Natural colorants and its recent developments

Natural colorants and its recent developments

9

Saminathan Ratnapandian Indian Institute of Handloom Technology, Salem, Tamil Nadu, India

9.1

Introduction

There is a growing awareness regarding the effect of human interaction, both short and long term, with the environment (Nicholson, 1987). The growing public awareness has been expressed through an expanding premium market for goods and services that carry “natural” or “eco-safe” or “green” or similar labels (Bansal and Roth, 2000; Esty and Winston, 2009). This provides an excellent focus point for the textile sector. Sustainable clothing includes all aspects of clothing manufacture and recycling. This chapter describes coloring matter derived from nature and highlights advancements in their applications onto textile substrates. The textile coloration sector uses dyes (coloring matter), chemicals (whose reaction ensures the coloring of textiles), and auxiliaries (chemicals that promote coloration) in addition to consuming large quantities of water. Unfixed dye, chemicals, auxiliaries, and water not picked up by the fiber material are collectively discharged at the end of coloration as effluent. Marine life would be adversely affected if these effluents, usually colored, were discharged without treatment into the water body (Walsh et al., 1980). Further, such effluents affect riverbeds, soil, and crops (Ajmal and Khan, 1985; Kaushik et al., 2005). The intimate relationship between textile coloration and environment offers enormous scope for development on the eco-conservation trend (Reid, 1996; Abrar et al., 2009). Efforts in this area may be summarized as: (1) Reducing the quantity of consumables by increasing process efficiency and reclaiming and reusing resources such as chemicals, water, and energy. (2) Using materials and processes with a lower environmental impact; and (3) Increasing the use of renewable resources such as natural fibers (silk, cotton, and wool) and natural dyes.

The growing niche market for sustainable products is prompting the reintroduction of natural dyes on a commercial scale. However, the dominance of synthetic dyes for the past 150 years has stifled in-depth studies on industrial-scale use of natural dyes. Hobbyists and craftspeople, a minority, who continue to use natural dyes for textile coloration, adhere primarily to the conventional home-scale exhaust dyeing method. Such methods are not readily compatible with commercial application methods. This chapter highlights the types of dyes used in textile coloration. A detailed description has been given on the type of natural dyes, their chemical structure, sources

Sustainable Technologies for Fashion and Textiles. https://doi.org/10.1016/B978-0-08-102867-4.00009-8 Copyright © 2020 Elsevier Ltd. All rights reserved.

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of extraction, and application methods. In addition, the recent developments and assisted techniques in natural dyeing are also included in this chapter.

9.2

Textile coloration

Colors not only increase the beauty of textile but also serve to identify, warn, or even indicate social status. Therefore, textile coloration has developed into an art and science. Textile materials acquire the desired color(s) by the application of appropriate dyes.

9.3

Dyes

Coloration of textile is achieved by application of dyes. Dyes are complex chemicals that are absorbed by and react with suitable substrates to yield a colored product. This process includes various methods and machineries. There are two broad dye categories based on origin, namely natural dyes and synthetic dyes. J. W. Slater published the first book in 1870 that provided comprehensive data on dyes and pigments used in textile coloration (Burdett, 1982). Today this information is available as the Color Index (CI) published jointly by the Society of Dyers and Colourists (SDC) and American Association of Textile Chemists and Colourists (AATCC) (Clark, 2011). The index scientifically classifies the wide variety of dyes based on chemical structure, color, application methods, fastness properties, manufacturer, synthesis route, and date invented.

9.3.1

Synthetic dyes

Dyes employed in the modern industry are termed as synthetic dyes because they are synthesized or manufactured to specifications usually from petroleum derivatives. These dyes are characterized by low cost, wide variety, standardized application method, repeatable results, and stringent performance standards (Trotman, 1984; Zollinger, 2003; AATCC, 2012; Standards-Australia, 2012). Consequently, this is a US$16.2 billion global industry (Freedonia-Group, 2009). Various synthetic dye classes and their prominent features are depicted in Fig. 9.1 (Ratnapandian, 2013). The scope of this chapter does not allow to discuss in detail on synthetic dyes. Hence, a detailed discussion of natural dyes will be covered in this chapter.

9.3.2

Natural dyes

The name natural dye applies to all coloring matter derived from natural sources, such as plants, animals, and minerals. The use of natural dyes has been recorded across the globe since ancient times. They were the only dyes available and used until 1856 when Sir W. H. Perkin discovered the first synthetic dye “mauviene,” a basic dye. Plants, animals, or minerals form the source for natural dyes. The sources have been identified across the globe since ancient time (Bechtold and Mussak, 2009; Ramakrishna, 1999). Tyrian purple is a well-known example used by Greek and Roman emperors. The use

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Synthetic dyes

Direct – simplest, wide variety of colors

Vat – insoluble in water, leuco form used to achieve dyeing

Sulfur – deep muted shades, dyes have complex structures

Azoic – bright shades formed in situ by coupling reaction

Reactive – fast colors, react with substrate, mono and bi-functional dyes Acid – used to dye wool, silk and nylon, classified as mordant and premetallized dyes Disperse – used mostly for polyester, requires unique application process

Basic – used in acrylic dyeing, cationic in solution

Pigments – insoluble in water, requires binders

Fig. 9.1 Classification of synthetic dyes.

of natural dyes started to decline after the discovery of “mauviene,” a basic dye by W. H. Perkin in 1856 (AATCC, 2012; Standards-Australia, 2012). Dye extraction and purification uses simple techniques such as boiling and filtering. Usually complex procedures are not needed. Indigo, however, is an exception. The dependence on nature imparts unavoidable inherent variation to quality and quantity to these dyes. Hence, repeatability in shades is possible by skillful dyers who utilize complex recipes. Generally, it is accepted that natural dyes yield unique shade for every batch. Researchers have demonstrated that natural dyes can produce diversity of colors similar to synthetic dyes. Today the Color Index includes natural dyes and classifies them in scientific manner (Bechtold and Mussak, 2009; Cardon, 2007; Hummel, 1898). The degree of affinity to textile forms the basis of classifying natural dyes into being either adjective or substantive. Substantive dyes, with a high affinity for textiles, maybe classified as vat, acid, direct, and pigment classes (Hummel, 1898; Patel, 2011). However, these are a minority, and majority require the use of mordants to fix the dye on to the fabric. Such dyes are called as mordants or adjective dyes (Gulrajani and Gupta, 1992). Mordants were believed to eat away the surface or open up the pores in a fiber and thus facilitate dye absorption (Bhattacharyya, 2010).

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They have now been identified as “bridging chemicals” that link dyes and fibers (Cardon, 2007; Bhattacharyya, 2010). Mordants, in turn, may be classified as single or bi-metallic salts, tannins, and oil mordants (Patel, 2011). Mordant dyes may further be divided into monogenetic and polygenetic dyes (Fereday, 2003; Bhattacharya and Shah, 2000; Bird, 1963). As the names suggest, the first type of dye yields only one shade, while the second yields different colors according to the mordant (metallic) employed (Hummel, 1898). Naturally occurring salts of aluminum, copper, potassium, and iron were commonly employed as mordants. Their quality, in terms of purity, affects the richness and durability of the shades obtained (Cardon, 2007). Advancements in chemistry have made aluminum sulfate, potash-aluminum sulfate, stannous (tin) chloride, copper (II) sulfate, and iron (II) sulfate to become the mordants of choice today. Chromium salts have been banned because of potential health hazard (Giles, 1944). At times, tanninemetal complexes and oilemetal complexes served as mordants for other natural dyes (Nalankilli, 1997). Mordants facilitate dye aggregation and free radical absorption, thereby increasing fastness properties (Gupta, 1999a). Gulrajani and Bhattacharyya have cited that mordanting may be classified as premordanting, metamordanting and postmordanting, which are discussed below (Gulrajani and Gupta, 1992; Bhattacharyya, 2010): PremordantingdIn this process, textile material is first treated with the mordant and then dyed. Intermediate drying allows storage of the mordanted material and helps in absorption of the dye liquor. Multicolored pattern may be obtained by using polygenetic dyes in combination with patterns printed with different mordants. Prolonged storage of premordant textiles can cause damage if the metal is corrosive. Another source of color variation is leaching of mordant into the dye bath. MetamordantingdIn this process, fabric is dyed in a single bath containing both mordant and the dye. Uneven dyeing is caused by aggregation of dye-mordant complex in the dye bath and the inability of resultant large complex to be absorbed by the substrate. PostmordantingdThe mordant is applied after the dye has been applied. This method yields deeper penetration of the dye, hence, a level shade. Prolonged storage of dyed fabric prior to mordanting has to be avoided in order to minimize loss of color by fading or rubbing. An intense area of research is that of determining the optimum amount of mordant required and developing a theory that explains the precise role of the mordant. The problem faced is complexity of dye molecule that hinders understanding of the binding mechanism and exact role of the mordant (Cardon, 2007). The major source for natural dyes is plants. In many cases, different parts of the same plant such as leaves, flowers, seeds, roots, and bark have been reported to yield different shades. Table 9.1 lists some typical colors obtained using natural dyes and their plant sources (Cribb and Cribb, 1981; Dyer, 1976). Mineral colors are naturally occurring metal salts such as chrome yellow, iron buff, and mineral khaki. Two ancient animal dyes are Tyrian purple obtained from shellfish (Murex brandaris) and cochineal obtained from insects (Dactylopius coccus). Lichen and fungi are also known to yield dyes but their slow rate of growth severely limits commercial

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Table 9.1 Typical shades of natural dyes. Shade

Source

Chemical group

Blue

Indigofera tinctoria

Indigo

Red

Caesalpinia sappan

Anthocyanin

Yellow

Bougainvillea glabra

Flavonoid

Green

Urtica dioica

Chlorophyll

Black

Haematoxylum campechianum

Brown

Acacia catechu

Tannins (Prorobinetinidin and Profisetininidin)

Orange

Bixa orellana

Carotenoid

exploitation. Some of the important chemical groups present in natural dyes are given in Fig. 9.2 (Ratnapandian et al., 2012).

9.3.2.1

Indigoid dyes

These dyes consist of indigo derived primarily from the indigofera, isatis, and polygonum species (Cardon, 2007; Balfour-Paul, 2006). Tyrian purple extracted from murex species of mollusks is another noted member of this group. The main shade obtained is blue. Indigoid dyes were the primary vat dyes with a typical two carbonyl (C¼O) structure. Consistent with existing practices, dyeing was possible only after O

O

N

O

N HO

N H

CH3

O

Anthocyanidin

Indigo

Betacarotene OH OH OH

Prorobinetinidin

O

OH OH OH

OH

O

OH

Profisetininidin

OH

Fig. 9.2 Typical chemical groups of natural dyes.

CCH3

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conversion into the leuco form. Usually this was brought about via inconsistent fermentation (Bechtold and Mussak, 2009; Cardon, 2007; Patel, 2011; Agarwal, 2011; Buhler, 1951; Vetterli, 1951).

9.3.2.2

Flavonoids

Weld, saw-wort, and fustic are the popular sources for these dyes. The chemical involved is luteolin with resultant yellow shade. It may be noted that flavonoids form the largest group of natural dyes. Luteolin is the primary colorant in these dyes. The major disadvantage is poor fastness to light. This has been proved by the selective fading of old tapestries (Bechtold and Mussak, 2009).

9.3.2.3

Anthocyanins

Red and blue color of flowers and fruits are the result of anthocyanins. These anthocyanins are water-soluble flavonoids widely employed as water color. The shade obtained is sensitive to pH and light (Melo, 2002). In the case of anthocyanidins the glycosides of the anthocyanins are replaced by hydroxyl groups. Logwood is an excellent example of this class of dyes.

9.3.2.4

Carotenoids

These colorants are named carotenoids as they were initially isolated from carrots. These are direct dyes producing yellow to orange-red shades. Common sources are carrots, sweet peppers, turmeric, and tomatoes, while saffron is an expensive source (Cardon, 2007).

9.3.2.5

Anthracenes

Safflower, madder, and cochineal are the well-known sources for dyes containing anthracenes. Diverse sources, such as plants (flowers, bark, and roots), lichens, and insects, exist for these dyes. The two main groups of this class are (1) anthraquinones with yellow, pink, and red pigments (Saidman et al., 2002) and (2) napthoquinones with brown, pink, or purple pigments (Gulrajani et al., 1992).

9.3.2.6

Chlorophyll

This is the essential green pigment used by the plants for photosynthesis. Commercially chlorophyll a and chlorophyll b are available in this class of dyes. Difficulty in application and poor light fastness (browning on exposure to light) make chlorophyll unpopular as a textile dye (Patel, 2011).

9.3.2.7

Tannins

Tannin is the term introduced by Seguin in 1790 to describe plant extractions that can convert rawhide to leather. They are primarily used for converting hides to leather. Their reactivity is rated based on the astringency to touch, taste, and the propensity

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to give a greenish or bluish-black hue in the presence of iron salts. Galling or sumaching is a textile application of this reaction, wherein the tannin serves as a mordant. At times they serve the dual purpose of mordant and colorant due to their close association with some of the other coloring groups such as flavonoids and quinones (Haslam, 1966). Tannins are identified as polyphenols. They may be subclassified as hydrolyzable (gallo-tannins and ellagi-tannins) and condensed (proanthocyanidins) tannins.

9.4

Fabric coloration

Textile coloration can be carried out at fiber, yarn, fabric, and garment stages. Fabric stage coloration is the most popular. This is achieved by employing a wide variety of machines and methods. Dyeing and printing are the broad classifications of textile coloration. A simplistic description of dyeing is that the fabric acquires a single color uniformly throughout. Similarly, printing may be defined as a process where one or more colors are applied in a desired pattern on the fabric (Clark, 2011; Broadbent, 2001). Fig. 9.3 schematically describes the different methods of dyeing and printing (Ratnapandian, 2013; Clark, 2011; Broadbent, 2001; Chakraborty, 2010).

9.5

Advancements in natural dyeing

A knowledge gap exists on how to use natural dyes in modern dyeing facilities. Their reduced use over the last 150 years has resulted in: •

Loss of knowledge about extraction and application process due to absence of written records Fabric coloration

Dyeing Continuous (padding)

Printing Batch-wise (exhaust)

Liquor circulating

Roller

Material circulating

Combined

Fig. 9.3 Fabric coloration methods.

Screen

Inkjet

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• •

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Extinction of some dye sources and nonidentification of new sources Slowing down of scientific research on natural dyes, leading to stagnation in recipe development, modernization, and optimization.

The increasing demand for natural dyes has produced scientific publications relating to dye sources, extraction and application methods, dye chemistry, and performance of natural dyes (Gulrajani and Gupta, 1992; Ratnapandian et al., 2012). The following subsections discuss some of these findings.

9.5.1

Sources and extraction

A wide variety of plant sources for natural dyes have been identified (Cardon, 2007; Cribb and Cribb, 1981; Dyer, 1976; Adrosko and Furry, 1971; Dean, 1999). In India alone, Mohanty et al. (1987) have described more than 300 potential sources. The plants range from undesirable weeds to those that can be cultivated. Efforts have been made to identify the plant parts such as leaves, flowers, bark, seed pods, and roots that yield the maximum color (Cribb and Cribb, 1981; Dyer, 1976). New sources namely by-products and waste products such as wood chips and sawdust (timber industry), grape pomace (wine making), and olive pomace (oil extraction) have been reported. (Bechtold et al., 2006; Meksi et al., 2012). Recombinant DNA technique is being explored to induce microorganisms to produce dyes (Nerurkar et al., 2011; Alihosseini, 2009; Han et al., 2008a,b). Simplest dye extraction technique is steeping, boiling, or fermentation followed by simple filtration, settling, and evaporation to obtain the final dye in paste, cake, granule, or powder form. This process not only reduces cost of transportation of raw material but also yields standardized product. Clustering of extraction and application industries would prove economically beneficial (Bechtold and Mussak, 2009). Spraydrying and supercritical carbon dioxide (SCCO2) extraction are the new developments in this sector. Commercial quantities of some natural dyes are now available. Companies such as Alps Industries and Atul Dyes in India are advertising availability of large quantities of natural dyes (Hill, 1997; Anonymous, 2001).

9.5.2

Application techniques

Traditionally, natural dyes are applied by long-liquor exhaust dyeing for textile coloration and hand or simple mechanical methods for printing. These methods are laborious, time-consuming, and sometimes convoluted compared to present-day coloration techniques (Broadbent, 2001). Recipe collection authored by enthusiasts are available for use by hobbyists (Dyer, 1976; Flint, 2008; Adrosko and Furry, 1971). Self-help groups conduct classes to promote use of natural dyes as source of income. Numerous conventions are held to popularize the natural-dyeing concept. Interest groups conduct classes to disseminate their knowledge as well. However, a common disclaimer among these protagonists is “These are my results. Your results will vary and you are welcome to try your own version.”

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Exhaust dyeing recipes have been reexamined, optimized, and standardized. The efforts in this area have eliminated unnecessary steps and auxiliaries, reduced the dyeing cycle time, and increased shade consistency and repeatability (Cardon, 2007; Chavan, 1995; Chavan, 2001; Gulrajani and Gupta, 1992; Gulrajani, 1999). The compatibility of natural dyes and commercial exhaust dyeing have been evaluated and determined to be feasible by Bechtold (Bechtold and Mussak, 2009). Cardon (Cardon, 2007) has collected and published information about natural dye chemistry and application techniques from across the globe. Gulrajani (Gulrajani and Gupta, 1992; Gulrajani, 1999) has evaluated diversity of colors that can be obtained by utilizing and reports that natural dyes are on par with synthetic dyes in this regard. The efforts of such researchers are providing increasing comprehension about chemistry of dyes and their interaction with textile substrates. In accordance with present-day demands, substrates not only include natural fibers but also include regenerated and synthetic fibers (Cardon, 2007; Gulrajani and Gupta, 1992; Kumaresan et al., 2012; Bechtold et al., 2003). Continuous dyeing method by padding technique has been investigated for coloration of textiles using natural dyes. Mordanting sequence, dye to mordant ratio, and padding parameters were the critical elements in these studies. The researchers concluded that the repeatability and uniformity of shade was possible using natural dyes from this technique. However, large-scale (industrial) trials and detailed work are necessary for individual dye-mordant combination and dye concentration (Ratnapandian, 2013; Shahid and Mohammad, 2013).

9.5.3

Performance of natural dyes

Natural dyes have been used to produce a variety of shades. However, with exceptions such as indigo, cochineal, and madder, most natural dyes cannot perform to the fastness standards set for synthetic dyes. There is a constant scientific effort to improve this deficiency. The role of mordants for increasing fastness ratings has been studied by several researchers. Certain natural dyes have been shown to impart antimicrobial activity and UV protection. Thus, research on natural dyes provides scope for innovative developments.

9.5.4

Objections to natural dyes

Common resistance to change has been seen with the idea of reintroducing natural dyes to industrial use. The question of whether natural dyes can provide the desired variety of shades has been answered positively by several researchers [10,28,32,98]. The limitations indicated are quantity and quality. There are doubts about the agricultural feasibility of replacing synthetic dyes with natural dyes. It should be noted that there is no move for complete replacement and natural dye consumption in Europe would probably constitute only 5% by volume of total dye consumed there. Research has revealed that this can be met by utilizing a small portion of nonagricultural semi-arable land in Europe. Modern farming methods have made dye crop cultivation a viable commercial proposition in some countries.

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Queries regarding sustainability, safety, and pollution with respect to natural dyes, for example, the use and discharge of metal salts for mordanting, have to be considered. The above arguments bring out the knowledge gap and highlights the need for research to commercialize natural dyeing.

9.5.3 9.5.3.1

Assisted dyeing techniques Use of enzymes

Enzymes are biological proteins that help in digestion and assimilation of food. These proteins have been utilized in industrial reactions including textile wet processing to improve cleaning (scouring and bleaching), increasing dye uptake, and improving fastness properties. Researchers have published data regarding improving the dye performance of natural dyes on various textile materials by employing enzymes in a pretreatment step. Enzymes serve to modify the physical and chemical surface properties or introduction of functional groups on the surface of textile fibers (Shahid and Mohammad, 2013). Vankar and coauthors studied the effect of three enzymes lipase, diasterase, and protease on dyeing characteristics of Terminalia arjuna, Punica granatum, Rheum emodi (Vankar et al., 2007), Acacia catechu, Tectona grandis, and Delonix regia (Vankar et al., 2008) on different types of textile fabrics. Natural fibers on treatment with enzymes such as tryptase, amylase, and others have been reported to exhibit enhanced dye uptake without changes with fastness properties when colored using different natural dyes (Akçakoca Kumbasar et al., 2009; Montazer et al., 2009; Parvinzadeh and Technology, 2007; Liakopoulou-Kyriakides et al., 1998; Tsatsaroni et al., 1998). Another important area is enzymatic reduction of indigo in both coloration and washing processes (Novo enzymes, private communication). An important consideration for increased benefits of enzyme treatment is dye-enzyme compatibility (Zhang et al., 2011). Enzymes are attracting increased consideration by the industry in view of their ecological benigness and improved process performance.

9.5.3.2

Use of chitosan

Chitosan is a bio-polysaccharide obtained by the deacetylation of chitin. Chitin is a by-product of seafood industry, as it is predominantly present in the inedible shells of crustaceans (crabs, lobster, and shrimp) (Sanford, 1989). As depicted in Fig. 9.4, it is poly-cationic with three reactive groups, namely the amino (eNH2) group at C2 and the two hydroxyl (eOH) groups at C-6 and C-3 in each repeat unit. This poly-cationic character of chitosan has been utilized in different areas, such as

Fig. 9.4 Schematic structure of chitosan (Sanford, 1989).

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cosmetics, weight loss, health care, water treatment, and textile dyeing (Sanford, 1989; Majeti and Kumar, 2000). The ability of chitosan to agglomerate dyes has been used to improve the dyeability of cotton and wool by pretreatment. It has been proposed that chitosan forms a film and increases the number of reaction sites on the substrates for dyes (Majeti and Kumar, 2000; Knorr, 1983). An improvement in dye uptake leading to darker shades has been reported for both synthetic and natural dyes (Canal et al., 1998; Gupta and Haile, 2007; Davidson and Xue, 1994; Kittinaovarat, 2004; Shin and Yoo, 2010; Giridev et al., 2009). Variations in shade due to fiber maturity and damage have been minimized with the help of chitosan (Rippon, 1984; Davidson and Xue, 1994). Ratnapandian (Ratnapandian et al., 2013) reported that careful use of chitosan while padding with natural dyes increases the depth of shade. However, beyond the critical point, chitosan interfered with the dyeing process. On a functional level, chitosan imparts durable antimicrobial properties to textile materials. Such an enhancement of properties opens the market for medical textiles or other similar materials.

9.5.3.3

Use of ionic liquids

Ionic liquids (ILs) are salts with melting temperatures below 100 C. They evolved from traditional high-temperature molten salts. Presently, they are being investigated for “green chemistry” applications (Wilkes and Zaworotko, 1992). ILs are used as catalysts, solvents, and electrolytes (Zhao et al., 2002). Pretreatment of wool with IL has been reported to increase the depth of shade and reduce contact angle so as to improve dyeing cycle efficiency (Yuan et al., 2010). Bianchini and Barbaro (Bianchini and Barbaro, 2002) investigated dyeing of cotton; wool and polyester with disperse red 13 using IL as the sole additive. Their results revealed that the use of IL significantly reduced the consumption of water and other auxiliaries, thereby lowering the environmental impact of dyeing process.

9.5.3.4

Plasma pretreatment

Plasma, also known as the fourth state of matter, is defined as a collection of nearly equal numbers of positive and negative charges obtained by ionization of a gas. This mixture of excited ions, molecules, electrons, neutrons, protons, and free radicals (Fig. 9.5) is highly reactive (Muguntharajan and Saminathan, 2009). The plasma gas is ionized by addition of energy in the form of temperature or the application of a high

Fig. 9.5 Schematic representation of plasma (Muguntharajan and Saminathan, 2009).

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electric field. Nonthermal or “cold” plasma, generated by the application of a high electric field, exists at near room temperature and is better suited for treating textile materials (Muguntharajan and Saminathan, 2009; Hwang, 2003). Shishoo (Shishoo, 2007) classifies plasma into low-pressure (vacuum) and atmospheric pressure plasma (APP), depending on the gas pressure at which the plasma is generated. The former operates at vacuum pressures between 102 and 103 mbars and generally used in nontextile applications. The latter, operating at atmospheric pressures, is subdivided into corona treatment, dielectric barrier discharge, and glow discharge, based on shape and positioning of electrodes. Glow discharge imparts uniform treatment by generating the plasma between two parallel-plate electrodes in an inert atmosphere. Treatment by either vacuum or APP yields comparable results in terms of the functionality achieved (Hwang, 2003; Ceria et al., 2010). APP for textiles is a fairly recent development, it is economical and convenient for continuous production as it avoids working under a vacuum (Shishoo, 2007). A typical characteristic of all plasma treatment is that it affects only the surface (<1000 Å, 1 Å ¼ 1  1010 m) and leaves the bulk properties unaffected. Desirable functionalities such as altered moisture relations (absorbance or repellence), antimicrobial property, soil repellence, stain resistance, soft handle, and improved dyeing are widely achieved on textiles by wet finishing processes. Such processes employ a variety of chemicals (Broadbent, 2001). Plasma pretreatment followed by wet finishing has been employed to impart similar functional finishes to textile materials with lesser amount of chemicals and in some cases without the use of chemicals or water (Sadova et al., 1983; Ulesova et al., 2008; Wakida et al., 1993, 1998; Sun and Stylios, 2004; € Shekar and Bajpai, 2000; Deshmukh and Bhat, 2011; Karahan and Ozdo gan, 2008). The effect of plasma treatment may be altered by varying process parameters such as supply frequency, discharge power, treatment time, type, and pressure of gas. For example, oxygen or helium plasma increases moisture absorbance, while fluorocarbon increases water repellence. Several surface phenomena such as adsorption, desorption, etching, cleaning, surface activation, and cross-linking occur singly or in combination € on exposure to plasma (Karahan and Ozdo gan, 2008; Subbulakshmi et al., 1998; H€ ocker, 2002). A concise comparison between traditional finishing and plasma treatment is given in Table 9.2 (Shishoo, 2007). Consistent with the above discussions, treatment of wool with plasma has been reported to affect the lipid layer and surface cuticle without changing the bulk properties. This results in higher wettability and increased dye uptake, leading to improvements in the depth of shade and evenness. The available literature relates only to different classes of synthetic dyes (Kan et al., 1998; Kan and Yuen, 2007; Lehocký and Mracek, 2006). It has also been suggested that intrinsic dye hydrophilicity is a deciding factor in this improvement (Naebe et al., 2010). On this basis of the APP, treatment of wool was proved to improve padding with natural dyes (Ratnapandian, 2013).

9.5.3.5

Ultrasound application

Ultrasound refers to all sound waves with the frequency above audible frequency of 20,000 Hz (Kadam and Nayak, 2016). Such sounds are widely used in medical imaging. Ultrasound is considered as part of the clean processing technique in textile

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Table 9.2 Plasma treatment versus traditional wet processing (Shishoo, 2007). Plasma processing

Traditional wet chemistry

Medium

No wet chemistry involved. Treatment by excited gas

Water based

Energy

Electricitydonly free electrons heated (<1% of system mass)

Heatdentire system mass temperature raised

Reaction type

Complex, multifunctional, and simultaneous

Relatively simpler, well established

Reaction location

Surface-specific, unaltered bulk properties

Bulk of material is usually affected

Potential for new processes

Rapidly developing field

Low technological growth

Equipment

Experimental, laboratory and prototype, rapid development

Mature, slow evolution

Energy consumption

Low

High

Water consumption

Negligible

High

industry. This technique works by means of cavitation in liquid medium in addition to other mechanical effects such as dispersion, degassing, diffusion, and intense agitation of liquid (Ahmed and El-Shishtawy, 2010). Cavitation is the formation, growth, and implosive collapse of small gas bubbles caused by ultrasonically induced alternating compression and rarefaction waves. The cavitation bubbles oscillate and implode, thus enhancing molecular motion and stirring effect in the dyebath. When cavitation occurs at fiberedyebath interface, the asymmetric implosion promotes mass transport in the direction of the fiber (Vankar et al., 2008). This increases the dyeing rate; economizes energy and time consumption without causing any apparent fiber damage (Vankar et al., 2008). Experiments have demonstrated that ultrasound-assisted natural dye extraction, mordanting, and dyeing are more efficient when compared to traditional methods (Mansour et al., 2011). The above statement has been verified by work on cochineal, lac, saffron (Shahid and Mohammad, 2013), catechu, tectona, and madder (Vankar et al., 2008). Xinsheng et al. (Xinsheng et al., 2008) have reported similar results for protein fiber coloration using natural dyes. An increase of nearly 50% dye uptake apart from improved fastness characteristics have been reported by Guesmi et al. (Guesmi et al., 2013) while working with indicaxanthin natural dye on modacrylic. Dye extraction from beetroots, green wattle barks, marigold flowers, pomegranate rinds, 4’O clock plant flowers, and cockscomb flowers has been reported to be increased with the assistance of ultrasound (Sivakumar et al., 2009, 2011). The benefits of ultrasonic-assisted natural dyeing may be summarized as: 1. Low temperature dyeing and reduced processing time resulting in energy savings. 2. Lower demand of auxillary chemicals and increased dye exhaustion reduces pollution.

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3. Minimizes damage to fibers during dyeing. 4. Increases industry competitiveness.

9.5.3.6

Advantages and key consideration of natural dyes

There are several advantages of using natural dyes, which includes reduced environmental impact, renewable resource, safe working, and safe to wear. As natural dyes are derived from nature, they are not harmful. The amount of toxicity of wastewater is much lower in the case of natural dyeing. In addition, natural dyes are biodegradable, hence, the final disposal of natural dyed clothes does not cause environmental pollution. The use of natural dyes will be more sustainable as they are obtained from renewable sources. For soft and soothing shade of textiles, natural dyes are preferred over their synthetic counterparts. Many of the natural dyes are safe to work and does not cause skin irritations while wearing. Even some natural colors such as carmine present in lipsticks are not harmful when ingested. In spite of these advantages, there are certain limitations of natural dyes, which needs to be overcome to replace the synthetic dyes. Some well-known drawbacks of natural dyes are insufficient supply, unavoidable variation due to changes in growing condition, low color yield, and low efficiency during cultivation and dyeing (Fereday, 2003; Dyer, 1976; Adrosko and Furry, 1971; Gulrajani and Gupta, 1992; Samanta and Agarwal, 2009). Although natural dyes can produce wide range of shades, most of them are not able to perform up to the standards of synthetic dyes in term of desirable properties. Indigo, cochineal, and madder are exception to these statements (Bechtold and Mussak, 2009; Cardon, 2007; Gulrajani et al., 2001). In an effort to overcome this deficiency, the role of mordants has been investigated (Gupta, 1999a,b; Cristea and Vilarem, 2006). Some natural dyes have inherent antimicrobial, insect repellent, and UV protection properties (Singh et al., 2004; Gupta et al., 2005). Thus research on natural dyes provides scope for innovative developments in future. Several queries regarding reintroduction of natural dyes on an industrial scale have been answered. Researchers have proved that natural dyes can provide wide variety of colors (Bechtold and Mussak, 2009; Cardon, 2007; Fereday, 2003; Gulrajani et al., 2001). The limitations indicated are quantity and quality. It should be noted that the natural dyes cannot replace synthetic dyes completely. Plant-based natural dyes would primarily been grown on semi-arable farm land and would prove to be source of supplementary income for farmers (Hill, 1997; Angelini et al., 2007; Guinot et al., 2006). Modern farming methods have made dye crop cultivation a viable commercial proposition in some countries (Hill, 1997). Queries exist regarding sustainability, safety, and pollution with respect to natural dyes, for example, the use and discharge of metal salts for mordanting, have to be considered (Glover, 1993).

9.6

Nontextile applications

Natural dyes have been widely accepted to possess medicinal properties. Therefore, they have been used as food colorant, for example, saffron and turmeric. They may

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be used to obtain hues ranging from green through yellow, orange, red, blue, and violet, depending on the source of colorants (Bakowska-Barczak and Sciences, 2005). Natural dyes are claimed to increase efficiency and extend the life of solar cell (Furukawa et al., 2009).

9.7

Conclusions

Natural dyes have been used for many centuries around the world. However, their use in textile coloration was temporarily stopped because of discovery of synthetic dyes. The colorants of the ancients, natural dyes, will not become extinct nor be eradicated. The growing consumer awareness about the environment and sustainability is creating resurgence in the industrial use of natural dyes. It is widely known that health issues and environmental pollution are caused by synthetic dyes. Therefore, interests on natural dyes will be growing continuously. The scientific database about chemistry and application of natural dyes continues to expand. Global research is been carried out not only to obtain an intimate understanding about natural dyes but also to merge these traditional dyes with emerging technology. Therefore, there will be no more hazards on people and planet by switching from the synthetic dyes to the natural dyes.

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