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Sustainable colouration concepts Part III. Issues in textile processing of cotton

In the previous article we have discussed an innovative water-free dyeing process for polyester, dyeing from supercritical CO₂ (scf-CO₂). Today we will look at cotton, which is a major contributor for polluted effluents. Cotton has ethical and ecological concerns, as well as requiring high water & energy consumption in the textile industry.

Cotton is by far the most important natural, renewable fibre, used for textiles. Cotton and cotton blends are almost 40% of the total textile fibre consumption. The second biggest natural fibre is wool, from animal hairs, with only 2% share.

Cotton is not based on depletable raw materials such as crude oil or coal tar, compared to synthetic fibres like polyester or nylon. In the view of a peak oil scenario, this may be an important factor in the future.

Cotton is a large scale commodity agricultural product. We have observed in 2011 how significant supply and demand affect prices of commodities, when cotton prices surged dramatically and caused in a shake-up of the market. Many textiles processors switched from cotton to cotton blends or polyester, and suppliers of dyes and chemicals for cotton were in serious trouble.
Nonetheless, despite being a natural fibre, there are many ecological and ethical compliance issues related to cotton, from child labor, pesticide consumption and high water demand for irrigation of cotton fields.

Pesticide use in cotton agriculture	Child labor in cotton agriculture
Water pollution from dyeing	Water depletion in Aral sea[1], due to cotton agriculture

Conventional cotton uses 10 percent of all agricultural chemicals and 25 percent of the world’s insecticides. Modern cotton seeds, genetically engineered cotton (GMO cotton), have reduced the amount of pesticides needed, and already captured a significant share of the market[2].

Some brands promote apparel from organic cotton which does not use any pesticides, however, the land area space and water required for irrigation for organic cotton is very high, “it is commonly reported that organic cotton will require more water”[3], in GMO cotton the harvested yield is reported 15-30% higher, and 10 to 45 percent more expensive than conventional cotton products [3].

Also, cotton processing is a major consumer of water in the textile industry. Compared to synthetic fibres such as polyester or nylon, more than double the amount of water is required for processing[4].

Not only is the water consumption high, there are also a severe issues with water pollution from dyehouse effluents. The Greenpeace detox campaign is one of the hottest stories in the textile industries in these days.  We have covered this in two detailed articles recently[5].

Reactive dyeing contributes to coloured effluent loads. The impact is huge, because of the huge volume of the reactive dyes consumed and because of intrinsic weaknesses of the technology. Reactive dyeing effluents contain high concentrations of salt, hydrolysed reactive dyes from incomplete fixation, traces of heavy metals and dyebath auxiliaries, contributing to high BOD/COD values. The effluent can be treated properly by various state of the art methods (for example, see review article[6]). Obviously, a proper treatment requires investments in infrastructure, continuous upgrading and adds cost. Not everybody takes this step, voluntarily.

The fashion and textile industry has made a painful learning experience, how easily big brands and retailers can be connected with serious pollution problems of their suppliers:  Target, Liz Claiborne, Tommy Hilfiger, Reebok, Nike and Walmart –  just to mention a few involved. We remember the year 2007, when one of the biggest textile dyers in the world, Chinese based Fountain Set, was charged with illegal discharge. According to a Wall Street Journal article[7], authorities “discovered a pipe buried underneath the factory floor that was dumping roughly 22,000 tons of water contaminated from its dyeing operations each day into a nearby river, according to local environmental-protection officials.”Eventually “Fountain Set has paid roughly $1.5 million in penalties and spent $2.7 million to upgrade its water-treatment facilities”.

Such connections may seriously put brands at jeopardy.

In Tiripur, South India, one of the biggest cotton processing hubs in India, officials temporarily closed of 700 dyehouse units in 2011, due to pollution problems. Now officials claim to have implemented zero discharge[8]: “Tirupur is the first textiles cluster in India to arrive at the Zero Liquid Discharge Technology i.e. no water pollution. ….. we are proud to say that first to arrive at Zero Liquid Discharge (ZLD) in Dyeing & Processing Technology. At present all Dyeing & Processing units are using 100 percent Zero Discharge technology.”, commented Apparel Export Promotion Council Chairman, Dr. Sakthivel in December 2012.

Zero discharge concepts are actually not really new. In Western Europe, 20 years ago, every industry conference had such topics at the top of the agenda. But mass textile production in Western Europe died, and the industry moved east, due to pressure from retailers to produce cheaper. The issue is now on the plate once again. While Europe has talked about it, it is remarkable that zero discharge concepts are now actually implemented in India.

Known methods for treatment of dyehouse effluents are for example reverse osmosis, nanofiltration, ultrafiltration, microfiltration, aerobic biological treatment with activated sludge, coagulation-flocculation, adsorption on activated carbon, oxidative chemical treatments, ozone treatment, electrochemical processes⁵. Depending on the production output, a combination of these methods is suitable.

A nice presentation in public domain for water conservation methods can be found here[9].

What can be done to reduce the effluent loads? Before we can answer this question, we should look at the technical and scientific background what happens in reactive dyeing and what causes the effluent loads.

Understanding the dyeing process- dye exhaustion and fixation in cellulose dyeing

Cotton is a natural fibre consisting of cellulose which is a polymer built upon glucose molecules, joined by 1,4 bonds. In the three dimensional structure, cellulose is held together by inter- and intramolecular hydrogen bonds.

These hydrogen bonds also determine the crystalline structure. In cellulose, there are crystalline and amorphous parts which are important for the tensile strength as well as for the dyeing. The crystal structure of the crystalline parts is well known[10]

Cellulose is mostly used in the form of cotton, so we will restrict the discussion to cotton only. In other cellulose fibres, there are differences in the morphological structures and in the crystalline parts which may cause differences in dyeing performance as well.

The dyeing process of cotton operates as follows, when looking at the microscopic or even molecular scale.

1. Diffusion of soluble dyestuff through pore structure of cotton. The rate of diffusion is influenced by an equilibrium of dye aggregation and monodisperse dyestuff, hydrated by a structure of water molecules. In cotton dyeing it is known from the work of Zollinger[11]  that the pore model of dyeing is predominant, as compared to many synthetic fibres in which the free volume model is more important. This depends on the structure of the polymers, the degree of crystallinity, the pore structure and the mobility of the dyes in the respective polymer.

   

2. Adsorption on the surface of cotton, in presence of high salt concentrations, preferably on surfaces of crystalline parts. Salt is required because Cellulose is slightly acidic, and the Cellosate (Cell-O^–)  ions  cause a negative charge on the fibre surface and electrostatic repulsion by anionic dyes, contained in reactive and direct dyes, which typically carry sulfo groups to facilitate water solubility. To compensate for this electrostatic repulsion, high amount of salt, 50-100 g/L sodium chloride (common salt) or sodium sulfate (Glauber´s salt), is applied for dye bath exhaustion.
   Further, there is diffusion (migration) into in amorphous parts of the cotton. Thermodynamically the absorption is driven by heat of adsorption and by entropy (increase of entropy by release of structured water around dye in solution vs. loss of entropy of dye molecule). Technologists call the ability of a dyestuff to be adsorbed on the fibre “substantivity”. Without alkali during neutral exhaustion phase the process stops here, and no dye is fixed (exception: neutral fixing dyes).
3. Fixation by forming a covalent bond between OH groups in cellulose and fibre reactive groups of the dyestuff. With the fixation the dye is immobilised and will not migrate further. When alkali is added, not only fixation starts, but also the adsorption of non fixed dyestuff is affected by dissociation of acidic OH groups, in cellulose or dyestuff, and repulsion of anionic charges in the dyestuff and the fibre would reduce the absorption. However, when the fixation starts, the equilibrium of absorbed to unabsorbed dye is disturbed, new dye molecules are absorbed, and the equilibrium is favourably shifted.
4. Wash-off unfixed dyestuff – after end of fixation phase, rinse to neutral pH and wash-off unfixed dyestuff. Depending on the amount of unfixed dye, the substantivity of the hydrolysed dye, and required wash fastness level, the amount of water, temperature and the number of soaping baths varies.

As pointed out in step 4, the essential function of a reactive dye is the fixation on the fibre. Dyestuff molecules react with cotton by forming a covalent bond, while being immobilized. Unless severe conditions are applied, this covalent bond is stable. This is the reason for excellent wash fastnesses of reactive dyes, providing that all hydrolysed dye is washed off.

There are many suitable fibre reactive groups known[12], especially halogentriazine, such as monochloro- (MCT) or monofluoro (MFT)- triazine, and vinylsulfone (VS). In particular, the most economical MCT and VS  reactive systems are used in large scale the market, being nowadays incorporated of most commercially available brands – the world market for reactive  dyes is approximately 250 tsd metric tons per annum.

In step 3, besides the desired fixation reaction, there is always a fraction of the reactive group lost, due to hydrolysis (reaction with water), occurring as competing reaction.  A high performing dye has a favorable ratio of fixation to hydrolysis (technical term : selectivity) and can achieve 90 % or more fixation yield. This is often achieved by integration of several reactive groups in a single dye molecule, which statistically improves the chance that at least one of the reactive groups would react with the fibre to form a covalent bond.

Fixation yields of reactive dyes

The table shows fixation yields measured for typical reactive dyes found in the market, in 1% and 5% depth at a liquor ratio of 1:10 and dyed on cotton at 60° in a temperature rise method, using a process with the amount of salt and alkali as recommended by the dyestuff suppliers (typically soda ash or soda ash/caustic method, alkali added after a period of neutral exhaustion phase). Fixation yields were assessed according to the known method of measuring fixation yields of reactive dyes [13].

Examples for fixation yields of reactive dyes [14]

MCT: monochlorotriazine, MFT: monofluorotriazine, VS: vinylsulfone, FCP: fluorochloropyrimidine

The fixation yields are linked with optimized dyeing conditions. Parameters reducing fixation yields are a longer liquor ratio, an increase in dye concentration (as shown in the table), and change of optimal dyeing conditions (salt and alkali amounts, temperature heat up curve, change of temperature, dosing of chemicals etc.).

Conventional reactive dyes systems, double anchor systems (“bifunctional”[15] MCT/VS) can reach up to 75% in paler shades, and 70% in darker shades, while high performance dyes can reach levels of above 90% fixation yield. This difference is significant. With 70% yield, 30% of dye ends up in the effluent, compared to only 10% in case of the 90% fixed product. This means the colour in the effluent can be reduced by 2/3 with choice of the proper dyestuffs.

As the table shows , 90% fixation yield or more can be achieved with double anchor systems containing at least two effective reactive groups of any one of the types of either fluoro  heterocycles,  fluorotriazines (MFT) or fluoropyrimidines  (FCP,TFP) and/or VS reactive groups. “Bifunctional” MCT/VS commodity dyes cannot reach higher levels since the MCT is not an effective reactive group at the typically used dyeing condition of 60°C.

High fixation dyes with > 90% fixation as shown in the table are already in the market since many years, despite claims of some distributors, fixation yields of  90% are not new. However, these dyes are often only used in pale shades due to higher cost level and used rather for light fastness than for ecological and high fixation reasons.  The market perception, especially in Asia, is even that fluoro containing reactive groups are dyes designed for high light fastness, although technically this is incorrect and there is practically no impact of the reactive system on the light fastness.
The reason why performance dyes are mainly used in pale shade is because of their higher prices. However, with the start of production of fluoro reactive dyes in India, high performance reactive dyes will also become commercially more attractive in the near future, and they will increase their market penetration.

State of the art: Best available technology

Best available technology, in reactive dyes, means high fixation dyes and dyes with excellent washing-off properties will allow to skip soaping baths and save water as well as energy. In combination with a proper effluent treatment system and water recycling, excellent results can be achieved.

Besides dyestuff selection, water savings can be supported greatly by modern machinery technologies : short liquor ratio, for example, and optimized dyeing & finishing processes will generally increase fixation yields and reduce water consumption.

Then there are some new, interesting concepts introduced in recent years, claiming further significant improvements.

“Saving the Earth” with Avitera SE

In 2010, Huntsman Textile Effects launched a promising new concept, the highly-soluble Avitera™ SE reactive dyes and Eriopon® LT clearing additive for exhaust dyeing processes[16]. According to Huntsman, “Avitera ® SE is a revolutionary, ground breaking technology that helps textile mills increase production outputs and significantly reduce water and energy consumption and CO2 emissions by up to 50%“[17]

Huntsman claims to reduce the water consumption from 30-40 ltrs to 15-20 ltrs for 1 kg of cotton, and carry out the soaping for washing off unfixed dyestuffs, aided by a special detergent, at a moderate temperature of only 60°, as compared to conventional operating procedures of  >90°C.

In the first wave of the launch market acceptance was limited due to very high prices and limitations in shades and unsatisfactory light fastnesses. The limited range and interaction with the detergent chemical also caused issues with levelness and dichroism, and problems with metamerism.

Despite these start-up problems, Avitera has potential to be a promising new concept for the future. Meanwhile a more extended range has been introduced and only time will tell about the progress.

Old concepts, rejuvenated

When direct dyes instead of reactive dyes are applied, cotton blends can be processed with higher productivity, while saving water, and in pale shades good light fastnesses can be achieved. They can be operated in one bath applications and shorter processing times can be achieved.

However, direct dyes were replaced on large scale by reactive dyes in the last decades. Since the beginning of mass production for reactive dyes, direct dyes have lost considerable market shares and are today considered as a niche area of the textile dyes market.

An example of a revival of old direct dyes concepts, including integrated water recycling, in the view of “sustainability”, is the Green Project W.S.T in Bangladesh[18].

Direct dyes have earned a bad reputation because of poor wash fastnesses and ecological problems, in fact many of the banned azo dyes are direct dyes based on benzidine. When the right benzidine free dyes are selected this concern would be unsubstantiated, but careful selection is required.

Wash fastness limitations of direct dyes can be fixed with cationic aftertreatments. However, the application of a cationic fixing agent may compromise the level of the lightfastness.

The problem why most reactive dyes fail in one-bath processes for CO/PES fibre blends are the lack of alkali stability of disperse dyes, and conditions of reductive clearing which are not healthy for reactive dyes.

Neutral fixing reactives dyes could possibly address these issue. These dyes can be applied at temperatures above 100°C and neutral pH values, comparable to polyester processing. Neutral fixing dyes are, for example, Kayacelon React dyes [10], a Japanese technology with a low market penetration. These dyes carry a carboxypyridiuniumtriazine reactive group. In principle it works, but unfortunately, so far the known dyes have a poor build-up and are only used for pale to medium shades.

The technical challenges and improvements in continuous dyeing of PES/CO blends are outlined in a recent paper of Tolksdorf[19].

Fibre modification concepts

Another approach to improve dyeing is to modify the cellulose substrate, in order to facilitate the dyeing process.

Cotton with increased reactivity

The hydroxyl groups in cotton are only low in reactivity, requiring alkali addition, soda ash or caustic soda, to convert to more reactive Cellosate groups (Cel-O^–).  The alkali system and pH value throughout the process is essential for the dyeing performance.

However, this creates two problems. Firstly, the alkaline pH increases the negative charge on the fibre surface (as explained above) and increases the electrostatic repulsion by anionic dyes, contained in reactive and direct dyes, and therefore high amount of salt is required for dye bath exhaustion. Secondly, the alkaline pH will cause hydrolysis as a side reaction. Therefore the fixation yield is often limited, and dye is lost due to hydrolysis.

Comparable, in wool there are amino and sulfide groups which are significantly more reactive. This allows wool to be dyes at lower pH and in higher yields.

To introduce comparable more effective functional groups in cotton would allow to dye without any salt, and at lower pH ranges.

Modified Cellulose

Various technologies have been developed to introduce functional groups in cellulose by chemical methods. By certain modifications a cellulose with enhanced reactivity can be made, allowing dyeing without salt, and substantially higher exhaustion and fixation rates.

It is beyond the scope of this article to discuss all known and explored methods, so I will only take a few particularly interesting examples and explain how the principle works. Many different systems have been explored in research, by academics as well as by commercial organizations, yet most of them failed due to cost, or performance limitations. For our readers who are more interested in the subject, please refer to excellent review article about  modified cellulose, by Lewis[20].

For example, Lewis introduced a chemical modification system based on AAHTAPC, a quarternary ammonium compound,  which significantly improved fixation yields for a variety of examined commercially available dyes, but did not achieve complete fixation without any colour in the effluent [21].

Cellulose with amino groups, incorporated by chemical modifications

Of particular interest is a cellulose containing amino groups, similar as wool.

Aminated cellulose can be made from tosylated cellulose and subsequent nucleophilic substitutions [22],[23].

In the 1990s, Hoechst developed and launched a working technology using sulfatoethyl-piperazine (SEP) [24], and a similar, at least academically interesting concept was based on aminoethylsulfuric acic (AES) as modifying agent [25]. AES-cellulose was not developed further. SEP-cellulose became a commercial product in the 1990s. SEP cellulose could be dyed without any salt and with complete fixation. However, it was not successful in the market at the time, due to various reasons, cost of additional equipment and fastness limitations. Time was too early for such ecologically advanced concepts, in the view of a cost driven environment.

There are some disadvantages, too, in chemical modification concepts, such as the need for a pretreatment (requiring equipment), incomplete fixation during pretreatment stage, and release of (colourless) organic chemicals from pretreatment stage, increasing the COD (chemical oxygen demand) values in the effluent treatment. Ideally the modification should be done in the same equipment, in a pretreatment step, in high yield.

Transgenic cotton plants with built-in amino groups

It should be mentioned in the context there are natural cellulose type fibres carrying amino groups : chitin (NH-COCH₃) and chitosan (NH₂). A new cotton fiber with a chitosan coating has even been developed in Japan[26].

The introduction of amino groups is also possible by bio-engineering. One particular interesting example is patented by Bayer Bioscience who introduced the amino group during biosynthesis of the cotton fiber in the form of N-acetyl glucosamine (GlcNAc) polymers[27].

In the vision of the inventors, one day a new, re-engineered easy-dyeable cotton plant may be born, which would be a truly game changing invention.

Summary

Cotton processing has various ecological problems, from polluted effluents to high water and energy consumption. Various concepts to improve this situation are discussed in this article. Best available technology can achieve zero discharge, especially by careful selection of dyes and processes, proper effluent treatment and recycling concepts. Future prospects include modified cellulose fibres, by chemical modifications, or bioengineered cotton.