Study on the Compatibility of a Binary Mixture of Babul Bark and Other Natural Dyes Used on Gallnut Pre-Mordanted Cotton Fabric for Developing Compound Shades

Abstract

Cotton fabric that has been pre-mordanted with gallnut extract was dyed using a binary mixture of aqueous extracts of the natural dyes such as babul bark with bixa (annatto) seeds, Rheum emodi (rhubarb) roots and pomegranate rind extracts to obtain compound shades. The compatibility of each binary pair of natural dyes was assessed by (a) a conventional process of analysis through progressive development of shades by changing time and temperature in one set, and variation in the total concentration of dyes in the mixture in the second set with 50:50 dye proportion and (b) by a newer approach of analysing the relative compatibility rating (RCR) of each pair of dyes by varying proportion of both the dyes (75:25, 50:50, 25:75) in the mixture. The study indicates that all three pairs of binary mixtures (babul bark with annatto seeds, babul bark with pomegranate rind and babul bark with Himalayan rhubarb roots) are found to vary in their respective degree of compatibilities and are differently compatible (moderated, fair and average) as determined by the newer RCR method. Interestingly the degree of compatibility found by the conventional method nearly matches with the newer method showing the efficacy of the RCR method/technique.

1. Introduction

Despite a number of drawbacks, natural colourants have been used to dye textiles since ancient times. In recent years, the use of natural dyes has experienced a significant resurgence due to the chemical hazards and harmful effects of some synthetic dyes [1]. Natural dyes offer a restricted range of shades; thus, textile dyers must create a wide range of colours to meet customer demands, colour forecasts, and fashion trends. Development of compound shades using natural dyes involves sequential dyeing in multiple baths, which is invariably time-consuming, expensive and associated with increased fabric damage. Therefore, there is a need for an alternate method that allows dyers to blend natural dyes in a single bath to create wider spectrum of colour shades.

Researchers have investigated the compatibility of binary and ternary synthetic dye combinations on a variety of textiles [2, 3, 4], but similar studies on natural dyes are scarce. Some such research has although been reported on cotton [1], jute [5] and wool [6].

Compatibility between two dyes in a binary mixture is a very important aspect for obtaining compound shades on textiles, leather etc. and the use of incompatible dyes in a mixture for compound shades is not advisable. Additionally, since the colour build-up of one dye and the colour build-up of the second dye in a binary mixture occur at different rates, it is difficult to control the development of compound shades to a desirable level to obtain a particular tone of colour. Thus, dyes having comparable or identical exhaustion properties are deemed to be compatible with one another [7]. There are several approaches of judging compatibility between two dyes in a binary/tertiary mixture [1, 4, 8, 9, 10, 11].

Though some techniques of determining the compatibility of dyes rely on the assumption that dyes are non-interactive and their rate of dyeing do not change in the presence of another dye, this supposition is not always true. The compatibility between two dyes can be assessed by the following methods [1, 2, 3, 4, 5]:

1. By determining the rate of dyeing of individual dyes.

2. By colourimetric analysis of progressive development of shade depth (colour build-up) under varying dyeing conditions i.e., either by varying dye concentration in the binary mixture or by varying time and temperature profiles keeping the dye concentration fixed through plots of K/S vs. ∆L and ∆C vs. ∆H.

3. By determining the diffusion coefficient of the dye by each dye in a pair of binary mixture.

All these three methods are subjective, highly time-consuming, analytical and skill-based requiring a precision control of time and temperature for progressive shade build-up. Additionally, they lack quantitative assessment of degree of compatibility rating.

Thus, it is preferable to use a compatibility test method in which the substrate is dyed with a mixture of dyes under realistic dyeing conditions as opposed to dyeing them with individual dyes and then predicting the compatibility from the rate of dyeing or other dyeing results because all dyeing process variables can affect the final dyeing results/outcomes [4]. The compatibility between two dyes can be assessed by a newer relative compatibility rating method based on the difference in colour difference index for dyeing the substrate with varying proportion of two dyes and a relative grading of degree of compatibility have been suggested [1, 5, 12]. Relative compatibility rating (RCR) method for binary dye mixtures is a more recent method that provides an easily usable way of determining the compatibility of binary combinations of dyes [1].

Previously established [1], this faster and more straightforward RCR method of dye compatibility testing is based on data for CDI differences (CDI_Max − CDI_Min) on dyeing results of varying proportion of individual dyes in a binary mixture following standard/optimised dyeing conditions and method. This newer RCR method can be used to test dye compatibility more quickly than the conventional method, which requires time-consuming, laborious and cumbersome plotting of different colour parameters for two separate sets of dyeing and the determination of the more difficult dyeing coefficient or rate of dyeing. The conventional method also does not provide relative quantitative grading for degree of compatibility.

The present study thus explores the application of newer RCR method of compatibility test of any two dyes, for choosing right compatible dyes only for development of newer shades using binary mixtures of natural dyes (babul bark with annatto seeds, babul bark with Himalayan rhubarb roots and babul bark with pomegranate rind) and the compatibility of the dyes used in the binary mixtures in different proportions on bleached pre-mordanted cotton fabric by both the conventional method of progressive colour build-up (with the plotting of K/S vs. ∆L and ∆C vs. ∆L) as well as the newer relative compatibility rating technique. Both methods have been compared for babul bark with annatto seeds, babul bark with Himalayan rhubarb roots and babul bark with pomegranate rind etc. to judge their efficacy for further endorsement.

2. Methodology

2.1 Fabric

Hand-spun and hand-woven, bleached 100% khadi cotton fabric from Khadi Silk Emporium, Kolkata was used in this study (Table 1).

Fibre	Weave	Warp/ends	Weft/picks	GSM	Thickness
Cotton	Plain	86 ends of 15 tex/inch	61 picks of 16 tex/inch	76 g/m^2	0.31 ± 0.2 mm

Table 1.

Fabric specifications.

2.2 Natural dyes, chemicals and auxiliaries

Dried gallnuts (Quercus infectoria) as a natural mordant, and natural dyes like dried babul bark (Acacia nilotica), annatto (Bixa orellana), dried pomegranate rind (Punica granatum) were obtained from Kangali Charan & Sons, Kolkata. Ready to use natural dye powder of Rheum emodi (Himalayan rhubarb) was sourced from AMA Herbal Laboratories Pvt. Ltd., Lucknow.

Gallnuts (Quercus infectoria) are collected from the oak tree or similar shrubs like the walnut tree after they are formed as a response to insect infestation. The galls from Quercus infectoria contain the highest level of naturally occurring tannins, approximately 50–70% [13]. They also contain 2–4% each of gallic and ellagic acid. The main colouring component in gallnut extract is ellagic acid [14], which has an affinity for dyeing substrates due to the presence of OH (auxochrome group) (Figure 1).

Figure 1.

Chemical composition of gallnut (Quercus infectoria).

The pod/leaves and bark of the Acacia Nilotica (babul bark) tree locally known as kikar or babul or Indian gum Arabic tree in India are rich in polyphenols being mainly composed of natural tannins containing gallic acid, ellagic acid and gallotannins having light yellow colour in condensed non-hydrolysable forms, some catechin and epigallo-catechin gallate having much darker yellow-brown colour gallate [15, 16, 17] besides few yellow-coloured amino acids (like NMT) and other minor constituents. The tannin content in babul bark ranges from 9 to 16.5% on dry weight (Figure 2) [17, 18].

Figure 2.

Chemical composition of babul bark (Acacia nilotica).

Bixa orellana (annatto) seeds is a tall, perennial shrub which is the source of 70% of all natural colourants consumed worldwide. The dye contains 80% of coloured unsaturated pigment i.e. bixin (monomethyl ester of the dicarboxylic carotenoid compound), and norbixin (9′-cis-6,6′-diapocarotene-6,6′dioic acid, C₂₄H₂₈O₄) as a yellow pigment [19]. In addition to bixin and norbixin, annatto seeds also contain isobixin, bixein, lutein, beta-carotene, cryptoxanthin, crocetin, zeaxanthin, bixol, ishwarane, threonine, tryptophan, phenylalanine, ellagic acid, salicylic acid and tomentosic acid (Figure 3) [20].

Figure 3.

Chemical composition of annatto seeds (Bixa orellana).

Rheum emodi (Himalayan rhubarb) is a powerful perennial herb having strong roots and a hollow stem with reddish streaks and a green colour. A variety of anthraquinone derivatives, including emodin, emodin-3-monomethyl ether, chrysophanol, aloe-emodin, and rhein, are present in Indian rhubarb [21] and physcion. Chrysophanolalso known aschrysophanic acid is the main colouring agent of rhubarb (Figure 4) [22].

Figure 4.

Chemical composition of Himalayan rhubarb roots (Rheum emodi).

Punica granatum (Pomegranate rind), is the most significant of two species of the Punica Genus that contains a significant amount of tannin in its flower, leaf, root-bark, fruit and peels [23]. The main colouring agent in pomegranate rind is granatonine, which is found as N-methyl granatonine [24]. The fruit’s rinds also contain rutin and quercetin. Condensed tannins make up 26% of the chemical content of the peel (Figure 5).

Figure 5.

Chemical composition of pomegranate rind (Punica granatum).

Other chemicals used like amylase enzyme (for desizing), acetic acid and sodium carbonate (for adjusting the pH), conc. sulphuric acid, hydrochloric acid, ferric chloride, aluminium chloride, chloroform, ammonia, and 1% picric acid were obtained from E-Merck (India). Reagents (Dragendroff, Mayer and Wagner) were obtained from Nice Chemicals Pvt. Ltd., Kolkata and non-ionic wetting agent (Axel NW 100) from Bharati Chemicals (Kolkata).

3. Methods

3.1 Pre-treatment of cotton fabric

The cotton fabric was desized using 2 ml/l of amylase enzyme, 5 g/l NaCl, 2 g/l non-ionic wetting agent, 1:30 MLR, at 50–90°C for 15 min as reported in earlier literature [25]. The treated samples underwent a thorough 15-min wash in hot water. Finally, 100°C was used to dry the textile samples to a consistent weight.

3.2 Extraction of colourants from natural resources

All materials from natural sources (gallnut, babul bark, annatto seeds and pomegranate rind) were dried in the sun and powdered using a mechanical grinder.

The aqueous extraction of gallnut powder was done at the optimised conditions reported earlier [26] at MLR—1:20, extraction time—45 min, temperature—80°C and at pH—11.

The aqueous extraction of babul bark was done at the optimised conditions reported earlier [16] at MLR—1:30, extraction time—45 min, temperature—60°C and pH—6.

Aqueous extraction of the colourants from powdered bixa and pomegranate rind were carried out under varying process conditions—pH (3–11), MLR (1:10–1:50), time (15–120 mins) and temperature (37–100°C) and optimization parameters identified based on the optical density with the greatest value at maximum absorbance wavelength of the solution (Table 2).

Natural materials	λmax	pH	MLR	Time (in min)	Temperature (°C)
Annatto seeds	410	11	1:20	75	80
Pomegranate rind	420	11	1:20	45	80

Table 2.

Optimised extraction condition parameters for different natural materials.

Ready-to-use dye powder of Himalayan rhubarb roots was extracted at 70–80°C for 60 min as recommended by the manufacturer.

3.3 Mordanting of cotton fabric with gallnut extract

Desized cotton fabric was pre-mordanted with gallnut under optimised conditions of mordant concentration—20% owf, 1:20 MLR, 11 pH at 80°C for 45 min as reported earlier [26].

3.4 Dyeing of pre-mordanted cotton using a mixture of two natural dye extracts

Gallnut extract (aqueous) pre-mordanted cotton fabric was dyed with aqueous extracts of both single or selected binary pairs of natural dyes in varying proportions (100:0, 75:25, 50:50, 25:75 and 0:100) as mentioned below:

1. B1: babul bark + annatto seeds (BB & AS)

2. B2: babul bark + Himalayan rhubarb roots (BB & HR)

3. B3: babul bark + pomegranate rind (BB & PR)

The binary mixtures of the dye extracts were applied on gallnut pre-mordanted cotton fabric maintaining the overall total concentration of both the selected dyes (in the binary mixture) at 40% (on the weight of source material) at 100°C for 60 min using MLR 1:20 and 10% owf of sodium chloride as additive using the exhaust process. Following dyeing, the samples were soaked in a 2 g/l soap solution for 15 min at 60°C, then rinsed and allowed to air dry in the shade.

4. Test methods

4.1 Phytochemical analysis

Analysis of the phytochemicals found in the aqueous and ethanolic extracts of natural mordants and dyes was done using the standard Trease and Evans and Harbone [27, 28] procedure.

4.2 UV-vis spectral analysis

The wavelength of maximum absorbance for 0.1% aqueous extracts of the natural dyes were identified at 410 nm for gallnut and bixa seeds; 420 nm for pomegranate rind; 430 nm for Rheum emodi and 510 nm of babul bark, separately, through UV-vis spectral analysis using UV-vis absorbance spectrophotometer (Hitachi-U-2000 model).

4.3 Surface colour strength

Surface colour strength of the control and dyed cotton samples was estimated using the Kubelka Munk equation [29] utilising a Premier Colour Scan reflectance spectrophotometer (model SC 5100A) and related colourlab plus colour matching software to measure the surface reflectance of each of the dyed samples at its respective λ_max.

From the measured K/S values for 100% individual dye applied on a pre-mordanted cotton fabric, the calculated K/S values (as K/S values are additive and liner) were obtained for samples dyed with specific properties of a selected pair of dyes using the following equation [5]:

$$\text{Calculated}\text{K}/{\text{S}}^{\text{a}}=\frac{\text{m}}{100}\text{K}/{\text{S}}^{\text{b}}+\frac{\text{N}}{100}\text{K}/{\text{S}}^{\text{c}}$$E1

Where (^a) is observed for the m:n proportion of the two dyes (A & B) in the binary mixture, (^b) is observed for 100% of dye A and (^c) is observed for 100% of dye B.

4.4 Colour interaction parameters

Using a Premier Colour Scan (model SC 5100A) reflectance spectrophotometer and associated Colourlab plus colour matching software, values for total colour difference (ΔE), lightness/darkness (ΔL*), redness/greenness (Δa*), blueness/yellowness (Δb*), change in chroma (ΔC*), and change in hue (ΔH_ab) were measured before and after dyeing to compare the shade depth and colour differences of each dyed sample against a particular undyed (bleached/mordanted) standard sample [29].

4.5 General metamerism index

Nimeroff and Yurow’s equation [30] was used to calculate the general metamerism index (MI) of the treated and dyed samples.

4.6 Colour difference index

The colour difference index (CDI) [1] was also used in the current work to understand the combined effects on total differences in colour strength with varying rate of colour build up and difference in hue and chroma and metamerism for different proportion of dyes or variations in different dyeing process variables by a single index value. CDI indicates the combined effects of various known individual colour difference parameters between any two samples when dyed with varying shade under various dyeing conditions. Relatively, higher CDI values indicate more criticality in the control of colour variation with a particular dyeing condition/variation in the individual dye proportion in their binary mixture for development of compound shades.

Thus, colour difference index (CDI) values represent the overall effect of colour variation arising due to variations in dyeing process variables in terms of the major colour difference parameters between sample-1 (standard) and sample-2 (produced) when dyed under different conditions using different proportion of binary mixture of dyes etc. This indicates overall dispersion of varied colour yield and colour differences in terms of hue, chroma, metamerism values. Colour difference index (CDI) was calculated by an established empirical relationship established earler [1]:

$$\text{Colour}\text{Difference}\text{Index}\left(CDI\right)=\frac{\Delta E\times \Delta H}{\Delta C\times MI}$$E2

4.7 Compatibility tests for selected binary pairs of natural dyes

4.7.1 Conventional method

The following selected binary pairs (50:50) of natural dyes were applied on the pre-mordanted cotton fabric using an overall 40% (owf) of the respective extracts.

Two sets of gallnut pre-mordanted cotton fabric samples were dyed in progressive depth of shade for each selected binary pair of dyes (babul bark with annatto seeds, Himayalan rhubarb roots, and pomegranate) taken in equal proportions (50:50).

B1: babul bark + bixa seeds (BB & AS).

B2: babul bark + Himalayan rhubarb roots (BB & HR).

B3: babul bark + pomegranate rind (BB & PR).

The shade was gradually/progressively developed in the first set of samples (Set-I) by adjusting the dyeing time and temperature. Three separate small pre-mordanted cotton fabric samples were dyed for each pair of dyes, using Lab Dyer MAG Solvics make with a temperature controller for different dyeing time periods (15–60 min). At intervals of 15 min starting at 60°C, the samples were taken out of the dye bath while still being heated at a rate of 1° per min until 90°. The first sample was removed after 15 min at 60°C, and the last one was removed 60 min later at 90°C, marking the end of the dyeing process. The incremental depth of darkness (progressive shade depth) in Set-II was created by changing the dye mixture’s overall concentration from 10 to 40%. Three separate small pre-mordanted cotton fabric samples were dyed in triplicate for each pair of dyes at intervals of 10% using 10–40% (owf) of each pair of dyes applied in equal proportion (50:50) at 90°C for 60 min.

The dyed fabrics for both dyeing processes (Set-I and Set-II) were washed, soaped, and rinsed before being air-dried. The differences in the CIELab coordinates, ∆L, ∆a, ∆b and ∆C for all the dyed fabrics using Set I and II methods in comparison to the standard undyed fabric sample were measured separately using the Macbeth 2020 Plus Reflectance Spectrophotometer and related colour matching software. The compatibility of a selective pair of dyes was judged from the degree of closeness and overlapping of the two curves i.e., ∆C vs. ∆L or K/S vs. ∆L corresponding to the two sets of dyeing (Set-I & Set-II).

4.7.2 Newer RCR method (alternative)

The related ∆E, ∆C, ∆H and MI values’ magnitudes, regardless of their sign or direction, may be utilised to produce the CDI (colour difference index) after the application of various ratios of binary pairs of dyes on the same fabric [4]. The compatibility rating (between 0 and 5, rating 5 shows maximum or excellent compatibility, rating 1 represents minimal or worst compatibility, and rating 0 is viewed as fully non-compatible) increases with the proximity of the CDI values for binary pairings of dyes.

4.8 Colour fastness properties

Fastness to washing of the dyed cotton was assessed using the Launder O Meter in accordance with AATCC Test Method 8-2009 [31]; fastness to light was assessed using AATCC Test Method 16-2004 [31]; fastness to rubbing (dry and wet) was evaluated using AATCC Test Method 8-2007 [31]; and fastness to perspiration (alkaline and acidic) was assessed using AATCC Test Method 15-2009 [31].

5. Results and discussions

5.1 Standardisation of extraction conditions for annatto seeds and pomegranate rind

The colouring matter from annatto seeds and pomegranate rind was extracted under various pH, MLR, time, and temperature conditions. The optical values for each parameter were established based on the highest optical densities of the corresponding solutions (Table 3).

Extraction process condition		Optical density at λmax
		Gallnut (at 410 nm)	Annatto seeds (at 410 nm)	Pomegranate rind (at 420 nm)
pH	3	1.2	0.5	1.6
	5	1.2	1.0	1.6
	7	1.3	1.6	1.7
	9	1.3	1.7	1.7
	11	1.4	1.8	1.8
MLR	1:10	1.1	1.8	1.7
	1:20	1.2	1.9	1.8
	1:30	1.1	1.6	1.8
	1:40	1.0	1.2	1.7
	1:50	0.9	1.1	1.6
Time	15	1.2	1.0	1.4
	30	1.2	1.3	1.6
	45	1.3	1.5	1.7
	60	1.2	1.6	1.6
	75	1.2	1.8	1.6
	90	1.2	1.8	1.5
	120	1.2	1.8	1.5
Temp	RT	1.1	1.1	1.5
	60	1.2	1.6	1.6
	80	1.3	1.8	1.7
	100	1.1	1.7	1.6

Table 3.

Optical densities at λ_max of aqueous extract of gallnut, annatto seeds and pomegranate rind.

Aqueous extraction of annatto seeds under alkaline pH gave a higher colour yield as shown in Table 2. pH 11 gave the highest optical density at 410 nm. A higher liquor ratio probably has a dilution effect thereby lowering the optical density of the extract. Thus, an MLR of 1:20 was considered optimum. The optical density of annatto seeds shows an increasing trend till 75 min after which the remains the same. Hence 75 min is considered optimum. The highest optical density was observed at 80°C, after which it declined. This indicates that most of the colouring matter from the annatto seeds is leached out at this temperature. Thus 80°C temperature is considered optimum.

Extraction of pomegranate rind is most favourable in an alkaline medium and gives better colour yield with the highest optical density value obtained at pH 11 (Table 3). Low MLR of 1:20 and 1:30 are best suited for extraction of pomegranate rind, but 1:30 requires a high amount of water and energy; hence, 1:20 was selected as optimum. After 45 min the optical density reduces, thus 45 min is considered optimum. With the increase in temperature to 80°C, the optical density diminishes. Thus, a temperature of 80°C was considered optimum (Table 4).

Natural resources	pH	MLR	Time (min)	Temperature (°C)
Gallnut	11	1:20	45	80
Annatto seeds	11	1:20	75	80
Pomegranate rind	11	1:20	45	80
Himalayan rhubarb roots	—	—	60	70–80

Table 4.

Optimised extraction conditions for aqueous extract of gallnut, annatto seeds and Himalayan rhubarb roots.

The extraction of Himalayan rhubarb roots was done as recommended by the manufacturer.

5.2 Phytochemical analysis of different natural resources

Gallnut, babul bark, annatto seeds, Himalayan rhubarb roots and pomegranate rind were extracted in two media—aqueous and ethanolic and phytochemical screening was done for each extract (Table 5).

Phytochemical screening	Gallnut		Babul bark		Annatto seeds		Himalayan rhubarb roots		Pomegranate rind
	Aq	Et	Aq	Et	Aq	Et	Aq	Et	Aq	Et
Alkaloids
Dragendorff’s reagent (potassium bismuth iodide)	2+	2+	2+	3+	3+	3+	2+	2+	2+	3+
Mayer’s reagent (potassiomercuric iodide)	1+	2+	2+	2+	2+	2+	1+	2+	2+	2+
Wagner’s reagent (iodo-potassium iodide)	2+	3+	2+	3+	3+	3+	2+	2+	2+	3+
Picric acid [(O2N)3C6H2OH]	2+	3+	2+	3+	3+	3+	2+	2+	3+	3+
Flavonoids
Ammonium	2+	3+	2+	3+	3+	3+	3+	3+	3+	3+
Aluminium chloride	2+	3+	2+	3+	3+	3+	3+	3+	3+	3+
Tannins
Ferric chloride	2+	3+	2+	3+	1+	1+	1+	1+	2+	3+
Terpenoids
Salkowski	2+	2+	1+	3+	3+	3+	3+	3+	2+	3+
Saponins
Emulsion	2+	2+	2+	2+	—	—	—	—	1+	1+

Table 5.

Qualitative examination of the phytochemicals in gallnut, babul bark, annatto seeds, Himalayan rhubarb roots and pomegranate rind.

Aq, aqueous extract; Bt, ethanolic extract; −, absence of compound; +, presence in low amount; ++, presence in moderate abundance; +++, presence in high amount.

The phytochemical screening of gallnut, babul bark, annatto seeds, Himalayan rhubarb roots and pomegranate rind extracts indicated the existence of certain compounds in their extracts (extraction in ethanol was done only for phytochemical analysis). Flavonoids, alkaloids, tannins, terpenoids and saponins are present in all five extracts prepared under different mediums. The extract of gallnut, babul bark and Himalayan rhubarb roots showed a moderate presence of alkaloids in both the mediums. Flavonoids were present in high amounts in the extraction extract of all five compounds, but it was present in moderate amounts in the aqueous extract. Tannins were present in high amounts when extraction was carried out in an ethanolic medium for gallnut, babul bark and Himalayan rhubarb roots, but it was present in moderate amounts in aqueous extract. Terpenoids were also present in high amounts in both the mediums (aqueous and ethanol) in bixa seeds and Himalayan rhubarb roots whereas in gallnut, babul bark and pomegranate rind is present in moderate amounts. Saponins were present in considerable amounts in gallnut, babul bark and pomegranate rind in both the extracts, but bixa seeds and Himalayan rhubarb roots do not report the presence of it.

5.3 UV-vis spectral analysis and UV-vis characterisation

UV-vis spectra with the corresponding UV-vis peak frequency at different wavelength (nm) of gallnut, babul bark, annatto seeds, Himalayan rhubarb roots and pomegranate rind aqueous extracts are shown in Figure 6(a–e) and characterisation of the different peaks in the visible and UV zone have been tabulated (Table 6).

Figure 6.

(a–e) Absorbance spectra of gallnut, babul bark, annatto seeds, Himalayan rhubarb roots and pomegranate rind extracts in the UV-range (100–1100 nm).

Sl. no.	Name of the natural source (aqueous extract)	Peaks in UV zone (nm) & content		Peaks in visible zone & content
1	Gallnut	212	Gallic acid and gallotannins	420	Gallic acid and gallotannins
		275	Gallic acid and gallotannins	—
2	Babul bark	240	Condensed gallotannins	510	Dark brown coloured catechin and epigallocatechin gallate
		270	Condensed gallotannins	—	—
		290	Condensed gallotannins	—	—
3	Annatto seeds	238	Caroteniod isomers	410–420	Bixin
		370	Some flavonoid and phenolic compounds	490–520	Nor-bixin
4	Himalayan rhubarb	348	Phenolic componds	430–450	Anthraquinone derivatives
		290	Condensed gallotannins	550–600	Anthraquinone compounds
		256–226	Hydroxy anthroquinone	—	—
5	Pomegranate rind	371	Presence of flavonoid and phenolic compounds	420	Presence of gallotannins
		364	Presence of C〓O group	—	—
		334	Presence of phenolic compounds	—	—

Table 6.

Identification and interpretation of peaks in the visible and UV zones.

5.3.1 Bixa orellana (annatto) seeds

Three peaks have been identified at 240 nm, 416 nm and 494 nm confirming the presence of carotenoid compounds [32]. Other researchers have [33] also found out, that the annatto seeds extract exhibits the highest absorbance between 400 and 500 nm (λ_max), confirming the presence of bixin and nor-bixin compounds.

5.3.2 Rheum emodi (Himalayan rhubarb) Roots

Absorption peaks in aqueous extract of Himalayan rhubarb roots (Rheum emodi) were identified at 226 nm, 256 nm, 290 nm, 348 nm, 430 nm and 600 nm. The absorption peaks around 226–256 nm indicate the existence of hydroxyanthraquinone group [34], absorption at 290–340 nm may be due to the occurrence of several phenolic compounds [35] while in the visible range, the absorption peaks of 430 nm and 600 nm show the presence of anthraquinone compounds and derivatives [36].

5.3.3 Punica granatum (Pomegranate) rind

It shows a significant peak between 330 and 370 nm confirming the presence of flavonoid and phenolic compounds [37]. A significant peak in the visible region 420 nm is visible and it indicates the presence of gallotannins [38].

5.4 Colour strength, related parameters and brightness index of pre-mordanted cotton fabric with gallnut dyed with selected binary pairs of natural dyes in different proportions

Colour strength, related parameters and brightness index of pre-mordanted cotton fabric with gallnut dyed with selected binary pairs of natural dyes in different proportions namely B1—babul bark + annatto seeds (BB & AS), B2—babul bark + Himalayan rhubarb (BB & HR) and B3—babul bark + pomegranate rind (BB & PR) are tabulated in Table 7.

Dye combination	Proportion of binary pairs of dyes
	100:0		75:25		50:50		25:75		0:100
	AD1	BD2	AD1	BD2	AD1	BD2	AD1	BD2	AD1	BD2
B1 (BB:AS)
Observed	5.5	6.6	5.3	6.9	5.4	6.8	5.5	6.7	6.1	6.9
Calculated	—	—	5.6	6.7	5.7	6.8	5.9	6.8	—	—
D1	—	—	0.3	0.2	0.3	0.0	0.4	0.1	—	—
D2	1.1		1.6		1.4		1.2		0.8
B2 (BB:HR)
Observed	5.5	6.1	5.1	5.4	4.7	4.8	4.4	4.5	4.5	4.7
Calculated	—	—	5.2	5.7	4.9	5.3	4.8	4.9	—	—
D1	—	—	0.1	0.3	0.2	0.5	0.4	0.5	—	—
D2	0.6		0.3		0.1		0.1		0.2
B3 (BB:PR)
Observed	5.5	6.3	5.6	5.3	4.7	5.1	4.5	5.0	4.4	4.3
Calculated	—	—	5.3	5.8	4.9	5.2	4.7	4.8	—	—
D1	—	—	0.3	0.5	0.2	0.1	0.2	0.2	—	—
D2	0.8		0.3		0.4		0.5		0.1

Table 7.

Observed and calculated K/S values and related parameters of cotton fabric pre-mordanted with gallnut and dyed with selected binary pairs of natural dyes in different proportions.

BB, babul bark; AS – Annatto seeds; HR – Himalayan rhubarb roots; PR, pomegranate rind; AD1, K/S at λ_max of BB; BD2, K/S at λ_max of BX/RE/PR; D1, difference between observed and calculated values; D2, difference between two sets of observed vales at the respective λ_max of the two dyes in the binary pair.

The observed K/S (surface colour strength) of cotton dyed with a mixture of babul bark and annatto seeds (BB + AS), Himalayan rhubarb (BB + HR), and pomegranate rind (BB + PR) is slightly lower than the theoretical K/S that has been calculated separately by adding the K/S for each proportion of the dye in the binary mixture (Table 7). This is true for all different ratios of the two dyes used in the binary mixture (i.e., 25:75, 50:50, and 75:25) although the difference varies from one pair of dyes to another and for different ratios utilised.

The metameric effect considering the differences in the two K/S values measured at λ_max of the respective dyes in the binary pair (D2) for each pair of natural dyes is found to be minimum for the B2 (BB + HR) mixture. The order of increasing difference between two sets of observed K/S values at two different wavelengths values for each pair of natural dyes is given in Table 7:

$$\text{B}2\left(BB+RE\right)<\text{B}3\left(BB+PR\right)<\text{B}1\left(BB+BX\right)$$

The surface colour strength for all binary pairs of dyes is highest for B1 (BB + AS) compared to other binary pairs B2 (BB + HR) and B3 (BB + PR), when K/S is assessed at the common wavelength of all the dyes used (410 nm). K/S values’ ascending order for the different dyes used in varying ratios in their binary pairs is

$$\text{B}1\left(BB+BX\right)<\text{B}3\left(BB+PR\right)<\text{B}2\left(BB+RE\right)$$

K/S reduces with decreasing amount of babul bark in the mixture for B2 (BB + HR)) and B3 (BB + PR) binary pairs (Table 8), but for the B1 (BB + AS) pair of dyes, the presence of more babul bark in the mixture decreases the surface colour strength (K/S) in general.

Dyeing conditions	Proportion	K/S at λmax (410 nm*)	ΔE	ΔC	ΔL	ΔH	BI	MI
Control (gallnut pre-mordanted cotton)		3.9	—	—	—	—	—	—
B1 (BB + AS)	100:0	6.6	18.6	9.9	15.2	9.9	10.4	3.8
	75:25	6.9	19.3	9.1	13.0	10.9	10.2	5.0
	50:50	6.8	19.5	12.2	10.8	10.8	10.4	5.3
	25:75	6.7	23.1	17.6	9.8	11.3	9.4	6.2
	0:100	6.9	37.9	23.8	10.4	13.6	5.2	7.7
B2 (BB + HR)	100:0	6.5	19.1	8.4	15.8	9.9	10.3	5.0
	75:25	5.8	17.2	4.8	13.9	8.9	10.6	3.8
	50:50	4.3	11.9	4.0	9.1	6.7	13.1	3.5
	25:75	3.6	6.9	3.8	7.5	3.7	16.0	2.8
	0:100	3.5	5.9	3.7	6.8	1.5	17.7	1.9
B3 (BB + PR)	100:0	6.6	18.2	7.9	14.8	9.8	10.5	4.9
	75:25	6.7	12.9	6.0	9.5	6.5	13.5	3.6
	50:50	5.9	7.3	5.4	7.3	3.6	17.9	2.6
	25:75	4.7	8.8	7.9	6.3	2.4	16.2	1.5
	0:100	4.4	6.6	6.0	5.3	1.1	18.1	1.1

Table 8.

Colour strength, metamerism and brightness index and other colour related parameters of cotton fabric pre-mordanted with gallnut and dyed with selected binary pairs of natural dyes in different proportions.

^*

410 nm—common.

BB, babul bark; AS – Annatto seeds; HR – Himalayan rhubarb roots; PR, pomegranate rind; Control, cotton fabric pre-mordanted with gallnut extract; ΔC, change in chroma; ΔH, change in hue; ΔE, total colour difference; MI, metamerism index; BI, brightness index.

Colour difference (ΔE) increases with a reduction in the proportion of babul bark in the binary mixture with annatto seeds (B1), but it reduces with an increase in babul bark proportion in the mixture with Himalayan rhubarb (B2) or pomegranate rind (B3). The binary pair of babul bark and pomegranate rind (B3) shows the lowest ΔE regardless of the ratio of each dye used in the binary mixture (Table 8). This is followed by B2 (BB + HR) and B1 (BB + AS) pairs of dyes. The increasing order of ∆E values for the three selected binary pairs of dyes are in the following order:

$$\text{B}1\left(BB+BX\right)<\text{B}2\left(BB+RE\right)<\text{B}3\left(BB+PR\right)$$

Change in chroma (ΔC) is positive indicating that it is lowest or minimum for all corresponding dye proportions (25:75, 50:50 and 75:25) used in the B2 (BB + RE) pair of dyes, while a maximum change in ΔC is observed for B1 (BB + AS) pair of dyes used in different ratios (Table 8). The increasing ∆C values for the three selected binary pairs of dyes are in the following order:

$$\text{B}1\left(BB+BX\right)<\text{B}3\left(BB+PR\right)<\text{B}2\left(BB+RE\right)$$

The B1 (BB + AS) pair of dyes has the highest growing order of magnitude for ∆H values across all ratios/combinations (Table 8). The increasing ∆H values for the three selected binary pairs of dyes are found to be in the following order:

$$\text{B}1\left(BB+BX\right)<\text{B}2\left(BB+RE\right)<\text{B}3\left(BB+PR\right)$$

Brightness index (BI), another important colour characteristic for dyed materials, is significantly impacted by surface sheen and specular reflectance. The brightness index values for the selected binary pair of dyes used in this study are found to be in the following order (Table 8):

$$\text{B}3\left(BB+PR\right)<\text{B}2\left(BB+RE\right)<\text{B}1\left(BB+BX\right)$$

Metamerism index (MI) is minimum for B3 (BB + PR) pair of dyes under all different ratios used in the binary pair. The order of increasing MI for the selected binary pair of dyes used in this study is found to be as follows:

$$\text{B}3\left(BB+PR\right)<\text{B}2\left(BB+RE\right)<\text{B}1\left(BB+BX\right)$$

5.5 Colour fastness of cotton fabric pre-mordanted with gallnut and dyed withselected binary pairs of natural dyes in different proportions

The fastness of cotton fabric pre-mordanted with gallnut and dyed with selected binary pairs of natural dyes in different proportions (Table 9) indicates that the light fastness of all combinations of dyes in the selected binary mixture are found to be good (rating of 4 and 5). Wash fastness for all the dyed samples also ranges between moderate (rating of 2–3) to good (rating of 3–4). All of the samples dyed with binary mixtures of natural dyes in various proportions have excellent (rating of 4–5) and good (rating of 3–4) dry and wet rubbing fastness, respectively, indicating that no superficial unfixed dye molecules are left on the fibre surface and the dyes have been well absorbed inside the fibre. For all binary pairs of natural dyes, fastness to acidic perspiration is either comparable to or significantly superior to alkaline perspiration fastness.

Dyeing conditions	LF	Wash fastness (ISO-II)			Crocking fastness		Perspiration fastness
							Acidic			Alkaline
		LoD	StC	StCW	Dry (STC)	Wet (STC)	LoD	StC	StCw	LoD	StC	StCw
Control*	4	4	5	5	5	4–5	3–4	4	4	3–4	3–4	4
75:25 proportion
B1 (BB + AS)	5	3–4	4	3–4	4–5	3–4	3	4	4	3–4	4	4–5
B2 (BB + HR)	5	2–3	3–4	3–4	4–5	4	3–4	4	3–4	3	4	4
B3 (BB + PR)	5	3	3–4	3–4	4–5	4	3–4	3–4	3–4	2–3	3–4	3–4
50:50 proportion
B1 (BB + AS)	5	3	4	3–4	4–5	3–4	3	4	3–4	3–4	4	3–4
B2 (BB + HR)	5	2–3	3–4	4	4–5	4	3–4	4	4	3	3	3–4
B3 (BB + PR)	5	3	3–4	3–4	4–5	4	4	4	3–4	3	3–4	3–4
25:75 proportion
B1 (BB + AS)	4	3	3–4	3–4	4–5	3–4	3	4	3	3–4	4	4
B2 (BB + HR)	5	3–4	4	3–4	4–5	3–4	3–4	4	3–4	3	4	3–4
B3 (BB + PR)	4	3–4	4	4	4–5	4	3–4	3–4	3–4	3	4	4

Table 9.

Colour fastness of cotton fabric pre-mordanted with gallnut and dyed with selected binary pairs of natural dyes in different proportions.

^*

Cotton fabric pre-mordanted with gallnut extract.

BB, babul bark; AS – Annatto seeds; HR – Himalayan rhubarb roots; PR, pomegranate rind; LoD, loss in depth of shade; StC, staining of adjacent cotton fabric; StCw, staining of adjacent cotswool fabric; LF, light fastness.

5.6 Compatibility tests for selected binary mixtures of natural dyes

Binary dye pairings exhibit a wide range of responses to different dyeing techniques. A pair of dyes may show compatibility under one set of dyeing conditions, but they may turn out to be incompatible under another set of conditions. When a certain fibre is dyed with one dye, it does not necessarily follow that the dye will behave the same way when mixed with another dye.

Plotting K/S vs. L and C vs. L for two sets of progressive depth of shade produced by dyeing pre-mordanted cotton with the selected binary mixture of natural dyes taken in equal proportion, and then judging the closeness and degree of overlap between two sets of curves (Set-I and Set-II) are the traditional/conventional methods for testing compatibility between dyes. To provide dyers the choice of selecting the right mixture of dyes to match a desired compound hue, it is important to test the relative compatibility of various binary mixtures of natural dyes using some type of quantitative term or rating system (Table 10).

Dyeing conditions		Time (in min)	Temperature (°C)	Dye concentration (%)	K/S	ΔC	ΔL
B1 (BB + AS) in 50:50
SET-I (dye concentration fixed at 40% owf with varying time and temperature)	BX1	15	60	40	6.5	11.8	−6.3
	BX2	30	70	40	6.6	11.6	−6.6
	BX3	45	80	40	6.9	11.8	−7.2
	BX4	60	90	40	6.7	12.9	−7.1
SET-II (time and temperature fixed with varying dye concentration)	BX5	60	80	10	6.5	12.1	−6.2
	BX6	60	80	20	6.6	10.4	−6.8
	BX7	60	80	30	7.0	12.9	−7.4
	BX8	60	80	40	6.9	12.5	−7.6
B2 (BB + HR) in 50:50
SET-I (dye concentration fixed at 40% owf with varying time and temperature)	BR1	15	60	40	4.3	7.1	−5.5
	BR2	30	70	40	4.1	5.5	−5.0
	BR3	45	80	40	4.1	6.7	−5.0
	BR4	60	90	40	4.4	5.8	−4.4
SET-II (time and temperature fixed with varying dye concentration)	BR5	60	80	10	3.9	6.3	−4.6
	BR6	60	80	20	3.9	6.1	−3.9
	BR7	60	80	30	4.2	6.4	−5.0
	BR8	60	80	40	4.4	6.8	−5.8
B3 (BB + PR) in 50:50
SET-I (dye concentration fixed at 40% owf with varying time and temperature)	BX1	15	60	40	5.1	5.5	−2.5
	BX2	30	70	40	5.6	6.1	−3.3
	BX3	45	80	40	5.7	6.3	−3.6
	BX4	60	90	40	5.9	6.7	−3.7
SET-II (time and temperature fixed with varying dye concentration)	BX5	60	80	10	4.6	4.8	−2.8
	BX6	60	80	20	5.0	5.6	−2.9
	BX7	60	80	30	5.6	5.9	−3.8
	BX8	60	80	40	6.0	6.3	−4.9

Table 10.

Colour strength and colour related properties of cotton fabric pre-mordanted with gallnut and dyed with selected binary pairs of natural dyes in 50:50 proportion under two sets of dyeing conditions—Set-I under varying conditions of time and 60 temperature, and Set-II under varying dye concentration.

BB – Babul bark; AS – Annatto seeds; HR – Himalayan rhubarb roots; PR – Pomegranate rind.

In the current investigation, two approaches of testing the compatibility of binary dye pairs have been employed. In the conventional method, the closeness and degree of overlap between the two sets of curvesin the plots ∆C vs. ∆L or K/S vs. ∆L produced using two sets of dyeing procedures (Set-I and Set-II) for the progressive build-up of shade have been compared. Also, the compatibility of various pairings of selected natural dyes has been established by an easy quantitative method based on relative compatibility rating (RCR) between any pair of natural dyes as described in the earlier study [5]. As suggested earlier the closer the CDI values of dyeing with different binary pair ratios of dyes, the greater their compatibility (RCR). Thus, the compatibility between the two approaches (traditional and proposed) for binary pair of dyes used in different proportions has been compared.

Figure 7 shows the K/S vs. ∆L (plots a–c) and ∆C vs. ∆L (plots a′–c′) plots for both sets (Set-I and Set-II) of dyed materials for three separate pairs (B1–B3) of natural dyes.

Figure 7.

Plots for K/S vs. ∆L (a–c) and ∆C vs. ∆L (a′–c′) for dyeing of three separate pairs (B1–B3) of natural dyes on pre-mordanted cotton fabric.

The two curves (Set-I and Set-II) for B1 (BB + AS) binary pair of dyes plots for K/S vs. ∆L of the dyed cotton samples exhibit a similar pattern but with a significant gap between the curves in the initial stage, which narrows and gets closer until it almost touches each other at the end of the dyeing cycle when dyeing equilibrium is reached. They indicated that for the K/S vs. ∆L plot for the Set-I curve, the colour build-up was initially slow at a lower temperature, while for the Set-II curve, it was moderately fast due to the use of higher temperatures (plot a, Figure 7). These observed variations in these two sets of curves could possibly be due to the difference in the molecular weights of the two dyes. However, both Set-I and Set-II curves are quite close to one another towards the end at the dyeing equilibrium indicating that the colour build-up is at par after enough time has passed during the dyeing process. In contrast, the two curves for Sets I and II show a similar pattern and run almost parallel on plots (a’) of ∆C vs. ∆L. Plots of the Set-I curve and Set-II curve show a similar rate of colour build-up with minor deviations, with the maximum increase occurring at 40% dye concentration, time of 60 min, and temperature of 90°C. This is likely due to the different major hues of these two dyes, which results in maximum shade developed by sufficient absorption of both babul bark and annatto seed natural dyes. In the case of binary pair B1 (BB + BX), the K/S vs. ∆L and ∆C vs. ∆L plots show that both curves for Sets I and II overlap with slight deviation, indicating a good degree of compatibility. In the proposed RCR method, this pair of dyes exhibits a Grade 3–4 (moderate) relative compatibility rating (Table 11).

Dye combination	CDI			CDImax − CDImin	RCR	Compatibility grade
	75:25	50:50	25:75
B1 (BB + AS)	4.2	4.0	3.9	0.3	3–4	Moderate
B2 (BB + HR)	5.5	5.2	4.6	0.9	2–3	Fair
B3 (BB + PR)	5.4	5.1	5.0	0.4	3	Average

Table 11.

Colour difference index (CDI) and relative compatibility rating (RCR) of cotton pre-mordanted fabric dyed selected binary pairs of natural dyes.

BB – Babul bark; AS – Annatto seeds; HR – Himalayan rhubarb roots; PR – Pomegranate rind.

Thus, the two curves for Set-I and Set-II in the plots of K/S vs. ∆L for the binary mixture B2 (BB + HR) do not exhibit a similar pattern for the build-up of colour from beginning to end. Set-I decreases with an increase in time and temperature and Set-II increases with an increase in dye concentration till 30% owf concentration after which it declines. But for plots (b′) of ∆C vs. ∆L, both the two sets show a similar trend.

Because of the closeness of the hues of these two dyes, it may be assumed that the observed gaps between Set-I and Set-II curves in ∆C vs. ∆L plots and K/S vs. ∆L plots become smaller with increasing time, temperature, and dye concentration. Consequently, regardless of the dyeing time and concentrations, the dulling effect does not diminish throughout the entire dyeing cycle in parallel sets, always maintaining a gap. In the case of the newer RCR method, higher CDI values for the binary mixture of natural dyes B2 (BB + HR) are revealed by the presence of chrysophanol and other anthraquinone derivatives, which differ greatly from the gallotannins of babul bark in terms of molecular weight. Therefore, there is little compatibility.

Thus, in the case of binary pair B2 (BB + HR), the plot of K/S vs. ∆L shows the two curves for Sets I and II to be widely spaced initially which gradually overlap with an increase in K/S values. The corresponding plots for ∆C vs. ∆L, also show the same trend, but the gap between the two curves for Set-I and Set-II is much less. Hence, both the set of plots i.e., K/S vs. ∆L and ∆C vs. ∆L for this pair of dyes (B1 – BB + AS) indicate a fair degree of compatibility as also corroborated by the RCR of 2–3 under the more recent relative compatibility classification system (Table 11). Thus, both the compatibility results obtained by the traditional method employing ∆C vs. ∆L and K/S vs. ∆L plots and the newer RCR method are similar.

In plots (c) of K/S vs. ∆L and (c’) of ∆C vs. ∆L for a binary mixture of B3 (BB + PR), the two curves for Set-I and Set-II exhibit nearly identical trend that increases with an increase in dye concentration, dyeing time, and temperature. The build-up of K/S (depth of shade) and the corresponding darkness (∆L) is similar (plot c, Figure 7) and therefore there is less gap between Set-I and Set-II curves, indicating moderate to fair compatibility between these two dyes in the binary mixture. For the ∆C vs. ∆L plots, the change in chroma (∆C) and darkness (∆L) do not build up systematically and change continuously with the increase in dye concentration as well as with the progress in dyeing time almost up to the end, indicating a moderate to average compatibility between this pair of dyes. These two colours are absorbed at various rates and have quite different molecular weights. Additionally, this may be because of the added dulling action of one colourant (gallotannins) in babul bark, which may occasionally be compatible with the colouring matter in pomegranate rind. According to a comparison of the chemical structures of natural dyes, these results may be interpreted as the result of closer molecular weights of the key colour components involved, even though there are significant variances in the dominant hues among them.

Thus, in the case of binary pair B3 (BB + PR), the plot of K/S vs. ∆L shows the two curves for Sets I and II to be widely spaced initially which gradually overlap with an increase in K/S values. The corresponding plots for ∆C vs. ∆L, also show the same trend, but the gap between the two curves representing Set-I and Set-II conditions of dyeing is much less. Hence, both the set of plots i.e., K/S vs. ∆L and ∆C vs. ∆L for this pair of dyes i.e., B1 (BB + AS) indicate a moderate degree of compatibility as also corroborated by the RCR of 3 (Table 11).

6. Conclusions

The optimised extraction recipe for annatto seeds is found to be pH 11, MLR 1:20, 75 min time at 80°C temp and for pomegranate rind, it is pH—11, MLR 1:20, 45 min time and 80°C temp. Phytochemical screening indicates the presence of tannins, flavonoids, alkaloids, terpenoids and saponins in the extracts of gallnut, babul bark, annatto seeds, Himalayan rhubarb roots and pomegranate rind. In terms of colour strength and colour related parameters B1 (BB + AS) gave the best results when compared to B2 (BB + HR) and B3 (BB + PR). All samples dyed with binary mixtures of natural dyes in various proportions exhibited good to excellent colour fastness properties.

Thus, comparing to methods of determination of compatibility of three pairs of binary mixtures of natural dyes, it is concluded that the B1 pair (babul bark with bixa seeds) is moderately compatible as per K/S vs. ∆L and ∆C vs. ∆L plots of colour build-up as observed in conventional method; while the newer relative compatibility rating (RCR) method also shows this B1 pair as moderately compatible. Similarly, B2 pair (babul bark with Himalayan rhubarb roots) is fairly compatible as per K/S vs. ∆L and ∆C vs. ∆L plots of colour build-up as observed in the conventional method; while the newer relative compatibility rating (RCR) method also shows this B2 pair to be fairly compatible. Lastly, and B3 pair (babul bark with pomegranate rind) is moderately compatible as per K/S vs. ∆L and ∆C vs. ∆L plots of colour build-up as observed in the conventional method; while the newer relative compatibility rating (RCR) method also shows this B2 pair to be averagely compatible. So, the newer and easier method of the RCR technique gives nearly matching results as the conventional method.