A novel synthesis,characterization and application of an anionic Cr-complexed azo dye based on environmental considerations

Abstract

A simplified novel one pot method of synthesis of a Cr-complex anionic azo dye and the subsequent minimization of the Cr-content in the final dye by ultrafiltration was carried out. Ultrafiltration allows the production of highly concentrated purified metal-complex dyes with drastically reduced free metal and electrolyte content.

All dyes were characterized by FT-IR, UV-Vis, NMR and their melting points. X-ray fluorescence analysis and atomic absorption measurements were performed for the determination of free and total Cr-content for the synthesized metal-complex dye before and after ultrafiltration.

All of the synthesized dyes (before and after ultrafiltration) were applied for the dyeing of wool and polyamide fibres, and colour measurements and fastness property tests were carried out. The wash and light fastness properties of the dyeings were excellent for the metal-complex dyes, but inferior for the non-metallized dye.

Keywords

Chromium complex azo dye wool-polyamide fibre ultrafiltration atomic absorption analysis XRF analysis

Introduction

Metal-complex dyes comprise the predominant dye class for the dyeing of wool, polyamide 6.6 and silk due to their superior fastness properties to wash and light compared to the non-metallized acid dyes.¹^–³ Metallization generally leads to bathochromic shift. Due to this metal-complex dyes are duller and are mainly used to produce deep dyeings with fastness properties which are unattainable with common acid dyes. Thus these dyes are fulfilling high customer demands.^{1, 3} The metal introduced in the dye molecule of an acid dye offers protection of the azo chromophore against ultraviolet degradation thus enhancing its light fastness.⁴^–⁶ Usually metal-complex azo dyes are predominantly complexes of Cr(III), Co(II, III), and Cu(II).⁴^–⁶ The earliest metal-complex dyes were produced directly within the fibre by reacting a metallizable dye with a chromium compound in situ.³ However, the past 20 years have been marked by a growing interest in the development and use of ecologically friendly dyes. Increasing regulations at local and international level, combined with public pressure, have led textile and dye manufacturing companies to examine the potential impact of their operations on human health and the environment. As a result, alternative dyeing processes and dye classes have now gained the researchers’ interest.⁷^–¹² Anionic acid and reactive dyes due to structural features of the wool fibre are significant dye classes for wool and polyamide fibres.¹³ However, natural dyes for example are regaining after almost 100 years popularity all over the world, possibly because of this increasing awareness of the environment. Due to the specific wool structure some natural dyes can dye the wool fibre with satisfactory colour strength and fastness properties.¹⁴^–¹⁸ Another consequence was that the use of mordant dyes has greatly declined, at the advantage of metal-complex dyes.^{3, 19} The increased popularity of the metal-complex dyes can be attributed to health and effluent hazards associated with the use of chromium in mordant dyeing and the much simpler application method employed for the metal-complex dyes. However, although the use of chromium in dyeing has greatly reduced when compared to mordant dyes still there is environmental concern with the application of metal-complex dyes to textile fibres.^{3, 19} High consumer awareness has also led to the introduction of high standards in terms of permissible heavy metal level in the textile articles.²⁰

The potential risks associated with introducing heavy metals, such as chromium, cobalt, copper into the environment, following either the synthesis or application of metal-complex dyes, have caused these heavy metals to be considered as main environmental pollutants.

Although the principal cause of the observed toxicity is chromium (VI) instead of chromium (III) used in the synthesis of metal-complex dyes, chromium is generally considered to be a priority pollutant, thus the disposal of dye effluents is a major environmental problem. Many techniques have been developed for the minimization of heavy metal content in the industrial effluents.²¹^–²⁴ Ultrafiltration technology is one of newest technologies applied for the treatment of textile effluents aiming at the elimination of toxic pollutants in the discharged effluents.²⁵^–²⁹

In this present work, a simplified novel one pot method using ultrafiltration technology is proposed for the synthesis of Acid Black 194. The use of ultrafiltration allows the production of highly concentrated, purified, metal-complex dyes with drastically reduced free metal and electrolyte content. The ultrafiltrated dyes have drastically improved solubility properties when compared to their non-ultrafiltrated counterparts.

The elimination of heavy metal and inorganic salts from the dye formulation allows the production of novel dyes of high concentration and purity with improved properties and more environmentally friendly compared to the ones conventionally made.

The dyes synthesized before and after ultrafiltration together with the commercial dye were used to dye wool and polyamide 6.6. Colour measurements and fastness properties were performed on the dyeings made on wool and polyamide 6.6.

Experimental section

Materials and method

Commercially available, lightweight (190 gm⁻²) wool serge, and knitted polyamide 6.6 (78F68) were kindly supplied by KYKE Hellas (Greece) and were used throughout this work. The wool and polyamide 6.6 samples were scoured using 1 g/L Widet MS/V (non-ionic surfactant supplied by Prochimica, Italy), for 20 mins at 60°C followed by thorough rinsing in distilled water. The wool and polyamide 6.6 samples were then left to dry in the open air.

A Shimadzu UV-2101 Spectrophotometer (Shimadzu Europa GmbH, Germany) was used for obtaining the absorption spectra. ¹H NMR spectra were recorded at 300 MHz on a Bruker AVANCE III 300 spectrometer and are quoted relative to tetramethylsilane as an internal reference in deuteriochloroform. Melting points were uncorrected and were determined using a Kofler hot-stage microscope. Fourier transform infra-red (FT-IR) spectra of the dyes were recorded by FT-IR spectroscopy (FT-IR Spectrum One, PerkinElmer, USA, resolution 4 cm⁻¹, 32 scans, 4000–600 cm⁻¹). All chemicals used were of analytical grade. The raw materials used for the synthesis of the dye such as 6-nitro 1,2,4-diazo acid, beta-naphthol, basic chromium sulphate and the commercial dye Neutrilan Black M-RX were kindly provided by Yorkshire Chemicals, France. TLC plates used for thin layer chromatography (TLC) were supplied by Polygram SIL G, Fluka.

Synthesis of Cr- complex

15 mL NaOH 29% solution were added within 1 h at 5°C under stirring in an aqueous solution of 0.1 mol (29.5 g) of 6-nitro 1,2,4-diazo acid, with the temperature remaining at 15°C. 15 g beta-naphthol was dissolved separately under stirring in 60 mL of water with the addition of 10 mL NaOH 29% at 50°C.

Coupling: The 6-nitro 1,2,4-diazo acid was added in 30 min to the solution of beta-naphthol, maintaining the temperature at 50°C and the pH at 9. Stirring was continued for 2–3 h till the end of coupling and then the mixture was heated at 75–80°C, ready for the metallization.

Metallization: A solution of basic chromium sulphate was prepared by dissolving 13.5 g basic chromium sulphate in 8.5 mL of water under stirring at 50°C. Prior to metallization the coupling mass prepared above was acidified at pH ∼5.5 with 80% acetic acid, maintaining the temperature at 75–80°C. The solution of basic chromium sulphate was added to the coupling mass. The mixture was then heated up, under stirring, at 95°C and the pH was adjusted to 4 using acetic acid while maintaining the temperature at 95°C until the end of metallization (∼10 h). The metallization reaction was followed with TLC until the azoic coupling disappeared. The eluent used for the TLC was a mixture of pyridine/25%, NH₄OH and pentan-1-ol in equal parts.

Ultrafiltration

A laboratory ultrafiltration unit equipped with a tubular membrane supplied by PCI Membranes (UK) was used throughout this work. The membrane used for the ultrafiltration process was the ES404, a polyethersulphone type membrane supplied by PCI Membranes (UK). Aqueous dye solutions at 1.5% w/v for Neutrilan Black M-RX and dye 1 were used.

Dyeing

Dyeing of both wool and polyamide 6.6 was carried out at 3% owf in a liquor ratio 20:1, at pH 4.5, using McIlvaine buffer at the boil. The dyeing method used is shown in Figure 1. All dyeings were performed in sealed, stainless steel dyepots of 100 cm³ capacity, housed in a Zeltex Vistacolor (model Lowboy) dyeing machine (Zeltex, Inc., USA). At the end of dyeing, all dyed samples were removed, rinsed and dried in the open air.

Figure 1.

Dyeing process of wool and polyamide fibres.

Colour measurement

Colour measurements were performed using a Macbeth CE 3000 spectrophotometer under D₆₅ illumination, 10° standard observer with UV included and specular component included. The samples were folded twice and four measurements were performed each time.

Fastness properties

Wash fastness tests were carried out according to BS 1006 1990 CO2.

Light fastness was determined according to BS 1006 1990 BO2 using a Q-SUN Xe-1-B xenon light fastness machine.

Colour changes for all samples were assessed visual using a VeriVide D65 (UK) light cabinet.

X-ray fluorescence (XRF) analysis

The elemental constituents were determined on Teflon filters by ED-XRF analysis using a SPECTRO-XEPOS bench-top XRF spectrometer (SPECTRO A.I. GmbH, Germany) with Pd end window X-ray tube. Element concentrations were determined by reference to calibration standards with appropriate corrections being made for instrumental errors and the effect of the matrix on X-ray emission intensities. A specialized software program (SPECTRO-X-LAB^PRO) was used for value normalization and error correction. The method detection limits were 100 ng·cm⁻² for Mg, 20 ng·cm⁻² for Al and K, 15 ng·cm⁻² for V, 10 ng·cm⁻² for Ca, Ti, Cr, Fe, 5 ng·cm⁻² for Si, Mn, Te, Ba, Sb and Pb, 3 ng·cm⁻² for Co, Cd and Sn, 2 ng·cm⁻² for Ni and 1 ng·cm⁻² for S, Cl, Cu, Zn, As, Se, Br and Sr. The estimated precision of XRF analysis for most elements was on average <5%.

Atomic absorption measurements

3 g of sodium dithionate were added to 100 mL of the permeate solution, which was previously buffered at pH 6 using a citrate buffer. The mixture was heated for 30 min at 70°C under stirring. The above reductive treatment was used to discolour the permeate solution. The final solution was passed through a series of cartridge filters connected to each other firstly through a Sartorius filter of 0.45 micron of cellulose acetate followed by two cartridges of Onguard Dionex P filters. The later two filters selectively filtered organic matter in the reduced permeate solution. The above procedure was repeated for all ultrafiltrated dyes in order to determine the total chromium content in the permeate solution.

A Perkin–Elmer model 5100 PC flame atomic absorption spectrometer equipped with a deuterium arc background corrector was used to measure the chromium content. A chromium hollow cathode lamp was used as a light source and was operated at 25 mA. The wavelength was set at 357.9 nm resonance line and the slit at 0.7 nm. A time-constant of 0.2 s was used for peak height evaluation. A flow spoiler was used in the spray chamber for all measurements. All measurements were carried out in air/acetylene flame.³⁰

Solubility test

The appropriate amount of synthesized metal-complex dye 1, before and after ultrafiltration, was dissolved by stirring in 200 mL of distilled water to give the desired concentration. The temperature was increased to 95°C within 5 min and kept at this temperature for 5 min. The solution was then placed in a thermostatically controlled water bath at 25°C where it was kept and stirred for 2 h. The solution was then filtered at 25°C under vacuum. The characteristics of the residue was examined in terms of insolubles and sediment left. The procedure was repeated for many concentrations of the dyes to find the maximum level of solubility of the dyes.³¹

Results and discussion

The symmetrical 1:2 disulphonated acid dye 1, as well as the non-metallized azo dye 2 (Figure 2), precursor of the Acid Black 194 (dye 1), were successfully synthesized with the novel one pot-one stage method as described in the experimental part.

Figure 2.

Structures of the metal complex dye 1 and the non-metallized dye 2.

Table 1 shows the melting points and the spectrophotometrical data of the metal-complex dye 1 and the non-metallized azo dye 2. The high melting points are in agreement with melting points for other acid dyes of similar structure (disulphonated Cr complex) and may result from the high symmetry of the molecules of Acid Black 194 (dye 1) and the azo dye 2.³²

Table 1.

Yield, mp and spectrophotometrical data for the dyes 1, 2

Dye	Yield %	Mp (°C)	λ_max	FT-IR	¹H-NMR
1	59.39	>350	571.5	−N = N−: 1556 cm⁻¹	–
				Naphthalene: 1457 cm⁻¹
				Cr-O: 740 cm⁻¹
2	–	>350	530.5	OH: 3399 cm⁻¹	OH(alcohol): 2.0 ppm
				−N=N−: 1585 cm⁻¹	C-OH(aromatic): 9.82 ppm
				Naphthalene: 1504 cm⁻¹	C-H(naphthalene): 7.32-8.93 ppm

A ¹H-NMR spectrum was obtained for the dye 2 where the presence of the naphthalene ring, azo double bond and hydroxyl group were identified as shown in Table 1 and correspond to the dye structure 2. However, it was not possible to obtain a ¹H-NMR spectrum for dye 1 due to the paramagnetic properties of the metal-complex dye 1.^{33, 34}

The synthesized metal-complex dye 1, as well as the commercial dye Neutrilan Black M-RX used as reference, was passed through the UF unit housed with a PCI ES404 polyethersulphone type membrane. Preliminary lab trials with a number of membranes showed that the most suitable membrane for the synthesized metal-complex dye 1 was the PCI membrane ES404 in terms of inorganic salt separation and dye purification. The XRF analysis of the above dyes before and after ultrafiltration was performed and the results are given in Table 2. The remarkable almost total elimination of chloride ions for both the synthesized metal-complex 1 and the commercial dye used (Neutrilan Black M-RX) clearly shows the effectiveness of the UF membrane to desalinate and purify the metal-complex dye. The elimination of the Na and S content in the commercial dye Neutrilan Black M-RX, is also very high, at about 80% and 67% respectively, almost double the same reduction obtained for the synthesized metal-complex dye 1. This can be attributed to the presence of inorganic additives, electrolytes and dispersing agents normally present in the standardized commercial dye Neutrilan Black M-RX as cutting agents and contain inorganic ions such as Na, S and Cl, which can be successfully eliminated after ultrafiltration. The above additives are not present in the synthesized metal-complex dye 1.

Table 2.

% reduction of the inorganic ions using XRF analysis

Dye	Na (%)	S (%)	Cl (%)
UFnbmrx	79.73	67.72	96.69
UF1	40.43	28.08	96.44

Atomic absorption measurements for the determination of the free Cr-content were made for the synthesized metal-complex dye 1 and the commercial dye Neutrilan Black M-RX in the permeate solution after the completion of the ultrafiltration process (Table 3). The significant reduction in free chromium achieved for both dyes can be attributed to the efficiency of the membrane to allow the permeation of the free chromium III ions through the membrane and thus achieving a cleaner dye in terms of free metal content in the final dye after ultrafiltration. The less amount of free chromium in the permeate of the commercial dye can be attributed to the fact that the commercial dye Neutrilan Black M-RX is already partly ultrafiltrated.³⁵ The atomic absorption measurements (Table 3) together with the XRF analysis (Table 2) clearly demonstrate the effectiveness of the ultrafiltration system employed to drastically eliminate the free chromium together with the inorganic ions present in the dyes. The elimination of all inorganic impurities results in the production of purer dyes as can be seen from the dye strength increase of the dyes after the ultrafiltration process which is of the order of more than 25% calculated from the corresponding λ_max of the dyes before and after ultrafiltration (Table 3). Additionally, the free chromium III elimination results in a more environmentally friendly metal-complex dye to the end used and to the environment. The dye strength increase (Table 3) coupled with the drastic improvement of the dye solubility (Table 4) clearly shows that the ultrafiltration has dramatically improved the quality of the dyes. This dye quality improvement opens new areas of applications for the ultrafiltrated dyes such as in ink jet digital printing where dye purity and solution stability is of paramount importance.

Table 3.

Elimination of chromium III for the synthesized dye 1 and the Neutrilan Black M-RX available dye before/after ultrafiltration

Dye	UV-Vis		Free Cr (ppm)	% Dye strength increase
Dye	λ_max	^cAbs.	Free Cr (ppm)	% Dye strength increase
Dye 1	571	0.74	223	27
UF 1	571	0.94
NBMRX	571	0.92	121	25
UF-NBMRX	571	1.16

Table 4.

Solubility of the synthesized metal complex dye 1 before and after ultrafiltration

Dye	Solubility at 95/25°C (g/L)
Dye 1	50
UF 1	250

Aqueous solutions of the synthesized dye 1 and 2, Neutrilan Black M-RX and the same dyes after ultrafiltration were used for the dyeing of wool and polyamide fibres. The colorimetric values of the dyed samples are given in Table 5.

Table 5.

K/S values and colour coordinates for the wool and polyamide fibres dyed with the dyes 1, 2, and Neutrilan Black M-RX, before and after ultrafiltration

Dye	Fibre	K/S	λ_max (nm)	L*	a*	b*	C	h
2	PA	8.3	520	31.1	9.1	−8.9	12.7	315.8
2	W	27.9	520	15.6	6.5	1.1	6.6	9.6
1	PA	22.5	580	19.0	0.02	−3.5	3.5	270.3
1	W	25.2	580	16.1	0.7	−1.3	1.4	298.7
NBMrx	PA	27.2	580	15.7	0.4	−2.8	2.9	277.0
NBMrx	W	28.6	580	14.4	0.7	−1.0	1.2	302.9
UF1	PA	24.9	580	17.5	0.4	−3.2	3.3	276.7
UF1	W	26.5	580	15.3	0.8	−1.2	1.4	303.5
UFNBMrx	PA	29.7	580	15.4	0.6	−3.1	3.2	280.9
UFNBMrx	W	30.1	580	13.7	0.6	−1.0	1.2	301.8

The synthesized dye 2 behaves as a typical monosulphonated acid dye showing high substantivity for wool rather than polyamide as can be seen from the much higher K/S value and the lower L* values obtained for wool dyeing (Table 5).

The colour strength obtained for the dyeing with the synthesized metal-complex dye 1 and Neutrilan Black M-RX after ultrafiltration is higher, as can be exemplified by the higher K/S and the lower L* values obtained for the ultrafiltrated dyes on both wool and polyamide application. The increase in dye uptake obtained for the ultrafiltrated dyes compared to the non-ultrafiltrated ones shows again that the ultrafiltration process resulted in dyes with higher colour yield. This can be attributed, as already mentioned above, to the elimination of inorganic compounds present in the dyes from the synthesis (both synthesized and commercial dyes) and additionally as cutting agents in the case of the commercial dye Neutrilan Black M-RX.

Table 5 also shows that the ultrafiltration process results in slightly redder dyeings as can be seen from the higher a* and h values obtained for the dyeing with the ultrafiltrated dyes. This can be attributed to the elimination of greenish dull components by the ultrafiltration process.

Both metal-complex dyes (synthesized and commercial) showed higher dye uptake on wool as this can be seen from the higher K/S values and the lower L* values on wool application. This can be attributed to the presence of two anionic sulphonic acid groups in the dye structure which make the dye more substantive to the wool fibre.

The wash and light fastness properties of the dyeing made with the synthesized dyes 1 and 2 and the commercial dye Neutrilan Black M-RX are shown in Table 6. As can be seen the metal-complex dyes 1 and Neutrilan Black M-RX showed excellent wash and light fastness whereas the non-metallized dye 2 showed reduced wash and drastically lower light fastness properties. The increased fastness properties of the metal-complex dyes can be attributed to the presence of metal in the dye structure. This also demonstrates the usefulness and significance of this dye class for the textile industry.

Table 6.

Wash and light fastness of the wool and polyamide fibres dyed with the dyes 1, 2 and Neutrilan Black MR-X, before and after ultrafiltration

Fibre	Dye	Wash fastness		Light fastness
Fibre	Dye	^aCC	^bCS
PA	NBMrx	5	diac = 5, c = 5, pa = 4–5, pes = 5, pac = 5, w = 5	>7
PA	UFNBMrx	5	diac = 5, c = 5, pa = 4–5, pes = 5, pac = 5, w = 5	>7
W	NBMrx	5	diac = 5, c = 5, pa = 4–5, pes = 5, pac = 5, w = 5	>7
W	UFNBMrx	5	diac = 5, c = 5, pa = 4–5, pes = 5, pac = 5, w = 5	>7
PA	1	5	diac = 5, c = 5, pa = 4–5, pes = 5, pac = 5, w = 5	>7
PA	UF1	5	diac = 5, c = 5, pa = 4–5, pes = 5, pac = 5, w = 5	>7
W	1	5	diac = 5, c = 5, pa = 4–5, pes = 5, pac = 5, w = 5	>7
W	UF1	5	diac = 5, c = 5, pa = 4–5, pes = 5, pac = 5, w = 5	>7
PA	2	3/4	diac = 5, c = 5, pa = 4–5, pes = 5, pac = 5, w = 5	1
W	2	3/4	diac = 5, c = 5, pa = 4–5, pes = 5, pac = 5, w = 5	3

CC = colour change.

CS = colour staining: diac = diacetate, c = cotton, pa = polyamide, pes = polyester, pac = polyacrylic, w = wool.

Conclusions

A novel simplified one pot method for the synthesis of Acid Black 194 (dye 1) was studied.

Ultrafiltration technology was used for the elimination and reduction of the excess Cr present in the Cr-complex dye. The dyes prepared, Cr-complex and non-metallized, were characterized by their spectrophotometric and analytical data. XRF analysis and atomic absorption measurements were performed for the determination of the inorganic ions and free Cr eliminated by the ultrafiltration process. A drastic reduction in the content of inorganic additives and free chromium was achieved by the ultrafiltration process. Ultrafiltration allows the production of highly concentrated purified metal-complex dyes with drastically reduced free metal and electrolyte content. The elimination of metals from the dye formulations and consequently from the dyehouse effluents is of high environmental importance.

The wash and light fastness properties of the dyeings obtained using the synthesized dyes and the commercial dye were excellent for the metal-complex dyes inferior for the non-metallized dye.

The implementation of ultrafiltration technology results in producing innovative dyes with greatly improved purity and solubility and opening new application areas for the ultrafiltrated dyes.

Colour measurements indicated that the ultrafiltration process slightly affects the hues of the dyeings.

Footnotes

Acknowledgements

We would like to thank Yorkshire Colours for their kind support throughout our work and their permission to disclose the structure of the metal-complex dye generously supplied to us.

Funding

The work was funded by the National Strategic Reference Framework ESPA 2007-2013 (COMPETITIVENESS Program EPAN II).

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