Denim is defined as being traditionally a 3/1 warp-faced twill fabric made from yarn-dyed warp and undyed weft yarn. Among all types of fabrics no other fabric has received such a wide acceptance like as denim. It has been used extensively by people of all ages, classes and genders. Denim can be considered as the most widely used garment in the apparel business. It is well known that denim and jeans have had a major influence on the lives of consumers since their inception. Denim processing or washing is an important operation for value addition of the final products in the apparel business. There are numerous operations exist for fulfilling of this treatment. In this article I will discuss about environment friendly denim processing or finishing technologies with briefly. Generally four types of technologies are used in sustainable denim processing. They are described below.

Sandblast finishing of denim:

We know that, to achieve a special look as well change the fashion, requisite washing methods of denim are required, including stone washing, sand washing and bleach washing. In the case of stonewashing, there are difficulties in removing residual pumice from the processed cloth items and in ensure a reduction in damage to equipment by an overload of the tumbling stones. Pumice stones and particulate materials can also clog machine drainage passages, sewer and drainage lines at the machine site.

On the other hand, sandblasting does not lead to these types of problems. Sandblasting is a modern technique applicable to specific portions of the indigo-dyed denim using special types of sand. The sandblast fading process for denim has been the most commonly used process in the industry to achieve a worn vintage effect. Actually, it is a mechanical process used to create a fading effect on heavy fabrics such as twill, denim, canvas, etc. Very fine granular powder and hard aluminium oxide has been used to sandblast denim. In the process, aluminium oxide sand mixed with dry air has been passed through the nozzle at very high speed to effect the sandblasting action. The sand removes colour from specific portions of the apparel and thus creates the fashionable effects.
After sandblasting, the apparel is washed with enzymes and silicon softener to increase the softness of the fabric. Because of the abrasion damage, the ends per inch and picks per inch of the denim fabric were found to increase, resulting in a decrease in the fabric’s surface density. Furthermore, the fabric tensile strength and seam strength were found to decrease owing to enzyme washing with sandblasting. Nevertheless, the stiffness of the denim fabric decreased after silicon washing, which indirectly ensured an increase in fabric softness. Here, the denim garments are placed in a closed chamber and are instantly faded by the frictional effect of the sandblast. The flow angle of the sandblast, an important parameter, is kept somewhere between 10 and 20 degrees on the surface. A properly set blowing angle is vital because a process with a higher blowing angle will result in a higher fading effect, in addition to higher fabric damage. After mechanical sandblasting of the denim fabric, a chemical wash such as stonewashing or a combination of stone and enzyme washing is required to meet the customer’s expectations for denim quality.

Numerous operations exist to create the fading effect of the fabric, but in the context of sustainable and environmentally friendly denim processing using waterfree or low water–based processing technologies, the use of laser, ozone and plasma are mentioned, as discussed subsequently.

Ozone fading of denim:

Like plasma-enhanced fading of denim fabric, ozone gas–induced fading is an approach in sync with low water–based denim processing. Ozone is released in the presence of sunlight as the result of the interaction of hydrocarbon, oxides of nitrogen, oxygen, etc.

Indigo-dyed denim fabric has been faded to yellow because of the ozone reaction after UV exposure. Although the ozone finishing of denim fabrics does not completely eliminate the use of water, it could noticeably reduce the consumption of water, energy, chemicals, enzymes and stones. The fading rate of indigo dye depends on the portion of dye in the fiber and its solubility, and the diffusion of ozone in the fiber. Besides, the absorption of ozone on the fiber surface is an aspect of ozone-fading in the denim fabric.

For denim washing by ozone, the garment is bleached with dissolved ozone in water in a washing machine. Denim garments can also be bleached or faded using ozone gas in a closed chamber. Some techno-environmental advantages of ozone-based denim processing are: 

(1) minimal loss of the fabric’s strength.
(2) a straightforward process.
(3) involvement of low water and chemicals.
(4) economy of process. 

The method is simple and environmentally friendly, because after laundering the ozonized water can easily be deozonized by applying UV radiation. Ozone characteristically acts as a mild bleaching and sterilizing agent.

Ozone fading can also be accomplished by plasma processing. During plasma treatment, high-energy electrons react with moisture in the air, generating a mixture of radicals. In the process of generating ozone plasma, a combination of charged particles, free radicals and UV light is also generated. UV light in the plasma zone can also contribute to the production of %OH radicals, which are the most oxidative radicals among radicals generated in the plasma and are mainly responsible for the degradation of indigo dye in textiles.

Plasma-assisted, ozone-treated denim fabric exhibits a paler shade than enzyme-desized denim fabric, because ozone in the plasma oxidizes indigo dye present on the fabric surface. During enzyme desizing, the enzyme reacts only with the sizing material of the fibers. It has no effect on indigo dye molecules; thus no significant shade change of the fabric takes place. However, in the case of cellulase enzyme treatment, it can penetrate effectively into the denim fabric in an aqueous medium. Enzymatic hydrolysis induced by cellulase in plasma-assisted, ozone-treated denim is more severe than that of the enzyme-desized denim fabric.

  textiles over the past decades, but a generally available and easily implemented technology has yet to be developed. Plasma treatment onto textile is an environmentally friendly inexpensive process, when compared to the traditional denim fading methods.

Plasma generation with the help of an electrical discharge using polymerizing to nonpolymerizing gaseous molecules was used for denim processing. However, efficacy of the process depends largely on the nature of gases used. The production of a faded and torn looks in the fabric using conventional technologies involves the consumption of large amounts of water and chemicals, which leads to environmental pollution. Dry finishing or processing is a promising solution to tackle these problems.

Laser and plasma technologies have shown a potential for creating different surface effects on fabric without using much water and chemicals. Chemically, a reactive plasma discharge has been used in various industries to modify the specific surface properties of the materials; in the past two decades, it has also been amply applied in textile processing. Washing of denim garments is a polluting activity in the textile industry; however, if plasma is applied to denim jeans, it minimizes or altogether avoids the use of water. So far, plasma exposure has mainly been used to fade indigo and sulphur- dyed denim garments. Mostly, it is the oxygen cold plasma that has been tried for desizing and colour fading of denim cloth. Washed denim is a desirable look in a garment that is mostly time consuming andpolluting process. Atmospheric pressure plasma is an environmentally friendly and cheap process.

Denim processing using water-free laser technology:

Similar to the plasma treatment of textiles, UV treatment has also been used for the surface modification of natural and man-made fibres to improve hydrophilic and other functionalities. A large number of lasers capable of operating at different wavelengths and different modes are available for the surface modification of various substrates, including textiles. Surface modification by laser can be carried out in a variety of ways, depending on the purpose of the surface modification, such as etching, ablation, deposition, evaporation and surface functionalization. The purpose of surface modification decides the type of laser to be used.

Although conventional technologies are much more beneficial from the point of mass customization, the production of discoloured jeans using those processes involves large quantities of water, which gets discharged when it is contaminated with a chemical admixture. On the other hand, the laser is a source of energy that can be directed onto the desired object, whose power as well as intensity can be controlled. This technology is computer-driven and replaces the use of water, chemicals and stones, yet it can produce precise, repeatable and intricate patterns. In addition, it can create excellent localized effects on denim fabric. However, major disadvantages of the process are that it is more costly, each garment is required to be positioned individually for treatment and there is a possibility of the garment receiving treatment only on one side.

 If we compare laser fading with manual dry processes, we get some points such as:
  1. Design consistency is high in Laser system.
  2. Effect quality is comparatively finer in Manual system than laser.
  3. Hairiness is comparatively less in Laser system.
  4. Hand feel is comparatively finer in Manual system than laser system.
  5. Manpower required – Laser: Manual= 1:3 (Approximately)
  6. Power Requirement is higher in Laser system (approximately 7.5 Kilowatt hour)
  7. Production cost is higher in Laser system.
  8. Rejection rate is 0% in Laser system whereas around 5% in Manual system.
  9. Tearing strength is higher in Laser faded denim (May differ in case of stretch denim)
  10. The laser beam would be harmful if it came into contact with skin or eyes.
  11. Working space required is less in laser system.

Reference: Collected from books & Internet.