Nanocellulose-Based Sustainable Dyeing of Cotton Textiles with Minimized Water Pollution.

This research aims at minimizing environmental pollution by effluents discharged from current textile dyeing processes. The reduction of pollution is approached with a nanofibrillated cellulose (NFC) dyeing method. In the commonly used exhaust reactive dye bath cotton dyeing process, water effluents are contaminated with unreacted dyes and dyeing formulation auxiliaries amid the consumption of 20 weight units of water per weight unit of colored textile products. It was recently demonstrated that using reactive dye-colored NFC hydrogels-an aqueous dispersion of the NFC pigment-a sustainable dye carrier-results in 6-fold reduction in consumption of water and auxiliaries. Here, we report further developments of this technology. Cotton fabrics and NFC hydrogels inherit a fraction of soluble polysugars that react and conjugate with the reactive dyes. These soluble dye-conjugated polysugars are released into the wastewater, thus resulting in water pollution and also in reduced efficiency of the dyeing process. We demonstrate here that post-treatment of NFC-colored cotton textiles with polycarboxylic acid secures permanent chemical grafting of the soluble dye-labeled polysugars and forms chemical cross-links with the NFC fibers on the cotton fabric via the esterification reaction. This combination leads to the improvement of dye fixation by 30% and reduces the dye discharge in the washing stage by 60%. This enhancement is approached without compromising the stiffness and breathability of the fabrics. The advanced textile method is tested for a series of reactive dyes covering the entire visual spectrum range.

Introduction

The exponential growth of the human population has created an increasing demand for the consumption of clothing, thereby amplifying the needs for textile production and textile dyeing. The current textile dyeing industry consumes water massively and releases a copious amount of untreatable hazardous effluents such as salts, hydroxides, and unreacted dyes to the environment. Unreacted azo dyes contained in the textile wastewater may metabolize into mutagenic products.[1,2] Generating a significant level of pollutants, the current textile dyeing industry is responsible for 1/5 of the global industrial water pollution.[3,4]

Cellulosic fabrics are dyed using reactive dyes—water-soluble synthetic chromophores with chlorotriazine and ethylsulfonyl functional groups in their chemical structures for covalent attachments to the cellulose surface. These dyes react with hydroxyl groups of cellulose, forming covalent bonds in alkaline medium at a high concentration of salt to maintain high ionic strength and pH 10. In conventional dyeing of cotton, only 80–85% of the initially loaded dyes are covalently bound to the cotton fabric. Hence, unreacted dyes and salts are discharged into the wastewater and should be removed before released to the environment as a safe practice. The government environmental regulations are concerned with textile dyeing, and the industry is responsible for the removal of residual dyes and other pollutants from industrial effluents. Environmental pollution associated with the textile industry is becoming a critical global issue, and novel technologies need to be established for the mitigation of the problem.

Recently, we introduced a new environmentally sound textile dyeing technology that explores a successful application of nanocellulose hydrogels of nanofibrillated cellulose (NFC) as an efficient carrier for textile dyes.[5,6] NFC is a nontoxic nanomaterial engineered from abundantly found cellulose sources. The manufacturing process of the NFC hydrogel involves mechanical homogenization when cellulose powder from a cut wood pulp passes through a homogenizer at high shear forces to facilitate the defibrillation of the cellulose fibers into nanofibrils and make a stable colloidal dispersion (hydrogels).[7−10] NFC is a thixotropic colloidal dispersion, which consists of entangled nanofibrils, several micrometers long and 10–50 nm in diameter with a specific surface area of 400–800 m2/g.

According to this novel method, NFC nanofibers are being dyed instead of cotton fabrics using reactive dyes and under dyeing conditions (pH, salt concentration, and temperature) typical for the traditional exhaust reactive dye bath cotton dyeing process. By conjugating NFC with reactive dyes, the nanocellulose fibrils are transformed into a colored NFC-dye pigment. The colored NFC hydrogel is then deposited on the surface of textiles using conventional deposition methods and annealed at an elevated temperature. Colored NFC hydrogels can be optimized to use for the existing batch and continuous dye deposition methods as well as for waterless dye-depositing methods such as spray dyeing. In comparison to the conventional dyeing of cotton and other fibers in a dye bath, the dyeing of NFC requires about six folds less water, salt, and alkali because of the high surface-to-volume ratio of NFC hydrogels. As compared with the conventional exhaust dyeing, the NFC dyeing technology is a proven green production strategy based on the life cycle analysis and the measures of water and dye auxiliary consumption.[5,6]

Water-soluble hemicelluloses which are present in NFC hydrogels and cotton fibers are typically extracted during dyeing at elevated temperatures in alkali solutions. However, hemicelluloses react with reactive dyes via the same chemical reactions as NFC and tend to be readily removed with wastewater. The disposal of the hemicelluloses conjugated with reactive dyes limits further improvement of the dye fixation of NFC dyeing. The release of the soluble hemicellulose–dye conjugates mainly associates with environmental pollution. We address this issue by grafting the hemicellulose–dye conjugates to the surface of NFC nanofibers and cotton fabric fibers using polycarboxylic acids (PCA). The combination of NFC dyeing and PCA post-treatment leads to a substantial improvement of dye fixation and reduction of dye contaminants in wastewater. Notable attempts have been reported using PCA for wrinkle-resistant post-treatment on cotton and had discussed the formation of chemical crosslinks/grafts with the submicron-scale cellulose fabric fibers using maleic, itaconic, and citric acids.[11−15] Naturally existing PCAs such as malonic acid,[16] cutin,[17] and suberin[18] can be used as potential green crosslinking agents for future research perspectives. As of polyesterification catalysts, niobium oxide[19,20] and zeolite[20] can be experimented for alternative catalyst systems. Therefore, the proposed chemical crosslinking post-treatment from PCA can be used as a solution for the substantial reduction of environmental pollution of wastewater with unreacted dyes generated by the current textile dyeing industry.

Results and Discussion

Textile Dyeing with NFC-Dye Hydrogels

The cotton textiles were dyed following the recently published NFC-based dyeing method.[5] The NFC dyeing method (Supporting Information, Figure S1) involves the dyeing of NFC hydrogels via reactions of cellulose hydroxyl groups residing on the surface of NFC nanofibers with chlorotriazine and ethylsulfonyl functional groups of the reactive dyes. A variety of colors can be achieved through combinations of blue, yellow, red, and black commercial reactive dyes (Supporting Information, Figure S2).

In the presence of sodium sulfate and sodium carbonate, the reaction of the dye with the NFC hydrogel takes place at elevated temperatures. The prepared NFC-dye hydrogel is then deposited on the cotton fabric surfaces following the conventional knife dye deposition method, and the colored fabrics samples are dried. Then, the NFC-dyed fabrics were washed, conditioned, and used to evaluate the efficiency of coloration. To demonstrate the color retention of the NFC-dyed fabric subjected to the wear and care conditions of the textile products, we selected a series of standard tests to evaluate the dye fixation and colorfastness. The dye fixation was assessed as the pivotal criterion to analyze the fraction of the reactive dye covalently bound to cellulose fabric fibers. The result of the tests, along with the visual appearance of the colored fabrics, is shown in Table and Figure , respectively. All the dyes show more than 90% retention after 10 laundry cycles, but black and blue dye underperformed in terms of dye fixation in comparison to red and yellow reactive dyes.

Table 1

Overall Dye Performance of Fabrics Dyed from NFC-Reactive Dye Systems (100% Cotton Woven)

reactive dye	blue	black	red	yellow
dye concentration, g cm–3	0.32	0.33	0.31	0.34
dye fixation, %	65 ± 4	77 ± 5	82 ± 3	84 ± 2
colorfastness to accelerated laundry, grade	4.5	4/5	5	5
colorfastness to crocking (dry/wet), grade	4.5/3.5	4.5/4	4.5/4	5/4
color retention after 10 laundering cycles, %	92 ± 4	95 ± 4	96 ± 5	93 ± 7

Figure 1

Photographs of fiber samples (100% cotton woven fabric) before and after five accelerated laundry cycle washing colored with NFC dyes (a) blue, (b) black, (c) red, and (d) yellow.

The bonding mechanism of reactive dyes to the cellulose matrix includes two steps; dye adsorption on cellulose surfaces and chemical reaction with the hydroxyl functional groups of cellulose. Both of these steps depend on the reactive dye chemical structure. The reactive dyes are anionic compounds that bear two to four sodium sulfonate functional groups (to provide solubility in water), chromogens, and reactive groups. NFC nanofibers are negatively charged because of (i) the slight acidity of hydroxyl groups, (ii) a small fraction of carboxylic functional groups formed by the oxidation of cellulose in the stages of NFC fabrication at high shear forces and elevated temperatures, and (iii) adsorption of salts.[21] Therefore, NFC nanofibers electrostatically repel similarly charged anionic dyes. The electrostatic repulsions are partially screened at high salt concentrations. However, dye adsorption is indeed limited by these repulsive electrostatic interactions. This effect depends on the number of ionizable groups and their special arrangements in the dye molecules.

The efficiency of the covalent binding of the dyes via the nucleophilic addition–elimination reaction between the cellulose hydroxyl groups and reactive groups of the dyes depends on the type and the number of reactive groups per dye molecule. The reaction of cellulose hydroxyls with reactive groups of the dyes competes with the hydrolysis of these groups in aqueous solutions at high pH.

NFC hydrogels and cotton fibers contain a fraction of water-soluble hemicelluloses (mainly xylans). These soluble polysaccharides react with the reactive dyes following the same mechanism as cellulose nanofibers. The interplay of all these factors results in different dye fixation efficiencies among the dyes (Table ). The typical range for the dye fixation efficiency of reactive dyes with one functional reactive group is 50–75% and dye fixation for two functional reactive groups is 75–90%.[22] Hence, 10–25% of the dye remains in the bath as hydrolyzed (nonreactive form) or bound to the soluble polysaccharides form. Contributing to the decrease of the dye fixation, nonreactive and soluble sugar bound forms of the reactive dyes remain dissolved in the medium. This leads to water pollution if the dye contaminated water is released to the environment without further purification (Supporting Information, Figure S3).

Analysis of Soluble Sugars in NFC Hydrogels

The plant cell wall contains up to 30% of soluble polysaccharides. Most of them are extracted at different stages of biomass treatments. However, the mechanical shearing applied to kraft pulp in the homogenization process of NFC production may result in the extraction of an additional fraction of the soluble sugars embodied at the cellular level.[14] Carboxymethylcellulose (CMC) is a water-soluble derivative of cellulose, bearing residual hydroxyl functional groups that contribute to the total soluble polysugars content in the NFC hydrogel. CMC is used as a plasticizer in the NFC hydrogel formulation to facilitate the defibrillation of bundles such as cellulose fibrils.[8,23,24] We used hydrogel samples with no CMC added for the analysis of soluble sugars to avoid the possible contribution of the CMC additive.

The soluble sugars and products of their reactions with reactive-dye molecules (the soluble sugar–dye covalently bound complex) are most likely to dissolve and exist in the aqueous phase of the NFC hydrogel. This soluble sugars fraction can be transferred into the supernatant during the centrifugation process. The presence of simple and complex carbohydrates in the supernatant of the NFC hydrogel was confirmed from the phenol–sulfuric acid method[25,26] by observing an orange-yellow color with a maximum absorbance at 490 nm (Supporting Information, Figure S4). The quantified total soluble sugar content is 1.2% of the cellulose weight and was estimated from the gravimetric analysis of the NFC supernatant. The MALDI mass spectroscopic analysis (Figure S5) of the supernatant extracted from the NFC hydrogel revealed two series of hexose and pentose containing oligosaccharides (Tables and 3).

Table 2

Oligosaccharides of Pentose Sugars

oligosaccharide	P7	P8	P9	P10	P11	P12	P13	P14
theoretical, m/z	966	1098	1230	1362	1494	1626	1758	1890
observed, m/z	968	1099	1231	1363	1495	1628	1760	1892

Table 3

Oligosaccharides of Hexose Sugars

oligosaccharide	H6	H7	H8	H9	H10	H11	H12
theoretical, m/z	1013	1175	1337	1499	1661	1823	1985
observed, m/z	1015	1178	1340	1505	1664	1825	1988

Oligosaccharide series of pentose sugars consist of 7–14 monomeric units (Table ), and oligosaccharides of hexose sugars series include 6–12 monomeric units (Table ). Considering the composition of the wood plant cells, we can speculate that the hexose oligosaccharides are glucose based, and the pentose oligosaccharides are xylose-based complex carbohydrates. Covalent bonding of the reactive dyes to the polysugars will lead to the loss of a fraction of reactive dyes during the washing process. This was further verified from the following experiments.

Effect of Soluble Hemicelluloses on Cellulose Dyeing

We hypothesized that the 65–85% dye fixation rate is affected by a combination of several side chemical processes: hydrolysis of the reactive groups in the basic aqueous solution, conjugation of the reactive dyes to the soluble polysaccharides present in the NFC hydrogel, and dye release from the cotton fabric under dyeing conditions (elevated temperature and alkaline dyeing liquor). Poor dye fixation is an effect of these side processes that could be minimized by covalent grafting of the water-soluble polysaccharide–dye conjugate to the cellulosic surfaces, either to NFC nanofibers or to the cotton fabric or both the materials.

We proved this hypothesis in several control experiments. In Experiment 1 (Figure ), we collected a solution of soluble polysugars from the supernatant after the centrifugation of the NFC hydrogel. This solution was mixed with the reactive-blue dye to synthesize soluble polysugar-dye conjugates under reaction conditions typically used for NFC dyeing. This conjugate solution was then used to dye cotton samples using two different methods: (i) the fabric was immersed into the conjugate solution with sodium sulfate and sodium carbonate at the same concentrations and temperature regimes as for NFC hydrogel dyeing, and (ii) after the exhaust dyeing as per step (i) we applied a post-treatment with maleic acid and sodium hypophosphite (SHP) catalysts followed by drying of the fabric samples at 120 °C. In the latter case, maleic acid reacts with hydroxyl functional groups of cellulose and soluble hemicelluloses, forming covalent bonds with the polysaccharides. Hence, the dye–hemicellulose conjugates could be grafted to the surface of the cotton fabrics. These experiments were compared with two control experiments when the exhaust dyeing of the fabrics was performed in a conventional dye bath, with and without maleic acid post-treatment. The results of these experiments are shown in Figure .

Figure 2

Control experiment 1: soluble sugars extraction, dyeing of soluble sugars, and the application of PCA.

Figure 3

Effect of grafting of soluble hemicellulose–dye conjugates on dye fixation: (a,b) control experiment of exhaust dyeing with reactive blue and, (c,d) dyeing with the supernatant of the NFC-dye hydrogel containing dissolved hemicellulose–dye conjugates; (a,c)—no post-treatment and (b,d)—post-treatment with maleic acid.

Results in Figure show that maleic acid has no effect on exhaust dyeing with the reactive-blue dye (control). The dyed fabrics with the soluble sugar dye-conjugated hemicelluloses (no post-treatment with maleic acid) show about 55% of dye fixation. Consequently, some fraction of reactive groups of reactive-blue conjugates remained intact after bonding to the hemicellulose. These residual reactive groups facilitate grafting of the conjugates to the cotton surfaces. The post-treatment provided a further improvement of dye fixation (up to 70%) due to the grafting of the soluble sugar-conjugates via the maleic acid crosslinker.

The above conclusions about the surface grafting of the soluble hemicellulose–dye conjugates to cotton using PCA post-treatment were further supported by the following control Experiment 2 (Figure ). The NFC-blue dye hydrogel was used as the control for the dyeing of cotton fabric samples. A sample of the same NFC-blue dye hydrogel was centrifuged and separated into two fractions: soluble hemicellulose–dye conjugates in the supernatant and the NFC-bound dye in the precipitate. These two fractions were separately used to dye cotton fabrics providing the same conditions. Both samples were post-treated with maleic acid. The dye fixation of each sample was examined after drying and washing. The results are shown in Figure .

Figure 4

Control experiment 2: NFC-blue hydrogels (not-centrifuged), extraction of soluble sugar–dye conjugates, and NFC-bound precipitates and application of PCA.

Figure 5

Effect of the post-treatment with maleic acid on dye fixation for the NFC dye (Reactive Blue) in the cotton dyeing process with (a,b) the NFC-dye hydrogel (control), (c,d) the supernatant of the NFC-dye hydrogel, and (e,f) the precipitate of the NFC-dye hydrogel; (a,c,e)—no post-treatment and (b,d,f)—post-treatment with maleic acid.

The dye concentration analysis in the washing effluents (rinsing water) of the dyed fabric samples (dyed from NFC-blue hydrogel) without and with post-treatment are shown in Figure . These experiments demonstrate a substantial improvement of the dye fixation in all three cases and a reduction of unbound dye concentration in the washing effluents when the PCA post-treatment was applied (Figure ). The surface grafting of maleic acid to the polysaccharides (Figure S6) was confirmed with FTIR-spectroscopy analysis (Figure S7).

Figure 6

Concentration of the unbound reactive-blue dye in washing effluents for dyeing with (a,b) NFC-dye hydrogel, (c,d) its supernatant, and (e,f) its precipitate; (a,c,e)—no post-treatment and (b,d,f)—post-treatment with maleic acid.

Figure 7

Refers to 100% cotton fabrics dyed from noncentrifuged NFC-blue dye with 0.83% percentage dye deposition on the weight of the fabrics. Discharged water from the washing process of NFC-blue-dyed fabrics without PCA (a1) and with PCA (b1). NFC-blue-dyed fabrics; (a2—before wash, a3—after wash). NFC-blue-dyed fabrics underwent PCA post-treatment; (b2—before wash, b3—after wash).

The difference between the experiments shown in Figures and 5, is that in the first case, soluble sugars were first extracted from the NFC hydrogel and then were conjugated with reactive dyes. Then, these soluble sugar–dye conjugates were used to dye the cotton fabrics. In the second case, the NFC hydrogel (containing both NFC nanofibers and soluble sugars) was dyed as the first step. Then, NFC-dye nanofibers and soluble sugar–dye conjugates were separated from the centrifuging process and used to dye the cotton fabrics. In both cases, the amount of dye conjugated to soluble sugars are the same. By comparing the results labeled as c and d in Figures and 5, we may conclude that dyeing efficiency in both cases is comparable. However, the dye fixation in the first case is more significant (Figure ). This difference is explained by the competition of soluble sugars and NFC nanofibers in the reaction with reactive dyes. The results confirm that a significant fraction of the dye is bound to NFC nanofibers than that to soluble sugars as the concentration of soluble sugars is much lower.

The obtained results explain the improvement of dye fixation in the presence of maleic acid. Maleic acid covalently binds both the NFC-dye and hemicellulose–dye conjugates to the surface of cotton fabrics. The crosslinking of NFC additionally improves the stability of the coating. With the PCA post-treatment, NFC-based textile dyeing secures a significant reduction of the unbound dye concentration in wastewater, as shown in Figure .

Textile Dyeing with NFC-Dye Hydrogels and PCA Post-treatment

The developed cotton dyeing method was tested for different types of PCA: citric, maleic, and itaconic acids. In these experiments, we varied PCA post-treatment formulations using equal concentrations of different PCAs. PCAs are commonly used to improve wrinkle resistance[11,12] of the cotton fabrics via crosslinking of cellulose molecules to minimize their swelling in water. In this research, we used the commonly studied PCAs for industrial applications to form crosslinks such as maleic, itaconic, and citric acids.[27−29] The crosslinking efficiency of the PCA esterification reaction elevates with the SHP[28,30,31] catalyst to reduce the esterification reaction temperature.

For all studied PCAs, the combination with SHP provided a higher blue-dye fixation as compared to 65 ± 4% dye fixation for the reference NFC-dyed fabric with no post-treatment (Figure ). Critic acid was found to deliver a slightly higher level of dye fixation followed by maleic and itaconic acids when the crosslinking reaction is favored with SHP. In the crosslinking reaction carried out in the absence of SHP, maleic acid shows the highest dye fixation. Crock-fastness and wash-fastness tests resulted in improvements of NFC-dyed fabrics with the PCA post-treatment.

Figure 8

Dye performance upon post-treatment with different PCAs: (a,b)—citric acid, (c,d)—maleic acid, and (e,f)—itaconic acid; (a,c,e)—no SHP catalyst and (b,d,f)—with SHP catalyst.

The crosslinking post-treatment improves dyeing for the all tested reactive dyes (Figure ). NFC-blue and NFC-black dyes indicate more significant improvement than the NFC-red and NFC-yellow dye. For all dyes, the dye fixation can be enhanced to the level above 85% with the PCA post-treatment.

Figure 9

Different dye system response to the post-treatment using citric acid and SHP; reactive dyes; (a,b)—blue, (c,d)—black, (e,f)—yellow, and (g,h)—red; (a,c,e,g)—no post-treatment and (b,d,f,h)—with post-treatment.

The PCA crosslinking post-treatment improves pivotal characteristics of NFC-dyed fabrics and makes less or no impact on fabric comfort properties (Table ). For example, stiffness of textiles is characterized by the mean bending length (higher the bending length, stiffer the fabric), and the air permeability of fabrics is closely related to the heat management and fabric comfort. Both these characteristics were not changed upon crosslinking. Having a minor impact with PCA crosslinking post-treatment on the textile characteristics can be explained by depositing a relatively thin (micrometer range) skin of NFC-dye coating on the surface of cotton fabrics (Figure ).

Table 4

Comparison of Dye Performance for the NFC-Blue Dye with and with No Crosslinking

	NFC-blue dye	NFC-blue dye with crosslinks
dye concentration, wt %	1.19	1.19
dye fixation, %	65 ± 4	89.1 ± 3
colorfastness to accelerated laundry	4.5	5
colorfastness to crocking (dry/wet), grade	4.5/3.5	5/4.5
color retention (after 10 cycles), %	92 ± 4	95 ± 4
air permeability, cfm	14.8 ± 2.7	13.7 ± 1.1
mean bending length, mm	28 ± 2	26 ± 1.2
tensile modulus (Figure S8)	113.6	130.1

Figure 10

SEM images of a cotton woven fabric at 1500 magnification: (a) control, 100% cotton, (b) ultra-thin NFC-dye coating prior to washing, (c) NFC-blue dye coating fabric after 5 laundry cycles, (d) NFC-blue dye coating with PCA post-treatment—after 5 laundry cycles and the coating retention with crosslinks.

The NFC-dye coating is made up of thin porous patches on the cotton surfaces formed at the microscopic level; hence, these colored structures appear as a uniform shade that delivers visual level of the color. The stiffness and the breathability are less affected with PCA crosslink formation because of the porous structure of the ultra-thin NFC-dye coating.

Conclusions

This study demonstrates that the PCA post-treatment promotes grafting and permanent crosslinking of NFC-dye and soluble polysaccharide-dye conjugates to the cotton fabric surface. The PCA post-treatment delivers up to 30% improvement in reactive dye fixation and up to 60% reduction in dye discharge. The proposed chemical crosslinking method, as a post-treatment, can be explored as a solution for a substantial reduction of wastewater pollution with dyes generated from the current textile dyeing technology. The PCA post-treatment improves the overall stability and permanent retention of the NFC-dye coating with no impact on other important textile characteristics such as stiffness and breathability as the deposited NFC-dye coating is formed by thin microscopic porous patches.

The efficiency of the PCA post-treatment is mainly governed by several factors: temperature, the presence of SHP, NFC concentration, the type of PCA, and the chemical structure of the reactive dye. Maleic acid demonstrated the highest dye fixation (up to 86%) with no SHP in the formulation. Citric acid is the highest performing PCA (up to 90% dye fixation) followed by maleic and itaconic acids with the presence of SHP at 120 °C.

Experimental Section

Materials and Reagents

Cellulose powder (Weyerhaeuser); carboxymethyl cellulose (CMC, MM = 90,000 g/mol, Sigma-Aldrich); sodium carbonate (Fisher-Scientific); sodium sulfate (Fluka); phenol (Fisher-Scientific); sulfuric acid (98%, Fisher-Scientific); SHP (Sigma-Aldrich); PCAs: maleic, itaconic, and citric acids (Sigma-Aldrich); reactive dyes: Everzol Red 3BS, Everzol Yellow 3RS, Everzol BRF, Everzol BHC, and 100% grey cotton knitted and woven fabrics (provided by Cotton Inc., North Carolina); and kraft pulp (Weyerhaeuser, Dacula, GA, USA) were used as received.

Preparation of NFC

The kraft pulp sheets were knife-milled and sieved three times using a sieve with a mesh size of 500 μm. The pulp powder was then stored at 60 °C to prevent moisture absorbance. In the procedure of making 2 wt % NFC hydrogel, 10 g of the pulp powder were mixed with 500 mL of deionized (DI) water into a slurry and processed through the homogenizer (APV 1000) at a high shear rate (650–700 bar pressure).

Preparation of the NFC-Dye Hydrogel

The NFC-dye hydrogel was prepared as reported previously[5] and is shown in a diagram (Figure , Supporting Information: Figures S1 and S2, Tables S1 and S2); the NFC hydrogel was mixed with the reactive dye and sodium sulfate. After heating to 60 °C, sodium carbonate was added, and the mixture was stirred for 75 min.

Figure 11

Dyeing steps for the NFC-dye hydrogel: the dyeing formulation with NFC, reactive dye, and sodium sulfate was heated to 60 °C for 25 min. After adding sodium carbonate, the mixture was stirred for 75 min before cooling.

Dyeing of the Cotton Fabric

The NFC-dye hydrogel was deposited using a conventional knife or padding method, which gave a homogeneous level of the color. Then, dyed fabrics were subjected to 100–120 °C drying temperature to remove moisture and to promote thermofixation of the NFC-dye fibrous pigment with the cotton fabric. Dyed fabrics were then washed, following five laundry cycle combined process of hot (at 60 °C) and cold (at 20 °C) washing (Figure S1). Washed fabrics were dried at 60 °C to prevent the boarding of cotton fibers. Then, the NFC-dyed fabrics were conditioned for 24 h before using for standardized testing.

PCA Post-treatment

PCAs (Figure S2) and SHP were dissolved in DI-water at 40 °C at a ratio of 8:3 at 5 wt% PCA in the solution.[30] The NFC-dye colored samples of fabrics were soaked into the solution and dried at 120 °C (Figure S9).

Extraction and Analysis of Soluble Hemicelluloses from the NFC Hydrogel

The NFC hydrogel 2 wt %) was centrifuged at 13,000 rpm for 45 min. Then, the supernatant was separated and filtered using a 0.2 μm (Fisher) filter. From the clear supernatant, 2 mL of the sample was mixed with 0.5 mL of 80 wt % phenol solution. Then, 5 mL of 98 wt % concentrated sulfuric acid was added. The occurrence of the orange-pink color was observed. The absorbance wavelength of the obtained color was evaluated using a UV spectrophotometer. The extracted supernatant was freeze-dried (LABCONKO freeze drying system, FREEZONE 4.5) to quantify the hemicellulose content in the NFC hydrogel after subtraction of the CMC weight. The samples were analyzed for oligosaccharides using a MALDI- TOF Bruker Microflex LT mass spectrometer. FTIR spectra for NFC-dyed fabric surfaces were measured using a Nicolet 6700-FTIR spectrometer using absorbance in 64 scanning cycles. The reactive dye concentration in water collected from the washing process was measured by UV–visible spectroscopy (UV-2401PC) at wavelengths: yellow 410 nm, red 520 nm, blue 630 nm, and black 580 nm.

Testing of Dyed Fabrics

Dye Fixation

Dye fixation is determined by the ratio of the color strength of the colored fabrics before and after wash off. Color strength (as a ratio of adsorption to scattering characteristics K/S) as defined by the Kubelka–Monk theory[32] was estimated from reflectance spectra of fabric samples acquired with a Macbeth Color-Eye 7000A spectrophotometer.

Colorfastness

Laundering and crocking (dry and wet) tests that show the retention of the color after washing and rubbing (for dry and wet samples) were conducted according to the AATCC 61-2013, 2A and AATCC 8-2016 test methods, respectively. Note, the colorfastness is reported in arbitrary grades with the highest grade of 5 and lowest of 1.

Air Permeability and Bending Length

The air permeability and bending length of the fabric were measured according to the ASTM D737-96 and ASTM D1388 tests, respectively.

Scanned Electron Microscope Imaging

Washed and dried fabric samples were gold coated and examined using the FEI Teneo FE-SEM instrument.