Abstract
The denim sector is booming worldwide, and it has brought a huge trend of fast-changing fashion. Denim washing and finishing has emerged as one of the important production ways toward meeting the fast-changing demands of the fashion industry and market. There are huge ecological concerns related to this sector as it is enormous. Approximately 1500 gallons of water is needed to make one pair of jeans out of 1.5 pounds of cotton. If this wastage continues, soon it will become a serious problem to drinking water supplies, which will lead to scarcity. It is therefore important to study and analyse the environmental impacts of denim and find alternative sustainable processes. This paper describes the different types of denim washing and finishing techniques. It also covers the environmental impact of denim dry and wet washing techniques, and the importance of sustainable washing techniques. The paper includes latest denim finishing technologies, comparing their impacts on the environment with those of the classic techniques.
Keywords: Denim, Environmental impact, Ozone fading, Sustainability, Textiles, Washing
1. Introduction
Denim is the oldest, widely used, fashionable, and most versatile fabric among other fabrics in the world. Though initially the fabric was developed for use in sailcloth, but at present denim has a wide range of use and is mainly used for producing garments, particular trousers for young people. Since denim is an apparel preferred by all age groups and due to this popularity, a lot of development work related to washing, finishing, dyeing has been done in the last few decades. Today’s consumers enjoy more casual, relaxed, sophisticated lifestyles and are interested in clothing that not only looks good but also feels great and comfortable. Due to these innovations, many varieties of denim are available to the customer in today’s times. The expectations from fabrics have changed with developments in textile technology and innovations and the consumers ’living standards. Denim is strong and versatile and gets softer and more comfortable when used more often by the wearer.
1.1. Denim and sustainability
Although, if we see manufacturing jeans in the conventional method, it may not seem to be a big issue in terms of sustainability and eco-friendliness, but if the size of the denim sector and the methods of its production are considered, it will make a big difference in every aspect. So, one can imagine the impact of this huge denim sector on the environment, if all the jeans in the world are produced by using eco-friendly methods and considering sustainability. Fortunately, many denim companies all over the world today are striving hard to embrace greener and sustainable methods for the production. Brands are trying to develop new ways and techniques of producing jeans, washing, finishing and dyeing as a part of their business strategies to save the environment. They have understood the importance and are aware of the need to build a sustainable business.
In addition, the denim companies should be clear of the practices carried out by the denim suppliers. Although, the supply chain is well spread in different countries but, when the company compromise on their ethical and environmental standards, the company’s reputation can get damaged. Currently, cotton prices are quite high and there is a rising demand from the customers for jeans and other denim products. In this tricky situation, denim manufacturers also need to stick to the idea of sustainability. Sustainable denim is not a concept that has been introduced only in developed nations, but also in developing countries like India and China. Many Indian designers have also come up with sustainable and eco- friendly denims for their customers.
Fig. Sustainable Denim Production
1.2. Denim washing and finishing
Traditionally, denim has been made only from cotton fibre, but now with technical improvements, many new fibers have been produced to replace cotton with many special features. These advances are encouraging the production of innovative and sustainable denim from different fibers. Then gradually several different washes have been introduced, such as stone wash, acid wash, moon wash, and monkey wash. In a few decades, India has probably seen the most dramatic and evolving changes in the washing of denim garments.
Denim washing can be achieved in many ways, for example by using pumice stones, hypochlorite, or some enzymes. Enzyme washing is mostly preferred as its very simple and also because it is eco-friendly in nature when compared with hypochlorite and pumice stones. Enzyme washing is a process, in which different enzymes are used to clean fabric, especially in the case of denims and other garments having a worn-in look. For regular cleaning process, enzymes carry numerous economic and environmental benefits. Looking at an industrial aspect, enzyme washing has replaced many laundering techniques such as stone washing, saving companies money and enhancing sustainability too.
2. Eco-friendly Processes
The major pollution problems generated by denim wet processing are usually due to water pollution. There are also minor problems related to air pollution, which is due to chemicals, lint, etc. The largest impact, with respect to water and air pollution occurs mainly during wet processing of denims. The denim industry consumes a huge amount of water and chemicals, and therefore its generation of wastewater is therefore also very high.
2.1. Sizing and Dyeing
The main purpose of sizing of yarn is to increase its strength by chemically binding the fibers to together. The protective size coating also prevents indigo dye in denim from rubbing off during production process. Normally, 8–12% size is applied in sizing. According to a report, the sizing process can contribute around 50% of the total pollution content.
Dyeing of denim is associated with many pollutants, such as dyes and chemicals like sodium hydroxide and sodium hydrosulphite. Pollutants are also associated with chemicals used in dyeing, maintenance of equipment, and cleaning. Usually, not all dyes used for denims get fixed on the warp sheets or layer. It is not possible to achieve 100% fixation while dyeing, so the unfixed dyes mix with the water bodies which results in high turbidity and this affects the ecosystem as it is toxic, carcinogenic, or mutagenic. Turbidity affects light transmission, severely affecting the photosynthetic capability of aquatic plants.
2.2. Environmentally Friendly Dyeing of denims
Because of the demand and limited production of natural indigo mostly synthetic indigo is used in the dyeing process. Currently, natural i.e. plant-based indigo is used for only 1% of all total indigo applications. Larger-scale production of plant-based indigo dyes will require large land areas for cultivation which is not viable. Therefore, synthetic indigo is produced from crude oil which is also known as aniline. But it is not possible to apply 100% of the dye to the warp sheets, so some of this synthetic dye is left over and gets discharged in wastewater, adversely affecting the ecosystem. So nowadays many organizations are promoting the use of natural indigo in their brands to help protect the environment at least to some extent. Indigo is insoluble in water, and it requires a huge quantity of alkali (NaOH) and a reducing agent such as sodium dithionite (Na2S2O4) to increase its water solubility. Utilization of alkali and a reducing agent will increase the BOD, and other pollution parameters.
Fig. Indigo Dyeing of denims
Many researchers and scientists are working on the electrochemical reduction of indigo dyes. They have studied electrochemical reduction of indigo dyes with the help of ultrasonics, to provide water-soluble indigo dyes in an eco-friendly manner, using an aqueous system during production. The reduction potential can increase with increasing pH, which means that mass transfer is better with increasing pH. The use of ultrasound in the process helps with the reduction of particle size. This is a green and eco-friendly technology, which offers many advantages over the conventional (chemical) reduction technique. Lower cost, greater energy efficiency, and no need for use of Na2S2O4 and NaOH are some of the advantages. This is a very novel route for environmentally friendly and sustainable indigo dyeing.
2.3. Stone Washing
Pumice stones are commonly used in stone washing. Pumice is a lightweight, highly porous, rocky substance, which can float on water due to its porosity. It is a product of volcanic eruptions. During stone washing, the fibers on the surface layer of the fabric are broken and hence removed, and the inner white fibers of the warp yarn, commonly known as “ring dyeing,” are gradually exposed.
2.4. Eco friendly Enzyme Washing Technique
With consideration of environmental factors and efficiency, various eco-friendly enzymes have come into the picture of denim processing. Chemically an enzyme is defined as a protein complex composing near about 200–250 amino acids. Enzymes can bring hydrolysis, oxidation, reduction, coagulation, and decomposition simultaneously, although the most common reaction among all is hydrolysis. Enzymes accelerate reactions, they act only on specific substrates, operate under mild conditions, easy to control, and are also biodegradable. Because of these special characteristics, enzymes are used extensively in denim processing. In the traditional stone washing process, denim color used to get faded by the abrasive action of pumice stones on the fabric to give a worn-out appearance. In bio stoning finishing, cellulase enzyme is used to accelerate the abrasive process and, in this process, the cellulase works by reducing the indigo dye on the denim surface. This enzymatic process offers a lesser pollution load and better ease of treatment in effluent plants.
Fig. Denim bio washing mechanism
2.5. Eco Friendly Bleaching Process
Looking at the current demand for bleached denim products, it is especially important to utilize alternative processes that do not affect our environmental systems. Therefore, as alternatives enzymatic bleaching, water jet fading, ozone bleaching and laser engraving are now used as environmentally friendly processes. These alternatives can possibly replace sodium hypochlorite which is commercially used for bleaching but none of them can provide the same effect as this chemical. In the last few decades, many researchers have worked on ozone bleaching, but it has not been commercialized because it requires a huge investment. Recently, use of laccases which belong to the oxidoreductase type of enzyme has been reported for bleaching indigo fabrics. Generally, these enzymes do not work independently, rather a chemical mediator needs to be employed between the enzyme and the indigo dye. In the presence of an aqueous medium, this laccase enzyme gets oxidized and attacks the mediator and converts it into free radicals. The free radicals attack the indigo and later convert it into oxidized products.
Fig. Schematic of ozone denim bleaching mechanism
3. Conclusion and Future Scope
In the denim processing line, there are many related processes that create environmental hazards. Researchers and scientists from the field of chemicals are trying to find alternatives for this issue. Though some alternatives are already available, their use will not be economically feasible unless the consumers are willing to pay more for sustainable denim products. New sustainable products are always costly as they require a huge investment in research and development that are required for the invention of any alternative process. If consumers i.e. the target market are willing to pay more for sustainable denims, then this will reduce the processing costs because when the market potential is increased, the product price will gradually decrease.
In the denim processing industry, the process of Eco-friendly fabrics or products cannot be attained immediately. Every stage of this supply chain must be implemented and analysed. It also requires participation and co-operation of all the stakeholders along with the consumers. The customers must involve and cooperate in the process of Eco-friendly denim for any crucial change to begin. More investment is required to clean the polluted water and other resources. The efficiency of sulphur dyes is around 50% left in the trough which contaminates the water sources and is most difficult to remove from water. It is one of the most severe problems in the denim industry. From the point of sustainability, the trend of less water-based technologies, such as a laser, plasma, ozone and sandblasting have also been explored for denim washing and have produced excellent fabric handle. With proper level maintenance in fastening and accurate production of the fabric by controlling the laser process parameters has been found as a useful tool for the color fading of denim fabric by the laser treatment.
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