DENIM WASHING OR WET PROCESSES

Introduction 


Today, washing plays an important role in the denim value chain. Lot of customers do not want to wear and tear their jeans themselves, but want the manufacturer to do it for them . To achieve this worn look, a lot of different treatments can be made and different kind of processes and machinery can be used. Some processes are easy and some are complicated and needs to be carefully controlled. Due to poor wet and dry rubbing fastness of the indigo dye, every step in the denim washing process can make a big difference.


STONE WASHING

Stone wash is a traditional washing process where volcanic rocks or pumice stone are added to the garments during wash as abradants. Stone washed products will have a worn look, and are generally a bit puckered at the seams. Usually, stonewashing is made on indigo dyed garments that easily loses colour during abrasion. Often ringdyed yarns are used, which means that only fibres on the surface
have been dyed and that the core remains uncoloured. Stone washing are made with natural and artificial stones. The artificial stones can be altered according to the wanted effect and can be made of coal or ceramic. Stone wash causes damage to fibres, the degree of damage depends on type of stones used and for how long the garment is stone washed.

ENZYME WASHING

Enzyme wash is a method when garments are washed in a cellulase-based liquid instead of being washed with stones. The cellulase enzymes are abrading the surface of the cotton fibre. During strict control, damage on the fibres’ strength and highlights on seam can be resisted. The same hand can be achieved with enzyme washing as with stone washing, but in a microscopic level, this process is more merciful towards the fibres.
There are four kinds of enzymes for washing:
·         Amylase
·         Cellulase
·         Laccase
·         Catalase
Cellulase enzymes are a mix of enzymes that depolymerise cellulose into glucose and other lower molecular. In fabric made of cellulose fires, the cellulase enzymes are by hydrolyze removing the fibres of the surface, even the ones who holds dye . The neutral enzymes gives less back staining and works best with pH value between 6,0- 8,0 and shows best activity at 55° C. Acid cellulose enzymes works best in the pH range of 4,5-5,5 and have optimum activity at 50° C.

Enzymes will attack a specific molecular group. For denim washing, mainly three types of enzymes are being used. That is neutral, acid and bio polishing enzymes. The process has to be strictly controlled because of the enzymes sensitivity to temperature, time and pH. These three parameters highly affect the result and too big variations can cause damage on the fabric.

After finishing an enzyme wash, a washing process that rinses the garment has to be done, to make sure there is no enzyme residue left. This process can be made in different ways and it will also give the garment a better appearance.

HEAVY STONE WASH

Heavy stone wash is a combination of stone washing and enzyme washing. The benefit of heavy stone wash is a shorter processing time and that almost 50 % less stones and enzymes are needed. The abrading effect of heavy stone wash depends on the type of stones, the enzyme type and the duration of the process.

ACID WASHING (ACID WASHED STONE WASH)

Acid washed garments are pre treated with stones that have been dipped in an oxidant. This method reduces the physical damage on the garment and reduces the time of washing. The oxidant will make the indigo molecules oxidate, which will destroy their ability to reflect blue wavelength. This process is also called dry bleaching. The size of the stones and the oxidation effect can be adjusted by demands. By neutralisation, the process can be stopped. Acid washing often gives light parts a light brown shade .
BLEACHING

Bleaching can be done in several ways, with several bleaching agents:
  • ·         Hydrogen peroxide
  • ·         Potassium permangate
  • ·         Sodium hypochlorite
  • ·         Calcium hypochlorite
Sodium- and calcium hypochlorite are commonly used for medium to vintage denim looks and potassium permangate is used for super vintage and light shade looks. Hydrogen peroxide can be used when a light bleach effect is wanted or if the or if fabric is Sulphur dyed. Bleaching can cause damage to fibres.

TINTING AND DYING

Tinting and dying are being done to change hue, cast or tone of indigo. Tinting is when only parts of the garment are dyed and dying is when the whole garment is dyed. Generally tinting is used to give denim jeans a worn and vintage look.

SOFTENING PROCESS

When selecting softeners there are some aspects that must be considered. Softeners may react with contaminants in high temperatures; this can cause fabric to start yellowing. Some colours, especially blue and red, are very sensitive towards softeners, and can after softening change shade. The softeners should be compatible with wetting agents, dispersing agents and other auxiliaries. Some softeners can have a solvent effect on certain dyestuffs.
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