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Processing, Dyeing & Finishing
  Types of stone wash & their effects on the denim fabric

There are many limitations and drawbacks associated with stone-washing process, which can be overcome by using new enzyme based washing technology that also helps to conserve water, time, energy and environment, emphasise S Sangita, P Siva Kumar, and M Ravi Chandran.

Denim is the most preferred clothing of today’s youth. Various items of denim like pants, shirts, skirts, jackets, belts and caps, etc, are available in the market. To give distressed denim look, many types of washing are given to denim fabric.

Pumice stone usage in stone wash
One of such washing is known as stone wash. In stone-washing the worn-out look is given purposely. The fabric is washed along with pumice stones. The stones and denim are spun together in large industrial washing machines. The longer they are spun together lighter the colour of the fabric with better contrasts would result. The time duration of this procedure is set beforehand so as to avoid the tear and wear of the fabric. Thereafter, the fabric undergoes various other processes of rinsing, softening and finally tumble-drying. These stone-washed fabrics are used for different purposes -- garment making as well as for upholstery purpose.

Disadvantages of pumice stone usage
Stone-washing the denim with pumice stones has some disadvantages. For instance, stones could cause wear and tear of the fabric, also it creates the problem of environmental disposition of waste of the grit produced by the stones. High labour costs are to be borne as the pumice stones and their dust particles produced are to be physically removed from the pockets of the garments and machines by the labourers. Denim is required to be washed several times in order to completely get rid of the stones. The process of stone-washing also harms big, expensive laundry machines.

Alternate methods for stonewashing
To minimise such drawbacks, stone-washing of denim is carried out with the aid of enzymes. The method of giving the denim a stone-wash look by use of enzymes like cellulase is known as- ‘Enzymatic Stone-washing’. Here cellulases are used to provide that distressed worn out look to the denim fabric.

Cellulase method
Cellulase is environmental-friendly compared to pumice stones. It reduces the percentage of damage caused to denim caused by the rough effect of stones on them. As there is huge demand of garments with distressed jeans look, stone washing with enzymes is being used increasingly. It is also known as bio-stone-washing. Enzymatic treatment has become another substitute for kilograms of stones, also the jeans stonewashed by this method has more shelf-life. It ensures the same result with minimum amount of water, waste, time, volume and damage to machines.

As jeans are made up of cellulosic fibres, the use of cellulase enzyme is successful in giving the stone wash look. This enzyme breaks down the surface cellulose fibres and removes them without causing harm to the jeans. Better finishing and look is achieved even with indigo dyed denim. In cellulase enzymatic wash, the denim is given an enzyme bath. Here certain amount of indigo dye and cellulose fibres are removed from the surface of the fabric. As enzymes are like yeast in nature, they eat the cellulose present in denims. When the jeans get the preferred colour, enzymatic reaction is stopped by changing the alkalinity of the bath or else the water is heated. Thereafter, the fabric undergoes rinsing and softening process. The number of rinsing process after enzymes treatment is less than pumice stone-washing. There is reduced amount of waste produced and overall costs for stone-washing is also less.

Disadvantages of cellulase treatment
There are certain disadvantages of cellulase treatment. It could leave marks of backstaining like blue threads becoming more blue or white threads becoming blue. To get rid of such unwanted re-coloration of threads, the jeans are rigorously washed adding surfactants to it. This process could result in colour-fading of jeans and there is added usage of water for the washing. Thus wastage of water and certain amount of backstaining could be experienced. The primary target of stone-washing the denim with pumice stones or enzymes is to provide the garment worn-out, old and aged look. Sometimes both stones and enzymes are used for the purpose.

Latest process of stone wash – Perlite
A new process of stone-washing has been found by a series of laboratory testings – Perlite. Perlite is the form of naturally occurring silicon rock. It has the distinctive property of expanding to 4 to 20 times its initial volume when heated at a particular temperature. This happens because the raw Perlite rock consists of 2-6% of water content in it. The crude Perlite rock when heated at the temperature above 870º C it gets swollen up and tiny glass sealed bubbles are formed. Its original colour which is black or gray changes to grayish white or else white. This heated form of Perlite is used for stone wash purpose.

It does the same function of stone-washing as stones. Perlite treatment reduces the rate of harm caused to large washing machines by pumice stones and gives the denim better supple and softer finish. Many jeans manufacturing companies, in the place of enzymatic treatment, use Perlite, which reduces the rate of wearing out of jeans when used. It gives throughout uniform worn-out and old look to the denim and not just the upper part of the garment. There are many grades of Perlite differing in sizes are used for giving the stone wash finish to denim right from largest to finest grades; some are very tiny just like grounded earth.

Modern stone-washed jeans, biostoneing technique
At first stone-washing involved using pea gravel, but pumice was discovered to float around with the jeans instead of lying in the bottom of the water and so manufacturers have switched. Turkish stone is commonly used for their porosity and cleanliness. There is beautiful pumice from Sicily but the supply is limited. Most of us are familiar with “Stone-washed” jeans. As the name implies, freshly dyed jeans are loaded into large washing machines and tumbled with stones. Adding pumice stones gives the additional effect of a faded or worn look. The pumice abrades the surface of the jeans like sandpaper, removing some dye particles from the surfaces of the yarn. Pumice has been used since the introduction of stone-washed jeans in the early 1980s.

However, stone-washing with pumice has some severe drawbacks. The quality of the abrasion process is difficult to control: Too little will not give the desired look. Too much can damage the fabric, particularly at the hems and waistbands. The outcome of a load of jeans is never uniform, with a significant percentage always getting ruined by too much abrasion. The process is also non-selective. Everything in the washing machines gets abraded, including the metal buttons and rivets on the jeans as well as the drum of the washing machine. This substantially reduces the quality of the products and the life of the equipment, and increases production costs.

Rinse (water) wash
This speaks for itself mostly. Like in the good old days the jeans will be washed at about 50º C. There is high risk of colour bleeding, so ideal to use for brightening up your old faded jeans by washing them together. One should make certain to wash separately from other garments the first few times. Some Jeans brands will not even use sanforized fabric, so that you can shrink them to fit in a hot bath. This was (again) very popular back in the 80s.

Stone wash
This is still the most famous of all washings. The jeans are washed with oval or round pumice stones which should all have roughly the same format. The pumice stones are very light with a rough surface. Sometimes, when the final quality inspectors at the jeans factory forget to clean the pockets, you may even find some residue of these stones in the pockets of your new jeans. During the washing process these stones will scrap off a thin layer of the denim thus showing some of the white threads from the part of the cloth where the indigo dyeing stuff was not able to penetrate. It also creates the effect called brilliance. One may also encounter words like deep stone or super stone wash, which are an indication of how long the jeans have been stone-washed. Ergo: The longer the wash, the lighter the jeans.

Overydye
This is an additional dyeing after the jeans have been sewn. This ensures a very deep dark or black colour and makes the sewing thread blend in with the jeans colour.

Dirty wash
After stone-washing the jeans or denim jackets, they will be dyed with special chemicals, thus creating a look in which the jeans will appear to be dirty.

Destroyed/damaged/used/whiskers:
There are several different techniques to make the jeans or denim jackets look old, worn and /or used. Most of these techniques involve actual sandblasting or abrading by some kind of power tool. Whiskers, which normally appear around the hip to crotch area of the pant, are usually made by using a grinder. Another popular way to make jeans appear damaged is to cut the edges at bottom, (back-) pockets, fly and knee area before the (stone) washing.

Laser marking/Spray painting, etc
From the high-tech side there has been a renewed interest in the jeans and garment industry in general. A few of the very cool new inventions are: The jeans laser marker and the Jeans spray robot, as shown below. However, due to the relative high cost of these machines they are hard to find in these parts of asia.

Objects of invention
It is a primary object of this invention to provide a method for creating a stonewashed appearance on denim fabric and garments.

It is a related object of this invention to provide a flexible process which avoids the many disadvantages of using pumice stones or permanganate-impregnated pumice stones to create colour and wear appearances on denim fabric or garments.

It is a further and related object of this invention to improve upon the cellulase-based treatment processes of the prior art.

It is a further and related object of this invention to provide an efficient, low-cost process using foamed cellulases to create a high-quality stone-washed appearance on denim fabric or garments using standard commercial washing equipment, which does not damage the fabrics or the machines in which the fabrics are processed.

Foaming agents
Acceptable foaming agents include, for example, exthoxylated alcohols, ethoxylated alkyl phenols, ethylene oxide and propylene oxide block polymers, carboxylic acid amides, sulfated amides, amine oxides, ethoxylated amines, sulfated, phosphated, or carboxylated alcohol ethoxylates, sulfated phosphated, or carboxylated alkyphenol ethoxylates, sulphated, phosphated, carboxylated or sulfonated alcohols, and alkyl carboxylates.

Foam stabilisers
Acceptable foam stabilisers include natural gums such as locust or guar gums, the carboxymethylated or ethoxylated derivatives of starch and cellulose, and synthetic polymers such as polyacrylic acid derivatives.
The invention is further described in the following Examples:

Denim garment processing: the eco-friendly way
The domestic readymade garment sector is booming, and garment processing has emerged as one of the important production routes towards meeting quick changing demands of the fashion market. In this article we look at processing of denim garments.

Advantages of garment processing
* Increases the opportunities available for differentiation of the end product.
* Offers option to retailer, which are not available in piece good finishing.
* Gives opportunity to impart unique looks and finishes that cannot be achieved in fabric form.
* It allows to hold colour and finishes until the last minute and enables you to shorten the time to market.
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, moonwash, monkey wash, show wash, frosted wash, white wash, mud wash, etc. Over the last 6-8 years, India has probably seen the most dramatic and exciting changes in the washing of denim garments.

Desizing
Traditional desizing is performed by using acid or oxidative desizing agents, which is associated with many drawbacks and limitations. Due to uncontrolled and non-specific reaction, the cellulose material gets damaged and loses strength. With the introduction of enzyme based desizing process, the limitation and drawbacks of traditional desizing process can be overcome. The enzymatic desizing process is performed by using alphaamylase enzyme.

Advantages of enzymatic desizing over traditional desizing
1. Due to very specific reaction of enzyme, there is no adverse effect on cellulose, therefore better strength retention.
2. Saving of water as multiple washing is not required to remove the residual chemicals after desizing.
3. Process time of desizing can be reduced.
4. Neutralisation is not required because same processing conditions are required in next process, therefore zero salt formation in ETP.
5. Saving of energy as desizing takes place at moderate temperature.
6. Feel of fabric is much softer and less hairiness on the fabric.
7. No adverse effect on other bath auxiliaries due to mild process condition.
8. No adverse effect on Lycra or polyester containing fabric.
9. Handling of chemical is safe and easy.
10. No adverse effect on machinery.

Stone wash effect
In traditional washing process, volcanic rocks or pumice stones are added to the garments during washing as abradant. Due to ring dyeing and heavy abrasion, fading is more apparent but less uniform.
The degree of colour fading depends on the garment to stone ratio, washing time, size of stones, material to liquor ratio and load of garments. Normally, after desizing, stone wash process starts with pumice stone addition in rotary drum type garment washer. Process time varies from 60 - 120 min.
Stone wash effect is one of the oldest but highly demanded washing effects.

Stone wash process gives “Used” look or “Vintage” on the garments, because of varying degree of abrasion in the area such as waistband, pocket, seam and body. There are many limitations and drawbacks associated with stone-washing process, which can be overcome by using new enzyme-based washing technology. This technology also helps to conserve water, time, energy and environment.

Enzyme wash
Cellulase enzymes are natural proteins, which are used in denim garment processing to get stone wash look on to the denim garments without using stones or by reducing the use of pumice stone. Cellulase attacks primarily on the surface of the cellulose fibre, leaving the interior of the fibre as it is, by removing the indigo present in the surface layer of fibre.

Cellulase enzyme is classified into two classes
Acid Cellulase: It works best in the pH range of 4.5 - 5.5 and exhibit optimum activity at 50°C.
Neutral cellulase: It works best at pH 6 however its activity is not adversely affected in the range of ph 6 - 8 and show maximum activity at 55° C.
Advantage of enzyme washing
* Soft handle and attractive clean appearance is obtained without severe damage to the surface of yarn.
* Inexpensive, low-grade fabric quality can be finished to a top quality product by the removal of hairiness fuff and pills, etc.
* Simple process handling and minimum effluent problem.
* Better feel to touch and increased gloss or lustre.
* Prevents tendency of pilling after relatively short period of wear.
* Can be applied on cellulose and its blend.
* Due to mild condition of treatment process is less corrosive.
* Fancy colour-flenced surface can be obtained without or a partial use of stone.

Denim bleaching
In this process, a strong oxidative bleaching agent such as sodium hypochlorite or KMnO4 is added during the washing with or without stone addition. Discoloration is usually more apparent depending on the strength of the bleach liquor quantity, temperature and treatment time.

Limitations
* Process is difficult to control, ie, difficult to reach the same level of bleaching in repeated runs.
* When desired level of bleaching reached the time span available to stop the bleaching is very narrow. * Due to harshness of chemical, it may cause damage to cellulose resulting in severe strength losses and/or breaks or pinholes at the seam, pocket, etc.
* Harmful to human health and causes corrosion to stainless steel.
* Required antichlor treatment.
* Problem of yellowing is very frequent due to residual chlorine.
* Chlorinated organic substances occur as abundant products in bleaching, and pass into the effluent where they cause severe environmental pollution.

3-E bleaching concept for denim
Intensive research is underway for the development of sodium hypochlorite bleaching alternative eg, glucose bleaching, bleaching with sulphinic acid derivatives, and recently with laccase (enzyme).

Laccase enzyme belongs to the oxidoreductase group. Laccase’s oxidative effect is complex, and it does not work independently. A mediator is necessary and a chemical mediator is employed between enzyme and indigo.

Advantages
* New Laccase based bleaching technique only affects the indigo and natural raw white of weft yarn is retained, giving the woven fabric a darker shade, which is not implicitly achieved with hypochlorite bleaching.
* The product is so specialised on indigo that it does not attack any other dyes.
* Laccases open up the door to bleach Lycra containing denim without losing the strength of the fabric. In case of hypochlorite bleaching Lycra containing product affects adversely by losing the tear and tensile strength.
* Finally, the process is based on enzyme so no risk of environmental pollution and harmful effluent discharge.

Limitations
* Expensive, compared to traditional process.
* Heavy faded look is difficult to achieve.

Enzymatic anti-backstaining agent-protease
The use of an engineered oxidatively stable alkaline protease that can tolerate a range of operating temperature and pH conditions offers flexible and alternative processes for backstaining clean-up, improved contrast of denim finishes, and reduced residual cellulase of fabric.

It is claimed that significant reduction in backstaining can be achieved at much lower temperature than conventional process by using 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 lower temperature also claims to achieve a significant reduction of residual cellulase.

Selection of denim fabric
The right selection of fabric can help minimise the cost of treatment and to solve environment related issues.
Different colour depth fabric: To cut processing time, effluent load, minimum damage to fabric and minimum use of chemical, there are different depths of indigo on denim. For example, in case of ice wash where we remove more than half the dye during washing, one can use lighter shade fabric which will help to cut the process time, chemical consumption, effluent load. It will help garment processor to process garment more economically and with minimum faults.
Flat look denim: Different chemicals and processes are used to get flatter look on the denim garments. To meet this requirement, special denim fabric has been developed which offers flat look after washing.
Quick wash denim: Quick wash denim fabric is dyed with modified technique of dyeing, so that during wash cycle, indigo dye can be removed quickly, giving washed look at shorter washing cycle. This results in more economical washes, ie, low water consumption, less usage of chemicals, less time and retaining fabric strength.
Tinted denim: With increase in demand of tinted/overdyed look on garment, garment processor is using an additional process of tinting/overdyeing, which is time consuming. It consumes large quantity of water and chemicals. It is also associated with the risk of patches and unevenness on garments. Now the denim fabric is also available in tinted form, which saves processors time and risk.
Soft feel denim: To meet such requirement, different varieties of denim fabric are available, which are having inherent softness. These fabrics require minimum application of softener at the garment stage.

New developments towards economical and ecological denim processing
Sand blasting
Sand blasting technique is based on blasting an abrasive material in granular, powdered or other form through a nozzle at very high speed and pressure onto specific areas of the garment surface to be treated to give the desired distressed/ abraded/used look.

Advantages of sand blasting process
- It is purely mechanical process, not using any chemicals.
- It is a water free process, therefore no drying required.
- Variety of distressed or abraded looks possible.
- Any number of designs could be created by special techniques.

Mechanical abrasion
- To give worn-out effect, abraded look or used look, some mechanical processes have been developed. These are based on mechanical abrasion by which the indigo can be removed. Some of these processes are sueding, raising, emeresing, peaching and brushing.

Advantages of these processes:
- Control on the abrasion
- Different look on the garment can be achieved.
- All are dry process.
- Economical, ecological and environmental friendly.

Ozone fading
By using this technique, the garment can be bleached. Bleaching of denim garment is done in washing machine with ozone dissolved in water. Denim garments can also be bleached or faded by using ozone gas in closed chamber. The advantages associated with this process are:
- Colour removal is possible without losing strength.
- This method is very simple and environmentally friendly because after laundering, ozonized water can easily be deozonized by UV radiation.

Waterjet fading
Hydrojet treatment has been developed for patterning and/or enhancing the surface finish, texture, durability, and other characteristics of denim garment. Hydroject treatment generally involves exposing one or both surfaces of the garment through hydrojet nozzles.

The degree of colour washout, clarity of patterns, and softness of the resulting fabric are related to the type of dye in the fabric and the amount and manner of fluid impact energy applied to the fabric. Particularly good results are obtained with blue indigo dyed denim.

As this process is not involved with any chemical, it is pollution free. By using water recycling system, the technique can be used as economical and environmental friendly denim processing. Color washout of dye in the striped areas produces a faded effect without blurring, loss of fabric strength or durability, or excessive warp shrinkage.

Single-bath stone-washing and tinting technique
Tinting of denim garment is usually done after the stone wash process. In this, garment has been lightly coloured in order to give the final denim appearance a slight shift. This is not true over dyeing but merely gives the impression of a change in overall colour of the fabric. This process consumes large quantity of water and chemical. To make this process economical and ecologically friendly, some novel colour based enzymes have been introduced in the market. By using this new technique, tinting and stone washing effect can be achieved in a single bath.

- Less process time to achieve tinted look.
- No extra chemical required, therefore making process more economical.
- Less water consumption.
- Less energy consumption.
- Less chance of patches or unevenness

Alternate to pumice stone
To overcome the shortcoming of pumice stones, synthetic stones have been developed. These are made of abrasive material such as silicate, plastic, rubber or Portland cement.
The advantage of using such type of products are:
- All major problems associated with the use of volcanic grade pumice stone can be overcome.
- Durability of such product is much higher and can be used repeatedly from 50-300 cycle depending upon type of synthetic stone.
- Reproducibility of washing is manageable.
- As stone discharge of the process is very less, therefore making process is economical and ecological.
- Less damage to machines and garments.

Note: For detailed version of this article please refer the print version of The Indian Textile Journal February 2010 issue.
S Sangita
Department of Apparel Technology
PSG College of Technology, Coimbatore

P Siva Kumar
Department of Textile Technology
PSG College of Technology, Coimbatore

M Ravi Chandran
Department of Textile Technology
PSG College of Technology, Coimbatore

published February , 2010
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