HISTORY OF NATURAL DYES
Color is one of the elements of nature that made the human living more aesthetic and fascinating in the world. They are supposed to be associated with emotions human qualities seasons festivals and passion in our life. In the past at dawn of the civilization the people tried to ornament their surroundings similar to that of natural colors observed in the plant soil sky and other sources. This gave birth to a new science of colors from natural origin.
The art of dyeing was as old as human civilization. From the historical records it is learnt that natural colorants were available to people during Greco-Roman periods. Our Vedas the Atharveda carries description of natural dyes. The use of natural dyeing materials is evident with the wall paintings of Ajanta Ellora and Sithannvasal and they still demonstrate the efficacy of dyeing craft that had been inherited from ancient times in India. Ancient Egyptian hieroglyphs contain a thorough description of the extraction of natural dyes and their application in dyeing. Further developments extending over many thousands of years led to rather complicated dyeing process and high quality dyeing.
Natural dyes have been used since ancient times for coloring and printing fabrics. Until the middle of last century most of the dyes were derived from plants or animal sources by long and elaborate processes. Among these Indigo Tyrian purple Alizarin Cochineal and Logwood dyes deserve special mention.
Natural dyes comprise those colorants (dye and pigments) that are obtained from animal and vegetable matter without chemical processing. Natural dyes fall into three categories on the basis of their origin
Plant/ vegetable Origin
Insect/ animal Origin
Plant/ vegetable Origin Colorants derived from root leaf bark trunk fruit and flowers of plants. In our country 500 plant species which have been identified as useful sources of dyes. Unfortunately most available publications refer to less than 200 species. Some of the examples of dye sources are turmeric mango leaves mesta calyx gulmohar poplar bark and ratanjot.
Insect/ Animal Origin Natural substances such as carminic acid kermesic acid and laccaic (popularly known as lac dye) obtained from either exudation or dried bodies of insects namely cochineal kermes and kerria lacca (kerr) respectively are well known and these acid compounds are used for dyeing purpose from ancient times. These acid compounds particularly carminic and laccaic acids find limited use for coloring food materials/ products and cosmetics. Some of the examples of animal dye sources include the urine of cow the camel dung Shellfish and Molluses.
Mineral Dyes The most commonly referred to and used mineral is Geru (Redchre) known for its characteristic shade. Oxides of iron tin and antimony have been used along with vegetable or insect dye to obtain the desired shades on fabrics salt like copper and iron sulphate are used as auxiliaries in the dyeing/ printing of fabrics as mordants which not only helps in dye molecule s adherence but also gives a gamut of colors with a dye.
Until the middle of Nineteenth century all the textiles were if necessary dyed/ printed with the use of natural products. Naturally various recipes/ procedures were in practice in different parts of the country depending upon the availability of local special vegetable products and stage of local standardization/ skill achieved by local craftsmen.
Nature manifests itself in a wide spectrum of rainbow colors. Man-fascinated by her glory strove towards harmonizing with her completely. He internalized color by responding to its vibrations emotionally and externally he drew from her vast storehouse to initially paint himself and then to dye the apparel he wore. Thus started the alchemy of color and India was forerunner in the art of natural dyeing-an art perfect during the era of the great Epics. In the epic period there are frequent references to PITAMBAR a yellow garment used by the gods. Atharveda carries description of natural dyes. Bhrigu Samhita is written using natural dyes. The references of Ajanta dating as far back as 1st century A.D. are painted with natural dyes. The later frescoes evidence the use of colorful garments by men and women alike. Polmey s descriptions carry reference to the multicolored flags that fluttered in the capital.
The evidence of use of natural dyes during Pre-Muslim and muslim period of Indian history is much better persevered in the form of dresses manuscripts and printings. Some of the records of the court historians (biographers like Firdusi) are written and illustrated with natural dyes. The palace decoration and the ceiling of the temples of Hallebid and Bellur (in Karnataka) widely testify to the mastery of the India craftsman in the use of natural dyes. The colored exquisite silk and muslin fabrics of India have been found belonging to the period of 16th and 17th centuries. Thus use of natural dyes is an old age history since ancient periods. A fragment of coarse madder dyed cotton fabric in a plain weave found during the excavation of the ancient Happan sites indicates that even the peoples of Mohanjodaro (c. 3000 B.C.) used natural dyes. Several historical examples found during excavation show that natural dyes are used since ancient time.
According to Mira Roy the tinctorical properties of vegetable substances recognized in the Vedic Period particularly in Atharvavedic and the succeeding periods (100 B.C. to 500 B.C.) were Kala or Asikin (possibly indicating Indigo) safflower madder and turmeric [Teli et al. 1994].
The period from the classical age (A.D. 300-700) up to the medieval period (in Indian history) acknowledges the tinctorial capacity of number of vegetable substances as well as of metal salts and minerals. The most prized red dyes of this period were kampillaka (Mallotus Philippinensis) pattanga (Caesaplinia/ Sappan) and jatuka (a species of oldenlandia). In addition of these some black dyes came to be known. These were derived from plant substances like abhaya (Terminala chebula) amalaka (Emblic myrobalan) nila (Indigofera tinctoria) ayas (Iron) kesesa (sulphate of iron) turrha (sulphate of copper) and anjana (sulphate of antimony). Apart from these sakala (a kind of black dye prepared from cow dung) was also used for this purpose.
The medieval period (from A.D. 900 onwards) was marked by the discovery of the color fixation property of tuvari (alum) and processes employed for the extraction of the coloring principles from the dyestuff. The late medieval period (18th century A.D.) introduced the application of iron mordant for the fixation of colors like blue green and violet and aluminum mordant for the fixation of red dyestuff.
Hamida Khatoon Naqvi provides an account of some of the processes employed in dyeing and printing of cotton fabrics in Mughal India (A.D. 1550-1800). Chakunda (Cassia tora) was found to be a cheaper substitute for indigo and was fixed by adding limewater. According to her most of the years she prepared her own decoctions. A list of a table from a translated version of 32 selected process of dyeing and makes an interesting observation about the use of Indigo. She also gets success in order to get effective alternatives of various costlier substances. It is observed that it is during the shades of number of colors in delicate tones come into the field of dyeing and printing with natural dyes. These were derived from flowers and fruits and added to the basic ranges of colors consisting of blue (indigo) red (manjistha) black (kalaka) yellow (haidra) etc.
The Beauliew manuscripts (C 1734) according to Schwariz provide a brief account of dyeing for black red and blue by mordants. Chayroot and indigo were used in India for cotton painting obviously to kalamkari work in the coastal region in the eighteen-century. Machilipatnam Sri Kalahastiand Tanjavur was some of the important places for kalamkari work in the eighteen century.
The manuscript of George Roque s according to Schwariz the cotton printing centers of western India in the late seventeenth century [A.D. indicates the use of roots of saranguy or Al (Morinda citrifolia or Morinda tinctorium)] for dyeing a beautiful red. In many parts of India the Al root were as cheap substitute for madder.
Bibhutibhusan Bandyopadhya s novel Ichhmati written in Bengali gives a clear existence of indigo house organized by the East India Company in the Bengal of that time for the forcible cultivation of Indigo by farmers for export with the help of local strong men. There were about seventeen indigo houses mainly in Jessore and Nadia districts. Their officers marked land owned by farmers especially for the cultivation of Indigo for export. These seventeen Neel Kuthis (indigo houses) which were closed down gradually with the appearance of synthetic indigo from Germany at very cheap rate in the year 1864.
A collection of 168 specimens of fabrics dyed by indigenous processes were kept in the Bengal Economic Museum Calcutta duplicates of some of the specimens formed the collection of 108 samples of fabrics dyed with Bengal dyes forwarded by Mr. Locke to the Secretary of State for India in 1878. These were probably transferred to the industrial section library of the India Museum Calcutta. There is a set of 15 volumes under the name Specimens of Fabrics dyed with India dyes by Thos. Wardle . Many of these colors are not permanent and the color has faded beyond recognition. Still some of these specimens are good while some of the specimens are missing.
With the influx of synthetic dyes for most of the traditional colors including blue in the nineteenth century A.D. the use of natural dyes has gradually gone out of existence from most parts of the country. However in a few places natural dyes continued to be used and in some places synthetic dyes are used in the old process particularly indigo and alizarin. Some of these places where natural dyes continued to be extracted or used in some form or other are Andhra Pradesh [Devi 2001] Cuddapah Uravakonda Machilipatnam Sri Kalahasti Sikkinaikenpet (Tanjavur district) Balarampuram Iika Guleguw Banhati Dhanmanka Ahmedabad Begru (Rajasthan) Sangarer Bharivgarh Berhampur (Orissa) Kptapad (Orissa) Imphal Kadampapi (Manipur) Khensa and Akoya (Nagaland) Darjeeling (Tibtan refugees) Gangtok and many north eastern hill areas for small commercial as well as domestic purpose. There are also many other small places where natural dye is still used in small scale and in some of the above-mentioned places the use might have been discontinued in recent years.
A painting work which is done in different places of India with vegetable colors is known as kalamkari [Udayini and Jacob 1988] which is continued still using natural dyes till recently through the efforts of Shri Radha Krishan Naidu. Natural dyeing processes were reportedly being followed in Balarampuram and other places in Kerala and in the parts of Tamil Nadu adjoining Kerala.
In Padamanabhapuram 55 kms. from Trivendrum on the way to Kanya Kumari the painting on the walls of the Palace there reported to be in vegetable colors indicate that vegetable colors were in use there in the 18th and 19th centuries A.D. At Dhamadka in Gujarat dyeing with an inferior type of natural indigo was no longer practiced.
In recent years few enthusiastic people keen on exporting textiles and garments are endeavoring to revive the processes of natural dyeing particularly because of the interest shown in export markets in the sophisticated fabric in which natural dyes are used. At a time when natural dyeing process are gradually being discarded even in few places where they have been kept alive for so long and when it seems clear that this knowledge will also be lost to the country in course of time the endeavor of these few people to revive the processes commercially is praise worthy.
Almost all the synthetic dyes have their origin from coal tar many synthetic dyes may lead to various harmful by-products during their manufacture. A number of azo dyes which release carcinogenic harmful amines have already been banned by most of the countries [Premi 1996]. However the effluent discharged from dyeing units is also causing lot of concern. Hence there is an increasing realization in the textiles industry as well as among the textile consumers to develop and demand eco-friendly methods of dyeing textiles. Natural dyes offer an important alternative in these regards as these are safer in use with minimum health hazards have easy disposability are biodegradable and can be used to make compost for agricultural purposes after the had been extracted. There is a long-time tradition of using natural plant-based dyes in India. The scientific names of many of these dye-yielding plants lie scattered in classical references such as Hooker [1876-1897] Watt [1896 and 1908] and a more modern version of these published in the eleven-volume Wealth of India (1948-1976). Among the more concise references to these plants mention may be made of the works of Dastur  Sundararaj and Balasubramanyam  Maheshwari and Singh  and Santapau and Henry .
A list of dye yielding plants of India given by Perkin and Everest  was reproduced Gulati (1949) who also gave corresponding common Indian names for easy identification of the plants.
In the excellently produced treatise Natural Dyeing Processes of India has given in the appendices the list taken from Liotard (1881) and also that from Perkin and Everest . Mohanty et al.  have also compiled from available literature on alphabetical list of botanical names in regional languages together with a number of illustrations of plants used as dyes and/ or auxiliaries in India.
PROMOTION OF NATURAL DYES
Office of the Development Commissioner for Handlooms Ministry of textiles (Govt. of India) with the help of its branch offices like Weavers Service Center Indian Institute of Handloom Technology (IIHTs) and National Handloom Development Corporation (N.H.D.C.) have been doing a lot since last few years. But in recent years serious efforts have been made so as to boost the use of natural dyes by implementing projects researches and seminars. It was also felt in seminars that the extracts of vegetable dyes should be chemically analyzed and trails shall be made to standardize them after extracting the dyes in the solution form as the marketable source and make these available in the market.
Since mostly the locally available natural products were utilized only for coloring textiles therefore depending upon local weather conditions stages of growth of the vegetable products their storing conditions and even quality of local water available for processing affected the final shades. The variation in tones of the leaves or flower petals or fruits one sees in one and the same plant species is the weakness of these natural products. Such a possibility of variation in final shades is one of the major issues considered in textile coloring.
Apart from this issue there were many other problems associated with the use of vegetable/ natural dyes. Following is the list of some of those problems
Non-availability of dye producing materials due to difficulty of collection.
Bulk isolation of dyestuff.
Standardization of dyeing procedure.
Complexity of dyeing process.
Reproducibility of shade.
Due to these problems encountered with natural dyes the development of synthetic dyes came into existence. The advent of synthetic dyes dates back to 1856 and still dominates the entire dyestuff industry. These synthetic dyes have received faster acceptability due to its ease in dyeing reproducibility and cost factors. It was only a decade ago when toxicological effects of dyes during wearing became more and more known and caused a great concern about the use of synthetic dyes. In the late 1994 Germany struck a severe blow to dyestuff industries and subsequently other European countries also executed ban on import of textiles and garments colored with a series of azo-dyes made from aromatic compounds which are carcinogenic allergenic and poisonous.
Even after this ban the textile processing units are one of the most polluting industries. The dyestuff industrial wastes in the form of atmospheric gases and wastewaters have been found to be polluting the neighboring areas. If the examples of the famous textile processing dominated industrial cities of Pali and Jaipur are any indication where because of pollution by industrial wastes even drinking water in the area was rendered unpotable and majority of the industries had to be de-registered or closed down. This aspect cannot be neglected in the coming days.
The pollution problem is the main inspiration behind the resurge of natural dyeing materials. Though after the advent of chemical colors people have got used to very bright and very fast range of shades which is still not possible with the natural dye materials available till date. However it is always not necessary or desirable to have only very bright or very fast shades on the cost of nature damage. The soothing effect of natural shades may at times win over in aesthetic aspects over the chemical dyestuffs.
People in general or the present day textile processors in particular are very much used to the idea of getting their dyes in ready powder form packed in tins/ bags. All the natural products are still not available in this form and hence these dyers/ printers do not readily come forward to handle these cumbersome natural products presently marketed in the form of barks stems roots leaves etc. and not as powders or extract of these products.
These shortcomings of the natural dyes can be overcome by making the efforts to streamline the supply channels of natural products and to improve the dyeing quality of these products. If sufficient research time is allotted to this subject recipes could be developed which could bring any of the desired colors at par with their competing synthetic colors.
Besides this many of these natural products have some medicinal or healing qualities for human bodies. The vegetable indigo gives a cooling effect to the skin even in hot summer. Myrobalan gives a soothing feeling to the skin. It is also said to be antiseptic in effect. If systematic studies were undertaken in these aspects surely the natural (dye yielding) products would win hands down over their synthetic counterparts.
Use of vegetable products for textile coloring at commercial level is to an extent a virgin area nearly seven years ago. In recent years a drive for using natural dyes is noticed all over the world because in the sophisticated fashion fabrics the natural dyes offer pastel color effect apart from being eco-friendly. The status of usage of natural dyes world over is enhancing every year. Up till now more than 500 natural dye sources are known. A very promising crop could be expected in this field if due efforts are put in for its growth and promotion. Exploration of plant parts for dye extraction which are otherwise waste like bark leaves and flowers etc. need to be exploited.
India is primarily an agricultural country. Nature has bestowed India with wide varieties of vegetables trees shrubs and different kinds of grasses which provide different natural dyes. In India some 500 varieties of plants are available that can yield natural colors. The natural dyestuff manufacturers have concentrated mainly on manjistha lac tissue pomegranate rind marigold henna turmeric catechu and Indian rose.
Natural dyes are mordant dyes. The mordant is the life for the vegetable colors except in the case of Indigo. Without mordant no color adheres in vegetable dyeing. It acts as an agent between the fiber and the color by helping the color and penetrates into the fiber permanently and makes it fast. Most of the natural dyestuffs will not by themselves adhere to yarn or cloth except as a surface stain which is easily washed away. A mordant usually of metallic origin introduced upon the prepared cloth unites with the dyestuff during the process of dyeing in the vat usually under heat to form an insoluble lake.
The traditional method of dyeing has been to boil the fabric or yarn in dye bath till the desired color is obtained. Enormous amount of heat is consumed in terms of heating the dye bath. Some dyes which are heat sensitive cannot be used in conventional dyeing because prolonged heating decomposes the dye molecules. The dye uptake by the fabric is also far from exhaustion as a result fair amount of dye is wasted however in case of ultrasonic dyeing the most advantageous part is that at low energy dyeing is carried out (mostly at room temperature no need for heating the dye bath).
The use of ultrasonic energy in textile processes is not a new idea there exists considerable amounts of literature on the improvement and acceleration of numerous textile processes with the aid of ultrasound. However until recently research into this technology has been limited due to the expenses associated with the generation of ultrasonic energy. Technological advances in the areas of inexpensive and reliable ultrasound generators has therefore prompted increased interest into effects of ultrasonic energy applications in industry. Research is being conducted into the beneficial effects of ultrasonic energy on dye application to cotton wool and other textile fabrics. Dyeing of textiles with low and high frequency sound waves has been the subject of many studies. Improved understanding of physicochemical changes caused by sound in dyeing systems is now known.
The mechanical agitation causes slight rise in temperature which helps in dyeing. Sonicator dyeing is very innovative technique and fuel saver methodology. In this method ultrasound energy of 20 KHz frequencies is utilized. Sonicator has high-energy sound waves which increase ultrasonic cavitations. This releases considerable amount of energy. Particularly in India this methodology is advantageous where energy resources are limited. Even heat sensitive dyes can be used in sonicator dyeing very comfortably without undergoing decomposition. The dye uptake is very good in sonicator dyeing. The same bath can be recharged and reused.
BASICS OF NATURAL DYEING
The knowledge and use of color or dye on cotton wool and silk began with the dawn of the civilization and was first developed in the East particularly in India. India has the long rich tradition of colored fabric design. There are many plants and some animal sources in nature that yield color and can dye fabric leather hair and other items. Humans started using dyes as soon as they were discovered 6000 BC or even earlier. It is not possible to precisely locate the place of antiquity where dyeing was first known as an art. Evidence leads us to believe that different civilizations had each its own methods practiced. It is said that the Egyptians learned this art as early as probably the Indians and Chinese. In the Medieval period there were certain plants that were heavily relied on for most colors till the invent of synthetic colors.
Color was considered by ancient people as a basic necessity as essential as food and water. The ancient people used exclusively dyestuffs of vegetable mineral and animal origin all easily obtained in their own vicinity. Natural vegetable dyes have been used in most of the ancient civilizations in different countries e.g. India Egypt Greece Rome etc. In India use of vegetable dyes in dyeing painting and printing goes back to the pre-historic periods.
In India according to the information collected so far there are nearly 300 dye yielding plants available. Based on this 30 raw materials were taken and some work was done by using these dyes on cotton silk and wool.
Advantages of Natural Colors/Vegetable Dyes
Natural dyes bearing Eco-mark are ecofriendly and acceptable in today s world
They are non-toxic & non allergic hazard free for skin.
Fastness can be achieved by the use of proper mordants.
They are safe the life environment fuel & time and other investment process.
For successful introduction of vegetable dyes into technical dyeing processes some additional demands have to be fulfilled
Increase of the number of available vegetable dyes with acceptable fastness properties suited for one -bath dyeing processes
Formation of an efficient supplier organization which is able to provide a dye-house with standardized dyes of constant quality and to generate an inventory of suitable vegetable dyes from application point of view
Availability of technical information about the use of the dyes collected from forest or locally grown plantation emphasis be made on production of plant material in sufficient amounts with modern agricultural methods which would include simple and environmentally clean extraction methods suiting the requirement of a dye-house
Determination of eco-friendliness of the vegetable dyes for suitability for wearing dyed fabrics
Determination of biodegradability of the waste generated after dye extraction from the plant sources.
It is of utmost importance to know the structure of the dye depending on the dyes structure the mordant and dye uptake is expected. Pretreatments are very important part of vegetable dyeing.
Natural Dyeing Principles
Application of natural dyes in today s scenario makes use of modern science and technology not only to revive the traditional technique but also to improve its rate of production cost effectivity and consistency in shades. It therefore requires some special measures to ensure even-ness in dyeing. Many factors have to be accounted for when one works with natural dyes. They are as follows
Nature of Material to be Dyed
Animal proteins like wool and silk dye best in acidic conditions and are weakened by alkaline. If an animal protein is dyed in alkaline conditions it is best to end with a diluted vinegar rinse to restore a slightly acidic pH to the fibers before they dry. Plant materials like cotton flax dye best in alkaline (basic) conditions and are weakened by acids. If cotton is dyed in acidic conditions it is best to end with a weak washing soda bath to restore the fibers to slightly alkaline before they dry.
Measurements of Mordants and Dyestuffs
Most dyeing procedures specify ingredients by weight rather than measure. Recipes will also specify the amount of fiber to be dyed or the other ingredients will be expressed as a ratio to fiber weight. This is because the amount of water in the dye-bath will not affect how strongly the fiber takes color but the amount of dyestuff in the dye-bath does. So if one gm of fiber has to be dyed with one gm of dyestuff and then one wants to reproduce the same color on 5 more gms of fiber the amount of dyestuff should be multiplied by 5 times as well. The water should always be enough to let the fibers move around freely water quantity should be sufficient to dip the fabric/fiber properly.
Different dyes work better at different temperatures. Most plant dyes benefit from being heated but some (i.e. madder) change colors if allowed to boil. Sappan wood also has a tendency to change color when heated for prolonged hours. Some dyes work best at lower temperatures (safflower and woad/indigo).
For getting even dye uptake one should move the fibers around as much as possible in the dye-pot. Unfortunately when wool is heated and agitated it tends to felt so one must be very careful about how much one should move it around. For most wools heating and cooling the dye-bath slowly and being gentle while moving the fibers is necessary to avoid felting.
Natural Dyes are Unpredictable
Books on natural dyeing can predict the range of colors that will most likely be given from a dye source but there are so many factors involved in the process that reproducing a color exactly can be very difficult unless those parameters are followed strictly. Some reasons for disappointing results could be insufficient heat or too much heat accidental iron or other metal contamination in the water bad growing conditions for the dye-plant plant harvested at the wrong time of year dyestuff allowed to dry out dyestuff kept in humid conditions dyestuff too old and dye obtained from different plantation in terms of climate and soil conditions. The point here is to list some reasons for failure which one would face if one does not get the expected color - the most experienced dyers in the world get accidental color sometimes. One can over-dye and get the desired colors.
Wet Fibers Look Darker
When trying to achieve a certain color it has to be always remembered that the color when wet will always appear darker and may be disappointing when the fibers dry. Also some color will rinse out after rinsing the fibers. Always dyeing to a darker shade in the dye-pot than what is required. Lifting the fiber out of the dye-pot to air is often good for the dyeing process to check the color.
Fibers should be rinsed after they have been dyed and some dyes will still bleed for several washings afterwards. As mentioned above it is advisable to add some washing soda to plant fibers or some vinegar to animal fibers to return them to their optimum pH in the last rinse.
Using Natural Dyes
The first step of the actual dyeing process is mordanting. A mordant is a chemical that when cooked with the fiber attaches itself to the fiber molecules. The dye molecule then attaches itself to the mordant. Different mordants give different colors when combined with the same dye. For example the dye cochineal when used with alum sulfate gives a fuchsia color when used with tin the color is more scarlet and when used with copper it is purplish. Mordants except for alum and iron are considered toxic and therefore should be avoided in the preparation of eco-textiles otherwise the whole exercise will be self defeating. As the mordants are toxic to the dyer and the disposal of the bath becomes an environmental problem. Therefore the choice of mordants is limited. Alum and iron are ideal safe mordants. Other chemicals known as assistants may be used in addition to dyes and mordants which help in coloration of the fabric in one way or the other for example- to change pH and hence the color sometimes to brighten the color to help in the absorption of the mordant metal or to slow down the rate of absorption of pigments or for evenness. These include potassium hydrogen tartrate (cream of tartar) oxalic acid tannic acid acetic acid formic acid ammonia sodium sulphate (Glauber s salts) sodium chloride (common salt) and sodium carbonate (washing soda). Treating cotton with tannic acid is useful as it prepares the fabric for effective absorption of the dye.
The word mordant comes from the French Mord and mordants can be described as metallic salts with affinity for both fiber and dyes stuffs and that improves the color fastness. Even some of the fugitive dyes have been used successfully with the help of mordants. Dyes are categorized as either mordant or adjective or Indirect dyes. Most of the natural dyes are mordant dyes except the very few direct dyes and vat dye such as Indigo. The latter dye needs no mordants.
In addition to adding substances to a bath for mordanting the vessel that is used may itself serve as mordant. The dyers use copper tin vessels to brighten the color and iron vat to dull the color. To get the effect of alum mordant now-a-days aluminum dye pot with a little soda is used. To get the basic original color of the coloring materials earthen or stainless steel materials are advisable.
There are two processes concerned with the dyeing of most vegetable colors. The first is mordanting and the second is the actual coloring. The mordanting prepares the material to be dyed to receive the dye. Mordant should not affect the physical characteristics of the fiber.
A mordant must not flatten i.e. dull or deaden the luster of the fiber but if anything raise or brighten it for which sufficient time must be allowed for the mordant to penetrate in the fiber thoroughly. Different mordants give different color with the same dyestuff. Alum Iron Tin Chrome and Copper mordants are used commonly.
Mordanting of Cotton
Mordanting is very important for cotton dyeing. Natural dyeing of cotton is more difficult than silk and wool. Cotton is not very porous and will not hold the dye stuff without a more complicated preparation for mordanting the fiber must be cleaned first.
Preparation of alum mordant - To prepare alum mordant first alum-powder and cream of tartar are mixed with little boiling water and then made up with the remaining required water. Stirring is continued well till the chemicals are dissolved and water should be lukewarm. The wetted fiber is entered slowly bring the bath to the required temperature and worked. Dyeing the fiber immediately or it should be dried in the shade and stored for further use.
Tin mordant - Dissolving cream of tartar or oxalic acid in a little quantity of hot water. When it is thoroughly dissolved some more hot water is added. Addition of stannous chloride and mixing well is continued till it dissolves. The remaining water is added. When it is luke-warm thoroughly loosened wetted fiber is entered and the heating to required temperature is done and continued to work for the specified time.
Copper mordant - Dissolving sulphate of Copper in lukewarm water and remaining required quantity of water is added. The fiber is entered and worked as tin mordant. If bright colors are desired cream of tartar may be added in the beginning itself in the copper sulphate.
Chrome mordant - Mordanting with potassium dichromate is best just before dyeing. Dissolving the potassium dichromate in little warm water and making up the solution with the rest of required water. When the bath is lukewarm then similar procedure is followed as stated above. Care should be taken while handling chrome mordant. Covering the pot with a lid except when the fiber is worked is necessary to avoid inhalation. Apart from this chrome is very sensitive to light. If light falls on any part of the fiber it will darken the fiber and result in uneven dyeing. Further procedure should be followed as before.
Iron mordant - Dissolving ferrous sulphate with a little warm and addition of cream of tartar to this and this should be mixed well. Addition of the remaining total water and entering the fiber and working for the specified time at specified temperature gives best results.
As mordanting is a very integral part of Natural dyeing more details are given in the next chapter.
MORDANTING THE TEXTILE FOR NATURAL DYEING
The term mordant is used for chemicals which usually have a metal with a valency of at least two or more they can also be other types of compound as well. Natural dyes also referred as mordant dyes do not readily adhere to cotton so mordants are used. Mordants are needed to set the color when using natural dyes. Different mordants will give different hue color with the same dye. A mordant is thus a chemical agent which allows a reaction to occur between the dye and the fabric. In textiles mordants are used to fix the color in dyeing or fabric printing especially for fabrics of plant origin (cotton). Mordant is added to the dye source to influence it it does not serve as a color source on its own. The fabric is impregnated with the mordant then during the dyeing process the dye reacts with the mordant forming a chemical bond and attaching it firmly to the fabric. The choice of mordant depends upon the fabric. An alkali mordant such as soda ash works well with cotton and acid mordant such as vinegar works well with wool.
Metal mordants can be defined as a polyvalent metal ion which forms coordination complexes with certain dyes. Two types of bonds are involved in the fundamental reaction between a mordant dye and a mordant. One is a covalent bond with usually hydroxyl oxygen and the metal atom. The other is a coordinate bond with the metal with the double bonded other oxygen also referred as chelation . It is possible however that the formation of dye-mordant complexes involving several molecules of dye can also form. Varying the amount of mordant with the dye is a way to exert some control over the change in hue color. The two commonest metals used in natural dyeing are aluminium and ferric ions both having valencies of three.
Treatment of fabric before dyeing
After removing the impurity of fabric then it is treated with 4 % (owf) solution of tannic acid in water. The fabric should be dipped in tannic acid solution for at least 4-5 hours. It is squeezed and dried. After mordanting the fabric is used for dyeing. Dyeing would depend upon the type of mordanting used. There are other types of pretreatment used these days which are ecofriendly. They are mainly two types
Enzymes like amylase trypsin cellulose and
Polyethylene glycol (PEG).
Both these types are used as depth improver for dye fixation.
Methods of Mordanting
The percentage of chemicals and the weight of the material to be dyed are very important. The details of chemicals to be used for various mordants with their quantity fixed temperature is to be maintained duration of time and the procedure to be followed after mordanting and before dyeing have to be followed strictly. There are three ways of mordanting. Mordants and dyes may be applied in three ways. They are as follows
Pre-mordanting where the mordant is applied first followed by dyeing.
Post-mordanting where the dyeing is done first and then mordanting is carried out.
Simultaneous mordanting where mordant and dye are mixed together and applied.
Mordants are commercially available commonly in the form of salts from metals such as chrome copper tin iron and aluminium. These mordants are listed in descending order of relative toxicity. Other types of mordants which are not metal mordants are tannins cream of tartar baking soda and vinegar. The later two serve to change the alkalinity and acidity respectively of the dye another property that influences the final color. Besides metallic salts tannins and other inorganic compounds sometimes oils such as oil of turkey and its type are also used as mordants. It is often remarked that the addition of a mordant to an appropriate dye solution results in a very sudden dramatic change in color. This is due to the incorporation of the metal atom into the delocalized electron system of the dye. Metals have relatively low energy levels so their incorporation into a delocalized system results in lowering of the overall energy. The absorbance of the hue and thus its color is related to this phenomenon.
Most dyers mordant the cloth and then apply the dyes as two separate steps. The advantage is that the mordant has a chance to bite into the fiber so that when the color is applied maximum amount of bonding takes place. Many dyers turned to natural dyes because they are safe and non-toxic so it is imperative to ascertain whether the mordants that are used to fix the dyes are safe or not. Most of the mordants that are used for natural dyeing are not seriously toxic.
The better results obtained in case of pre-mordanting with stannous chloride and ferrous sulphate are attributed to the empty d-orbitals of ferrous and stannic ions. The mode of binding of dye seems different with iron and aluminium ions.
The pre-mordanting with ferrous sulphate showed least color discharge after washing because of the pretreatment of fabric with tannic acid which shows 100% iron binding efficiency in terms of tannic acid equivalents. The iron binding by phenolics increases with increasing number of -OH groups. A flavonoid ring B and a 3 4 -dihydroxy group is required for Fe-binding (I) and in case of aluminium ion binding the 3-hydroxychromane groups are required as shown in the structure (II) below.
Cotton has very low affinity for natural dyes. The tannins play an important role in cotton dyeing and are largely used for preparing cotton so as to enable it to retain coloring matter permanently. Most common mordant for cotton is thus tannin or tannic acid. It occurs in many tannin containing substances especially in gall nuts which has about 60-70% tannic acid. The aqueous solution of tannic acid gradually decomposes on standing by fermentation. Addition of boric acid inhibits the decomposition. It is also considered as primary mordant before mordanting the cotton fabric with metallic salts. Treatment with tannic acid helps the cotton fabric to absorb all types of metallic mordants. The metallic mordants form complex with the carboxylic groups of tannic acid.
Frequently the purpose of preparing the cotton fibers with tannin is not so much to fix the coloring matter although that is the final goal as to fix certain metallic salts such as alum copper tin and iron in the form of insoluble tannates. The metal tannates present on the fabric form insoluble lakes with the natural dyes during dyeing. A study was carried out to see the effect of mordant on dye uptake and enhancement of fastness properties. The figure -1 shows that treatment of the fabric with only tannic acid tannic acid and alum show gradual increase in dye uptake although blank (un-mordanted) fabric shows good dye uptake but it all washes off thereby showing the importance of mordanting.
STANDARDIZATION OF VEGETABLE DYES
Natural dyeing has the problem of standardization. In 1971 Rita Adrosko wrote about vegetable dyes in NATURAL DYES AND HOME DYEING Craftsmen are becoming increasingly enthusiastic about this out-dated and time-consuming process for one of the reasons that manufacturers rejected it because of difficulty of standardization. Natural dyestuffs produce offbeat one-of-a-kind colors. No two dye lots are identical each having subtle differences due to impurities peculiar to the particular plant material used.
The problem of course with raw botanicals is that the numerous chemical ingredients that make up plants vary widely. Not only do the variations occur between plants of the same species but also from part to part of the same plant so that for instance in madder the dye is contained in the roots not the leaves. The type and quantity of chemicals present are affected by such things as soil species weather time of harvest as well as the part of the plant used. The manner in which they are stored and processed also has a profound effect. Color varies greatly with plants grown in different areas due to mineral content of the soil and various other factors of growth.
Dyers learned by trial and error what to pick and when and where to pick it. They passed down their knowledge from generation to generation often keeping trade secrets from outsiders. Dyers of course used color as a control. They kept trying to determine which plant which part of the plant which species what growing conditions and what time of harvest would produce the color closest to the one they wanted but they also had many other variables to worry about such as the water and utensils used. Dyes prepared in a tin pot give a color different than the same ones prepared in an iron pot. To obtain the desired color time after time the dyer had to know all this. If he didn t get what he was looking for he knew that something was wrong with the raw materials used or with the manufacturing process and had to figure out what and how to adjust it.
In an effort to standardize colors dye plants were often cultivated rather than gathered wild. Many were grown commercially. In order to get a standard color from a particular species of dye plant 400 years ago the farmer would have had to have worked empirically by selecting & cultivating plants that produced a dye that got closer and closer to the color he wanted. He would also have had to have tried growing the plants under different conditions to see what type of soil etc. gave the best results. When he reached his goal he would then have had to have maintained the results by always growing the same species under the same conditions using color as his control. By keeping good records and adjusting the variables he learned by experience how to obtain the desired color but it wasn t easy or exactly the same each time. Even today it is not possible to precisely match color from batch to batch not even with synthetic dyes.
To produce fully standardized eco-friendly vegetable dyes from potential herbal/vegetable resources evaluation of natural colors for eco-friendliness standardization of processes for various textile substrates large scale production and development of commercial natural dyes there are certain specifications that need to be followed. They are-
Specifications in terms of
Water soluble matter
pH of water extract
Ash content and
Colour component and its tinctorial value 8. Total suspended solid content.
Procedures for the extraction of coloring matter in aqueous medium has to be standardized at pilot and bulk scale plant. Shade cards having shades of fabric and dyed yarns having good light wash and rub fastness need to be produced for each natural dye. But still the problem of reproducibility arises. Dye from one company does not necessarily give the same shade as the same dye from another company. Comparative study has been carried out for three dyes- 1. Acacia catechu (Catechu) 2.Rubia cordifolia (Manjistha) and 3. Quercus infectoria(Gall nut). Table -I shows the difference in their pH and total suspended solid contents in the dyes supplied by different companies. The dyes show that UV spectrum Visible spectrum as well as FT-IR spectrum shows difference which goes to show that there is definitely difference in their optical density which is the root cause for their difference in dyeing ability and color. This exercise of standardization was carried out to confirm this fact.
METHODS OF DYE EXTRACTION
Experimental trials were carried out in domestic gardens in collaboration with botanists mainly focusing on the best conditions for the growth of dye plants in regard to soil and climatic factors. Modern cultivation system for getting maximal dye yields including optimal seeding and harvesting time optimal fertilization procedures were adapted. The utilizable plant parts were subjected to specific dehydration processes or the dyestuff was extracted as per the given strategy.
The traditional method used to extract the dyestuffs from all other plants mentioned earlier where the plant material is added directly to the dye bath. This has been used by dyers for centuries and is still used by many dyers in north eastern states of India.
The disadvantages of this method are
The plant material has to be separated from the textile
It is not applicable to modern textile fabrication machines (pumps and spinerettes will be choked)
Hard plant material such as madder roots or barks of Cassia amla are dufficult to extract
The low density of the dried material requires high processing volume
Disadvantage has to be solved for use by modern mills. For industrial use the best method is to provide extracts. Aqueous extracts are not especially favorable for dye plants such as Parkia Alkanet and Tulsi where we have used 50 50 water methanol extract for dyeing. The reason being that 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 such plants is to boil the powdered material with methanol for one hour. This method is used for quantitative determination of the dye content. 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.
Innovative Method for Extraction of dyes Efficient extraction of the dye from the plant material is very important for standardization and optimization of vegetable dyes. Utilizing a) Soxhlet b) Supercritical fluid extraction c) Sub critical water extraction and d) Sonicator methods
SCFE is a two step process which uses a dense gas as solvent usually carbon dioxide above its critical temperature (31°C) and critical pressure (74 bar) for extraction. The natural product is powdered and charged into the extractor. Carbon dioxide is fed to the extractor through a high-pressure pump (100-350 bar). The extract charged carbon dioxide is sent to a separator (60-120 bar) via a pressure reduction value. At reduced temperature and pressure conditions the extract precipitates out in the separator. The extract free carbon dioxide stream is introduced several times for effective extraction of all the dye material from the natural product.
Why SCFE is superior over the traditional solvent extraction of natural dyes? Firstly it uses a clean safe inexpensive nonflammable nontoxic environmentally friendly nonpolluting solvent-carbon dioxide (CO2). Secondly the energy costs associated with SCFE are lower than the conventional techniques.
GRAPHICAL Subcritical Water Extraction
Subcritical Water Extraction was performed with some plants to extract natural colorant. The water was purged with nitrogen to remove dissolved oxygen prior to the extraction. Deoxygenated water was used in an HPLC pump programmed for a constant flow of 1-3 ml/min-1. A 10.4 ml µm frit at the inlet and outlet was connected to a 1 m cooling loop (in ice water) outside of the oven. A pressure control valve was placed between the cooling loop and the collection vial. The extraction was carried out in efficient manner.
Innovative Method for dyeing The traditional method of dyeing has been to boil the fabric or yarn in dye bath till the desired color is obtained. Enormous amount of heat is consumed in terms of heating the dye bath. Some dyes which are heat sensitive cannot be used in conventional dyeing because prolonged heating decomposes the dye molecules. The dye uptake by the fabric is also far from exhaustion as a result fair amount of dye is wasted however in case of ultrasonic dyeing the most advantageous part is that at low energy dyeing is carried out (mostly at room temperature no need for heating the dye bath). The mechanical agitation causes slight rise in temperature which helps in dyeing. Sonicator dyeing is very innovative technique and fuel saver methodology. In this method ultrasound energy of 20 KHz frequencies is utilized. Sonicator has high-energy sound waves which increase ultrasonic cavitations. This releases considerable amount of energy. Particularly in India this methodology is advantageous where energy resources are limited. Even heat sensitive dyes can be used in sonicator dyeing very comfortably without undergoing decomposition. The dye uptake is very good in sonicator dyeing. The same bath can be recharged and reused.
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 60°C. 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 meters per piece were dyed with the help of local company. The test showed that all natural textiles can be dyed with natural dyes without problems using jiggers or jets.
Color is one of the elements of nature that made the human living more aesthetic and fascinating in the world. They are supposed to be associated with emotions human qualities seasons festivals and passion in our life. In the past at dawn of civilization the people tried to ornament their surroundings similar to that of natural color observed in the plant soil sky and other sources. This gave birth to a new science of colors from natural origin.
Natural coloring matters are broadly classified in three categories.
Vegetable origin Colorants derived from root leaf bark trunk fruit and flowers of plants.
Animal origin Lac Cochineal and kermes have been the principal dye yielding insects.
Mineral origin Various inorganic metal salts and metal oxides
Only after 1856 the development of synthetic dyes came into existence and still dominates the entire dyestuff industry. These synthetic dyes have received faster acceptability due to its ease in dyeing reproducibility and cost factors. But in the late 1994 Germany struck a severe blow to dye-stuff industries and subsequently other European countries also executed ban on import of textiles and garments colored with a series of azo dyes made from aromatic compounds which are carcinogenic allergenic and poisonous.
With the present national and international awareness of environmental ecology and pollution controls natural dyes appear to be the ideal choice since they are chosen from the non toxic lot and can be handled very easily and safely however it is not so simple there are some problems encountered in the use of vegetable/natural dyes as well. The are as following
Non-availability because of difficulty of collection.
Bulk isolation of dye-stuff.
Standardization of dyeing procedure.
Complexity of dyeing process.
Reproducibility of shade.
But in the twenty first century maintaining a safe environmental balance becomes even more important. The co-operation of individuals communities and countries to make this happen become a global necessity and the following properties are often considered to be advantages of natural dyes.
They are obtained from renewable resources.
No health hazards sometimes they act as health cure.
Practically no or mild reactions are involved in there preparation.
No disposal problems
They are unsophisticated and harmonized with nature.
Lot of creativity is required to use these dyes judiciously.
Selection of Plant Sources for Dye Extraction
Color like so many other attractive elements in nature attracted and continued to fascinate mankind from the earliest times. They were not content to enjoy color through their eyes. They wanted to feel it enjoy it intimately with which began personal adornment. No doubt flowers and leaves served for a while but they faded and shriveled away shedding their color to use.
India is an advantageous position since the country holds a rich reservoir of natural products. Thus started in India a chapter in chemistry which has over the years grown into specialized area. Different parts (leaves bark seed flowers roots and wood) of number of plants have been reported to yield the dye but a large number of them hitherto remain unexplored. As the uses of natural dyes do not cause pollution it is of immense importance to explore the additional sources of natural dye from rich flora of our country which are abundantly occurring plants. Research work done has led to the new color yielding plants namely Al (root) Alkanet Amaranthus (Flower) Balsam (Flower) Babool (Bark) Bougainvillea (Flower) Canna( Flower) Carthamus (Flower) Catechu Cassia fistula (Bark) Cosmos (Flower) Eclipta ( Weed) Eucalyptus (Bark) Gompherena (Flower) Hibiscus (Flower) Hollyhock (Flower) Lawsonia (Leaves) Nerium (Flower) Nyctanthes (Flower) Plumeria (Flower) Tectona (Leaves) Terminalia (Bark) and Tulsi (leaves). It is a specialized technique of dyeing of the fabric/ yarn of cotton by natural dyes for the lovely shades using common mordants like alum and salts of iron tin and chrome with good wash fastness and light fastness. It is really stands for the pride and glory of the craft of India as this art of natural dyeing has been in Indian as this art of natural dyeing has been in Indian culture from a very long time. Because of the beauty of its results those who use them claim that no chemical dye has that luster the under glow of rich color that delicious aromatic smell and the soft light and shadow that gives so much pleasure to the eyes. These colors are alive as all beauty is alive.
EXTRACTION OF COLORANTS
Natural or vegetable sources like leaves fruits seed bark etc. could be used as such for dyeing of textile materials. The dye matter has to be extracted in any one of the following methods.
Plant part were soaked in water and heated at 60°C for certain time. The colorant present in plant part were transferred to the aqueous solution. Then the dye solution extracted from the sources were filtered and collected.
In case of balsam for aqueous extract 40 gm flower soaked in 150 ml of water for 2 hours and give dark peach dye extract.
For Bougainvillea s extract 5 gm of bracts in 100 ml water leave for one hour in sonicator. These bracts give bright magenta color.
5gm of cassia fistula s bark powder was taken in 100 ml water and boiled it. Extraction takes one hour and gives dark brown color extract.
Dark yellowish orange extract were obtained from 5 gm of frozen cosmos flowers dipped in 100 ml water till all dye bleached out from the flowers at room temperature or on mild heating.
But some sources give different color in different medium e.g. Bougainvillea. In acidic medium and basic medium it gives dark magenta and bright yellow respectively.
Sometime colorants which are present in natural sources do not come in the aqueous medium. For that case soxhlet was used to extract the natural sources in organic solvent. Mainly the solvent used was methanol. Plant parts are cut in to small pieces and were refluxed in soxhlet in methanol till it discharged color. The process takes 4 - 6 hours. This method was used for balsam cosmos and tulsi.
5 gm balsam flower gives deep peach color in 4 hours with soxhlet extraction in methanol.
Dark yellowish orange extract was obtained from 5 gm flower refluxed with 150 ml methanol.
5 gm tulsi took 6 hours in soxhlet extraction and give dark green color.
EQUIPMENT USED FOR DYEING AND ANALYSIS OF DYED FABRIC AND THEIR PRINCIPLE
Model Make Julabo-SRO5
The sonicator used is of 20 KHz frequency which is found to be suitable for introducing cavitations. High energy ultrasonic waves cause cavitations. When dye bath is irradiated with high energy ultrasonic cavitations occurs which releases considerable amount of energy and collapse of the bubbles. This increases with the surface tension at the bubble interface and decreases with the vapor pressure of the liquid. Since the aqueous dye bath has water which has comparatively high surface tension it is very effective medium for cavitations. Interestingly cavitations in alcohol solution (methanolic extract of dye in our case) is considerably high because of increase in vapor pressure. Here the later plays a dominant role. We thus were interested in looking into this aspect of dyeing cotton.
ULTRAVIOLET AND VISIBLE SPECTROPHOTO-METER
Model Make Perkin Elmer Lambda 40
When a material is illuminated by light specific wavelengths are absorbed depending on the molecular structure present. This is caused by electrons in the ground state molecule absorbing light energy and moving to an excited state. The absorption intensity depends on the wavelength and the absorption spectrum (curve measuring absorption intensity changes accompanying wavelength changes for monochromatic light illuminating a material) is characteristic of a specific material. Analyses of materials based on this principle are called absorptiometry. These analyses can be used for various purposes such as
Electronic state analyses
In addition molecules absorbing light and entering the excited state later lose energy through thermal dissipation collisions with other molecules or other processes and return to the ground state. These processes are called radiation less transitions and may include the emission of the absorbed light energy in the form of light. Such re-emission processes include fluorescence and phosphorescence. Analyses using these phenomena are called fluorescence photometry.
Bouguer-Beer law a basic principle of quantitative analysis is also called the Lambert-Beer rule. The following relationship is established when light with intensity Io is directed at a material and light with intensity I is transmitted. Here k is proportionality constant. In this instance the value I/Io is called transmittance (T) and then value I/Io X 100 is called transmission rate (T%). The value log (1/T) = log (Io/I) is called absorbance (Abs).
As can be seen from the above formulas transmittance is not proportional to sample concentration. However absorbance is proportional to sample concentration (Beer s law) along with optical path (Bouguer s law). In addition when the optical path is 1cm and the concentration of the target component is 1mol/l the proportionality constant is called the molar absorption coefficient and expressed using the symbol å. The molar absorption coefficient is a characteristic value of a material under certain specific conditions.
Finally stray light generated light scattered light and reflected light must not be present in order for the Bouguer-Beer rule to apply.
FOURIER TRANSFORM INFRA RED SPECTROSCOPY
Model make Bruker Vector 22
The IR absorption spectroscopy is based on the absorption of infra red radiation by molecules and is most widely used for the identification of the organic compounds. The atoms in molecules vibrate constantly in a variety of stretching and bending motions. The different types of motion are called vibrational modes. Atoms that are connected by covalent bonds can stretch or bend at natural resonance frequencies which depend on the strength or stiffness of the bonds.
The double and triple bonds are stronger than a single bond and have correspondingly higher energies of vibration. Similarly stretching modes have higher energies than bending modes for the same atoms. These vibrational modes can be excited to higher energy states which cause the atoms to vibrate with greater amplitude that is a greater displacement from their average position. Vibrations can be excited by increasing the temperature or by absorption of photons of the appropriate energy. The energies of the vibrational modes are quantized can be excited only with discrete amounts of energy. A photon that has the same energy as vibration is said to be in resonance with that vibration and can be absorbed.
Infrared radiation typically 4 000-400 cm-1 is in the energy range that can excite molecular vibrations to higher vibrational energy levels. When an IR photon of the same energy as a vibrational mode passes by a molecule the molecule can absorb that IR photon. The energy of the photon is converted to greater vibrational amplitude. Eventually this energy is transferred to the surroundings e.g. by collision with solvent molecules resulting in an increase in the temperature of the sample. Since different functional groups in molecule vibrate at distinctly different frequencies IR spectra are useful for qualitative identification of molecular compounds.
GAS CHROMATOGRAPH MASS SPECTROMETER
Model make Finnigan Fison
The GC-MS is an analytical instrument combining a gas chromatograph (GC) and a mass spectrometer (MS). Gas chromatographs have excellent separation capabilities and achieve a very high degree of separation when using a capillary column. Mass spectrometers can be used to determine the molecular structure of a substance from the mass spectrum of the substance. FID and ECD conventional gas chromatograph detectors were only able to provide qualification information of the retention time. Mass spectrometers excel at qualifying components and when used as a detector of gas chromatograph the mass spectrum data can also be used at times other than during the retention time making it possible to perform peak identification and quantification.
A mass spectrometer consists of the ion source quadrupole mass separator Ion detectors and a data-processor. Each of the sample molecules separated by the gas chromatograph are introduced into the ion source. In the ion source thermoelectrons emitted by the filament accelerate to 70eV and collide with the sample molecules (electron impact ionization method EI). As a result an electron is knocked out of the sample molecules creating positive charged ions (molecular ions). At the same time energy from the electrons is transferred to the sample molecules so the molecular ions split creating positive charged ions. This process is called fragmentation and the decomposed ions that result from this process are called fragment ions. The molecular ions and fragment ions formed in this way are pulled from the ion source as an ion bundle and introduced into the quadrupole mass separator. The quadrupole mass separator consists of four metal rods (electrodes) that are parallel with respect to each other. The ion bundle is introduced into the units in the direction of the long axis. The relative electrodes are charged with a direct current with a 180° phase shift and the electrodes are also in a high-frequency electric field. The mass to charge number ratio (m/z) of the ions that pass through the path between the electrodes in the direction of the long axis is dependent on the voltage applied to the electrodes. The ions that can pass through the unit are detected and amplified in the detector. This information is then converted into electric signals that are processed by the data-processor.
There are two methods of measurement used by mass spectrometer the scanning method and the selective ion monitoring method (SIM method). With the scanning method a specific mass number range is scanned at set intervals to measure the mass spectrum. This method is used to obtain total ion chromatograms mass chromatograms and mass spectrums. With the SIM method the type of ion to be measured is set in advance of measurement and this limitation results in improved sensitivity. This method is used primarily to quantify trace components.
CHEMISTRY OF DYE
BASIC CONCEPT OF DYES COLOR
Color is not an independent phenomenon it is the human detection and perception of electromagnetic radiation. Dyes are aromatic organic compounds and as such are based fundamentally on the structure of benzene. Benzene appears to be a colorless fluid. In fact it absorbs electromagnetic radiation just as dyes do but it does so at about 200 nm so that we do not see it.
The perception of color is an ability of some animals including humans to detect some wavelengths of electromagnetic radiation (light) differently from other wavelengths. Normal daylight or white light is a mixture of all the wavelengths to which we can respond and some to which we cannot in particular the infrared and ultra-violet rays. We respond to wavelengths between about 400-700 nm. When an object absorbs some of the radiation from within that range we see the waves that are left over and the object appears colored. In reality this range we see makes up only a very small fraction of the electromagnetic spectrum.
In scientific terms there is nothing special about the wavelengths in the visible range other than being the major components of sunlight which are not removed by the earth s atmosphere. Their special importance is based exclusively on the ability of human retinas to respond to them and to discriminate between them to a significant degree. These discriminations are what we call color.
Wavelengths just outside the visible range are considered colorless even though there is no substantive difference between them and the limiting wavelengths inside the range. Some animals (bees for example) can see these other wavelengths but because humans do not we consider them colorless. The point is that color is a subjective phenomenon and thinking of color as something objective is misleading. For that reason we should refer to the wavelengths involved rather than describe the human response to them.
When some of the wavelengths found in white light are absorbed then we see what is left over as colored light. The color that we see is referred to as the complementary color of the color that was removed. For instance if the red rays are removed from white light the color we detect is blue-green. Blue-green is complementary to red and red is complementary to blue-green.
The perception of color is merely a human evolutionary adaptation to the absence of some wavelengths in white light. Suppose however that the same thing happens outside the range to which our eyes respond. Suppose a chemical removes radiation which has a wavelength about 200 nm as benzene does.
RELATION BETWEEN COLOR AND CONSTITUTION
Like the physical and chemical properties of organic compounds there is a definite relationship between the color and constitution e.g. Benzene is colorless whereas its isomer fulvene is colored. The following theories have been proposed to explain the observed general relationships existing between color and constitutions.
Witts Theory (Chromophore-Auxochrome Theory)
In 1876 Witt put forward a theory according to which the color of a substance is mainly due to the presence of an unsaturated group known as chromophores (Greek chroma - color and phores - bearing). The important chromophores are
C = C -
C = N -
C = O -
N = N -
The compounds possessing chromophores are known as chromogens. The chromopheric groups are of the following two types.
Witt also pointed out another type of groups which while themselves are unable to produce the color but can deepen it if the molecule possesses a chromophore. To such group he called auxochrome (Greek auxein-to increase and chroma-color). The important auxochrome are Acidic -OH SO3H -COOH Basic NH2 NHR NR2 etc.
Since the auxocromes are capable of forming salts either with a basic or acidic groups their presence also convert a colored compound (devoid of salt forming groups) into a dye which must fix permanently to the fiber i.e. it must be fast to water light soap and laundering when fixed to the fiber. The permanent fixing of dye to the fiber is generally due to the formation of chemical bond between the fiber and the auxochrome. This can best be exemplified by the following examples.
Armstrong Theory (Quinonoid Theory)
Armstrong in 1885 suggested that all coloring matters may be represented by quinonoid structures (p- or o-) and thus believed that if a particular compound can be formulated in a quinonoid form it is colored otherwise it is colorless. Some of the important compounds the coloring properties of which can be explained on the basis of this theory are given below.
On the basis of this theory we can see that benzene is colorless where as benzoquinones are colored.
But the quinonoid theory is not sufficient to account for the coloring characteristics of all the compounds. For example iminoquinone and di-iminoquinone both posses a quinoid structure even then they are colorless.
On the other hand a number of colored compounds are known which cannot be represented by quinonoid structures e.g.
The above two theories were discussing the relationship between color and constitution are found to be only empirical. The next two important theories which explain plausibly the relation between color and constitution require somewhat theoretical background about the effect of light on the molecule.
Valence bond theory
Molecular orbital theory
Valence bond theory The various postulates of this theory are as follows
Chromophores are groups of atoms the ð-electrons of which may get transferred from ground state to excited state by the absorption of radiation thus producing the color.
Auxochromes are groups which tend to increase resonance by interacting the unshared pair of electrons on nitrogen or oxygen atoms of the auxochromes with the ð electrons of the aromatic ring. This increase in resonance increases the intensity of absorption of light and also shifts the absorption band to longer wavelength. Hence there occurs the deepening of the color. From this it is evident that increase in resonance must deepen the color and actually it has been found to be so.
The dipole moment changes as a result of oscillation of electron pairs. The following order has been observed for the case of excitation of different groups.
Resonance theory explains the relation of the color and the symmetry of the molecule or transition dipole of the molecule because as the number of charged canonical structures increases the color of the compound deepens. The more the possibility and longer the path for a change to oscillate in a compound the longer wavelength of light will be absorbed and therefore deeper would be the color of the compound.
Resonance theory also explains that pure p-nitrophenol is colorless but yellow in alkaline solution nitrophenol exists as nitrophenoxide ion in which only the charged structures are contributing to the resonance hybrid and therefore the compound absorbs light of higher wavelength.
Molecular orbital theory According to this theory the excitation of a molecule means the transference of one electron from an orbital of lower energy to that of higher energy. These electrons may be s p or n (non-bonding) electrons. The higher energy states are commonly known as anti-bonding orbitals. The anti-bonding orbitals associated with s and p bonds are called s * and p* orbitals respectively. However there are no anti-bonding orbitals associated with n (non-bonding) electrons because they do not form bonds. Chart of the simplest form the essential types of energy are given below.
The electronic transitions can occur by the absorption of ultraviolet and visible radiation. Although transitions are possible only the following types are allowed
A transition takes place when a bonding s -electron is excited to an antibonding s -orbital i.e. s*. This type of transition requires a very large amount of energy as s -electrons are very tightly bond. Hence the compounds like saturated hydrocarbons which do not have any p or s electrons may undergo only s ® s* transitions. However these transitions do not take place by absorbing in the ordinary ultra-violet region e.g. ethane absorbs at 135 mµ.
CHARACTERIZATION OF NATURAL DYES
Studies on the analysis/ identification of natural dyes started as early as nineteen hundred thirties [Pfister 1935 and Knecht et al. 1893]. A French chemist Pfister used a micro chemical analysis in which he achieved the result by color reactions with different chemicals. Abraham et al. 1964 reported a method using infrared structural analysis. Many workers have used thin layer chromatography to identify natural dyes in textiles [Karbade and Agarwal 1985]. UV- Visible spectroscopic studies were carried out by Schweppe 1988. Identification of dyes in historic textiles using chromatographic and spectrophotometric methods as well as by using sensitive colours reaction was done by Schaffe. A non-destructive method was reported of faded dyes on textiles fibers through examination of their emission and excitation spectra. High performance liquid chromatography (HPLC) has been used [Walker et al. 1986] to identify synthetic as well as natural dyes.
Various techniques that are used for characterization of natural dye are given below
Solubility of extracted dyes is determined in different solvents extraction is done on the basis of the polarity e.g. ether methanol alcohol acetone ethyl acetate dilute acid and alkali. The solubility is determined both at room temperature and higher temperature (50-60°C).
Thin Layer & Column Chromatographic Studies
Thin layer chromatography is a versatile technique for identification of natural dyes T.L.C. studies carried out on dye extract using suitable eluent system for a specific dye. The spots are visualized in visible light as well as in iodine chamber. The possible constituents of the extracts are identified by comparing the TLC data i.e. color of the spot and Rf values of known compounds. Column chromatography is used to separate the colored components form single dye or mixture of dye after eluting with a suitable solvent. This is also used as clean-up procedure for the subsequent instrumental analysis.
Ultra Violet-Visible Spectrophoto-Metric Studies
The dye is dissolved in a suitable solvent system and scanned through UV-Visible spectrophotometer. Identification of the dye by this method involves as empirical comparison of the details of the spectrum i.e. maxima and minima point of the unknown with those of pure compounds. A close match is considered to be good evidence of the chemical identity particularly if the spectrum contains a number of short and well-defined peaks.
Fourier Transform - Infra-red Studies
Functional groups were identified by the FT-IR of purified dye extract. Major peaks were identified for different types of C-C C-H C-O stretching and bending vibrations. Absorption in the infrared region is due to molecular vibration of one kind or another the spectrum is generally very complicated and contains many absorption peaks.
High Performance Liquid Chromatographic Studies
The applicability of high performance liquid chromatography (HPLC) to analyze ancient textile dyes was first successfully demonstrated [Wouters 1985]. HPLC linear gradient elution method was first described for the analysis of Indigoid dyes [Wouters and Verhockem 1991]. Identification of blue and purple indigoid dyes was also described using HPLC techniques [Wouters 1991]. Analysis of manjistha alizarin turmeric sandalwood etc. were carried using HPLC techniques [Bhattacharya 1999].
Gas Chromatography-Mass Spectrophotometric Studies
Gas chromatography with mass spectrophotometer (GC MS) is an important detection method for natural products providing chemical fingerprints from the peaks. Electron impact source (EI) and automated library searching makes chemical identification easy. The gas chromatograph serves a method to separate a mixture so that they enter the mass detector one at a time for identification.
MORDANTS USED IN DYEING
Mordants [Chattopadhyay et al. 1997] are considered as an integral part of the natural dyes or to be more precise the natural dyeing process by the dyers of natural dyes. This is an anomaly which continues to be perpetuated by different authors and practitioners of natural dyeing.
A close look at the chemical structures of the natural dyes isolated would show that these dyes are capable of forming complexes with metals or not. Many natural dyes have good affinity for the fiber however their uptake as well as hue can be further modified by pre treatment or post treatment with the metal salts called mordants. Among the naturally occurring mordants are the tannins. Pre treatment with tannic acid followed by metal salt treatment to cotton introduces additional hydroxyl and carboxyl groups in the fiber. These groups by themselves can only increase the dye uptake. A subsequent treatment of the tannin treated cotton with metal salts such as alum introduces aluminium ions in the fiber. The tannin treated cotton at the hydroxyl or carboxyl groups absorbs these ions either by forming metal-complex or metal salts. These metal ions then provide sites for the mordant dyes. Hence introducing metal ions in the fiber either directly or as tannin-metal complexes can increase the affinity of cotton towards mordant dyes. It is apparent that the tannins by themselves do not act as mordants but tannin-metal salt combination can only act as a mordant for the natural dyes.
There are different mordants like alum stannic chloride stannous chloride ferrous sulphate oxalic acid and zinc oxide are used in the vegetable color dyeing and printing. Out of the above mordants alum is the major mordant which was used from the beginning in the vegetable colors in India particularly in Machilipatnam and the Coromondal coast. The dyers and printers of this area have been using only alum as the mordant to obtain different colors. Some dyestuff will yield a variety of colors with different mordants.
The different tones of colors can be obtained depending upon the amount of mordant applied. If alum in a different strength in different places in the same places in the same fabric With the same fabric in a vat of pomegranate bark myrobalan flower moduga flower and bark of mango tree give different yellows in different tones. If you develop the same fabric in a vat ratanjot you can get greys and khakies. If one develops the same fabric in a vat of suridi chekka one can get different vats. Thus using only alum as mordant one can get many different colors has been used as the main link between the color and fiber. Therefore without a mordant there is no color in vegetable dyestuffs.
Mordant are classified in three classes.
Tannins and tannic acid
Metal salts or metallic mordants
Oils or oil mordant
TANNINS AND TANNIC ACID
The term tannin was introduced by Seguin in 1796 to describe the substances present in number of vegetable extracts which are responsible for converting putercible animal skins in to the stable product by tanning process.
The use of vegetable tannins in the manufacture of leather probably predates recorded history and there is creditable evidence that they were in use in Egypt as far back as 5000 B.C.
Tannin [Wali and Razdan 1970 and 1999] may occur in almost any part of a plant including roots stems or trunk bark leaves fruit and even hairs. It may occur either in isolated individual cells in groups or chains of cells (the more common occurrence) or in special cavities orsacs. It may also be present in latex vessels and lactiferous tissue accompanied by other substances.
Dyeing with natural dyes tannins play very important role. It improves the affinity of fibers towards different dyes. With different natural dyes it gives different shades like yellow brown grey and black.
Tannins are complex organic materials and frequently have very large molecules and high molecular size still not certain whether they might better be considered macro-molecular substances i.e. those with very large molecules and high molecular weights which break down into smaller fragments. Tannins were at one time classed with glycoside because of the sugar groups that most of them contain but they are now more often regarded as constituting a class by themselves as some e.g. the hemlock tannins do not have the sugar group in the molecule. In addition to C H and O some N P as well as traces of inorganic ions may be present. Because they are extremely complex substances vegetable tannins are difficult to classify however they may be divided into two different classes on the basis of the type of phenolic nuclei present and the process they are linked together.
Hydrolyzable tannins are identified by having as a core of polyhydric alcohol such as glucose. These hydroxyl groups are esterified either partially or wholly by galic acid or its congeners. The class of this type tannin can be readily hydrolyzed by acids bases or enzymes and they give carbohydrate and a number of isolable crystalline phenolic acids as a product. The ellagic acid isolated from hydrolyzable tannins. Gallotannins and ellaitannins are two groups of the hydrolyzable tannins of vegetable origin.
Condensed tannins contain only phenolic nuclei on reaction with hydrolytic reagents it gets polymerize particularly in acidic medium to yield insoluble amorphous often red colored known as phlobaphenes. Mostly condensed tannins are formed by the condensation of two or more molecules of flavan-3-ols like catechin.
METAL SALTS OR METALLIC MORDANTS
Originally only naturally occurring metal salts were used as mordants. Now-a-days metal salts of aluminum tin iron copper and chromium are used. Some of the common mordants are alum stannic chloride stannous chloride ferrous sulphate copper sulphate and potassium dichromate.
Alum The name alum was originally coined to denote their double sulphate of aluminum and potassium which crystallize in the shape of octahedral with 24 molecules of water crystallization. This salt is also called potash alum (Al2K2(SO4)4. It contains about 36 % aluminium sulphate. The name alum was later extended to a whole class of double sulphates of analogous constitution and isomorphous form. Thus a great number of alum is known such as
For mordanting 10 kg of cotton 1 kg of alum and 0.5 Kg of sodium hydroxide are dissolved in 200 litres of water give basic aluminium sulphate of required strength.
Two different methods are employed mordanting cotton with basic aluminium sulphate. They are as follows
The material is impregnated directly with the mordant and the later fixed by other chemically or by ageing.
The material is impregnated with a substance which attracts the basic sulphate and forms and insoluble compound with alumina.
Stannic chloride Stannic chloride is prepared by the oxidation of stannous chloride with chlorine or potassium chlorate. It is highly soluble in water. It is an important mordant for cotton and silk.
Stannic chloride is extensively used as mordants for cotton. Many natural dyes e.g. logwood fustic quercitron and weld are fixed on cotton with stannic oxide produced on mordanting. Generally the material is first mordanted with tannin and then worked in a dilute solution of stannic chloride and finally washed before dyeing in these cases tannic acid present in the tannin acts as a fixing agent for the stannic hydroxide which is the actual mordant.
Stannous chloride Stannous chloride is prepared by dissolving tin in hot hydrochloric acid.
From the aqueous solution monoclinic crystals are obtained which contain two molecules of water of crystallization and which are known commercially as tin crystals SnCl2.2H2O. Stannous chloride has fairly high solubility in water. The clear solution becomes turbid on dilution with water insoluble basic stannous chloride is formed
Tin crystals are oxidized in the same way on exposure to the air. Stannous chloride is very powerful reducing agent and is used in discharge printing. Due to its being very powerful reducing agents it can not be used along with oxidizing mordants such as copper sulphate. The natural dyes which are susceptible to reduction should not be dyed on the tin mordants lest they may get decolorized.
Stannous salts are not frequently employed as mordants on cotton. Persian berries yield a good yellow to orange shade on the materials previously mordanted with tannin and stannous chloride.
Ferrous sulphate Ferrous sulphate FeSO4.7H2O is also known as Copperas or Green vitriol. Ferrous sulphate is prepared on the large scale from iron pyrites. It is also obtained as a by-product in various manufacturing processes. Ferrous sulphate is readily soluble in water 100 parts of water dissolve 115 parts of the crystallized salt FeSO4.7H2O at 25°C. Solution of ferrous sulphate do not dissociate either on heating or on cooling. It is one of the most important and one of the oldest mordants known and is still extensively employed.
Iron salts are very extensively applied in dyeing and printing. The application of iron salts in dyeing for mordanting purposes and for increasing the weight of the silk which is to be dyed black is of great importance.
Cupric Sulphate Cupric sulphate (CuSO4.5H2O) is also known as blue vitriol or copper sulphate. Cupric sulphate is manufactured by roasting ores which contain copper and by dissolving them in sulphuric acid. From this solution crystals having different degrees of purity are obtained. Cupric sulphate crystallizes in transparent blue triclinic crystals which contain 5 molecules of water of crystallization. It is readily soluble in water and 100 parts of water dissolve 49 parts of cupric sulphate at 30°C
The light fastness of many colors is vastly improved by an after treatment with copper sulphate e.g. Copper salts are employed in cotton dyeing as oxidizing agents for the production of cutch browns and logwood blacks. Copper sulphate is frequently employed in the dyeing of black shade on cotton and for this purpose it is fixed by means of tannin. However copper sulphate is not considered as eco-friendly mordant.
Potassium dichromate Potassium dichromate is also referred to as Red chromate or Dichromate of Potash or chrome. It is manufactured by heating chrome iron ore with lime and potash when the atmospheric oxygen oxidizes the chromium oxide chromates of calcium and potassium are formed. Potassium dichromate crystallizes in the form of large orange triclinic prisms.
For the successful application of chrome mordant it is essential that dichromate be reduced to chromic oxide before the mordanted material is dyed. In case the goods are treated with dichromate after dyeing the chromic acid produced acts as the oxidizing agent and chromic oxide which is thus generated acts as the mordant. This is also not eco-friendly.
Oil mordants are used mainly in the dyeing of turkey red color from maddar. The main function of the oil mordant is to form a complex with alum used as the main mordant. Since alum is soluble in water and does not have affinity for cotton it is easily washed out from the treated fabric. The naturally occurring oils contain fatty acids such as palmitic stearic oleic ricinlic etc. and their glycerides. The sulfonated oils which possess better metal binding capacity than the natural oils due to the presence of sulfonatic acid group binds metal forming a complex with the mordant dye to give superior fastness and hue.
TECHNIQUES USED FOR DYEING
Conventional dyeing is carried out by boiling the fabric in dye bath for 4-5 hours and often the dye uptake is still not complete. Enormous amount of heat is consumed in terms of heating the dye bath.
Utilization of ultrasound energy to aid wet processing of fabrics particularly is well documented in the literature [Datar et al 1996]. The process of increasing dye transfer from the dye-bath to fabric using ultrasound energy is a function of the acoustic impedance characteristics of the fabrics. Fundamental investigation of ultrasonic effects in textile wet processing by [Beckham et al 1996] shows many potential advantages such as-
Energy saving by dyeing at lower temperatures and reduced processing times
Environmental improvements by reduced consumption of auxiliary chemicals
Process enhancement by allowing real-time control of color shade
Increased color yields thereby causing lower overall processing costs
In ultra-sonication the following phenomena are known to occur-
Decreasing aggregation of particles in solution
Destruction of the diffusion layer at dye/fabric interfaces
Generation of free radicals
Dilation of polymeric amorphous regions
Enhancing transport of the dye to the fiber surface by reducing the boundary layer thickness.
Microwave dyeing takes into account only the dielectric and the thermal properties. The dielectric property refers to the intrinsic electrical properties that affect dyeing by dipolar rotation of the dye and the influence of microwave field upon dipoles. The aqueous solution of dye has two components which are polar. In the high frequency microwave field oscillating at 2450MHz it influences the vibrational energy in the water molecule and the dye molecules. This causes frictional heating while materials other than water may be dipolar or may behave as dipoles due to the stress of the electric field water usually dominates probably because it is pervasive and at high concentrations in dye mixtures. The other heating mechanism is ionic conduction which is a type of resistance heating depending on the acceleration of ions through solutions and resulting in collision of dye molecules with molecules of the fiber [Majetich et al 1995].
MECHANISM OF DYEING
Natural dyes work best with natural fibers such as cotton linen wool silk jute and sisal [Gulrajani et al. 1999 and Gupta 2001]. Amongst this wool is by far the easiest to take up dyes followed by cotton linen silk and then the coarse fibers such as sisal and jute. Nearly every plant will yield some of color whether we use leaves bark wood roots or fruits. Nearly all require or are enhanced by some sort of mordant. The trick then is to determine which plants or which part of the plants will give not only beautiful tones but colorfast shades as well. A coloring material that has the strength to bind itself to a fiber and remain there by staining the fiber is considered to be the best.
So natural dyes can neither dye animal nor vegetable fibers directly but require a mordant which depends upon the nature of the dye. If the dye is acidic the mordant must be basic (the most common basic mordants are the salts of Cr Al Sn Fe) on the other hand if the dye is basic the mordant must be acidic (the most common acidic mordant is tannin or tannic acid containing some amount of tartar emetic). The fabric to be dyed is first soaked into a solution of the metallic salt (i.e. mordant) and then steamed or other wise treated to form the insoluble metallic hydroxide. The fiber so obtained is known as mordanted fiber it is dried then placed in a solution of the dye when the latter is held by the hydroxide of the metal on the fiber by means of chelation. Such chelated complexes can be formed only when the resulting dye has a five- or six- membered ring which is again possible only when OH group in the dyestuff is present ortho to one of the following groups
The chemistry of bonding of dyes to fibers is complex it involves direct bonding H-bonds hydrophobic interactions. Mordants to this effect increase binding of dye to fabric by forming a chemical bridge from dye to fiber.
The mordant has affinity for both fiber and the dye. Thus those dyes which do not have any affinity for a fiber can be applied by using mordants. Thus improves the staining ability of any dye along with increase in fastness properties. Mordant forms an insoluble compound of the dye within the fiber. The mordant dyes include those differing widely in the origin but those form more or less insoluble compounds with metal salts. Presence of certain functional groups in suitable position in the dyes molecule helps in coordination of the metal salt. Generally either two hydroxy groups ortho to each other or one hydroxy groups ortho to carbonyl nitrso or azo groups are the main features of mordant dyes. They produce a wide range of hues of remarkable resistance to wet treatments but the shades lack brilliancy.
A chromium atom can combine with alizarin by covalency and co-ordinate valency to form the Lake. In the first step chromium combines with the hydroxy groups in the same way as sodium combines with phenol to form sodium phenate. In the subsequent step the oxygen atom of the adjacent quinone group donates a lone pair of electrons to the chromium atom and forms a co-ordinate bond. Chromium being trivalent combines with three molecules of alizarin. An example of direct bonding is given below.
With protein molecules the interaction between the dye and the fabric of the following type where the polypeptide linkages have H-bonding with the dye. A representative dye example of Alizarin has been shown to demonstrate the nature of bonding.
A substance which is resistance to light water and soap called dye. So it is a fundamental requirement that colored textile should withstand the conditions encountered during processing following coloration and during their subsequent useful life [Knecht et al. 1933 Mayer and Cook 1943 Shore 1990 Venkataraman 1978 Stevens 1979 Grierson et al. 1985 Nasu et al. 1985 Forrester 1975 Taylor 1986 Sewekow 1988 Gulrajani et al. 2001 and Seerangarajan and jayabal 2001]. When a colored textile is subjected to particular conditions e.g. light or washing one or more of several things may happen. As far as the color of the material is concerned there may be alteration in hue value or intensity. In certain cases there may be alteration in all three. Thus a red material may become pales yellowish and duller. Further under certain conditions e.g. during washing adjacent white material may become colored and colored material may acquire new color due to the transfer of dye from the original dyed material.
The color fastness of a colored textile is therefore defined as its resistance to these changes when subjected to a particular set of conditions. It follows that color fastness must be specified in terms of thee changes and expressed in terms of their magnitude. Fastness properties are divided in to two classes.
Fastness properties of natural dyes
Fastness properties of dyed material
Fastness Properties of Natural Dyes
In early times clothing was infrequently washed and the fading of colors on clothing was accepted as inevitable. A study of the older literature shows that early in the history man was aware of the fleeting nature of natural dyes available to him and was perpetually making efforts to improve the fastness properties of these dyes. Pliny writing in the first century AD records in great detail a method by which the Egyptians smeared white cloth with a series of colorless drugs (Mordants) and plunged the whole into boiling dye-bath. After the dyeing was complete the cloth was multi-colored the variations in hue being dependent on where each drug had been placed. There is evidence that Egyptians had learned this technique of mordanting from India. This is as far back as first century man was using mordants to improve the fastness of this dyeing and for shade development.
Although mordanting and certain after treatments improved fastness the inherent instability of the chromophores of the natural coloring matters resulted in low fastness to washing and light. Old textiles dyed with natural dyes have acquired an overall brownish hue. Greens produced by over-dyeing indigo with higher light fastness of indigo component. These effects are readily observed in old tapestries.
In recent as well as classical studies it has been reported that some natural dyes have not very good light stability and hence the colors in museum textiles are often different from their original colors. It has also been observed that some natural dyes undergo marked changes in hue on washing shown to be attributable to even small amounts of alkali in washing mixture highlighting the necessity of knowing the pH of alkaline solutions used for cleaning of textiles dyed with natural dyes.
The light and wash fastness ratings assessed with the blue/ grey scale of some important natural dyes on wool have been tabulated.
From the above information it is evident that most yellow dyes have a fastness grading equal to or less than 5. But some red blue brown and black vegetable dyes exhibit good to excellent fastness on wool. The differences in fastness characteristics as influenced by various factors for different classes of dyes. Different colors are derived from different sources. Based on color the dyes can be categorized as follows
Yellow Dye Yellows obtained from plant materials are usually pale i.e. the depth of shade is low and so the fading is quicker. The few dyes which give full deep yellows such as turmeric and beriberi are rendered susceptible to light because they emit fluorescence. Thus the brighter a yellow shade is the less fast it is to light.
About ninety percent of all yellow dyes are flavonoids the fading of these dyes to yellow brown hue in museum textiles can be attributed to their inherent tendency to form quinones on exposure to light.
Their fastness is also affected by the nature and position of substituents generally hydroxy on the chromophore the substituents can shift the fastness either way. For example luetolin the famous yellow dye was extracted by ancient Europeans from weld it has light fastness rating much higher than that of its 3-hydroxy counterpart quercetin (flavonol) found in sunflower chrysanthemum etc. This is due to the extreme photosensitivity of the -OH in position 3.
Mordants greatly influence the fading of yellow dyes. Use of tin and alum mordants results in significantly more fading than when chrome iron or copper mordant are used as in case of tessu dolu and onion. Indeed some researchers have concluded that the mordant is more important than the dyes itself in determining the light fastness of colored textile. Wash properties of yellow dyes range fair to excellent.
Red Dyes Natural red dyes are almost invariably based on anthraqunone and its derivatives. The light stability of this chromophere has been well established in synthetic dyes. With synthetic anthraquinones it has also been observed that the nature of substrate and substituents have little effect on their fastness to light.
The hydroxy anthraquinones such as alizarin and its derivatives are fixed on textiles by complexing the ionized o-hydroxy groups with transition metal oxides or salts used as mordants (a). These highly colored complexes comprising of large insoluble molecules are stable to light and washing.
Choice of mordant in some cases may affect the wash fastness for example the wash fastness of cochineal dyeing is much higher when wool is mordanted with chromous acetate than when alum is used as a mordant.
Blue Dyes Blue color among natural dyes is mainly obtained from indigo. An unusual facet of the photochemical behaviour of indigo in the fact that its light fastness on wool (7-8) is generally much higher (3-4 points) than on cotton and 1-2 points higher than on silk. It is so because an oxidative pathway is involved in the fading of indigo-dyed cotton. As stated earlier fading on non-protein substrates is reductive thus the indigoid chromophore which is resistant to photoreduction shows high fastness on wool.
They also exhibit excellent fastness to washing. The dye is applied in the soluble leuco form but once it is inside the fiber the dye gets oxidized to insoluble form and gets firmly held by the fiber.
Brown and Black Dye Brown grey and black shades are generally obtained from plant sources which are rich in tannins e.g. pomegranate skin myrobalan etc. Some yellow dyes such as dolu also give deep rich browns when mordanted with copper salts.
Tannins combine with ferrous salts to form complexes which give a range of grey and grey-brown shades. Since the colors are deep and dark the light fastness is generally good.
Moreover tannin being complex polyhydric phenols of high molecular weight [b c] have large molecules and have good affinity for cellulosic and protein fibers which confers on them good fastness to most agencies including light and washing.
Fastness Properties of Dyed Materials
The factors affecting light fastness of dyed materials such as the nature of dye fiber and state of dye inside the fiber also in general have a similar effect on the wash fastness properties but the physical chemistry determining wash fastness properties is relatively simpler and less challenging than that of light fading.
The affinity of dye for fiber molecules reduces the rate of absorption and desorption from the fiber. The dye-fiber attractive forces tend to keep the dye molecules attached to the fiber molecules and retard their diffusion along the pores of the fiber. The superior wash fastness of metal-complex dyes is due to the ability of dye molecules to associate into large aggregates in the fiber which have low absorption rates and not because of the additional forces of attraction between wool and metal ions as is generally believed.
EVALUATION OF ECO-FRIENDLINESS
Although dyes are derived from nature the metallic salts used as mordants for better dye fixation on textile and to improve fastness are not always eco-friendly [Gill 1993]. Health hazards as well as the environment friendly behavior of natural dyes have been investigated [Sewekow 1988 and Ali 1993]. Very little work has been carried to assess the toxicity of natural dyes. Only one or two have been identified as posing potential problems. For example quercetin is considered to mutagenic [Chavan 1995].
Eco-friendliness of natural dyes is done by assessing the eco-parameters viz. toxic heavy metals pesticides formaldehyde pentachlorophenol azo-dyes based on carcinogenic amines or banned amines etc by analyzing the dye extract.
The mordant that are used for fixation and development of color on textiles are mainly Alum (Potassium aluminium sulphate) Tin (Stannous chloride and Stannic chloride) Iron (Ferrous sulphate) Chromium (Potassium dichromate) and Copper (Copper sulphate). Out of these copper and chrome are red listed and have been restricted to some stipulated limits by various eco-labels. On the basis of analysis of some natural dyes like Katha Jackwood Turmeric and Indigo show the presence of arsenic lead mercury copper and chromium less than 0.2 ppm which is much below the stipulated limit except for chromium. This shows that the natural metal contaminants in the dyes are very low and so can be used safely. But the concentrations of mordants used in dyeing are sometimes very high. Therefore optimization of mordants is necessary.
Contamination of natural dyes and fibers by chorine-based pesticides may occur during the growing of plant from soil or during the storage. The analysis of some natural dyes like Manjistha Jackwood and Indigo by Gas chromatograph using ECD detector.
The presence of any of the banned amines in natural dyes is ruled out because most of the natural dyes whose structures are known as based on quinones flavonoids anthraquinones alkaloids napthaquinone etc. and not based on azo-linkages [Bhattacharya et al. 1995].
So the conclusion is that natural dyes are safe and eco-friendly as textiles dyed with natural dyes are almost free from hazardous chemicals. Red listed mordants may be either avoided or may be optimized as per eco-standard without impairing the desirable properties (e.g. fastness) of the textiles.