Thermodynamics, Kinetics, and Multifunctional Finishing of Textile

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Perspective pubs.acs.org/journal/ascecg

Thermodynamics, Kinetics, and Multifunctional Finishing of Textile Materials with Colorants Extracted from Natural Renewable Sources Shahid-ul-Islam*,† and Gang Sun‡ †

Department of Textile Technology, Indian Institute of Technology, New Delhi-110016, India Fiber and Polymer Science, University of California, Davis, Davis, California 95616, United States



ABSTRACT: Because of a continued ban on some toxic synthetic dyes, textile and polymer scientists are presently in a constant search for alternative greener agents that can reduce pollution in textiles industries. Natural dyeing is one such promising technology that has the potential to clean up environmental pollution arising due to excessive use of toxic synthetic dyes and chemical auxiliaries. Dyeing compounds extracted from natural materials have produced sober and elegant shades on different types of fabrics over the past few years and are currently investigated as novel functional agents in the production of highly active textile surfaces having deodorizing, antioxidant, antimicrobial, antifeedant, and UV protection properties. The present perspective is intended to outline the recent research in functional finishing of different textile substrates with colorants and functional agents exploited from natural renewable sources. To the best of our knowledge, this perspective discusses for the first time thermodynamic and kinetic studies of natural colorants in order to elucidate dyeing mechanisms or understand dye−fiber interactions arising during the dyeing process. Finally, it also highlights the limitations of this technology and provides the scope for further investigations to corroborate existing gaps in order to make natural textile dyeing less vulnerable and widely acceptable in modern dye houses. KEYWORDS: Coloration, Kinetics, Natural extracts, Biomordants, Textile materials



INTRODUCTION With the advancement in color science and discovery of synthetic dyes in 1856, the textile industry has developed into a million dollar industry and is one of the largest growing sectors to manufacture a wide range of products on a large commercial scale.1,2 In addition to their largest use in the retail apparel market, it is worth noting that natural and synthetic fibers with multifunctional properties such as self-cleaning, antimicrobial, antioxidant, UV-protective, waterproof, mothproof, flame retardant, and stain-resistant properties are widely desired nowadays for various hygienic and medical applications.3−5 As a consequence, textile and polymer scientists have employed a lot of synthetic dyes and innovative chemical finishing agents for sizing, scouring, bleaching, mercerizing, dyeing, printing, and finishing applications.6,7Among all textile production processes, coloration or dyeing is one the most chemical-consuming processes since it involves the use of different dyeing compounds, auxiliaries, metal ions, and surfactants. Unfortunately, some of the colorants, particularly those containing azo groups, may cause skin allergies or produce toxic wastes and therefore pose a serious direct threat to both human health and ecology.8,9 Because of growing environmental problems, government authorities, scientists, and industrial firms are working around the globe to mitigate the pollution caused by synthetic dyes and toxic finishing agents currently used in textile applications. It © 2017 American Chemical Society

has been advised that green products based on renewable sources should be exploited since they are biocompatible and highly biodegradable.10,11 History has highlighted the use of crude extracts from different plant parts such as flowers, leaves, roots, barks, and seeds in the dyeing of cotton, wool, and other substrates.12 The presence of different classes of coloring compounds in naturally occurring materials, in addition to their recently discovered properties such as antimicrobial, antioxidant, and UV-protection, has resulted in creation of more awareness of their reintroduction into textile dyeing and finishing.13 Natural dyes from plants are considered biodegradable, nontoxic, and nonallergic and are being used extensively in coloration of textile materials.14,15 The literature shows that a wide range of beautiful hues including black, olive green, brown, burgundy, yellow, and many more can be produced from different natural sources.16 To reduce costs and make natural dyeing more economically viable and acceptable, presently, scientists have turned their focus toward exploring colorants from waste materials. Substantial investigations have revealed the production of natural colorants from agricultural byproducts such as walnut husk, orange peel, pomegranate peel, and almond shells.10,17 Received: May 11, 2017 Revised: June 22, 2017 Published: July 13, 2017 7451

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ACS Sustainable Chemistry & Engineering Of all the textile materials, natural fibers such as wool, silk, and cotton are more prone to attack by microorganisms due to several intrinsic characteristics including high moisture content, oxygen, and nutrients.10,11 Herbal extracts are known to be rich sources of polyphenols and other potent secondary metabolites.18,19 However, their potential is yet to be fully explored to combat the growth of deadly microorganisms on different textile fabrics. Plant-based colorants are a sustainable and abundantly available source of antimicrobials which are a subject of considerable interest for researchers.20,21 It is worth pointing out that the textiles finished using herbal extracts find applications in medical garments, sanitary garments, military garments, napkins, socks, disposable wipes, and carpets.22,23 Considering their importance for the development of multifunctional textiles, this Perspective is intended to summarize some outstanding research studies conducted in this realm. To the best of our knowledge, the thermodynamic and kinetic investigations of natural dyes have also been deliberated for the first time in this Perspective.

values reported for free energy (ΔG°) and enthalpy (ΔH°) terms indicted that the lac dyeing for silk is spontaneous and exothermic. Kongkachuichay et al.31 studied the thermodynamic properties of adsorption of laccaic acid on silk. The mechanism for adsorption of the dye compound to silk was stated to be following the Langmuir isotherm, and the heat of dyeing and entropy were found to be −13.20 kcal/mol and −0.03 kcal/mol/K, respectively. The pretreatment of silk with memecylon mordant provided significant advancement in dyeability as well as improved silk lac affinity. Rattanaphani et al.32 also studied the effect of pH, the initial dye concentration, and the material to liquor ratio (MLR) on adsorption capacities of lac dye on cotton previously treated with chitosan. The chitosan was noticed to enrich cotton with cationic sites and therefore enhanced its interaction with lac dye molecules. The optimum conditions for highest adsorption were found at pH 3, a material to liquor ratio of 1:100 (w/v), and a contact time of 3 h. Experimental results followed the Langmuir isotherm with an enthalpy change (ΔHo) of −17.43 kJ. Vinod et al.33 evaluated the kinetic and thermodynamic studies on the dye uptake by silk using the bark extract of Albizia lebbeck as a source of natural dye. The results showed that the use of alum mordant and a premordanting technique significantly enhances the color fastness values. The regression results in their experiments showed that the Langmuir model fitted the experimental results well. Changes in the standard affinity, enthalpy, and entropy suggested that the adsorption of Albizia lebbeck dye onto silk was a spontaneous and endothermic process. The dyeing studies and adsorption isotherms of Cuminum cyminum L extract on silk has been explored in a recent research work by Tayade and Avidenkar.34 Pseudo firstorder and second-order kinetic models were used to describe the dyeing mechanism, and it was observed that the pseudofirst-order kinetic equation better represents the experimental data than the pseudo-second-order kinetic equation. From enthalpy, entropy changes, and activation energy, it was noticed that the adsorption of Cuminum cyminum L on silk follows chemisorption and is a spontaneous and endothermic process. In order to understand the mechanism of tea polyphenol dyeing, Tang et al.35 studied the adsorption behavior of tea polyphenols (TP) on wool, silk, and nylon. Their results showed that pH 4 is favorable for adsorption of tea polyphenols onto selected fabrics, and electrostatic interaction and hydrogen bonding seemed to play major roles in the adsorption process. It was found that adsorption followed Nernst- and Langmuirtype isotherms. In another research, Hou et al.36 used a natural dye derivative, namely sodium copper chlorophyllin, to study its dyeing, kinetics, and adsorption characteristics on silk. Dyebath experiments were carried out as a function of pH and NaCl, and it was observed that the use of sodium chloride salt significantly improved dye uptake as well as the depth of silk. It was shown that the pseudo-second-order kinetic equation was best fitted with the experimental results. The values obtained for thermodynamic parameters such as adsorption affinity (−Δμ°) and enthalpy change (ΔH°) showed that dyeing with sodium copper chlorophyllin was exothermic and spontaneous. An adsorption study of alum−morin dye extracted from the heartwood of Maclura cochinchinensis was studied on silk by Septhum et al.37 The effect of important variables such as pH, initial dye concentration, and temperature on adsorption was investigated, and it was found that the increase in temperature resulted in higher adsorption of alum−morin dye onto silk. The pseudo-second-order kinetic model was best fit to the



ADSORPTION ISOTHERMS: UNDERSTANDING THE DYEING MECHANISM Despite the fact that adsorption isotherms help to understand the dyeing mechanism and to control the finishing process, only limited research works have been carried out so far on the thermodynamic and kinetic aspects of natural dyeing preventing their industrial-scale production. Most coloration processes involve both adsorption and absorption of colorants in fibers, and such processes are difficult to be clearly differentiated in a dyeing process. Thus, the term of sorption of the colorants can be employed in research methods and was measured to study thermodynamics and kinetics of the processes.13,24 Adsorption of colorants on fibers follow three most general isotherms, mostly determined by interactions between colorant molecules and polymers, and absorption of colorants into fibers is a little more complicated, affected by colorant molecules, dyeing conditions, and concentrations.25 Several theoretical models were found effective in describing the overall sorption processes regardless of the complexity of molecular interactions and sizes of colorants.26,27 However, the adsorption and absorption processes are reversible, i.e., sorption and desorption of coloration occur simultaneously, the equilibrium processes. Thus, most thermodynamic and kinetic investigations of the dyeing processes have been mostly based on these equilibrium processes as well. Several researches have been carried out on a number of herbal extracts over the past several years. To generate the scientific data, Gulrajani et al.28 investigated the thermodynamic and kinetics of wool and nylon dyeing using alcoholic extracts of red sandalwood without using mordants. They found that the dyeing is endothemic for wool and exothemic for nylon. In another research work, the kinetics and thermodynamics of wool, human hair, silk, nylon, and polyester dyeing with juglone compound extracted from the hulls of raw walnut were studied by Gupta and Gulrajani.29 The absorption of juglone on all the selected fibers was found to follow a partition mechanism. Similarly, the thermodynamics and kinetics of sorption of lac dye on silk was investigated at pH 3, a material to liquor ratio of 1:100 (w/v), an initial dye concentration of 450 mg/L, and 60 min dyeing time by Chairat et al.30 Their study showed that pseudo-second order kinetic equation agreed well with the experimental results with an activation energy of 47.5 kJ/mol. Furthermore, the negative 7452

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ACS Sustainable Chemistry & Engineering Table 1. Thermodynamic and Kinetic Investigations Performed by Different Researchers

Thermodynamic parameters Colorant or Plant Sodium copper chlorophyllin Juglone

Fabric

Equilibrium model

Kinetic model

Optimum conditions

ΔH (kJ/mol)

ΔS (J/mol K)

ref

pseudosecond-order −

pH 8

−6.32

−9.01

36

pH 4.6, M:L ratio 1:800

0.65

82.0

29

pH 3.5, M:L ratio 1:50, time 180 min pH 4.5, M:L ratio 1:1000 pH 3, M:L ratio 1:50





45

−23.48 −

−22.05 −

41 27

pH 4, M:L ratio 1:50 pH 4, M:L ratio 1:100, 80 °C, time 120 min pH 4, M:L ratio 1:800

− −

− −

35 40

Partition

pseudosecond-order − pseudosecond-order − pseudosecond-order −

−0.29

77.34

42

Partition Langmuir

− −

14.06 −

65.38 −

43 46

Nernst Langmuir

− −

pH 4 pH 7, M:L ratio 1:50, 90 °C, time 60 min pH 4.2, M:L ratio 1:800, 90 °C pH 3, M:L ratio 1:100

13.36 −8.55

99.49 −17.62

44 38

silk

Langmuir Partition

Terminalia arjuna

wool, human hair, silk, nylon, and polyester wool

Arnebia nobilis Adhatoda vasica

wool wool

Tea polyphenols Terminalia chebula

wool, silk, and nylon wool

Partition and Nernst Redlich-Peterson and Hill isotherm Langmuir and partition Langmuir

Lawsone Rheum emodi Acacia nilotica

wool, hair, silk, tussah, and nylon wool and silk wool

Madder Madder

nylon wool

Redlich−Peterson

phenomenon might be a result of small molecular sizes and rapid diffusion of colorants into fibers.

experimental data with an activation energy (Ea) of 45.26 kJ/ mol. The adsorption of alum−morin on silk was spontaneous and endothermic as depicted by free energy and entropy change values of −17.73 kJ/mol and −45.7 J/molK, respectively. Farizadeh et al.38 likewise examined extraction and sorption studies of colorants exploited from madder on natural polyamide fibers (wool). The experimental data were analyzed through Langmuir, Freundlich, BET, and Temkin isotherms, and it was shown that the adsorption process follows the Langmuir isotherm. Thermodynamic parameters such as standard affinity, entropy, and enthalpy have also been evaluated, and it was observed that adsorption of madder dye on wool is a spontaneous and exothermic process. Sun and Tang et al.39 reported the adsorption and UV protection of chlorogenic acid, which is an active component of honeysuckle onto wool. They noticed that the amount of chlorogenic acid on wool increased with a decrease in pH values, indicating that electrostatic interactions between oppositely charged functional groups control the adsorption process. The standard deviations of Freundlich, Langmuir, Redlich−Peterson, and Langmuir− Nernst isotherms were determined, and the results revealed that Redlich−Peterson and Langmuir−Nernst fitted well with the experimental results. Kinetic analyses were also conducted using pseudo-first-order, pseudo-second-order, Elvoich, and intraparticle diffusion equations, and the standard deviation results showed that the adsorption kinetics were best represented by a pseudo-second-order model. The honeysuckle-coated wool fabrics further demonstrated excellent UV protection properties. Some important thermodynamic and kinetic parameters which have been calculated by researchers are summarized in Table 1. According to the information summarized in Table 1, adsorption of natural colorants on protein fibers such as wool and silk generally follow the Langmuir isotherm due to the acidic conditions and ionic nature of the interaction between protein and colorants.27,35,36,38,40 Another interesting finding was that sorption of smaller natural colorants into fibers may undergo different processes, such as partition or Nernst isotherm, if only adsorption is considered.29,41−44 Such a



RENEWABLE COLORANTS AND MULTIFUNCTIONAL FINISHING Different classes of compounds such as tannins, carotenoids, anthocyanins, naphthoquinones, etc. present in herbal extracts have been proposed as multifunctional finishing agents for textile materials.47 As previously discussed, the herbal colorants are abundantly available in fruits, flowers, leaves, shells, husks, stems, etc. which are presently extracted using safer methods and applied to different fabrics.11,48 Many different herbal extracts have been investigated as promising functional agents for wool, cotton, silk, and other synthetic textile materials. Table 2 summarizes the molecular structures and colorimetric data of some important plant extracts.



MULTIFUNCTIONAL EFFECTS ON WOOL The production and exploitation of natural colorants in wool dyeing is currently being investigated worldwide. Application of different natural dyes on wool has been the aim of scientists to produce various functional effects besides novel and elegant hues. Bhattacharya and Shah61 studied the effect of various metal sulfates on wool fabric dyed with catechu extract and observed that the metal salts form strong complex with dye molecules resulting in high color strength and fastness properties. Mirjalili et al.62 applied natural dye extracted from weld using Soxhlet apparatus on the wool fabric, producing beautiful shades with acceptable fastness properties for commercial dyeing. They compared the dyeing results of weld with an acid dye and found that weld can be a viable alternative to toxic synthetic dyes. Bechtold et al.63 used several techniques to characterize natural dye extracted from Canadian golden rod samples and studied its dyeing properties on wool. The Canadian golden rod displayed promising results. Bechtold et al.64 also carried out the dyeing of wool with an aqueous extract of ash tree bark in the presence of iron sulfate using a metamordanting method. They found that metamordanting with iron produces good shade reproducibility and fastness 7453

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ACS Sustainable Chemistry & Engineering Table 2. Molecular Structures and Colorimetric Data of Some Herbal Extracts

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ACS Sustainable Chemistry & Engineering properties. In another study, Bechtold et al.65 likewise reported dyeing properties of wool using colorants extracted from waste materials such as pressed berries, grapes, distillation residues, and peels. The authors used iron and alum mordants and observed that both these mordants are effective in improving dye uptake and fastness properties. Baaka et al.66 identified polyphenolic and flavonoid compounds from grape pomace using FTIR spectroscopy and investigated their dyeing properties on wool using ultrasound technique. It was observed that an ultrasound technique not only reduced dyeing time and temperature but also improved color strength values. Moiz et al.67 studied the application of tea natural dye on wool using premordanting, metamordanting, and postmordanting methods with different metal salts in order to improve color strength and fastness properties of wool. It was interesting to note that mordanting, particularly postmordanting, produced deep shades on wool and delivered the best results for 2% and 5% tea concentration. Meksi and colleagues68 obtained colorants from olive mill wastewater and studied their dyeing and fastness properties on wool. Their results showed that olive mill wastewater in the presence of mordants could be used as a potential dye source to develop deep shades with acceptable fastness. Yusuf and co-workers69 applied madder dye as an ecofriendly natural colorant on woolen yarn to produce a range of beautiful shades. Shams-Nateri70 suggested that a madder dyebath could be reconstructed to produce cost-effective dyeing. As observed from spectroscopic data, it was clear that a madder dyebath could be suitably used to produce the same quality of shades with good fastness properties. Parvinzadeh71 introduced pretreatment of protease enzymes on wool to investigate its role in dyeing and other mechanical properties. A protease enzyme significantly enhanced colorimetric properties by increasing madder adsorption onto the wool surface. In 2016, Shabbir and colleagues56 reported the dyeing of woolen yarn with colorants extracted from Terminalia chebula. They observed that premordanting with traditional mordants significantly enhanced dye exhaustion and improved the color strength and fastness properties. Guesmi et al.72 reported the first application of chlorophyll-a as a biomordant on wool dyed with natural dye, namely, betanin extracted from the Opuntia ficus-indica plant. They used conventional and ultrasound dyeing methods to investigate the dyeing potential of betanin pigments. Biomordant concentration was found to strongly influence the color strength values. Their results also demonstrated that ultrasound or sonication methods improved dyeing efficiency from 30% to 60%. Wakida et al.73 reported dyeing of wool fabrics with colorants extracted from cochineal, Chinese cork tree, madder, and gromwell previously pretreated with oxygen, carbon tetrafluoride, and ammonia low-temperature plasmas. They found that compared to untreated wool cochineal, Chinese cork tree-dyed and plasma-treated samples displayed brighter shades. Tsatsaroni and Kyriakides74 studied dyeing and fastness of chlorophyll and carmine pigments on enzymatic pretreated wool and cotton and compared their results with metal mordanted samples. In their research, to increase the dye uptake of textile materials cellulase, α-amylase and trypsin enzymes were employed before being dyed with selected natural colorants. It was observed that the pretreatment with enzymes is far better than metal salts in terms of fastness properties. To further investigate the role of greener techniques in dyeing and finishing of wool, Kamel and colleagues75 carried out the dyeing of wool with an aqueous extract of insect-

derived lac dye using conventional and ultrasonic techniques and observed that higher color strength values and good fastness properties were obtained with ultrasound rather than conventional heating. Nagia an El-Mohamedy76 studied the effect of dye bath pH, salt concentration, time, and temperature on the dyeing of wool with two anthraquinone compounds named 2-acetyl-3,8-dihydroxy-6-methoxy anthraquinone or 3acetyl-2,8-dihydroxy-6-methoxy anthraquinone isolated from Fusarium oxysporum. They found that the dyeability of wool was highest at acidic pH which was ascribed to the ionic interactions between protonated amino groups of wool and anionic dye structures. Both of these two coloring compounds produced deep shades on wool samples with acceptable fastness properties. Different classes of dyeing compounds such as anthraquinone, naphthoquinone, flavones, and tannin structures present in madder, cochineal, walnut, weld, white onion, red onion, and pomegranate were tested by Montazer et al.77 on wool to evaluate color parameters in the absence and presence of ammonia after-treatment. Ammonia after-treatment produced a wide range of shades and caused a decrease in (lightness) L* values with an increase in ammonia percentage indicating darker or deeper shades. They also observed that ammonia-developed color on wool was irreversible when treated with an acid. Likewise, in another investigation conducted by Islam et al.,59 the influence of ammonia posttreatment on dyeing of wool with carotenoid compounds present in Bixa orellana seeds were determined. Three different concentrations, viz., 1%, 3%, and 5% of ammonia solution, were used to develop a variety of deeper and yellowish elegant shades on wool. The shades developed with ammonia showed good wash fastness results with a slight decrease in light fastness rating. The decrease in light fastness was ascertained to the blockage of protecting groups in bixin by ammonia molecules. Wool is also known as a potential vector to transmit Grampositive and Gram-negative bacteria and fungi. A curcumin dyeing compound was studied by Han and Yang78 to introduce color as well antimicrobial activity to wool. They noticed that the curcumin dye displays 45% reduction of E. coli and 30% of S. aureus after 30 washing cycles. Dev and colleagues50 reported a study to investigate wool dyeing and its antimicrobial activity with henna dye using chitosan pretreatment. Chitosan treatment increased the dye uptake and showed strong antimicrobial activity against S. aureus and E. coli. The dyeing compound in henna is lawsone (2-hydroxy-1, 4-naphthoquinone) which has inherent antibacterial and antifungal activity. Yusuf et al.79 recently investigated the dyeing and antimicrobial finishing of woolen yarns with the leaf extract of henna and discovered that the henna dye produces beautiful orange− brown to light yellow shades on wool and inhibits growth of E. coli, S. aureus, and C. albicans. They also observed that the use of metal mordants increases the wash durability of color and antimicrobial activity by forming a complex with the functional groups of the henna dyeing compound. Ghaheh and coworkers80 investigated the antibacterial properties of wool treated with green tea, madder, turmeric, saffron petals, and henna as natural dyes. As could be predicted, dyed wool fabric samples showed different interesting colors and good inhibition against S. aureus, E. coli, and P. aeruginosa. Wool dyed with the above-mentioned colorants displayed good fastness properties but poor durability of antimicrobial activity to washing and light. However, the wool fabrics pretreated with aluminum sulfate showed improved fastness properties and notably excellent durable antibacterial activity maintained up to five 7455

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mordants alum and iron, and it was found that alkalineextracted henna colorants had better color strength and good fastness properties. Haddar et al.90 investigated the dyeing properties of Foeniculum vulgare leaves extract on cationized cotton and discovered that the dyeing time of 59.38 min, pH 8.22, and temperature 99.83 °C are the optimum dyeing conditions to get the best results on modified cotton. Vankar et al.91 noticed that the use of natural dyes from Eclipta alba exhibited high dye uptake and 7−9% higher dyeing efficiency on cotton by the use of a sonication method than conventional dyeing. Vankar et al.92 also studied the biomordant dyeing properties of cotton with Rubia cordifolia natural dye using an ecofriendly sonicator dyeing method and found that Eurya acuminata DC var. euprista Karth biomordant imparts very good fastness properties to cotton and could be used as an alternative to metal mordants in natural dyeing. Kamel et al.,93 likewise, studied the effect of ultrasound power, particle size, extraction temperature, and time for the extraction of coloring matter from cochineal dye. Various dyebath factors including pH, salt concentration, ultrasound power, dyeing time, and temperature were also investigated to study the dyeing of cationized cotton with an extracted colorant. Their study showed that sonication method yields higher color strength and better fastness properties than the conventional method. Guesmi et al.94 used a sonicator reaction for the cationization of cotton cellulose with bromoacetyl bromide. The cationized cotton was characterized by FTIR spectroscopy and was used for dyeing with natural dye from Acacia cyanophylla yellow flowers. The modified cotton after dyeing with an ultrasound technique was found to show improved dye uptakes and very good fastness properties. Enzymatic pretreatment of cotton with cellulase, α-amylase, and trypsin enzymes was introduced by Tsatsaroni et al.95 to enhance the dye uptake after the application of crocin and curcumin pigments. Wangatia et al.96 studied the application of mango bark mordant to cotton using a premordanting or postmordanting method for the development of ecofriendly shades. They utilized natural dye from bitter leaves and found that postmordanting with mango yields better color strength and fastness properties than premordanting. Recently, natural dyes from pomegranate peels, nutshell, orange tree leaves, and alkanet roots have been applied to cotton previously pretreated with an ozone and ultrasound combination, and good wash fastness properties were reported by Benli and Bahtiyari.97 In a subsequent publication, the same authors used ultrasound technology to investigate the role of enzymes in the dyeing of cotton using the above-mentioned natural colorants. The dyeings were carried out at 80 °C, and it was noticed that enzymes and selected natural dyes in a single bath could produce deep shades on cotton and may prevent environmental pollution since there was a marked decrease in the use of chemicals, water, and energy consumption. Sinha and colleagues98 used a natural dye from the bark of Terminalia arjuna to investigate its dyeing properties on cotton previously treated with a tannic acid−alum mordant combination. By using Response Surface Methodology (Central Composite Design), they found that the dyeing time of 82.17 min, temperature of 69.34 °C, pH 8.7, mordant concentration of 3.75% (o.w.f), and dye concentration of 8.9% are the optimum dyeing conditions to obtain maximum color strength values. In 2015, Chairat et al.99 described coloration of cotton yarns using flavonoid compounds of Combretum latifolium stems as a source of yellow natural dye. It was observed that pretreatment of cotton yarns with chitosan in the presence and absence of a

washing cycles. Likewise, the dyeing property and antimicrobial activity of gallnut extract on wool has been investigated by several researchers. Hou et al.81 studied the dyeing and UV protection properties of wool fabrics dyed with orange peel extract and discovered that orange peel extract had a strong UV protection property. They observed that the dyeing time of 120 min, pH 3 for direct dyeing, pH 7−8 for mordant one, and temperature of 100 °C are the optimum conditions to obtain bright yellow and brown/black color shades with acceptable color fastness to light and good fastness to washing with soap and rubbing. Ghoranneviss and colleagues82 reported the use of plasma-sputtering treatment to modify wool prior to application of natural dyes from madder and weld. It was shown that plasma-sputtering treatment had a drastic effect on color strength and antibacterial activity of dyed wool against S. aureus and E. coli. Rather et al.46 has recently reported the fluorescence finishing of wool yarns using different metal salts and Acacia nilotica natural dye. They used FT-IR spectroscopy to investigate the dye−fiber interactions. Metal salts used in their study resulted in the formation of strong wool−metal− dye chelate complexes thereby enhancing the fluorescence property of the dyed wool. The resultant woolen yarn samples showed an interesting fluorescence property and good durability to many washings. Kilinc et al.83 evaluated wool dyeing and antimicrobial finishing with colorants extracted from the cone of Chamaecyparis lawsoniana and found that the color and fastness properties of wool were improved by the use of natural mordants. The dyeing and antimicrobial activity of wool was investigated in a recent work by El-Ksibi et al.84 employing phenolics compounds extracted from pepper waste. It was found that pepper waste after application on wool was highly active against P. aeruginosa and S. aureus.



FUNCTIONAL FINISHING OF COTTON TEXTILES Presently, several research studies have focused on the application of colorants from natural materials on cotton. Many efforts have been spent to modify the cotton surface. Natural dyeing of cotton with turmeric, myrobalan, madder, and red sandalwood was also studied by Samanta et al.85 They employed alum mordant using premordanting, postmordanting, and simultaneous-mordanting techniques and observed that all the natural extracts produced beautiful shades with high color strength values. Adeel et al.86 discovered that the colorants extracted from pomegranate peel could be effectively used to produce colored cotton with acceptable fastness properties. Vankar et al.57 noticed that the extracts of Terminalia arjuna, Punica granatum, and Rheum emodi improved the color strength and fastness properties of cotton previously pretreated with an enzyme−tannic acid complex. Premordanting, metamordanting, and postmordanting were carried out by Deo and Desai87 to study the feasibility of using an aqueous extract of tea for the dyeing of cotton and jute. The results for wash and light fastness were good to excellent, and medium shade depth in the acidic medium was obtained for cotton fabrics. Various colorful shades of blue, yellow, red, black, green, and fawn with good fastness properties were obtained on cotton with Kerria lacca, Indigofera tinctoria, Terminalia chebula, Quercus infectoria, Punica granatum, and Acacia catechu natural dyes alone and in combination by Gulrajani et al.88 Likewise, Ali and colleagues89 reported a study to extract natural dyes from henna leaves under alkaline conditions and with distilled water. The extracted dyes were applied on cotton in the presence and absence of metal 7456

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dyeing results with unirradiated samples and found that gamma radiations had significant effect on color and fastness properties. Batool et al.108 studied the dyeing property and color fastness of cotton fabrics treated with chicken gizzard leaves extract after being activated with different dosses of gamma radiations and discovered that cotton fabrics treated with a dose of 10 kGy result in deep shades and efficient dyeing parameters. Gulzar and colleagues109 investigated the dyeing behavior of cotton with quercetin after being irradiated to different adsorbed doses using a Co-60 irradiator. They found that 20 kGy is the effective dose for cotton surface modification. Optimum dyeing of cotton at 60 °C, 40 min, pH 6, liquor concentration of 1:30, 1 g of electrolyte concentration, 7% of tannic acid as premordant, and 5% of copper sulfate as postmordant are the best conditions to get maximum color strength and good fastness properties.

cross-linking glyoxal solution yields higher color strength with better fastness properties than untreated samples. Their results also proved that cotton yarns post-treated with Memecylon scutellatum biomordant and dyed with Combretum latifolium have much improved light fastness and wash fastness properties than control dyed samples. Furthermore, functional finishing imparting UV protection and antimicrobial properties to cotton fabrics is presently the subject of considerable research. Cotton-based textiles provide a suitable substrate for the growth of harmful bacteria and fungi. Scientists have more recently discovered that the antimicrobial properties and other functional characteristics of several natural colorants could be exploited to impart multifunctional properties to cotton. Rheum emodi and Lithospermum erythrorhizon roots were employed by Feng et al.100 to produce a UV protection finishing of cotton. Both of these dyes were able to absorb about 80% of the UVA and UVB rays, demonstrating excellent activity. Kim et al.,101 likewise, successfully applied polyphenolic compounds of green tea extract as a dyeing and UV protecting agent onto cotton in the presence and absence of a chitosan mordant. It was observed that chitosan-pretreated cotton fabrics display excellent UV protection and show a good dyeing property. They concluded that increasing the chitosan concentration results in more adsorption of catechin polyphenol onto cotton which is responsible for higher dye uptakes and enhanced UV protection. Ibrahim et al.102 studied the effect of fabric structure, type, and concentration of mordant and kind and percent of natural dye extract on the dyeing and multifunctional properties of cotton knits. Pitisak et al.103 applied tannin-based natural dyes extracted from Xylocarpus granatum bark to cotton. Before the application of the dyes, they utilized whey protein isolate as a pretreatment agent to cotton fabrics to improve dyeability, fastness properties, physical characteristics, and ultraviolet protection. Reddish-brown shades with good to excellent fastness properties to washing, water, seawater, perspiration, and hot pressing were observed for all the dyed samples. The mechanism proposed was based on a complex formation between whey protein isolate and tannin along with strong contribution or stabilization through hydrogen bonding and hydrophobic interactions. Boonroeng et al.104 modified curcumin with glycidyltrimethylammonium chloride to investigate metal-free dyeing and other functional properties onto cotton. The modified dyeing compound after application onto cotton pretreated with citric acid and sodium hypophosphite showed an excellent UV protection property which was maintained up to several home launderings. The use of alternative and innovative treatments to chemical methods in natural dyeing, particularly for application of natural dyes to cotton textiles, are gaining widespread interest worldwide. Several researches have been undertaken on the use of irradiation methods as alternatives to metal salt mordants. Radiation treatments offer energy savings, low environmental impact, and several other advantages over chemical approaches.105 Powders of onion shells and cotton fabrics were exposed to dosses of 2, 4, 6, 8, and 10 kGy using a Cs-137 gamma irradiator by Rehman et al.106 The extracted quercetin dyeing compound from onion shells was then applied to gamma ray-treated cotton. It was observed that resultant fabrics display satisfactory color and wash and light fastness properties. In another research investigation, Rehman et al.107 studied cotton dyeing using lawsone dye extracted from henna leaves with gamma radiations. The authors compared the



COLORED AND MULTIFUNCTIONAL SILK Silk is a natural fiber derived from a silk worm, Bombyx mori. Silk is mainly composed of sericin and fibrin natural macromolecular proteins which help in cocoon formation.110,111 Owing to its biocompatibility, biodegradability, and other useful characteristics, silk has been dyed and modified with numerous plant extracts over the past several years. Punrattanasin et al.112 studied the dyeing of silk fabric with mangrove bark using the exhaustion dyeing process. They employed premordanting, metamordanting, and postmordanting to modify the silk surface with different metal salts. Color parameters were described in terms of CIEL*a*b* (where L* describes lightness, a* measures redness or greenness, and b* measures yellowness or blueness) and color strength (K/S) values, and it was shown that pale to dark reddish-brown color shades are produced for unmordanted samples, while a wide range of other hues were developed with metal mordants. Torgan et al.113 studied the dyeing property of Helichrysum arenarium extracts on silk fabrics. They utilized several mordants such as alum, ferrous sulfate, stanium chloride, calcium nitrate, and potassium bitartrate to study their role in the dyeing process. The authors also employed reversed-phase high-performance liquid chromatography to identify the natural colorants in the dyed silk as well in extracts. They concluded that Helichrysum arenarium extracts produce good dyeing results on silk with good wash and light fastness properties. Mongkholrattanasit et al.114 in their research work studied dyeing, color, and wash fastness properties of silk and wool substrates treated with natural dye extracted from eucalyptus leaves. Their results showed that natural dyes from eucalyptus leaves can be successfully applied to silk to produce pale yellow to brown shades with good wash fastness properties. They also showed that the shade ranges can be extended to dark grayishbrown by the use of ferrous sulfate mordant with an enhancement in fastness properties. Madder (Rubia tinctorium L.) and walloon oak (Quercus ithaburensis) extracts have also been tested as potential dyeing materials on silk by Deveoglu et al.115 They used reverse-phase high-performance liquid chromatography to identify the colorants in dyed silk fabrics and reported their good dyeability after application onto silk. Ajmal et al.116 noticed that use of gamma irradiations is an efficient method to extract natural dye from pomegranate peel. They irradiated both pomegranate peel powder and silk fabrics with 20, 25, 30, 35, and 40 kGy doses and reported that gamma radiations at a dose of 40 kGy results in deep shades with acceptable fastness properties. They further found that dyeing 7457

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ACS Sustainable Chemistry & Engineering of silk with extracted colorant at 50 °C, 40 min, pH 6.0, and salt concentration of 10 g/L were the optimum conditions to get better dyeing results. Vankar et al.117 used onion skin available as kitchen waste and studied its dyeing property on silk. In the presence of 2% metal mordant, they noticed acceptable color and wash fastness properties imparted by 5% onion dye. Shankar and Vankar118 introduced textile dyeing including silk with an aqueous extract of Gulzuba flowers (Hibiscus mutabilis) in order to develop ecofriendly shades on selected fabrics. They evaluated the effect of mordants and material to liquor ratio and found that pretreatment with 2−4% metal mordants and M: L ratio of 1:40 (w/v) are optimum dyeing conditions to obtain high color strength and wash fastness properties. Lee and Kim119 carried out the dyeing of silk and other fibers with Cassia tora L. colorants and found that premordanting of silk with metal salts at 60 °C for 60 min results in good wash fastness properties. Likewise, Coffea arabica L. extracts were used to develop a variety of shades on a number of natural fibers including silk by Lee, and it was found that Coffea arabica L. in the presence of metal salt mordants induces a good dyeing and deodorizing property to silk. Mansour and Heffernan120 studied the dyeing property of Sticta coronata lichen onto silk using alum and catechu mordants. They found that the use of ultrasound results in higher dye uptakes, and the application of mordants alone or in combination impart high color strength, deep hues, and good light fastness to silk fabrics. More recently, Toussirot et al.121 investigated dyeing, coloring compounds, and antioxidant activity of leaves of Hubera nitidissima onto silk and other textile fabrics. Mangiferin and homomangiferin along with quercetin glycosides were found to be the main dyeing compounds and were reported to impart a yellow color and antioxidant activity along with a good light fastness property. Silk has been rendered antimicrobial by the use of selected natural extracts. Prusty et al.122 reported the preparation of colored and antimicrobial silk fabric by the use of four different natural dyes such as root bark of M. citrifolia, waste leaves of T. catappa and T. grandis, and waste leaves and heart wood of A. heterophyllus. The resultant silk fabric exhibited high antimicrobial activity against E. coli and fungal strain Aspergillus niger and showed good and durable wash fastness properties. Prabhu et al.123 investigated the coloration and antibacterial property of Emblica officinalis G. dried fruit as a biomordant on silk. The natural colorant-dyed silk using a biomordant showed good color strength and antimicrobial activity; however, the silk that was pretreated with an Emblica officinalis biomordant and metal mordant combination showed an improved dyeing effect with an enhancement in antibacterial activity against S. aureus and E. coli. Pan and colleagues124 reported the after-treatment of ferrous-rich mud onto silk previously dyed with a Chinese herbal extract to produce multifunctional silk. FTIR spectroscopic analysis of finished silk revealed the modifications induced by ferrous-rich mud by causing the disappearance of hydrophilic sites on one side of the silk fabric. In the presence of ferrous mud, it was noticed that the coated silk displays strong antibacterial activity against S. aureus and E. coli and had much improved antioxidant activity. Jia et al.125 studied the dyeing and multifunctional finishing of silk with natural extracts from chestnut shell and black rice bran. Their results showed that the ratio of selected natural plant extracts and the type of metallic mordants used had a significant influence on color, ultraviolet protection, and antioxidant properties of dyed silk. It was demonstrated that the deep shades along with good UV protection performance (UPF > 30) and antioxidant activity

could be imparted to silk at pH 3 using a combination of both chestnut shell and black rice bran extracts. In a very recent work, Yin et al.126 also reported a new natural colorant for silk dyeing by extracting anthocyanins from purple sweet potatoes using an ultrasound-assisted ammonium sulfate/ethanol system. A color strength value of 4.5 was obtained on silk, and it was found that the use of alum mordant improved the dyeing efficiency of extracted anthocyanins by more 40% compared to direct dyeing. The dry wet fastness grading was also found in the acceptable range. A bacterium strain of Vibrio sp. isolated from marine sediments could produce bright red colorants of prodiginine compounds, which could be employed in dyeing of protein and synthetic fibers as well as providing antibacterial functions.127,128 In general, natural extracts containing anthraquinone, prodiginine, and polyphenol structures could absorb visible light and also provide antimicrobial functions if they are leached out from the fabrics. Some of natural colorants of polyphenols and carotenoids will bring antioxidant functions to colored textiles, while most of the colorants could also improve the UV (UVA or UVB) protective function as well.



SYNTHETIC TEXTILE MATERIALS Synthetic textiles such as polyester and nylon have been predominantly functionalized by using disperse dyes, acid, and vat dyes.129,130 So far little attention has been focused on the application of colorants derived from renewable sources onto synthetic textile surfaces. It is worth mentioning that textile and polymer scientists have carried out some research over the past few years in this realm. Among different natural dye sources, the Bixa orellana plant is well known for its yellow dye production. The carotenoid bixin dye extracted from seeds of this plant has been significantly studied as a dyeing principle for different textile substrates. Gulrajani et al.131 investigated the dyeing studies and mechanism of Bixa orellana dye on nylon and polyester and discovered that annatto colorants had good affinity for both these synthetic textiles and imparted moderate wash fastness properties. A high-temperature dyeing method was employed by Raisanen et al.132 to study the dyeing property of emodin and dermocybin compounds isolated from the fungus Dermocybe sanguinea. It was interesting to observe that emodin produced bright yellow and dermocybin produced bright reddish-orange shades on polyamide with excellent fastness properties. Likewise brownish-orange and wine-red hues with moderate fastness properties were obtained onto polyester fabrics with respective colorants. Gulrajani et al.133 also applied ratanjot (Onosoma echoides) natural dye on nylon and polyester fabrics to understand its mechanism and dyeing properties. A partition mechanism was found to contribute to the dyeing rate, and it was observed that ratanjot produces black shades on nylon and pink shades on polyester with good colorfastness to light and washing. Gupta et al.44 applied purpurin (1,2,4-trihydroxyanthraquinone), a coloring compound present in the roots of Indian madder (Rubia cordifolia), onto the nylon surface. Various dyeing parameters such as dye uptake, diffusion coefficient, standard affinity, heat of dyeing, and entropy were determined, and it was observed that nylon can be effectively dyed at 70 °C. In a similar study, Gupta and colleagues134 evaluated the dyeing properties of nordamncanthal, a coloring compound present in the roots of Indian madder, on nylon and polyester. It was shown that the application of Indian madder coloring compound behaved 7458

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ACS Sustainable Chemistry & Engineering Table 3. Some Recent Applications of Renewable Colorants To Develop Colorful and Multifunctional Fabrics Author

Extracted from

Fiber

Function imparted

ref

Shahid et al. Ticha et al. Ren et al. Nazari Jose et al. Zhou et al. Baaka et al. Rather et al. Mariselvam et al. Nakpathom et al. Zhou et al. Adeel et al. Ozdemir Hou et al. Rym et al.

Lonicera japonica Parthenocissus quinquefolia Prodigiosins nanomicelles walnut hull and henna A. catechu, A. tinctoria, and C. longa Scutellaria baicalensis Tamarix aphylla Acacia nilotica Syzygium cumini Camellia oleifara Curcuma longa L Tagetes erecta Juniperus oxycedrus L. Sorghum husk extracts Vitis vinifera L.

silk cotton, wool, and silk polyester wool silk, cotton silk cotton wool cotton cotton silk cotton wool wool cotton

color, antioxidant activity color color, antimicrobial activity mothproof activity color antioxidant, antibacterial, and UV protective properties color color, antioxidant activity antimicrobial activity pigment printing color, antibacterial, and UV protection color color color, UV protection, and fluorescence color

138 139 140 141 142 143 144 53 145 146 147 148 149 150 151

Figure 1. Schematic representation of possible mordant and natural dye extract interactions: (a) cellulose and (b) wool.

more like a disperse dye and exhibited good affinity to both these fibers. Joshi et al.135 in a research experiment modified a polyester/cotton blend fabric using seeds of the neem tree (Azadirachta indica) to produce antimicrobial activity in the blended fabric. To get maximum cross-linking in the blended fabric with glyoxal, they optimized the resin and catalyst concentrations prior to the application of neem extract. They observed that the treated fabric had high activity against Grampositive bacteria (Bacillus subtilis) and Gram-negative bacteria (Proteus vulgaris). The coloring potential of onion (Allium cepa), lac (Laccifer lacca), and turmeric (Curcuma longa) natural colorants on nylon have been assessed by Lokhande and Dorugade.136 On the basis of their results, a good wash fastness grading of 4 was observed for all the tested colorants, and a lightfastness rating of 4−5 was achieved for onion and lac, while the turmeric dye showed only 3−4 ratings. Guesmi et al.137 reported the use of a sonicator for studying the coloration of modified acrylic using natural indicaxanthin pigments extracted from fruits of Opuntia ficus-indica. The dyeing of modified fabric was carried out at different pH values, salt concentrations, temperatures, durations of dyeing bath, and ultrasonic powers in order to examine their influence on the coloring potential of

indicaxanthin dye. Optimum dyeing conditions to get deep shades with high color strength values were found at 80 °C, dyeing time of 30 min, and pH 3. Park et al.51 carried out the dyeing of poly(ethylene terephthalate) fabrics with an aqueous extract of Caesalpinia sappan L. wood using chitosan and plasma pretreatment. They observed that the aqueous extract of C. sappan L. wood had low affinity with poly(ethylene terephthalate) fabrics resulting in fair to good wash fastness properties. They concluded that the pretreatment with chitosan or low-temperature plasma improved the uptake of C. sappan L. wood and produced deep shades with good wash fastness properties. Some recent applications of renewable colorants to develop multifunctional textiles are shown in Table 3.



DEVELOPMENT OF BIOMORDANTS TO REPLACE TOXIC METALLIC SALTS Mordanting is one of the important processes that is extensively employed in natural dyeing technology to fix various types of natural extracts to textile surfaces.152,153 Various types of mordanting methods such premordanting, simultaneous mordanting, and postmordanting have been developed over the past few decades to produce beautiful shades and improved 7459

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ACS Sustainable Chemistry & Engineering

Figure 2. Chemical structures of some important polyphenolic biomordants.

dye uptakes.13 Metallic salt mordants such as aluminum potassium sulfate, cobalt sulfate, magnesium sulfate, potassium dichromate, stannous chloride, ferrous sulfate, zinc chloride, copper sulfate, tannic acid, and oil mordants are commonly being used because of their strong metal complex-forming ability and easy availability.154,155 Most of the plant dyes are known to form stable complexes with the metal ions resulting in enhanced dye fixation as well as improved wash fastness properties.156,157 The possible interactions of mordants with natural dye extracts are depicted in Figure 1. The use of mordants in natural dyeing has endowed textiles with novel and durable functionalities and boosted their role in the development of clothing and textiles for various end users including the sportswear, fashion apparel, medical sector, and carpet industries. Nevertheless, their usage in dyeing applications has been reduced recently because of the consumer’s enhanced awareness about the toxic and carcinogenic effects associated with certain metal mordants.11,158 Therefore, there has been a growing need to explore alternative environmentally benign agents for functional modification of textile materials. Considering these facts, biomordants exploited from medicinal plants emerge as an attractive alternative to metal mordants because of their remarkable properties such as increased

sustainability, abundant availability, soft chemistry, renewability, and inherent antimicrobial activity.159 New biomordants from various plant species are gaining importance. Vankar et al.92 studied the application of E. acuminata DC as a source of biomordant to cotton afterward dyed with Rubia cardofalia dye for the development of colored fabrics. They found that E. acuminata biomordant is rich in alum content and had improved dye uptake by 23.5% and also enhanced colorimetric data and wash fastness properties. In 2009, Vankar and Shankar160 reported the potential of Pyrus pashia biomordant plus enzyme pretreatments for silk dyed with Delonix regia extract. Pyrus pashia species containing copper ions produced good color strength and higher color coordinate values after application on silk. In 2014, Ismal and co-workers161 examined the potential of wool dyeing with almond shell extracts in the presence of biomordants derived from Quercus ithaburensis, Punica granatum rind, Rosmarinus officinalis, and Thuja orientalis. They compared their results with metallic salts and noticed that R. officinalis can be used as an alternative to alum (premordanting and postmordanting), iron II sulfate (premordanting), and copper II sulfate (premordanting, simultaneous mordanting, and postmordanting). The other biomordants selected in their work produced shades similar to that of 7460

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washing cycles when only tamarind seed-coated tannins were used as biomordants. By using tamarind seed-coated tannins in combination with copper sulfate, the washing durability of antibacterial activity was found to be enhanced significantly up to 20 laundering cycles. It is very important to mention here that the application studies of biomordants are only at a preliminary level and conducted on laborator- scale experiments. More research to explore newer sources of biomordants and to investigate their multifunctional effects instead of focusing only on coloration is highly important to increase market readiness and make them widely acceptable in modern houses.

alum and were effective to improve both light and wash fastness properties. Ismal et al.162 likewise reported the use of an ultrasonic bath for the dyeing of wool using the same colorants and biomordants. It was shown that ultrasound did not produce any considerable change in color yield increment but improved wash fastness properties. Guesmi et al.72 introduced the first application of chlorophyll-a as a biomordant for wool dyed with betanin dye and found that chlorophyll-a at acidic pH significantly enhanced the dyeability of wool. Chattopadhyay and co-workers163 have described good color strength and excellent wash fastness properties of jute fabrics obtained by the double mordanting method. They used Terminila chebula and Punica granatum biomordants in combination with chemical mordants for jute dyeing with manjistha, annatto, babool, and ratanjot natural dyes and found that the combination imparted very good UVB protection properties. In 2016, Rather et al.164 tested the biomordant potential of gallnut, pomegranate peel, and babool bark on wool and compared their results with metallic salt mordants. Their study showed that wool dyeing with biomordants offer similar color strength and light and wash fastness properties to that of metal salt mordanted samples. Yusuf et al.23 used Rubia cordifolia dye to develop a range of beautiful colorful shades with acceptable light and fastness properties on wool yarns previously premordanted and postmordanted with different concentrations of gallnut extract (viz., 1−5%). They found that premordanting imparts better colorimetric and wash fastness properties than the postmordanting method. Mansour et al.165 investigated the biomordant effect of tannic acid and pomegranate peel extract onto linen and jute dyed with an aqueous extract of Vitis vinifera L. leaves. They obtained a wide range of soft and light colors with excellent washing and rub fastness properties and good to excellent light fastness properties. The chemical structures of some polyphenolic biomordants are shown in Figure 2. To date, all the studies that investigated the anchoring ability of biomordants only paid attention to their dyeing and fastness properties. There are only a few studies reported in the literature that evaluated the role of natural mordants in the development of multifunctional textiles. Haji166 investigated the biomordanting potential Rumex hymenosepolus onto wool followed by their application with berberine colorant extracted from Berberis vulgaris. Their results showed that the tannins present in Rumex hymenosepolus could interact with the functional sites in wool and colorant molecules and had the ability to improve wash, light, dry, and wet fastness properties. The biomordant treatment also resulted in high antibacterial activity of wool fibers. Prabhu et al.123 applied Emblica officinalis G. dried fruit tannins as an ecofriendly biomordant to cotton and silk afterward dyed with natural dyes and observed that treated fabrics had high color strength and good activity against S. aureus and E. coli compared to control dyed fabrics; however, the imparted antibacterial activity was lost on washing. By using Emblica officinalis with 0.5% or 1% copper sulfate mordant, they claimed that the antibacterial activity was maintained even up to 20 consecutive washing cycles. Prabhu and Teli,167 likewise, studied the application of turmeric and pomegranate rind natural dyes to cotton, wool, and silk previously premordanted with tamarind seed coat tannins alone and in combination with copper sulfate mordant in order to develop colored and antibacterial fabrics. They reported that the dyed fabrics had high color strength values and were highly active against S. aureus and E. coli, and the activity was maintained up to five



CURRENT CHALLENGES AND FUTURE DIRECTION Presently, the use of colorants from natural materials in the coloring of wool, cotton, and silk as well as leather is becoming popular because of the environmental concerns arising due to excessive use of synthetic dyes. With the exception of only few such as indigo and logwood, most of the colorants extracted from natural sources have poor to moderate light fastness properties and hence prevent their use in modern day applications.168,169 Therefore, understanding the fading of dyestuffs by light radiation is one of the hot topics in the scientific community today. Recent research in this area has suggested that chemical structures of dyes and pigments, dye concentration, nature of fibers, and nature of mordants are important parameters which have been discovered to have influence on photofading of natural dyes. Literature has shown that several studies have been undertaken to investigate the light fastness of natural colorants used in textile coloration and finishing. The photofading of two dyeing compounds, namely, purpurin (1,2,4-trihydroxyanthraquinone) and munjistin (1,3dihydroxy-2-carboxyanthraquinone) isolated from Indian madder roots, on nylon have been investigated by Gupta et al.170 However, there has been little emphasis on improving the light fastness of natural colorants, with only a handful of recent studies investigating the application of UV absorbers to prevent the photofading. One way to control the photofading of natural dyes can be done by metallic salts employed during functional finishing. By using metal salt mordants, strong metal dye complexes are formed thereby preventing photofading. A study conducted by Crews171 showed that wool fabrics dyed with 18 natural colorants including some yellow dyes such as turmeric, fustic, and marigold previously premordanted with certain metal salts faded less significantly than unmordanted wool. In another research work, Lee and colleagues172 studied the use of sulfonated hydroxybenzotriazole UV absorber (UVA 1) and sulfonated hydroxybenzophenone UV absorber (UVA 2) to improve the light fastness property of mordant dyed wool and silk with goldthread, amur cork tree, gromwell, and redwood natural dyes. They showed that the UVA aftertreatment in the presence of mordants exhibited improved photostability of selected colorants. Also, to improve light fastness of carthamin natural colorant, Oda173 in a series of reactions evaluated the role of nickel hydroxyarylsulfonates in suppressing the rate of photofading of carthamin. It was shown that the presence of substituents in the nickel sulfonate complexes, mainly the hydroxyl groups, have more beneficial effects. In another investigation which was conducted by Cristea and Vilare,174 the effect of various UV absorbers and antioxidants on light fastness of madder, weld, and woad natural dyes were determined. After treatment with UV absorbers and antioxidants, an increase in light fastness of dyed cotton was observed. It is worth pointing 7461

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ACS Sustainable Chemistry & Engineering out that vitamin C and gallic acid were most effective agents to improve the light fastness of selected dyes. Considering these facts, improving the light fastness of natural dyes using UV absorbers has opened a new field for researchers working in this field. More studies are still required to explore other green UV agents before it can be predicted that UV absorbers and aftertreatments can improve the light fastness of colorants derived from renewable sources and may offer full chances in their revival for the development of colorful and multifunctional textiles in the near future.

Shahid-ul-Islam received his Ph.D in Chemistry from Jamia Millia Islamia (A Central University), India, in 2016. He then joined the Department of Textile Technology at Indian Institute of Technology Delhi (IITD) where he is currently DST-SERB National Postdoctoral Fellow. His research interests include green chemistry, dyes and pigments, thermodynamics and kinetics of colorants, and textile finishing using polymeric nanocomposites. He is a recipient of several academic awards and competitive fellowships. He has published numerous peer-reviewed research articles in journals of high repute including contributions to several internationally recognized books published by John Wiley & Sons, Springer, and Elsevier. He is on the editorial board of two international journals and also associated with many professional activities such as a regular reviewer of many topranked journals published by Springer Nature, American Chemical Society, Wiley, and Elsevier.



CONCLUSIONS Undoubtedly, the search for new colorants extracted from natural renewable sources for the development of colorful and multifunctional textiles has become the major focus of textile and polymer scientists. In recent years, many researchers have tried to elucidate the dyeing mechanism by investigating the thermodynamic and kinetic aspects of plant extracts. It is noteworthy to mention that adsorption of plant-derived colorants on the surface of natural and synthetic textile materials is created due to electrostatic forces arising between functional finishing agents and textile surfaces causing relatively strong bonding interactions. Renewable colorants have opened new interesting fields and have raised the interest of researchers to produce fabrics with deodorizing, antimicrobial, UV protective, and antioxidant properties. Despite the notable progress made in this field, there are still many obstacles to be addressed before dyeing with natural or herbal extracts can be accepted in modern textile industries. The use of new advanced techniques for isolation and identification, the use of novel UV absorbers and after-treatment agents to improve fastness properties, and the designing of proper standardization techniques for dyeing of natural and synthetic textile materials are probably the next few steps needed to fill existing gaps in this area. Moreover, more studies are also required to evaluate toxicity and safety issues which can make natural textile dyeing less vulnerable and widely acceptable.



Gang Sun is a professor of Fiber and Polymer Sciences in Division of Textiles and Clothing at University of California, Davis, and has been conducting research on functional textiles, textile chemistry, and nanotechnologies since 1995. Most of his research activities and efforts have been devoted to development of novel antibacterial textiles and polymers for personal protection equipment, including medical, chemical, and military protective clothing and products. He is a recipient of the Olney Medal in 2016, the highest science award by American Association of Textile Chemist and Colorist (AATCC). He has published over 200 peer-reviewed journal articles and is serving on editorial boards for several major textile professional journals.

AUTHOR INFORMATION

Corresponding Author

*Tel: +91-9716136386. E-mail: [email protected].



ORCID

ACKNOWLEDGMENTS Authors express their appreciation to the Science and Engineering Research Board (DST-SERB), India, for providing a National Postdoctoral Fellowship (Grant PDF/2016/ 003859) to Shahid-ul-Islam.

Shahid-ul-Islam: 0000-0001-9198-7530 Notes

The authors declare no competing financial interest. Biographies



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(1) Zhuo, J.; Sun, G. Antimicrobial Functions on Cellulose Materials Introduced by Anthraquinone Vat Dyes. ACS Appl. Mater. Interfaces 2013, 5, 10830−10835. (2) Montazer, M.; Pakdel, E. Functionality of nano titanium dioxide on textiles with future aspects: Focus on wool. J. Photochem. Photobiol., C 2011, 12, 293−303. (3) Shahid, M.; Mohammad, F.; Chen, G.; Tang, R.-C.; Xing, T. Enzymatic processing of natural fibres: white biotechnology for sustainable development. Green Chem. 2016, 18, 2256−2281. (4) Hou, A.; Feng, G.; Zhuo, J.; Sun, G. UV light-induced generation of reactive oxygen species and antimicrobial properties of cellulose fabric modified by 3, 3′, 4, 4′-benzophenone tetracarboxylic acid. ACS Appl. Mater. Interfaces 2015, 7, 27918−27924.

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DOI: 10.1021/acssuschemeng.7b01486 ACS Sustainable Chem. Eng. 2017, 5, 7451−7466