Chemistry of Natural Dyes

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Wesley Alexander
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The advent f synthetic dyes caused rapid decline in the use f natural dyes, which were cmpletely replaced by the frmer within a century.

1 GENERAL I ARTICLE Chemistry f Natural Dyes Padma S Vankar Padma S Vankar is at the facility fr eclgical and analytical testing in lit, Kanpur. Nature expresses itself in a wide spectrum f clurs all arund us. The alchemy f clurs started frm an early time. Indians have been frerunners in the art f natural dyeing. The advent f synthetic dyes caused rapid decline in the use f natural dyes, which were cmpletely replaced by the frmer within a century. Earlier understanding f dyeing techniques and their applicatins was empirical and was nt backed by scientific reasning. Natural dyeing had develped essentially as a flk art. Hwever, in recent times the dyeing technique is interpreted n sund scientific principles, and the interactin between the dye and the dyed material is well understd. Frm ur recent wrk, we have tried t explain the nature f bnding in certain dyes by lking at their structures and using metal ins fr chelatin. Surces f Natural Dyes There are primarily fur surces frm which natural dyes are available. Specialised plant and animal surces: Many plants and sme animals have been identified as ptentially rich in natural dye cntents, and sme f them have been used fr natural dyeing fr quite sme time. Varius parts f plants like rts, stems, barks, leaves, fruits and seeds may cntain cluring matter which can be explited. Nrmally natural dyes are extracted frm the rts, stems, leaves, flwers, fruits f varius plants, dried bdies f certain insects and minerals. Sme plants may have mre than ne clur depending upn which part f the plant ne uses. The shade f the clur a plant prduces will vary accrding t time f the year the plant is picked, hw it was grwn, sil cnditins, etc. The minerals in the water used in a dye bath can als alter the clur. Sme natural dyes cntain natural mrdants ~ RESONANCE I Octber

2 GENERAL I ARTICLE By-prducts (especially lac dye): The lac industry gives lac dye as a by-prduct, which is extracted frm the effluent. Similarly, frm Cassia tra, used in gum manufacturing, a brwn dye is btained as a by-prduct. Chemical synthesis: This invlves synthesis f dyes with mlecular structures identical t thse f natural dyes. We have tried t make such synthesis envirnmentally less hazardus by using micrwave technlgy. Tissue r cell culture by DNA transfer bitechnlgy: Certain fungi such as Drechslera and Trichderma prduce anthraquinne derivatives as secndary metablites. As anthraquinnes are a very imprtant class f dyes, expliting the fungi wuld be advantageus ver their chemical synthesis. If genetic mdificatins can be achieved, it is pssible t develp fungi that prduce substituted anthraquinnes. Classificatin f Dyes Dyes are classified based n their structure, surce, methd f applicatin, clur, etc. We briefly describe belw tw types f classificatin. Based n Chemical Structures Indigid dyes: This is perhaps the mst imprtant grup f natural dyes, btained frm Indigfera tinctria. Anthraquinne dyes: Sme f the mst imprtant red dyes are based n the anthraquinne structure. They are btained bth Indig Anthraquinne ~ RESONANCE I Octber 2000

3 GENERAL I ARTICLE OH a-hydrxynaphthquinne flavne frm plants and insects. These dyes' are characterised by gd fastness t light. They frm cmplexes with metal salts and the resultant metal-cmplex dyes have gd wash fastness. Dihydrpyran Alpha-hydrxy-napthquinnes: The mst prminent member f this class f dyes is lawsne r henna, btained frm Lawsnia mermls. Fla'Vnes: Mst f the natural yellw clurs are hydrxy and methxy derivatives f flavnes and isflavnes. Dihydrpyrans: Clsely related t flavnes in chemical structure, are substituted dihydrpyrans like the ne shwn here. Anthcyananidins: Carajurin btained frm Bignnia chica. Cartenids: In these the clur is due t the presence f lng cnjugated duble bnd. Annatt and saffrn are examples f this. OCH3 Anthcyananidin Beta-cartene -RE-S-O-N-A-N-C-E--I-O-ct--be-r ~ '

4 GENERAL I ARTICLE Based n clur Varius natural dyes culd present all the clurs f the visible spectrum. The natural clur and hue f a dye can be altered by treating with metal salts. If the dye is f plant rigin, the clur may vary depending n the sil prperties, part f the plant, seasn f harvesting, cultivatin practices, etc (Table 1). Red clur: Mst red dyes are fund in rts r barks f plants r camuflaged in the bdies f dull grey insects. Unlike the wide abundance f yellw, the surces f red clur are limited. Cchineal is an imprtant red dye and it is the brightest f all the available natural red dyes. Manjith and Kusumbar amng the vegetable surces and lac and Kermiz amng the animal surces give red clurs. Yellw clur: Yellw is the liveliest and perhaps the mst abundant clur in nature. The plants which yield yellw dyes utnumber thse yielding ther clurs. Mrdants Dyes d nt cmbine directly with the material they are in- Table 1. Surces f Different Clured Dyes. Clur Btanical N arne Parts used Mrdant Red Dyes Safflwer Caesalpina * Maddar Lac * Carthamus tinctrius Caesalpinia sappan Rubia tinctrium Cccus lacca (insect) Flwer Wd chips Wd Twigs inhabited by these insects Alum Alum Stannic chlride Yellw Dyes Bugainvillea * Glden rd Teak Marigld Parijata Bugainvillea glabra Slidag grandis Tectna grandis Tagetes species Nyetanthesar brtristis Flwer Flwer Leaves Flwer Flwer Tin Alum Alum Chrme Chrme Table' cntinued ~ RESONANCE I Octber 2000

5 GENERAL ARTICLE Blue Dyes Black Dyes Brwn Dyes Green Dyes Clur Btanical Name Parts used Mrdant Indig Indigfera tinctria Leaves Wad Isatis tinctria Leaves Suntberry Acacia niltica Seed pds Pivet Ligustrum vulgare Mature berries Alum and irn after frst Water lily Nymphaea alba Rhizmes Irn and acid Lac* Cccus lacca (insect) Twigs inhabited Ferrus sulphate by these insects Alder Alnus glutinsa Bark Ferrus sulphate Rfblamala Lranthus pentapetalus Leaves Ferrus sulphate Custard apple Anna reticulata Fruit Harda Terminalia chebula Fruit Ferrus sulphate Caesalpina* Caesalpinia sappan Wd chips Ferrus sulphate Bugainvillea * Bugainvillea glabra Flwer Ferrus sulphate + acid/ neutral Balsam* Impatiens balsamina Flwer Alum Marigld Tagetes species Flwer Chrme Black berries Rubus fructicsus Berries Irn Tulsi* Ocimum sanctum Leaves Ferrus sulphate Bugainvillea * Bugainvillea glabra Flwer Alum + Base and Ferrus sulphate+acid Canna* Flwer Alum and Ferrus sulphate Lily Cnvallaria majalis Leaves and stalk Ferrus sulphate Nettles Urtica dicia Leaves Alum OrangelPeach Dyes Bugainvillea* Bugainvillea glabra Flwer Stannus chlride + Acid; Alum + base Balsam* Impatiens balsamina Flwer Tin Dahlia Dahlia species Flwer Alum / chrme Annatt Bixa rellana Seeds The surces marked with an asterix have been used by us fr the first time fr dyeing cttn fabric mainly by utilising ultrasnic energy in the dye bath. We have even attempted t dye synthetic fabric terryct with lac dye ~ RESONANCE I Octber 2000 n

6 GENERAL I ARTICLE tended t clur. Natural dyes are substantive, requlflng a mrdant t fix t the fabric, and prevent the clur frm either fading with expsure t light r washing ut. These cmpunds bind the natural dyes t the fabric. They are f three types. Metallic mrdants: Metal salts f aluminium, chrmium, irn, cpper and tin are used. Tannins: Myrbalan and Sumach are the cmmnly used tannins emplyed as mrdants in the dyeing f textile fibres. Oil mrdants: Oil mrdants are used mainly in the dyeing f turkey red clur frm maddar. The main functin f the il mrdant is t frm a cmplex with alum used as the main mrdant. The sulfnated ils, which pssess better metal binding capacity than the natural ils due t the presence f sulfnic acid grup, bind t metal ins frming a cmplex with the dye t give superir fastness and hue. Limitatins f Natural Dyes Sme f the natural dyes are fugitive and need a mrdant fr the enhancement f their fastness prperties. Tedius extractin f cluring cmpnent frm the raw material, lw clur value and lng dyeing time push the cst f dyeing with natural dyes cnsiderably higher than with synthetic dyes. In case f sappan wd, prlnged expsure t air cnverts the clurant brasiline t brasilein, causing a clur change frm red t brwn. T vercme this drawback we used a snicatr, and fund that the dye extractin was much faster. Sme f the natural dyes are fugitive and need a mrdant fr the enhancement f their fastness prperties. Sme f the metallic mrdants are hazardus. Bnding Natural dyes wrk best with natural fibres such as cttn, linen, wl, silk, jute, ramie and sisal. Amng these, wl takes up dyes mst easily fllwed by cttn, linen, silk and then the carse fibres such as sisal and jute. Nearly all f them require sme srt f a mrdant. The trick is t chse the right dye frm ~~ R-E-S-O-N-A-N-C-E--I--O-c-t-b-e-r

7 GENERAL I ARTICLE the right surce that gives nt nly beautiful tnes, but clurfast shades as well. The chemistry f bnding f dyes t fibres is cmplex. It invlves direct bnding, H-bnds and hydrphbic interactins. Mrdants help binding f dyes t fabric by frming a chemical bridge frm dye t fibre, thus imprving the staining ability f a dye alng with increasing its fastness prperties. Mrdants frm insluble cmpunds f the dye within the fibre. The presence f certain functinal grups in suitable psitins in the dye mlecule causes its crdinatin t the metal in. Generally, tw hydrxy grups r a hydrxy grup with a carbnyl, nitrs r az grup in adjacent psitins are respnsible fr crdinatin. The mrdant dyes prduce a wide range f hues f remarkable resistance t wet treatments, but the shades lack brilliancy. An example is the chrmium-alizarin cmplex. A chrmium in can cmbine with alizarin by cvalency and c-rdinate valency t frm the 'Lake'. Chrmium, being trivalent, cmbines with three mlecules f alizarin. The fibres made f prteins, such as wl and silk, retain the dye thrugh hydrgen bnding between the plypeptide linkages and the dye. An example f alizarin is given t shw the nature f bnding with Nyln-6, which als has peptide linkages. OH 3 + Cr +++ O~ 1:--0 Alizarin OH --Cr + 3H+ " Frmatin f chrmium cmplexes -R-ES-O-N-A-N-C-E---I-O-c-t-b-e-r ~~~

$GENERAL I ARTICLE OH ----- O==C~ H 2 C" '" /CH 2 H 2 C \ N-H /CH " 2 Hydrgen bnded dye interactin with Nyln-6 Imprving the Quality f Natural Dyes Pr light fastness f sme f the natural dyes is$ We have imprved the washing fastness f natural dyes by treatment with ec-friendly mrdants such as alum, stannic chlride, stannus chlride and ferrus sulphate. We have als used tannins with mrdants.

We have imprved the washing fastness f natural dyes by treatment with ec-friendly mrdants such as alum, stannic chlride, stannus chlride and ferrus sulphate. We have als used tannins with mrdants.

8 GENERAL I ARTICLE OH O==C~ H 2 C" '" /CH 2 H 2 C \ N-H /CH " 2 Hydrgen bnded dye interactin with Nyln-6 Imprving the Quality f Natural Dyes Pr light fastness f sme f the natural dyes is attributable t phtxidatin f the chrmphre. We have tried t prevent!,minimize such phtxidatin by frming cmplex f the dye with transitin metal. We have imprved the washing fastness f natural dyes by treatment with ec-friendly mrdants such as alum, stannic chlride, stannus chlride and ferrus sulphate. We have als used tannins with mrdants. Treatment with metal salts alters the light absrptin characteristics f tannins in additin t making them insluble in water with the fabric acquiring washing fastness. As research in the directin f vercming the limitatins f natural dyes cntinues, we may cnclude by saying that natural, ec-friendly dyes are certainly welcme. Address fr Crrespndence Padma 5 Vankar Incharge Facility fr Eclgical and Analytical Testing(FEAT) Indian Institute f Technlgy, Kanpur , India. Suggested Reading [1] M L Gulrajani,Intrductin t Natural Dyes, Indian Institute f Techn 1- gy, New Delhi, [2] M L Gulrajani and Deepti Gupta, Natural Dyes and their Applicatin t Textiles, Indian Institute f Technlgy, New Delhi, ~ RESONANCE I Octber 2000

Chemistry of Natural Dyes

Chemistry of Natural Dyes Padma S Vankar Padma S Vankar is at the facility for ecological and analytical testing in IIT, Kanpur. Nature expresses itself in a wide spectrum of colours all around us. The