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AIR2-CT94-0981
Cultivation and Extraction of Natural Dyes for Industrial Use in Natural Textiles Production |
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Proposal No: | AIR2-CT94-0981 |
| Date Prepared: | September 1999 | |
| Source: | Final Summary Report June 1997 |
Summary
Up to the end of the 19th Century dyes from plants were the only textile colours used other than inorganic pigments and dyes of animal origin. With the development of the cheaper synthetic colours, natural dyes were completely replaced and the cultivation of dye plants ceased. Now interest in natural dyestuffs has revived, not only in Europe but also in Japan and the United States. This is due in part to increasing cases of allergic reaction against synthetic dyes and in part to changing environmental considerations. However, to date most of the naturally-dyed textiles sold on the European market is produced in India or Thailand. The reintroduction of natural dyes into the European textile industry was not possible because of technical problems and availability, reflecting antiquated cultivation methods based on hand-working and long-terming processes of dye preparation. The present project aimed to investigate if these problems could be solved.
The main objectives of the project were:
Reliable cultivation methods using modern agricultural techniques were investigated for the following important dye plants: woad, Polygonum tinctorium (blue), reseda, dyer's camomile, Canadian golden rod (yellow) and madder (red). These methods enable mass production of plant material with a high quality under Central European conditions for some crops. Results with woad were less satisfactory. This plant was the only one used for the production of blue dye in the Middle Ages in Europe. However, it contains indigo precursors only in small amounts and the ability to produce dye is not good. For that reason it was difficult to regard woad as a suitable dye plant. Polygonum, on the other hand, can produce about ten times more dyestuff (about 100 kg/ha Indigo) and can compete on the European market with the natural Indigo from Indigofera species from tropical countries. The various species investigated could produce pigments ranging from blue and violet to red as well as to shades of yellow and olive. First calculations of the production costs for the different species show that they are lower than the current market prices.
In the blue-dye plants the indigo precursors must be extracted with water and oxidised to indigo immediatly after the harvest because high loss of pigments occurs during drying. The other dye plants such as reseda, golden rod, dyer's camomile and madder can be dried and then stored for years without extensive loss of pigments. The harvested material must be dried quickly and at moderate temperatures. Hence, in addition to investigations concerning cultivation, it was also necessary to revive the partly-forgotten dyeing techniques and to develop these to modern standards.
It was shown that it is possible to get a wide range of shades without using heavy metals in the mordants. The dyeing conditions could be optimised by establishing the best pH, the mordant required, the relationship between mordant and textile weight and by the use of complexing agents, so that a maximum yield and dyeing to the required shade was possible. Dyeing could be carried out using aquaeous extracts conserved with food preservatives. Good results were also achieved in dyeing with slightly crushed or pulverised plant material. The plants can be used directly in contact with the fabric or put into a bag. All these methods are well suited for small-scale activities such as in trade factories or in applied art where they are already used. However, these methods are not suitable for large-scale industrial processes. The natural dyeing process would lead to unrealistic needs such as the necessity to invest much money in suitable textile machines, the problems of changing industry to use plant material and the difficulties in achieving reproducible results with variable natural pigments. Nevertheless, a way could be found to overcome these difficulties and to make natural pigments applicable for established dye-works. This requires the production of standardised pulverised extracts of red and yellow dyeing plants as are already used in indigo dyeing. Experiments with reseda led to very good pigment powders obtained by counter-current extraction and spray drying.
Textiles dyed with natural dyes show a resistance against sweat, washing and rubbing which satisfies industrial requirements. However, at present the resistance to photo-bleaching is not sufficient. Hence the possibility of finding plants providing pigments with a better fastness to light was investigated. The fastness to liqht of different dye plants including species of Reseda, Chrysanthemum, Centaurea, Agrimonia, Luteola and Cadhamus were investigated. Results indicated that, in spite of widespread opinion, natural dyes are not dull but can be sparkling and brilliant.
Introduction
With the discovery of synthetic dyes at the end of the 19th Century the cultivation and application of natural dyes disappeared. Nowadays, the colouring of textiles (and also wood, leather and other commodities) by dyes from plants is receiving increased attention. There are several textile companies using natural dyes. Hence the chance for the re-establishment of dye-plant cultivation and use in Europe. A prerequisite for that is the development of modern cultivation systems for important dye plants growing in Europe in the past, the winning of dyes out of these plants and the working-out of optimised dyeing procedures for industrial use. These were the objectives of the project. Up to now most of the naturally-dyed textiles on sale in Europe were imported from Third World countries. India still is a major producer.
Materials and methods
The main areas of investigation concerned the four dye-plant species madder (Rubia tinctorum) for red, weld (Reseda luteola), Canadian golden rod (Solidago canadensis) for yellow and woad (Isatis tinctoria) for blue. These are well known and widespread in Europe, with background knowledge and historical experience of use at the time the project was conceived. In addition a number of minor crops was also investigated but less intensively. In fact, woad was found unsuitable as the dye content was too low for an economical production of dye. Hence, this was replaced by Polygonum tinctorium, the classical Japanese plant used for blue colour. For each crop the best sowing time and density, fertilizer requirements, best harvest time in regard to biomass and dye yield and the required post-harvest measures for a safe conservation were determined. The harvested crops were used as source material for the dye production and the dyeing experiments.
The best method to apply for the dyestuff is to produce extracts, in most cases simple water extracts. The dependence of the efficacy of the extraction on the water temperature, pressure and pH was determined.
For optimisation of dyeing procedures the influence of the bath ratio (g textile/ml of dye bath), pH, dyeing temperature, time of dyeing, and mordant type and amount were examined. For quality control, the fastnesses to light, washing, rubbing and perspiration were determined using standard procedures. Dyeing at an industrial scale was done by using several different types of machines. To cope with industrial applications more shades were developed by mixing the dyestuffs. Finally, methods of removing dyes, mordants and other dyeing chemicals from associated waste waters were developed.
Results
Development of modern cultivation processes Cultivation methods were investigated for Isatis, Polygonum, Reseda, Solidago and Rubia. Woad, for blue dye, had already been superseded in the 17th Century by imported indigo from Bengal derived from Indigofera sp. with a higher content of dye precursors. Even so, for woad an excellent cultivation system was elaborated that will be used by farmers for production of an antifungal product. This resulted in cultivation in Thuringia of around 100 ha in 1996, using conventional methods for sowing, weed control and harvest. However, even in the best variety, the theoretical yield of dye is only around 30 kg indigo/ha in very favourable years. In many years the dye yield is very much lower. The variation in yield is very high, so the plant might be improved by breeding. However, even if it was possible to select woad with the highest biomass yield and an indigo content of 1% of the dry weight, the increase in yield would only be 60 to 70 kg indigo per hectare.
This productivity is already exceeded by Polygonum tinctorium, the classical Japanese plant used for blue dyeing. Over more than four field trials it had more or less steady leaf yields of about 45 dt/ha dry biomass with an indigo content from 2 to 4 %, corresponding to between 90 and 180 kg indigo/ha. The crop grows very well under Central European conditions after sowing at the end of April at around 5 kg seed per ha. Polygonum must be harvested in two or possibly three cuts and processed imediately after harvesting. If allowed to dry indigo extraction is difficult. Production of seeds is variable: as a strictly short-day plant, Polygonum begins to flower in the middle of August and in poor years the seeds do not ripen. In years with a long, warm autumn the yield of seeds is very high (up to 25 dt/ha).
Weld has been known since antiquity as the most widely-used plant for yellow dyeing. It may be grown and harvested on a large scale using standard farm equipment. The biomass yields reach from 35 to 70 dry tonnes per ha. One- or two-year cultivation is possible. There are no differences in the yield, but because of the better weed control the first possibility is preferable. An increase in the plant-available N from 80 kg per hectare to 120 kg at the beginning of vegetation has no effect on the biomass yield of weld, but over the three years of the project a continuing decrease in the dye content of about 10 % was observed in the higher fertilized plots in each year. The harvest time and conditions are important for the dye content of weld. The best time to harvest Reseda lies about ten days after the beginning of blossom.
After the cutting the material must be dried carefully and quickly. The highest loss of dyestuff occurs in direct sunlight. For the production of seeds it is necessary to kill the plants with Roundup when the seeds are brown in the lower half of the main inflorescences. In the upper part then they are still white. The yield in the last 4 years was 5 dt/ha on average (from 2.5 to 8.9 dt / ha in the various years). Waiting for the plants to ripen lowered the yield to zero.
Observations for weld are valid for golden rod. The harvest of golden rod must be done at the time of full flowering of the crop. After harvest the material must be dried quickly and carefully in the dark. Although a high fertilization with N gives a significant increase in biomass yield, golden rod should only be moderately fertilized with N since again dye content decreases with the increased N. Due to the very small seed size, sowing of golden rod using a drill machine is impossible. The seeds must be sown in a greenhouse and the young plants set out into the field later at a distance of 30 x 20 or 30 x 30 cm. It should be possible to use a field of golden rod for up to 15 years. The biomass can be very high (> 100 dt dry mass/ha and year).
At present madder is the only plant producing red dye that can be cultivated. For plants established by seed it takes three years before the first harvest is possible. In each year of cultivation the root yield more than doubles. If the madder plants are sown on a small plot and set out into the field in the next year, or plants are propagated from stolons as was the practice in the past, the cultivation time may be a vegetation period shorter, but this is uneconomic.
The harvesting of madder roots requires highly-specialised equipment which still has to be developed. A potato harvester does not have the required depth. The seed set of madder is only high in warm, dry years; in years with low temperatures (< 15oC) at the time of flowering, seed production is almost zero.
A number of suitable dye-plant species was grown in small experimental plots and investigated for yield of biomass and dyestuff. In dyeing experiments their dye shade was determined and their quality (fastnesses) investigated. More investigations are required if they are to be cultivated on the larger scale. It could be that one or the other species could be better suited for dyeing purposes because of a higher dye quality and a higher dye yield.
Dye production
In woad and polygonum the plants do not contain the dyestuff as such, but the pigments occur as water-soluble glycosides. During drying some of the glycosides are cleaved into sugar and the slightly water-soluble aglycone is produced. The reaction is not quantitative. Indigo is insoluble in water and soluble only in a few organic solvents such as chloroform or ethyl acetate. Hence it is difficult to remove the indigo from dried leaves. Therefore emphasis has been put on precursor extraction and subsequent indigo formation. The woad leaves are harvested from the field.
Prior to the extraction they were put into mesh bags (for easy handling and avoiding the problem of wet leaves blocking mechanical pumping devices and tank outlets). The bags of leaves (approximately 10 kg per bag) were put into a steeping tank and covered with hot water (about 60oC) to damage the wax surface sufficiently to facilitate precursor extraction, and the pH was adjusted to 3.5 with either hydrochloric or sulphuric acid. A weight was put on the bags to hold them under the water, preventing the entry of air. The water was circulated around the tank using a water pump to increase the efficiency of the extraction. After 24 hours the leaves were removed, the extraction liquor was pumped into a smaller tank, alkali added to pH 9-10 and the system aerated to produce indigo.
The main indigo precursor of woad, isatan-B (indoxyl-5-ketogluconate), could not be stored for a long period in the aqueous phase because it is highly reactive. The best way to extract isatan-B from woad is to boil the leaves for a few minutes and then to cool off the extract to room temperature, but this cannot be used commercially due to a high energy requirement. The extraction method developed is a compromise between the extraction duration and the use of energy. The analysis of the indigo samples has shown that the dye content of the raw indigo is between 5 and 40 % of the dry weight of the extracted solid matter.
For indigo production Polygonum tinctorium is more suitable. The indigo precursor indican (indoxyl-beta-D-glucoside) is more stable in an aqueous solution. So, the time of extraction can be longer (up to three days) and the temperature lower (room temperature). The indigo produced in this way corresponds with the synthetic compound in regard to its chemical qualities.
The traditional method used to extract the dyestuffs from weld, golden rod and madder is to add the plant material to the dye bath. This has been used by dyers for centuries and is still used by many dyers in developing countries. The disadvantages of this method are:
The first two disadvantages could be overcome by putting the plant material into bags and these into the container used as the dye bath. However, due to the size of the bags, only a restricted quantity of textiles can be dyed. Disadvantage has to be solved for use by modern mills. For industrial use the best method is to provide extracts. Watery extracts are not especially favourable for dye plants such as weld, golden rod and madder. The flavonoids,anthraquinones and aglycones are poorly soluble in water and therefore are extracted only partially. The remaining material always contains a considerable amount of dyestuffs. A method to extract the dyestuffs from weld is to boil the powdered material with methanol for one hour. This method is used for quantitative determination of the dye content. Nearly the same quantity of dye can be extracted by hot water under the pressure of 1.3 bar. A solution of 0.1% Na2CO3 (1:100W/V) also gave good results. Also, an alkaline extraction of madder as a first step gave promising results. Because of their slightly acidic character flavonoids and anthraquinones are soluble in alkaline solutions and, after drying, also in water. This method gives good reproducible relations between the dye content and the dyeing power.
Optimisation of the dyeing procedures
Dyeing at the laboratory scale was investigated, although techniques and recipes are known from books and dye-house records. In the first step the essential parameters for the dyeing process, such as dyestuff concentration, dyeing duration, influence of temperature, mordant and its concentration, and the dye bath ratio (g textile/ml of dye bath) for Reseda, Solidago and Rubia, were determined. The largest group of natural dyes is that of the mordant dyes. Well-known mordants are compounds of aluminium, ferrum, tin, copper and chromium. During the dyeing process aslightly-soluble complex between the fibre, the metal ion and the dyestuff is formed. Nowadays for ecological reasons only mordant solutions without heavy metals are used. Based on the results, standard procedures were developed for Reseda, Solidago and Rubia using textiles which had always been boiled to remove all added textile-processing aids first.
Dyeing procedures
In a two-bath process the textiles were first treated with alum or ferric chloride as mordant for 30 minutes at 90oC. After a good rinsing the textile was heated in the dye bath solution for 60 minutes at 90oC. For increasing the dyeing power of Reseda and Rubia, citric acid was added to the dye bath (1.5 % of citric acid in relation to the mordant weight gives the best results).
The two-bath mordanting procedure is normally required for natural dyes but it is time-consuming and labour-intensive. So, dyeing in a single bath was investigated and compared with two-bath methods in respect of dyeing and fastnesses. In the single bath process both dyeing and treatment with mordant are done in the same solution, but the addition of mordant may vary. In one process the dye solution and mordant are mixed together and the textile is treated in this mixture for 60 minutes at 90oC. In an alternative process the textile is treated in the dye solution for 30 minutes at 90oC, then the mordant is added and the textile treated for a further 20 minutes.
Dyeing with bottom mordant gives more intensive shades than the first single-bath method. For the second single-bath method the intensity is not very different but the shade can be changed. Single-bath dyeing gives greater fastnesses than with a bottom mordant.
As after-treatment all dyed textiles were washed with soap for 30 minutes at 60oC. The soaping will improve fastness to washing.
Dyeing on an industrial scale was done using several different types of machines. Depending on the type of textile, up to 400 metres per piece were dyed. The test showed that all natural textiles can be dyed with natural dyes without problems using jiggers or jets. Only for indigo as a vat dye is it better to dye on foulards.
Plant dyeing will be accepted by the textile industry only if the results with regard to the fastnesses of the dyes are similar to the currently-needed synthetic dyes. The optimising of the dyeing process has led to fastnesses which are comparable with the fastnesses of synthetic dyes.
Wastewater
It is well known from the past and from current activities in the Third World that dyeing procedures may cause huge problems for the environment. Therefore environmentally-friendly dyeing processes require an effective recycling system for all waste streams. The natural dye compounds and the plant residues can be composed by natural processes but the required addition of mordants and dye auxiliaries makes special reconditioning necessary. Waste-water treatment was started by using different filter materials and different modern methods for all used dye extracts. A careful examination of the effectivity and a calculation of all costs was carried out to find out the best method for different application ranges. The most elegant method proved to be reversing membrane-filter diffusion. Chamber-filter presses were found useful for smaller quantities of waste water. Deep-bed filtration was only economical for small quantities.
Discussion
Of all problems related to the re-introduction of natural dyes for dyeing purposes the question of cultivation is most easy to solve, as has been done for woad, polygonum, weld, madder, golden rod, dyer's camomile and safflower. Although most dye plants available are at the moment still typical wild plant species, they can be cultivated under middle European conditions very successfully with the standard equipment currently used by farmers. Their biomass yields per area are high or very high, as is the dye content. A prerequisite for the latter is that the dye plants are harvested at the right time and their conservation and storage is carried out carefully.
There are enough farmers who are willing and able to cultivate dye plants and offer them to the market at prices which are 50 % to 80 % lower than the current prices. They would always profit. The argument of people in the textile industry is that there is not sufficient land to cultivate them - this is not true. There is enough land available which will not be needed in Western Europe for food production. The old techniques and recipes as gleaned from books and dye- house records can be rapidly reconstructed and adapted to modern dyeing processes and machinery relatively simply.
It was possible, after optimisation of the dyeing procedure in the laboratory, to transfer the methods to an industrial scale. This requires the use of mordants. Some traditional mordants such as tin, copper and chrome are heavy metalsand, because of their toxicity, unacceptable. This can be overcome by using other dyes with the same shade. However, aluminium and iron can also produce excellent results in dyeing. Generally, naturally-dyed textiles should not have a poorer fastness, a weaker colour strength and duller shades than synthetic dyes. They are able to compete with synthetic dyes in many standards. All tests showed a wide range of application for the industrial use of natural dyes. However, to convince industry of the viability of the concept, further efforts are required.
Weld, like golden rod, gives a broad spectrum of yellow shades which can be varied to yellow-green or olive by changing the mordant combination. Textiles can be dyed with madder in red shades with a yellow tint, in red with a blue tint and in brown. The combination of weld and madder gives different orange shades. Unfortunately the blue colour of indigo can not be combined with madder or weld in one process. Missing basic colours are green and black and bright red. For successful use in many applications the range of shades must be enlarged. This can be done by testing and optimising old dye plants known from the past, and by looking for new dyes from plants now used only in certain regions within the EU.
First tests with a number of "minor crops" like sawwort, liverwort, celandine and rayed knapweed have shown that several of them give not only a good dyeing yield on different textiles but also produce unique colour shades with high fastnesses, especially to light.
Another open question is the application of natural dyes for dyeing purposes. Synthetic dyes are generally solid and water-soluble and in this way are easy to use. At the moment natural dyes are sold and used as chopped or powdered water-insoluble material. The question is how to use this material on an industrial scale. The standard for smaller-scale dyeing is the use of dyeing bags containing the plant material. The bags are put into the container for the dye bath; but many dyers have no container big enough for such a volume. Because of the quality differences of different provenances of the dye plant, the results of dyeing are not in every case reproducible. For industrial use the best method is to provide solid extracts. The simplest method is to make a watery extract. The hydrous solution is accepted by the dyers. The problems of this method are the transport of greater volumes, the non-correspondence of dye content and dye power, and the high requirement of energy for the boiling part of the process. The extraction of the dyestuff is never complete. Using organic solvents gave hopeful results in the laboratory, but the resulting solid extracts are not completely water-soluble. A better way to get water-soluble powder extracts seems to be to use an alkaline extraction. For madder this gives a good reproducible relationship between dye content and dyeing power. Further research is needed to standardise the extraction and drying procedure.
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Updated
by CPL Press:
03/07/2007
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