Natural dyes in modern textile dyehouses — how to combine experiences of two centuries to meet the demands of the future?

Journal of Cleaner Production 11 (2003) 499–509 www.cleanerproduction.net

Natural dyes in modern textile dyehouses — how to combine experiences of two centuries to meet the demands of the future? T. Bechtold a,∗, A. Turcanu a, E. Ganglberger b, S. Geissler b a

Leopold-Franzens University Innsbruck, Institute of Textile Chemistry and Textile Physics, Hoechsterstrasse 73, A6850 Dornbirn, Austria b Institute of Applied Ecology, Seidengasse 13, A-1030 Vienna, Austria Received 12 February 2002; accepted 1 July 2002

Abstract Plant materials which are available from farming regions in the moderate Austrian climate were investigated to serve as sources for natural dyes in textile dyeing operations. The extraction of the dye components from the plant materials was performed with boiling water without addition of chemicals or solvents. Based upon a rigorous selection of possible plant sources, a selection of natural dyestuffs applicable in a one-bath dyeing step was established. A broad variation in shade and color depth can be achieved by applying mixtures of natural dyestuffs in various combinations of iron- and alum-mordants. More than 60% of tested dyeings achieved acceptable fastness properties. On the basis of the developed natural dyestuff-based dyeing procedures, a comparison was made between the effluents from processes based upon them and those based upon the current ‘state-of-the-art’ techniques utilizing synthetic dyes. The comparison revealed that a lowering of the chemical load released with waste water can be expected by shifting to the plant-based dyes.  2002 Elsevier Science Ltd. All rights reserved. Keywords: Natural dyes; Textiles; Mordant; Dyeing

1. Introduction The colored drawings on the walls on the Altamira cave in Spain are dated at 15,000–9000 before Christ. The drawings were performed with inorganic pigments which can last a very long time. Dyed clothes have been produced in all cultures since a very long time ago but the durability of these products is limited. Thus, very old samples of dyed textiles are rare, e.g., dyed textile material from Egypt could be dated to 3200 BC, in India dyed textiles were dated at 2000 BC. Depending on the climate, various plants served as sources for natural dyes, e.g., indigo plant, madder, barberry [1]. 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

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and more reproducible techniques of application. As a result, a distinct lowering in the dyestuff costs per kg of dyed goods was achieved [1–3]. The predominance of synthetic dyes hindered a continuous development and adaptation of natural dyeing to the changing requirements of modern dyehouses. As a result, now a considerable gap exists, separating the knowledge about natural dyes from the demands of commercial dyeing processes. In the last decade, investigations about possible use of natural dyes in textile dyeing processes have been performed by various research groups. The dyeing of cotton and jute with tea as a natural dye using alum, copper sulfate, or ferrous sulfate mordants has been studied by Deo and Desai [4], Bhattacharya et al. investigated the properties of selected natural dyes on jute [5]. Nishida and Kobayashi reported properties of natural dyes on silk, cotton, and cashimilon using alum or ferrous sulfate mordants [6]. Bru¨ckner et al. investigated the color depth and fastness properties of selected natural dyes on wool and on synthetic fibers, e.g., polyester, polyamide, and polyacylonitrile [7]. Lokhande and

0959-6526/03/$ - see front matter.  2002 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0959-6526(02)00077-X

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Dorugade presented results with selected natural dyes on polyamide using various mordants, e.g., alum, ferrous sulfate, stannous chloride and tannic acid [8]. In general, the authors described encouraging results with regard to color depth, shade, and fastness properties. The dyeing procedures are mainly two-bath dyeings including a separated mordanting step, so such processes are rather difficult to be handled in a modern dyehouse. In the study of Bru¨ ckner [7], the final conclusion focuses on the unsatisfying fastness properties of the natural dyes, which must be understood as an indicator for a distinct need for research to overcome these problems. In the application of the dyes, different techniques of mordanting and post-treatment were used to improve color fastness properties [1,3–6]. As a result, a broad set of variations in the dyeing recipes is given in the literature, and an optimization of the dyeing conditions with regard to the type of natural dye is quite common. The numerous variations of plant sources and dyeing processes proposed in the literature make an introduction of natural dyeing into full-scale technical dyeing processes rather difficult. The rapid changes in trends and fashion and the demand for good fastness properties on different substrates requires a basic database describing possible applications of natural dyes, otherwise too much parallel optimization work has to be done by each dyehouse. This situation has led to a rather controversial discussion of expected advantages resulting from future use of natural dyes. While some experts focus on numerous difficulties which could hinder the successful introduction of natural dyes into regular dyeing processes [9– 12], others concentrate more on expected advantages of technologies which are based upon sustainable sources [13,14]. The introduction of natural dyes into modern dyeing procedures then can be seen as one step of a continuous development of textile dyeing and finishing processes towards increased sustainability with regard, for example, to water, chemicals, and energy consumption. As a result of the use of natural products with low toxicity, a decrease in the overall reduction of exposure to harmful chemicals is expected for both textile workers and wearers of the cloths. As long as the natural dyes remain chemically unchanged, the released dyebaths will fit into the natural pathways of biodegradation. To achieve at least a partial replacement of synthetic dyes by natural dyes, the technical aspects of dyeing, defined by the demands of a modern dyehouse, have to be considered at the same time as the demands of the producer/manufacturer of the dye, e.g., the farmer. In this paper, results from a study to evaluate sources for natural dyes available in Austria are presented [15]. Starting with more than 50 different types of plants which could be used as raw materials for dyestuff extraction, a selection was performed with regard to the following requirements:

앫 production of the plant material in sufficient amounts with modern agricultural methods including simple and environmentally clean extraction methods to obtain the dyestuffs, 앫 formation of a suitable class of dyes which is, in its applicability, comparable to the classes of synthetic dyes in use at the present. During the dyestuff selection, a one-bath dyeing process with the addition of the mordant into the dyebath was investigated to serve as the general dyeing procedure instead of a two-bath dyeing step with separated mordanting. The selection of a one-bath dyeing procedure was made with regard to the demands of the textile dyers, who would reject a two bath dyeing process with the arguments of handling, time consumption, and risks of lower reproducibility. Shades and fastness properties of dyeings on linen or wool obtained with the proposed dyeing method are presented in this paper. The possibility for a variation in color depth and shade with use of dyestuff mixtures and mixed mordants was found.

2. Selection of plant sources To keep transportation to a minimum, a concentration on plant sources available from plants growing in the moderate Austrian climate was done. The number of possible plant sources was reduced by rigorous selection considering the main aspects given in Table 1. A general overview of the dyestuff production and dyeing step is given in Fig. 1. Besides the possibility for agricultural production of the plant materials, the extraction of the dyestuffs had to be performed by a simple aqueous extraction. The use of chemicals or non-aqueous solvents for improvement of the dyestuff extraction was excluded to maintain the possibility for further use of the extracted residue, e.g., as fertilizer or as animal feed. Also, the use of solvents such as ethanol has to be considered with great care because on average, a mass of approximately 100 kg plant material has to be extracted for the dyeing of 100 kg goods. Thus, even if solvent losses in the extracted plant mass could be reduced to 5% o.w. of the plant mass, the absolute amount of ethanol lost in the process will be substantial. Only simple equipment was used and simple experimental procedures were applied to permit a primary extraction with inexpensive apparatus near the site of harvesting to avoid long-distance transportation of the bulk of the plant materials. For the first selection, literature information was used to generate a basic set of dyes containing a yellow/red/blue and black shade [1]. The dyes had to be applicable as direct dyes, and/or with iron or alum mordants. Dyeings with the use of mordants such as Co, Sn or Cr salts will cause problems with the effluents released from the dyeing process because of the waste

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Table 1 The main requirements for a basic set of natural dyes Agricultural demands Reasonable requirements for production and harvesting of the plant materials Easy handling and storage of the raw materials High dyestuff content Easy extraction with water Requirements defined by a technical dyehouse Simple and rapid dyeing process, no intermediate drying steps, etc. One-bath dyeing Broad range of shades formed by a basic set of brilliant dyes, including dark shades (black) Easy correction of deviations in color depth and shade Acceptable fastness properties Applicability in dyeing machines in use today Observance of existing waste water limits, e.g., heavy metals No use of mordants based upon Cu, Sn, or Cr salts Bio-degradability of dyes in waste water treatment plants Non-toxic properties of dyes and non-allergic potential of dyed material Consumption of chemicals and energy comparable or lower than the current state-of-the-art systems based upon synthetic dyestuffs

Fig. 1.

Dyestuff extraction and dyeing step.

water limits defined for the concentrations of heavy metals [16]. Such mordants were, therefore, not used in this research. Table 2 gives a summary of the tested plant sources. As can be seen from Table 2, different sources

of raw materials were studied. While part of the plants will be produced by farming, others, for example, barks, are by-products from the wood processing where bark is removed prior to sawing.

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Table 2 Plant sources, chemical basis of dyestuff, and range of shade Plant

Used part

Chemical basis, main component [1] C.I. Natural Yellow 18, basic dye, berberin flavonoid dye, quercetin

Barberry

Berberis vulgaris L., Berberitze

branches and roots

Canadian golden rod

buds, leaves

Madder

Solidago canadiensis L., Kanadische Goldrute Rubia tinctorum L., Krapp

Hollyhock

Alcea rosea L., Schwarze Malve

Privet

Ligustrum vulgare L., Liguster

Walnut tree

Juglans regia L., Walnuß

Ash tree

Fraxinus excelsior L., Esche

Sticky alder tree

Betula alnus L. var. glutinosa L., Schwarzerle

3. Experimental 3.1. Plant material — storage and extraction Depending on the type of raw material, various ways of storage were chosen to avoid time-dependent changes of the raw plant material. While barberry branches and roots, Canadian golden rod buds, hollyhock buds, ash tree barks, and sticky alder tree barks were dried to give a storable form, privet berries and green walnut were stored in a freezer to avoid changes during drying of the plant material. Before extraction of the dyestuffs, the plant materials were ground to pieces of about 1 mm. The dyestuff was then extracted with boiling water by applying a liquor ratio of 1:20, corresponding to a ratio of 1 g plant material to 20 ml of water. The duration of the extraction was fixed at 60 min. The insoluble residue was separated by sedimentation and filtration through a stainless steel filter fabric (0.3 mm mesh). The resulting extract was used for the dyeing experiments. For the dyeing experiments with madder, dried and powdered extract was used as supplied from the manufacturer (Cebeco Groep, Rotterdam).

roots

C.I. Natural Red 8anthrachinone dye, alizarin-2-primverosid buds anthocyan dye, malvidin3,5-glucosid berries C.I. Natural Black 5anthocyan dye, malvidin-3glycosid green walnut, brown nut C.I. Natural Brown shell 7naphtochinon dye, juglon bark flavonoid dye, quercetin-3rutinosid bark gallotannin dye, tannin

yellow–brown yellow–olive red–brown

brown–green blue–green

brown beige–black beige–black

a dyebath volume of 20 ml is applied). Ten grams of bleached linen fabric were used as cellulosic material. Ten grams of bleached wool yarn were used as protein fiber substrate. The dyeing experiments were performed in beakers according to the temperature dyeing diagram given in Fig. 2. In the experiments with addition of mordant, FeSO4.7H2O (technical grade ⬎96% purity, Riedl-deHaen) and alum KAl(SO4)2.12H2O (puriss. p.a. Fluka), were added as concentrated solution (50 g L–1) to give a final dyebath concentration of 2.5 g L–1 or 5 g L–1 mordant. After dyeing, the unfixed dyestuff was removed by rinsing three times with cold water (5 min, room temperature, liquor ratio 1:20). The CIELab values of the dyeings were measured with a tristimulus colorimeter (Minolta Chroma-Meter CR 210, sample diameter 10 mm). The colors are given in CIELab coordinates, L corresponding to the brightness (100 = white, 0 = black), a to the red–green coordinate (positive sign = red, nega-

3.2. Dyeing procedures The dyeings were performed by the exhaustion method1 using a liquor ratio of 1:20 (For 1 g of goods 1 In exhaustion dyeing, the process is performed as a batch treatment. First the dyestuff is dissolved in the dyebath, then it adsorbs on the fiber and finally is fixed thereon, e.g. by formation of a metal complex.

Fig. 2. Temperature time diagram of the one-bath dyeing process.

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tive sign = green) and b to the yellow–blue coordinate (positive sign = yellow, negative sign = blue). 3.3. Fastness properties Wash fastness 40 °C: The wash-fastness was determined at 40 °C according to DIN 54014 [17]. A solution containing 0.45 g L–1 FAS, sodium laurylsulfate (Ufarol NA30, Jauk GmbH, Vienna) and 0.5 g L–1 APG, alkyl polyglycoside (Glucopon ECS 650, Henkel) was used as washing liquor. The samples were treated for 30 min at 40 °C in a liquor ratio of 1:50. After rinsing and drying, the change in color of the sample and the bleeding to white fabric (cellulose, wool) was determined. The changes were related to the standard grey scale (marks 1–5, 1 = poor, 5 = excellent). Wet fastness 37 °C: The wet fastness was determined according to DIN 54006 [17]. After thorough wetting in distilled water, the samples were treated for 4 hours minumum at 37 °C. After drying, the change in color of the sample and the bleeding to white fabric (cellulose, wool) were determined. The changes were related to the standard gray scale (marks 1–5, 1 = poor, 5 = excellent). Light fastness: The light fastness was determined using artificial illumination with a xenon arc light according to DIN 54004 (Xenotest, Hanau, Germany) and was related to the standard scale of blue dyeings (marks 1–8, 1 = poor, 8 = excellent).

4. Results and discussion 4.1. One-bath dyeing A general dyeing method was used for all investigated natural dyes without significant changes. Optimization of the procedure with regard to a single plant material was not performed to maintain all dyestuffs within the same group of application. The dyeings were performed both on wool (protein fiber) and linen (cellulose fiber) to get information about the influence of the substrate on color depth, shade, and fastness properties. The first step of the dyeing procedure was independent of substrate and type of mordant. The material was immersed into the dyebath containing the dyestuff extract. After 10 min of wetting at room temperature, the temperature was raised to 95 °C within 15 min. After a dyeing time of 15 min at 95 °C the mordant was added to the dyebath. The dyeing temperature was then held at 95 °C for a further 35 min. After cooling of the dyebath to 60 °C, the liquid was removed and three washing steps at room temperature were applied to remove unfixed dye. At this stage of screening with parallel evaluation of fastness properties, the dyeings were performed as single experiments. A detailed study of standardization and reproducibility of selected colors is the topic of an ongoing

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project. The direct addition of the mordant into the dyebath might cause losses of dyestuff due to partial precipitation of dyestuff, but such a proceeding forms the basis for a one-bath dyeing process. From the point of dyestuff consumption, a two-bath process seems favorable. For a dyehouse, the use of separated baths for mordanting and dyeing is very undesirable with regard to time of dyeing, further treatment, or recycling of the mordanting baths. The proposed introduction of a one-bath procedure can thus be understood as a technical compromise required to implement the technology into a modern dyehouse. To illustrate the range of colors achieved, the CIELab coordinates of the different dyeings are shown in Table 3 and a verbal description of the shades is given. As can be seen from the data, the use of an iron mordant results in a distinct shift of color depth and shade compared to the shade of the dyeings without mordanting, while the use of an alum mordant does not change the shade to such an extent. The iron-mordanted dyeings are generally darker compared to the other methods; this is of particular interest for dark shades, e.g., dark gray (no. 11, 35, 41, 47). To examine the properties of the dyeings in possible further applications, selected fastness properties were determined. In Table 4 light fastness, wet fastness, and wash fastness at 40 °C are shown. Besides the change in color, the bleeding onto accompanying fibers is also given. The results indicate that a rigorous evaluation and selection of the properties of dyeings with different plant materials and different mordants is required to obtain a group with acceptable fastness properties. While shade and intensity of the yellow dyeings with barberry look excellent, the poor light fastness of these dyeings will hinder the use of this source as a raw material (no. 1–6). Dyeings on the basis of the Canadian golden rod (7–12), hollyhock (19–24), privet (25–30), and ash tree (37–42) all show similar behavior: Iron mordanting generally increases the fastness properties; an improvement of the light fastness after application of an iron mordant is of particular interest. The use of alum mordants does not affect the light fastness to such an extent (no. 12, 21, 24, 27, 30, 39, 42). Madder (no. 13– 18), walnut ( no. 31–36) and sticky alder (no. 43–48) give dyeings with generally higher levels of fastness properties. However, the use of mordants — namely iron mordanting — will increase the level of the fastness properties achieved. 4.2. Mixtures of dyes/mordants From the point of view of a dyer, the formation of a class of dyes with compatible procedures of application is of great importance. The formed group of dyes enables the dyer to produce a broad variation of shades. For this purpose, the dyeing process was designed as a one-bath dyeing step with direct addition of the mordant in the dyebath. In such a dyeing step, mixtures of dyes and

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Table 3 CIELab coordinates of the natural dyeings on linen and on wool (direct = application as direct dye, 5 Fe = 5 g L–1 iron sulfate, 5 Al = 5 g L-1 alum) (∗∗use of solid madder extract) No.

Source mass

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Extract vol.

Barberry

g 61

ml 1200

Canadian golden rod

76

1500

Madder

1.2∗∗

1200

Hollyhock

64

1200

Privet

63

1200

Walnut

76

1500

Ash tree

75

1500

Sticky alder tree

61

1200

Substrate

Mordant

L

Fl Fl Fl Wo Wo Wo Fl Fl Fl Wo Wo Wo Fl Fl Fl Wo Wo Wo Fl Fl Fl Wo Wo Wo Fl Fl Fl Wo Wo Wo Fl Fl Fl Wo Wo Wo Fl Fl Fl Wo Wo Wo Fl Fl Fl Wo Wo Wo

direct 5 Fe 5 Al direct 5 Fe 5 Al direct 5 Fe 5 Al direct 5 Fe 5 Al direct 0.5 Fe 0.75 Al direct 0.5 Fe 0.75 Al direct 5 Fe 5 Al direct 5 Fe 5 Al direct 5 Fe 5 Al direct 5 Fe 5 Al direct 5 Fe 5 Al direct 5 Fe 5 Al direct 2.5 Fe 2.5 Al direct 2.5 Fe 2.5 Al direct 5 Fe 5 Al direct 5 Fe 5 Al

81.32 72.61 81.70 73.64 56.02 79.97 83.27 59.58 80.73 71.07 27.05 69.93 76.89 63.18 68.56 62.91 52.28 62.26 54.90 46.05 47.40 35.13 29.48 34.78 79.40 52.39 66.80 63.94 43.53 44.67 64.17 55.64 56.86 41.09 25.53 43.28 83.71 65.78 79.42 73.27 22.50 68.69 69.98 51.23 68.70 62.14 26.31 66.88

mixtures of mordants can be applied and a broad range of variation in shade and color depth can be achieved. This finding is of enormous importance for a technical application because the dyer is able to correct deviations in the desired shade during the run of the dyeing by the addition of selected dyestuffs or mordants. To give an idea of the range of shade–shade possibilities with the

a

–7.13 –0.99 –6.18 –5.06 2.15 –8.25 –1.10 –2.93 –6.74 –0.70 –0.75 –3.02 16.41. 2.77 24.70 22.09 6.82 24.40 6.30 –0.21 2.34 5.72 –1.87 –1.21 3.55 –1.96 1.47 1.41 –3.96 0.02 2.43 0.74 3.04 7.84 2.73 9.59 –0.52 –1.89 0.49 0.95 0.02 –1.41 6.24 –1.88 3.05 7.10 0.74 4.85

b

Shade

34.34 27.60 27.69 54.45 –8.25 43.37 16.43 11.16 33.75 33.26 8.43 57.63 12.10 5.85 11.60 18.13 13.91 18.11 –4.75 –1.97 –5.11 1.57 2.16 0.19 2.18 –3.90 –4.03 7.76 –1.32 –8.78 11.32 5.74 9.10 18.66 3.74 17.62 11.80 4.56 13.60 24.47 0.70 31.24 14.96 7.39 20.39 22.52 –1.10 25.29

yellow beige yellow yellow brown yellow yellow olive yellow beige olive yellow red brown red red brown red violet gray gray violet green green beige gray gray beige gray blue brown brown brown brown brown brown beige olive beige beige olive yellow brown olive brown brown brown beige

use of the presented dyeing method, exemplary series of dyeings were performed with systematic variation of the composition of the iron/alum mordant or with the use of dye mixtures in combination with one type of mordant. Fig. 3 shows the change of the shade and color depth obtained on wool with sticky alder dye as a function of the composition of mordant. By variation of the mordant

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Table 4 Fastness properties of the natural dyeings on linen and on wool (Light = light fastness against artificial light; 1 = poor, 8 = excellent, Wet = wet fastness; 1 = poor, 5 = excellent, Wash = wash fastness; 1= poor, 5 = excellent) color = change of color, bleed. = bleeding both 1 = poor, 5 = excellent, accompanying fiber Cell. = cellulose fiber, Fl = linen, Wo = wool No

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48

Natural source

Barberry

Substr.

Linen

Wool

Canadadian gold rod

Linen

Wool

Madder

Linen

Wool

Hollyhock

Linen

Wool

Privet

Linen

Wool

Walnut tree

Linen

Wool

Ash tree

Linen

Wool

Sticky alder tree

Linen

Wool

Dyeing

direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al direct Fe Al

Light

⬍1 ⬍1 ⬍1 ⬍1 ⬍1 ⬍1 4 3–4 3–4 ⬍1 4 ⬍1 3 3 3 2 3–4 3 1 2–3 1 ⬍1 2–3 1 1–2 2–3 1 3 1–2 1 4 3–4 4 4 3–4 3–4 4 3–4 2–3 1 4 2 3 3–4 3–4 3 ⬎3–4 ⬎3–4

from iron sulfate to alum, the color can systematically be shifted from gray to beige. Even in the presence of relatively high amounts of alum, the uptake of iron will lead to a distinct shift in shade. Fig. 4 shows CIELab coordinates of colors that can be obtained on linen using

Wet

Wash

color

bleed. Cell.

bleed. Wo

color

bleed. Cell.

bleed. Wo/Fl

1–2 2–3 2 4 4–5 4 3–4 3 2–3 3–4 4 4–5 3–4 4 3–4 2–3 3 4 1 2–3 1 3 4–5 2 1–2 4 4 4 5 2–3 2–3 4–5 3–4 3–4 3–4 3 4 4 4–5 — 4 4–5 3 3 2–3 4–5 4–5 4–5

2–3 2 1–2 1–2 1–2 2 3 4 3–4 3 4–5 3–4 3–4 4 4 3–4 4 4 2–3 3–4 2 2–3 4 3–4 4–5 4 4–5 4 4–5 3–4 3 4 2–3 4 4 4 4–5 4 4 — 4 4 3 3 3–4 4 4 4

3 2 2 1 2 2–3 2–3 3 2–3 3 4–5 3 3 3–4 3–4 3–4 3–4 3–4 3 2 2 2–3 3–4 3 4–5 4 4–5 4 4 3–4 2 3 2 2 2–3 2–3 4 4 3–4 — 4 4 2–3 2–3 3 4 4 3–4

1 1–2 1–2 1–2 3 1–2 3–4 2–3 2 3 3 4 3 4 3 3 2–3 3–4 1 4 1 3–4 4 4 1 3–4 4 4 4–5 1 3 3–4 3 3 4 3 4 3 4 — 4 3–4 2–3 3 3 4 4–5 3–4

4 4 4 4 4 4 3 4–5 4 3 4–5 4 3–4 4 4 4–5 4–5 4–5 3–4 4 4 3 4 4 4 4 4–5 4 4 3–4 4 4 4 4–5 4–5 4–5 4–5 4–5 4–5 — 4–5 5 4 4 3–4 4 4 4

3 Wo 4 Wo 3–4 Wo 2–3 Wo 2–3 Wo 2–3 Wo 3 Fl 4 Fl 3 Fl 2–3 Wo 4–5 Wo 3–4 Wo 4 Wo 4 Wo 4 Wo 4 Wo 4–5 Wo 4–5 Wo 3–4 Wo 4 Wo 3–4 Wo 4 Wo 3–4 Wo 4 Wo 4 Wo 3–4 Wo 4 Wo 4 Wo 4–5 Wo 4 Wo 4 Fl 4 Fl 4 Fl 4–5 Wo 4 Wo 2–3 Wo 4 Fl 4 Fl 4 Fl — 4 Wo 4–5 Wo 4 Wo 3–4 Wo 3–4 Wo 4 Wo 4–5 Wo 4–5 Wo

a mixture of ash tree and hollyhock with iron mordant. The results show a systematic variation of the shade as expected for a dyeing system where the dyes behave chemically independently and do not influence each other. For comparison, a single dyeing is presented in

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Fig. 3. Change of CIELab coordinates obtained with sticky alder on wool as a function of the composition of mordant (62.4 g bark 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).

Fig. 4 obtained with the use of a mixture of both dyes but with the application of alum mordant. In Fig. 5 the change in color due to different ratios of ash tree and privet on linen using an iron mordant is shown. Similar to Fig. 4, a single dyeing using a dyestuff mixture on the basis of alum mordant is presented.

5. 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 [18]. One main difficulty hindering an immediate interpretation of such data for a discussion of the ecological impact of natural dyes is caused by the fact that such data normally are determined in the total stream of effluents. Thus, depending on the extent of other treatments performed in a dyehouse, waste water from the dyeing step is diluted more or less by effluents released, for example, from desizing, scouring, and bleaching. Thus, calculations were performed to demonstrate the impact of effluents from the dyeing step and to identify differences between the techniques more clearly. In Table 5 the calculated chemical load in the waste water

Fig. 4. Change of CIELab coordinates obtained with mixtures of ash tree and hollyhock on linen using iron mordant and change in color with use of Al = alum mordant (61.1 g ash tree extracted with 1200 ml water, 62.1 g hollyhock extracted with 1200 ml water — liquor ratio 1:21), CIELab coordinates: L = brightness (100 = white, 0 = black), a red–green coordinate (positive = red, negative = green), b yellow–blue coordinate (positive = yellow, negative = blue).

for dyeings with natural dyes and for selected conventional dyeing processes in use today are shown. Numerous dyeing methods are applied technically for a certain fiber, so two representatives were selected for each fiber. For cellulose fibers, dyeing with reactive dyes and direct dyes have been taken into consideration, while for wool, metal complex and reactive dyeing were chosen [19–25]. The most relevant parameters of the waste water limits fixed in Austria for textile effluents are also given in Table 5 [16]. To facilitate a comparison of the methods, the expected chemical load in the waste water was calculated. Differences with regard to water consumption due to a different number of washing baths required to achieve the final fastness properties were not considered because too many variations currently exist. The final concentration of the chemicals was calculated for a dilution of the dyebath by a factor of four (dyebath and three rinsing baths). The values for reactive dyeings on cellulose fibers were calculated from an average recipe for 1–2 % of color depth applied in exhaust dyeing at a liquor ratio of 1:20 [24]. The limitation of the sulfate

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the Cr content. For this element, the limit for textile waste water is fixed at 0.5 ppm Cr in Austria. The use of reactive dyes for wool is a rather low pollution method but careful selection of the dyestuffs is required and the method of application needs great care to obtain acceptable uniformity of the dyeing. Even without detailed optimization of the described dyeing process, a chemical load can be reached in the waste water which is comparable to the best technologies used at present. From the point of sustainability, all compared conventional techniques based on synthetic dyes produced from non-regenerable sources, while in case of natural dyeings, the dyestuffs are extracted from regenerable sources. The possibility of generating the dyeing matter from renewable natural sources makes natural dyes an interesting class of colorants.

6. Conclusions

Fig. 5. Change of CIELab coordinates obtained with mixtures of ash tree and privet on linen using iron mordant and change in color with use of Al = alum mordant (60.3 g ash tree extracted with 1200 ml water, 63.8 g privet extracted with 1200 ml water — liquor ratio 1:21), CIELab coordinates: L = brightness (100 = white, 0 = black), a red– green coordinate (positive = red, negative = green), b yellow–blue coordinate (positive = yellow, negative = blue).

content was caused by the possible corrosion of concrete tubes due to the sulfate content of the waste water. In the case of cellulose dyeing with reactive dyes, NaCl can be used instead of Na2SO4 but there exists an increased risk of metal corrosion when NaCl is used. For cellulose fibers, the examples given show a distinct lowering of the chemical load in the waste water when natural dyes are used. With reactive dyes, both the alkalinity of the waste water and the salt content are rather high; with the use of direct dyes no alkali is required but the salt content remains twice the concentration released from mordant dyeing [25]. While the Na2SO4 is lost at the end of the dyeing process, the added iron sulfate or aluminum sulfate are compatible with the subsequent waste water treatment where such salts are added for phosphate elimination. A comparison of natural dyeing processes with conventional wool-dyeing methods leads to two different results: The use of metalcomplex dyes often requires the addition of Na2SO4 as a leveling agent, which causes similar sulfate loads in the waste water. A bigger problem can arise from the metal content in the metal-complex dyes mainly due to

The use of natural dyes is often linked to the terms “poor fastness” properties and rather laborious procedures of application. The results of the research presented in this paper indicate that a general one-bath dyeing process can be established for various natural dyes and acceptable fastness properties can be achieved both on wool and on linen as substrates. Despite the small number of different sources investigated in this work, a broad range of shades was obtained. Depending on the type of dyestuff, different fastness properties were obtained. While some dyes generally show a high level of fastness, e.g., madder, others require careful selection of the dyeing process to reach acceptable fastness properties, e.g., Canadian golden rod. The possibility of using mixtures of dyes and varying the composition of the mordant enables the dyer to achieve various shades in a dyeing process, comparable to the methods in use today. The common application of dyestuffs and mordant in the same bath permits the dyer to adjust color depth and shade by selection of the composition and amount of dyestuff and by use of a certain concentration and mixture of mordants. Such a procedure will improve reproducibility of the dyeing compared to pre-mordanting or after-treatment methods which are both twobath techniques. In addition, such a process permits the correction or fine adjustment of color depth and shade by further addition of dyestuff or mordant to the dyebath. An estimation of the chemical load that would be released into the waste water from different dyeing processes in use today indicates that the use of the proposed dyeing process will result in ecological improvements. However, a detailed comparison should be performed in direct work with the dyer considering the dyeing process to be replaced. At this stage of discussion a detailed comparison of aquatic toxicity of both systems will be required. In addition to the savings of process chemicals,

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Table 5 Estimation of the chemical load released into the waste water. Natural dyes considered as direct dyes, with iron mordant or alum mordant and for comparison with average values for selected dyeing methods in use today. The limits given are valid for waste water released to a communal waste water treatment plant in Austria [16] Substrate

Dyestuff

Chemicals in dyebath

Final concentration in the waste water

Legal limits for textile effluents [16]

Cellulose fibre (Linen)

Natural dye direct Fe mordant

— 5 g L–1 FeSO4.7H2O

— — 0.1 g L–1 SO42–

Al mordant

5 g L–1 KAl(SO4)2.12 H2O

Reactive dye

0.4 ml L–1 NaOH 50% 3 g L–1 Na2CO3 40 g L–1 Na2SO4 (or NaCl)

Direct dye Natural dye direct Fe mordant

5–10 g L–1 Na2SO4

— 0.25 g L–1 Fe2/3+ 0.43 g L–1 SO42– 0.07 g L–1 Al3+ 0.50 g L–1 SO42– 0.1 ml L–1 NaOH 50% 0.75 g L–1 Na2CO3 10 g L–1 Na2SO4 (NaCl) 0.85–1.7 g L–1 SO42–

— 5 g L–1 FeSO4.7H2O

Al mordant

5 g L–1 KAl(SO4)2.12 H2O

Metal complex dyes

2.5 g L–1 Na2SO4 Cr content of dye pH adjustment pH adjustment

Protein fibre (Wool)

Reactive dyes

the use of regenerable natural sources for textile dyeing replaces synthetic dyes by renewable materials. Particularly, the use of by-products from other agricultural forestry processes, e.g., the timber industry, will be of particular interest for further development. For a successful introduction of natural dyes into technical dyeing processes, additional organizational demands have to be fulfilled: 앫 increase of the number of available natural dyes with acceptable fastness properties suited for one-bath dyeing processes; 앫 formation of an efficient supplier organization which is able to provide a dyehouse with standardized dyes of constant quality; 앫 availability of technical information about the use of the dyes collected in a technical description suited to the requirements of a dyehouse; 앫 determination of toxicological data with regard to humans working with the materials and wearing the dyed fabrics; and 앫 determination of the biodegradability of the products in the waste water. The list indicates the demand for extensive investigation of the toxicological properties of natural dyes with regard to possible irritation of humans and concerning ecotoxicological data of the various products. Based upon the results presented in this article, we were provided the opportunity to start a cooperation project with two textile dyehouses to investigate the possibility of

— 0.25 g L–1 Fe2/3+ 0.43 g L–1 SO42– 0.07 g L–1 Al3+ 0.50 g L–1 SO42– 0.42 g L–1 SO42–

— 0.1 g L–1 SO42– pH 6.5–9.5 0.1 g L–1 SO42–

0.1 g L–1 SO42– — — 0.1 g L–1 — 0.1 g L–1 0.1 g L–1 0.5 mg L–1 Cr pH 6.5–9.5 pH 6.5–9.5

scaling up these processes within their factories. The research activities are ongoing at present and part of the huge amount of required information about natural dyes is expected to be available by the end of the project.

Acknowledgements The authors wish to thank to Fa. JM Fussenegger Textil (Dornbirn) for the determination of the light fastness of the dyeings and the Cebeco Groep, Rotterdam for providing a sample of madder extract. The authors also wish to thank Christoph Mader for collaboration in the dyeing experiments and fastness properties.

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