Anti-back staining for denim garments

The Reason behind

Abstract:

Denim and its various items like pants, shirts, jackets, belts, caps, etc, are the most preferred clothing for todays 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- 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. - 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.

Measure

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.

Why Anti Back Staining agent ?

1) Back staining will reduce the garments value in the world market. Today Major banded denim business is fashion driven where denim looks cannot be compromised. Whether it is front side or reverse side of denim, as denim brands place them as status symbols and denim with poor contrast will result in Brand De-valuation.

When a consumer purchases a pair of denim, the on check he performs is to check the reverse side to know whether dye has bleeded on back or not. If there is indigo dye re-deposited on back side of denim, it will create an impression in minds of consumer that the fabric will fade away after wash. This will hamper the sale of denim garments. The higher the whiteness of the pocketing fabric better the visual image that denim in the minds of customer. This leads to increase in brand value in minds of consumer which will enchase customer retention and better future sales.

2) Has good compatibility with enzymes, pumice stone ,hence can improve denim look after washing.

3) Improve effect of washing.Better contrast effect.This leads to less usage of washing chemicals which leads to cost saving.

4) It is formulation of eco-friendly chemicals, making your denim a next step towards green earth.

5) To avoid yellowing and colour change after wet processing as Loose indigo on un-dyed weft is prone to yellowing

Types of yellowing

Phenolic yellowing :This is the most common type of textile yellowing and it generally occurs during storage. Phenolic yellowing is characterized by its bright yellow colour and has maximum absorption at 420 450 nm. This yellow colour fades when exposed to ozone. The yellow products are soluble in hot water and polar solvents such as alcohols, and become colourless in acid medium at pH 5,0 or below. Plastic wrapping materials such as poly bags contain Butylated Hydroxy Toluene (BHT) as an anti oxidant to prevent it from ageing. These can be transferred to textile surfaces. BHT reacts with nitrogen oxides present in the atmosphere and form nitro phenol compounds or stillbene quinone. These are yellow in colour and impart yellowness to the fabric. Alkaline pH is a must for this process to take place. The sources of alkalinity on white or pale shaded fabrics are the residual chemicals used in the bleaching or dyeing process, which are not properly neutralized.

Position of dyestuff in the fibre

The yellowing due to ozone is directly proportional to the amount of back staining generated in the wash process. Back staining is the re-deposition of indigo dye released during various wash process. Redeposited dye has more loose sites for oxidation resulting in the yellowing of denim garments, the use of anti re-deposition chemicals in the de-size and abrasion baths can prevent the re-deposition of the released dye by suspending the dye particles in the bath and not allowing the re-deposition to happen.

Ozone Fading

One of the key noxious by-products of urban photochemistry is ozone, and this can be reached dangerously high levels of 0.5 ppm. In presence of UV light, there is an interaction between hydrocarbons, oxides of nitrogen and oxygen that causes of the release of ozone. The release is during day time due to the presence of sun light. Ozone induced yellowing is a major problem on denims. This is due to the sun's UVradiation. The increased concentration of NOx and SOx gases due to pollution can potentially react with ozone and reduce its concentration tremendously in the ozone layer. As a result a portion of the sun's UV radiation reaches to the earth's surface and generates ozone gas from the atmospheric oxygen. Ozone (O3) being a very strong oxidizing agent decomposes indigo dye in the following manner.

The ozone induced oxidation of indigo produces certain compounds which are 'isatin, anthranillic acid, and the complex of the two products.' which is yellow in colour. This is the reason of yellowing due to ozone fading.

Back-staining is usually generated right from the beginning of the jeans washing process. In fact a big amount of indigo dyes is released during the prewash of the Denim Jeans and tend to redeposit on the garment. To prevent this redeposition, we should use an auxiliary in each step of the jeans washing process, and this product is called Zyter Base PW SP.

Optimization of back staining---Additional not required

1) The back staining level is also depending on the type of water and quality of indigo dye. The usage of very hard water can increase the level of back staining. Semi hard waters are the best ones.2 ) The stabilization of the indigo dye on the fabric also matters a lot in the level of indigo re-deposition.

The lowest level back staining is achieved in between the pH levels of 6.5 to 7.5. Since the acid enzymes work on acidic pH, the level of back staining will be much higher with these kind of enzymes. However it can be resolved by using Zyter base pw sp