Cationization of cotton with ovalbumin to improve dyeing of modified cotton with cochineal natural dye

Abstract

Natural dyeing of cotton is commonly associated with the use of metallic mordants, which are related to harmful effects on the environment and human health. For this reason, more environmentally sustainable processes should be investigated. In this work, cotton was cationized with ovalbumin by cross-linking in order to improve its dyeing with natural cochineal dye. Different pretreatments with chitosan, synthetic cationizer, and metal mordants were also carried out at different dyeing pH values. The best dyeing results were obtained at pH 3 at 55℃ on fiber pretreated by padding with a liquor concentration of 150 g L⁻¹ ovalbumin and 90 g L⁻¹ cross-linker. This cationization of ovalbumin by cross-linking was confirmed by Fourier transform infrared spectroscopy, energy dispersive X-ray spectroscopy, changes in the isoelectric point of the fabrics, and scanning electron microscopy. The kinetic study showed that the pseudo second-order model best represented the experimental data, indicating that the dyeing can be controlled by the chemisorption process. The equilibrium data were easily adjusted by the Langmuir model, indicating the formation of a dye monolayer in the cationized cotton. The thermodynamic study indicated that dye adsorption was spontaneous and exothermic. The cationized fabric with ovalbumin presented low wash fastness, good light fastness, and increase in tensile strength and crease recovery, with a decrease of hydrophilicity when compared with other treatments. Thus, ovalbumin is a viable and eco-friendly alternative in the dyeing of cotton with cochineal.

Keywords

natural dye cotton cochineal cationization ovalbumin

Natural dyes have been widely used in textile coloration since ancient times. Nevertheless, after the discovery of the first synthetic dye in 1856 and its industrialization in the late nineteenth century, the dye market was dominated by such compounds. The benefits of synthetic dyes include lower cost, higher fastness, color variety, ability to dye synthetic fibers, and availability on a large industrial scale.^1,2 Unfortunately, there are also some significant drawbacks of synthetic dyes, such as the generation of toxic effluents. Mutagenic, carcinogenic, and toxicological properties have been attributed to synthetic dyes, as well as contact dermatitis in humans, albeit with a low incidence.³ Due to these deleterious consequences, along with the growing awareness about cleaner surroundings and healthy lifestyle, there has recently been a worldwide interest in the coloration of textiles with natural dyes, which, in general, are environmentally benign and nontoxic.^1,4

Cochineal is a natural dye extracted from a scale insect, Dactylopius coccus, which lives as a parasite on prickly pear cacti (Opuntia cacti) mainly in South America. Cochineal is considered one of the main natural sources of red dyes. The main coloring compound present in the body of this insect is carminic acid, which consists of an anthraquinone derivative, with anionic character mainly due to a large number of hydroxide groups.^2,5,6 Its chemical structure is shown in Figure 1.

Figure 1.

Chemical structure of carminic acid.

Cotton is the most common textile fiber in the world. When cotton fibers come into contact with water, a negative charge is produced due to ionization of hydroxyl groups in the cellulose molecular chain. The negative charge on the fiber results in repulsion of the negatively charged anionic dyes during the dyeing process, which causes problems in fiber–dye bonding, especially with natural dyes.⁷ The lack of affinity of cotton for natural dyes is improved by the mordanting technique, which consists of treating the fabric with metal salts. These mordants form metallic complexes with dye and cotton, improving dyeing. Many of the metal mordants are toxic, however, which does not sit well with the main reason for natural dyeing, which is to be a means of sustainable coloration with low environmental impact.^4,8

An effective method for improving the affinity of cotton for anionic dye is to perform cationization prior to dyeing. Cationization is the chemical modification of cotton to produce cationic (positively charged) dyeing sites in place of existing hydroxyl sites. This treatment increases the substantivity for anionic dyes due to the Coulombic attraction between the positive charge on the fiber and the negative charge on the anionic dyes. Many cationization treatments make use of quaternary ammonium compounds that can display a range of health effects, from mild skin and respiratory irritation to severe caustic burns on the skin and gastrointestinal lining, nausea, vomiting, convulsions, hypotension, coma, and death.⁹ Therefore, there is a growing interest in the development of cationizers that do not present health problems; this basically consists of the application of biopolymers such as chitosan and proteins.^7,10 Because protein fibers like wool and silk were known to have good dyeability, efforts were made, as early as the mid 1800s, to animalize cotton by depositing protein material onto it to make it more dyeable.⁹ Different proteins of animal origin and their amino acids, including those extracted from milk and wastes of silk, chicken feathers, wool, and cattle hoof and horn, as well as collagen and bovine serum albumin, have been investigated to improve the dyeing of cotton.^7,11–22 Proteins from vegetable sources such as those extracted from soybean were also investigated for the same purpose.^23,24 Ovalbumin is the main protein found in egg white;²⁵ its use in cotton cationization has not yet been reported for improvement in dyeing. For protein fixation in cotton, cross-linkers are generally used such as those based on methylolamide (DMDHEU and DMeDHEU)^12–14,19 and citric acid.¹⁵

In order to make the entire cotton dyeing process more sustainable, the present work aimed to investigate the use of ovalbumin protein to cationize cotton to improve its dyeing with cochineal natural dye. Citric acid and DMDHEU cross-linkers were evaluated for protein fixation in cotton. The best conditions were obtained for ovalbumin and cross-linker concentrations and for the dyeing temperature. Chemical and morphological characterizations were performed to study this cationization process. Kinetic and equilibrium data were obtained as well as thermodynamic parameters of dyeing. The functional characteristics and fastness properties of the cationized cotton with ovalbumin were determined and compared with those obtained by other techniques used for the improvement of natural dyeing, such as cationization with chitosan and the synthetic cationizer CHPTAC, and also mordanting with the metal salts potassium alum and iron(II) sulfate.

Materials and methods

Materials and chemicals

The plain weave cotton fabric (210 g m⁻²) ready for dyeing, i.e. scoured and bleached, used in all the experiments was donated by Paranatex Textile (Apucarana, Brazil). Ovalbumin 85% with molecular weight from 42 to 45 kDa purchased from Maxxi Ovos (Indaiatuba, Brazil), medium molecular weight chitosan (190–310 kDa), and the synthetic cationizer (3-Chloro-2-hydroxypropyl)trimethylammonium chloride (CHPTAC) solution 60 wt. % in H₂O supplied by Sigma-Aldrich, were applied in the cationization tests. For fixation of ovalbumin in cotton, citric acid (C₆H₈O₇) in conjunction with sodium hypophosphite (NaH₂PO₂) was evaluated, and also Fixapret ELFB, a cross-linker based on 1.3-dimethylol-4.5-dihydroxyethyleneurea (DMDHEU) donated by Archroma (Sao Paulo, Brazil), was used in conjunction with magnesium chloride (MgCl₂). Potassium alum (KAl(SO₄)₂·12(H₂O)) and iron(II) sulfate (FeSO₄) were used as mordants. Carminic acid powder 95% (FCC II) purchased from NatColor (Comas, Peru), was used as a dye in the dyeing process, in which sodium hydroxide and acetic acid were used for pH correction of the dyeing solution. The non-ionic detergent Nionlab CELM was used to wash the fabrics after dyeing.

Cotton pretreatments

Cationization with ovalbumin fixed with citric acid: The treatment solution was prepared by dissolving in water 100 g L⁻¹ of ovalbumin, 40 g L⁻¹ of citric acid, and 10 g L⁻¹ of sodium hypophosphite. First, a sample of 4 g of cotton was treated in the infrared dyeing machine using the treatment solution. The treatment was performed at a material-to-liquor ratio of 1:30, for 60 minutes at room temperature. Then the fabric sample was padded to about 80% pickup and dried in a stenter dryer at 100℃ for 10 minutes and subsequently cured also in the stenter at 160℃ for two minutes for cross-linking to occur.

Cationization with ovalbumin fixed with DMDHEU: A fabric sample was padded to about 80% pickup with aqueous solution comprising 100 g L⁻¹ of ovalbumin, 60 g L⁻¹ of DMDHEU, and 30 g L⁻¹ of magnesium chloride. The padded sample was then dried in a stenter dryer at 120℃ for two minutes and subsequently cured also in the stenter at 130℃ for four minutes for cross-linking to occur.

Cationization with chitosan: The padding solution was prepared by dissolving 20 g L⁻¹ of chitosan in an aqueous solution of 1% (v/v) acetic acid with stirring for 60 minutes at 60℃. The fabric sample was padded to about 80% pickup and then dried in the stenter dryer at 100℃ for two minutes and cured at 150℃ for three minutes.

Cationization with CHPTAC: A fabric sample was padded to about 80% pickup with aqueous solution comprising 75 g L⁻¹ of CHPTAC and 35 g L⁻¹ of sodium hydroxide. After padding, the fabric was dried in the stenter at 100℃ for two minutes and curing was performed at 115℃ for four minutes. The sample was then rinsed several times with water and neutralized with an aqueous solution of 2 g L⁻¹ acetic acid. After neutralization, the fabric was again rinsed several times with water and dried at room temperature.

Mordanting with metal salts: Samples of 4 g of cotton were treated in the infrared dyeing machine using aqueous solutions containing 10 g L⁻¹ of mordant. Metal salts of potassium alum and iron (II) sulfate were used as mordants. The treatment was performed at a material-to-liquor ratio of 1:50 for 60 minutes at 80℃. After the treatment, the samples were removed and dried at room temperature.

Dyeing process

Untreated and treated fabric samples were dyed with aqueous solutions of 2 g L⁻¹ carminic acid. In order to evaluate the effect of pH on dyeing, the pH of these solutions was adjusted to the values of 3, 4, and 5 with acetic acid and sodium hydroxide. Samples of 1 g of the fabric were dyed in the infrared dyeing machine (Model TC-2200, Texcontrol Ltda., Brazil) at a material-to-liquor ratio of 1:50, at 70℃ for 60 minutes. After dyeing, samples were submitted to washing with 6 g L⁻¹ non-ionic detergent at 40℃ for 10 minutes at a material-to-liquor ratio of 1:50, rinsed in cold water and dried at room temperature.

Adsorption of dyeing: The concentrations of dye in solutions were determined by standard curves obtained at the different pH values by UV-visible spectrophotometer (Model Cirrus 80, Femto, Sao Paulo). The amount of dye adsorbed per gram of fabric (q) (mg g⁻¹) was determined using the following mass balance:

q = (C_{0} - C_{f}) \frac{V}{W}

(1)

where V is the volume of dye bath (L), W is the mass of fabric (g), and C₀ and C_f are the initial and final concentration of the dye solution (mg L⁻¹).

Color strength: The color strength (K/S) of the dyed fabric samples was determined in a reflectance spectrophotometer (model CM-3600A, Konica Minolta, Japan). The reflectance values at the maximum wavelength (R_λmax) of the dyed fabric were converted to the corresponding K/S value using the Kubelka-Munk equation:

\frac{K}{S} = \frac{(1 - R_{λ max})^{2}}{2 R_{λ max}}

(2)

Optimal conditions

The influences of ovalbumin concentration, DMDHEU concentration, and dyeing temperature, respectively, were investigated. A statistical experimental design was adopted using 2³ complete factorial design, adopting the amount of dye adsorbed in milligrams per gram of fabric (q) as the response variable. All runs of dyeing were conducted in the infrared dyeing machine with a dyebath at pH 3 containing 2 g L⁻¹ of carminic acid for 120 minutes at a material-to-liquor ratio of 1:50. The ratio 2:1 was maintained for the amounts of DMDHEU and magnesium chloride. After dyeing, the samples were submitted to washing and rinsing processes. Table 1 lists the variables and levels used in the factorial design. Statistical analysis was performed using Design-Expert® software version 11.0 (Stat-Ease, USA).

Table 1.

Factorial design for dyeing of cationized cotton with ovalbumin

Variable	Lower level	Higher level	Central point
Ovalbumin concentration (g L⁻¹)	50	150	100
DMDHEU concentration (g L⁻¹)	30	90	60
Dyeing temperature (℃)	55	85	70

Kinetic of the dyeing process

Kinetics were performed at temperatures of 55℃, 70℃, and 85℃, at pH 3 and at the best concentration results as shown in Table 1. Kinetic experiments were conducted by putting fabric samples in contact with the dye solution. Previously weighted fabric samples were immersed into flasks with a known volume of the dye solution. Such systems were maintained in contact for different running times, from five to 180 minutes.

The fabrics were then removed and the concentration of the final solution was analyzed through UV-VIS spectrophotometer, as already discussed. The amount of dye adsorbed per gram of fabric (q_t) (mg g⁻¹) was calculated using equation (1) where C_f stands for the value of the concentration at time t (C_t). Experimental data were adjusted to pseudo first- and pseudo second-order models.

The pseudo first-order model is expressed as:

q_{t} = q_{e} (1 - e^{- k_{1} t})

(3)

where q_e and q_t are the adsorption capacities in equilibrium and in time t (mg g⁻¹), respectively, and k₁ is the rate constant of adsorption of pseudo first-order (min⁻¹).

The equation of the pseudo second-order model is represented as:

q_{t} = \frac{k_{2} {tq}_{e}^{2}}{1 + k_{2} {tq}_{e}}

(4)

where k₂ is the rate constant of adsorption of the pseudo second-order model (g mg⁻¹min⁻¹). This equation indicates that chemisorption is the rate-controlling step.^26,27

Activation energy: Rate constants from the pseudo second-order model at different temperatures were used to estimate the apparent activation energy of the adsorption of carminic acid dye onto cationized cotton with ovalbumin using Arrhenius equation:

\ln k_{2} = \ln A - \frac{E_{a}}{RT}

(5)

where E_a is the activation energy of adsorption (kJ mol⁻¹), k₂ is the rate constant of adsorption of the pseudo second-order model (g mg⁻¹min⁻¹), A (g mg⁻¹min⁻¹) is the Arrhenius factor, T is the absolute scale of temperature (K), and R is the gas constant. E_a can be determined from the slope of the plot of ln k₂ versus 1/T.^28,29

Adsorption isotherms

Equilibrium data was obtained using dye baths with different concentrations, at pH 3 with the best concentration results for ovalbumin and DMDHEU and at temperatures of 55℃, 70℃, and 85℃. The equilibrium data were fitted to Langmuir and Freundlich models.

The Langmuir isotherm is expressed by the equation:

q_{e} = \frac{q_{max} K_{L} C_{e}}{(1 + K_{L} C_{e})}

(6)

where q_max (mg g⁻¹) represents the maximum amount of dye per unit mass of fiber to form a complete monolayer adsorption of the surface, C_e is the equilibrium concentration of dye in the liquid phase (mg L⁻¹), q_e is the amount of dye adsorbed per gram of fiber (mg g⁻¹) at equilibrium state, and K_L is the Langmuir constant related to the affinity of binding sites (L mg⁻¹).³⁰

The empirical equation that describes this Freundlich isotherm is given by:

q_{e} = K_{F} C_{e}^{n}

(7)

where K_F is the adsorption equilibrium constant of Freundlich (L g⁻¹) and n is the heterogeneity factor.³¹

Thermodynamic studies

Adsorption isotherms were also used to obtain the thermodynamic parameters of free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°) of dye adsorption. For charged adsorbates, such as carmine acid dye in the anionic form in solution, these thermodynamic parameters can be calculated from the equilibrium constant (K_c) using the following equations:³²

K_{c} = \frac{C_{ad, e}}{C_{e}}

(8)

Δ G^{\circ} = - {RTlnK}_{c}

(9)

{lnK}_{c} = \frac{Δ S^{\circ}}{R} - \frac{Δ H^{\circ}}{RT}

(10)

K_c is the equilibrium constant, and C_ad,e and C_e are the dye concentration adsorbed at equilibrium (mg L⁻¹) and the concentration of dye in the exhausted bath at equilibrium (mg L⁻¹), respectively. T is the solution temperature (K) and R is the gas constant. Enthalpy (ΔH°) and entropy (ΔS°) of the adsorption are calculated from the slope and intercept of Van’t Hoff plots of ln K_c versus 1/T.³³

Morphological and chemical characterization

Fourier transform infrared spectroscopy (FTIR): DMDHEU, ovalbumin, carminic acid, and fabric samples were chemically characterized using a FTIR spectrometer with an attenuated total reflectance attachment (model Cary 600 Series, Agilent Technologies, USA).

Morphology and EDS analysis of fabrics: The morphology of the fabrics was investigated by scanning electron microscopy (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) (model 6390LV, JEOL Inc., USA). The SEM samples were coated with thin gold film prior to observation.

Isoelectric point of biopolymers and fabrics: For the determination of the pH of the isoelectric point (pH_IEP) of the ovalbumin and chitosan biopolymers, solutions containing 0.5% (w/v) of these biopolymers were prepared in deionized water with the pH being varied by automatic titration using solutions of sodium hydroxide and hydrochloric acid. The zeta potential was determined by the system for analysis of charge characteristics (Stabino, Microtrac GmbH, Germany). For the fabric the pH_IEP analysis was investigated by electrokinetic analyzer for solid surface analysis (SurPASS, Anton Paar GmbH, Austria). Solutions of sodium hydroxide and hydrochloric acid were used to adjust the pH of the solution by titration.

Functional characterization of fabrics

Vertical wicking test: For measuring water transport rate, the vertical wicking test described by standard JIS L1907 was used. Dyed samples were cut into strips (200 mm × 25 mm). One end of a strip was immersed about 3 mm in water. The height of water transported along the strip within 10 minutes was then measured.

Tensile strength test: Tensile strength for weft and warp of fabric samples was measured by the strip method as per ABNT NBR 11912:2016 standard using a CRT-type tensile strength tester (Maquitest, Brazil).

Crease recovery: Dry crease recovery angle of weft and warp of the fabrics was evaluated using the AATCC test method 66–2008. Specimens were prepared in 40 mm × 15 mm swatches and 500 ± 5 g of weight was loaded on the folded specimens for 5 minutes ± 5 seconds. The recorded vertical angle guidelines were aligned and the recovery angles were measured.

Fastness properties

The wash fastness of the dyed samples was evaluated according to the standard ISO 105-C06:2010 (A1S) using a neutral soap. The light fastness test was performed under irradiation by artificial light with Xenon arc fading lamp test for 40 hours according to ISO 105 B02:2013 (method 5). Fading due to washing and light, and staining on a white test cloth, were assessed using the gray scale.

Results and discussion

Wavelength of maximum absorption UV-VIS

The UV-VIS wavelength ranges of the dye solutions at pH 3, 4, and 5 are shown in Figure 2. In the evaluated pH range, the curves showed practically no variation in absorption. In the visible spectrum, peaks at 492–493 nm indicate that the dyeing solution has a reddish hue and agrees with the absorption of carminic acid under acidic conditions.^2,34,35 At pH 3 spectrophotometric readings were performed at the peak at 492 nm and at pH 4 and 5 at 493 nm.

Figure 2.

UV-VIS wavelength ranges of the carminic acid solutions at pH 3, 4, and 5.

Dyeing pretreated samples

Table 2 shows the results of the dyeing at pH 3, 4, and 5 of the cotton samples submitted to different pretreatments

Table 2.

Adsorbed dye (q), color strength (K/S), coefficient of variation (CV) of K/S and color representation obtained in the dyeing of samples with different pretreatments

Sample	pH	K/S	CV of K/S	q (mg g⁻¹)
Untreated	3	0.73	6.69	1.35
	4	0.59	5.54	1.05
	5	0.45	6.69	0.90
Treated with citric acid cross-linker	3	0.49	5.89	2.79
	4	0.35	8.12	2.24
	5	0.33	7.71	0.83
Cationized with ovalbumin fixed with citric acid	3	4.99	1.18	13.15
	4	2.68	1.57	7.54
	5	1.93	2.12	3.63
Treated with DMDHEU cross-linker	3	0.79	5.72	2.25
	4	0.64	3.94	1.95
	5	0.47	8.69	0.15
Cationized with ovalbumin fixed with DMDHEU	3	7.50	1.87	19.34
	4	2.75	2.36	8.24
	5	1.98	3.71	5.54
Cationized with chitosan	3	5.08	1.47	29.83
	4	3.98	1.55	12.48
	5	3.19	2.33	8.5
Cationized with CHPTAC	3	13.39	1.53	44.75
	4	9.44	0.97	26.08
	5	7.04	0.94	24.39
Mordanted with potassium alum	3	0.79	3.27	7.46
	4	0.55	5.11	2.42
	5	0.48	5.13	1.68
Mordanted with iron(II) sulfate	3	2.33	4.63	7.83
	4	1.22	2.43	2.05
	5	1.08	4.04	1.86

Comparing the dyeing results of the samples in which ovalbumin was fixed with the citric acid and DMDHEU cross-linkers, it was observed that DMDHEU was more effective because it resulted in higher color strength in the fabric and higher dye adsorption. Thus, DMDHEU was used as cross-linker in this work.

In addition to the tests performed at the cationizer concentrations recommended in the literature (Table 2), for comparison purposes, these tests were also performed at the same concentration of 20 g L⁻¹ of cationizer agent. The results of these tests are presented in Table 3.

Table 3.

Adsorbed dye (q), color strength (K/S), coefficient of variation (CV) of K/S and color representation obtained in the dyeing of samples treated with the concentration of 20 g L⁻¹ of cationizer

Sample	pH	K/S	CV of K/S	q (mg g⁻¹)
Cationized with ovalbumin fixed with DMDHEU	3	1.45	2.88	8.39
	4	0.75	3.93	3.63
	5	0.68	3.04	1.95
Cationized with chitosan	3	5.08	1.47	29.83
	4	3.98	1.55	12.48
	5	3.19	2.33	8.5
Cationized with CHPTAC	3	6.72	0.51	18.18
	4	5.38	1.03	12.85
	5	4.92	1.08	9.31

As for the influence of the pretreatments, when comparing cationizer agents at the same concentration (Table 3), ovalbumin resulted in lower color strength, lower than pretreatments with CHPTAC and chitosan. But when comparing cationizers at the concentration recommended in the literature (Table 2), these were classified according to the increase of the color strength obtained in the dyeing in more acidic medium (pH = 3) in the following order: CHPTAC > ovalbumin > chitosan > iron(II) sulfate > potassium alum.

The presence of the cross-linked ovalbumin in the fabric introduces amine groups onto the surface of the cotton fiber, derived from the amino acids of that protein. The DMDHEU cross-linker has two N-methylol groups which can bind to different cellulose molecules causing their cross-linking. These N-methylol groups may also cause cross-linking between two protein molecules. In the cotton cationization process with ovalbumin, one of the N-methylol groups of the DMDHEU cross-linker reacts with the hydroxyl groups of the cellulose (scheme (a) of Figure 3). The other N-methylol group can chemically bind with carboxyl groups by the esterification reaction or with the amine groups of ovalbumin, as shown in schemes (b) and (c) of Figure 3, respectively.

Figure 3.

Reaction between cotton, DMDHEU, and ovalbumin: (a) between DMDHEU and cellulose, (b) between the N-methylol group of DMDHEU and the carboxyl group of ovalbumin, and (c) between the N-methylol group of DMDHEU and the amine group of ovalbumin.

As can be seen below, the pH_IEP of untreated cotton increased from 2.9 to 3.46 after cross-linking with DMDHEU, possibly due to the presence of N-methylol groups in the cross-linked cotton. This increase in pH_IEP indicates a cationization effect on DMDHEU cross-linked cotton, making it more favorable for the occurrence of ionic attraction with the anionic dye. This explains the increase in adsorption of the cross-linked cotton when compared with the untreated cotton. The pH_IEP of untreated cotton increased from 2.9 to 4.8 after cationization with ovalbumin. Thus, cationized cotton has a positive charge in dyebaths with pH below 4.8 since the amine groups (–NH₂) of the protein present in the fabric become protonated (–NH₃⁺). Carminic acid has more than one ionic dissociation step, the first occurring in its carboxylic group (–COOH), where pK_a1 = 2.81.³⁶ Therefore, in a pH solution higher than 2.81, carminic acid is predominantly in its ionized form with the presence of carboxylate groups (–COO^-). In view of this, the pH dyeing interval between 2.81 and 4.8 was the most favorable for the occurrence of ionic attractions between the dye and the fiber. The best dyeing results were obtained within this interval and occurred at the lowest evaluated pH value (pH = 3). This more acidic medium increased the amount of the –NH₃⁺ groups present in cotton cross-linked with ovalbumin, which led to greater ionic attraction with the –COO^- groups in the dye.

A scheme for the ionic interactions between the molecules of the carmine acid dye with the cationized cotton by ovalbumin and the DMDHEU cross-linker in an acid dyeing medium is proposed in Figure 4.

Figure 4.

Scheme of interaction between cotton, DMDHEU, ovalbumin, and carminic acid.

The same explanation can be given for the cationizations with chitosan and CHPTAC, since they increased the pH_IEP values of the treated fabrics, but at different values. However, treatment with chitosan resulted in less color strength (K/S) than CHPTAC treatment, although it presented a higher value of the isoelectric point (pH_IEP = 6.96). As chitosan becomes bound to cellulose through hydrogen bonds and van der Waals forces,³⁷ the process of soaping and washing after dyeing can cause the elimination of the chitosan in which the dye is ionically bound. In cationization with CHPTAC, the formation of an ether-like bond between CHPTAC and cotton occurs.¹⁰ This cationized fabric presented the highest K/S, probably due to its pH_IEP of 5.58 being greater than that of the cationized fabric with ovalbumin.

The untreated cotton presented pH_IEP of 2.9 and it did not present favorable conditions for the occurrence of ionic attractions with carminic acid, rather they tended to repel each other, mainly with the increase of dyeing pH. The small amount of dye adsorbed in the dyeing of untreated fabric can be attributed to the formation of non-electrostatic interactions as hydrogen bonds between the hydroxyl groups present in the dye and cellulose and also by the attraction of Van der Waals forces.^6,38

Samples submitted to the mordanting processes showed lower K/S than those submitted to cationization processes. Probably the metallic complexes formed between the metal ion, the fiber, and the dye by mordanting were weaker at capturing and retaining the cochineal dye than the ionic interactions that occurred in the cationized fabrics. As can be seen in Table 2, pre-mordanting with iron(II) sulfate caused a dimming of the coloring, which changed from carmine to a grayish hue, and the potassium alum caused a color whitening to a rose-colored hue. Indeed, iron sulfate is reported as a dulling mordant and the potassium alum is a brightening mordant because it produces a paler and brighter color.³⁹ Each mordant gave different colors on the fabric samples because the individual metal ions display unique complex formation. Briefly, metals have relatively low energy levels. The incorporation of the metal atoms into the delocalized electron system of the dye results in a lowering of the overall energy which affects the absorbance of the dye-mordant complex.⁴⁰

Optimal conditions

Table 4 shows the results of the experimental design. All samples presented a carmine coloration. The highest color strength was given in test 4, which corresponds to the use of 150 g L⁻¹ of ovalbumin, 90 g L⁻¹ of DMDHEU in the padding solution, and dyeing at 55℃.

Table 4.

Results of amount of dyestuff adsorbed (q), color strength (K/S), and color representation of the 2³ factorial design for the dyeing with carminic acid of cationized cotton with ovalbumin

Test	Ovalbumin concentration (g L⁻¹)	DMDHEU concentration (g L⁻¹)	Temperature (℃)	q (mg g⁻¹)	K/S
1	50	30	55	11.19	4.30
2	150	30	55	22.33	6.72
3	50	90	55	14.29	4.53
4	150	90	55	26.03	8.15
5	50	30	85	8.19	3.33
6	150	30	85	19.14	6.05
7	50	90	85	9.49	3.37
8	150	90	85	23.13	7.32
9	100	60	70	16.14	4.88
10	100	60	70	16.54	4.75
11	100	60	70	17.04	4.97

The effect and the contribution percentage on the response variable (q) is shown in Table 5.

Table 5.

Effect list of the factorial design for the dyeing of cationized cotton with ovalbumin

Term	Standardized effect	Contribution (%)
A - ovalbumin concentration (g L⁻¹)	11.87	86.12
B - DMDHEU concentration (g L⁻¹)	3.02	5.59
C - temperature (℃)	−3.47	7.37
Interaction AB	0.82	0.42
Interaction AC	0.42	0.11
Interaction BC	−0.37	0.09
Interaction ABC	0.52	0.17
Curvature	−0.16	0.02
Pure error	–	0.12

The percentage contribution of each variable of the experimental design showed that the ovalbumin concentration has the greatest contribution in increasing the response variable (q), followed by dyeing temperature and DMDHEU concentration. Furthermore, it can be noted that the effect of dyeing temperature is inverse, meaning that an increase in this variable decreased the value of response variable (q). This was also in agreement with the experimental data.

Analysis of variance (ANOVA) was applied to evaluate the significance of each variable. The results of ANOVA for factorial design for the dyeing with carminic acid of cationized cotton with ovalbumin are shown in Table 6. P-values lower than 0.05 indicate that the model and the terms are statistically significant. P-value <0.05 presented by the model fit indicates that this proposed model is significant. The values of the terms A, B, and C also showed P-value <0.05, indicating that these terms produce significant effects in the model.

Table 6.

ANOVA results for 2³ factorial design

Source	Sum of squares	Degrees of Freedom	Mean square	F-value	P-value
Model	324.04	3	108.01	219.12	<0.0001
A - Ovalbumin concentration	281.65	1	281.65	571.36	<0.0001
B - DMDHEU concentration	18.28	1	18.28	37.08	0.0009
C - Temperature	24.12	1	24.12	48.93	0.0004
Curvature	0.0518	1	0.0518	0.1051	0.7568
Lack of fit	2.55	4	0.6379	3.14	0.2558
Pure error	0.4061	2	0.2031	–	–
Total	327.05	10	–	–	–

Note: F value is a value on the F distribution that can be used to determine whether the test is statistically significant.

Table 7 shows the statistical model equation that can be used to make predictions for adsorption (q) in the analyzed region. The adjusted and predicted R² values are in agreement, confirming the suitability of the model.

Table 7.

Equation of the statistical model obtained by factorial design

Model equation	R² adjusted	R² predicted
q = 9.93905 + 0.118669A + 0.050382B – 0.115754C	0.9864	0.9660

To understand the interaction of the medium components and estimate the efficiency of cationized cotton dyeing parameters over independent variables, three-dimensional (3D) response surface curves were plotted by statistically significant model. The 3D surface graphs and contour plots between the factors are presented in Figures 5–7.

Figure 5.

Effect of ovalbumin and DMDHEU concentration on dye adsorption.

Figure 6.

Effect of ovalbumin concentration and temperature on dye adsorption.

Figure 7.

Effect of DMDHEU concentration and temperature on dye adsorption.

The increased concentration of ovalbumin in the padding solution is presumed to be related to the higher protein content on the fabric surface. Thus, more amine groups were incorporated onto the cotton fiber surface and served as dye adsorption sites, increasing the K/S values of the dyed fabrics.

High dyeing temperatures can promote the weakening of the attractive forces between the dye and the fiber, that is, there is a reduction in the affinity of the dye for the cotton cationized with the temperature increase.^2,29 This contribution of temperature decrease in adsorption indicates that the dyeing process is exothermic. This result is consistent with other studies using cochineal dye in dyeing cotton,⁴¹ wool,^2,28,42 and polyamide.⁶

The direct influence of the concentration of the DMDHEU cross-linker on the padding solution in the increase of the dye adsorption may be related to the increase in the availability of N-methylol groups which during the cross-linking bind with both the cellulose and ovalbumin. Similar concentrations of the DMDHEU and DMeDHEU cross-linking agents were enough for the fixation of the sericin protein in cotton in the works of Kongdee, Bechtold and Teufel¹² and Kongdee and Chinthawan,¹³ respectively.

Kinetic of the dyeing process

Figure 8 presents the kinetic results and nonlinear adjustments to the pseudo first-order and pseudo second-order models for temperatures of 55℃, 70℃, and 85℃. It was seen that the equilibrium time was 50–60 minutes in all evaluated temperatures. Adsorption was very fast in the early stages of contact time and gradually decreased with time until it remained constant.

Figure 8.

Nonlinear fit for kinetic models at different temperatures: (a) pseudo first order and (b) pseudo second order.

Concerning the model adjustments, it was observed that the pseudo second-order model best described the experimental data. As seen in Table 8, the percentage differences between the amount of dye retained experimentally and the estimated amount of dye retained in the equilibrium (q_e,exp/q_e,cal) were lower for the pseudo second-order model at all temperatures. In addition, the highest values of the correlation factor (R²) and lower values for the Akaike information criterion (AIC) occurred in the pseudo second-order model.

Table 8.

Comparison of kinetic models: pseudo first and pseudo second order

Model	Parameters	Temperature (℃)
	Parameters	55	70	85
	q_e,exp (mg g⁻¹)	26.10	24.30	22.53
Pseudo first order	q_e,cal (mg g⁻¹)	25.51 ± 0.3207	23.96 ± 0.1947	22.24 ± 0.1392
	% q_e,exp/q_e,cal	2.308	1.427	1.295
	k₁ (min⁻¹)	0.1837 ± 1.657.10⁻²	0.2705 ± 2.182.10⁻²	0.3760 ± 3.116.10⁻²
	R²	0.9488	0.9781	0.9844
	AIC	17.23	−3.484	−17.51
Pseudo second order	q_e,cal (mg g⁻¹)	26.62 ± 0.1754	24.57 ± 0.1046	22.66 ± 7.013.10^-2
	% q_e,exp/q_e,cal	1.961	1.207	0.569
	k₂ (g mg⁻¹min⁻¹)	1.288.10⁻²± 9.083.10⁻⁵	2.753.10⁻²± 1.899.10⁻³	4.903.10⁻²± 3.541.10⁻⁴
	R²	0.9903	0.9952	0.9973
	AIC	−19.42	−39.39	−55.83

These data suggested that the mechanism of adsorption of the pseudo second-order model was predominant and that the overall rate of adsorption of cochineal dye was probably controlled by the chemisorption process due to the electrostatic attraction between the dye and the fiber. This is consistent with other studies that described the adsorption of the cochineal dye in compounds having amine groups, such as wool^28,43 and polyamide,⁶ and in the marine sponge Hippospongia communis that has protein composition.⁴⁴

Activation energy: The result obtained for dyeing of cationized cotton with ovalbumin with cochineal at different temperatures is 43,58 kJ mol⁻¹. The physisorption processes usually have energies in the range of 5–40 kJ mol⁻¹, while higher activation energies (40–800 kJ mol⁻¹) suggest chemisorptions.²⁸ Therefore, it can be concluded that both the physisorption and the chemisorption processes occurred partially.

Adsorption isotherms

The nonlinear fits of isotherms for the Langmuir and Freundlich models for dyeing at temperatures of 55℃, 70℃, and 85℃ are shown in Figure 9. It was found that the best fit was obtained for the Langmuir model.

Figure 9.

Nonlinear fit for isotherm models at different temperatures: (a) Langmuir and (b) Freundlich.

Parameters of model adjustments are presented in Table 9. As shown in Figure 9, the Langmuir model provided the highest R² and lowest AIC values and predicted the maximum amount of dye adsorbed (q_max) close to the experimental data (q_max_,_exp) at 55℃, 70℃, and 85℃. Moreover, higher values of adsorbed dye are seen with increasing temperatures, which is a consequence of the exothermic dyeing process.

Table 9.

Parameters of isotherms for Langmuir and Freundlich models

Isotherm	Parameters	Temperature (℃)
	Parameters	55	70	85
	q_max,exp (mg g⁻¹)	36.85	34.23	31.19
Langmuir	q_max (mg g⁻¹)	40.15 ± 0.2228	37.24 ± 0.2176	34.08 ± 0.1752
	K_L (L mg⁻¹)	1.215.10⁻³± 3.931.10⁻⁵	1.145.10⁻³± 3.807.10⁻⁵	1.063.10⁻³± 3.007.10⁻⁵
	R²	0.9989	0.9988	0.9991
	AIC	−20.17	−21.41	−27.89
Freundlich	K_F	5.873 ± 1.201	5.020 ± 0.9644	4.210 ± 0.8242
	N	0.203 ± 2.345.10⁻²	0.2115 ± 2.199.10⁻²	0.2203 ± 2.237.10⁻²
	R²	0.9698	0.9745	0.9747
	AIC	22.43	18.21	15.76

It is known that the Langmuir model considers the formation of a monolayer of the adsorbate on the surface of the adsorbent and represents the equilibrium data of dyeing involving ionic interactions between ionic dyes in ionic fibers with opposite charge.⁴⁵ In this case, the monolayer was due to ionic interactions between the anionic carboxylate groups (–COO^-) of the carminic acid dye and the protonated amine groups (–NH₃⁺) present in the cationized cotton with ovalbumin.

The adjustment of the Langmuir model in the adsorption of cochineal is also reported in other adsorbents of protein origin such as wool and marine sponge.^44,46

Thermodynamic studies

The results of the thermodynamic parameters, ΔG°, ΔH°, and ΔS° of cochineal dye adsorption are shown in Table 10.

Table 10.

Thermodynamic parameters for the adsorption of cochineal dye in cationized cotton with ovalbumin

Temperature (℃)	K_c	ΔG°(J mol⁻¹)	ΔH°(J mol⁻¹)	ΔS°(J mol⁻¹ K⁻¹)	R²
55	9.892	−6252
70	8.886	−6232	−6840	−1.785	0.9997
85	8.018	−6198

The negative values of ΔG° and ΔH° indicated that dye adsorption was spontaneous and exothermic. The highest value of K_c was obtained at 55℃, indicating the most favorable equilibrium among the temperatures studied herein. Such results were in agreement with isotherm experimental data and the factorial design. When the temperature increases, the negative values of ΔG° decrease, indicating less driving force and hence resulting in lower adsorption capacity at higher temperatures. ΔS° also presented a negative value, suggesting a decrease of randomness, that is, the dye became more restricted in cotton fiber treated with ovalbumin. Negative values for thermodynamic parameters were reported in cochineal dyeing of wool, silk, and polyamide.^2,5,6

Morphological and chemical characterization

The FTIR spectra for the reagents and fabrics are shown in Figure 10. In Figure 10(d) the spectrum of untreated cotton shows characteristic peaks of cellulose, which is the main component of cotton fiber.⁴⁷ The spectrum of DMDHEU-cross-linked cotton (Figure 10(e)) is like that of untreated cotton. The 1699 cm⁻¹ band is observed in the cross-linked cotton corresponding to the peak at 1693 cm⁻¹ referring to the C=O stretching of the DMDHEU spectrum (Figure 10(a)). The additional peak at 2899 cm⁻¹ is associated with the stretching of CH₂ present in the cross-linker. This spectrum is in accordance with that reported for DMDHEU cross-linked cotton.^12,48

Figure 10.

FTIR spectra of (a) DMDHEU, (b) ovalbumin, (c) carminic acid, (d) untreated cotton, (e) DMDHEU-cross-linked cotton, (f) cotton cross-linked with ovalbumin, and (g) cationized and dyed cotton.

In the ovalbumin spectrum (Figure 10(b)) the bands in between 3000 and 3400 cm⁻¹ correspond to the amide A of the protein. The peak at 1644 cm⁻¹ is attributed mainly to the C=O stretching vibration of amide I. The minor peak at 1536 cm⁻¹ is the characteristic vibration of amide II, belonging to the combination of N–H bending and C–N stretching vibrations.⁴⁹ The carminic acid spectrum (Figure 10(c)) contains C–C stretching, C–OH bending and C–H folding vibrations at 1568 cm⁻¹; C–C stretching, CH₃ bending, and C–H bending at 1427 cm⁻¹; C–OH bending, C–H bending, and C–C stretching at 1222 cm⁻¹.^50,51

The spectrum of cotton cross-linked with ovalbumin (Figure 10(f)) shows a small peak at 1741 cm⁻¹ which may correspond to the C=O stretching of the ester group (–CH₂O–OC–) resulting from the esterification reaction between the N-methylol groups present in the cross-linked cotton and the carboxyl groups of ovalbumin, confirming the reaction of the scheme of Figure 3(b). In this spectrum is observed a deviation to 1644 cm⁻¹ of the 1699 cm⁻¹ band present in the spectrum of DMDHEU cross-linked fabric, corresponding to the C=O stretching of DMDHEU. This deviation band can be caused by reactions between the N-methylol groups of DMDHEU and amine groups in the main chain of the protein, represented in the scheme of Figure 3(c). Moreover, with the presence of ovalbumin on the surface of the cotton there appeared an absorption band at 1546 cm⁻¹ corresponding to the N–H bending vibration characteristic of the amide II of the protein. Spectra and similar discussions are presented in other papers for cotton cross-linked with DMDHEU and protein.^12,13,48

The spectra of the cotton dyed with carminic acid (Figure 10(g)) shows most of the characteristic peaks of the cotton cross-linked with ovalbumin, however, higher peaks are noted at 1564, 1548, and 1531 cm⁻¹. In addition, there was an increase in absorbance in the range of 1500 to 1700 cm⁻¹ corresponding to the greater presence of the C=O groups in the dyed fabric from the anthraquinone structure of the dye.

The surface morphology of the untreated and treated cotton fabrics is shown in Figure 11(a)–(c). An apparent difference between untreated and treated fabrics was observed. Figure 11(b) shows the presence of cross-linker deposits, forming an irregular film on the DMDHEU cross-linked cotton fibers, whereas none of these changes were observed in the untreated fibers (Figure 11(a)). In the cotton fibers cross-linked with ovalbumin (Figure 11(c)) the surface of the fiber presents an irregular film with granular deposits that can be attributed to the presence of the protein on the fiber, confirming the fixation of ovalbumin on cotton.

Figure 11.

SEM images of cotton fabrics: (a) untreated cotton, (b) DMDHEU-cross-linked cotton, and (c) cotton cross-linked with ovalbumin.

The EDS results are shown in Table 11. The cross-linked cotton presented 5.9% nitrogen, which was derived from the DMDHEU used in the cross-linking, and did not occur with untreated cotton. The cotton cross-linked with ovalbumin had a higher nitrogen content (8.56%), confirming the presence of chemical groups containing nitrogen on the cotton surface after the ovalbumin cross-linking process. The nitrogen element comes from amine groups of the ovalbumin protein.

Table 11.

EDS elemental analysis of ovalbumin and cotton fabrics

Sample	Weight percent (%)
Sample	C	N	O
Ovalbumin	68.09	15.83	12.90
Untreated cotton	78.26	–	21.74
DMDHEU-cross-linked cotton	68.85	5.90	21.79
Cotton cross-linked with ovalbumin	68.62	8.56	19.97

Figure 12 shows the plot of the zeta potential versus pH for the biopolymers and fabrics analyzed, the isoelectric point pH (pH_IEP) results are presented in Table 12. The pH_IEP of ovalbumin is located in the acidic region because this protein has approximately twice as many acidic side groups as glutamic acid and aspartic acid when compared with basic groups such as lysine and arginine.^25,52 In turn, chitosan has a pH_IEP in the neutral region. Untreated cotton had a low isoelectric point (pH_IEP = 2.9) due to the presence of hydroxyl and carboxyl groups, which rose to pH_IEP of 3.46 due to the presence of N-methylol groups in the DMDHEU-cross-linked cotton.

Figure 12.

Graph of determination of the isoelectric point with values of the zeta potential versus pH for the biopolymers and fabrics.

Table 12.

pH of the isoelectric point of biopolymers and fabrics

Sample	pH_IEP
Ovalbumin	4.89
Chitosan	7.84
Untreated cotton	2.90
DMDHEU-cross-linked cotton	3.46
Cationized cotton with ovalbumin	4.80
Cationized cotton with chitosan	6.56
Cationized cotton with CHPTAC	5.58

The cross-linking of the cotton in the presence of ovalbumin caused the pH_IEP to be raised to 4.8 because of the presence of the amine groups from ovalbumin. Thus, it was found that ovalbumin cationized the cotton because it increased the zeta potential values, making its surface positively charged at a low pH. Already the cationized cotton samples with chitosan and CHPTAC presented even higher pH_IEP.

Functional characterization of fabrics

As shown in Table 13, there were no appreciable differences in wicking height and tensile strength for the samples treated with CHPTAC, potassium alum, or iron(II) sulfate when compared with untreated fabric, and only a small improvement was observed for the angle of recovery to crease. The cationized cotton with chitosan showed a 30% decrease in wicking height when compared with untreated fabric, one of the lowest values of tensile strength, and greatest crease recovery.

Table 13.

Wicking height, tensile strength, and crease recovery angle of fabrics

Sample	Wicking height (cm)	Force at break (kgf)		Crease recovery angle (degree)
Sample	Wicking height (cm)	Weft	Warp	Weft	Warp
Untreated and undyed	8.5	37.34 ± 1.12	78.86 ± 2.28	55.33 ± 3.68	37.67 ± 3.09
Untreated and dyed	8.5	36.93 ± 1.80	77.34 ± 2.11	56.33 ± 3.86	37.01 ± 3.56
Treated with cross-linker	6.9	32.65 ± 1.17	71.07 ± 2.94	74.98 ± 4.32	51.67 ± 3.09
Cationized with ovalbumin and dyed	7.5	40.88 ± 1.95	78.09 ± 3.67	71.67 ± 2.49	46.33 ± 4.78
Cationized with chitosan and dyed	6.0	35.95 ± 1.97	61.01 ± 3.13	71.02 ± 1.41	48.33 ± 2.62
Cationized with CHPTAC and dyed	8.5	37.11 ± 1.99	75.26 ± 3.04	63.01 ± 3.56	44.33 ± 3.30
Mordanted with potassium alum and dyed	8.5	36.03 ± 2.54	77.48 ± 2.18	58.33 ± 3.68	39.03 ± 3.27
Mordanted with iron(II) sulfate and dyed	8.5	37.71 ± 2.16	74.60 ± 3.03	58.99 ± 3.74	41.33 ± 3.86

The cross-linking of the cotton with DMDHEU caused the greatest crease recovery and a 19% decrease in wicking height as compared with the untreated fabric, that is, due to the cross-linking reaction which forms cross-links between adjacent cellulose chains which increase the crease resistance and decrease the degree of accessibility to water.^48,53 The cross-linking caused one of the largest decreases in tensile strength since cross-linking reduces the slippage of fibers and yarns, resulting in a less efficient redistribution of stress concentrations throughout the sample.⁵⁴

With the presence of ovalbumin in the reticulated fabric, this decrease passed to 12%. This indicates that the presence of hydrophilic groups of the protein contributes to improved wicking height. There was also a decrease in the crease recovery; possibly this occurred because some of the N-methylol groups of the DMDHEU reacted with the protein and did not participate in cross-linking with the cellulose chains. On the other hand, the tensile strength increased; probably the presence of ovalbumin forms a more ductile film on the fibers allowing greater mobility during tensile stress. This decrease in crease recovery and increase in tensile strength occurred in other cases after the use of protein material.^12,55,56

Fastness properties

All the dyed fabrics showed low wash fastness for color change. However, except for cationized cotton with chitosan, the other treatments increased the color wash fastness when compared with untreated fabric (Table 14). The cationized cotton with ovalbumin has medium wash fastness (2–3). The best result was obtained by the cationized cotton with CHPTAC; possibly the presence of cationic groups in the cotton improved the binding of the carminic acid dye to the fiber increasing the wash fastness.

Table 14.

Color fastness of the dyed cotton fabrics

Sample	Wash fastness			Light fastness
Sample	Color change	Staining on cotton	Staining on wool	Light fastness
Untreated	2	4–5	4	1–2
Cationized with ovalbumin	2–3	5	3	4
Cationized with chitosan	2	4–5	3–4	2
Cationized with CHPTAC	3–4	4–5	3–4	3
Mordanted with potassium alum	2–3	5	4	1–2
Mordanted with iron(II) sulfate	2–3	4–5	4–5	4

In general, it was observed that a white test cloth composed of wool showed more staining than the cotton test cloth. Wool is a protein fiber having amine groups that can ionize and serve as binding sites for anionic dye molecules. Cotton fiber, on the other hand, exhibits electrostatic repulsion of molecules of negatively charged dyes.⁵⁷

Cotton mordanting with iron(II) sulfate and cationization with ovalbumin resulted in good light fastness, higher than the untreated fabric and other treatments.

Conclusions

Cotton cationized with ovalbumin can be easily dyed with cochineal, as is normally the case in protein fibers such as silk and wool. DMDHEU cross-linker was more effective than citric acid to fix ovalbumin on cotton. In the more acidified dyeing medium evaluated (pH = 3), cationization with ovalbumin was highlighted by greater dye retention than the use of chitosan or metallic mordants in concentrations recommended in the literature. This dye retention was below that achieved with synthetic cationization, however. The best dyeing results were obtained in this more acidified dyeing medium, as it increased the amount of the protonated amine groups present in cotton cationized with ovalbumin, which led to greater ionic attraction with the carboxylate groups in the dye.

The best dyeing results occurred at a temperature of 55℃, concentration of 150 g L⁻¹ of ovalbumin, and 90 g L⁻¹ of cross-linker. The ovalbumin concentration was the variable that most contributed to the adsorption of dye onto cationized cotton, followed by temperature and DMDHEU concentration. This cross-linking of ovalbumin in cotton by the DMDHEU cross-linker was confirmed by the FTIR and EDS analyses, as well as morphological alterations detected by SEM images. Ovalbumin introduced amine groups into the cotton, causing its cationization, as it raised the pH of the isoelectric point from 2.9 to 4.8.

The kinetic data on the dyeing were better represented by the pseudo second-order model, indicating that the dyeing can be controlled by the chemisorption process. The Langmuir model represented the equilibrium data better, indicating the formation of a dye monolayer on the cationized fiber. Negative values for enthalpy, Gibbs free energy, and entropy changes allow the conclusion that the dyeing process is exothermic and spontaneous, with a decrease in the randomness indicating that the dye became more restricted in cotton cationized with ovalbumin. The cationized cotton with ovalbumin presented low wash fastness but good light fastness, as well as increased tensile strength and crease recovery, and a decrease in hydrophilicity when compared with other treatments.

The use of ovalbumin as a natural cationizer for cotton has shown promise as it improved the dyeing results and the physical properties of the fabric samples when compared with untreated fabric. Cationization with this biopolymer can clearly replace the use of metal mordants and even synthetic cationizers, both of which cause environmental damage and human health problems which does not sit well with the main reason for natural dyeing, which is to be a means of sustainable coloration and with low health risks. In addition, ovalbumin offers a number of advantages over chitosan, which is the most widely used biopolymer for the purposes of natural cationization of cotton.

Footnotes

Authors' Note

Fernando Giacomini is now affiliated to Textile Technology Coordination, Federal Institute of Santa Catarina, Brazil.

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.

ORCID iD

Fernando Giacomini

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