Classification of Denim Washing Techniques

General finishing sequence of denim

There are almost countless variations of processing techniques used by designers and textile chemists to achieve fashionable looks that are distinctive and desirable. The number of variations is very large and the evolution of chemical and mechanical techniques is continuing. There often are some secret and proprietary methods. Regardless of the specific look and name chosen, the following are the process steps normally used to attain the desired results.

In denim garments, preparation consists primarily of desizing, which enables subsequent chemical and mechanical treatments (washing/abrasion) by removing the previously applied warp size. Untreated denim garments are extremely rigid due to the size applied at the yarn stage for increasing weaving efficiency, by coating the yarn with a protective outer layer which retards yarn breakage. The sizing remains in the yarn after the fabric is woven and, therefore, provides fabric stiffness necessary for more efficient sewing of garments. Sizing is typically made up of starch (or polyvinyl alcohol/starch), binders and waxes and lubricants. Additionally, finishes containing starch and/or polyvinyl alcohol are applied to denim as a topical finish before the fabric is shipped to the sewing plant.

After desizing, denim garments are rinsed to remove all size materials before washing/abrasion. Different washing/abrasion techniques are introduced in the following. Again, after the washing/abrasion processes, denim garments are rinsed to remove unwanted materials from fabric surface. Then a softening process enhances the hand feel and softness of the denim garments. Finally, the denim garments are dried and packed for delivery.

Desizing

The most popular method of removing starch from denim garments is to use amylase enzyme. This product can break down the long starch molecular chains (water insoluble) into smaller molecules (water soluble) which can be more easily washed away.

Regular Washing

Regular washing is the simplest and most commonly used washing method for denim garments. The degree of color fading using regular washing is comparatively slight, but it provides uniformity, depending on whether it is deeply dyed classic denim or only moderately dyed with poor penetration. Detergent is used for regular washing for about 15min at temperatures between 60°C and 90°C. Softening is applied after the washing process.

Bleach Washing

Bleach washing is normally carried out with a strong oxidative bleaching agent such as sodium hypochlorite (NaOCl) or potassium permanganate (KMnO4). The bleach washing effect and deculturation usually depend on strength of the bleach liquor, liquor quantity, temperature and treatment time. The bleached fabric materials should be properly antichlored or after washed with peroxide to reduce the subsequent yellowing or tendering of the bleached denim fabric.

Stone Washing

Volcanic rocks or pumice stones are added during washing as abradants. The color fading is more apparent but less uniform. The degree of color fading depends on the washing time, stone ratio (weight of stones relative to weight of the garment), size of stones, liquor ratio (∼10:1) and garment load. The world’s major pumice stone supplying countries include the USA, Turkey, Italy, Germany, Iceland, New Zealand, Japan, Indonesia and Philippines.

Enzyme Washing

In enzyme washing, cellulase enzymes are used. Hydrolysis of the cellulose, which is catalyzed by cellulase, causes the surface fibers to become weakened and later they get removed when there is either fabric-to-fabric abrasion or fabric-to-stone abrasion during washing. The temperature and the pH used are specific to the type of cellulase employed.

Acid Washing

Pumice stones are first presoaked in a solution of a strong oxidizing agent (either sodium hypochlorite or potassium permanganate) and are then applied to the garments by means of dry tumbling. This results in a localized washing effect with clear blue/white contrast. This technique is also termed ‘acid washing’, ‘snow washing’ or ‘ice washing’.

Ozone Fading

Ozone typically acts as a mild bleaching agent as well as a sterilizing agent. In this technique of denim washing, the garment is bleached with ozone dissolved in water in a washing machine. However, this technique can also be carried out in a closed chamber by using ozone gas. The advantage of this method is the minimum loss of strength and it is a simple method that is environmentally friendly.

Dip Dyeing

In dip dyeing, dip dyeing machine is used to achieve special effects on denim garments. Direct dye is commonly used but the color fastness is comparatively poor and therefore fixation is required to improve fastness. Pigments can also be used for dip dyeing. If a bleaching agent is used, bleaching effect can be achieved on garments.

Pigment Washing

Pigment washing is done on garments to achieve special shades using pigments. Pigments needs fixation treatment to ensure the color fastness of garments.