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A new concept for sustainable denim production

15th March 2013


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The depletion of natural resources in order to ensure adequate living standards for the planet’s growing population has become a pressing issue. Miguel Sanchez reports

In today’s highly connected world, the effect of a lack of the basic essentials of food and water on impoverished regions is evident to all, and the corresponding need to reduce the impact of today’s manufacturing, distribution and consumption infrastructure has become clear.

Yet a higher demand for goods and services can only be met by increasing production, which in turn requires new infrastructure, transportation systems and land to grow on. This in turn suggests higher inputs of all raw materials, including minerals, fuels, wood, fibres, food and water.

Despite living on the ‘blue planet’, water, as the basic element of life, is basically a non-renewable resource and its availability is already scarce in some regions of the world, as can be seen from the by well-known data below (Fig. 1).

All of our activities, including production, largely depend on this relatively small amount of available water.

Water textile industry

Today’s textile industry requires vast amounts water:  in some steps of the process of converting a fibre into a final article, the water is required for dissolving and applying chemicals, and eventually for removing those that are unfixed or not required for subsequent treatments. In this sense, the operations of pretreatment, dyeing and finishing of the fibres and fabrics are critical. The World Bank1 estimates that 20 per cent of industrial pollution comes specifically from these three processes.

The water required for each type of article depends greatly of the number of steps required in its production. National Geographic2 made an estimation of the liters required for the whole processing of common items used every day, including some basic textiles –from obtaining the fibre to the point where the article is ready for final use and presented its conclusions in a very graphic way (Fig. 2).

Denim is on top of the pyramid of water consuming textiles, with a total volume of 2.900 Gallons (approx. 11.000 litres) per pair of jeans required.

The denim industry

Cotton is the most basic fibre for denim production, even if some other man-made fibres can be used together with it for certain looks and effects. In the previously mentioned volume of 11,000 litres required for the production of a pair of jeans, an estimated 40-50 per cent is needed for the plant’s growth. This leaves more than 5.000 litres of water used for the manufacturing steps, from fibre spinning to the final garment treatments and finishing2.

Some good alternatives for obtaining cotton with the lowest use of resources, including BCI (Best Cotton Initiative), the use of recycled fibres or the implementation of artificial fibres from natural polymers are gaining acceptance by denim brands and end consumers. The same is occurring with the methods for obtaining the final effects that make jeans so special. Hence the use of a new generation of clever enzymes is replacing the traditional stone-wash or sand-blasting wash-down techniques. At the same time, treatments with ozone and laser beams are being employed instead of chemicals like sodium hypochlorite or potassium permanganate to achieve both water savings and prevent the contamination of effluents with hazardous chemicals2.

Nevertheless, the  colouring of the warp yarns in denim production, which is crucial to acquiring the desired depth, cast and final contrast effects, is still mainly based on traditional dyeing technologies involving the use of Indigo and associated chemicals such as Sodium Hydrosulphite. As shown in practical analyses conducted by Clariant in many different countries, the total volume of water required for dyeing and removing of unfixed dyestuff requires volumes of water between 15-25 l/Kg dyes CO warp in a slasher denim range.

Indigo warp dyeing

The warp for the denim fabric used in jeans is usually dyed in large ranges where the cotton ends are presented to the machine either in ropes (12, 24, 36 groups of 400-450 running threads) or in a slasher (open width, with each single end running parallel to the rest to form the final weaving beam, with 3,800-4,200 threads).

In a normal application process for blue denim, the ends are pre-wetted in a first box with a solution normally containing an alkali, a surfactant and a dispersing agent. The fabric then moves on to a series of boxes for washing off the saponified impurities.

The ends subsequently enter into a series of boxes containing the Indigo. Indigo is a unique dyestuff due to its particular dyeing performance, capacity for obtaining ring-dyeing effects and the special colours and effects that can be achieved through mechanical or chemical treatments, normally once the fabric has been converted into garment.

The boxes of Indigo can typically contain from 300l to 2.000l of dye solution, plus the recirculation bath required for keeping the same concentration of dyestuff in each of the boxes. There ususally can be a volume of 10,000 to 20,000l of indigo solution going around during the dyeing.

Fixation of the indigo layers, obtained by air oxidation -skying- of the leuco form, takes place after each of the impregnations.

In the third segment of the dyeing range, the unfixed Indigo and the remains of the reducing chemicals and auxiliaries used in the dyeing are removed by overflow rinsing. The typical water consumption in this particular process can be 3-5l/Kg of yarn per rinsing box.

Advanced denim concept

The Advanced Denim concept is firstly based on the use of state-of-the-art sulphur dyes (Diresul RDT).

Sulphur dyes have also been employed in denim warp dyeing for many years. Like indigo, their chemistry is based on a reduction-oxidation balance. Nevertheless, while Indigo requires strong reducing conditions and hence is easily oxidised to its insoluble form, sulphur dyes can be solubilised with mild reducing agents. On the other hand, sulphur dyes need strong oxidation conditions for their fixation and full shade development.

Sulphur dyes have been long time partners of Indigo either to complement it for special shades and effects (by bottoming or topping) or used as single elements for black and colour denim.

Sulphur dye molecules have two main active groups: the Thiol group, responsible of the fixation of the dye on the cotton by forming hydrogen bonds with the cellulose, and the Quinonic one, that allows the developing of the real shade of the dyestuff from the reduced leuco form (Figs. 3 and 4).

Having higher affinity for the cellulose when in reduced form, it is possible to apply them using a small amount of dye in the dyebath, in a single step impregnation.

In standard conditions, after the dyeing of the warp in the denim range, a rinsing off is carried out for the removal of unfixed dyestuff before its fixation by the use of an oxidant agent. However, the use of  sulphur dyes allows the rinsing before oxidation to be skipped. It is possible to completely insolubilise the dyestuff to a very stable form by using a cationic fixing agent. As a result, after the impregnation with the dyebath, and without any intermediate rinsing, an immediate cationic treatment can be carried out.

Thanks to the ionic character of the Thiol group in the dyestuffs, a strong ionic bond is formed with a bi-reactive cationic agent. Hence we can have two types of dye-cationic compound form:

CEL – Dye – Cationic agent – Dye –

CEL – Cationic agent – Dye – Cationic agent –

Unfixed dye, normally in the surface of the fibre, stays in a stable insoluble form

Dye – Cationic agent – Dye –

Graphic details can be seen on fig. 5:

The steps of the process can be then described as follows:

Impregnation of the fibre with the dye solution, containing reducing agent, alkali and auxiliaries.

Short air oxidation (Skying).

Fixation bath containing an oxidant agent and a cationic fixing agent. pH to be adjusted to the type of oxidant product, normally acid (pH 4-5).

The intermediate skying between the dyebath impregnation and the fixation ensures a regular migration of the dyestuff and also improves the efficiency of the fixation by the oxidation of most of the still active reducing chemicals.   On the other hand, the presence of an oxidant agent in the fixation bath is required for the development of the actual shade of the used dyestuff. If a cationic agent is used on its own, the shade obtained will be that of the leuco form at the moment of the fixation.

A simple diagram of this Denim-Ox process can be as shown In Fig. 6.

Pad-Sizing/Ox process 

The preparation of the warp for the weaving of the denim fabric requires a sizing, intended to protect the fibres from the friction forces that occur at the loom and thus obtaining a good weaving efficiency.

In the search for even lower volumes of water when colouring denim warp, the possibility of combining the oxidation/fixation of the dyestuff, as done in the Denim-Ox process, with the sizing was considered and implemented. This process is called Pad-Sizing/Ox and reduces the water input to a mere 7 per cent of what is required for a conventional denim dyeing process, with the additional feature of completely preventing the generation of waste waters, since no rinsing before the oxidation/fixation/sizing is required. This is technically possible by the use of a sizing agent (Arkofil DEN-FIX) that combines a good fibre coverage and elasticity with some fixing properties. The complete dye fixation is ensured by the addition in the sizing bath of some specific cationic agent (Direfix SD). This fixation is carried out in acidic (pH 4-4.5) conditions.

The advantages of the Pad-Sizing/Ox process go beyond this extremely high water saving. It also reduces the energy required for dyeing in a 25 per cent and the generated cotton waste in more than 60 per cent. This is due to the fact that Pad-Sizing/OX is applied in a very compact mechanical set up. As for the Denim-Ox process already discussed, only one bath of impregnation is required. After the subsequent skying, the oxidation/fixation takes place in the standard sizing range.

In slasher ranges, where the yarns run continuously from the dyeing range to the sizing, the Pad-Sizing/Ox cane be directly implemented.  In rope dyeing ranges, the dyed warp is rebeamed for the eventual oxidation/fixation/sizing in the separated sizing unit.

A graphical summary of the different processes presented is shown is Fig 7.

Technical considerations

The Denim-Ox and the Pad-Sizing/Ox processes developed for the Advanced Denim concept, are based on a direct fixation in acidic conditions of the pre-reduced dyestuff applied in the impregnation step. Therefore, it is very important to prevent the presence in the dyebath of chemical species that can have an undesired – and even dangerous- reaction with the acid.

At this particular point, it is also very important to prevent any sodium sulphide (a standard reducing    agent for classical sulphur dyes) being transported with the fabric into the fixation bath. Diresul RDT dyes are suitable for this process as they present two important technical features:

* While still pre-reduced sulphur dyes, the Diresul RDT dyes have a very low sodium  sulphide content, which is fully oxidized during the impregnation- skying process. The fabric is sulphide free when entering in contact with the acid fixation bath

* The use of sulphide free reducing agents, such as glucose derivatives, is also required for the same reason.

The key technical aspects of the Advanced Denim are reflected in the Fig. 8.

Resource savings

Industrial experience with the defined Denim-OX and Pad-Sizing/Ox processes allows the actual water, energy and cotton that can be saved when compared to a conventional denim dyeing procedure to be quantified with accuracy.

The absence of wastewater to be treated in the Pad-Sizing/OX process means additional economical savings in the effluent treatment plant. The Denim-Ox process generates a certain volume of water, but since it is completely colourless and free from any interfering chemical species, it could be reused, eg in a new rinsing bath. This low impact on effluents is reflected in Fig. 9.

The cotton savings are related to the shorter length of yarn that remains in the machine when switching batches or when running the range at low speed until the intended mechanical conditions are reached. Here the longer the cotton yarn passage, the more cotton waste generated.

The Advanced Denim concept not only offers the variety of resource savings already mentioned, along with an extremely high sustainability  profile, it also has clear technical advantages and opens new doors to making the ideas of designers and fashion setters possible. The main features are:

* Easy and versatile dyeing possibilities

* Simple application in already existing equipment (slasher ranges for Pad-Sizing/Ox)

* High reproducibility-  batch-to-batch and lab-to-bulk

* High productivity with short colour changing times

* A wide palette of colours, tones and casts (eg Diresul 'Indi' RDT dyes)

* The possibility of new effects by simply modifying application conditions

* Suitability for wash-down treatments considered as (ozone, laser …)

* High overall fastness levels

The listed properties put Advanced Denim in line with the higher quality standards that the textile industry is, and increasingly, demanding.

Summary

Advanced Denim is a multipurpose concept for denim warp dyeing and denim garment processing, allowing a significant reduction in resources, particularly water consumption. It offers clear answers to modern ecological and technical requirements in the denim market and also in the field of fabric and garment finishing, with the development of specific products. Advanced Denim offers advantages in the field of water use and treatment as well as in the fashion (such as a large variety of shades, special ring or solid dyeing effects and stay-colour shades) and technical aspects.

Advanced Denim is a clear example of Clariant’s commitment to enable our customers to strengthen their own sustainability efforts. We consider in Clariant that all efforts, present and future, must be done at all concerned levels:  textile producers, retailers, end consumers and chemical companies in order to ensure the sustainability of our industry. Advanced Denim is just one step in this direction.

Miguel Sanchez is Head of Golbal Product Line Special Dyes, BU Textile Chemicals, Clariant, Muttenz, Switzerland. www.clariant.com

References: (1)  The World Bank Report 2010. Textile Exchange, Barcelona 2012; (2) The National Geographic Magazine, Issue April 2010: “Water, our thirsty world"

 


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