A novel method for natural dyeing of cotton fabrics with anthocyanin pigments from Morus rubra fruits

Abstract

A novel pathway to improve the substantivity of anthocyanin pigments from Morus rubra fruits onto cotton fabrics is proposed by preparing and applying an anionic agent (sodium, 4-(4,6-dichloro-1,3,5-triazinylamino)-benzenesulfonate) to fabrics. Successful modification of cotton fabrics with few negative effects was carried out and evidenced by the results of Fourier transform infrared spectroscopy, tensile strength and the whiteness index. The optimum amount of anionic agent was identified to be about 70 mg/g under the following conditions: 20% (owf) anionic agents, 1:20 material-to-liquor ratio, 100 g/L sodium sulfate, 30℃ for 60 min and pH 11.0. A temperature of 40℃ and a time of 60 min were found to be the optimal conditions for cotton fabrics dyeing with anthocyanin pigments. Dyed cotton fabrics had a shade of brilliance and saturation red with acceptable fastnesses for potential commercial applications. The influences of pH and temperature on the stabilities of the extracts were also investigated.

Keywords

cotton fabrics anthocyanin pigments natural cationic pigments

Manufacturing of synthetic dyes largely depends on petrochemical sources and many of those are toxic, resulting in environmental pollution.¹ On the other hand, most dyes obtained from plants, insects, animals and minerals are considered to be eco-friendly, biodegradable and low in toxicicy.² In recent years, the need for natural dyes per year has increased to about 10,000 tons, which is equivalent to about 1% of synthetic dye consumption.³ Anthocyanins are considered secondary plant metabolites, commonly found in flower petals, fruits and leaves, that produce orange, red, violet and blue shades.⁴ Flavylium cation (Figure 1) with a red shade is the predominant structure of the anthocyanins in acidic solution.⁵ Anthocyanins have strong oxidation resistance and antimicrobial property, and provide ultraviolet (UV) protection.^6–8 Anthocyanins can also inhibit the growth of cancerous cells and inflammation.⁹ Textiles dyed with anthocyanin extracts can be very useful in developing clothing that protects people against allergies and skin dysfunctionalities. The shades of anthocyanin extracts from berries and flowers also make them a promising source for textile dyes. A few researches have been carried out to investigate the potentials of anthocyanin-containing flowers and fruits as sources of natural dyes.^10,11 However, there is still little scientific data in the literature about the application of anthocyanin pigments in textile dyeing, unlike anthraquinones, berberine, indigoids, tannins, carotenoids and curcumin, which have been investigated much more.^12–18 Morus rubra fruit is a plant native to Asia that has been widely cultivated in Southern Europe for centuries and has already been widely used in the production of wine and fruit juice.¹⁹ The fruit has been reported to be rich in various polyphenolic compounds, such as anthocyanins and flavonoids.²⁰
Figure 1.
Chemical structure of cyanidin 3-glucose or cyanidin 3-rutinose.

Cotton is considered to be one of the most important natural fibers, especially in the textile industry. At the molecular-structure level, cotton is a carbohydrate homopolymer consisting of β-D-glucopyranose units joined together by β-1,4-glycosidic linkages.²¹ From the perspective of environmental protection, it is best not to use any synthesis agent for the natural dyeing of cotton. However, natural cotton cellulose possesses only hydroxyl groups in its structure, which causes the problem of fiber–dye bonding (especially with natural dyes).²² The pretreatment of the cotton by synthetic chemicals to introduce a new chemical group has been the alternative, and has been investigated by many researchers, especially for natural dyeing, in recent years.^23–26 The hydroxyl groups of cellulosic fibers belong to weakly ionized groups. In a neutral and alkaline dyebath, fibers gain negative charges when salts, such as sodium chloride and sodium sulfate, are added to increase the adsorption of dyes containing anionic sites. In an acidic dyebath, especially when the pH value is low, the hydroxyl groups of cellulose fibers can barely ionize. In other words, cellulose fibers can barely gain enough negative charges to help the adsorption of the cationic natural dye. Berberine is a cationic colorant extracted from Rhizoma coptidis, phellodendron and Berberis vulgaris and has been successfully used to dye wool fabrics.^27–29 Anthocyanin is another cationic colorant but its direct application in textiles as a natural dye is limited due to its low affinity to cellulose. To our knowledge, so far no satisfactory dyeing result of such natural pigments for cotton has been reported. Thomas Bechtold et al.¹¹ applied anthocyanin extracts from Blauer Portugieser Zweigelt and Blauer Burgunder to cotton fabrics after 30% tannin pre-mordanting (dye concentration TAC, 24.5–126 mg/L) and obtained dyed fabrics with K/S values of 0.63–1.95, whose dyeing effects (K/S) are not satisfactory. To improve the K/S values of cotton fabrics dyed with anthocyanin pigments, we propose the application of a synthetic reactive anionic agent to cellulosic fiber to introduce a strong anionic group, -SO₃^-, to the cellulosic substrate. This will enable the cellulose fibers to gain enough negative charges to improve the adsorption of cationic anthocyanin pigments in a strongly acidic dyebath.

The study was conducted with the synthesis of a potential anionic agent containing a strong ionized group (-SO₃^-) and its application onto the cotton fabric. Subsequently, the anionic cotton fabric was dyed with the cationic anthocyanin pigments extracted from the Morus rubra fruits. Different factors affecting the modification and dyeing of cotton fabrics were thoroughly investigated. Comparison of dyeing properties of control and pretreated cotton fabrics are also presented.

Experimental details

Materials

The fabric used for the work was a 190 g/m² plain woven 100% bleached cotton fabric (42 yarns/cm in the warp direction and 19 yarns/cm in the weft direction) that was obtained from local markets in Hangzhou, China.

Chemicals

Ethanol, 4-amino-benzenesulfonic acid, sodium carbonate, 2,4,6-trichloro-1,3,5-triazine, hydrochloric acid, tannic acid and other chemicals were obtained from Aladin (Shanghai, China). All the above chemicals were analytical grade. Acetonitrile, acetic acid, methanol and formic acid with high-performance liquid chromatography (HPLC) gradient grade were obtained from Merck (Darmstadt, Germany). High-purity water was obtained by filtering passing water though a Milli-Q treatment system (Millipore, USA), and the HPLC mobile phase was prepared using Milli-Q water.

Synthesis of the anionic agent

Solution 1: 11.56 g (0.05 mol) 4-amino-benzenesulfonic acid and 5.30 g (0.05 mol) sodium carbonate were dissolved in 100 mL distilled water, and then the solution was cooled to 0–5℃ in ice-water bath.

Solution 2: 14.75 g (0.08 mol) 2,4,6-trichloro-1,3,5-triazine was dissolved in 100 mL acetone, and then 200 mL water (0–2℃) was added to the mixture. The pH value of the mixture was adjusted to 1–2 by 2N hydrochloric acid, and then the system was kept in an ice-water bath.

Solution 1 was poured into solution 2 slowly, and the pH value of the mixture was adjusted to 6.0–6.5 by 100 g/L aqueous solution of sodium carbonate. The mixture was stirred at 0–5℃ for 90 min. After that, the white precipitated solid was filtered, washed with acetone to remove unreacted 2,4,6-trichloro-1,3,5-triazine and dried in a vacuum drying oven at 30℃ for 24 h. The yield of reactive anionic agent (16.53 g) was 82.40%. The synthesis scheme of the reactive anionic agent is shown in Figure 2.
Figure 2.
Synthesis scheme of the reactive anionic agent.

Modification of cotton fabrics

In a mixture solution containing reactive anionic agent (1–60%, owf) and sodium sulfate (0–200 g/L) with a liquor ratio of 1:20–60, the cotton fabric was impregnated at different temperatures (20–70℃) for different times (15–90 min). After a half-time modification, the pH value of the mixture solution was adjusted to 7–12 using sodium carbonate.

At the end of the modification, the cotton fabric was washed in 100 mL distilled water and dried at room temperature. The modification scheme of the reactive anionic agent and cotton cellulose is shown in Figure 3.
Figure 3.
Modification scheme of the reactive anionic agent and cotton cellulose.

The exhaustion of the anionic agent onto cotton fabric was determined by high-performance liquid chromatography with diode-array detection (HPLC-DAD). Using the established peak area (mAu.s)/weight (mg) relationships of the reactive anionic agent or hydrolyzed anionic agent, the amount of anionic agent in the modified solution, the modified residual solution and the solution for washing modified cotton fabrics was calculated. The extent of fixation (F, %) is determined using equation (1), where D₀, D_e and D _r are the amount of anionic agent in the modified solution, the modified residual solution and the recycling washing solution, respectively
$F (%) = (D_{0} - D_{e} - D_{r}) / D_{0} \times 100$
(1)

Extraction of anthocyanin pigments from Morus rubra fruits

After carefully washing by tap water, the fresh Morus rubra fruits were crushed with a mortar and mixed with solvent (ethanol/trifluoroacetic acid/H₂O 60/0.5/39.5 v/v/v), keeping a 1:2 material-to-liquor ratio. The extraction was carried out at room temperature for 24 h and filtrated thereafter. Degreased by petroleum ether (60–90℃, bp), centrifuged at 5000 r/min for 25 min, the extracts were further processed by a rotary evaporator to evaporate ethanol at 40℃. The remaining aqueous phases were then diluted with distilled water and used for dyeing afterwards.

The concentration of anthocyanin pigments in the dyeing solution was calculated by the total anthocyanins content (TAC) using the pH differential method and expressed as cyanidin 3-glucoside equivalents.^30,31 The TAC in the dyeing solution was 60.79 mg/L.

Separation and identification of the anionic agent and anthocyanin extract

The separation and identification of the anionic agent and anthocyanin extracts were carried out by HPLC-DAD and a mass spectrometer using an electrospray ionization (HPLC-DAD/ESI-MS). HPLC-DAD experiments were performed using a HPLC machine (1260 Infinity, Agilent Technologies, CA, USA). The separation of the anionic agent was carried out on an Agilent Zorbax SB-C₁₈ column (4.6 mm × 250 mm, 5 µm) equipped with a guard column (Agilent Zorbax SB-C₁₈, 4.6 mm × 12.5 mm, 5 µm). The column oven temperature and flow rate were set to 30℃ and 1.0 mL min⁻¹, respectively. For analysis of the anionic agent, the mobile phase was 70% water containing 5% formic acid (solvent A) and 30% acetonitrile (solvent B). The detected wavelength was 280 nm. For analysis of tannic acid, the mobile phase was 25% water containing 5% acetic acid (solvent A) and 75% methanol (solvent B). The detected wavelength was 276 nm. For analysis of anthocyanin extracts, the mobile phase was 10% formic acid (solvent A) and 100% CH₃CN (solvent B). The elution conditions of anthocyanin extracts were set as follows: isocratic elution 0% B, 0–10 min; linear gradient from 0% B to 10% B, 10–20 min; 12% B, 20–40 min; 100% B, 40–50 min, 100% B, 60 min. The detected wavelength was 515 nm. Mass spectrometry (MS) experiments were performed using a mass spectrometer (LCQ Fleet, Thermo Fisher Scientific, PA, USA) and electrospray ionization (ESI). The MS analytical parameters were as following: polarity, negative or positive; ion source, turbo spray (ESI); capillary temperature, 350℃; capillary voltage, 60 V; spray voltage, 4.5 kV; full-scan range, m/z 200–800. A MS² scan of the most abundant ion uses relative collision energy of 20%.

Stability of the Morus rubra fruit extracts

The extracts were diluted with deionized water to 18.35 mg/L stock solutions. The pH values were adjusted to 1.0–7.0 by 1 M HCl and 1 M NaOH, respectively, monitored by a microprocessor pH/mV meter (PHS-3C, Shanghai Precision & Scientific Instrument, Shanghai, China). The absorbance spectrum (Δλ = 1 nm) of each sample was recorded with an ultraviolet-visible (UV/Vis) double-beam spectrophotometer (TU-1950, Beijing Purkinje General Instrument, Beijing, China). Reference blank measurements were performed with the cuvette filled with distilled water. The whole visible spectrum (380–750 nm) was recorded. The colorimetric method was used to characterize the color properties of the samples. D₆₅ standard illuminant and 10° standard observer were considered in the calculation.

To determine the effect of temperature and pH on the thermal degradation of the extracts, the evaporated extracts were diluted with distilled water and then adjusted by 0.2 M acetic acid or 0.2 M sodium carbonate solution to yield the dyeing solution (TAC = 60.79 mg/L) with pH values of 2.0, 3.0 and 4.0. After 1, 2, 3, 4, 5 and 6 h, the tubes were immediately cooled in an ice-bath for 2 min to stop thermal degradation. The anthocyanin concentration of each sample was analyzed by HPLC-DAD.

Dyeing with anthocyanin pigments

The evaporated extracts were diluted with distilled water and then further adjusted to pH 2.0 using 0.2 M acetic acid or 0.2 M sodium carbonate solution. In a dye bath containing 60.79 mg/L anthocyanin pigments with a liquor ratio of 1:50 and pH 2.0, the anionic and control cotton fabrics (1.0 g) were dyed at different temperatures (30–50℃) for different times (5–180 min). At the end of dyeing, the cotton fabrics were removed, rinsed thoroughly in tap water and air-dried at room temperature.

The exhaustion (E, %) of anthocyanin pigments onto cotton fabric is determined by HPLC-DAD using equation (2), where A₀ and A_e are the total peak areas of anthocyanin pigments in the dyeing bath before and after dyeing, respectively
$E (%) = (A_{0} - A_{e}) / A_{0} \times 100$
(2)

Pre-mordanting with tannic acid

Tannic acid is a common biomordant in natural dyeing, and it was used in this paper to compare the dyeing effect with the reactive anionic agent on cotton fabrics.^32–34 In a pretreatment solution containing tannic acid (1–60%, owf) and sodium sulfate (0–200 g/L) with a liquor ratio of 1:20–60 and pH 4.0–9.0, the cotton fabrics were pretreated at different temperatures (20–70℃) for different times (15–90 min). The pH value of the tannic acid solution was adjusted using 1 M acetic acid or 1 M sodium carbonate solution. After the process, the cotton fabrics were rinsed in tap water and dried at room temperature. The exhaustion (E, %) of tannic acid onto cotton fabrics was also determined by HPLC-DAD using equation (2). The dyeing process of pre-mordanted cotton fabrics with anthocyanin pigments was the same as that for anionic and control cotton fabrics.

Fourier transform infrared spectral analysis

The structural change occurring in the cotton fabrics was monitored by Fourier transform infrared spectroscopy (FTIR). A PerkinElmer System 2000 FTIR spectrophotometer (PerkinElmer Inc., MA, USA) was used to collect the transmission spectra of all powder samples in the wavenumber range of 400–4000 cm⁻¹. The FTIR spectra of the powders were collected using the KBr pellet technique. The resolution for all the infrared spectra was 4 cm⁻¹ and there were 100 scans for each spectrum.

Fabric performance properties

The tensile strength of cotton fabric was measured according to ASTM method D5035, and tested in the warp direction only. Five specimens were used for each data point. The whiteness index of the cotton fabric was measured according to standard AATCC 110-2005.

Color analysis

Colorimetric data of cotton fabrics were calculated on a Datacolor SF600 spectrophotometer (Datacolor International, NJ, USA). The sample diameter was 10 mm, with 10° standard observer and D₆₅ illuminant.

Fastness testing of dyed fabrics

The fastnesses of dyed cotton fabrics was tested according to AATCC standard methods. Light fastness tests were carried out according to AATCC 16-2004. Color changes of the samples were assessed against the gray scale and blue wool standard. Color fastness to washing, crocking fastness and perspiration fastness tests were carried out according to test no. 1b of AATCC 61-2009, AATCC 8-2007 and AATCC 15-2009, respectively. The colors of the specimens were rated by reference to the Gray Scale for Color Change. The staining on each original fabric was rated by means of the Gray Scale for Staining.

Results and discussion

Separation and identification of the anionic agent

The main research purpose of this paper is to dye cotton fabrics with anthocyanin pigments, for which the anionic sites (-SO₃^-) are proposed to modify cotton fabrics. As shown in Figure 2, the synthesized anionic agent has a dichloro reactive group, which can be covalently bonded to cotton fabrics by a nucleophilic substitution reaction. Thus, the adsorption is driven by strong ionic interaction (Coulombian force) between the negatively charged -SO₃^- groups (Figure 3) anchored at the anionic cotton fabrics and cationic anthocyanin pigments (Figure 1) in solution. In the synthesis process of the reactive anionic agent, purification was not carried out. Therefore, separation and identification of the anionic agent need to be discussed.

The major compounds (monitored at 280 nm) were separated and identified by HPLC/MS. Figure 4 shows the HPLC-DAD profile of two major compounds (peak 1 and peak 2) in the anionic agent. Their retention times were 3.462 and 2.256 min, respectively. Peak 1 and peak 2 take up to 97.53% and 2.47% in the chromatogram respectively. After separation, the MS (Figure 5) of the majority compound (peak 1) in the negative ionization mode shows molecular ions (319.38 [M-Na]⁻, 100%, 321.09 [M-Na+2]⁻, 70.7%, 323.10 [M-Na+4]⁻, 14.5%). The data of MS indicate that peak 1 is assigned as sodium, 4-(4,6-dichloro-1,3,5-triazinylamino)-benzenesulfonate. In the artificial synthesis of organic compounds, unwanted byproducts cannot be completely removed; thus, further purification is always required. Since, in this research work, the contents of byproducts in the chromatogram are less than 5% in the anionic agent, the purification may not be necessary for commercial applications.
Figure 4.
High-performance liquid chromatography with diode-array detection profile of major compounds in the anionic agent.

Figure 5.
Mass spectrum of the major compound (peak 1) in the anionic agent.

It is found from the DAD spectrum (Figure 4) that the maximum wavelengths of peak 1 and peak 2 (byproduct) are observed at 278 and 280 nm, respectively. Peak 1 and peak 2 do not absorb any visible light in the range of 400–600 nm, which indicates that the synthesized anionic agent is white and will not cause any unwanted shade changes of the cotton fabrics. The colorimetric data of the modified and control cotton fabrics are shown in Table 1, as well as the color differences (ΔE) between them. It can be clearly observed that the anionic agent caused very little change of shade on cotton fabrics. Meanwhile, it is evident from Table 1 that the shade of the fabric pre-treated with tannic acid shifted from white to brown.
Table 1.
CIELab color parameters (L, a, b, C and h) and color differences (ΔE) of modified and control cotton fabrics. Modification conditions: amount of anionic agent (20% owf) or tannic acid (20% owf); material-to-liquor ratio, 1:20; temperature, 30℃ for the anionic agent (AA) and 50℃ for tannic acid (TA); time, 60 min

Cotton fabrics L* a* b* C* h* ΔE*

Control 95.52 −1.63 0.54 1.72 161.85 –

TA 20% 93.21 0.47 2.04 2.09 77.31 3.46

AA 20% 94.11 −0.44 0.73 0.85 121.14 1.85

Characterization of the anionic cotton fabrics

FTIR analysis

The FTIR spectra are used to identify the presence of functional groups in cotton fabrics and the results are shown in Figure 6. A strong O–H stretching absorption at 3405.7 cm⁻¹ with a broad peak appears in the spectrum of cotton fabrics. The band at 2900.5 cm⁻¹ is assigned to C–H bonds stretching absorption of cellulose. The C–O–C stretching absorption is around 1062.6 cm⁻¹. The characteristics of FTIR absorption are consistent with those of the typical cotton cellulose. Moreover, a new peak at 1569.8 cm⁻¹ is observed in the spectra of cotton fabrics modified with the anionic agent (20% owf), which is associated with stretching of –N=C bonds (1,3,5-triazine cycle). The peaks at around 1255.5, 1178.3, 1130.1, 1037.5 and 1010.5 cm⁻¹ generated by the –SO₃Na group are not identified in the FTIR spectra because they overlap with that generated by the ether C–O–C stretching of cellulose at 1000–1300 cm⁻¹. Even so, the presence of –N=C at 1569.8 cm⁻¹ indicates that the cotton cellulose has been successfully modified with the anionic agent.
Figure 6.
Fourier transform infrared spectra of control (A) and anionic (B) cotton fabrics. Modification conditions: amount of anionic agent, 20% owf; material-to-liquor ratio, 1:20; temperature, 30℃; time, 60 min.

Tensile strength

Table 2 shows the effect of anionic agent concentration on the performance properties of the modified cotton fabrics. A slight decrease in the tensile strength at the break point of cotton fabrics modified with anionic agent is observed from Table 2. Only 7.1% loss of the tensile strength is obtained at the concentration of 20% (owf) anionic agent in pretreatment solution. Different from the tensile strength, which slightly decreases with increasing anionic agent, the variations of the elongation and whiteness index are both small and irregular. The negligible negative effect of the anionic agent on the cotton fabrics’ performance properties is considered due to the following two reasons. Firstly, the basic conditions (pH 11.0) may not lead to significant cellulose degradation during anionic agent modification. Secondly, the synthesized anionic agent is white, and does not change the whiteness index of cotton fabrics by a noticeable amount.
Table 2.
The effect of anionic agent concentration on cotton fabric performance properties (average values and standard deviations of cotton fabric performance properties measured in five different specimens). Modification conditions: amount of anionic agent, 1–60% owf; material-to-liquor ratio, 1:20; temperature, 30℃; time, 60 min

Amount of anionic agent/% Tensile strength/N Elongation/ mm Whiteness index

Control 622.4 ± 29.9 29.7 ± 2.3 55.7 ± 0.2

1 599.3 ± 23.7 30.0 ± 3.8 55.6 ± 0.3

5 585.6 ± 16.8 28.5 ± 1.6 56.1 ± 0.1

10 581.3 ± 19.3 25.5 ± 2.2 56.3 ± 0.2

20 578.2 ± 11.2 22.6 ± 3.0 56.6 ± 0.1

30 569.5 ± 21.8 24.5 ± 2.2 56.5 ± 0.3

40 572.0 ± 18.1 25.0 ± 1.3 56.7 ± 0.2

50 570.1 ± 19.3 26.2 ± 2.4 56.5 ± 0.3

60 570.1 ± 9.3 25.8 ± 2.7 56.8 ± 0.3

Treatment conditions of agents

To determine the optimum treatment conditions of the agents onto cotton fabrics, the fixation of the anionic agent and exhaustion of tannic acid were obtained at various temperatures, liquid–solid ratios, time and pH values. In the range of 20–70℃ (Figure 7(a)), it is evident that the fixation of the anionic agent increased with decreasing temperature, which means that the reaction between the chlorines of the anionic agent and cellulosic substrate can take place even at room temperature. On the contrary, the exhaustion of tannic acid increased with increasing temperature and a maximum exhaustion value was obtained at the temperature of 50–70℃. In the case of treatment time, proper values of the fixation and exhaustion were sufficiently achieved in 60 min (Figure 7(b)). It is found from Figure 7(c) that the maximum values of the fixation of the anionic agent and exhaustion of tannic acid were observed at about pH 11.0 and pH 7.0, respectively. Figure 7(d) shows that when the liquid–solid ratio increased, the values of fixation and exhaustion decreased for both treatment agents. Compared to the effect on the exhaustion of tannic acid, neutral salt (sodium sulfate used in this paper) could greatly affect the adsorption of the anionic agent on to the cotton fabrics, as shown in Figure 7(e). Figure 7(e) shows that the fixation of the anionic agent increased with the increasing concentration of sodium sulfate and a maximum fixation was obtained at the concentration of 100–200 g/L. From the view of industrial applications, the amount of 100–200 g/L was too excessive to employ practically, but 50 g/L of sodium sulfate was considered appropriate and is used in subsequent experiments.
Figure 7.
The effects of temperature, time, pH, liquid–solid ratio and neutral salt (sodium sulfate) on the fixation of anionic agent (20%, owf) and exhaustion of tannic acid (20%, owf) on to the cotton fabrics. Modification conditions: (a) material-to-liquor ratio, 1:20; temperature, 30–70℃; time, 60 min; sodium sulfate, 100 g/L; pH 11.0 for the anionic agent and pH 7.0 for tannic acid; (b) material-to-liquor ratio, 1:20; temperature, 30℃ for the anionic agent and 50℃ for tannic acid; time, 15–90 min; sodium sulfate, 100 g/L; pH 11.0 for the anionic agent and pH 7.0 for tannic acid; (c) material-to-liquor ratio, 1:20; temperature, 30℃ for the anionic agent and 50℃ for tannic acid; time, 60 min; sodium sulfate, 100 g/L; pH 7.0–12.0 for the anionic agent and pH 4.0–9.0 for tannic acid; (d) material-to-liquor ratio, 1:20–60; temperature, 30℃ for the anionic agent and 50℃ for tannic acid; time, 60 min; sodium sulfate, 100 g/L; pH 11.0 for the anionic agent and pH 7.0 for tannic acid; (e) material-to-liquor ratio, 1:20; temperature, 30℃ for the anionic agent and 50℃ for tannic acid; time, 60 min; sodium sulfate, 0–200 g/L; pH 11.0 for anionic agent and pH 7.0 for tannic acid.

Agents on the cotton fabrics

To investigate the fixation of the anionic agent and exhaustion of tannic acid onto cotton fabrics, the effect of application concentration in the finishing solution is shown in Figure 8. Figure 8 shows that the fixation (%) or exhaustion decreased with the increasing concentration of agents. At about 20–60% (owf) of the agents, the fixed amount of anionic agent (mg/g) and exhausted amount of tannic acid (mg/g) reached the maximum values of 70–100 mg and 35–50 mg per 1 g of cotton fabrics, respectively.
Figure 8.
The effect of the amount of agents on the cotton fabrics (1 g). Modification conditions: amount of agents, 1–60% owf; material-to-liquor ratio, 1:20; temperature, 30℃ for the anionic agent and 50℃ for tannic acid; time, 60 min.

Separation and identification of anthocyanin pigments from Morus rubra fruits

Anthocyanins in Morus rubra fruit extracts were identified by HPLC-DAD/ESI-MS. Figure 9 shows the HPLC profile of the anthocyanins presented in Morus rubra fruits. The specific data obtained by HPLC-DAD/ESI-MS analysis are summarized in Table 3. Since the individual anthocyanin standard is unavailable, the identification was carried out by comparison of the molecular ion and fragment pattern of the individual anthocyanin. Peak 1 (RT = 24.508 min) and peak 2 (RT = 25.246 min) present the two major anthocyanins in Morus rubra fruits. The MS spectrum of peak 1 indicates that the peak had a molecular ion at m/z 449, and the MS/MS spectrum shows that the molecular ion at m/z 449 was fragmented to an l product ion, 287 ([M-C₆H₁₀O₅]⁺), which correspond to cyanidin 3-glucoside and cyanidin, respectively. Peak 2 has a molecular ion at m/z 595 and, MS/MS product ions of m/z 449 ([M-C₆H₁₀O₄]⁺) and 287 ([M-C₆H₁₀O₄-C₆H₁₀O₅]⁺), which correspond to cyanidin glucoside and cyanidin, respectively. Peak 2 is thus established as cyanidin 3-rutinoside. The specific data of the two main anthocyanins identified are in accordance with the previous results.^27,35
Figure 9.
High-performance liquid chromatography anthocyanin profiles (monitored at 515 nm) of the dyeing solution.

Table 3.
Retention time (RT) and percentage peak area of chemicals in high-performance liquid chromatography anthocyanin profiles (monitored at 515 nm), maximum absorption (λ_max) of chemicals in ultraviolet-visible spectra, the m/z value of the molecular ion ([M⁺]) and fragmentation pattern in mass spectrometry spectra, as well as the deduced chemicals in the extracts

Peak RT/min Peak area/% λ_max / nm [M⁺]/m/z MS² of [M⁺]/m/z Compound

1 24.508 65.08 516, 331, 279 449 287 cyanidin 3-glucoside

2 25.246 33.03 517, 330, 280 595 449, 287 cyanidin 3-rutinoside

Among the anthocyanins in Morus rubra fruit extracts, cyanidin 3-glycoside (cyanidin 3-glucoside and cyanidin 3-rutinoside) is the major anthocyanin structure and takes up 98.11% of the total anthocyanin content in the chromatogram. Even without further purification, the Morus rubra extracts still can be considered as single component anthocyanin pigments for cotton fabric dyeing in this study.

Influence of pH and temperature on the stability of the extracts

In acidic aqueous solution, anthocyanins exist in the form of four main equilibrium species (Figure 10(b)): red flavylium cation; blue quinoidal base; colorless carbinol or pseudobase; and yellowish chalcone.⁴ Under acidic conditions (pH < 3.0), the anthocyanins exist primarily in the form of a red flavylium cation. Increasing the pH value causes fast loss of the proton and produces quinoidal base forms, blue or violet. At the same time, hydration of the flavylium cation occurs and the carbinol or pseudobase are generated, which slowly reaches equilibrium and produces the chalcone in faint yellow. The relative amounts of the above four forms of anthocyanins at the equilibrium condition vary according to pH values. The color variation in the aqueous solutions of extracts from Morus rubra fruits was studied within the pH range 1.0–7.0. The visible spectra of the extracts at different pH values are shown in Figure 10(a). It is observed that the spectra of extracts at pH 1.0 and 2.0 have a very high absorption peak at 517 nm. With increasing pH values, the absorbance of extracts at 517 nm decreased and finally disappeared at pH 5.0. However, with only these data we do not have enough information to know the color variation at different pH values. It is well known that CIELab* parameters (L, a, b, C and h) determined following CIE1964-XYZ and CIE1976Lab of the Commission Internationale de L’Éclairage are very helpful to describe the properties of dyes, including the color expression.^36,37 The CIELab* parameters (L, a, b, C and h) of the extracts at different pH values are shown in Table 4. At the lowest pH value (pH ≤ 2), where anthocyanins existed in flavylium form, the extracts showed reddish hues (h^= 14.08°). The extracts underwent small decreases of the hue angle (h) and increases of lightness (L) as the pH increased from 1.0 to 3.0. Meanwhile, large decreases in hue were found as the pH increased from 3.0 to 4.0. At pH 5.0–7.0, the form of anthocyanins changed from chalcone to quinoidal bases, resulting in the hue of the extracts gradually changing to yellow (h* values between 70° and 110°). Comparing the ΔE* values at different pH values, it is observed that the reddish hues of extracts are clearly stable at the most acidic pH (1.0–3.0) without great loss of color.
Figure 10.
Visible spectra of the Morus rubra fruit extracts (a) and structure of cyanidin 3-glycoside (Bb) at different pH values. The total anthocyanins content of the extracts was 18.35 mg/L.

Table 4.
CIELab* color parameters (L, a, b, C and h) and color differences (ΔE, calculated between the initial pH value (pH 1.0) and after each increase of pH) of the extracts from Morus rubra fruits

pH values L* a* b* C* h* ΔE*

1.0 74.17 56.08 14.06 57.81 14.08 –

2.0 74.38 55.00 13.12 56.54 13.42 1.45

3.0 76.78 48.13 8.17 48.81 9.63 10.23

4.0 89.10 18.10 0.10 18.10 0.30 43.13

5.0 93.58 4.15 −0.44 4.17 353.99 57.30

6.0 91.71 −1.00 3.12 3.27 107.74 60.71

7.0 89.54 −1.44 8.51 8.64 99.62 59.80

The thermal degradation of anthocyanin pigments from Morus rubra fruits were studied at 50℃, 70℃ and 90℃. The results obtained are presented in terms of C/C₀, representing the initial anthocyanin concentration (C₀) and anthocyanin concentration (C) after time t treatment at a set temperature. Figure 11 shows the curves of ln (C_t/C₀) versus heat treatment time. The rate constant (k) and half time of anthocyanin degradation (t_1/2) of the anthocyanin pigments are listed in Table 5. It is clear that the degradation of the extracts increased with increasing heating temperature and time. A linear relationship was obtained that indicated that the degradation of the anthocyanin pigments followed a first-order kinetic model (ln (C_t/C₀) = −kt + b) with a good regression coefficient (0.97 < R²< 0.99). The same behavior was also found for the heat degradation of extracts from other natural sources, such as mangosteen peel extracts,³⁸ raspberry extracts,³⁹ eggplant peel and strawberry.⁴⁰ Table 5 shows that there was greater degradation with higher rate constants (k) of the extracts when subjected to heat at higher pH values. The data in Table 5 clearly indicate that the half-life time of the extracts increased between two and four times with pH decreasing from 4.0 to 2.0 at the same temperature.
Figure 11.
Degradation of anthocyanin pigments from Morus rubra fruits. The total anthocyanins content of the extracts was 60.79 mg/L.

Table 5.
Rate constant (k), half time (t_1/2) and first-order kinetic model of anthocyanin degradation in the Morus rubra fruit extracts at different pH values and temperatures

Temperature/℃ pH k/min⁻¹×10⁴ t_1/2/min First-order kinetic model R ²

50 2 0.85 8082.24 ln (C_t/C₀) = −0.000085 t − 0.003451 0.98

3 2.05 3313.80 ln (C_t/C₀) = −0.000205 t − 0.013003 0.99

4 2.99 2263.09 ln (C_t/C₀) = −0.000299 t − 0.015557 0.97

70 2 3.25 2072.97 ln (C_t/C₀) = −0.000325 t − 0.018762 0.99

3 7.71 857.05 ln (C_t/C₀) = −0.000771 t − 0.032354 0.99

4 8.97 738.19 ln (C_t/C₀) = −0.000897 t − 0.030871 0.99

90 2 30.00 258.97 ln (C_t/C₀) = −0.003000 t + 0.083762 0.98

3 42.50 195.29 ln (C_t/C₀) = −0.004250 t + 0.136841 0.99

4 51.90 176.20 ln (C_t/C₀) = −0.005190 t + 0.221351 0.98

The temperature sensitivity of the rate constant is analyzed using the Arrhenius equation, k = A exp (−E_a/RT). This equation can be further derived as ln k = – E_a/RT + ln A. k is the rate constant (min⁻¹), A is the pre-exponential factor (min⁻¹), E_a is the activation energy (kJ/mol), R is the ideal gas constant (8.314 J/mol/K) and T is the absolute temperature (K). To determine the effect of temperature on the parameters studied, the constants obtained were fitted to an Arrhenius equation in each of the kinetic models studied (Figure 11(b)). The average activation energies for anthocyanin degradation in the range of 50–70℃ at three different pH values were 86.31 kJ/mol for pH 2.0, 73.65 kJ/mol for pH 3.0 and 69.19 kJ/mol for pH 4.0 (Table 6). It can be concluded from these results that the increasing of pH value resulted in the increasing of the sensitivity to temperature.
Table 6.
Activation energy (E_a) and Arrhenius equation of anthocyanin degradation in the Morus rubra fruit extracts at different pH values

pH E_a/kJ/mol Arrhenius equation R ²

2 86.31 ln k = −10,381.59 / T + 22.59 0.94

3 73.65 ln k = −8858.87 / T + 18.84 0.98

4 69.19 ln k = −8322.22 / T + 17.51 0.95

The rate constants (k) presenting the degradation of anthocyanin pigments increased drastically with increasing pH value and temperature, indicating that anthocyanin red color was retained at a lower pH value. Thus, the application of “anthocyanin red” from Morus rubra fruits as natural dyes needs to be carried out in an acid dyeing bath with low pH values. In this paper, pH 2.0 of the dyeing bath was chosen and applied in the following natural dyeing experiments. Although a low pH was used in this paper to make anthocyanin dyes with the form of a flavylium cation absorptive to fabrics, the strength of the fabric was found to decrease by less than 5%. This amount of decrease is acceptable for better dye uptake and the necessary shade.

Dyeing with Morus rubra fruit extracts

To investigate the effect of the amount of anionic agent on the exhaustion of anthocyanin pigments, the cotton fabrics were pretreated with different amounts of anionic agent and then dyed with anthocyanin pigments from Morus rubra fruits. The results are shown in Figure 12. It reveals that the equilibrium exhaustion (60 min dyeing time applied) was greatly improved with an increasing amount of anionic agent. Considering the efficiency of the anionic agent, the optimum amount of anionic agent was regarded to be 20% owf, where the acceptable exhaustion (36.77%) of anthocyanin pigments had been reached.
Figure 12.
Exhaustion of anthocyanin pigments to the cotton fabrics modified with different amounts of treatment agent. Modification conditions: amount of treatment agent, 1–60% owf; material-to-liquor ratio, 1:20; temperature, 30℃ for the anionic agent and 50℃ for tannic acid; time, 60 min. Dyeing conditions: dye concentration, total anthocyanins content 60.79 mg/L, material-to-liquor ratio, 1:50; temperature, 40℃; time, 60 min.

Tannic acid also can provide carboxylic acid groups (–COOH) as anionic sites to cotton fabrics, and is compared with the anionic agent for anthocyanin dyeing on cotton fabrics to demonstrate the efficiency of the proposed anionic agent. In Figure 12, the anthocyanin pigment exhaustion could only achieve 3.28% for untreated cotton fabrics due to the absence of anionic sites. Equilibrium exhaustion was slightly improved with increasing tannic acid and then tended to be constant when the amount of tannic acid increased to 50% (owf), where the exhaustion reached 8.86%. Meanwhile, when the anionic agent in the pretreatment solution was used for cotton fabric modification, anthocyanin pigment exhaustions were significantly increased and reached 44.36% at 50% owf (AA 50%). The exhaustion of anthocyanin pigment values was 10–14 times higher than that of the untreated cotton fabrics, and five to six times higher compared to the treated cotton fabrics with an equal amount of tannic acid. The results can be explained by the following two reasons. The first reason is the dissociation of sodium salts of the anionic agent, which is much stronger than that of the carboxylic acid groups of tannic acid. The second reason is the synthesized anionic agent with a dichloro reactive group, which can be covalently bonded to cotton fabrics by a nucleophilic substitution reaction. Meanwhile, the adsorption of tannic acid on cotton fabrics is physisorption, which would result in the desorption of tannic acid from cotton fabrics to the dyeing solution during the dyeing process. Therefore, in the dyeing process, the number of anionic sites (dyesites) of anionic cotton fabrics available is greater than that of cotton fabrics treated with tannic acid.

Effects of dyeing time and temperature on the adsorption of anthocyanin pigments on cotton fabrics

The effects of dyeing time and temperature of anthocyanin pigments on anionic and control cotton fabrics are shown in Figure 13. In the initial stage, it was found that a higher dyeing temperature resulted in higher dye adsorption for the first 80 min for anionic cotton fabrics dyed with anthocyanin pigments. Meanwhile, for the control cotton fabric dyeing, the time was 20 min. The result is considered due to high mobility of dye ions with high temperature that enhances the interactions between dye ions and active sites at the surface of the cotton fabrics.¹⁶ Moreover, because the –OH groups of cellulose fibers can barely ionize in our acid dyeing bath, the adsorption was mainly driven by the intermolecular weak interaction, such as the hydrogen bond and van der Waals force, between control cotton fabrics and cationic anthocyanin pigments. Such an absorption process could be considered as physisorption, which is fast and achieves equilibrium quickly.⁴¹ Meanwhile, the absorption of cationic anthocyanin pigments onto anionic cotton fabrics was mainly controlled by electrostatic forces between positive and negative electric charges and needed a longer time to reach equilibrium. After the initial stage, the amount of dye adsorption increased at the lower temperatures and finally reached the equilibrium in the cotton fabric dyeing process using anthocyanin. That is, a longer time was needed to reach the equilibrium in the dyeing at lower temperatures. As for adsorption of anthocyanin on anionic cotton fabrics, the equilibrium times were 30, 80 and 140 min at dyeing temperatures of 50℃, 40℃ and 30℃, respectively. Moreover, the rate of adsorption of anthocyanin pigments on control cotton fabrics was faster at the initial stage of dyeing. The equilibrium times were 10, 30 and 50 min at dyeing temperatures of 50℃, 40℃ and 30℃, respectively.
Figure 13.
The effect of dyeing time and dyeing temperature of anthocyanin pigments on anionic (a) and control (b) cotton fabrics at an initial dye concentration; total anthocyanins content 60.79 mg/L, material-to-liquor ratio 1:50. Modification conditions: amount of anionic agent, 20% owf; material-to-liquor ratio, 1:20; temperature, 30℃; time, 60 min.

Color measurement and fastness

In this investigation, we have focused our study on the pigment “anthocyanin red” belonging to anthocyanin classes. The above data can evince that a pH value of around 2.0 might be suitable to obtain “anthocyanin red” with the form of a flavylium cation. However, a highly acidic dyeing bath can damage the fabrics. Firstly, we carried out some preliminary dyeing experiments by soaking cotton fabrics into Morus rubra fruit extracts for a sufficient time (about 2 h). The strength of cotton fabrics was found to decrease by less than 5%. This decrease is acceptable for gaining a better dye uptake and shade of “anthocyanin red”.

Table 7 shows the colorimetric values of dyed cotton fabrics with anthocyanin pigments after being pre-treated with the anionic agent and tannic acid. The color strength (K/S) value, calculated from a diffuse reflectance measurement at 530 nm, was slightly improved from 0.39 to 0.91 for the dyed cotton pretreated with tannic acid (TA 50%). However, at the same concentration of modification agent in pretreatment solution, the color strength (K/S) value of dyed anionic cotton fabrics was sharply increased to 10.61. The results demonstrated that the color strength (K/S) value of cotton fabrics was affected more significantly by the anionic agent than by the tannic acid. The increased color strength (K/S) was reflected in the lower L, b values and higher a, C values in the anionic agent than in tannic acid in this paper. Moreover, higher a* and lower b* values of dyed anionic cotton fabrics (AA 20% and 50%) meant that the modification of cotton fabrics by the anionic agent would lead to a higher degree of brilliance and saturation of the red shade after dyeing with Morus rubra fruit extracts. It is well known that many natural dyes, such as madder and cochineal, are capable of dyeing red shades on cotton fabrics, but it is difficult to obtain a higher degree of brilliance and saturation of red. Madder root is considered the most important natural red source and has been successfully applied to cotton fabrics; however, it only yields brown and yellowish red shades (a* 19.7, b* 14.4 and a* 20.5, b* 8.4)⁴²; cochineal directly applied to cotton fabric yields yellowish red shades (a* 11.72, b* 4.82).⁴³ Thus, the brilliance and saturation of the red shade (a* 46.29, b* –0.40) of cotton fabrics (AA 20%) provided by the proposed method is an important supplement to the red line of natural dyes in the dyeing of cotton fabrics.
Table 7.
CIELab* color parameters (L, a, b, C and h) and color strength (K/S) of dyed cotton fabrics with anthocyanin pigments from Morus rubra* fruits. Modification conditions: amount of anionic agent (AA; 10%, 20% and 50%, owf) or tannic acid (TA; 10%, 20% and 50% owf); material-to-liquor ratio, 1:20; temperature, 30℃; time, 60 min. Dyeing conditions: dye concentration, total anthocyanins content 60.79 mg/L, material-to-liquor ratio, 1:50; temperature, 40℃; time, 60 min

Cotton fabrics L* a* b* C* h* K/S Color obtained

Control 74.19 8.37 −5.64 10.09 281.51 0.39

TA 10% 71.69 13.31 −6.95 15.06 296.06 0.56

TA 20% 69.35 19.09 −5.12 19.76 288.42 0.75

TA 50% 67.23 21.03 −4.38 21.48 289.67 0.91

AA 10% 54.59 32.81 −5.83 33.33 349.93 2.75

AA 20% 46.78 46.29 −0.40 46.29 359.50 6.97

AA 50% 40.96 45.33 1.39 45.35 1.75 10.61

This shade was also evident in terms of a hue angle (h) that shifted from the blue region (281.51° for control cotton) to the red region (359.50° for anionic cotton fabrics cotton (AA 20%)). The color strength (K/S) and brilliant red shade suggest that the anionic agent plays an important role in increasing the uptake of the “anthocyanin red” dyes and “anthocyanin red” dyes have been successfully used for dyeing cotton fabrics.

As for the washing, perspiration, crocking and light fastness of dyed fabrics, the results are shown in Table 8. In Table 8, the fastness to washing, perspiration and crock were mostly in the range of 3–4.5 (fair to very good), except for the control fabrics, whose ratings were 2 and 2.5 (poor). The light fastness of dyed fabrics tended to slightly increase when the color strength (K/S) increased. The good light fastness might be attributed to the presence of a benzene ring in the anthocyanin molecule that remains stable on the surface of cotton fabrics upon exposure to light.⁴⁴ All these fastness results of anthocyanin pigments from Morus rubra* fruits dyeing on cotton fabrics are fairly acceptable in natural dyeing.
Table 8.
The color fastness results of the dyed cotton fabrics. Modification conditions: amount of anionic agent (AA; 10%, 20% and 50%, owf) or tannic acid (TA; 10%, 20% and 50% owf); material-to-liquor ratio, 1:20; temperature, 30℃; time, 60 min. Dyeing conditions: dye concentration, total anthocyanins content 60.79 mg/L, material-to-liquor ratio, 1:50; temperature, 40℃; time, 60 min

Cotton fabrics Washing
Perspiration
Crock
LF

CC Staining CC Staining Dry Wet

Control 2 3.5 3 2.5 4.5 3 2.5

TA 10% 2.5 3 3 3 4.5 3 3

TA 20% 2.5 3 3 3 4.5 3 3

TA 50% 3 3.5 3 3 4.5 3 3.5

AA 10% 3 3 3.5 3 4.5 3.5 3.5

AA 20% 3 3.5 3.5 3 4.5 3.5 4

AA 50% 3.5 3.5 3.5 3 4.5 3.5 4

CC: color change; LF: light fastness.

Conclusions

This paper investigated an eco-friendly natural dyeing process of cotton fabrics with anthocyanin pigments from Morus rubra fruits. A reactive anionic agent was synthesized and applied to the cotton fabric. The majority of anionic agent compound (sodium, 4-(4,6-dichloro-1,3,5-triazinylamino)-benzenesulfonate) and two anthocyanins (cyanidin 3-glucoside and cyanidin 3-rutinoside) from Morus rubra fruits were separated and identified by HPLC/MS spectra. Successful modification of cotton cellulose with few negative effects using an anionic agent was carried out by analyzing the results of FTIR, tensile strength and the whiteness index. The adsorption of anthocyanin pigments from Morus rubra fruits onto anionic cotton fabrics sharply increased compared with control cotton fabrics. The exhaustion value of anthocyanin pigments onto anionic cotton fabrics (AA 50%) was 14 times higher than that of the control cotton fabrics, and five times higher than that of the cotton fabrics treated with an equal amount (50% owf) of tannic acid. The color strength (K/S) of cotton fabrics was also greatly improved from 0.39 to 10.61 for the cotton fabrics pretreated with 50% owf anionic agent. In terms of gray scale assessment, the washing perspiration, crock and light fastness of the dyed samples were all at a lower limit of 3 to 3–4, which is fairly acceptable for potential commercial applications in natural textile dyeing.

Cotton fabrics	L*	a*	b*	C*	h*	ΔE*
Control	95.52	−1.63	0.54	1.72	161.85	–
TA 20%	93.21	0.47	2.04	2.09	77.31	3.46
AA 20%	94.11	−0.44	0.73	0.85	121.14	1.85

Amount of anionic agent/%	Tensile strength/N	Elongation/ mm	Whiteness index
Control	622.4 ± 29.9	29.7 ± 2.3	55.7 ± 0.2
1	599.3 ± 23.7	30.0 ± 3.8	55.6 ± 0.3
5	585.6 ± 16.8	28.5 ± 1.6	56.1 ± 0.1
10	581.3 ± 19.3	25.5 ± 2.2	56.3 ± 0.2
20	578.2 ± 11.2	22.6 ± 3.0	56.6 ± 0.1
30	569.5 ± 21.8	24.5 ± 2.2	56.5 ± 0.3
40	572.0 ± 18.1	25.0 ± 1.3	56.7 ± 0.2
50	570.1 ± 19.3	26.2 ± 2.4	56.5 ± 0.3
60	570.1 ± 9.3	25.8 ± 2.7	56.8 ± 0.3

Peak	RT/min	Peak area/%	λ_max / nm	[M⁺]/m/z	MS² of [M⁺]/m/z	Compound
1	24.508	65.08	516, 331, 279	449	287	cyanidin 3-glucoside
2	25.246	33.03	517, 330, 280	595	449, 287	cyanidin 3-rutinoside

pH values	L*	a*	b*	C*	h*	ΔE*
1.0	74.17	56.08	14.06	57.81	14.08	–
2.0	74.38	55.00	13.12	56.54	13.42	1.45
3.0	76.78	48.13	8.17	48.81	9.63	10.23
4.0	89.10	18.10	0.10	18.10	0.30	43.13
5.0	93.58	4.15	−0.44	4.17	353.99	57.30
6.0	91.71	−1.00	3.12	3.27	107.74	60.71
7.0	89.54	−1.44	8.51	8.64	99.62	59.80

Temperature/℃	pH	k/min⁻¹×10⁴	t_1/2/min	First-order kinetic model	R ²
50	2	0.85	8082.24	ln (C_t/C₀) = −0.000085 t − 0.003451	0.98
3	2.05	3313.80	ln (C_t/C₀) = −0.000205 t − 0.013003	0.99
4	2.99	2263.09	ln (C_t/C₀) = −0.000299 t − 0.015557	0.97
70	2	3.25	2072.97	ln (C_t/C₀) = −0.000325 t − 0.018762	0.99
3	7.71	857.05	ln (C_t/C₀) = −0.000771 t − 0.032354	0.99
4	8.97	738.19	ln (C_t/C₀) = −0.000897 t − 0.030871	0.99
90	2	30.00	258.97	ln (C_t/C₀) = −0.003000 t + 0.083762	0.98
3	42.50	195.29	ln (C_t/C₀) = −0.004250 t + 0.136841	0.99
4	51.90	176.20	ln (C_t/C₀) = −0.005190 t + 0.221351	0.98

pH	E_a/kJ/mol	Arrhenius equation	R ²
2	86.31	ln k = −10,381.59 / T + 22.59	0.94
3	73.65	ln k = −8858.87 / T + 18.84	0.98
4	69.19	ln k = −8322.22 / T + 17.51	0.95

Cotton fabrics	L*	a*	b*	C*	h*	K/S
Control	74.19	8.37	−5.64	10.09	281.51	0.39
TA 10%	71.69	13.31	−6.95	15.06	296.06	0.56
TA 20%	69.35	19.09	−5.12	19.76	288.42	0.75
TA 50%	67.23	21.03	−4.38	21.48	289.67	0.91
AA 10%	54.59	32.81	−5.83	33.33	349.93	2.75
AA 20%	46.78	46.29	−0.40	46.29	359.50	6.97
AA 50%	40.96	45.33	1.39	45.35	1.75	10.61

Cotton fabrics	Washing	Perspiration	Crock	LF
Control	2	3.5	3	2.5	4.5	3	2.5
TA 10%	2.5	3	3	3	4.5	3	3
TA 20%	2.5	3	3	3	4.5	3	3
TA 50%	3	3.5	3	3	4.5	3	3.5
AA 10%	3	3	3.5	3	4.5	3.5	3.5
AA 20%	3	3.5	3.5	3	4.5	3.5	4
AA 50%	3.5	3.5	3.5	3	4.5	3.5	4

Footnotes

Declaration of conflicting interests

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

Funding

The authors received no financial support for the research, authorship and/or publication of this article.

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Cotton fabrics	Washing		Perspiration		Crock		LF
Cotton fabrics	CC	Staining	CC	Staining	Dry	Wet	LF
Control	2	3.5	3	2.5	4.5	3	2.5
TA 10%	2.5	3	3	3	4.5	3	3
TA 20%	2.5	3	3	3	4.5	3	3
TA 50%	3	3.5	3	3	4.5	3	3.5
AA 10%	3	3	3.5	3	4.5	3.5	3.5
AA 20%	3	3.5	3.5	3	4.5	3.5	4
AA 50%	3.5	3.5	3.5	3	4.5	3.5	4