Natural dyes in technical textile dyeing operations

In Section 19.2.1 the general aspects to be considered in textile dyeing are discussed. Section 19.2.2 presents the requirements defined by the dyeing process, and the selection criteria for dyes and mordants are highlighted in Sections 19.2.3 and 19.2.4.

19.2.1 General aspects

The selection of an appropriate dyeing procedure and the suitability of a certain dyestuff is determined by the fibre type to be dyed and the desired final properties of the product. In 2004 the total production of textile fibres reached a volume of 68.6 million tonnes.1 This amount is divided into different classes of material: natural fibres (e.g. cotton, flax, wool) and man-made fibres (Table 19.1). Man-made fibres hold a share of approximately 55% of the total amount of textile fibres produced. Important representatives for synthetic fibres are polyester, polyamide and polyacrylic fibres. Another class of man-made fibres is based on regenerated cellulose (for example lyocell, viscose and modal fibres).

With the exception of white textile goods and materials that are used inside a certain textile construction (e.g. fill fibres), coloration of the materials is an important step to achieve access to the customer. Figure 19.1 presents the relevant steps carried out in textile dyeing. As can be seen the most relevant difference is found in the nature of chemicals used for dye-stuff fixation. For synthetic dyes - depending on the type of dyestuff - alkali, salt or reducing agents are added while in the case of natural dyes mordants are used for dyestuff fixation. Up to the end of the nineteenth century natural dyes were the main colorants available for textile dyeing procedures. The development of synthetic dyes at the beginning of the twentieth century led to a more complete level of quality and more reproducible techniques of application. As a result, a distinct lowering in the dyestuff costs per kilogram of dyed goods was achieved.2-4

During the twentieth century numerous groups of dyes for the various classes of fibres were developed and introduced into full-scale production. During the second half of the century a concentration on the main groups of dyes occurred and today a few classes of dyes form the major part of

Table 19.1 Production of textile fibres in 20041

Million tonnes

%

Natural fibres

Cotton

22.0

32.1

Wool

1.2

1.7

Jute

3.2

4.7

Flax, linen

0.8

1.2

Cellulosics

3.1

4.5

Synthetics

Polyester

24.4

35.6

Polyamide

4.1

6.0

Acrylics

2.7

3.9

Polyolefines

6.3

9.2

Others

0.8

1.1

Total

68.6

100

Textile goods (fibre, yarn, fabric, garment)

Pretreatment steps (washing, desizing, scouring, bleach)

Natural dyes

Pre-mordanting i

Dyeing step (exhaustion)

Fixation (temperature, time, mordant)

Synthetic dyes

Dyestuff application (exhaustion, continuous)

Fixation (temperature, time, chemicals)

Fig.

Washing steps to remove unfixed dyestuff

Dyed goods

19.1 Relevant steps in dyeing processes with synthetic dyes and natural dyes.

dyestuffs consumed; representatives are disperse dyes, reactive dyes, acid dyes, vat dyes or synthetic indigo. An estimation of the total annual dyestuff production for textile fibres can be made assuming an average colour depth of 1% of shade, which means an amount of dyestuff of 1% of the mass of the textile good is applied. The worldwide production of textile dyes thus can be estimated to reach a scale in the order of 600 000-700 000 tonnes per year. The predominance of synthetic dyes hindered a continuous development and adaptation of natural dyeing to the changing requirements of modern dyehouses. As a result, nowadays a considerable gap exists, separating the knowledge about natural dyes from the demands of commercial dyeing processes.

At present natural dyes are mainly used for the traditional or so-called 'green products'. In many cases these products are near to industrial art and require a lot of manual work, which makes them expensive and technically not suited for scale up. The expectations from the reintroduction of natural dyes into technical textile dyeing can be summarised as follows:

• improvement of the ecological situation of textile dyeing processes;

• possible use of wastes from food, beverage and forestry to obtain valuable products;

• introduction of new crops and products into modern concepts of farming;

• formation of synthetic chemistry on the basis of sustainable resources;

• replacement of 'hard products' (synthetic chemistry) by 'soft' products (natural sources).

The re-introduction of natural dyes into commercial dyehouses has to be achieved on a competitive basis, where the modern synthetic dyes define the parameters that have to be fulfilled. Representative parameters that have to be considered are summarised in Table 19.2.

Traditionally natural dyes were obtained from direct farming of crops to produce the dyestuff, typical examples are natural indigo, madder and weld. At present the production of natural dyes by direct farming results in considerably high specific costs per kilogram of plant material, or per kilogram of dyed material.5'6 New strategies are required to establish technically and commercially competitive processes.7-9 A promising concept for the production of natural dyes with lowered specific costs is based on the use of different plant sources for the extraction of natural dyes:

• direct farming yields rather expensive plant material that is not available from other sources;10

Table 19.2 Important factors in evaluating the use of a natural dyestuff

Farming

1. Simple farming procedures (standard equipment, easily accessible sources, e.g. forestry, food industry)

2. Low consumption of energy, fertiliser and chemicals during farming, handling, transport and storage

Dyestuff preparation

3. Easy handling of plant matter

4. High dyestuff content in plant material

5. Standardised raw products

6. Optimised balances (energy, water/solvents, wastes) in dyestuff extraction

7. Database describing properties of the plants, products, dyes and dyed goods

8. Information about toxicological properties

9. Formation of a supplier organisation collecting the materials and providing dyehouses with standardised qualities

Dyeing processes

10. Application for a broad range of textile fibres (cellulose, protein, synthetic fibres)

11. Complete colour gamut: yellow, red, blue, black

12. Application in a set of dyes with common application (direct dye, iron- and alum-mordant)

13. Acceptable level of fastness properties: light fastness, wet fastness, wash fastness

14. Robust design of dyeing processes

15. Application of existing equipment with minimal modification

16. Optimised consumption of energy, water and chemicals

• use of cheap by-products from other agricultural activities, e.g. barks from timber industry;1112

• use of dyestuff-containing wastes, e.g. those released almost free of charge from the food and beverage industries.5'6

Great care has to be taken to ensure that a significant improvement of the overall consumption of energy, chemicals and water is reached in relation to the state of the art processes, which have been optimised continuously for over a hundred years. The plant material has to permit simple handling and storage. The harvested crop should contain a high dyestuff content, which is easy to extract with water. The farming and the formation of a storable intermediate or dyestuff product has to be performed with a minimum consumption of energy. The dyestuff extracted from a plant source should be applicable to a maximum range of different fibres (e.g. cellulose fibres, protein fibres, polyamide fibres). A very important aspect is the possible range of colours that can be obtained from the sources. For a technical application a basic gamut containing brilliant yellow, red and blue dyes and a set of dyes for dark shades (grey/black) is needed.

In an optimum situation the dyestuff is applied as direct dye without addition of any mordant. The function of metal-salt-based mordants, e.g. iron salts or alum, can be explained by the formation of coordinative complexes that show higher affinity to the fibre and respectively exhibit lower solubility in water. Tannin-based mordants can be understood as chemicals used to increase the adsorptive properties of the textile material to be dyed.

Fastness properties

An acceptable level of fastness properties has to be achieved (Table 19.3). Fastness properties are of major interest as far as the quality of dyeing and dyestuff is concerned. The colour has to be resistant to various daily treatments, like exposure to light, contact with water and repeated washing cycles. There are several standardised tests to assess the fastness properties of dyed textiles by comparing treated samples and untreated originals.13 Effects of light, water and detergents on coloured substrates lead to changes in colour shade and/or colour depth, 'bleeding' of dye to adjacent material and variation of physical and chemical properties of the textile fabric. While water and wash fastness properties are mainly dependent on the structure

Table 19.3 Relevant selection criteria for fastness properties of dyeings with natural dyes

Property

Method13

Range, poor - excellent

Acceptable lower limit

Fastness to light

DIN 54 004

1-8

3

Water fastness

DIN 54 006

1-5

3-4

Wash fastness, 40 °C

DIN 54 014

1-5

3-4

of the dye molecule, and therefore on the fixation of the dye on the substrate, light fastness is related to the photochemical properties of the colour molecule. The absorption of energy by the dye molecule causes excited states. There are different ways back to the electronic ground state: energy release with or without radiation as well as photochemical reactions that destroy the dyestuff.14 Light-induced degradation of dyestuff results in changes of colour shade or colour depth, so-called 'fading'.15,16

In fact all dyes are more or less sensitive to light. However, textiles and fibres are also destroyed by irradiation. Therefore colorants require sufficient stability such that they do not fade visibly during a garment's lifetime.17 Plant dyes predominantly show good to very good fastness values according to water and laundry variations, but light fastness seems to be the decisive factor for textile applications.18-20

Processes

For successful introduction of natural dyes into a modern dyehouse different dyes have to be applicable in a common procedure, which permits a mixing of different dyes and mordants in a dyeing operation. At present, classes of synthetic dyes are applied in standardised dyeing apparatus and the run of a dyeing process is adapted to the chemistry of the dyes (e.g. reactive dyes, acid dyes). Similarly dyeing processes with natural dyes have to fit on the existing range of apparatus and only minor changes in the equipment will be acceptable, for example installations required to extract the dyestuff from the plant material. Longer duration of processes or rather complex handling will lead to a considerable increase in costs and can also be identified as potential sources for deviations in the final colour of the dyed goods.

A supplier structure has to be established that is able to provide the dyehouse with standardised qualities of natural dyestuffs or plant material in a short time period (within 1-2 weeks after ordering). Similar to the presentation of modern synthetic dyes a database is required which contains:

• technical information about farming and harvesting including conditions of storage;

• information concerning dyestuff extraction and standardisation;

• toxicological properties of plants, dyes and dyed goods;

• technical recommendations for dyeing processes and high-quality relevant data about the dyeings obtained (e.g. shade, fastness properties);

• information about ecological profiles (energy, chemicals, wastes) compared with the state of the art.

19.2.2 Dyeing processes

For dyeing with natural dyes, numerous variations have been in use in traditional dyeing operations. The dyeing process is dependent on the type of goods (flax, jute, wool, silk) and in particular on the plant material to be

Table 19.4 Important groups of dyeing procedures for natural dyes

Type of process

1. Step

2. Step

Direct dye

Dyeing

-

Pre-mordant

Mordanting

Dyeing

Meta-mordant

Dyeing-mordanting

-

After-mordant

Dyeing

Mordanting

Vat dyeing

Indigo vat

-

Direct dye, the dyestuff exhausts without any addition of fixatives; pre-mordant, the mordanting step is performed in a separate bath before the dyeing; meta-mordanting, the mordant is added into the dyebath; after-mordanting, the mordanting step is carried out after dyeing in a separate bath; vat dyeing, application requires addition of reducing chemicals/alkali.

Direct dye, the dyestuff exhausts without any addition of fixatives; pre-mordant, the mordanting step is performed in a separate bath before the dyeing; meta-mordanting, the mordant is added into the dyebath; after-mordanting, the mordanting step is carried out after dyeing in a separate bath; vat dyeing, application requires addition of reducing chemicals/alkali.

used. As a result a great variety of dyeing procedures had been in use. Dyeing operations can be classified into different groups with regard to the chemistry during the application of the dyes; important classes are direct dyes, mordant dyes and vat dyes (mainly natural indigo). In Table 19.4, a classification of relevant natural dyeing operations is shown. Wash baths have not been considered.

Processes that permit the direct addition of the mordant into the dyebath are favourable, because this technique forms the basis for a one-bath dyeing process. However, losses of dyestuff due to partial precipitation of dyestuff can occur. Dyes that do not show high affinity to the substrate require pre-mordanting. This requires the introduction of a two-bath process and in some cases the intermediate storage of mordant baths is necessary. For a dyehouse, the use of separate baths for mordanting and dyeing is quite undesirable with regard to time of dyeing, further treatment or recycling of the mordanting baths.

Indigo represents the most important blue natural dye. As indigo is a vat dye the application requires the use of a special dyeing procedure. Traditionally this was an anaerobic bacterial reduction (vatting), which is nowadays replaced by use of powerful reducing agents, e.g. Na2S2O4/NaOH. However, the dyeing of blue and green shades needs the introduction of an additional dyeing step which makes justification of advanced ecological processing quite difficult. Introduction of an additional bath for vat dyeing with indigo and a pre-mordanting bath to dye a green shade for example would mean replacing a one-bath dyeing procedure, based on synthetic dyes, with a series of three separate treatment steps.

19.2.3 Selection requirements

In the application of dyes in the past, different techniques of mordanting and post-treatment were used to improve colour fastness properties.2,21-24

Today the criteria to select a certain plant source as a possible raw material are more strict, and rigorous evaluation and comparison with the state of the art dyeing technology is necessary to form a class of natural dyes that meets existing requirements.

Selection of mordant and procedures

Mordants increase the fixation of dye molecules on the substrate by building metal complexes. Whilst natural colorants themselves are generally harmless, some mordants are not environmentally acceptable. Consequently selection has to be carried out, to avoid heavy metal contaminants in waste water and to fulfil governmental limits.2526 Traditional mordants, like iron salt (ferrous sulphate, Fe) and alum (Al), do not cause environmental or ecological problems.27 Dyeings using mordants such as cobalt, tin or chromium salts will cause problems with the effluents released from the dyeing process because of the wastewater limits defined for the concentrations of heavy metals.26

Formation of dyestuff groups

The formation of a class of dyes with compatible procedures of application is of great importance, because this enables the dyer to produce a broad variation of shades. One-bath dyeing with direct addition of the mordant in the dyebath is most favourable. In such a dyeing step, mixtures of dyes and mixtures of mordants can be applied and a broad range of variation in shade and colour depth can be achieved. Furthermore, a dyer is able to correct deviations in the desired shade during dyeing by further addition of dyestuff or mordant. Figure 19.2 shows an example of the colour shade variation achievable by systematic variation of the composition of the iron/ alum mordant, using a one-bath dyeing method.

19.2.4 Ecological impact

The textile industry is one of the biggest industrial consumers of water, so extensive data about the effluents have been collected and are available from the literature.28 Depending on the extent of other treatments performed in a dyehouse, waste water from the dyeing step is diluted to a greater or lesser extent by effluents released, for example from desizing, scouring and bleaching.29

Table 19.5 shows the calculated chemical load in the waste water for dyeings using natural dyes and those using selected conventional dyeing processes. Numerous dyeing methods are applied technically for a certain fibre type, so two representatives were selected for each fibre. For cellulose fibres, dyeing with reactive dyes and direct dyes has been taken into consideration, while for wool, metal complex and reactive dyeings were chosen.3^36 The most relevant parameters of the wastewater limits for textile effluents in Austria are also given in Table 19.5.26

Composition of mordant (g L-1)

Fig. 19.2 Change of CIELab-coordinates obtained with Canadian Golden Rod (Solidago canadensis L.). CIELab-coordinate on wool; combination of mordants, FeSO4.7H2O and KAl(SO4)2.12H2O; 62 g plant material extracted with 1200 ml, dyeing liquor ratio 1 : 20. CIELab-coordinates: L*, brightness (100 = white, 0 = black); a*, red-green coordinate (positive = red, negative = green); b*, yellow-blue coordinate (positive = yellow, negative = blue).

Composition of mordant (g L-1)

Fig. 19.2 Change of CIELab-coordinates obtained with Canadian Golden Rod (Solidago canadensis L.). CIELab-coordinate on wool; combination of mordants, FeSO4.7H2O and KAl(SO4)2.12H2O; 62 g plant material extracted with 1200 ml, dyeing liquor ratio 1 : 20. CIELab-coordinates: L*, brightness (100 = white, 0 = black); a*, red-green coordinate (positive = red, negative = green); b*, yellow-blue coordinate (positive = yellow, negative = blue).

To facilitate a comparison of the techniques, the expected chemical load released directly into the waste water was calculated. The final concentration of chemicals was calculated for a dilution of the dyebath by a factor of four (dyebath plus three rinsing baths). Differences with regard to water consumption due to the different number of washing baths required to achieve the final fastness properties were not considered, because too many variations currently exist.

Cellulose fibres

Values for reactive dyeings on cellulose fibres were calculated from an average recipe for 1-2% of colour depth applied in exhaust dyeing at a liquor ratio of 1 : 20.35 An upper limit for the sulphate concentration has been fixed, to avoid possible corrosion of concrete tubes. In the case of cellulose dyeing with reactive dyes, NaCl can be used instead of Na2SO4 but there is an increased risk of metal corrosion if NaCl is used. For cellulose fibres, the examples given show a distinct lowering of the chemical load in the waste water when natural dyes are used. With reactive dyes both

Table 19.5 Estimation of the chemical load released into the waste water. Natural dyes were considered as direct dyes, with iron or alum mordant; for comparison, average values for selected dyeing methods in use today are given. Limits given are valid for waste water released to a communal wastewater treatment plant in Austria26

Table 19.5 Estimation of the chemical load released into the waste water. Natural dyes were considered as direct dyes, with iron or alum mordant; for comparison, average values for selected dyeing methods in use today are given. Limits given are valid for waste water released to a communal wastewater treatment plant in Austria26

Substrate

Dyestuff

Chemicals in dyebath

Final concentration in the waste water

Legal limits for textile effluents26

Cellulose fibre (linen)

Natural dye direct Fe mordant

Al mordant

1-5 g/L FeSO4-7H2O 1-5 g/L KAl(SO4)2-12H2O

0.05-0.25 g/L Fe2/3+ 0.086-0.43 g/L SO42-0.014-0.07 g/L Al3+ 0.10-0.50 g/L SO42-

0.2 g/L SO42-0.2 g/L SO42-

Reactive dye Direct dye

3 g/L Na2CO3

5-10 g/L Na2SO4

0.75 g/L Na2CO3 10 g/L Na2SO4 (NaCl) 0.85-1.7 g/L SO42-

pH 6.5-9.5 0.2 g/L SO42-0.2 g/L SO42-

Protein fibre (wool)

Natural dye direct Fe mordant

Al mordant

1-5 g/L FeSO47H2O 1-5 g/L KAl(SO4)242H2O

0.05-0.25 g/L Fe2/3+ 0.086-0.43 g/L SO42-0.014-0.07 g/L Al3+ 0.10-0.50 g/L SO42-

0.2 g/L 0.2 g/L

Metal complex dyes Reactive dyes

2.5 g/L Na2SO4 Cr content of dye pH adjustment pH adjustment

0.1 mg/L Cr6+, 1 mg/L Cr3+ pH 6.5-9.5 pH 6.5-9.5

the alkalinity of the waste water and the salt content are rather high; with the use of direct dyes no alkali is required, however the salt concentration remains twice the concentration released from mordant dyeing.36 Furthermore, iron sulphate or aluminium sulphate are compatible with the subsequent wastewater treatment, where such salts are added for floccula-tion and phosphate elimination.

Wool fibres

A comparison of natural dyeing processes with conventional wool dyeing methods leads to two different results: the use of metal complex dyes often requires the addition of Na2SO4 as a levelling agent which causes similar sulphate loads in the waste water. The bigger problem can arise from the metal content in the metal complex dyes, mainly due to the Cr content. For this element, the limits for textile waste water are fixed at 0.1 mg/L for Cr6+ and 1 mg/L for Cr3+. The use of reactive dyes for wool causes low pollution, but careful selection of the dyestuffs is required and the method of application needs care, to obtain acceptable uniformity of the dyeing. As can be seen from Table 19.5 a lower chemical load is released from natural dyeing processes compared with technologies in use at present. From a sustainabil-ity viewpoint all the conventional techniques are based on synthetic dyes that are produced from non-regenerable sources while in the case of natural dyeings the dyestuffs are extracted from sustainable sources.

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