Colorimetric Characterisation and Process Standardisation for Application of Natural Dyes on Textiles: A Research Review

Abstract

Colorimetric characterisation and to standardise the dyeing processes for applying natural colourants (dyes) on different textiles materials and related investigations have been highlighted here with a broad coverage. Lack of sufficient colorimetric characterisation data for different natural dyes led to the researchers to take a stock of total work done in this field. Without characterisation of colour components and process standardisation of specific natural dye and natural fibre combination, reproducibility, uniformity, maximum or optimised colour yield and acceptable grade of colour fastness could not be possible to achieve for natural dyeing of different textile materials. Hence, a systematic review of standardisation of extraction conditions of natural dyes and finishes, type of mordant and mordant application conditions and finally standardised dyeing process variables have been elaborated. Worldwide eco concern of consumers have created scope of newer research for revival of natural dyes and natural finish application on textiles with standardised methods/process conditions to obtain reproducible dyeing results with acceptable colour fastness. Functional finishing of textiles for antimicrobial and UV resistant finishes by natural resource materials are the two relatively newer areas, needing further research. In the concluding remarks advances in natural dyeing and natural finishing have also been dealt with futuristic need to explore.

1. Introduction

Natural colourants (dyes) are extracted from various natural bio-logical resources. The origin may be any parts of vegetable plant or animal. Although, there are exceptions where the source of colouring matters is of mineral origin. ‘The Society of Dyers and Colourists’ (SDC) in their 3rd edition specified ‘Colour Index” as natural colouring/dyeing substances and defined as: “The natural dyes and pigments comprise of all the colours obtained from vegetable and animal matters with no or very little chemical treatments”. These natural colourants are mainly anionic type dyes having mordant-able phenolic-hydroxy or carboxylic groups rendering necessary coordinating capacity with metallic and other type of mordants, with few exception having either cationic nature (barberry) or class of insoluble vat dye type (natural indigo), where they can be classified under application based classification of textile colourants such as anionic direct dyes (turmeric), as cationic basic dyes (barberry), as anionic acid dyes (madder/Manjishtha) with or without mordanting ability, also as non-ionic insoluble vat dye type (natural indigo) and few disperse dye/solvent dyes type (Henna/Lawson) besides few are also available as pigment colour like Azurite/Ash/Limonite [1, 2, 3, 4, 5, 6]. Majority of the natural dyes are non-substantive for most of the textile fibres, which means that they have little attraction towards any fibres i.e., they have no colouring power to dye textiles by applying only themselves, and hence require the aid of some anchoring agent called “mordant”. These mordants along with other additives used for dyeing of textiles materials with natural colourants require to be environmentally-friendly as well. Some of the natural substances have medicative values too and may incorporate either microbicidal or UV radiation protection or both, which need to be explored for natural finishing of textiles along with natural dyeing.

The production process of synthetic chemicals is associated with series of chemical reactions which consumes high energy. In addition, many unwanted bio-hazardous by-products generate during the chemical reactions [6, 7, 8, 9]. These toxic or bio-hazardous by-products are released into different natural streams of water, lakes or in the biosphere. These negative impacts of the man-made synthetic colourants have encouraged researcher to find alternative environmentally-friendly dyes. Now a days consumers are very aware about the ecology and demanding bio-friendly natural products as well as eco-friendly processes like dyeing of natural fibres with natural dyes, which has thus become indispensable. However, all natural colourants may not be necessarily eco-friendly. Hence, ecological hazards associated with any natural colourants should always be examined/asserted before use. The production of man-made dyes involves series of hazardous chemical reactions and produces lot of effluents and objectionable gases causing water and air pollution and are usually conducted at high temperature and pressure causing lot of energy consumption and is carbon negative process, using so many primary chemicals isolated from petroleum derivatives [10, 11, 12, 13], while natural dyes and pigments have manifold advantages over synthetic dyes and pigments, as they are eco-friendly without any toxicity and safe for human. Majority of the natural colourants are evidenced to be ecologically safe without toxicity except few. Hence, although synthetic dyes show better performance, people are more attracted for application of natural colours for textiles due to growing knowledges that most of the natural colours have neither allergic consequences nor toxic effects. In addition, textile products dyed with natural colouring matters are comfortable for the wearer with soothing colouring effect [14, 15, 16, 17, 18, 19, 20].

However, the present scenario there are an estimated around 30 million tonnes of consumption of textiles across the global and such mammoth quantity of textile products may not be coloured with only natural colourants [19]. Hence, the use of ecologically green synthetic dye-stuffs cannot be avoided or fully replaced by natural dyes, but a small part of dyed textiles may be handled by bio-friendly natural colourants, ensuring green measure for colouring for a portion. Application of natural colourants will also render new avenue for engaging rural population because plants of natural colourants are normally grown at country side at the barren lands. So, those waste lands can be used to grow the respective dye producing plants or herbs [19]. However, for reproducibility, acceptable colour fastness and uniform dyeing in either single or compound shade using combination of two or more combination of natural dyes, standardisation of dyeing input variables and analysis of compatibility for use of blend of these natural colours are crucial [21, 22].

Today, when the world is endangered with environmental pollution-oriented destruction from the man-made toxic chemicals (including hazardous synthetic dyes and its intermediates), awareness about sustainability should be the main focus, otherwise, it would be impossible to save our planet earth from annihilation. That is why it is important to revive the ancient art of colouring with “Natural Colourants and Pigments “ [10, 21, 22]. Some other associated advantages of natural dyes and pigments include expected non-toxicity and anti-allergic properties and some medicinal value [23] as well as anti-microbial property and UV protective character [24]. But, many times natural dyes and pigments lack in uniformity and reproducibility of shade. Limited availability of such natural dyes and pigments is major drawback in addition to availability of handful scientific information on mechanism [25] and standardise dyeing process controlling variables [21, 26] for deciding proper methods of dyeing with natural dyes and pigments and colour fastness [27] achievable for such natural dyed textiles, which are based on chemistry of such natural dyes and pigments and their possible interaction with mordants and textile fibres on which those are applied. For Improving dyeing yield and dyeing performances, for dyeing textiles with natural dyes, many researchers attempted either modification of textile fibres to improve colour yield on modified textile fibres or attempted to adopt newer methods/techniques of dyeing textile fibres with those natural dyes [28, 29, 30, 31, 32, 33]. To assure that a textile substrate is dyed purely with natural dye, there is need of developing suitable methods and standards for identification [34] of natural dyes and pigments identifiable from such dyed textiles for consumers’ assurance and satisfaction. Use of extract of neem bark/leaf [35] or chitosan [36] derivative and extract of tulsi leaf [37] have been referred to have antimicrobial character and extract of anar-peel [38] has been referred to have UV protective character besides their colouration effect on textiles except chitosan.

2. Principles of application of natural dyes on textiles

‘The Society of Dyers and Colourists’ (SDC) in their 3rd edition specified natural colouring substances as ‘The natural dyes and pigments comprise of all the colours obtained from vegetable and animal matters inclusive few vat, disperse/solvent dyes, pigment colour and a few direct, basic and acid dyes’ [1, 2, 19].

Majority of natural dyes are mostly non-substantive towards textiles, and hence, it is normally be used along with a mordant or so called mordanting assistant compounds. A mordant usually a metallic salt or other coordinating complex forming agents having an attraction towards both the colouring component and the fibre and thus able to form complexes combining the fibre-mordant-natural dye, forming an insoluble complex as precipitate or lake of complex compounds remain anchored on the specific textile fibres. Application wise, natural dyes include mostly either direct dyes or acid dyes or other classes of mordantable dye type. There are some natural dyes akin to vat dyes, a few are as solvent dyes and also as pigments also. Only one natural basic dye and one or two disperse dye type natural dyes are known, but natural dyes akin to sulphur dyes, azo dyes are still unavailable.

Natural colourants are usually non-substantive, needing metallic or other types of mordants, or other complexing agents as so-called mordanting assistant, requiring for attracting and fixing natural dyes to the fibre substrate. Hence natural dyeing of textiles has the following three important stages for successfully applying it on textile fibres.

2.1 Extraction process of natural dyes from source materials

Extraction process of natural dyes from source materials by applying aqueous method i.e., extraction with the help of boiling/hot water with or without added any acidic/alkaline/alcohol containing chemicals in extraction bath within fixed ML ratio, time, pH and temperature range followed by distillation, dehydration and vacuum drying etc. or by supercritical carbon dioxide fluid extraction process or solvent extraction using soxhlet apparatus extraction by mixture of alcohol and benzene or by rotatory vacuum pump or extraction under low pressure etc. [1, 2, 39].

2.2 Use of mordant/mordanting assistants for creating substantivity and fixation of natural dyes to the fibres

The origin of word ‘mordant’ is from ‘modere’ (Latin) meaning ‘to bite’. The mordant attaches with the fibre surface so that a dye can sink in it. It can be tannic acids or tannins, sulphonated oils, metal salts or any natural product containing tannic acid/tannates.

These mordants not only fix the natural dyes on fibre but also govern their colour produced on the fibres and thus provide a variety of colours with a same dye material by using different mordants. Attached mordants play an important role in fastness and intensity of colour properties during the subsequent dyeing operations. Mordants thus fulfil following functions as described below:

1. Dye fixing on the fibre,

2. Increase the fastness properties of dye,

3. They themselves may act as a colouring matter,

4. They may ease dyeing/enhance dye absorption.

The followings chemicals/extracts of natural agents may be used as mordanting assistants:

1. Tartaric acid: help to acidify dye bath solutions and can used to nullify calcareous lustre effect.

2. Tannic acid and chebulinic acid: the natural ‘tanning agent’ seems usual in natural dye fixation as mordanting assistant. A number of tannins containing substances are employed as mordants or mordanting assistant during natural dyeing process. Among these raw tannins, myrobolan (harda) containing chebulinic acid, gall nut and Sumach containing tannic acid are most important.

3. Tartar cream: it is a whitish crystalline material normally utilised for colligation with mordants to brighten the colours.

4. Glauber’s salt/common salt: it acts as levelling agent used extensively in silk dyeing.

5. Boiled-off liquor: during use of degumming solution of silk in the dye-bath of silk, it helps the colouring substance to be distributed evenly and it also helps it to preserve lustre.

6. Enzymes/alkali: use of compatible alkali/enzymes combination in pre-treatment processes or into the dye bath containing natural colourants, improve colour yield.

2.3 Dyeing methods and principle/mechanism of fixation of natural dyes on textiles

Natural colourants frequently display non-consistent shades due to varying dyeing parameters or not adopting standard dyeing conditions. So, standardisation/optimisation of colouring/dyeing process for particular dye-fibre-mordant co-relations are necessary. Being mordantable, dyeing of majority of the natural dyes is accomplished by aqueous extracts of dyes in normal/hot dye-bath. Colouring may be executed by pre-mordanting, post-mordanting or concurrent mordanting, followed by standard dyeing process in normal/hot aqueous bath or HTHP technique with controlled pH range (i.e., acidic or alkaline), or using advanced ultrasonic dyeing technique etc. [1, 2, 39], using single or more natural colours extracted appropriately and found compatible scientifically.

Among different metallic salts most common eco-friendly mordant largely used in natural dyeing is alum (potash alum and ferric alum) besides uses of ferrous sulphate, stannous chloride etc. with or without harda/myrobolan as mordanting assistant, two types of natural potash alum are available in the market in two forms—potash alum containing 10.8% of Al₂(SO₄)₃ and ammonia alum containing 11.9% of Al₂(SO₄)₃.

Usually only alum or ‘phatkiri’ means potassium-alum, which is a hydrated complex of potassium-aluminium-sulphate with formula KAl(SO₄)₂·12H₂O. In cases of mordant-able natural dyes, aluminium sulphate or other metals chemically combine with some ▬OH or ▬COOH or other specific groups present in the dye and then further bound by co-ordinated/covalent bonds or hydrogen bonds and other interactive forces with the accessible functional groups present in the fibre [2] to form fibre-mordant-dye complex (for fixation of natural dyes on fibre substrate).

2.4 Advantages and limitation of natural dyes

In present days, there is an increase in demand of natural colourants. Although synthetic dyes have superior fastness properties, good durability and relatively easy to apply, the natural colours have certain limitations over synthetic dyes. Some of advantages [1, 2, 39] are as follows:

1. Moderately non-polluting and less toxic.

2. Soft, lustrous eye soothing shades to eye.

3. Vast range of colours. A little variation in dyeing conditions or by using other mordants for a particular dye can produce a new colour, which is impossible with man-made dyes.

4. Natural dyestuffs produce uncommon colour ideas with harmony.

5. Natural dyes are biodegradable as well as replaceable with saving of energy.

6. In few cases, like harda, indigo etc., the wastage can be utilised as fertiliser for agricultural fields.

7. As many plants/herbs of natural dye are grown on wastelands, utilisation of wasteland is an additional advantage for the natural colourants. Dyes like madder developed as host in tea gardens. So, no extra cost required for its cultivation.

8. Being highly labour-intensive, it can create new job opportunities like cultivation, extraction of natural colours and their application.

9. It has a potential to uplift the textile export of the country and thus may increase earning of foreign exchange [2].

10. Naturals dyes are safe for skin being anti-allergens and non-hazardous.

11. Natural dyes are good moth resistant.

12. Some natural dyes become more attractive with ageing, while synthetic dyes fade away with time [1, 2, 18, 26].

13. During bleeding, for any reason, natural dyes do not stain the adjacent fabric except turmeric.

14. Natural dyes can good UV protector without altering wear properties.

15. They are utilised in repairing and preservation of historic textiles materials.

16. They can be an ideal replacement synthetic dyes for food-stuffs without any hazards.

Despite these advantages, natural dyes possess some in-built disadvantages [1, 2, 39], such as:

1. Due to non-availability of sufficient database and standardising recipe for different shades on different textiles, there is doubtfulness about final shades and different fastness properties to be achieved and the same variation from plant from different sources, between crops and different seasons (i.e., variation due to different location, different crops and different seasons) and even for different mordant.

2. The extraction of dyes, preparation of textile, mordanting need competency which is expensive. Application of costly mordants, use more dyestuffs with long dyeing time to compensate low colour yield raise the dyeing cost for natural colouring as compare to man-made dyes but it has a niche market for bio-friendly, sustainable products dyed with natural colours particularly expensive products of silk, jute, wool, cotton and even for nylon etc.

3. Insufficient knowledge base due to less exploration it needs more studies/researches for commercialization.

4. Limited knowledge available on dye wise extraction process and fibre-wise dyeing techniques for natural colours cause uncertainty for shade depth and various colour fastness characters.

5. Natural dyes have a unique characteristic to change its colour during exposure to sun light (UV ray), perspiration, moisture and air [39] which may be considered disadvantageous.

6. Nearly all-natural dyes require to use of metallic mordants as dye fixing agent to fabric. During colouring, a prominent part of the metallic mordant remains in the residual dye bath unexhausted and these metals are threat to environment. So, hazardous metallic mordants containing metal like copper/chromium etc. should to be eliminated.

7. Being fugitive nature, these natural dyes are applied in conjugation with a mordant causing inadequate fastness to colour which is mis-match with textile end usage. Acidic pH during dyeing or post treatment processes for natural colours to enhance fastness to colour is hazardous for biosphere.

8. Few natural dyes have a little bit toxicity but there are limited studies available to assess their toxicity. There are only one or two reports available on source of natural dyes/colour which have been figured out as a potential threat to ecological balance due to toxicity, e.g., quercetin is studied to be mutagenic.

9. There are very few trained professionals available for application of natural dyes. So, there is a need for in-depth training and interaction programme for successful application of natural dyes.

10. Non-availability of pure natural dyes in the form of powder or in liquid form for direct application for developing known shade depth.

In spite of these limitations associated with application of natural dyes, it has a niche export market for dyed/printed fibres provided complying with the GOTS standards originated from the developed countries combining perniciousness, eco-friendliness and pollutant norms. For revival [40, 41, 42, 43, 44, 45, 46] of application of natural dyes these disadvantages are to be partially or fully eliminated/reduced utilising colorimetric analysis of sufficient scientific research data.

2.5 Colour fastness for natural dyed textiles

Fastness to colour is the resistance of colour component to alter its any colour features or to shift its colourants to neighbouring materials or both. Fading indicates the alteration of depth of colour (lightening or darkening) during laundering or friction during rubbing or during human perspiration or on exposing to light. Some reports in literature [47, 48, 49, 50] has described effective ways and means for improving colour fastness to wash and light by using different pre or post treatment with natural or eco-friendly fixing agents including tannin/chitin or quaternary compounds or UV absorbers related natural or substituted eco-friendly materials. There are many known techniques available to improve wash-stability, fastness to rubbing and light for synthetic dyed products, but such techniques for natural colours are still very limited. So, exploring fastness improving techniques for natural colourants are essentially needed as compared to synthetics.

Fading during exposure to UV light is generally boosted up in presence of vapour, thermal energy, oxygen in air and also depend on many other factors, which are shown in Table 1. Light fastness property of natural colours depends on its UV-soaking property of respective dyes [49, 50]. For example, UV absorption character of pomegranate rind i.e., anar peel extract (Punica granatum L.) has been proven and highlighted by many researchers in past [51, 52, 53, 54, 55], which caused improved light fastness and higher Sun protection factor for pomegranate rind extract dyed natural textiles.

Factors	Examples with and comments
Energy level and exposure time to light	Strictly regulated by light fastness test methods (Sun-light, Xenon arc, MBTF lamp)
Dye-stuff or pigment: type, density and dispersion	Superior light fastness with higher concentration and low specific surface area (pigment shows good light fastness than molecular dispersed dyes). Matching dye distribution over the fibre cross-section shows superior light fastness than ring dyeing
Dyestuff combination i.e., mixture of dyes	Catalytic fading (one dye catalyses photo-degradation of another dye), rarely it gives light fastness protection sometimes
Fibre type	Some dye gives poor light fastness to jute than onto cotton or acrylics due to self-fading behaviour of the fibre itself for jute
Moisture	Better is light fastness with lower moisture content. However, the level of humidity is usually fixed for standard methods of light fastness testing
Heat	Less is the light fastness rating with higher temperature; however, temperature is pre-fixed by standard light fastness testing method

Table 1.

Factors on which light fastness of natural dyed or synthetic dyed textiles depends.

Poor fastness to washing for majority of the natural colours is mainly associated with weak dye-mordant-fibre bonds. Breakages of dye-(metallic) mordant-fibre compound during washing by ionisation of the natural colours is responsible for change in hue [12]. Due to ionisation of many phenolic hydroxy groups present in natural dyes under acidic or alkaline condition, fabrics coloured with natural colourants alter its colour characteristics during laundering with alkaline surfactants. In most of the cases weak hydrogen bonds is responsible for poor washing fastness. In such cases, dyeing isotherm shown may be Nernst isotherm type showing high value of partition coefficient, like absorption of disperse dye on polyester fibre. Factors on which light fastness of any natural or synthetic dye depend are summarised in Table 1.

Rubbing fastness is measured by change in colour on the surface of rubbed textile or staining to the abrader cloth in both dry and wet conditions (for wet rubbing fastness, bleeding of colour may occur due to migration) after completion of specific abrasion cycles. Depending on the tensile strength of the fibre, small coloured particles may bleed out showing poor fastness to rubbing. If the dyes are not affixed properly on the fibre surface, that will stain the abrader fabric during rubbing. Numerical rating for rubbing fastness ranges from 1 (very poor) to 5 (best) depend on the following parameters:

1. Molecular structure and size, chemical characteristics and mode of fixation of natural dyes,

2. Mordants type and concentrations

3. Concentration, shade percentage and diffusion of dyes

4. Dyeing process, fixation percentage and degree of surface dyeing and ring dyeing

5. Amount of unfixed surface deposited dyes not removed after washing

6. Application of dye fixing chemicals, softener, silicones and cross-linking agents.

3. Colorimetric evaluation and process standardisation of natural colouration for textiles

There are various difficulties associated with natural colourants due to insufficient knowledge and lack of scientific information to attain uniform and optimum colour yield with acceptable colour fastness characteristics for silk fabrics. No such integrated study is available with pomegranate rind (anar peel) applied on silk, except one recent study on standardisation of dyeing process variables using pomegranate rind on cotton fabric [55].

Unknowingly natural dyes had been in use form primitive age of civilization in different regions of the world. It has been used since pre-historic times in order to beautify various materials like foods, drinks, household articles, fabrics etc. Almost all the dyes were of natural origin till 1800 B.C. As mentioned in literature [56], William Perkin synthesised first man-made dye ‘Mauviene’ by in 1856. Some studies on the use of synthetic dyes had reported that these dyes are not only non-biodegradable but also pose a threat to human health in terms of causing irritation, genotoxicity and even carcinogenicity. With the present sustainable concept to use of environmentally friendly and bio-degradable substances, the application of natural dyes on natural textiles have gained the momentum again and awareness of consumers about eco-friendly textile merchandise, use of natural colours is slowly reviving again [40, 41, 42, 43, 44, 45, 46]. There are several advantages of natural dyes over synthetic colourant/dyes [1, 2, 12, 39, 57] and such eco-friendly, nontoxic natural dyes are safe even for body contact [58]. Many scientists had already reported about the medicinal value of many natural dyes [51, 59, 60]. Yellow natural colourants extracted from rhizome of turmeric was traditionally utilised for medication in case of anti-inflammation [61].

The natural colours are extracted from the vegetables/plants/herbs, minerals or insects [62]. Majority of the natural colours show inferior to average light fastness property whereas synthetic dyes exhibit moderate to excellent light fastness property [63]. Dyeing of cellulosic fibre with leaf-extract of Beilschmiedia fagifolia had been reported in literature [64], where sonicator assisted dyeing method, that was used to dye cellulosic fibre with B. fagifolia extracts. The researchers had observed that cotton pre-treated with 1–2% metallic mordant and dyed with 5% of plant extract developed good fastness by use of sonicator assisted dyeing process. In another study [65] marigold flower dye extract was utilised to dye cotton fabrics.

Natural dyes leaching out from Jatropha integarrima flowers was used for colouring of different textiles made of cotton, wool, silk [66]. Silk dyeing with Onosma echiodes (Golden drop) had also reported in literature [67]. Areca nut palm extract was used as natural colourant [68] to dye cotton with suitable mordant. Colouring of natural cellulosic material and silk with advance ultrasonic technology by Nerium oleander flower petal extract had already been studied [69]. Colour extracted from waste leaves of Aroto carpus betero phyllus has been reported to dye various textile materials made of cotton and silk to obtain a consistent golden yellow shades [70]. Successful colouring with natural colourants extracted from turmeric, catechu, henna, madder, tea leaves, Indian rhubarb and pomegranate exocarp on man-made polyamide fibre had been reported in literature with details of procedure and dyeing performances [71]. Some successful researches had also been carried out on application of Lac dyes and its absorption criteria [72] on different fibres. Mulberry silk was successfully dyed with natural dye annatto [73]. Some studies [74, 75, 76] on application of natural dye of Indian madder, Spathodeac ampanulata and lac dyes on silk had been reported. Cost effectiveness of dyeing silk in different shades with different natural dyes was analysed and reported by Patel et al. [77].

The substantial scientific document on natural colouring matter was initiated by Perkin and Everest [78]. Studies on contemporary processes for applying natural dyes on textiles as practiced in different states of India along with the chemistry of such dyes and traditional dyeing methods has been well documented by Mohanty et al. [13]. This book is probably an important authenticated document on the processes of natural dyeing on textiles of ancient India till today. The present section of literature review thus covers some important research work on application of natural colouring matters on textiles under the following sub-heads:

1. Characteristic and bio-chemical/chemical anatomy of some important natural dyes.

2. Methods of extraction of colourant component from natural dyes.

3. Effects and methods of application of different mordants.

4. Different methods of natural dyeing and variation of dyeing parameters.

5. Study of dyeing kinetics and dyeing process variables.

6. Exploring new shades with natural dyes and colour parameters analysis for dyed textiles.

7. Colour fastness properties of natural dyes.

8. Miscellaneous recent studies.

3.1 Characteristic and bio-chemical/chemical anatomy of some important natural dyes

Studies on analysis of natural colours and their application on textiles are a challenging subject which attracted of scientist and academicians for compiling it in a form of scientific literature and books as early as in 1918 since the publication of book by Perkin and Everest [78]. Since then, among all review and books published on the chemistry of natural colours and pigments and their application on textiles. Some ancient and present literature reports are specially mention worthy as reported by Gulrajani and Gupta [1], Samanta and Konar [2, 10], SERI [12], Mohanty et al. [13], Vankar [39] and others like Perkin and Everest [78], Sahid and Mohammad [79] and Mayer and Cook [80], to utilise as study materials for further reading.

Structures of quinoids based natural colourants is described in Thomson’s book [81]. Earlier review work of many authors on natural dyes and pigments also include the review report by both Parris [82] and by Hofenk and Graaff [83], where the latter collected data about fastness to colour and history of their uses. A detailed review work was done by Samanta and Agarwal [4], Samanta and Konar [2, 10], and in a recent book of Vankar [39].

Samanta et al. [84, 85, 86], had analysed the thermodynamics of rate of dyeing, half dyeing time, internal energy of system (i.e. enthalpy), dyeing isotherms, free energy etc. as a physico-chemical attributes of colouring for ligno-cellulosic material with natural dyes like red-sandal wood, jackfruit wood and tesu (palash) etc.

Mass spectroscopic analysis of dyeing of cellulose polymeric material with indigo has already studied and finger print region of it was used for identification of natural indigo [87] with the supporting analysis of TLC and UV VIS spectroscopic analysis. However, there are contradictory reports on non-iso-labelling/non-identifying natural indigotin differentiating it from synthetic indigo as reported by De Wong [88], who was unable to identify 6,6-dibromo-indigotin (with the help of direct ingredient index analysis), but later it was resolved after the 6,6-dibromo-indigotin had been separated from natural indigo by subtractive extraction process with sodium hydro-sulphite. Koren [89] had analysed the natural madder and natural indigoid dyes by high performance liquid chromatography (HPLC) to figure out its chemical functional nature.

Thin layer chromatography (TLC) analysis is very useful tool for researchers for identification of natural colourants in textiles and Parris reported TLC [82] different vegetable and insect dyes of yellow, red and blue shade. Guinot and Roge [90] also used TLC as simple chromatographic analysis technique to study extracts of different parts of herbs/plants bearing flavonoids (flavonols, flavones, flavanones, chalcones/aurones, anthocynanins), hydroxy-cinnamic acids, tannins and anthraquinones, which are the active phyto-colouring substances found in those plants. The colour elements were separated from most of the barks containing bio-flavonoid moieties.

Physico-chemical dyeing attributes of natural colours extracted from red sandal wood and its compatibility behaviour with other colours were also examined and analysed by Samanta et al. [91, 92]. Analysis of UV VIS spectroscopy of extract from the bark of neem tree [93] extracted colourant showed dual maximum absorption bands at 275 and 374 nm respectively; while extract of beet sugar exhibited three absorption bands at 220, 280 and 530 nm. The visible spectrum of extract of ratanjot was studied by Gulrajani et al. [94] under acidic medium and it displayed highest absorption around 520–525 nm, while under alkaline pH absorption band was shifted to 570 nm with an additional peak at 610–615 nm. Extract of Red sandalwood [94] showed a substantial absorption crest at 288 nm but highest absorption was observed at 504 nm and 474 nm at pH 10 in methanol extracted solution. Colourant extracted from Gomphrena globosa flower showed single major peak at 533 nm and there were almost no changes observed in visible spectra at pH 4 and 7; however, there was little shift of peak to 554 nm as observed by Shanker and Vankar [95].

Extraction, identification and assessment of dyeing behaviour on wool of natural dyes obtained from barks of Musops elengi and Terminilia arjuna were studied by Bhuyan et al. [96] and they observed the dye absorption by wool fibres varied for respective dyes extracted from Mimusops elengi and Terminalia arjun depending on their dye contents. The dye absorbed by wool fibre varied in between 21.94–27.46% and 5.18–10.78% respectively for the said two natural dyes.

Analysis of colorimetric parameters including K/S values, DE values, L, a, b, DC, DH, DL values etc. was studied for use of binary dye mixture for obtaining compound shades along with analysis of compatibility in between different pairs of natural colour combination applicable for both conventional methods and a newer method for compatibility test of natural dyes based on analysis colour difference index (CDI values), a newly defined and established index of colour differences parameters [97, 98] as a new indicator called ‘Colour Difference Index’ (CDI), calculated by an empirical formula presented by them. By this newer route, the differences of CDI values for dyeing with binary mixture using combination of any two dyes in different proportions has made it possible to determine dye compatibility rating from the chart given by them, as much simpler and easier.

Identifying the colourants in historical textiles materials by the help of modern spectrophotometric and chromatographic techniques and by sensitive colour responses were studied by Blanc et al. [99], who also studied the retentivity behaviour of natural colour components like carminic acid, indigotin, corcetin, gambogic acid, alizarin flavanoid, anthraquinone and purpurin after dyeing. There is another non-destructive way via study of emission and excitation spectra, which was reported for recognising faded dyes on textiles fabrics. Zin et al. [100] characterised purified extract of natural dye components extracted from bark of mango tree for application in wool fibres.

Walker and Needles [101] studied the method for segregation and identification of natural colours from natural protein fibre like wool by reverse phase HPLC using a C-18 column. Binary and quaternary solvent systems were utilised to find chromatograms of dyes, isomers and minor products present in the dyed sample. A linear gradient elution method was used in HPLC analysis for natural colour extracted from insect, plant/herbs/trees, for red anthroquinonoid mordantable colours, red purple natural colour, molluscan blue and indigoid vat dyes [89]. This technique was very useful for identifying various chemical categories of natural colourants from the same elution programme. In addition, it is much time saver for natural anthroquinonoid dyes over those previously published.

Cristea and Villemar [50] reported successful quantifiable analysis of Weld by HPLC and concluded that 0.448% luteolin, 0.357% luteolin 7-glucoside and 0.233% luteolin-3′7-diglucoside in aqueous methanol solution can be after identified within 15 minutes. Some other authors [102] reported analyses of indigo by HPLC that with increase in dyeing time, it changes structure of indigo compound causing a reduction in colour intensity of polyester fibre-based textiles. Jain and Vashanta [103] analysed antimicrobial nature of bio-friendly natural colourant with arcea nut with natural mordanting add-on like myrobolan, lodhra and pomogrenate rind and observed that pomegranate rind showed optimum anti-bacterial performance and Lodhra renders best washing fastness to among all the tannin based natural mordants/mordanting assistant/additives used.

Mondhe and Rao [104] had attempted to develop azo-alkyd natural colours by reducing nitro-alkyds, followed by diazotization of amino-alkyds and coupling with various phenolic substances present in situ in the seed oil of Jatropha curcas and analysed it functional nature by IR spectra.

Majority of the natural colours are non-toxic and eco-organo-friendly. The toxicity analysis [105, 106] for each of natural dyes is therefore necessary and it may also provide positive or negative evidences about the skin friendliness or its adverse effect to human body for any such natural dyed textiles. Few natural dyes and dyed products may also contain toxic component in its composition not skin friendly to human. Primary concerns of toxicity are the human skin toxicity, irritation on skin or eye and sensitization potentiality. In addition, possible long-lasting effects such carcinogenic, reproductive toxicity or mutagenic toxicity had also been identified from many natural products. The LD50 test is popular for its perniciousness. It is said the ‘lethal dose for 50% of the test animals’ which is the amount of material in kg/kg of body mass, can kills 50% of the animals. Another report describes the toxicity and antimicrobial test data of raw methanolic extracts from different parts of the plant Artocarpus Hetrophyllus including stem, roots, leaves, fruit, seeds [107] and their subsequent partitioning with organic solvents like petrol, dichloromethane, ethyl acetate and butanol fractions, rather exhibited a wide spectrum of anti-bacterial performance. The butanol fractions of root/bark and fruit were also witnessed to be good antimicrobial nature but no anti-fungi property.

Tannins was extracted from gall nut and leaf from oak plant (i.e., Oak leaf) by Mishra and Patni [108] from the different plant at Himalayan region. These extracts from gall nut and Oak leaves are rich of gallic acid and tannic acid for better dye fixation for application in colouring of cotton, woollen and silk textiles in presence of various metallic mordants to obtain better colour fastness without any dermatological problems. Jain [109] has shown supremacy of few natural mordants for colouring of cellulosic fabrics with extract of jamun tree and its by-products, inspiring further study on such bio-mordanting and bio-additives on cotton, jute, wool and silk textiles.

Benencia and Courreges [110] studied chemo preventative aspect of red sandalwood oil on papilloma skin of mice and also studied its preventive nature against development of skin tumour for CD-1 mice and anti-viral function against herpes simplex virus-1 and 2. Singh et al. [111] examined anti-microbial activities against pathogens like Escherichia coli, Bacillus subtilis, Klebsiella pneumoniae, Proteus vulgaris and Pseudomonas by Acacia catechu, Kerria lacca, Quercus infectoria, Rubia cordifolia and Rumex maritimus. Minimum repressive/inhibitory density was observed varying from 5 to 40 μg per kg of textile materials. Analysis of natural components of colours for dyed historical textiles was also reported by Ferreria et al. [112] by different methods. Another analytical report from Hauluk et al. [113] render analytical data for ellagic acid content in European oak wood for its use as natural mordanting additives as dye fixative in natural dyeing of different textiles. A realistic evaluation of dyeing of different textiles with vegetable colourants with metallic mordants need to be subjected for determining their toxicity for objectionable metal present in the metallic mordant, which was reported for use of different metallic mordants during natural dyeing process [114].

3.2 Methods of extraction of colourant component from natural dyes

It is reported in literature by Dayal and Dobhal [115] that natural colourant extracted from eucalyptus leaves, Cassia tora seeds and bark of Grewiaoptiva, shorearobusta by using aqueous medium under different conditions leads to variety of attractive light shades on cotton. Later, Vankar and another group of researchers [28] used the supercritical carbon-dioxide fluids to taking out/extraction and purification of natural dyes from bark of eucalyptus. Extraction of natural dyes/colourants from bio-mass products of cutch, ratanjot and madder were described by Khan et al. [107]. Ray Maulik et al. [116] extracted natural dyes/pigments from Hinjal, Himalayan rhubarb and Jujube bark for dyeing of wool. Teli et al. [71] extracted natural colourants from the coffee-seed for dyeing of nylon. Pan et al. [117] applied natural colours on ligno-cellulosic (Jute) fabric with extraction of deodar leaf, eucalyptus leaf and wood of jackfruit by soaking and boiling it for approximately 4 hrs separately in soft water. Verma et al. [118] attempted to develop dye powder from wattle bark by soaking it overnight in deionised water, boiled in pressure vessel followed by passing through filter to get 15–20% (w/w) dye residual powder. Saxena et al. [119] initially extracted antioxidant components of natural colourants from leaves and fruits of confers cultivated in Iran and also from marigold and chrysanthemum flower petals by simply boiling dried petals in acidic aqueous solution. While, Sarkar et al. [120] extracted natural colour from marigold flower and applied on cotton, wool and silk. Later Sarkar et al. [121] again made solvent assisted extraction of yellow vegetable colouring matter including extraction of marigold flower for its application on even hydrophobic textile substrate. Deo and Paul [122] extracted natural colour/dye components from onion skin by adding of 5% solution of sodium chloride in a ratio of 1:2 and applied with natural mordant combination for dyeing ecru denim fabrics. Dixit and Jahan [123] extracted euphorbia continifolia leaves under acidic pH to dye silk fabric and later evaluated its various colour fastness properties.

Sudhakar et al. [124] derived natural colours from nuts of Areca catechu, which is profusely grown in India and can be used for colouration of natural protein fibre like silk or wool. It was reported by Radhika and Jacob [125] that natural colour/dye extracted from jatropa seeds under alkaline conditions, which when is applied on cotton fabric, it gives an attractive range of soft, bright and uniform shade on cotton fabric. Extraction of natural colours from this seed waste/food waste can be achieved better with ethanolic or acidified (with 40 M oxalic acid) extraction. Hagerman and Wilson [126] has described a method for quantitative estimation of ellagic acid from tannin containing natural colouring agents for precise estimation of the same, which are believed to be have good property of natural dye fixation on textile substrate for presence of tannate/ellagic acid component along with colour component(s).

Extraction dye from the teak leaves in methanolic solution was successfully carried out by Nanda et al. [127] to develop a brick red shade for silk/wool dyeing with different mordants. Bhattacharya et al. [128] studied the effects of mordants for dyeing polyamide fabrics with some eco-friendly natural colourants and attempted to standardise natural dyes extracted from arjun bark, babul bark and pomegranate rind for reducing the effect of dyeing process variability. Onal [129] attempted to dye wool, cotton and leather with onion skin/peel (Allium cepa L.) extract as natural colourant source.

Agarwal et al. [130] studied superior dyeing properties on polyester by organic solvent (solvent:water ratio 1:9) assisted dyeing using the said solvent dyeing medium, where colours of the selected natural dyes was extracted from henna leaves, and directly used in dyeing of polyester in the solvent dye-bath. Houlton et al. [131] studied the dyeing of jute-cotton blends by extracts using grape skin waste.

Teli et al. [132] had dyed cotton with concentrated tea extract after different pre-treatment of cotton. It was studied by Padmaja et al. [133] to extract natural colourant from Hibiscus flower to apply it on pre-mordanted silk textiles.

Thus, the role of solvent/acid/alkali in both extraction and dyeing with selective natural dyes is understood from above few case studies of earlier work. Hence it is felt appropriate to study the extraction of natural dyes or natural finishes under solvent based/acid based or alkali based extraction, for dissolving out or taking out colour components and other active functionality species for extracting colours from specific natural colourants. Extraction of natural finish components and colour components from extract of catechu and gall nut and eucalyptus leaves need to be studied in depth.

3.3 Effects of different methods of mordanting and applying different mordanting assistants for natural dyeing

Mordanting with selective metal salts can be accomplished by pre, simultaneously and post-mordanting process. Various metallic and non-metallic bio-mordants may be used on different textiles to enhance colour absorption and dye uptake of natural dye. Without mordants, natural colourants have generally no affinity for textile fibres. Extensive work on type of mordants and mordanting conditions including application of few bio-mordants before applying natural colourants/bio-colours on several textiles have been studied in few current literature [134, 135].

Dayal and Dobhal [115] studied the effect of copper sulphate (CuSO₄) and potassium di-chromate (K₂Cr₂O₇) on natural protein fibre like silk, wool and also for cellulosic fibres as well. They, analysed their colour yield and related results on colour fastness properties of natural dye extract from Shore Robusta bark. Wool was reported mordanted with metallic ions like Al (III), Cr (VI), Cu (II), Fe (II), Sn (II) and rare earths elements such as La (III), Sm (III) to colour extracted from beet sugar and the dyeing could satisfy colour fastness standards of BIS as reported by Mathur and Bhandari [136]. Agarwal et al. [137] optimised the mordant concentrations or mordant combinations and concluded that most beneficial shades that may be obtained with selective mordant combinations only such as 0.15% alum, 0.08% copper-sulphate and stannous-chloride mixture for optimised dyeing process with natural colour obtained from turmeric (Curcuma longa) for wool and also used 0.04% ferrous-sulphate and 0.06% potassium-dichromate for dyeing of mulberry silk fabric. But copper above a certain limit is not eco-friendly, and dichromate is not at all environmentally friendly. Hence, current research are focussed on the search of results of application of suitable bio-mordants or dual mordants applied as mixture or in sequences applying one after another for different natural dyeing. Hence, the study on effect of application of dual mordants preferably with bio-mordants combination in different ratio is needed to be explored for improving colour yield and colour fastnesses.

Irrespective of methods of mordanting, silk treated with potash-alum natural alum salt exhibited enhancement in colour, while silk mordanted with copper sulphate, potassium di-chromate and ferrous-sulphate evidenced very good colour fastness to light [138]. But, application of potassium-dichromate and copper-sulphate as mordant is not recommendable for environmental concern. Silk fabric mordanted with magnesium sulphate [139] produced lower depth of shade but at the same time, when it is mordanted with copper sulphate, it shows highest depth of shade. Copper above a certain limit is not environmentally safe. Das et al. [140] studied chemical alteration of cellulosic fabric in presence of acrylamide and K₂S₂O₈ to increase dyeability for natural dyes and to improve colour fastness to light and rubbing for these pre-treatment, although the said chemical modifications do not enhance washing fastness of such natural or synthetic dyed textile materials. Latter Das et al. [141] reported that application of aluminium sulphate and ferrous sulphate as mordants enhancing washing fastness of textile materials dyed with tea extract. Chan et al. [142] also reported that the effect of natural dye effluent on environment for wool dyeing with four varieties of tea and found that the protein fibres turned blackish in presence of mordant like ferrous sulphate. Gulrajani et al. [143, 144] reported the outcome of various mordanting agents on natural yellow colours such as kapila, onion, tesu, and dolu on different textiles.

Some mordants incorporated good wash stability to cotton, when it is dyed with golden rod using tin as a mordant and dyed with marigold by using chrome as a mordant and dyed with onion skins using alum and tin as mordants as reported by Vastrad et al. [145]. Bhattacharya and Lohiya [146] reported dyeing of cotton and polyester with catechu, nova red, turmeric and pomegranate rind as natural dyes with different eco-safe mordant.

Vat dyeing of cellulosic fibre in presence of iron (II) salt and complexing agent such as gluconic acid has been studied earlier by Chavan and Chakraborty [147], where they have reported to use of iron (II) salts complex with tartaric acid and citric acid as ligands at room temperature for its application on cotton using natural indigo. One report by Kumar and Bharti [148] showed that wash fastness and light fastness can be enhanced by using metal salts in presence of tannic acid or natural tannates for application on cotton fabrics. Pre-mordanted cotton yarns with potash-alum, ferrous-sulphate, potassium-dichromate and copper-sulphate are when treated with Acalypha extract [138] as natural dye, showed excellent colour fastness to light and wash, but potassium-dichromate and copper-sulphate are not eco-safe and ferrous sulphate always gives a darker grey to blackish shade.

A recent report described natural dyeing of cotton using gall nut extract as bio mordant (without any metallic mordants in combination) having natural tannates in extract for dyeing with extracts from babul bark as natural dye [149]. Dual pre mordanting with 15% concentration of harda and aluminium sulphate (in ratio 75:25) was applied in succession on cotton fabric and was subsequently dyed with aqueous extract from bark of babul [150] gave very good result in dye uptake/colour yield and colour fastness, proving that cotton fabric pre-treated with harda (myrobolan) as mordanting assistant (containing chebulinic acid) showed higher dye up take of respective natural dye for additional complexing of chebulinic acid of harda with metallic mordant like alum to form big giant complex of [Fibre-Harda-Metalic mordant-Natural dye] complex of much bigger size, improving wash fastness too. Double Pre-mordanting route also found to favourable for manjistha and other different natural dyeing of jute fabric as per earlier reports by Konar et al. [151].

Analysis of few tannin-based natural bio-mordants which can fix natural dyes on the fibres by forming a fibre-bio-mordant-natural dye complex/adduct, are already reported by Samanta [134], and also by Yusuf et al. [152]. Bio-mordants have advantages of its natural origin, eco safe and biodegradability criteria, free from toxicity causing no allergic reaction to human skin and sustainability [41]. Different bio-mordants (used with or without metallic mordant) have also been applied for eco-friendly natural dyeing of textiles in presence of their tannins/flavonoid content with mordantable capacity of poly-phenols/carboxylic acids which may assist dye fixation on fibres by forming larger (giant-sized) fibre-bio-mordant-metallic mordant(optional)-natural dye bigger giant complex. Examples of few such bio-mordants are gall-nut [149, 152]; myrobolan/harda [153], tamarind seed coat [154], pomegranate rind [155], aloe vera [156], lemon juice [157], mango bark [158] and banana sap [159], though use of different type of bio mordants and method of bio-mordanting with or without metallic mordants in combination having significant effect on rate and extent of colour yield and colour fastness to wash and photo fading behaviour due to presence of tannates.

According to one study, use of copper as metallic mordant with or without bio mordants showed high resistance to washing and photo fading; but stannous-chloride or alum combination with other bio mordants did not show such characteristics. However, light fastness/resistance to photo fading was found to improve after post-mordanting with copper ion, whereas, pre-mordanting with stannous-chloride or alum was found superior as reported by Gupta et al. [160].

Binary mordants combination with harda or tartaric acid was observed to be the very good mordant combination followed by tannic acid-harda and tartaric acid-tannic acid formulation as reported by Deo and Paul [161]. A synergistic effect of these mordant combination was found for use of any of the said binary combinations of mordants. According to this report, meta-mordanting exhibited superior results for harda-tartaric acid and tartaric acid-tannic acid formulation, while pre-mordanting showed good results for tartaric acid-harda formulation. Bains et al. [162] also studied the results of various combinations of mordants on colour fastness of wool dyed with golden drop is reported to show encouraging results.

Lots of literature are available on effect of different mordants or their combinations prior to natural dyeing on colour yield and fastness performance, particularly use of double or dual mordants with variation in their ratio, development of shade and colour yield and other colour interaction parameters [135, 163, 164], on cellulosic(cotton), ligno-cellulosic (jute), protein (silk and wool) and synthetic (nylon and polyester) fibres. But combination of Natural tannate based bio mordants along with natural metallic mordant i.e. potash alum or fitkari are not explored much, which need further study in this area for natural dyeing of cotton, jute, silk and wool textiles.

3.4 Methods of natural dyeing and effects of variation of dyeing process parameters

Ghorpade et al. [165] reported dyeing of cotton with sappan wood using ultrasound waves instead of simple aqueous bath dyeing. Tiwari et al. [166] studied natural dyeing of cotton with extract of tulsi leaves using ultrasonic waves. Lokhande et al. [167] reported dyeing of polyamide with three distinct natural dyes with different mordants by two different techniques (open bath and HTHP) and found HTHP dyeing is better than open bath dyeing. Bhattacharya and Lohiya [146] also utilised HTHP technique for dyeing cotton-polyester textiles with pomegranate rind, catechu and turmeric.

Tiwari et al. [166, 168] used the unconventional natural dyeing with alkanet root bark using both microwave and sonicator. Kamel et al. [169] studied colouring of wool fabric by the liquid extract of terminilla arjun fruit and chochineal mixture. Neetu and Jahan [170] reported the dyeing of combination of natural colours derived from onion skin and kilmora root. Samanta et al. [5] studied the use of mixture of turmeric and madder for cotton and observed synergistic effect for enhancing colour strength and improves washing fastness for mixed shades of turmeric and madder than that obtained for application of only turmeric on cotton. Compatibility of binary mixture of dyes for producing compound shades are studied by Samanta et al. [92, 97, 98, 164].

However, there are only sporadic report and very less scientific or technological reports on either statistical optimization/standardisation of dyeing process variables [149, 150, 151], studies on dyeing kinetics [85, 86], dye compatibility tests with conventional and newer colorimetric CDI based methods [92, 97, 98] and effect of bio mordants [149] and bio-finishes [134] for enhancement of colour strength, colour uniformity and colour fastness to wash by use of single and binary mixture of natural dyes on various natural textile fibres. Hence, there are ample scope of research in this area, particularly on standardisation of the dyeing process variables for natural dyeing using bio-mordants and also to study the effects of different bio-finishes for improving antimicrobial and UV-resistant, as wellness properties.

Computer aided colour matching system has become an integral part of textile dye houses as an important tool in textile processing unit for dyeing any textiles with particular class of synthetic dyes. The prediction of colour matching formulation using natural dyes is practically difficult and not that easy. Hence, it is not systemised yet for commercial applications for the following difficulties. And variation of colours for natural dyeing due to:

1. Varying shades, depending on source, extraction method of colour and non-reproducible nature.

2. Lack of standardised dyeing process parameters and hence variation in colour after dyeing.

3. Besides colour, the shade and hue is also much dependant on type and concentration of mordants.

4. Usually for purified synthetic dyes, plot of dye concentration vs. K/S value is linear and K/S are additive, but for natural dyes, in most of the cases, this linearity is not maintained due many variations in the natural resourced materials and also due to additional process of mordanting etc. Hence separately linearization of data base, standardisation of mordanting and dyeing process conditions to obtain uniform and reproducible dyeing etc. are to be assured.

The above difficulties can be partly or fully eliminated by taking extracted purified natural dye source of known concentration only for calibration dyeing for preparation of correct database eliminating colour variations, using fixed type and concentration of mordants following optimised standard process of mordanting and dyeing, to obtain linear curves for purified natural dye concentrations vs. K/S values, applying linear least square curve fitting principles, and then one can use that data base for colour match prediction with known and purified natural dyes combination for colour match prediction with natural dyes allowing higher tolerances of colour differences.

3.5 Physico-chemical studies on dyeing process variables and dyeing kinetics

Different natural colours have different chemical constituents, which are determined by LC-MS or GC-MS analysis [149], while its extraction of colour components and finishing components or both depends on extraction media, temperature, pH, time, solvent and other conditions/factors of natural dye source like which part of the plant is taken, from where or which source the dye was extracted, area/soil of the field of cultivation/growth, atmospheric conditions of the growing areas, time of harvesting, extraction method or technology applied etc. Some researchers have reported discrete and piece-meal experimental inferences of laboratory testing regarding techniques and method of extraction including temperature, time, solvent-vegetal ratio, type of solvent used and extraction methods etc. Similarly colour strength produced with those extracted natural dyes depends on methods and types of mordanting, dyeing process variable parameters and nature of textile substrate etc. [171]. Hence, it is essential to study fibre-wise and dye-wise optimization of extraction parameters and standardisation/optimization of process variables of dyeing for different natural fibres to be dyed with different natural dyes using different single and double mordants and single and compatible binary mixture of dyes.

The effects of dye extraction mode with variation in pH, media, time and temperature used during extraction, and similarly for pre-mordanting stage, effect of type of mordant and mordant concentration and method of mordanting and mordanting conditions, and finally for Dyeing, effects of dye concentration and method of dyeing and variation in all dyeing process parameters are most important. For silk dyeing with some specific natural colourant effect of dyeing process parameters/conditions have been studied by Grover et al. [172], Dixit and Jahan [123] and Samanta et al. [149, 150, 151]. From the study made by Grover et al. [172], it was inferred that the acidic pH showed highest absorption for jatropa, lantana, hamelia and euphorbia dye, on the other hand kilmora and walnut exhibited better outcome in alkaline pH. The results shown in their various experiments lead to the optimization of a standard recipe for particular dye-mordant-fibre combination. Dumitrescu et al. [173] also studied the dyeing parameters which gave optimised and best result of colour yield and colour fastness on wool. The optimum concentration of colourant from beet sugar for wool dyeing was found to be 0.03 gm/gm of wool at pH 4, 5 at 97.5°C [39]. Deo et al. [174] described dyeing of cellulose and ligno-cellulosic materials with tea extract. Samanta et al. studied optimization of dyeing process variables [149, 150, 151] and compatibility [92, 97, 98] of binary mixtures of dyes applied on jute and cotton textiles with single and double mordants for different natural dyes. Das et al. [141] have reported that the colouring constituent of tea showed maximum attraction for both wool and silk at pH 2–4 in presence of ferrous-sulphate and aluminium-sulphate as mordants. Optimisation of dyeing process parameters for wool dyeing with natural dye derived from turmeric has been analysed by Agarwal et al. [175]. Gupta et al. [176] reported the kinetics and thermodynamics of fuglone dyeing and showed that linear isotherm for wool, human hair, silk, nylon and polyester indicated a partitioning dyeing mechanism. The slope of isotherms was raised with the rise of temperature for all cases. Both ∆H and ∆S value were found positive for all the dyeing. The apparent diffusion co-efficient was maximum for wool and minimum for silk. Samanta and Agarwal et al. [85, 86] have studied dyeing kinetics for dyeing jute and cotton with jackfruit-wood and red-sandal-wood natural colour.

A latest report on review of Scientific aspects and revival of incorporation of natural colours on textiles and a brief accounting of latest research done on natural dyes for textiles covering different scientific issues has been compiled and edited by Vankar [39] and also another two chapter contributed by Samanta [163, 164] in an edited book, covering all the aspects of natural dyeing including its extraction, characterisation and application on textiles.

A comprehensive current review on sources and composition of different natural dyes [135] and standardising method of application of natural colourants on different textiles and its characterisation with strategy for its revival are available in current literature [177, 178].

3.6 Colour fastness characteristics of natural dyes

Colour fastness is defined as the resistance of a material to alter its colour features or, transfer of its colour to adjacent materials in contact, or both, under different atmospheric condition and or any process like laundering, dry cleaning, etc. or exposing to various stimuli like heat, light etc. Fading means alteration/loss in colour depth after exposure to any environment/agency/process either by lightening or darkening of the shades.

Extensive works have been accomplished to increase the light fastness features of textiles dyed with natural colour. Cook [48] has reported a wide-range of review on various attempts taken for enhancing colour fastness of various natural dyes applied on several textile fibres by different techniques and methods. The said review also addressed after-treatments linked with tannin for enhancing the washing stability and colour fastness to light of such mordantable natural dyes suitable for cotton. Few of these chemical treatments might be relevant for specific natural colours and few are commonly applicable. Better alternative is to find out suitable natural resource based after-treatment, if possible, e g., to improve wash fastness of natural colourants by treatment of dyed fabrics with chitosan in acidic media and to improve light fastness, natural resource based after-treatment with antioxidant/UV resistant natural materilas like pomegranate rind extract or ashwagandha extract or Eucalyptus leaf or bark extract etc.

The light fastness is also associated with resisting the effect of UV light exposure initiated oxidation/reduction of natural/synthetic dyestuffs [49]. The ultraviolet light (UV) is a part of the electromagnetic radiation spectra having shorter wavelength (λ) than visible light. UV light may be divided into three groups e.g., UV-A (λ—320–400 nm), UV-B (λ—280–320 nm) and UV-C (λ—100–280 nm). As per law of physics, shorter wavelength (λ) has higher energy (E) level (as per Einstein’s rule: E = hν = [h X c/λ) causing more detrimental effect on dyestuff or human skin. Luckily, high concentration of ozone in the stratosphere (around 15–30 km above the surface) absorbs UV-C radiation totally. UV-B is less than 1% of the radiation of the sun reaching to earth and is not much damaging, while UV-A (λ—320–400 nm) consists of around 5.6–6% of total radiation which reached to the earth surface. So, identified UV absorbing (UV-A component) natural resource based materials are known to have better UV protective action and hence after treatment with UV resistant dyes or finish materials can cause improving light fastness better than other dyes/finishes, not having this UV absorbing character.

Majority of the natural colours showed inferior to moderate wash stability and also inferior to moderate light fastness on UV light exposure (with respect to the comparable best synthetic dyes) and that is why, developed shades are sometimes lighter or different from their original colours. A comparative light fastness for a range of natural colourants was examined by Padfield and Landi [179], who also revaluated the development in qualitative fashion for selective natural dyes. These lightening or otherwise alteration in shade after washing for natural dyed wool textiles were measured quantitatively by Duff et al. [180], who evaluated the colorimetric analysis to note the changes of colour values in the Munsell scale and also with respect to the CIE colourimetric terms. Wool coloured with nine different natural colourants was exposed to Microscal MBTF fading lamp and finally the colour fastness ratings were found in line with proposition of Padfield and Landi [179] on exposure to daylight fading. During evaluation for light fastness (LF) as per blue wool standards, yellow natural colourants (old fustic and Persian berries) showed inferior light fastness with 1–2 rating; reds colours (cochineal with tin metallic mordant, alizarin with alum and metallic stannous mordant, lac with stannous metallic mordant) showed moderate LF with rating 3–4; indigo showed LF with rating 3–4 or 5–6 some times based on the mordant and logwood black used as natural colourants, showing LF of 4–5 with chromium containing mordant or 6–7 with other mordants. Gupta [181] reported that light fastness and all other colour fastness behaviour also depends on chemical constituent/structure of relevant natural dyes like ratanjot.

A mordant is required for majority of the natural colour components to be fixed on the textile substrate. Nature, type and concentrations of mordant has a significant role on wash stability and light fastness. The results of various mordants have a crucial role in fading of 18 different yellow natural dyes as studied by Crews (Crews P. C. 1982) [182]. Turmeric, marigold and fustic dyes have more tendency to fade than other natural dye of yellow shade. However, application of stannous metallic mordant and alum mordant combination showed higher fading than the mordants containing iron, copper or chromium. Hence, an inference may be drawn that the type of mordant is very crucial factor for determining the wash fastness (WF) and light fastness (LF) of textiles dyed with natural-colours. The LF of such natural colourants on wool has been compared with ‘imported’ dyes of similar shades by Duff et al. [180], again using Micro-scale MBTF fading lamp for simulated UV light exposure.

Oda reported [183, 184] the influences of various additives/add-ons on the photo-fading behaviour for carthamin natural dyes applied on cellulose acetate film and cotton cellulose yarns for improvement of light fastness. The presence of nickel hydroxy-aryl-sulphonates can suppress photofading intensity remarkably, while UV-absorbing dyes or finishes or such additives can retard the light fading rate remarkably. Cristea and Vilarem [185], Lee et al. [186] and Micheal et al. [187] have attempted to increase the LF of various natural fibre-based and natural dyed textile fabrics.

Duff et al. [180] studied wash fastness rating of few natural dyed old textiles with respective study of wash stability. Experiments were conducted under 50°C and at 20°C temperature with a washing combination suitable for preservation work for refurbishing of old textiles materials. Some colours showed remarkable alteration in their hue after washing with addition of small amounts of soda-ash or alkali in washing bath formulation, highlighting the importance of the pH for cleaning of any textiles containing natural colourants. In general, natural colourants (on wool) shows moderate wash stability, if evaluated as per ISO-II method.

Hofenk and Graaff [83] examined the nature of fading for natural dyes for both light and wash fastness targeting to retaining/restoration of the original colours of natural dyed-old church or museum textiles or painted flags for conservation. The effects of wash bath surfactant solution for conservator work for natural coloured textiles have been discussed by Hofenk and Graaff [83]. A liquor containing 1gpl of sodium-poly-phosphate was felt to be best as reported by Duff et al. [180] for test of wash stability of old natural dyed textiles remained in the old textiles of church.

Any small increment in cleaning efficiency attributed by the alkali, should be equalised to resist probable changes of shades of natural colours, apart from probable damage of protein fibres in alkaline pH conditions. Duff et al. [180] also examined the wash stability for natural dyes extracted from native Scottish locality/source as compared to imported natural dyes. The fastnesses of the logwood and indigo were found superior against other natural dyes like water-lily root and privet berries respectively when tested as per ISO-II method, but in comparison of imported vs. native natural colourants for yellow, reds, red/purples, greens and browns shades, the difference of wash fastness rating was observed to be minor in between these two groups.

Gulrajani et al. [188] described application of red-sandal wood extract to dye wool and nylon as compared to some other selective natural dyes determining all physico-chemical parameters and rate of dyeing and colour fastness properties. The said fabrics were coloured with four distinct natural colours (turmeric, myrobolan, madder, red sandalwood) with pre, post and simultaneous-mordanting techniques with Aluminium-sulphate mordant. Few samples were also coloured with a mixture of turmeric/madder or turmeric/red-sandalwood and myrobolan mixed with madder or red-sandalwood in various ratio to obtain a compound uncommon shade. Selected mordanted and natural coloured samples were post-treated with normal cationic dye fixing agents to improve wash fastness. Being a direct dye, Turmeric exhibited maximum colour strength during concurrent mordanting with single or combination of other colourants. Turmeric exhibited inferior wash stability, which was enhanced a little during application of a cationic dye fixing agent in after-treatment process or when applied in combination some other natural dyes of higher wash fastness with turmeric having lower rating of wash fastness, to obtain a compound shade of good washing fastness. Combined of turmeric with other natural dyes by simultaneous mordanting method exhibited good colour yield for development of darker shade and observed higher colour strength values than the calculated or expected values. Myrobolanin mixed with other dyes showed more prominent shades with higher colour strength value during its application by post-mordanting process. In concurrent mordanting process, myrobolan did not exhibit any synergistic effect with respect to observed and calculated K/S values, but post-mordanting gave the best results.

Konar et al. [189] studied application of dye extracted from tesu on jute fabric with varying aspects of mordants as well as with varying dyeing process variables to standardise this dyeing process. Application of selective pre-mordant (single and double mordants) on 6% H₂O₂ (50%) bleached jute fabric was carried out using myrobolan (harda) and eco-safe metallic salts mordants (like potash-alum or fitkari and aluminium-sulphate) and subsequent dyeing with aqueous extract of tesu (palash flower petal) under different dyeing conditions for optimization of the dyeing process variables. It was observed that 1st pre-mordanting with 20% myrobolan followed by 2nd mordanting with aluminium-sulphate (20%), in succession, is the most potential double pre-mordanting system rather than when used separately as single mordant, considering the results of textile related characteristics and colour yield for this dyeing. Effect of important process variables regarding dyeing (e.g., time, temperature, pH, MLR, concentration of mordant, dye and salt) on surface colour strength has been evaluated to determine the optimum dyeing conditions. Along with washing, rubbing and light fastness results. Generally, this dyeing is observed to be very sensitive to pH for selective fibre-mordant-dye combinations and it is observed here that dyeing at pH of 11 renders better colour yield and overall all round better colour fastness properties. Enhancement in washing and light fastness can be achieved with suitable post-treatment with cationic dye fixer and UV stabiliser compounds. Dyeing kinetics for dyeing tesu on jute was also reported by Konar et al. [86].

Samanta et al. [85, 190, 191], studied physico chemical parameters of natural dyeing on jute and cotton including its dyeing kinetics and also compatibility of a series of binary pairs of natural colourants by determining all the colour interacting parameters and colour fastness properties. Bleached jute and cotton fabrics (after consecutive premordanting with myrobalan and aluminium-sulphate) have been coloured with a definite ration and concentration of purified mixture of jackfruit wood (JFW) with other natural dyes, like manjistha (MJ), red sandalwood (RSW), marigold (MG), sappan wood (SW) and babul (BL) bark to get desirable combined shades with different colour strength (K/S values) and improved colour fastness. To increase the light fastness and wash-fastness of these binary mixture of natural dyed cotton fabrics, it need additional after-treatment with selected cationic dye-fixing agent along with an UV-absorbing compound, respectively. Post-treatment with 1% CTAB or 1% Sandofix HCF showed one level higher washing fastness for these dyed fabrics and 1% benztriazole treatment showed increment in LF rating approximate by one unit. Singhee and Samanta [192] has studied standardisation of dyeing process variables for application of Tesu as natural colour on silk fabric. Besides natural colouration, UV protective finishing action of anar peel (pomegranate ring) as a bio-dye cum bio-finishing material, applied on cotton khadi fabric, was reported by Sinnur and Samanta et al. [153] as a newer approach of bio-dyeing and bio-finishing, in which field, there are very few scanty reports on similar work with other bio-dyes and bio-finish combinations, need to explore further.

However, from consumer protection point of view. It was highly needed to standardise test methods for identification of exact natural dyes from such natural dyed textile substrate, to assure customers that the dyed product is natural dyed materials. With long painstaking effort for required R&D work and inter lab test trial report in this endeavour, a group of Indian researchers have developed and established dye-wise test methods for identification of specific natural dyes, e g. test method to differentiate between natural indigo and synthetic indigo for identifying natural indigo, which is published by BIS-India as new Indian standards for identification of natural Indigo [193]. A similar Standards on identification of natural madder differentiating it from synthetic alizarin colourant is also published by BIS as another new BIS standard [194] and later in a series, more than eight more natural dye test standards for identification of that specific individual natural dye extracting from that natural dyed textiles are published by BIS for the benefits of consumers by establishing some form of natural dye mark scheme, required both nationally and globally, for assurance of natural materials used.

4. Concluding remarks

Thus from the above said literature review, it is understood that there are still many lack/gaps in scientific studies on effects of different bio-mordants (such as tannin based natural complexes) and mordanting assistants (gallic acid from gall nut or chebulinic acid from harda-based natural complexes) and standardisation/optimization of dyeing process variables as well as effects of different natural resource/plant based after-treating or finishing compounds as dye fixatives/UV absorbers for improving wash fastness, Sun-light/UV light fastness and rubbing fastness besides attempts to increase antimicrobial and UV protection properties of such natural dyes by applying selected post treatments with different natural agents or eco-friendly synthetic agents. It is also necessary to understand the possible dye-fibre interactions and function of various pre and post treatments/additives for increasing colour yield (with respect to K/S values, uniformness of colour yield) by CV% of K/S values as well as antimicrobial grading and UV protection factor for various mordant–fibre-natural colourants application system particularly for cellulosic fibre (cotton) and protein fibres (like silk) when colouring with aqueous or aqua-alcoholic extract of chosen natural colourants. Hence, for revival of application of natural colourants on producing natural bio mordanting-bio dyeing and bio-finishing for producing natural resource based medical textiles and skin friendly high value apparel textiles.

However, one must remember that natural colours are not a replacement of man-made synthetic dyes. Natural dyes have their own customer base and any further enlargement of this customer base for natural-coloured products will not squeeze down the market of any man-made synthetic dyed textiles. So, further studies are required to meet the above-mentioned gaps for applying natural dyes and natural finishes on textile fabrics for improving wellness properties like antimicrobial, mosquito repellent, anti-odour and UV-resistant properties in apparel textiles.