Stonewash 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 mins.
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.
- 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 fluff and pills, etc.
- Simple process handling and minimum effluent problem.
- Better feel to touch and increased gloss or luster.
- 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.
- More reproducible effect can be obtained.
- It allows more loading of the garment into machines.
- Environmental friendly treatment.
- Less damage to seam edges and badges.
- Wear and tear of equipment is minimum due to absence of stone.
- Use of softener can be avoided or minimised.
- Easy handling of floor and severs as messy sludge of stones does not interfere.
- Due to absence of stone, labour intensive operation of stone removal is not required.
- Homogenous abrasion of the garments.
- Puckering effect can also be obtained.
- Problem of pumice powder contamination on garment is not there.
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 i.e. 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.
Enzymatic Bleaching:
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, 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 loosing the strength of the fabric. In case of hypochlorite bleaching Lycra containing product affects adversely by loosing 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.
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