Some Notes on Denim Washing

There are four main processes in the Denim washing

- Pretreatment

- Stone or Enzyme Wash- To adjust the surface effect

- Bleaching- To adjust the color

- Finishing- To adjust the handfeel of the garment

Pretreatment involves removal of impurities from the garments and desizing the garment. It also involves prevention of creases in the garment. Wetting Dispersing agent is used in desizing process. It should be able to rapidly wet the jeans so that it can prevent white lines of creases and prevents back staining. Back staining is the redeposition of indigo dyed short fibers, or loose indigo, removed during desizing, stone-washing or enzyme washing. Backstaining reduces the contrast of warp and undyed weft. It also stains pockets and labels and it is more prone ot ozone and yellowing. An anticrease agent is added to prevent crease formation during the washing process. 

Enzyme in garment Industry

Enzyme is a kind of protein. They help the chemical reaction but themselves do not take part in chemical reaction. The starting molecule in a reation is called substrate and the yield molecule is called substate. Enzymes and substrate work like lock-key model so only one enzyme is useful for one type of substrate. Enzymes are better than chemical catalyst because they act in mild conditions such as room temperature. They are also biodegradable. Many enzymes are used in garment washing. An enzyme called Amylase is used in removing starch in desizing. Celullase is used in breaking and removing cellulose fibers. Laccase is used in biobleaching and catalase is used as an anti peroxide. 

Stone and Enzyme Wash

Cellulase weaken the surface fibers which are then mechanically torn off during processing, taking with them indigo. However, they need mechanical action to work. Hence they are used with stones. Cellulase is also used in biopoloshing, which removes surface fibers and make the surface smooth. 

Bleach 

Laccase enzyme decolorises indigo without using bleach. It provides very good contrast and since it attacks only indigo dye and not the fiber so it gives excellent tensile strength. 

Yellowing

The discolouration of textiles, i.e. a change of shade or loss of whiteness, giving a yellow tint, is commonly known as YELLOWING.Yellowing is due to many reasons. Cotton, yellows with age. However aging cannot make severe yellowing happen. Certain lubricants used in weaving and knitting can cause yellowing. The Anti oxidants present in these oils can cause a type of yellowing called as phenolic yellowing. The anti oxidants are also present in packaging materials and silicon softeners. Also temperature of drying and curing during processing can scorch the fibers and cause yellowing. 

Indigo dyed fabrics are more prone to yellowing. Indigo when exposed to NOx or Ozone can produce yellow colored compounds. Indigo itself through simple oxidation can transform into yellow colored compounds. 

There are specialised anti-yellowing softeners available. This work by either forming a protective filem, or by reacting the pollutants to form colorless compounds.

The source of these notes this presentation. This also contains images and chemical reactions. You can also view images of various denim washes here. One more resource is here.

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Resources for Textile Technology Students

If you are a student of textile technology, you will find this site very useful. The owners of the sites are themselves textile students who have put together all the resources at one place.

While surfing I came across one more site, again very useful for textile students.

Please do not miss their blog.


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Some Selected Notes on Textiles: Part -5

Flammability Test

This is important for nightwear as they are finer and thinner than regular wear and are more prone to flammability than other fabrics.

In the testing, among other things time required for the flame to travel a certain distance is determined. It is now mandatory to have a flammability test if the fabric GSM is less than 88 grams.


Samples are tested both as submitted and after one cycle of washing and dry cleaning to obviate any instance of flammbility finish that might get washed of after one washing. Also iginition behaviour of the fabric is observed. Normally 5 specimens are tested. If they do not get ignited than 10 specimen are tested.

Butane gas of specified parameter is used. Based on the ignition behaviour, fabric is classified into Class I, class II and Class III. You can learn about the classification here.

You can read some facts about fabric flammability here.


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Some Selected Notes on Textiles- Part 4

- Colorfastness to Light: In this the samples is exposed to a light source along with blue wool sample. The fading is observed and the rating is given.  The main difference between American and European Standards is that American standard is time bound. In European standards it is not a time bound test but fading bound test- means you have to keep on exposing the sample till the sample fades to that of blue scale. Factors affecting lightfastness are the type of dyes used, depth of shade, surface structure and finishing chemicals. 

- A cotton pigment printed fabric cannot be dry cleaned . In dry cleaning solvent called Perc- Perchloro Ethylene or Tetrachloroethylene. Pigments are very susceptible to dry cleaning- they will come out. Perc is the only solvent that do not blast. In exports they use MTO . The problem with perc is that it damages Ozone layer. Perc will take out the pigments from the print. However, if you dry clean with MTO, nothing will happen.  Perc does not smell as much as MTO does.  To test for dry cleaing, a bag is prepared where instead of steel balls, stainless steel discs are added. 

- To remove smell from pigment printed fabric, it has to be cured well. To do it, maintain a temperature of 150 deg and allow the fabric for 10 minutes. It will cause phosphoric acid to get liberated and pigment get fixed- for that Diammonium phosphate needs to be added – so fixation takes place under acidic condition.  

- When testing for shrinkage, the fabric is conditioned for 24 hours. Then it is subjected to washing process for 45 minutes to 1 hours. Then it is dried and reconditiond before remeasuring.

- When testing for shrinkage, soft water is used. Alum can be used for softening. Just move the alum on the upper surface for a few times. Now decant the upper portion and use the rest of the water for testing. One can also use wetting agent. 

- Drying can be line drying, wet drying, tumble drying or drip drying. 

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