Abstract
Enzymes have been used in textile processing and finishing since a very long time to get rid of starch-based sizing, but it’s in the past two decades that a serious attention has been given to enzymes for a wide range in textile applications. Jeans which is manufactured from denim fabric, is one of the world’s most popular clothing item. The famous “stonewash look” has been traditionally achieved by removing the indigo dye using a local process in which a pumice stone is added to the washing drum for the abrasion of the garment. This stone-washed finish on denim fabric virtually damages the machinery items and also cause a lot of water pollution. The concept of biotechnology in this process can be utilized for the removal of indigo dye from the yarn or fabric surface. The new eco-friendly process which is known as bio-stoning, uses enzymes to wash/bio-stone the denim material. This produce a distressed appearance, without harming any machinery items or the environment. The bio-stoning method has benefitted in cost saving and has also improved the quality of denim.
Keywords: Denim, sustainability, Bio-stoning, Pumice stone, Stone wash, Indigo
1. Introduction
Denim jeans which are dyed with natural or synthetic indigo, are one of the most popular garments among all the age groups and approximately one billion pairs of jeans are being produced in one year. Traditionally, denim is woven with cotton in twill weave (warp face), in which the warp is blue, and the weft is white. Dyeing of denims is usually performed with pure indigo, or indigo dye mixed with a sulphur dye to decrease the costs which is caused by the expensive indigo dye. Indigo dye is quite popular because it washes down to bright blue shades without staining any of the white weft yarns.
Most of the denim jeans or other denim materials are subjected to a wash treatment to get a slightly worn look. In the traditional stonewashing technique, which is done by pumice stone, the blue denim is faded out by an abrasive action of the stones on the garment surface. This process removes some of the indigo dye. Stonewashing whereas added a whole new dimension to the denim garments in the late 1970s: the process facilitated the artificial ageing of denim fabrics which imparted a fashionably aged look. As the name ‘stonewashing’ suggests, the blue jeans were washed with pumice stones to give a faded look. However, the usage of pumice stones has many limitations, like damage of the machine parts, blockage of the drainage system and difficulty in removal of the residues. Moreover, too much abrasion can damage the fabric, particularly the hems, seams, and waistbands.
Figure 1: (a) Original Indigo dyed denim(Raw) (b) Bio-stone washed denim
The finishing process of denim fabric and garments has been revolutionized by the application of enzymes as an alternative to pumice stonewashing process. Currently the cellulolytic enzymes are applied in textile processes where mechanical action is always present like in jets or rotating drum washers. Cellulase treatment of cotton/denim fabrics is an eco-friendly way of improving their property. It is already recognized that cellulases with strong EG activity is preferred for achieving the worn-out look of denims and this effect is best obtained in machines that provide fast beating action. It is an obvious advantage in letting out pumice stone and the possibility of significantly increasing the capacity of the laundry, that convince people to use biostoning. These advantages are obtained at a lower or similar cost per unit compared to the traditional stonewashing methods.
2. Biostoning of denim
Cellulase enzymes were introduced in the 80s to assist denim washing to get a faded and abraded look different from what is traditionally provided by pumice stones. Cellulase enzyme fastens the abrasion by a process known as ‘bio stone washing’. The use of these enzymes allows the denim garment washing procedure to be carried out under mild conditions without the use of pumice stones and other harsh chemical agents. As a result of this process, different visual effects may also be fused on fabrics. A small amount of enzyme can replace several kgs of pumice stones. It is now feasible to fade a denim fabric to a greater degree without any possibility of damaging the garment. Productivity can also be increased because the laundry machines contain lesser stones and more garments. There is also no sediment in the wastewater so there will be reduced block drains.
Figure 2: Bio-washing/bio-stoning of denim
A life cycle assessment was performed on three methods of reducing the indigo and the three steps specifically related to the production of stonewashed jeans. The three steps are stone washing, wash off (removal of the stones), and wash (clean-up of the garment). The biostoning washing method scored eight out of ten environmental parameters. A cost comparison based on environmental regulations was also made for the three washing methods. The highest costs arise from wastewater treatment was for the pumice method and the lowest were for biostoning. A range of cellulase for denim finishing, having their own distinctive properties, is available in the market.
The first commercial cellulase enzyme introduced in the market was extracted from the Trichoderma family, a fungus which has the longest history in cellulase research. The second process in the development of cellulase was the introduction of products based on another fungus called Humicola insolens. These cellulases was then known as the neutral cellulases as they could work in a neutral pH environment. A special feature of the neutral cellulases is the ability to provide the stonewashed look with a very minimal indigo re-deposition during the treatment process. Thus, the jeans and other denim garments will have a higher contrast been white and blue yarns, and the inside pockets as well because the leather labels would not be stained with indigo. However, the reaction time of this enzyme is slow, and the use requires a longer processing time.
3. Process
In enzyme washing, Hydrolysis of the cellulose, which is catalysed by enzyme cellulase, causes the surface fibres to get weak and later they get removed when there is either fabric-to-fabric abrasion or fabric-to-stone abrasion during the washing process. The temperature and the pH used are quite specific to the type of cellulase enzyme employed. Usually neutral cellulases at pH 6–7 is applied, while acid cellulases are applied at a pH 4.5–5.5. However, the result is a greater extent of back staining, as it is more penetrative. An enzyme dose of 2–4 g/L is usually sufficient, provided that the enzyme activity is not at all impaired.
The steps of denim enzyme washing is shown below:
Loading of garments
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De-sizing
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Rinse (at least Two times)
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Addition of Cellulase enzyme
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Adjusting PH for Acid Cellulase or enzyme (4.5-5.5) by adding Acetic acid (CH3COOH).
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Required temperature kept for different enzyme
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Tumbling for 30-60 minutes (depending on shade)
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Rinse (two times)
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Softening
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Extracting
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Drying
Temperature is one of the most important factors in enzyme treatment. Enzyme reaction increases with temperature, but it is only activated within a temperature range in which its structure remains unchanged and stable. The enzyme activity decreases sharply beyond this optimum range because the protein structure of cellulase enzyme get tangled through a thermal agitation. There is an increase of colour fading effect when the temperature increases from 50 °C to 60 °C. It’s because the enzyme reaction is activated by higher temperature within an optimum temperature range and thus more surface fibres are hydrolysed by cellulase. The weakened fibres are removed by the abrasion of fabrics and some mechanical agitation. The indigo dye particles are also removed with the cotton fibres.
The mechanical action can provide a colour fading effect on denim garments by enzyme washing. The sample treated without any agitation has a better colour fading effect than the treated one because the fibres gets weakened after treatment with enzymes alone and are not well removed by any mechanical agitation. The surface of the denim fabric becomes hairier and a layer of fuzz is formed on its surface. Longer enzyme treatment time prolongs the enzymatic degradation of cellulose. The color fading effect increases with an increased treatment time. The increase in colour fading effect is mainly due to the desorption of dye particles. The increased color fading effect is also due to the fuzziness of fabric which is caused by the prolongation treatment of the enzyme. Cellulase effectively hydrolyses fragments of cotton fibrils with a longer treatment time, and thus the fabric is less fuzzy than the original.
The mode of action of cellulases will be explained as follows– denim garments are dyed with indigo, a dye that penetrates only the surface of the yarn, leaving the centre light in colour. Cellulase enzymes are natural proteins that are utilized in denim garment processing to urge a stone wash look on to the denim garments without using pumice stones. Cellulase attacks totally on the surface of the cellulose fiber; the cellulase molecule binds to an exposed fibril (bundles of fibrils form up a fiber) on the surface of the yarn and hydrolyzes it, leaving the inside of the fiber because it is, by removing the indigo present within the surface layer of fiber. When the cellulases partly hydrolyze the surface of the fiber, the blue indigo is released, aided by mechanical action, from the surface and light-weight areas appear.
Figure 3: Bio-washing/bio-stoning of denim with cellulase enzymes
One of the issues related to the denim bio-stoning is back-staining. Back-staining is defined because the re-deposition of released indigo onto the clothes. This effect is sort of important in denim finishing. The washing out of the indigo dye from the blue yarn may stain the white yarn because of the enzymatic treatment. Most consumers prefer prime quality jeans with low back-staining. Application research during this area is concentrated on preventing or enhancing back-staining counting on the fashion required. Back-staining at low pH values (pH 4-6) is comparatively high, whereas it’s significantly lower within the neutral pH range. Neutral cellulases are therefore often used when the target is minimal back-staining.
4. Conclusion and future scope
Recently, to understand specific applications and to remain pace with the most recent fashion inclinations of the denim industry, research and development activities are focused to customize on a replacement of cellulase enzymes whose configuration has been reformed through biotechnology to deliver higher abrasion contrast, improved fabric strength, retention reduced back staining, and extended operating pH and temperature ranges. the event of latest biotechnology brought new implements for scientists to craft novel, improved cellulase products for textile applications.
The various cellulases available because the Deni Max® product range, developed by Novozymes, for modifying the surface of denim give fashion designers a pallet of potentials for creating novel shades and finishes. Bleaching or fading of the blue indigo color can even be attained by use of another enzyme product (Deni Lite®) supported a laccase and a mediator compound. this method along with dioxygen from the air oxidizes and thereby bleaches indigo, creating a faded look. The mixture of latest looks, lower costs, shorter treatment times and fewer solid waste has made abrasion and bleaching with enzymes the foremost widely used fading processes today.
Novozymes (earlier Novo Nordisk) has also patented a technique within which a stone-washing effect of denim was obtained by using xyloglucan polymer before dyeing and afterwards creating the abraded. The xylo-glucanase isn’t ready to hydrolyze the cellulosic fabric, and so no strength loss is resulted [12]. Valumax A 838 may be a high-performance, direct-to-use cellulase enzyme for the abrasion of denim (bio-stonewashing). This product allows new shades and finishes to be created easily and value effectively, in an environmentally friendly way. The outstanding features of this innovative cellulase enzyme, developed by Novozymes, are high color contrast finish, low degree of indigo back staining, optimum strength retention, high degree of reproducibility and reliability, maximized fabric strength retention, improved wash look or creation of recent looks, easy handling and value effectiveness.
5. References
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- Novozymes report (2006)
