Sustainability in Textile Processing

Dr. Asim Kumar Roy Choudhury,
Director Academic, KPS Institute of Polytechnic, Belmuri, Hooghly (W.B.), India
Ex-professor and Ex-HOD (Textile)
Govt. College of engineering and Textile Technology, Serampore (W.B.), India
Email: [email protected]

Introduction:
“The clothing industry contributes up to 10% of the pollution driving the climate crisis.” This claim appeared in an Instagram post published by the clothing company Patagonia in late November 2020, and has received more than 502,000 views. Although the post does not indicate the source of the claim, scientific studies estimate that clothing contributes 3 to 10% of greenhouse gas emissions that drive climate change (Hertwich and Peters, 2009).

One of the most polluting industries is textiles and clothing; its detrimental ecological footprint is caused by high energy, water and chemical use, the generation of textile waste and micro-fibre shedding into the environment during laundering (Niinimäki et al., 2020). Furthermore, it has been estimated that up to 20% of industrial wastewater pollution is caused by textile dyeing and finishing (Morlet et al., 2017).

The environmental impacts of the fashion industry are widespread and substantial. For example, although there is a range of estimates, the industry produces 8-10% of global CO₂ emissions (4-5 billion tonnes annually). The fashion industry is also a major consumer of water (79 trillion litres per year), responsible for ~20% of industrial water pollution from textile treatment and dyeing, contributes ~35% (190,000 tonnes per year) of oceanic primary microplastic pollution1 and produces vast quantities of textile waste (>92 million tonnes per year), much of which ends up in landfill or is burnt, including unsold product. The rising environmental impact (and awareness thereof) can be attributed to the substantial increase in clothing consumption and, therefore, textile production. Global per-capita textile production, for instance, has increased from 5.9 kg to 13 kg per year over the period 1975–2018. Similarly, global consumption has risen to an estimated 62 million tonnes of apparel per year, and is projected to reach 102 million tonnes by 2030. As a result, fashion brands are now producing almost twice the amount of clothing today compared with before the year 2000.

In 1975, some 34 million metric tons of textile fibers were produced worldwide. By 2020, this amount had more than tripled, amounting to 109 million metric tons and was expected to reach 146 million metric tons by 2030 (https://www.statista.com/statistics/1250985/global-textile-fiber-production/).

However, textile industries are backbone of human civilisation. Hence, all efforts are made to improve its sustainability. The use of eco-friendly textile materials, dye and chemicals, adaptation of new, improved processes with strict process controls, waterless dyeing are some of the ways by which sustainability can be improved.

Sustainable textiles should be environmentally friendly and should satisfy the rational conditions to respect social and environmental quality by pollution prevention or through installation of pollution-control technologies.

When considering both the volume generated and the effluent composition, textile wastewater is considered to be the most polluted water among all the industrial sectors. The global textile supply chain that is very complex, involving many variable stages. The companies and brand owners may, therefore, be the best placed to bring about the required change.

Water charged with chemicals is released as wastewater, which, if not treated thoroughly, may pollute the environment thermally by virtue of the high temperature of the effluent, and chemically by extreme pH conditions and/or by contamination with dyes, diluents of dyes, auxiliaries, bleaches, detergents, optical brighteners and many other chemicals including carcinogenic heavy metals used during textile processing.

Sustainability:
Sustainable textiles should be environmentally-friendly and should satisfy the rational conditions to respect social and environmental quality by prevention of pollution through various pollution-control technologies.

Eco certification is a voluntary process. Any entity conducting a business for which a standard exists may ask to have its output or services certified. Certification is a procedure through which a third party, the certification body gives a written assurance that an organisational system, process, person, product or a service complies with requirements specified in a standard or benchmark. Certification is awarded for a limited period, during which the certification body carries out monitoring. Third-party certification bodies and governments have issued restricted substances lists that link production ecology to human ecology (Oecotextiles, 2012).

Six Issues of Sustainability:
The following six issues have the potential to make the life cycles of textiles and clothing unsustainable (Sherburne, 2009):

1. Use of toxic chemicals
2. Consumption of water
3. Consumption of energy
4. Generation of waste
5. Transportation
6. Packing materials

1. Use of toxic chemicals
Many dyes and chemicals, used in the textile industry, are toxic and non-eco-friendly. A large number of dyes are used in the dyeing and printing of textiles; the Colour Index International lists 27,000 individual products under 13,000 Colour Index Generic Names (SDC, 2016). Volatile chemicals pose particular problems because they evaporate into the air or are absorbed into foods or through the skin. Some chemicals are carcinogenic or some are harmful to children even before birth, while others may trigger allergic reactions in some people.

Lists of restricted substances are constantly changing as more information from scientists and health professionals becomes available, leading to an enhanced understanding of chemicals and their effect on human health and the environment. The substances listed in the Restricted Substances List (RSL) are based in large part on global legislation regulating chemicals usage in the manufacturing of apparel products. The European Union has developed the organisation namely “Regulation Concerning the Registration, Evaluation, Authorization and Restriction of Chemicals” or “REACH” which is aimed at ensuring a high level of protection of human health and the environment from the risks that can be posed by chemicals (AAFA, 2016).

Other countries which have developed similar lists of restricted substances are China, Canada and South Korea. In the United States, several states, including California, Washington, and Maine, have adopted laws regulating chemicals in consumer products. These regulatory requirements are incorporated into the RSL.

On June 7, 2018, “Environmental Protection Agency (EPA)”, USA finalized a rule that adds a category of 13 specific nonylphenol ethoxylates (NPEs) to the Toxics Release Inventory (TRI) list of reportable chemicals. NPEs are nonionic surfactants used in adhesives, wetting agents, emulsifiers, stabilizers, dispersants, defoamers, cleaners, paints, and coatings. NPE degrades in the environment into substances including nonylphenol (NP), which accumulates in the bodies of fish and disrupts their hormones, harming fertility, growth and sexual development. Use of NPE in textile manufacture in Europe was banned over 10 years ago but the substance is still released into the aquatic environment through imported textiles being washed. EU member states have agreed to ban Hormone disrupting NPE widely found in clothing because it poses an “unacceptable risk” to the environment (The Guardian, 2015).

Various enzymes are being used to substitute a number of hazardous chemicals in the textile industry. The global market for industrial enzymes increased from US$ 1.5 billion in 2000 to US$ 2.25 billion in 2007 (Roy Choudhury, 2014).

2. Consumption of water
After agriculture, the textile and related industries are considered to be the second highest consumer and polluter of clean water (Oecotextiles, 2012). Water is used in many steps of the textile dyeing process both to convey the chemicals used during the step and to wash them out before beginning the next step. In a traditional dyeing and finishing operation for example, one tonne of fabric could result in the pollution of up to 200 tonnes of water by a suite of harmful chemicals and in the process consume large amounts of energy for steam and hot water (Moore and Wentz, 2009). With the industry now centred in countries with still-developing environmental regulatory systems, such as China, India, Bangladesh, and Vietnam, textile manufacturing continues to have a huge environmental footprint.

The reutilization of waste water can present very important savings, namely in reduction of water, energy and chemical consumption. The recycling of waste water is affected in process baths and rinsing waters, before water is taken for treatment for removal of remaining chemicals and other effluents generated. Steam condensate and cooling water are easily recoverable as they are clean. Their thermal energy can very quickly pay back the investment.

With the continuous growth of global production and consumption of textile-related products and services, there are both opportunities and challenges for the textile industry.

3. Consumption of energy
The textile industry is a major energy-consuming industry with low efficiency in energy utilization. About 23% of the total energy is consumed in weaving, 34% in spinning, 38% in chemical processing and another 5% for miscellaneous purposes. Thermal energy dominates in chemical processing while electrical power dominates the energy consumption pattern in spinning and weaving. Thermal energy in textile mills is mainly consumed for the heating of water and drying of textile materials (Rupp J.  2008).

The textile industry is one of the largest sources of greenhouse gases (GHG’s), not least because of its enormous size. In 2008, the annual global production of textiles was estimated at 60 billion kilograms of fabric with associated (estimated) energy and water needs of 1,074 billion kWh of electricity (or 132 million tonnes of coal) and between 6 to 9 trillion litres of water (Oecotextiles, 2011).

A large quantity of non-renewable energy sources are eventually consumed in the form of electricity, not so much in the process of textile production (15-20%) but mostly in subsequent laundering processes during consumer use (75-80%) (Sherburne, 2009). It is reported (Ray and Reddy, 2008) that the total thermal energy required per metre of cloth is 18.8-23 MJ and the electrical energy required per metre of cloth is 0.45-0.55 kwh. The energy used is mostly consumed by heating process water or for use in laundering and drying materials after laundering. Whilst data on energy usage for the textile industry is readily available, complications arise in estimating the associated CO2 emissions that arise from the sources (coal, electricity, natural gas or other sources) from which  the energy is produced because the textile industry is a fragmented and heterogeneous sector dominated by small and medium enterprises (SMEs).

Energy is one of the main cost factors in the textile industry. Especially in times of high energy price volatility, improving energy efficiency should be a primary concern for textile plants and various energy-efficiency opportunities exist in every textile plant, many of which are cost-effective, but not implemented because of limited information or high initial cost. For example, the use of electricity is associated with in-built inefficiencies as compared with the direct use of thermal energy and the use of steam is less-efficient than direct-fired gas heating in a mill. The share of total manufacturing energy consumed by the textile industry in a particular country depends upon the structure of the manufacturing sector in that country. For instance, the textile industry accounts for about 4% of the final energy use in manufacturing in China (LBNL, 2007), while this share is less than 2% in the USA (U.S. DOE, 2010).

Electricity is the main energy consumed in the textile industry, being used for driving machinery, cooling, temperature control, lighting and office equipment, whereas fuel oil, liquefied petroleum gas, coal and city gas are widely used to generate steam. Efficiencies have been achieved; between 1990 and 2005, the carbon emission intensity in the textile industry decreased for grey cloth, jute goods and polyester chips by 1.90, 2.07 and 0.72 percent respectively. On the other hand, cotton yarn showed the highest increase in emission intensity of 7.37%, which means that cotton yarn continues being produced inefficiently (ECCJ, 2007). Emission intensity is the average emission rate of a given pollutant from a given source relative to the intensity of a specific activity; for example grams of carbon dioxide released per mega joule of energy produced, or the ratio of greenhouse gas emissions produced to GDP. Emission intensities are used to derive estimates of air pollutant or greenhouse gas emissions based on the amount of fuel combusted, the number of animals in animal husbandry, on industrial production levels, distances travelled or similar activity data. Emission intensities may also be used to compare the environmental impact of different fuels or activities. The related terms emission factor and carbon intensity are often used interchangeably, but “factors” exclude aggregate activities such as GDP, and “carbon” excludes other pollutants (Wikipedia 2013).

Spinning consumes the greatest share of electricity (41%) followed by weaving (including weaving preparation) (18%) whereas wet-processing preparation (desizing; bleaching) and finishing together consume the greatest share of thermal energy (35%). A significant amount of thermal energy is also lost during steam generation and distribution (35%), but these percentages will vary by plant. Such analysis of energy-efficiency improvement opportunities in the textile industry points to advantages to be gained from retrofit/process optimization, not just from complete replacement of current machinery with state-of-the-art new technology (Hasanbeigi, 2010).

There are various possibilities for using renewable energy in the textile industry. Four examples from a range of possible measures are:

1. Installation of wind-powered Turbo Ventilators on production plant roofs;
2. Use of direct solar energy for fibre drying; and
3. Use of solar energy for water heating in the textile industry;
4. Solar electricity generation

4. Generation of waste
Like any other industry, the textile industry generates all categories of industrial wastes namely liquids, solids and gases. For greener processes, non-renewable wastes need to be recycled and renewable wastes need to be composted if recycling is not an option. Various useful materials can be recovered from textile process wastes.

The recovery of chemicals such as sodium hydroxide from mercerization baths is achievable by heating to concentrate the solution; following such a step, 90% of the sodium hydroxide can be recovered (Indiamart, 2013). The EVAC vacuum suction system in the textile dyeing process recovers hot alkaline hydrogen peroxide, additives and finishing chemicals (TIFAC (DST), 2012). EVAC is the trade name of a vacuum system which has been introduced in Thailand from the United States. This equipment is installed at the finishing stage to suck excess chemical solution from the fabric, and then transfer it to the storage tank for recovery and recycling (UNEP, 1994).

Desizing effluent containing Polyvinyl alcohol (PVA) has high chemical oxygen demand (COD) and hence, contribute significantly to a textile plant’s Primary Oxygenation Treatment of Water (POTW) operation, and being biologically inert, it presents a threat to the environment. Unfortunately, no effective and efficient means to treat PVA desize effluent has been implemented in the textile industry. Ultrafiltration (UF) reverse osmosis technology for the recovery and recycling of PVA size is more than 40 years old, but it is not used widely because of its many disadvantages. The situation necessitates a new technology for the recovery and recycling of PVA size which can reduce energy and water consumption in an economical and environmentally-friendly manner. A new technology that would eliminate the disadvantages of the current ultrafiltration process in the recovery of PVA from desize effluent is vacuum flash evaporation (VFE). The VFE process for recovery and concentration has been used in a variety of other industries, but has never been demonstrated for size recovery in the textile industry (Gupta, 2009).

Waste Water Pollutants
Some waste-water is still being disposed of in an environmentally unfriendly way, into the sewage networks where available, or else in cesspools, with no regard to the biochemical oxygen demand (BOD), chemical oxygen demand (COD) and/or heavy metal content of the waste-water. The untreated waste-water generated from textile production and processing can vary greatly depending on the chemicals and treatment processes involved and may include materials with a high BOD and COD, high total suspended solids, oil and grease, sulphides, sulphates, phosphates, chromium, copper and/or the salts of other heavy metals (TFL Ledertechnik AG, 2016) of these, the most important are considered to be COD (Chemical Oxygen Demand), BOD (Biochemical Oxygen Demand), pH, fats, oil, nitrogen, phosphorus, sulphates and SS (suspended solids) (Tüfekci et al., 2007). The total suspended solids (TSS) levels are low in raw textile dyeing wastewater compared to many other industries. On the other hand, Biochemical oxygen demand (BOD) and chemical oxygen demand (COD) are relatively high in effluents from sizing operations, and wet processing and therefore, are more important pollution prevention targets (Visvanathan et al., 2000). Sulphates and phosphates are toxic at very high concentrations. Problems caused by sulphates are most frequently related to their ability to form strong acids which changes the pH whereas in surface waters, phosphates cause eutrophication.

Solid Waste
Industrial solid wastes from textile production include the following:

• Ashes and sludge,
• Cardboard boxes, bale wrapping film or non-recyclable soiled fabric,
• Plastic bags carrying chemical raw material,
• Non-reusable paper cones and tubes,
• Waste fabrics, yarns, and fibres from non-recyclable processing.

Unmanaged solid waste is likely to be dumped as landfill.

The textile and footwear industries consume high amounts of water and also threaten the water quality in China. The latest discharge standard, GB 42872012, serves as a way to enforce China’s Environmental Protection Law, and to encourage the improvement of production methods used in the textile dyeing and finishing industry and the improvement of pollution control technology. The standard covers the discharge limits, the requirements for monitoring and controlling for water pollutants in the textile dyeing and finishing industry or production facility.

The industries should follow the requirements for the pollution discharge and control in the standard and adapt necessary measures to ensure normal operation of the pollution control systems. From 1 January 2013 to 31 December 2014, current and all new factories should have been meeting the suggested discharge limits for water pollutants. Starting on 1 January 2015, current factories should meet the discharge limits set by CEPL. The Ministry of Environmental Protection (MEP) of the People’s Republic of China released a modification list for the existing GB 42872012 on 27 March 2015 (Louann, 2015).

In India, The National Green Tribunal’s (NGT) circuit bench of Jodhpur has ordered the closure of 739 textile units in Balotra and its surrounding areas of Jasol and Bithuja (Rajasthan state) till July 9, 2015. It has also ordered the trust operating the Common Effluent Treatment Plant (CETP) to renew the Consent to operate the plant and obtain the hazardous waste disposal authorization from the Rajasthan State Pollution Control Board. It is claimed that between them the units have been discharging 17-18 million litres of untreated effluent per day into the Dravyawati river. Local courts have ordered 213 units to close by May 31, 2015 and the remainder by June 30, 2015 with local police asked to support the order (Mathews, 2015.).

In 2012, more than half of India’s $1.25 billion worth of textile exports to the U.S. came from the southern city of Tirupur. While the business has brought economic benefits, its environmental and social costs are many. Downstream of Tirupur and its more than 300 textile factories, the Noyyal River had become foamy and discolored. Pollution from this industry is blamed for causing illness among local people and sapping the productivity of nearby farms.

Tirupur is not an isolated case. According to the World Bank, 20% of water pollution globally is caused by textile processing. Nongovernmental organizations (NGOs) such as environmental groups say parts of India and China are among the most polluted.

Major chemical companies as well as start-ups are responding by developing less harmful textile-processing chemicals. But such substances are often more expensive than ones in use. Lower-cost reagents and better enforcement by regulators are required, textile chemical executives say, along with a shift in consumer behaviour away from opting for the cheapest products.

The textile industry uses more than 8,000 chemicals to make the 400 billion m² of fabric sold annually around the world. Many are toxic and persist in the environment. They include heavy-metal-rich dyes and fixing agents, bleaches, solvents, and detergents.

Making textiles is also a water-intensive business. Producing a pair of jeans requires about 1,800 gal of water; a T-shirt takes 700 gal. Treating such large volumes of waste water is costly—if it is treated at all.

Air Emissions
Burnt fossil fuels contribute to the emissions of carbon dioxide, a primary contributor to the greenhouse effect. The textile manufacture is also responsible for the following emissions:

• Nitrogen oxides and sulphur oxides (from fossil-fuel heated boilers) which create acidity in the natural environment (freshwater lakes, rivers, forests and soils) and lead to the deterioration of metal and building structures. They also contribute to smog formation in urban areas.
• Solvent escaping into the air from drying ovens used in solvent coating operations.
• Solvents released from cleaning activities (general facility clean-up and maintenance, print screen cleaning).
• Emissions of volatile hydrocarbons which include non-methane hydrocarbons (NMHC) and oxygenated NMHC (e.g., alcohols, aldehydes and organic acids).

5. Transportation
Long-distance transport is required to move the finished products from the factories located in low labour-cost countries to the consumer in a developed country, thus adding to the overall quantity of non-renewable fuel consumed.

6. Packing materials
Packaging is the science, art, and technology of enclosing or protecting products for distribution, storage, sale, and use. Packaging also refers to the process of design, evaluation, and production of packages. Packaging can be described as a coordinated system of preparing goods for transport, warehousing, logistics, sale, and end use. Packaging contains, protects, preserves, transports, informs, and sells (Soroka, 2002).  For consumer packaging, the packaging used to present products in stores, the materials often used are plastic, paper, metal, aluminium, cotton, hemp and biodegradable materials. Companies implementing eco-friendly actions are reducing their carbon footprint by using more recycled materials, increasingly re-using packaging components for other purposes or products, employing recycled materials (e.g. paper, cotton, jute, hemp, wood), biodegradable materials, natural products grown without the use of pesticides or artificial fertilizers and re-usable materials (e.g. cotton bags or hemp). Reducing packaging waste is one of the best ways of minimizing environmental impact. EU Directive 94/62/EC specifies a number of requirements relevant for packaging and packaging waste. It also sets specific recycling targets and maximum levels for heavy metals (Tufekci et al., 1998). Sustainable packaging is the development and use of packaging which results in improved sustainability. At the end stage of design it involves increased use of life cycle inventory (LCI) and life cycle assessment (LCA) which considers the material and energy inputs and outputs to the package, the packaged product (contents), the packaging process, the logistics system (Zabaniotou and Kassidi, 2003).

Improvement in Sustainability:
Sustainability of textile processing can be improved by several ways such as:

1. Substitution of unsustainable textile materials and chemicals by greener organic and biodegradable materials.
2. Elimination or minimisation of the use of toxic chemicals in production and packing
3. Minimisation of the use of water and chemicals and recycling them.
4. Minimisation of consumption of energy and fuel in production and transport.
5. Minimisation of waste and easy waste disposal.
6. Maintaining environmental management systems strictly.

Greener Textile Materials:
The basic raw material for textile manufacture is textile fibers which are of two types – natural and man-made (Farrington, 2008). The major natural fibres are of vegetable or animal origin. They are not inherently green. They are biodegradable, but more biodegradable polymers can be made by biological means. A major defining difference between biopolymers and other polymers can be found in their structures. Biopolymers often have a well-defined structure. In contrast most synthetic polymers have much simpler and more random (or stochastic) structures. Synthetic polymer fibres are not biodegradable and hence must preferably be recycled to minimise accumulation.

Because of the increased awareness of the polluting nature of textile effluents, social pressures on textile processing units are increasing. Awareness about eco-friendliness in apparel textiles has become one of the important issues in recent times. Owing to the demands of global consumers, research is being carried out to establish new eco-friendly technology. Problems related to toxicity and other health hazards have resulted in the replacement of chemical processing by more eco-friendly physical methods (The Textile Institute, 2002). Plasma, biotechnology, ultrasonic treatment, super critical carbon dioxide and laser technologies for example, are technologies which are gaining ground because they offer many advantages against wet techniques with fewer or no harmful chemicals in wet processes or waste water (although there may still be some pollution; plasma treatment, for example, can give rise to some air pollutants from the degradation of fibre surfaces).

Plant-based fibres and eco-friendly chemicals can be used to manufacture green composites. Among natural fibres the bast fibres, extracted from the stems of plants such as jute, kenaf, flax, ramie and hemp are widely accepted as the best candidates for the reinforcement components of composites due to their good mechanical properties, with hemp shown to have particular promise (Kolybaba et al., 2003).

Organic Cotton
Organic cotton production is more environmentally friendly, better for the health of the community and for the local economy than GM cotton. Organic cotton is cotton from plants that are not genetically modified and are certified to be grown without the use of any synthetic agricultural chemicals, such as fertilizers, pesticides or defoliants, produced according to the internationally recognized organic farming standards

Organic Linen
The flax plant (Linum usitatissimum) is one of the oldest fibre crops in the world and has been used in the production of linen for over 5,000 years.

Organic linen refers to linen made from flax fibres grown without the use of toxic pesticides or chemical fertilizers.

Organic Wool
The requirements for certification of organic wool by the Organic Trade Association (OTA) in North America are as follows (OTA, 2011):

• Livestock feed and fodder used from the last third of gestation must be certified organic.
• The use of synthetic hormones and genetic engineering is prohibited.
• The use of synthetic pesticides (internal, external and on pastures) is prohibited.
• Producers must encourage livestock health through good cultural and management practices.

Organic Silk
Organic silk must not only be obtained by feeding the silkworms with the leaves from mulberry bushes that have been grown organically, but in which no ‘cruelty’ has been employed (i.e. not by the conventional production method of placing the cocoons containing the live silk worms into boiling water). Thus, in the so-called ‘peace’ or ‘vegetarian’ varieties, the silkworms are allowed to develop and emerge as moths. As a consequence, the silk is obtained in the form of short staple instead of continuous filament, yielding a fabric with a different appearance and handle, but with a warmer handle. For organic silk, silkworms are given feed which is free from harmful substances.

Greener Regenerated Cellulose Fibres
Regenerated cellulose filaments and fibres such as viscose rayon, modal and cupro fibres are manufactured by dissolving cellulose in carbon disulfide in the case of viscose and modal fibres and cuprammonium hydroxide solvent for cupro fibres followed by precipitation in filament form in a suitable solution (usually mineral acid). Both solvents cause environmental problems, hence, attempts have been made to develop an alternate process.  Lyocell fibres are produced by regenerating cellulose in an organic solvent, N-methylmorpholine-N-oxide (NMMO) hydrate, which is non-toxic, biodegradable and is almost completely recycled (Albrecht et al., 1997). The lifecycle of a lyocell fibre has minimal environmental impact and is significantly more sustainable than oil-derived synthetic fibers (e.g., polyester, nylon, acrylic etc.) or natural fibres such as cotton, using less land, irrigation, pesticides or fertilizers to cultivate the eucalyptus or beech trees from which lyocell fibres are made if the forests are sustainably managed than is the case for cotton (White et al., 2005). Lyocell fibres are available as Tencel (Lenzing, Austria), LyoCell (Lenzing, Austria) and NewCell (Akzo-Nobel, Germany); 98% of lyocell fiber production is attributable to Lenzing.

Synthetic Polymer Fibres
A large quantity of adipic acid (HOOC(CH₂)₄COOH) is used in the production of nylon 66, polyurethanes, lubricants and plasticizers. Conventionally it is produced from benzene (a known carcinogen).

However, natural glucose can be converted into adipic acid by an enzyme discovered in genetically modified bacteria.

Polyurethane polymers are presently produced from toxic diisocyanates, but a series of polyurethanes based on bis-carbamate diols can be synthesized using the Candida antarctica lipase B as catalyst (McCabe, 2004).

Greener Preparatory Processes:
Textile materials possess a variety of impurities. All such impurities have to be removed before dyeing or printing processes by preparatory processes (or pre-treatments), which may vary from fibre to fibre.

Enzyme processing
The most important application of green chemistry in textile preparatory processes is the use of enzymes. Enzymes are biological catalysts. They consist of complex three-dimensional proteins composed of polypeptide chains. They range from individual proteins with a relative molecular mass (RMM) of around 13,000 catalyzing a single reaction, to multi-enzyme complexes. Additionally some enzymes require some specific small non-protein molecules, known as cofactors, in order to function as catalysts. Enzymes are relatively fragile substances and they are susceptible to denaturing, that is, degradation due to temperature, ionizing radiation, light, acids, alkalis, and biological factors and thereby become inactive.

Commercially enzymes are obtained from three primary sources: animal tissue, plants, and microbes. Natural enzymes are not readily available in sufficient quantities for industrial use. They are largely manufactured by fermentation, the technique  is well known for more than 3000 years. Microorganisms producing enzymes important for the textile industry are listed in Table 1 (Shaikh, 2010).

Specific enzymes, alone or as mixture, are used in various textile-processing steps as shown in Table 1 (Holmes, 1998).

Table 1: Applicability of enzymes at various textile processing steps

The enzymes may be applied in various stages of textile processing namely desizing, scouring, bleaching, dyeing, finishing and also later, when the product is being disposed of by composting (Preša and Tavčer, 2009).

Bio-desizing
Warp sizes mostly consist of starch, modified starch or organic polymers such as polyvinyl alcohol (PVA), and polyacrylates. Conventionally, starch desizing is done by hydrolysis with water, by mineral acids or by oxidation with sodium bromite or per-oxy compounds. The alternative eco-friendly method is to treat with amylase enzymes in the presence or absence of lipase (to remove lipids). Amylase enzymes are obtained mainly from malt, bacterial, or sometimes pancreatic sources.

Amylases were the first and most successful enzymes used in the textile industry for desizing. They were first used in the early 1900s when malt extracts from barley containing the active enzymes were applied to greige fabrics. In 1917 bacterial enzymes were isolated and ultimately bacteria became the source for industrial production.

A regular amylase may be applied at a pH 5.5–7.0 and at a temperature of 25–55 0C. Amylases for use at medium temperature can be used in the range of 50–95 0C, whereas high temperature amylase can be used successfully above 95 0C and also in the pad-steam process. The end-products of the enzymatic desizing process are various types of sugars and dextrins which are nontoxic. However, they have a negative impact on the BOD of the wastewater.

Bio-scouring
Scouring is an alkaline cleaning process for raw fibres (mostly natural) before the coloration process. Bioscouring has a number of potential advantages over traditional scouring. It is performed at neutral pH, which reduces total water consumption, the treated yarn/fabrics retain their strength properties, the weight loss is reduced or limited compared with processing in traditional ways, and it increases cotton fibre softness. The temperature of bioscouring is much lower compared to classical scouring; the optimal temperature is from 40 to 60ºC. Bioscouring can create the desired hydrophilicity in the cotton fabric, but at the same time leave sufficient wax materials on the fabric surface to lessen the required addition of softeners for consumer satisfaction.

An extremely powerful alkaline pectinase has been isolated recently. The major benefit of this enzyme in bio preparation is that the enzyme does not destroy the cellulose of the cotton fibre. The enzyme is a pectate lyase, and as such very rapidly catalyzes hydrolysis of salts of polygalacturonic acids (pectins) in the primary wall matrix. Pectinase, as the name suggests, hydrolyzes pectin present in cotton as a non-cellulosic impurity. The best kinds of pectinase are those that can function under slightly alkaline conditions even in the presence of chelating agents. Such enzymes are called ‘‘alkaline pectinases’’. Most conventional pectinases are usually inactive under these commercially useful conditions, their optimum activity lying in the slightly acidic region (Sundar et al., 2007).

Cellulases are especially suited for scouring of cotton fabrics. Some pectinase enzyme preparations contain cellulase. Those impurities are then removed by subsequent washing. However, the combined actions of both types give greater weight loss and strength loss as compared to the action of pectinase or lipase alone.

Cotton fibers, or their blends with other fibres, can be treated with aqueous solutions containing protopectinases for 18 h at 40ºC to give scoured yarn with good tensile strength retention. Pectinases and cellulases are very effective compared to the proteases and lipases.

The combined enzyme system for simultaneous desizing and scouring may contain amylase, lipase, and pectinase enzymes to achieve the necessary fabric properties without the use of harsh chemicals (Karmakar, 1998).

BioPrep 3000L (Novo Nordisk, Denmark) is an alkaline pectinase, free from Cellulases  (Lange, 2000). It works optimally at pH 7–9.5 and at temperature up to 60 0C in exhaust systems and at somewhat higher temperature in pad systems. Biological oxygen demand (BOD) and chemical oxygen demand (COD) of enzymatic scouring process are 20–45 % as compared to alkaline scouring (100%). Total dissolved solid (TDS) of the enzymatic scouring process is 20–50 % as compared to alkaline scouring (100 %). Handle is very soft in enzymatic scouring compared to the harsh feel in the alkaline scouring process.

Enzymatic scouring makes it possible to scour fabric effectively without negatively affecting the fabric or the environment. It also minimizes health risks in as much as operators are not exposed to aggressive chemicals (Pawar et al., 2002).

Despite all the research on bioscouring, it has yet to be applied on an industrial scale. There is a need for pectinases with higher activity and stability at high temperatures and alkaline conditions.

Bio-bleaching
At the University of Auburn (USA) glucose oxidase enzyme (GOx; EC 1.1.3.4) was used for bleaching when the whiteness index was improved with lower strength loss (Shaikh, 2010).

In the presence of molecular oxygen, glucose is oxidized by the enzyme glucose oxidase to gluconic acid and hydrogen peroxide. D-Gluconic acid acts as a sequestering agent during bleaching. Amyloglucosidases, pectinases, and glucose oxidases are compatible concerning their active pH and temperature range and were selected. A combination of two or all three preparation steps with minimal amounts of treatment baths and rinse-water showed compatible results in whiteness, absorbency, dyeability, and tensile properties of the treated fabrics. Recently studies were done of biobleaching of wool under both oxidative and reductive conditions. The studies showed that hydrogen peroxide bleaching in the presence of protease preparation, Bactosol SI (Clariant), considerably improved whiteness and hydrophilicity.

Peroxide killer
Catalases (CATs), more correctly hydroperoxidases, catalyze the degradation of H₂O₂ to H₂O and O₂. Catalase or peroxidase is a oxidoreductive class of enzyme produced by a variety of different microorganisms including bacteria and fungi (Mueller et al.,1997) and most have optima at moderate temperatures (20–50ºC) and neutral pH. In addition to the protein part of the molecule, catalase enzyme contains a nonprotein part, which is a derivative of heam and includes metal iron. The advantage of the catalase enzyme (Jensen, 2000). is that it attacks only hydrogen peroxide and nothing else. The reaction is as follows:

H₂O₂ + catalase → 2H₂O + O₂ + catalase        [22.4]

Laccases
Laccases are a group of oxidative enzymes whose exploitation as biocatalysts in organic synthesis has been neglected in the past, probably because they were not readily commercially available. They are glycoproteins, which have been reported as present in higher plants and in virtually every fungus that has been examined for them (Barreca A.M. et al. 2003). Laccases are oxidoreductases (EC 1.10.3.2, p-diphenol: dioxygen oxidoreductase) belonging to the so-called blue-copper family of oxidases (Riva, 2006), the multinuclear copper-containing oxidases; they catalyse the mono-electronic oxidation of substrates at the expense of molecular oxygen.

At present, the main technological applications of laccases are in the textile dyeing or printing industries, in processes related to decolourisation of dyes (Li et al., 1999) – and in the pulp and paper industries for the delignification of woody fibres, particularly during the bleaching process (Galante, and Formantici. 2003). In most of these applications, laccases are used together with a chemical mediator.

Colorants
Azo (-N=N-) groups, such as those incorporat­ed in benze­noid compounds, and which form the basis of the majority of synthetic dyes, are not found in naturally occurring products. Approximately 70% of all dyes (belonging to various dye-classes) used in the textile industry are azo dyes (Carliell et al., 1995). Under reductive conditions some azo dyes may produce carcinogenic amines (Lacasse and Baumann, 2004).

Following the introduction in 1994 of the German Consumer Goods Ordinance that restricted the use of certain azo dyes in consumer goods, several other EU member states introduced similar but different regulations (EC, 2002). A small number of aromatic amines are classified as being carcinogenic or potentially carcinogenic to humans, hence, in order to protect human health, those azo dyes which can break down under reductive conditions to release any of a group of defined aromatic amines, were prohibited from being used in consumer goods considered to have regular skin contact.

Four banned carcinogenic amines are (a) 4-aminodiphenyl; (b) benzidine; (c) 4-chloro-o-toluidine; and (d) 2-naphthylamine. Cleavable arylamines, carcinogens, allergens and other banned colourants should not be detectable in the finished articles or in the dyed parts thereof. Further information is available in the Oeko-tex website (Oeko-tex, 2013).

The list of the 187 banned azo dyes (106 direct dyes, 30 acid dyes, 11 basic dyes, 11 azoic colours, 27 disperse dyes and 2 other dyes) out of about 2000 is available elsewhere (EC 2002); such lists are difficult to keep up-to-date because of new chemical developments and sector changes, and for that reason, it is recommended to check periodically the internet links (Oeko-tex, 2013) where the update list of prohibited dyes is available.

Due to the known ecological and toxicological issues surrounding the use of chromium and cobalt, research has been carried out into their substitution with more environmentally benign metals such as iron and aluminium. As environmentally friendly alternatives to Cr(III) and Co(III) metal complex acid dyes, a range of 1:2 iron complex formazan dyes was synthetised, which produce violet, blue, black and brown shades on wool and polyamide with good wet- and light-fastness (Carliell et al., 1995)

Reactive dyes
Metal-free reactive dyes are currently quite popular in the textile dyeing and printing trade – they are comparatively safe and not many adverse effects have been reported (Hannemann and Runser 1999). However, the most obvious deficiency in the conventional reactive dyestuffs lies in the fact that the dye-fixation on the fibre is no more than 70% even in printing applications. The remainder of the dyestuff undergoes hydrolysis instead of reaction with the fibre increasing the pollution load in the effluent. More recently reactive dyestuffs have been developed which provide much higher degrees of fixation than the conventional reactive dyes (Morris et al., 2008).

These high-fixation (above 90%) reactive dyestuffs contain multiple (mostly two) reactive groups attached to each dye molecule. The dye may contain two similar reactive groups i.e. homo-functional dyes e.g. Procion HE® dyes (DyStar) or two different reactive groups i.e. hetero-functional dyes e.g. Procion ME® (DyStar).

Natural dyes
Natural dyes can be placed into three categories on the basis of origin – from plants (such as indigo), from animals (like cochineal), and those obtained from minerals (such as ochre). The major types of natural dyes and their origin have been tabulated by Hill (1997).

Prior to the advent of synthetic dyes, natural dyes were widely used, often together with mordants such as alum, to dye natural fibres like wool, linen, cotton and silk, but their use declined following the discovery of synthetic dyes. In recent years, however, interest in natural dyes has been revived due to increasing demands on manufacturers to produce more environmentally-friendly alternatives to petrochemical-derived dyes. One main issue associated with the use of natural dyes in the colouring of textiles is their poor to moderate light fastness, and despite their long tradition, not all natural dyes are especially environmentally-friendly. Some natural dyes have no or little affinity for the textile materials and they require heavy-metal salts as mordants for fixation and colourfastness. Natural dyes may be sustainable, but need water and land to produce and there is insufficient dye yield per acre of plant material to sustain industrial-scale production.

Natural dyes can be classified into two groups, depending on method of application, namely substantive and non-substantive dyes. The substantive dyes require no pre-treatment to the fabric (e.g. indigo, orchil, turmeric) and there are three types: direct (for cotton, e.g. turmeric, safflower; acid (for silk and wool, e.g. saffron, lac) or basic (for silk and wool, e.g. berberine).The non-substantive dyes, by contrast, can only dye already-mordanted material or work with the addition of a mordant to the dyebath (e.g. logwood, madder/ alizarin, cochineal or fustic)

The fixing agents recommended are alum, stannous chloride, tartaric acid, ferrous sulphate and Turkey Red Oil. Tannins are used as natural mordants in the natural dyeing process, as they can enhance the affinity of different dyes. Tannins are naturally occurring phenolic compounds and those having o-dihydroxy groups form metal chelates after pre-treatment or post-treatment of the substrate with metal salts. Tannin-metal salt combinations, rather than the tannins themselves, can be considered as the mordant for natural dyes. Some important raw materials for tannins are oak bark and wood (Quercus infectoria), pomegranate rind (Punica granatum) and cutch (Acacia catechu) (Roy Choudhury, 2006).

Greener Dyeing Processes:
To become environmentally responsible, dyers need to adopt the well-established “3R” principles of pollution prevention i.e. reduce, re-use and recycle; the most effective pollution prevention practice in textile wet processing is “right-first-time” dyeing. Corrective measures like shading additions, or stripping and re-dyeing processes, consume additional dyes and chemicals, water and energy (Holmes, 2012).

There are two basic approaches to pollution control:

1. Effluent treatment—or end-of-pipe solutions
2. Waste minimization—or source-reduction solutions

The first approach has no financial payback whereas the waste minimisation approach not only reduces environmental impact but also minimises the cost of production and has the potential to make dyeing processes greener in several ways:

1. Process optimization aimed at reducing process time and energy consumption, for example, by the use of combined scour/dye processes)
2. Reduced consumption of water (ultra-low liquor ratio dyeing – 3:1 to 4:1 for polyester and 6:1 for cotton), electrical power and steam consumption.
3. Promoting high exhaustion e.g., through the use of polyfunctional fibre-reactive dyestuffs which can give exhaustion in excess of 95% thus decreasing pollution.
4. Continuous preparatory and dyeing methods instead of batch-wise methods minimizing consumption of energy, water and chemical to less than half.
5. Semi-continuous cold pad–batch dyeing processes with reactive dyes instead of batch-wise or pad-steam methods (Kehry and Strohle, 2006).
6. Chemical-free water-free denim processing using laser technology to burn away quickly the surface of the dyed denim fabric or a pair of jeans on a mannequin to replicate an authentic worn look. Conventionally, this is done with stones, oxidizing agents or enzymes by aqueous methods (Tarhan and Sariisik, 2009).
7. Sustainable digital printing and heat transfer printing which require far less water and produces far less waste than the traditional printing methods (Fashion-incubator, 2010).
8. Cold transfer printing process (Cooltrans) of reactive dyes at room temperature on pre-treated cotton, viscose, linen and silk thereby saving water and heat (Thiry, 2010).
9. Supercritical carbon dioxide dyeing or waterless dyeing (Miah et al., 2013).

Dyeing in Supercritical Carbon Dioxide:
A supercritical fluid, a substance above its supercritical temperature and pressure (which for supercritical CO₂ are 31.1°C and 73.8 bar respectively (Cid et al., 2005), has the properties midway between a gas and a liquid. When a gas is compressed it becomes increasingly dense and finally it is nearly as dense as a liquid, expanding to fill its container like a gas but with a density similar to that of a liquid. The practical application of supercritical fluids as dyeing media received considerable attention from researchers in the last decade of the 20th century, as illustrated by the review by Bach and co-workers (Bach etal, 2002). Carbon dioxide, has so far been the most widely used, because of its convenient critical point , cheapness, chemical stability, non-flammability, stability in radioactive applications and non-toxicity. Owing to small viscosity of supercritical

Carbon dioxide and to high diffusion coefficient of dyestuff molecules in this condition, disperse dyestuff molecules easily penetrate into fibres and as a result supercritical carbon dioxide dyeing provides a better dyestuff transportation compared to dyeing in water. The CO₂ used is a waste product of combustion, fermentation and ammonia synthesis, so it does not have to be specially produced for dyeing.

The main advantages (Poliakoff and Licence, 2007) of using supercritical CO₂ (scCO₂) instead of water in a dyeing process are:

1. The easy separation of the CO₂ and the unused dye remaining after the dyeing process. Depressurization leads to precipitation of the excess dye and gives clean, gaseous CO₂, so that both compounds can be recycled and no waste is generated.
2. After the dyeing process, the textile does not need any energy-intensive drying step as in the case of aqueous dyeing.
3. Because scCO₂ is a non-polar solvent, no dispersing agent is needed when non-polar dyes are used, an additional advantage of scCO₂ in the disperse dyeing of polyester. This means that simpler dye formulations can be used than in aqueous polyester dyeing where dispersing agent constitutes around 50% of the whole dye powder.
4. One of the reasons for the superior economy of the supercritical process over the aqueous process is the higher rate of dyeing.

Two industry leaders – Huntsman Textile Effects and DyeCoo Textile Systems – joined forces in October 2012 to develop and grow supercritical CO₂ (scCO₂) textile processing technology. The joint collaboration is set to create more sustainable products which will benefit the industry as a whole. By using carbon dioxide as the application medium, DyeCoo’s innovative technology completely eliminates the use of water in the textile dyeing process. Huntsman Textile Effects is working with DyeCoo to develop and deliver innovative dye and chemical products to support the waterless dyeing process and to obtain the high level of colour fastness and performance that consumers demand. DyeCoo uses scCO₂ gas rather than water to infuse fabric with colour. Special temperature-controlled pressure chambers force the carbon dioxide to act as a fluid similar to water (the supercritical fluid CO₂) which causes the polymer fibre to swell allowing the dispersed dye to easily diffuse within the polymer, penetrating the fibers, and carrying the dyes into the fabric and causing perfect dyeing. The research is already under way to apply the technology to other natural and synthetic fabrics (Huntsman, 2016).

Future Trends:
The majority of chemicals are used during wet processing, such as dyeing, washing, printing and fabric finishing. According to surveys as much as 200 tonnes of water is consumed for every tonne of textiles produced. Many of the chemicals used in textile production are non-hazardous, but a relatively small proportion of these chemicals are potentially hazardous.

Sustainable production and consumption can only be achieved if industrialists, retailers and consumers take ecological factors into account in every decision-making process. There has been an increasing awareness that many products could be produced under better conditions with greater respect for the environment, for example, by using less energy, attaining better yields, creating less pollution of water or air, generation of less waste and fewer (or no) unwanted by-products. There is now an international set of standards under the ISO 14000 series concerning the assessment and improvement of environmental management within industry (Boiral, 2007).

As globalization of the wet-processing part of the textile industry continues, it is clear that there continues to be scope for it to continue to ‘green’ its processes and the chemistry involved. In almost every case of pollution, the fundamental problem for the textile industry is found to be low process inefficiency and a poor understanding of the life cycle of textile chemicals.  The application of the principles of green chemistry and other aspects of clean technology will increasingly lead to more environmentally-compatible manufacturing systems

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Founder & Editor of Textile Learner. He is a Textile Consultant, Blogger & Entrepreneur. He is working as a textile consultant in several local and international companies. He is also a contributor of Wikipedia.

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