Colorimetric studies of some newly synthesized bisazo reactive dyes

2.1 General

All the chemicals used in the dye synthesis were of commercial grade and were further purified by crystallization and distillation. Melting points were determined by the open capillary method. The purity of all the dyes was checked by TLC (Fried and Sharma, 1982). The visible absorption spectra were measured using a Shimadzu UV-1700 spectrophotometer. A Perkin-Elmer model 881 recording infrared spectrophotometer was used for recording the FTIR spectra of the dyes as KBr pellets in the range between 4000 and 400 cm⁻¹ and ¹H NMR spectra on a Bruker Avance II 400 instrument using TMS as internal standard and DMSO as solvent. Elemental analysis of C, H and N were carried on Carlo Erba 1108 instrument. The light fastness was assessed in accordance with BS: 1006-1978 (Standard Test Method, 1978). The rubbing fastness test was carried out with a Crockmeter (Atlas) in accordance with AATCC-1961 (AATCC Test Method, 1961) and the wash fastness test in accordance with IS: 765-1979 (Indian Standard, 1979). Colorimetric data (L^∗, a^∗, b^∗, C^∗, H^∗, K/S) were recorded on Reflectance Spectrophotometer Gretag Macbeth CE: 7000.

2.2 Synthesis of 4,4′-methylene-bis(2-methyl-5-nitroaniline) (1) (Patel and Patel, 2011)

2-Methyl-5-nitroaniline (15.2 g, 0.1 mol) was first mixed with sufficient HCl to reach pH 4 of the medium. Then, the pale yellow, colored solution was put into a three-necked flask fitted with a stirrer and was mixed at 50 °C temperature. Formaldehyde (35 ml, 3%, v/v) was added at interval of 10 min over a period of 1 h and the stirring was continued for a further 10 h maintaining temperature at 60 °C. Then, the stirring was stopped and the product was washed with NaOH (2%, w/v) first and then with water till the mixture became neutral. Finally, the yellowish colored product (1) was dried in an oven at 90 °C for about 4 h (13.61 g, 86.04%). Yellow powder, m.p. 202 °C (recrystallized from acetic acid). R_f = 0.75 (PhMe:EtOAc, 3:1, v/v). IR (KBr): ν_max (cm⁻¹): 3455, 3363 (N–H), 2925, 2845 (C–H), 1568, 1356 (N⚌O). ¹H NMR (400 MHz, DMSO-d₆) δ ppm: 2.58 (2H, s, CH₂), 2.75 (6H, s, N–CH₃), 6.67 (4H, s, NH₂), 6.95–7.25 (4H, m, Ar–H). Anal. Calcd. for C₁₅H₁₆O₄N₄ (316.31 g/mol): C, 56.96; H, 5.10; N, 17.71%. Found: C, 56.78; H, 4.93; N, 17.60% (Scheme 1).

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Scheme 1

Synthesis of reactive dye 4a by using cyanurated H-acid (3a) as coupling component.

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2.3 Tetrazotisation of 4,4′-methylene bis(2-methyl-5-nitroaniline) (2)

4,4′-Methylene-bis(2-methyl-5-nitroaniline) (1) (1.58 g, 0.005 mol) was stirred in a mixture of water (25 ml), conc. HCl (1.88 ml, 0.015 mol) and ice (10 g). The reaction mixture was cooled to 0–5 °C using an ice bath. NaNO₂ (0.35 g, 0.005 mol) dissolved in water (10 ml) was then added dropwise. The solution was stirred for 30 min and excess HNO₂ was decomposed by adding sulphamic acid. Activated carbon was added with stirring and the mixture was filtered at 0–5 °C to give the clear yellow solution (2) (Scheme 1).

2.4 Cyanuration of H-acid (3a)

H-acid (3.19 g, 0.01 mol) was dissolved in water (15 ml) at pH 7.5, using 20% (w/v) Na₂CO₃. A solution of cyanuric chloride (1.85 g, 0.01 mol) in acetone (20 ml) was cooled to 0–5 °C and added dropwise to the stirred H-acid solution at 0–5 °C. After 10 min, the solution was adjusted to pH 7 by adding 20% (w/v) Na₂CO₃, and the reaction was continued for 1 h at 0–5 °C. The reaction progress was followed by TLC using n-PrOH:n-BuOH:EtOAc:H₂O (2:4:1:3, v/v), in which the product 3a had R_f = 0.70 (Scheme 2).

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Scheme 2

Preparation of cyanurated H-acid (3a).

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2.5 General method for the synthesis of dye 4a

Freshly prepared tetrazonium salt solution (0.005 mol) (2) was added dropwise to a well-stirred solution of cyanurated H-acid (3a) (0.01 mol). The solutions were maintained at pH 9 by adding 20% (w/v) Na₂CO₃ and the coupling step was continued for 4 h at 0–5 °C. Then, 10% (w/v) urea was added (Ravikumar et al., 1998) and the dyes were isolated by salting out of solution using NaCl (12 g). The pH was adjusted to 7 using HCl (6%, w/v) and stirring was continued for 2 h. The dye was collected by filtration and washed with NaCl (5%, w/v). Salt was removed by stirring the crude dyes with dimethylformamide (DMF), followed by the dye precipitation by adding EtOAc to the filtrate. The dye 4a was collected, washed with EtOAc and dried in oven at 45 °C for 12 h (5.71 g, 84.0%). The eluent system for TLC was 2-BuOH:EtOH:NH₄OH:pyridine (4:1:3:2, v/v). Dye 4a had R_f = 0.43, with minor impurities at R_f = 0.23 (Scheme 1).

Following the above procedure other reactive dyes 4b–h were synthesized using various cyanurated coupling components such as K-acid (3b), Gamma acid (3c), N-methyl-Gamma acid (3d), J-acid (3e), N-methyl-J-acid (3f), N-phenyl-J-acid (3g) and Chicago acid (3h). The structures of all the cyanurated coupling components are shown in Table 1 and the characterization data of all the synthesized dyes are recorded in Table 2.

Table 1 Various cyanurated coupling components (3a–h): arrow indicates coupling position.

C.C.a	Structure	C.C.a	Structure
3a		3e
3b		3f
3c		3g
3d		3h

a Cyanurated coupling components.

3.1 Spectral data of dyes

The structures of dyes 4a–h were confirmed by various spectroscopic techniques including FTIR (Colthup et al., 1991) and ¹H NMR (Bassler et al., 1991) spectral data. Dyes 4d, 4f and 4g showed broad band at 3455–3522 due to the O–H stretching vibration. Dyes 4a–c, 4e and 4h showed overlapped absorption band at 3360–3490 cm⁻¹ region corresponding to the O–H and N–H stretching vibration. Two medium absorption bands at 2920–2945 cm⁻¹ and 2850–2860 cm⁻¹ showed C–H stretching vibration of methyl and methylene groups. The absorption band at 1560–1570 cm⁻¹ confirmed the presence of azo group. All the dyes showed two bands at 1190–1200 cm⁻¹ and 1040–1050 cm⁻¹ which showed asymmetric and symmetric S⚌O stretching vibration of sulfonic acid group. Dyes 4a–h showed bands at 1520–1530 cm⁻¹ and 1340–1360 cm⁻¹ showed asymmetric and symmetric N⚌O stretching vibration of nitro group. The chloro group was confirmed by the band at 765–775 cm⁻¹. The IR spectral data are summarized in Table 3.

The ¹H NMR spectra of dyes 4a–h displayed two signals at 1.32–1.48 δ ppm and 2.22–2.34 δ ppm was due to the methyl and methylene proton. Dye 4d and 4f showed signals at 2.52 δ ppm and 2.58 δ ppm were due to the N–CH₃ proton. All the dyes showed broad signals at 3.62–3.78 δ ppm due to the OH proton. All the dyes except 4d, 4f and 4g showed singlet at 4.02–4.18 δ ppm due to the secondary amino group. The aromatic protons were showed signal at 6.58–7.88 δ ppm as multiplet. The ¹H NMR spectral data are summarized in Table 3.

3.2 Visible absorption spectra of dyes

Absorption maxima (λ_max) of all the dyes (4a–h) were recorded in water (10⁻⁶ M) and are shown in Table 2. The absorption maxima values (λ_max) are directly proportional to the electronic power of the substituents in the coupler ring system. The values of molar extinction coefficient maxima (ε_max) are shown in Table 2. Dye 4a has the highest ε_max value 28,248.80 l mol⁻¹ cm⁻¹ which is due to the greater co-planarity of dye 3a.

The introduction of electron donating or electron attracting group in the coupler moiety results in additional color shifts, i.e. shifting of λ_max towards higher wavelength (bathochromic shift) or lower wavelength (hypsochromic shift). Thus the introduction of methyl group in the dye 4d (λ_max = 485 nm) resulted in bathochromic shift of 10 nm compared to the dye 4c (λ_max = 475 nm), the same effect is observed by the introduction of methyl group in the dye 4f (λ_max = 490 nm) resulted in bathochromic shift of 12 nm compared to the dye 4e (λ_max = 478 nm) and the introduction of phenyl ring in the dye 4g resulted in bathochromic shift of 22 nm compared to the dye 4e (λ_max = 478 nm). Dye 4a (λ_max = 530 nm), 4b (λ_max = 465 nm) and 4h (λ_max = 468 nm) have the same substituents but the position is different, hence in dye 4a there is more place for NH and SO₃Na groups, which cause faster electron oscillation and hence neutralization of electron is faster than dye 4b and 4h, this effect is responsible for higher λ_max of dye 4a. The visible absorption spectra of all the dyes 4a–h are shown in Fig. 2.

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Figure 2

Visible absorption spectra of reactive dyes (4a–h).

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3.3 Dyeing of fibres

All the dyes (4a–h) were applied on hydrophilic fibres such as silk, wool and cotton in 2% (owf) shade according to the following procedure. These dyes are given yellow to purple hues with brighter and deeper shades with excellent levelness on the fabric. The variation in the hues (yellow to purple) of the dyed fabric is due to the alteration in the coupling components, e.g. when H-acid is used as coupling component it gives purple hue, J-acid gives yellow color, N-methyl-J-acid gives orange color and N-phenyl-J-acid gives red color.

3.4 Wash-off process

The dyed fabrics (silk, wool and cotton) were rinsed in warm water, scoured with 2 g/l Lissapol detergent at 90 °C for 5 min and rinsed again in warm water. The dyed fabrics afforded color in the first warm water rinse, less color in the scouring bath and practically no color in the second water rinse. This indicated that the unfixed dyes were easily removed from the fibre surface. The amount of the hydrolyzed dye has low substantivity released easily from the substrate after two or three washes.

3.5 Exhaustion and fixation study

The percentage exhaustion and fixation data are calculated according to the known method (Patel and Patel, 2005) and are summarized in Table 4.

The percentage exhaustion of 2% (owf) dyeing on silk fabric ranges from 67% to 78%, in which dye 4a shows maximum exhaustion (77.18%) while dye 4h shows minimum exhaustion (67.20%); for wool percentage exhaustion ranges from 64% to 77%, in which dye 4c shows maximum exhaustion (76.10%) while dye 4h shows minimum exhaustion (64.22%) and for cotton fabric the percentage exhaustion ranges from 67% to 76%, in which dye 4d shows maximum exhaustion (75.10%) while dye 4c shows minimum exhaustion (67.15%).

The percentage fixation of 2% (owf) dyeing on silk fabric ranges from 82% to 91%, in which dye 4c shows maximum fixation (90.84%) while dye 4b shows minimum fixation (82.34%); for wool fabric the percentage fixation values ranges from 81% to 91%, in which dye 4a shows maximum value of fixation (90.04%) while dye 4b shows minimum value of fixation (80.97%) and for cotton fabric the percentage fixation ranges from 87% to 92%, in which dye 4g shows maximum exhaustion (91.46%) while dye 4b shows minimum value of fixation (87.30%).

The data summarized in Table 4 show that the exhaustion and fixation values are very good. This is due to the fact that the diffusion of the dye molecule within the fabric is rapid, so the rate of diffusion is high. The lower exhaustion is due to the lower substantivity due to the lower hydrophobicity (Dawson, 1991). The introduction of reactive group like s-triazine ring to the dye molecule improves the exhaustion value.

3.6 Fastness properties

The light fastness property of all the dyes rating 3–6 (Grey scale) for silk, wool and cotton shows that it ranges from moderate to very good. The wash fastness and rubbing fastness properties of all the dyes rating 3–5 for silk, wool and cotton show that these properties of all the dyes range from good to excellent. The good wash fastness properties may be due to the better dye penetration and good covalent fixation with fabrics (Samanta and Das, 1992). The solubility of the dyes and rate of movement of the dyes out of the fibre during washing depends on the size of the dye molecule and also on the nature and position of the substituent present on the coupler ring. The data of light, wash and rubbing fastness properties of all the dyes (4a–h) are shown in Table 4.

3.7 Colorimetric data (CIE lab data)

The color of a dye on silk, wool and cotton fibres is expressed in terms of CIE lab value (Table 5) and the following CIE lab coordinates were measured, lightness (L^∗), chroma (C^∗), hue angle form 0° to 360° (H), a^∗ value represents the degree of redness (positive) and greenness (negative) and b^∗ represents the degree of yellowness (positive) and blueness (negative). A reflectance spectrophotometer was used for the colorimetric measurements on the dyed samples. K/S values given by the reflectance spectrophotometer are calculated at λ_max and are directly correlated with the dye concentration on the substrate according to the Kubelka–Munk equation (Billmeyer and Saltzman, 1981): $$K/S={(1-R)}^2/2R\text{,}$$ where K is the absorbance coefficient, S the scattering coefficient and R is the reflectance ratio.

The color coordinates (Table 5) indicate that the dyes have good affinity to silk, wool and cotton fibres.

For silk fibre, Table 5 showed that the hue obtained using dye 4b was greener, lighter and brighter than dye 4a, while dye obtained using dye 4d was greener, darker and duller than dye 4c. The dyeing obtained using dye 4f was redder, darker and brighter than dye 4e, while the dyeing obtained using dye 4g was redder, darker and brighter than dye 4e. Dye 4a has the highest value of color strength (K/S) of 10.14. The K/S value for silk fibre follows the order, $$\mathbf{4}\mathbf{a}>\mathbf{4}\mathbf{g}>\mathbf{4}\mathbf{f}>\mathbf{4}\mathbf{d}>\mathbf{4}\mathbf{e}>\mathbf{4}\mathbf{c}>\mathbf{4}\mathbf{h}>\mathbf{4}\mathbf{b}$$

For wool fibre, Table 5 showed that the dye obtained using dye 4b was greener, lighter and duller than dye 4a, the dyeing obtained using dye 4d was redder, lighter and duller than dye 4c. The dyeing obtained using dye 4f was greener, lighter and brighter than dye 4e. The dyeing obtained using dye 4g was redder, lighter and brighter than dye 4e. Dye 4g has the highest value of color strength (K/S) of 15.88. The K/S value for wool fibre follows the order, $$\mathbf{4}\mathbf{g}>\mathbf{4}\mathbf{a}>\mathbf{4}\mathbf{f}>\mathbf{4}\mathbf{c}>\mathbf{4}\mathbf{d}>\mathbf{4}\mathbf{b}>\mathbf{4}\mathbf{e}>\mathbf{4}\mathbf{h}$$

And for cotton fibre, Table 5 showed that dyeing obtained using dye 4b was greener, lighter, duller than dye 4a, and the dyeing obtained using dye 4d was greener, darker and brighter than dye 4c. The dyeing obtained using dye 4f was redder, darker and duller than dye 4e while the dyeing obtained using dye 4g was redder, lighter and brighter than dye 4e. The dye 4a has the highest value of color strength (K/S) of 9.70. Wool fibres possess higher K/S value than silk and cotton due to the high substantivity of the dyes with wool fibre. The K/S value for cotton fibre follows the order, $$\mathbf{4}\mathbf{a}>\mathbf{4}\mathbf{g}>\mathbf{4}\mathbf{f}>\mathbf{4}\mathbf{e}>\mathbf{4}\mathbf{d}>\mathbf{4}\mathbf{c}>\mathbf{4}\mathbf{b}>\mathbf{4}\mathbf{h}$$

CIE lab graph of b^∗ vs. a^∗ for silk, wool and cotton fibre are shown in Figs. 3–5, respectively. The graph of K/S value of all the dyes for silk, wool and cotton fibres are shown in Fig. 6.

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Figure 3

CIE lab graph of b^∗ vs. a^∗ for silk fibre.

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Figure 4

CIE lab graph of b^∗ vs. a^∗ for wool fibre.

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Figure 5

CIE lab graph of b^∗ vs. a^∗ for cotton fibre.

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Figure 6

Graph of K/S value for dyes 4a–h.

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