Special color change mechanism of Disperse Red 60 in alkaline dyeing system

Abstract

Color Intex (C.I.) Disperse Red 60 is a bright red anthraquinone dye, but it is easy to produce dyeing defects (a kind of discoloration phenomenon, the partial color is random blue patches) in the polyester dyeing process, and the mechanism of blue patches is unknown, resulting in a low right-one-time of dyeing. To investigate the mechanism of this particular color change, the interaction between C.I. Disperse Red 60 and borax was investigated by absorbance, and samples with blue patches were prepared; the presence of B-N bonds was confirmed by testing the X-ray Photoelectron Spectroscopy (XPS) of the dyed samples. The functional additive LV200 (a polymer containing polyester and polyether) was selected to control the interaction between the Disperse Red 60 and LV200. The results showed the co-existence of the red component (1-amino-2-phenoxy-4-hydroxy-9,10-anthraquinone) and the blue component (1-amino-2-phenoxy-10-hydroxy-4,9-anthraquinone) of Disperse Red 60 was the main reason for the random blue patches (dye defects) during the dyeing process of C.I. Disperse Red 60. By using the functional auxiliary LV200 and the H-bonding of the polar group of the dye, the conversion of the red and blue components of the reciprocal isomers can be controlled, and both bright red polyester-dyed fabrics and purplish red polyester-dyed fabrics can be produced. This provides a theoretical guide to improving the dyeing authentic rate of C.I. Disperse Red 60.

Keywords

Polyester fiber disperse dyes C.I. Disperse Red 60 1-amino anthraquinone isomers dyeing defects

Disperse dyes are widely used in printing and dyeing processes for textiles such as poly(ethylene terephthalate) (PET) fibers, poly(trimethylene terephthalate) (PPT),¹ polyoxymethylene (POM) fibers,² polylactic acid (PLA) fibers.^3,4 In recent years, with the continuous development of eco-friendly disperse dye printing and dyeing technologies, such as supercritical CO₂ dyeing without water consumption,^5–7 microencapsulation dyeing,⁸ low temperature and low toxicity carrier dyeing (e.g. vanilla),⁹ silicone solvent dyeing systems,^10,11 alkaline and alkaline buffer system dyeing,^12–14 liquid disperse dyeing without reduction cleaning and low dispersant,¹⁵ micro printing with liquid dyestuff with ultra-low water consumption.^16–18 The development of these technologies has significantly reduced the status of heavy pollution of the environment and the consumption of water resources in the printing and dyeing industry.

C.I. Disperse Red 60 is an important anthraquinone-based dye that is irreplaceable among red-phase disperse dyes. It is a weakly water-soluble anthraquinone-based disperse dye, and increasing its water solubility improves the dyeing properties such as its solubility which is significantly improved in supercritical carbon dioxide fluids.^19–21 And the adsorption of the dye in PET fibers increases slowly with increasing carbon dioxide pressure,²² but the equilibrium adsorption of the dye increases with increasing pressure and temperature.²³ When the solubility of C.I. Disperse Red 60 is high, it is not conducive to dyeing the fibers, due to the hydrophobic nature of polyester fibers.²⁴ During the dyeing process, polar solvents can generate interactions and H-bonds with C.I. Disperse Red 60.²⁵ Also, C.I. Disperse Red 60 can be used to improve the absorption and dyeing rate of the dye by ultrasound.²⁶

Although C.I. Disperse Red 60 can be dyed under weak alkaline conditions, it also produces discoloration (dyeing defects, random blue patches) during the improper dyeing process. This phenomenon may be related to the residual alkaline substances on the fabric (such as alkaline scouring treatment and alkali weight loss treatment). However, the advantages of alkaline staining are obvious. It can reduce cleaning containing reducing agents (such as insurance powder), and it is also beneficial for hydrolyzing and removing oligomers from polyester fibers, which not only shortens dyeing processing time but also reduces dye floating on the fiber surface and improves washing and rubbing resistance color fastness.^13–14

However, the discoloration of C.I. Disperse Red 60 is particularly severe in an alkaline solution, which is difficult to remedy in the subsequent process and causes great loss to the factory. In this article, the above-mentioned staining and discoloration phenomena in the process of C.I. Disperse Red 60 dyeing and its staining and discoloration mechanism have been studied intensively and reported. The above dyeing defect fabric (fabric with discolored blue spot) was prepared with C.I. Disperse Red 60 and borax. The color change mechanism of C.I. Disperse Red 60 reciprocal isomerization was proposed, and the reciprocal effect of functional additives on reciprocal isomerization was verified. The research results provide theoretical guidance for the future modification of C.I. Disperse Red 60 dyestuff, which can improve the reproducibility of industrial dyeing success and has more important theoretical and practical significance.

Materials and methods

Materials

Liquid Disperse Red 60-I (DR60-I) and liquid Disperse Red 60-II (DR60-II)

The DR60-I dye with an average particle size of about 0.8 μm was prepared by mixing C.I. Disperse Red 60, disperse emulsion agent DE-20, LV200, and deionized water in a zirconium oxide grinding device for 90 min and filtration under neutral conditions at pH 6.5. The DR60-II dye was made by taking an appropriate amount of DR60-I dye, adding a small amount of NaOH (adjust the pH to 10), and mechanical stirring (1500 rpm, 20 min).

Liquid Disperse Red 60-III (DR60-III) and liquid Disperse Red 60-IV (DR60-IV)

The DR60-III dye with an average particle size of about 0.8 μm was prepared by mixing C.I. Disperse Red 60, disperse emulsion agent DE-20, LV200, and deionized water in a zirconium oxide grinding device for 90 min and filtration under alkaline conditions at pH 10 (NaOH adjustment). The DR60-IV dye was obtained by taking an appropriate amount of DR60-III dye and adding a small amount of HCl (adjusting the pH to 6.5) (Figure 1).

PET fabric was supplied by Jiangsu Sheng-Hong Group (Jiangsu, China). The fabric was plain weave (8.33 tex × 8.33 tex, 76.0 g/m²). C.I. Disperse Red 60 (raw materials, commercial dyes) was supplied by Jiangsu Yabang Dyestuff Co. Ltd (Jiangsu, China). LV200 (functional auxiliaries) and disperse emulsion agent (DE-20; a mixture of anionic and non-ionic dispersant and emulsion) were supplied by Suzhou Changchunteng Import and Export Co. Ltd. (Jiangsu, China). N, N-dimethylformamide (DMF) (Analytial reagent (AR)), borax (sodium tetraborate decahydrate, AR), and NaOH (AR) were supplied by Sinopharm Chemical Reagent Co. Ltd (Shanghai, China).

Figure 1.

Schematic diagram of dye preparation.

Interaction of dyes with borax

C.I. Disperse Red 60 was taken and dissolved in a mixture of DMF and water (100 ml, V(DMF): V (H₂O) = 2:1) as solvent. The above dye solution was taken and placed in a volumetric flask, adding 4 g/l borax aqueous solution dropwise with a burette, and testing the absorbance of the dye solution with a UV-Vis spectrophotometer (TU-1810, Beijing Purkinje General Instrument Co. Ltd, Beijing, China), adding dropwise until the absorbance of the dye at 600 nm no longer increased.

Preparation of blue patch sample (dyed defective product)

The PET fabric was dyed at a high-temperature dyeing machine (XC-E7, Xiamen Rapid Precision Machinery Co. Ltd, Fujian, China) with Disperse Red 60 2% (o.w.f.), the value of pH was 9.5 (borax adjustment), the liquid ratio was 1:50, the temperature was 120°C, the dyeing time was 30 min. After dyeing, the samples were washed fully in hot water (50°C, 10 min) twice.

The above samples were tested for elemental content of blue patch specimens on an X-ray photoelectron spectrometer (ESCALAB 250 XPS, Thermo Nicolet Corporation, USA), monochromatic Al Ka (hυ = 1486.6 eV), power 150 W, beam spot 500 μm, energy analyzer fixed transmission energy 20 eV.

Dyeing

The PET fabric was dyed on the above dyeing machine with liquid disperse dye (DR60-I, or DR60-II, or DR60-III, or DR60-IV) (concentration: 2% (o.w.f.), liquid ratio: 1:50, temperature: 120°C, time: 30 min). After dyeing, the samples were washed fully in hot water (50°C, 10 min) twice.

The functional group information of LV200 was tested on an Fourier Transform Infrared Spectrometer (FTIR Spectrometer) (NICOLET 5700, Thermo Nicolet Corporation, USA) by grinding a dried KBr powder in an agate grinding body, acting under a pressure of 20 MPa for 15 s, pressing a translucent pressed sheet, applying the liquid sample to be tested with a capillary tube to the surface of the pressed sheet, and drying it in an infrared drying oven. The wavenumber range of the test was 4000∼600 cm⁻¹, the number of scans was 32, and the resolution was 4 cm⁻¹.

Results and discussion

Blue spot phenomenon of dyed fabrics and hypothesis on the mechanism of the color change of isomers

The random blue phenomenon of C.I. Disperse Red 60 in dyeing polyester fabric often occurs in the following situations.

When the dye concentration is high, the blue spot phenomenon is less likely to occur during the dyeing process. Too low a dye concentration (e.g. 0.02% (o.w.f.)) and the above discoloration will not occur.

When the pH value of the dye solution is between 8–12, fabric color is bluish, causing the color change to occur easily when PET fabric is at high-temperature heat setting.

During the soaping treatment of dyeing completion, PET-dyed fabric is prone to irregular local color change.

In the process of placing undried polyester dyed fabric, the surface of the fabric in contact with the air will be partially blue.

The particular color change phenomenon of C.I. Disperse Red 60 (that is, the appearance of local blue spots) is initially believed to be related to the alkali on the fabric, but the mechanism of this particular color change is rarely reported.

The effect of alkali on disperse dyes is more complex in alkaline or alkaline buffered systems. It depends on the type of alkali and the pH value. For example, when NaOH is chosen as the alkali dyeing agent, the −CN group on the benzene ring (from the diazo component of the mono azo disperse dyes) is not alkali resistant and cyanide-containing dyes will hydrolyze, failing to dye the PET fabric or lighter color. While the −CN group on the aniline (from the coupling component of the mono azo disperse dyes) has better alkali resistance and can be used for alkali dyeing of polyester. When choosing borax as an alkali dyeing agent, −CN (on benzene ring and aniline) substituents has good alkali resistance and can be used for polyester alkali dyeing. The reason is that borax is a strong base and weak acid salt, and [OH⁻] is present in the form of a coordination bond ([B(OH)₄]⁻) with boron, which is different from the free [OH⁻] that is completely ionized in the strong base NaOH.

Due to the special characteristics of borax alkaline, the phenomenon of the color change of C.I. Disperse Red 60 in a borax alkaline system was investigated. It is preliminary speculation that the special color change phenomenon of C.I. Disperse Red 60 in the alkaline system is caused by the reciprocal isomerism of 1-amino-4-hydroxy-anthraquinone, as shown in Figure 2. The molecular formula of C.I. Disperse Red 60 has reciprocal isomerism, structural formula I (1-amino-2-phenoxy-4-hydroxy-9,10-anthraquinone, red component), and structural formula IV (isomer, 1-amino-2-phenoxy-10-hydroxy-4,9-anthraquinone, blue component) are coexistent. And they could be interconverted under certain conditions. However, the blue component (structural formula IV) is extremely unstable and difficult to separate.²⁷ The Huckel charge in Figure 2 was calculated using Gaussian software. The rationality and reliability of the color change mechanism was analyzed as follows.

Figure 2.

Molecular structure of tautomerism of C.I. Disperse Red 60.

Spectral phenomena and interactions of basic dye systems

To understand the color change of C.I. Disperse Red 60 in alkaline solution, borax was added to C.I. Disperse Red 60 solutions in successive drops, and the change of absorbance of the dye by the amount of borax (or pH) was investigated, see Figure 3. The interactions between the boron elements and the dye were tested by XPS photoelectron spectroscopy.

Figure 3.

Effect of borax dosage on the absorbance of Disperse Red 60 solution.

As shown in Figure 3, there are two characteristic absorption peaks (X-band and Y-band) with high absorption intensity for C.I. Disperse Red 60 (pH 8.58). The X-band (long band) is 550 nm, which is caused by the quinone chromophore (longer conjugated system) and is attributed to the naphthalene ring structure with stable structure, which belongs to the n → π* leap. The Y-band (short wavelength band) of 520 nm is caused by the combined action of acetophenone chromophores and 1-amino and 4-hydroxy groups (easily polarizable) and belongs to the π → π* leap. The color is related to the π → π* leap, but also the charge spectrum contribution of the lone pair of electrons on the 1-amino or 4-hydroxy group and the intramolecular hydrogen bonding.^28,29

As the pH value increases, the color of the dye changes from bright red to purplish red and deep red to light blue, and the color change is a gradual process, the higher the pH value, the more blue components. The pH value of C.I. Disperse Red 60 solution is changed, the effect on the 555 nm characteristic peak is small, such as strong alkaline solution (pH > 12), the maximum absorption peak is only displaced to the long wavelength direction about 5 nm, which indicates that the alkali does not affect the quinone-type chromophore, the dye is a naphthalene ring structure with a long conjugation system.

The pH value affects the Y-band and the absorbance of the characteristic peak in the Y-band (520 nm) starts to decrease with increasing pH values from 2.48 (pH = 8.58) to 2.46 (pH = 10.30), 2.35 (pH = 11.10), and 2.13 (pH = 11.86), respectively. As the pH increases to 12.26, the Y-band (520 nm) starts to disappear, while a new characteristic absorption peak (∼595 nm) is generated at the long wave. The absorbance of this new Y-band (595 nm) increases with increasing pH, which indicates that strong alkalinity cannot affect the acetophenone chromophore but affects the co-chromophores, such as the 4-hydroxy group which is prone to protonation and proton charge transfer under alkaline conditions to produce the 1-aminoanthraquinone intermediate compound (see Figure 2, structural formula II and III), which is prone to the 1-aminoanthraquinone isomer (structural formula IV) due to the instability of this intermediate compound.

In addition, the presence of intramolecular hydrogen bonds (−NH…O=C and −OH…O=C) in Disperse Red 60 (structural formula I) increases the stability of the dye aggregates in the presence of borax (a double hex ring condensed from two H₃BO₃ and two [B(OH)₄]⁻), which is a strong base and weak acid salt, and ionized borax ([B(OH)₄]⁻) can generate intermolecular hydrogen bonds with the isomer (structural formula IV) (see Figure 4), which increases the stability of the dye aggregates in the quinone nature of the isomer,^30,31 resulting in a significant blue shift of the Y band toward the long wavelength.

Figure 4.

The interaction between borax and C.I. Disperse Red 60 (H-bond).

In borax alkaline conditions, C.I. Disperse Red 60 undergoes interchangeable isomeric structural changes, leading to a shift in the color of the dye solution from bright red to deep reddish blue, which is also confirmed by the disappearance of electronic interactions existing with the aromatic ring of 1-amino-4-hydroxyanthraquinone. For example, the characteristic absorption peak at 505 nm decreased from 2.51 (pH = 8.58) to 2.46 (pH = 10.30), 2.13 (pH = 11.10), nd 1.81 (pH = 11.86) with pH increase. As the pH continues to increase, the interaction of amino and hydroxyl groups on the same aromatic ring becomes weaker or even disappears, while it is difficult for amino and hydroxyl groups on both aromatic rings to interact.

XPS of borax-dyed polyester fabrics can verify the presence of interaction between borax and Disperse Red 60. Figure 5 shows the XPS of borax alkaline system dyed fabrics (samples with blue spots). Figure 6(a) and (b) show the XPS split peaks of elements O and B, respectively. The binding energies of C_1s, O_1s, N_1s, and B_1s, of the PET fabric samples are 284.4 eV, 530.4 eV, 398.4 eV, and 181.4 eV with the elemental percentages of 46.90% (C), 30.82% (O), 0.34% (N), and 21.66% (B), respectively (Figure 5). The relative intensity of the N_1s spectrum is too low to show the N_1s spectrum, due to the low concentration of dye on the fiber surface.

Figure 5.

X-ray Photoelectron Spectroscopy (XPS) of blue spots on fabric.

Figure 6.

(a) O_1s and (b) B_1s spectra of polyester fiber.

The O_1s fractional peak has C=O, C-O-H, and C-O-C chemical shifts, but no N-O chemical shift (Figure 6(a)). The B_1s fractional peak has B-O-B, B-O-H, B-C, and B-N (179.6 eV) (Figure 6(b)). Although the peak of the B-N bond was small, it was a chemical shift of the B-N bond. It was indicated that the dye molecules interacted with borax root ions. Borax is soluble in water and its aqueous solution is alkaline due to the hydrolysis of [B₄O₅(OH)₄]²⁻, and the solution contained equal amounts of H₃BO₃ and [B(OH)₄]⁻. And the electron deficiency of the B atom leads to the presence of sp³ hybridization in [B₄O₅(OH)₄]²⁻ (B atom is the center of a tetrahedron) and sp² hybridization (B atom is the center of a planar triangle),^32,33 while the N atom in the dye was sp³ hybridized. When a B-N bond was formed, there may be a B-N bond with sp³ and sp² hybridization (179.6 eV). As the result of sp3 and sp2 hybridization in borax, it can also combine with C to form B-C bonds.

The B-N bond was generated based on the interaction between borax and C.I. Disperse Red 60. In an alkaline aqueous solution, alkaline protons ([OH⁻¹], [B₄O₇ ²⁻], etc.) could attack the 1-amino group and 4-hydroxyl group of Disperse Red 60 (Figure 2: structural formula I), resulting in a higher positive charge of the N atom and a higher negative charge of the O atom, which could easily cause the protonation of the 4-hydroxyl group to generate intermediates (Figure 1: structural formula II; shown in Figure 2). Because of the high charge density on the N atom (1-position) and O atom (4-position) in structure (II), which was an unstable state, part of the charge would be transferred to the 2-position ether and carbonyl O atom, resulting in reciprocal isomerization to produce 4,9-anthraquinone isomers (structure III). Similarly, structure (III) was an unstable reciprocal isomer, and part of the charge would continue to be transferred to the carbonyl O atom, producing a reciprocal isomer to generate the 1-amino-10-hydroxy-4,9-anthraquinone isomer (structure IV), restoring the partial hydrogen bond.

So, there are resonance isomers of C.I. Disperse Red 60, and the red component (1-amino-2-phenoxy-4-hydroxy-9,10-anthraquinone) and the blue component (isomer, 1-amino-2-phenoxy-10-hydroxy-4,9-anthraquinone) could undergo charge transfer and reciprocal isomerization. Meanwhile, the absorption wavelength was shifted to the long-wave direction extremely obviously, from 520 nm in the red region to 595 nm in the blue region, due to the contribution of the charge spectrum of the N and O atoms on the 1-amino and 9-carbonyl groups of 10-hydroxy-4,9-anthraquinone and the contribution of intramolecular hydrogen bonding. The degree of interconversion of the above reciprocal isomers was related to the type and concentration of the base, and the production of blue spots during the staining process was related to the blue component of the conversion (isomer, structural formula IV).

The effect of functional auxiliaries in dyes

Although [OH⁻] and [B(OH)₄⁻] can be ionized from borax in an aqueous solution, the concentration of free [OH⁻] is less in aqueous borax solution. [B(OH)₄⁻] could form weak ion-dipole interactions and hydrogen bonds with the dye, which would prevent the interconversion of charge transfer and interconversion.While, aqueous solutions containing [OH⁻] (e.g. NaOH) could cause a specific discoloration of C.I. Disperse Red 60. The degree of conversion of the blue isomer depends on the [OH⁻] concentration.

Because the blue isomer is an unstable 4,9-anthraquinone structure, it cannot be separated from the dye solution. It is known that amino, ester, and ether groups can form H-bonds with amino, carboxyl, and hydroxyl groups. Therefore, the functional polymer (LV200) was selected to prevent or inhibit the reciprocal isomer conversion of 9,10-anthraquinone to 4,9-anthraquinone isomers by using the polar groups on the LV200 molecular structure to form H-bonds and interact with 9,10-anthraquinone dye (neutral conditions) and 4,9-anthraquinone isomers (basic conditions), respectively. So, by use of the polyester dyeing test, we were able to prepare bright red and fuchsia colored polyester dyed fabrics respectively.

The infrared spectrum of LV200 is shown in Figure 7. LV200 was an aliphatic amino (3429 cm⁻¹, 1647 cm⁻¹), ester (1450 cm⁻¹, 1350 cm⁻¹, 1088 cm⁻¹, 1030 cm⁻¹) and ether (1244 cm⁻¹, 950 cm⁻¹) functional additive. Thus, in the presence of polymer LV200, LV200 can form H-bonds with 9,10-anthraquinone dyes to weaken and prevent the attack of [OH-] on the 1-amino and 4-hydroxyl groups to form a stable red component (9,10-anthraquinone, structural formula I); and it can also form H-bonds with 4,9-anthraquinone isomers to weaken and prevent the attack of [OH-] on the 1-amino and 10-hydroxyl groups to form a relatively stable blue component (4,9-anthraquinone, structural formula IV). This is because the polar groups in the polyether monoamine dispersions interact with the C.I. Disperse Red 60 to increase the stability of the dye-polyether derivatives.³⁴

Figure 7.

Infrared spectrum of function auxiliary LV200.

Based on the above theoretical analysis, four dyes were prepared by pH-adjusted isomeric conversion of the red component (9,10-anthraquinone, structural formula I) and the blue component (4, 9-anthraquinone, structural formula IV) as well as the relatively stable H-bonds and interactions formed with LV200, respectively. Table 1 shows the performance of polyester-dyed fabrics with the four dyes and the pH of the dye residue.

Table 1.

Difference of dyeing property of four liquid C.I. Disperse Red 60 dyes

Liquid dye	Dye bath pH	Dye residue pH	L*	a*	b*	K/S
DR60-I	6.5	6.8	43.04	58.24	0.74	13.83
DR60-II	10.0	8.0	43.22	58.22	0.76	13.85
DR60-III	10.0	8.1	42.56	56.24	−0.96	13.31
DR60-IV	6.5	6.7	41.46	53.03	−3.66	12.88

Note: K/S: the apparent color depth of the fabric. a* and b*: the color direction.+a*: the red direction, −a*: the green direction, +b*: the yellow direction, −b*: the blue direction.

Comparing the dyed fabrics of DR60-I and DR60-II, the K/S values and color characteristic values are almost the same. The effect of NaOH in the dyeing solution on the C.I. Disperse Red 60 was extremely small. Because the aqueous solution is neutral to preparing liquid to disperse dyes, the red component (structural formula I) does not transform into the isomer (structural formula IV), and LV200 forms H-bonds with the polar groups (amino, carbonyl, and hydroxyl) on structural formula I. Meanwhile, because of the interaction between the dye and the auxiliary, even under alkaline dyeing conditions, the transformation of the reciprocal isomer caused by [OH-] becomes difficult. And the dyeing process was mainly based on the red component (structural formula I) staining and fixing on the fiber, and the dyed fabric had a heavy red light (+a* value) and a slightly yellow light (+b* value). In addition, when LV200 was dyeing with an alkaline solution, there was alkaline hydrolysis of the ester bond, which led to a decrease in the pH of the dyed residue and would decrease the occurrence of [OH⁻] induced dye interconversion isomerization.

The dyed fabrics dyeing with DR60-IV showed more blue light and less red light than with DR60-III. The K/S value between them is very different. The reason was that the red component (structural formula I) and the blue component (structural formula IV) were present simultaneously. When the dyeing medium was alkaline (DR60-III, pH = 10), there was a reciprocal isomeric transition from structural formula IV to structural formula I, and the dyed fabric had a less blue light. And when acidic conditions were chosen for dyeing (DR60-IV, pH = 6.5), the reciprocal isomeric conversion of structural formula IV to structural formula I became more difficult, and the dyed fabric was heavier in blue. And as the blue light increased, the red light of the fabric was weakening. In addition, the dyeing performance of the red component (structural formula I) was slightly better than that of the blue component (structural formula IV), as shown by the variation of K/S values.

C.I. Disperse Red 60 has an isomeric structure in alkaline media, and the isomers can convert to each other. Because polymer LV200 can form H-bonds and interact with the dye, it can effectively control or inhibit the mutual transfer of the two isomers of C.I. Disperse Red 60, so that the bright red dyed fabric (DR60-I) and the purplish-red dyed fabric (DR60-IV) can be prepared respectively, see Figure 8. The color saturation (C*) of the DR60-I dyed fabric is 58.2, which is better than that of the DR60-I dyed fabric (C* = 53.2), where the former is a red component (structural formula I) on the dyed fiber, and the latter is a blue component (structural formula IV) and red component on the dyed fiber together.

Figure 8.

Isomeric conversion of C.I. Disperse Red 60 and color change of poly(ethylene terephthalate) (PET) dyeing.

In summary, the appearance of blue spots in the staining process of C.I. Disperse Red 60 is caused by the blue component isomer (structured formula IV). Disperse dyes of 1-amino-4-hydroxy-9,10-anthraquinone structure, under the conditions of polar solvents and bases, etc., as 1-amino-4-hydroxy-9,10-anthraquinone (structural formula I, C.I. Disperse Red 60) and 1-amino-10-hydroxy-4,9-anthraquinone (structural formula IV, isomer) can transform each other by mutual isomerism, and structural formula IV is unstable and reversible. This directly causes irregular blue spots to appear during the dyeing of C.I. Disperse Red 60, and the degree of blueness corresponds to the 1-amino-10-hydroxy-4,9-anthraquinone isomer.

The color-changing of PET fibers dyed using C.I. Disperse Red 60 can be analyzed in terms of changes in molecular structure. For example, undried dyed PET fabric usually tends to show blue spots in the process of placement. Because the water in the fabric is a polar solvent, PET fabric contains residue of alkali solution on the surface. The undried dyed PET fabric can adsorb CO₂ in the air (slightly soluble in water), which can generate [CO₃²⁻]. [CO₃²⁻] can interact (such as ion-dipole force) with the polar groups (amino, carbonyl, hydroxyl) of the dye, resulting in the dye on the fiber occurring in the electron cloud density rearrangement and mutually variable isomeric transformation, and it could stabilize this blue isomer on the fiber, the formation of blue spot dyeing defects. Another example, when there is residual alkaline material on the dyed fabric, in the process of heat setting at high temperature, the discoloration phenomenon of blue spots will appear, which is caused by the isomeric conversion of red and blue components. However, if the dyed fabric is finished with a softener, the blue spots on the dyed fabric would be difficult to make appear. This is because the softener can prevent the conversion of the isomers of the said mutual change.

Conclusion

C.I. Disperse Red 60, which was an extremely important bright red dye, tends to appear in blue spots on the surface, resulting in dyeing failure and huge economic losses. This article summarized the phenomenon of dyeing defects in the dyeing process of C.I. Disperse Red 60, hypothesized and verified the existence of reciprocal isomerism of C.I. Disperse Red 60 and the mechanism of color change by XPS analysis of dyed defective fabrics in borax alkaline medium, and confirmed the mechanism of the color change of C.I. Disperse Red 60 by functional additives (LV200). C.I. Disperse Red 60 has a reciprocal isomeric structure of blue and red components. When the solution is alkaline, 1-amino-2-phenoxy-4-hydroxy-9, 10-anthraquinone can convert to 1-amino-2-phenoxy-10-hydroxy-4, 9-anthraquinone, because polar molecules (solvents, water, bases, etc.) can form weak ion-dipole interactions and hydrogen bonds with the dye polar groups. Therefore, the functional auxiliaries (such as LV200, and EDTA) that form H-bonds with the polar groups of the dye molecule can control the generation of blue and red components and the interconversion of isomers, improving the dyeing one-time success rate.

Footnotes

Acknowledgements

The authors of this study are grateful to the Hubei Key Laboratory of Biomass Fibers and Eco-dyeing and Finishing.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

ORCID iDs

Li Ai

Yawei Zhu

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