrates depending upon the shade and customer requirements.