Types of smart dyes used in textile industry.
Our future will be based on functional and AI based smart products, where every industry wants to develop these kinds of products. Textile industry also cannot remain untouched with this technological innovation. Dyes have been utilising for coloration of textiles since ancient time. But at present with various advancement in technology as well as requirements of consumers, the need for functional and smart dyes arises. Various current researches are based on application of smart and functional dyes on textile to develop smart and functional textiles. The dyes which add the functional and smart properties to the textiles can be called as functional and smart dyes. Functional and smart dyes are available in both synthetic and natural form. But with the environment concern, the researchers are going on to find out natural source of these dyes. Functional dyes such as UV protective dye, antimicrobial dye, moth repellent dye offer specific function after application on textiles. Smart dyes like photochromic, thermochromic, electrochromic and solvatochromic etc. are playing very imp role to develop a smart textile which can offer reversible colour change which leads to impart various properties such as thermoregulation, camophlage properties into the textiles. Functional dyes generally limited to the textiles sector but smart dyes are not just restricted to it. Application of smart dyes is extended to various fields such as automobiles, robotics, aircrafts, medicine and surgery etc. This chapter will mainly deal with the types, application methods and application area of functional and smart dyes especially in connection with textiles.
- functional dyes
- smart dyes
- photochromic dyes
- thermochromic dyes
- electrochromic dyes
- solvatochromic dyes
- UV protective dyes
- antimicrobial dyes
- smart textiles
- functional textiles
Smart and functional textiles is the need of the future. There are various methods to develop smart and functional textile which start right form the fibre stage and end in the dyeing and finishing stage. Among these methods, application of smart and functional dyes is considered the most affordable method to develop smart and functional textiles. Traditionally, Dyes has been utilising in textiles generally for coloration purpose. Earlier, consumer wants to buy the textiles mainly due to its aesthetic aspect. Presently, consumer has smart choices. A textile product only having aesthetic property could not allure them. In near future, the product should be smart and functional one to attract the consumer. Smart textiles refer, a textile which can act according to their nearby environment while functional textiles refer, a textile which provides a particular function specially protection from nearby environment. Smart and functional textiles are considered as the part technical textiles. Since last few decade the researchers have been searching the functional as well as smart aspect of dyes. The dyes which can offer smart and functional textiles can be considered as smart and functional dyes. Earlier dyes were categorised as natural dyes and synthetic dyes. Natural dyes were categorised as based on origin (vegetable, mineral and animal), colour (yellow, red, blue), chemical constitution, (indigoid, anthraquinone, alphanapthaquinone, dihydropyrans, anthocynidins, cretonoids), application method (direct, basic, acid, vat, disperse, reactive, mordant) while synthetic dyes were classified as water soluble (direct, basic, acid, reactive), water in soluble (vat, disperse, sulphur) and in situ colour formation (azoic dyes) [1, 2]. But Presently and in near future, the dyes can as be categorised as smart dyes and functional dyes on the basis of providing AI based smartness as well as functionality to the textiles. The types of smart and functional dyes are being described in this chapter.
Smart dyes can sense the nearby environment and make changes accordingly . Smart dyes can change the colour due to various external stimuli such as pH, pressure, temperature, solvents, moisture and electricity. These changes can be permanent or temporary on the basis of need . This phenomenon is known as chromism . There are different types of chromism such as photochromism (induced by sunlight or UV rays), thermochromism (induced by changes in temperature), solvato chromism (induced by polarity of the solvent), hygrochromism (induced by moisture), ionochromism (induced by ions), halochromism (induced by pH value), acido chromism (induced by acids), chemochromism (induced by specific chemical agents like dangerous gases, warfare agents, etc.), electrochromism (induced by electricity), piezochromism (induced by pressure), mechanochromism (induced by deformation of substances) . In the field of textiles mainly photochromism, thermochromism, electrochromism and solvatochromism types of chromism is reported [7, 8, 9, 10]. Smart dyes are based on these different kinds of chromism. Several natural and synthetic smart dyes have been discovered. On the basis of external stimuli smart dyes are classified as photochromic dye, thermochromic dye, solvato chromic dye, hygro chromic dye, iono chromic dye, halo chromic dye, acido chromic dye, chemo chromic dye, electrochromic dye, piezo chromic dye, mechnao chromic dye which changes colour respectively due to UV rays, temperature, polarity of solvents, moisture, ions, pH value, acids, specific chemical agent, electricity, pressure, deformation of substance . Despite of having a large group of these dye only a few one makes a way towards textile. Several factors such as bonding with textiles, end uses and comfort of the wearer etc. are considered while application of smart dyes on textiles. Smart dyes like photochromic dyes and thermochromic dyes, are mainly used in textiles sector. Earlier in 90’s, the thermochromism and photochromism were widely used particularly in fashion designing field [12, 13]. Thermochromic dyes have better stability in comparison to photochromic dyes hence these are mostly used dyes in textiles in comparison to other smart dyes . Very few researches related to the use of electrochromic dyes, solvatochromic dyes and mechanochromic dyes on fabric are available. Researches related to the use of electrochromic dyes on textiles are still going on . Different parameter of dye such as maximum absorption, no. of wash cycle, different fastness describes the properties of smart dyes . Application of smart dyes are generally extended to fashion, sport and defence and medical related sector. After application of smart dyes, a simple textile can act as a smart textile which can perform the various function such as Camouflage, thermoregulation etc. [17, 18, 19].
Functional dyes can add a specific function to the textile such as protection from UV radiation, microbes as well as insects and moth [20, 21, 22]. On the basis of these mentioned function, functional dyes can be categorised into UV protective dyes, antimicrobial dyes and insect and moth repellent dyes. Synthetic and natural dyes can provide these functions but presently more attention is being given to natural dyes due to several environmental concerns. Presently due to climate change issue researches on UV protective dyes and antimicrobial dyes have been increased.
Despite of being constant researches on smart and functional dyes, commercial application is very limited. To widen the application area of smart and functional dyes, it is necessary to use a proper application method to impart these dyes onto textile surface. The knowledge of proper bonding mechanism is important in this regard. The relation between fabric properties and application method should be well known to the researcher. It is also important to work on the light stability and washing stability aspect of these dyes. Researches related to preparation and application of these smart and functional dyes on textiles should be carried out in a significant number. Functional group can be added to a traditionally used dye to enhance its functionality. These kinds of researches would be help in the current scenario and also helpful to increases its commercial application.
2. Types of smart dyes
As discussed in introduction part, only few smart dyes such as photochromic, thermochromic, electrochromic, solvatochromic paved a way for its application in textiles industry. Although researches are still going on to utilise more no. of smart dyes in textile sector. Types of smart dyes that are being used in textiles sector are as follows:
2.1 Electrochromic dyes
Electrochromic dyes are based on electrochromism in which reversible changes of colour occurs due to electricity (gain and loss of electron) . This process occurs generally with some transition metal oxides which conduct both electrically and ions . It is reported that strong electric field can alter the colour of certain dyes . Phthalocyanine dye is a good example of electrochromism . Common electrochromic materials are Polyaniline. In a study, conducting polymer polyaniline layer was formed electrochemically on conducting woven textile substrate which exhibited reversible colour change . During application of electrochromic dyes on textile, performance parameters such as electrochromic contrast, coloration efficiency, write-erase efficiency, switching speed, stability, cycle life, and optical memory are considered. Polyethylene terephthalate (PEPES) membranes were coated with poly-3,4-ethylenedioxythiophene polystyrene sulfonate (PEDOT:PSS) to develop electrochromic textile system . Application of Electrochromic dyes can also be extended to develop smart window curtain which filter sun light accordingly . Other smart application such as display of information or for camouflaging purposes of electrochromic dyes have also been reported .
2.2 Thermochromic dyes
Thermochromic dyes causes reversible changes in colour within the absorption spectrum of a thermochromic molecule, usually in the visible light range. These changes are induced by heat. Thermochromic material can be organic, inorganic polymers and sol-gels in nature . Presently, only two type of thermochromic system are used in textiles i.e. liquid crystal type and molecular rearrangement type. Organic leuco dyes also create thermochromic system . Organic thermochromic systems perhaps occur due to equilibrium between molecular species like acid–base, keto-enol, lactim-lactam, stereoisomers or between different crystal structures . Among inorganic thermochromic substances, few examples of metal oxides are known such as indium oxide, zinc oxide, chromium hemitrioxide-alum earth . Application of thermochromic dye on textile can be done using 3 methods which can be used exhaust, continuous method, microencapsulation and printing. In exhaust method thermochromic pigment are applied on to textile with a cationic agent, non-ionic dispersing agent and binder using Material to liquor ratio 1:30 at 70 o C temperature for half hour . In continuous method thermochromic dye, cationic and non-ionic dispersing levelling agent are applied with acrylic soft binder solution. This solution is applied on textiles through pad dry cure machine. Drying of samples is generally done at 80°C for 3 min while curing is done at 140°C for 2 to 3 minutes . In microencapsulation method, microcapsules were created using colourless dye, precursor and colour developer . These materials melt and solidifies with respective to the application of heat and cold. On melting these material changes its colour, while on solidification come back to its original colour.
Various novel colour effects as well as camouflage designs can be produced using combination of thermochromic, non-thermochromic and mixture of both on the conductive cotton . These thermochromic dyes can take heat from there nearby environment such as sun rays and cause a reversible change in the colour of textiles. These dyes are also establishing their presence in the field of Protective clothing for military . Protective clothing using thermochromic colourant can mimic the colour of its surrounding environment with the change in temperature. Even sometimes colour can change with body temperature. Thermochromic dyes on textiles also used to protect a brand from fake imitation . Microparticles (made of thermochromic and photophoresent dyes) and binder are used to make the brand logo. These logos can be temporary and permanent based on the binder used. The logo made up of thermochromic dye change colour with body temperature . Thermoregulation is also a very imp application of thermochromic dye which is based on heat absorbance (in case of dark colours) and reflectance (in case of light colours). A suitable example of this phenomenon is coating of thermochromic dyes on firemen’s uniform on high temperature. The Colour of uniform will be converted to white colour which reflect the heat. Thermochromic dyes can also do thermoregulation via expanding or shrinking the textiles fibre. This Shrinking and expanding of fibres in a fabric causes opening or closing of pores which help regulate the passage of air to the body according to external environment .
2.3 Photochromic dyes
Photochromic dyes change its chemical structure due to UV rays. Photochromic effect was first time seen on the tetracene and after that it was observed in potassium salt of dinitomethane . In 1960 first photochromic spiropyrans was developed using printing technique by Ksnebo ltd, Japan. In 1998, photochromic dyes were first time used in textiles to produce camouflage effect. Photochromic dyes also decolorise. This decolourisation is called negative photochromism . Various metal oxides, alkaline earth metal sulphides come under the category of Photochromic dyes. Photochromic dyes are water insoluble hence cannot be applied on wool and cotton . Presently Researches are also going on Water-soluble photochromic dyes. These dyes are relatively cheaper option for textiles and leather. Spirooxazine-based Water-soluble photochromic dyes having sulphonate group have been developed. Sulphonate gp impart water solubility. These water-soluble photochromic dyes are mainly for proteins fibres . The wash fastness and photostability of these dyes are moderate .
Application of photochromic dyes can be done on the window curtains and facades. This would be helpful to control sunlight . Heat sensitive photochromic dyes also used as temperature indicator [49, 50]. Dupont company are also working on the camouflaging property of these photochromic dyes to develop camouflaging clothing for armies [51, 52]. Flexible UV sensors based on Photochromic dyes also change its colour due to UV rays which makes a UV detective fabric which help to tell the intensity of UV rays to get protection accordingly .
Different application methods have been reported to apply Photochromic dyes and pigments. In a study exhaust method (direct Coloration), photochromic colourants were used with auxiliaries such as dispersing agent and ceramic balls (250 g) and aqueous acetic acid. The dyeing was started at 40o C temperature and temperature raised to 60 o C and then to 90 o C .
Photochromic colourants as disperse dyes has also been used to dye polyester using exhaust dyeing technique. Photochromic colourants are insoluble in water therefore organic solvents such as Acetone is used to dissolve the photochromic colourant. After dissolution of colourant, dispersing agent is added. Acetic acid is also used to maintain the weak acidity (4.5) of solution. This weak acidity helps to minimise the degradation of colourant. The material to liquor ratio of solution was kept to 1:50. The fabric is dyed at 120 o C for 45 min. Further, rinsing in cold water was done to reduce the temperature to 70 o C. This 70 o C temperature was maintained for 20 minutes with in the solution of sodium hydroxide, sodium dithionite and a non-ionic surfactant. Material to liquor ratio was maintained to 1:30.
Photochromic colourants can also be applied through pad dry cure method. In this method the fabric is dried at 80 o C for 2 minutes in hot air, followed by 140°C of curing temperature for 3 minutes. Microencapsulation was also used for application of photochromic colourants [55, 56, 57].
2.4 Chemo chromic dyes
Chemo chromic dyes change its colour due to differences in pH. Phthalides, fluoranes, triarymethines, and simple azo dyes are the examples of pH sensitive dyes .
Conventional exhaust dyeing, solgel method as well as prior addition to electrospinning solution has been reported as methods of application for chemo chromic dyes. Chemo chromic dyes can be used in identify the proteins (of certain microbes) with change of colour. This identification with changing colour indicates the presence of microbes in textiles . Therefore, Application area of Chemo chromic dyes can be widened to medical textiles bandages .
2.5 Solvatochromic dyes
Solvatochromic dyes work on the principle of Solvatochromism which corresponds to reversible changes of colour due to solvent polarity. The change in colour is occurred due to shifting in maximum absorption of different solvents . There are two types of solvato chromism i.e. positive solvatochromism and negative solvatochromism. Positive solvatochromism corresponds to a hypsochromic shift induced by a decrease of solvent polarity“ and “negative solvatochromism corresponds to a hypsochromic shift induced by an increase of solvent polarity” . Common example Solvatochromic dyes are pyridinium, merocyanine, and stilbazolium dyes. Solvatochormic dyes can be applied in the textiles using microencapsulation technique. Assembly of microsphere on fabric causes colour change on drying and wetting of the fabric. Application of Solvatochromic dyes in textiles is very limited. These dyes can be used to identify the stale or toxic food due to colour change of packaging .
2.6 Mechanochromic dyes
Mechanachromism is a phenomenon in which a polymer changes it colour due to deformation such as elongation and compression. Elongation and compression occur due to the change in pH, temperature. Mechanachromic dyes are the organic dyes which is applied on polymer and causes changes in colour due to certain mechanical pressure. Mechanochormic dye cannot act alone it require a polymer (material) because only polymer can deform (elongate or compress). In a study 1,4-bis (R-cyano-4- methoxystyryl)-2,5-dimethoxybenzene9 (C1-RG, F) has been incorporated in polyethylene using melt processing technique . In another research mechanochromic polymers have been developed by incorporating a dye filled microcapsules . In a study Polydiacetylenes (PDA) was synthesised by thermal polymerisation of diphenyl sulphide containing bisdiacetylene. It is reported in that study that PDA changes its colour from blue to red due to elongation and compression. In the cool (non-extended state) form, the material has a particular colour, this particular colour changes due to mechanical abrasion which causes heating of surface. Mechanochromic colourants can be physically dispersed in form of supramolecular aggregates in a matrix in a polymer. Covalent insertion of chromophoric units into the macromolecule backbone or side chains has also been used to apply mechanochromic colourants . Application area of mechanochromic dyes can be done in footwear and shaped garment industry in which product can change colour due to deformation (Table 1).
|S. No||Smart Dyes||External Stimuli responsible for colour change||Suitable methods of application for textiles||Application in textiles||References|
|1.||Electrochromic dye||Electricity (gain and loss of electron)||Coating||For displaying information or for camouflaging purposes||[23, 24, 25, 26, 27, 28, 29, 30]|
|2.||Thermochromic dye||Heat (absorption spectrum of a thermochromic molecule)||Exhaust, Continuous method, microencapsulation||Thermoregulation, Brand forgery detection||[31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42]|
|3.||Photochromic dye||Light||Exhaust and pad dry cure method||Camophlaging textiles, window curtains, UV detective fabric||[43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57]|
|4.||Chemo chromic dyes||pH||Conventional exhaust dyeing, solgel method, prior addition to electrospinning solution.||Medical textiles||[58, 59, 60]|
|5.||Solvatochromic dye||UV rays||Microencapsulation||Packaging||[61, 62, 63]|
|6.||Mechanochromic dye||Deformation (elongation and compression) of polymer on which applied||Melt processing technique, physically dispersed in form of supramolecular aggregates in a matrix in a polymer, Covalent insertion of chromophoric units||Footwear and shaped garment industry||[64, 65, 66]|
3. Types of functional dyes
Functional groups like -OH, -NH2, − COOH of dyes offer various functional properties when applied onto the textile . Both natural and synthetic dyes can perform various functions after application on textiles. Now days more emphasise is being given to the green and sustainable functional dyes which directly comes from nature. Types functional dyes are as follows:
3.1 Antimicrobial/antibacterial dye
Microbes and bacteria cause several kinds of dermal infection, body odour and several other severe health issue . Antimicrobial dyes provide the protection from variety of micro-organisms like gram-positive bacteria such as
Perspiration cause formation of bacterial colonies on textiles, which led to bad odour . Various natural dyes can act as a barrier to form these colonies. Natural dyes such as pomegranate, coffe arabica,
There is no. of methods to impart antimicrobial dyes on textiles. For proper bonding of textile and dye, the textile surface can be modified through various treatments such as treatment with chemicals, chitosan, enzymes, UV radiation, ultrasound . Application methods can be altered with regard to type of fibre dye and the end use. Therefore, researchers should consider the structure of dye and fibre. The researchers should also have knowledge how this bonding of fibre and dye affect the fabric properties. The products of Health, hygiene as well as medical textiles comes under the application area of antimicrobial dyes.
3.2 UV protective dyes
Presently UV ryas are causing various harmful effects. UV protective dyes enhance the UPF (Ultraviolet Protection Factor) of the textiles. UPF means how much a fabric can protect the wearer from harmful UV rays. In general, all dyes act as a UV absorber because spectral region falls into UV region. Various kind of synthetic dyes are commercially available to enhances the UPF of the fabric. Direct, vat and reactive dyes increases the UPF of fabric [85, 86, 87]. Various researches reported that natural dyes can also enhances the UPF of the fabric. Absorption characteristics of natural dyes generally determines the UPF of the fabric . Phenolic compounds in natural dyes work as UV protective agent as these molecules absorb the UV radiation. For instance,
In a study exhaust method was used for the application of herbal plant extract to enhance the UV protection of the fabric. Madder and cutch dye was applied on nettle fabric using exhaust dyeing method . Pad dry cure method were also used for application of UV protective dye .
Application of UV protective dyes can be extended mainly to the clothing of outdoor activities such as fishing, farming, horticulture, gardening, building construction, road construction, postcard distribution, oil production field, military defence services, skiing, police work, professional cycling, surfing . UV protective dye can also be applied to the clothing of Indoor workers who are potentially exposed to UV radiation for example in hospitals where UV radiation is required for some kind of treatments in some laboratory works, plasma torch operating, printing, lithographing, painting, wood curing, plastic working, in some cases food industry also . Army personnel who have been working in extreme climate conditions also experiences intense solar radiation with terrible heat stress also requires protection from UV rays .
3.3 Moth proof and mosquito repellent dyes
Moth proof and mosquito repellent dyes provides protection against moths and mosquitos after application on textiles. Synthetic moth proof or mosquito repellent are generally available in colourless form. Therefore, moth proof and mosquito repellent dyes available in natural form. Various natural dyes contain tannin which can also act as a moth proofing agent. It has been reported in a research that the natural dyes having more than about 40% tannin is effective as an anti-moth agent. In various studies, it was reported that natural dye extracted from Saffron flower waste, onion skin, henna, myrobalan, silver oak leaf, madder, wall nut, dholkanali and yellow roots provides anti-moth properties after application on textiles . Application of dye extracted form pomegranate peel with polyvinyl alcohol can act as mosquito repellent . The mosquito repellent property of synthesised
Moth proof dyes were applied using exhaust method in which condition such as (Concentration of colourant-5%, temperature. 90°C degree, M:L- 1;:40, and pH 5–6, were maintained. Simultaneous dyeing with mothproofing agent on wool fabric were also reported. Result of this study showed that undyed and only mordanted fabric provides lesser protection from
Mothproof and insect repellent dyes is textile museum and library to protect the textiles and books. Mosquito repellent dyes can also be applied to the children’s clothing, pram and curtains  (Table 2).
|S. No||Functional Dyes||Source||Suitable methods of application for textiles||References|
|1.||Antimicrobial dyes||Treatment of textiles surface with chemicals, chitosan, enzymes, UV radiation, ultrasound||[70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84]|
|2.||UV protective dyes||Exhaust, pad dry cure and microencapsulation||[85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101, 102, 103, 104]|
|3.||Moth repellent dyes||Saffron flower waste, onion skin, henna, myrobalan, silver oak leaf, madder, wall nut, dholkanali and yellow roots||Simultaneous dyeing||[105, 106, 107, 108, 109, 110]|
|4.||Mosquito repellent dyes||Exhaust, microencapsulation|
This high technological era is not only based on beautiful products but it is based on “beauty with artificial intelligence”. Various researches are being conducted to develop a smart and functional textile which is not only appealing due to its looks but also have an artificial intelligence to give signal according to the change in nearby environment. Dyes can play important role to develop smart and functional textiles. Besides the functionality, sustainable and green aspect of dyeing are also being considered during synthesis and application smart and functional dyes. In comparison to the conventional dyes smart dyes can add a special intelligence to textile fabric such as thermoregulation, camouflaging. Similarly, functional dyes application on textile provides protection form UV rays, unhygienic conditions and insects. Despite of having such an intelligence of to perform according to nearby environment, various smart dyes have lost their ability to develop colour after several molecular transformation. This phenomenon is known as fatigue resistance. Similarly, functional dyes also have limitation with regards to durability and comfort properties of the fabric. For instance, UV protective dyes have less stability in light and laundry. The fabric dyed with antimicrobial dyes as well as moth and Mosquito repellent also do not have very good wash fastness. These stability-related issue of smart and functional dyes can be enhanced to utilise proper application methods. Different application methods like surface modification (plasma treatment and UV irradiation, etc.) and microencapsulation to enhance the stability of these functional as well as smart dyes. Application method should be such type that could make a balance between comfort properties and stability.
|UPF||Ultraviolet Protection Factor|
|M:L||Material to liquor ratio|