Sunday, December 31, 2017

Enzyme Washing Process of Denim Garments

Enzyme Washing:
In modern textile technology finishing processes, enzyme washing is a new concept. It can replace a number of mechanical and chemical washing. Enzyme is a living biochemical substances. In enzyme washing process, enzyme works as a catalyst in the washing process.


Because of the problems with pumice, alternative methods for stone washing of denim fabrics were developed. Cellulases have been used in denim washing for many years and it is estimated that about 80% of denim washing is done in this way. The most important cellulase-producing organisms are fungi of the genera Trichoderma, Penicillium and Fusarium. The cellulases used are chemically complex and consist of at least three enzyme systems working together synergistically. Endo-β-(1,4)-gluconase or endocellulase hydrolyse cellulose chain molecules randomly, preferentially attacking the internal regions of the polymer. They generate more chain ends which are further hydrolysed by exo-β-(1,4)-gluconases (also termed cellobiohydrolases) to produce cellobiose. These exo-cellulases may assist in the disintegration of crystalline regions, making the region more susceptible to hydrolysis by endocellulases. The cellobiose and other small oligomers are then hydrolysed by the third enzyme, β-(1,4)-glucosidase, into glucose. All of these enzyme components act synergistically during the degradation of cellulose.

Enzyme washed jeans
Fig: Enzyme washed jeans
During enzymatic treatment, the indigo that is removed can redeposit on the white yarn of denim fabric. This process is called back staining and it can destroy the look of a denim garment. An ideal bio-stoning enzyme possesses highly abrasive activity as well as low back staining. Although there have been some studies on back staining which focused on the mechanism of indigo redeposition on denim garments and the reduction of the effect of back staining, it continues to be a problem for manufacturers.

It is claimed that a significant reduction in back staining can be achieved at a much lower temperature than a conventional process by using a small amount of protease either at the end of the cellulase washing step or during the rinsing step. By adding the protease at the end of the cellulase wash step, one rinse step is eliminated, offering savings in time and energy. This process at a lower temperature also claims to achieve a significant reduction in residual cellulose.

The finishing and washing effects delivered by cellulases are always applied in processes in which a strong mechanical action on the fabric is provided.

The basic steps of denim enzyme washing are shown as follows:

Desizing (10–15 min)

Rinsing

Enzyme washing, e.g. 30–60 min at 50–60 °C with pH 4.5–5.5

Hot rinsing, e.g. 80 °C

Softening

Enzymatic washing treatment of denim:
During enzymatic treatment, the indigo that is removed can redeposit on the white yarn of denim fabric. This process is called back staining and it can destroy the look of a denim garment. An ideal bio-stoning enzyme possesses highly abrasive activity as well as low back staining. Although there have been some studies on back staining which focused on the mechanism of indigo redeposition on denim garments and the reduction of the effect of back staining, it continues to be a problem for manufacturers.

Cellulase are available in three categories:
  1. Neutral
  2. Acidic
  3. Hybrid enzymes
Acid cellulases exhibit the greatest activity, generally in the pH range 4.5–5.5 at 45–55°C, whereas neutral cellulases require a pH of 5.5–8.0 at 50–60°C. Neutral enzymes give a better salt and pepper effect with much less back staining; they are generally available in powder form and the fabric retains better strength than with acidic cellulase. Acidic cellulase gives faster results but with too much back staining; it cuts down the indigo colour and affects the strength of the fabrics.

Laundry workers need faster results in less time and less expensively; hence chemical suppliers combine neutral and acid cellulase in such way that it works faster and with better results than acid cellulase. It is a cost-effectiveness solution known as a hybrid enzyme.

Enzymes, which are proteins present in all living cells, speed up chemical processes that would run slowly if at all. They are nontoxic and are readily broken down. Enzymes are used in textile processing, mainly in the finishing of fabrics and garments. Enzymes such as yeast are used to eat away cellulose physically in cotton. The colour in denim fabric is actually on the outer surface of the yarn owing to poor exhaustion. When the denim is washed in a cellulose enzyme bath, the indigo is removed along with the fibre. When the desired colour is achieved, enzymes are stopped from reacting, either by changing the alkalinity of the bath or by heating the bath. A rinsing and softening cycle follows. This process is more environmentally friendly than stonewashing because strip-mined pumice stones are not used.

Laccases (benzenediol:oxygen oxidoreductase, EC 1.10.3.2) have a broad substrate specificity with respect to the electron donor. They catalyze the removal of a hydrogen atom from the hydroxyl group of ortho- and para-substituted mono- and polyphenolic substrates and from aromatic amines by one electron abstraction to form free radicals capable of undergoing further depolymerization, repolymerization, demethylation or quinone formation.

The group of enzymes called laccases, or phenol oxidases, has the ability to catalyze the oxidation of a wide range of phenolic substances, including indigo. Laccases are extracellular, multicopper enzymes that use molecular oxygen to oxidize phenols, and various aromatic and nonaromatic compounds by a radical-catalysed reaction mechanism.

Application of laccases in the textile industry is growing quickly. The first commercial use of laccases in the textile industry was in the denim washing process. Laccases in combination with redox mediators were used to reduce back staining, enhance abrasion levels and bleach indigo.

The enzymatic treatment uses a chemoenzymatic approach to remove indigo dye selectively from denim fabric. Laccase is applied together with a suitable mediator to create a bleached look to jeans. A better contrast between white and blue yarns can be achieved. In this process, the indigo chromophore is transformed into Isatin and back staining is reduced or avoided. Laccases have been reported to bleach indigo fabrics and laccase-based systems are able to degrade indigo both in solution and on denim, leading to various bleaching effects on the fabric. There have been many studies regarding indigo degradation with laccase from Polyporus sp., Sclerotium rolfsii, Trametes hirsute and T. versicolour. However because of the high cost and short shelf life, laundries do not prefer it.

Most denim garments are abraded using enzymes or a combination of enzymes and pumice stones. A wash processing concept from Novozymes, called Novozymes Denimax ore, makes it possible to integrate the abrasion process and the preceding desizing process, with abrasion results equal to or better than conventional processing.

It is assessed that the environmental impacts of moving from traditional enzymatic denim washing processing to the combined denim wash process. A savings of 50% of the water used for desizing and abrasion in the traditional process was reported. Heat use was reduced by 50% and electricity by 12%–15%. The environmental benefits of shifting to the combined denim wash process can be expected to be even larger for textile laundries that shift from a pumice stone treatment.


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  2. Super White Wash on Cotton Garments
  3. Stone Wash on Denim Garments
  4. Denim Manufacturing Process from Fiber to Finishing
  5. Different Types of Denim Fabric Used in Apparel Manufacturing