Thermochromic colors in textiles

Source: Americos Industries India
Web: www.americosind.com

Abstract

Increasing awarenessamong the consumers of textiles paves the path way for novel developments quiteapart from the conventional research focus. The thrust is emphasized on microand nano technology for special purpose applications like military and medical,and space applications and also for general applications like self-cleaningeffects on garments which are expected out of the phenomenon called LotusEffect. The chromic colors are interesting in that they change the color inresponse to external stimuli and hence they produce novel effects on fabricsand garments. Of the various chromic colors, thermochromic colors are widely studiedand used in textiles for many novel applications. This paper aims at discussingvarious aspects of thermochromic dyes such as principle, problems associated,recent developments and its various applications. The article also highlightsthe contributions from Americos in the specialty application usingthermochromic dyes.

Introduction

Chromic colors are thosewhich undergo reversible change in color. Generally the color change is basedon alteration of the electron states of molecules, especially the π- ord-electron state. This phenomenon is induced by various external stimuli whichcan alter the electron density of substances. The process of such reversiblecolor change is commonly known as chromism. Chromism is classified by what kindof stimuli is used. Accordingly, the major kinds of chromism are thermochromism induced by heat, photochromism induced by light irradiation,electrochromism induced by the gain and loss of electrons and solvatochromismwhich depends on the polarity of the solvent. Some of the other known chromismsare ionochromism color change induced by ions, halochromism color changeinduced by pH, tribochromism color change induced by mechanical friction andpiezochromism color change induced by mechanical pressure.

Of these, thermochromiccolors are the most common colors used for various applications of which mood ring is a good example. The thermochromiccolors are derived fundamentally from liquid crystals and leuco dyes. Due to veryhigh temperature sensitivity, liquid crystals are used in precisionapplications, however, their color range is limited by their principle ofoperation. In contrast, leuco dyes allow wider range of colors to be used, buttheir response temperatures are more difficult to set with accuracy.Thermochromic colors are used in flat thermometers, battery testers, clothing,and the indicator on bottles of maple syrup that change color when the syrup iswarm. The most well-known line of clothing utilizing thermochromics wasHypercolor. The thermometers are often used on the exterior of aquariums, or toobtain a body temperature via the forehead.

Photochromiccolors generally belong to one among the following organic chemical groups:triarylmethanes, stilbenes, azastilbenes, nitrones, fulgides, spiropyrans,naphthopyrans, spiro-oxazines and quinones. Similarly, substances of silver andzinc halides which are inorganic in nature are also extensively used. Inparticular, silver chloride is extensively used in the manufacture ofphotochromic lenses for sunglasses, as found in eye-glasses which is one of themost famous reversible photochromic applications. Photochromic dyes aregeneraly used for making novelty items such as toys, cosmetics, clothing andother industrial applications. Some of the high-tech applications ofphotochromic colors being investigated with a few commercial successes are asmolecular switches in supramolecular chemistry, as data storage in 3D opticaldata processing.

Polyaniline,viologents, polyoxotungstates and tungsten oxide are some of the electrochromicmaterials of which polyaniline is commonly known. Electrochromic materials areused to control the amount of light and heat allowed to pass through windowsknown as smart windows. Tungsten oxide is the main chemical used in theproduction of such electrochromic windows. Electrochromic colors are also usedin the automobile industry for automatic tinting of rear-view mirrors undervarious lighting conditions. Viologen in conjunction with titanium dioxide (TiO₂)is used for developing small digital displays which might even have thepotential to replace LCDs as the dark blued viologen has a high contrast to thebright color of the titanium white, providing a high visibility of the displaythereby.

Depending upon the difference in dipole moment of the solvatochromic color molecule between its ground state and excited state, there are two types of salvatochromism i.e. negative solvatochromism that corresponds to hypsochromic shift, positive solvatochromism that corresponds to bathochromic shift with increasing solvent polarity. 4,4'-bis(dimethylamino)fuchsone (orange in nonpolar toluene, red in slightly polar acetone, and red-violet in more polar methanol) is a proper example of positive solvatochromism while the examples of negative solvatochromism include 2-(4'-hydroxystyryl)-N-methyl-quinolinium betaine (ink-blue in nonpolar chloroform and blood-red in polar water) and 4-(4'-hydroxystyryl)-N-methyl-pyridinium iodide (violet in n-butanol, red in 1-propanol, orange in methanol, and yellow in water).

As far as applications of chromic colors in textile are concerned, thermochromic and photochromic colors are widely used compared to other chromic colors. Since, the subject matter of thermochromic and photochromic dyes and their various aspects are vast, in this paper the primary focus is restricted only to thermochromic colors. This paper first outlines the principle of the thermochromic colors and then it addresses the various problems associated with these colors in effectuating their commercial applications. It also reviews the recent research works conducted by various groups of researchers to develop new thermochromic compositions. Finally, it presents the summary of the paper with special reference to Americos contribution in this specialty textile application.

Thermochromic Dyes

Thermochromic and photochromic encapsulated dyes were developed a number of years ago, and primarily incorporated into plastic or textile colorants for wide commercial applications. Thermochromic dyes undergo a color change over a specific temperature range. The dyes currently available change from a particular color at low temperature to colorless at a high temperature (e.g. red at 29.5C and colorless at above 32C). The color change temperature can be controlled, such that the color-change can take place at different temperatures (eg. just below a person's external body temperature so that a color change occurs in response to a human touch). The thermochromic dye manufacturers are able to manipulate the critical temperature for the color change.

Dye Composition

Certain dyes undergo keto-enol type of tautomerism. Such tautomeric rearrangements can lead to an increase in the conjugation and formation of a new chromophore, leading to color development [1]. Such rearrangements can be induced by a change of temperature, resulting in thermochromism. These dyes are extensively used for textile applications. The most common types are fluorans, crystal violet lactones, and spiro pyrans. All of these dyes undergo ring opening rearrangements. The equilibrium of crystal violet lactone is shown in Figure 1. A similar color change mechanism of fluorans (leuco dye) is shown in Figure 2 [2]. The use of leuco dye and acid developer process is most common in thermal paper preparation because it affords a snow-white paper and a vivid developed picture. The picture develops through the reaction of colorless leuco dye with acid component in polymer matrices as shown in Figure 2.

Figure 1: Tautomers of crystal violet lactone

Figure 2: Color development of fluoran dye with acid developer

Reversible thermochromism composition is made from such dyes, along with a color developer containing acidic protons capable of proton transfer or strong H bonding to the dye molecule. The dye and the developer are dissolved in a nonvolatile solvent, and the ternary composition is encapsulated. On heating, the organic solvent melts, resulting in color change. Some commonly used developers are bisphenol A, bispheno B, 1,2,3-triazoles, thioureas, etc. Various compounds have been used as solvents, but the most common are aliphatic alcohols like stearyl alcohol. A puzzling feature of this system is that they are colored at low temperature and turn colorless at high temperature. The color development of the ternary system depends on the melting point of the solvent used. In order to ensure a homogeneous mixture throughout the color development stage, it is necessary to keep it in a closed system by microencapsulation. In a microcapsule, which is small solid particle of 1 1000 m size, there is a core containing the thermochromic system and a coating or shell of a polymeric material. The two processes which are prevalent in the literature for microencapsulation of thermochromic dye systems are complex coacervation and interfacial/in situ polymerization.

Thus, a thermochromic pigment consists of an electron-donating, color former, an electron-accepting developer and a color change controlling agent. In other words, thermochromic pigments or materials are formed using an organic dye, an acid activator and a low melting solid such as ester or alcohol that acts as a solvent when liquified. These three agents are generally put inside microcapsules (Fig.3). If the temperature is below the melting point of the solvent, the color forming components are in contact and due to the electron interaction a visible color occurs. However, if the temperature is above the melting point of the solvent, the color forming components are separated leading to no electron interaction and hence no visible color occurs.

Figure 3: Principle of thermochromism

The microcapsular form is essential in order to keep its function unaffected by outside conditions as the excellent color changing function is obtained only when the three components system is strictly at a constant ratio. This composition is capable of producing commercial products having increased value in chromatic effect and function since it offers more diverse colors and higher coloring densities in comparison with thermochromic materials such as metal complex crystals and cholesteric liquid crystals and since dramatic changes occur between colored and colorless states. The thermochromic pigments are applied over fibre or yarn or fabric through coating using a binder.

Role of Acid Activators/ Shelf Life/ Toxicity

The selection of acid activators is quite important to preserve the durability of a thermochromic dyes as it may cause an irreversible color change in due course of time. The acid activators used for the color changing pigments or materials include phenols, carboxylic acids, phosphinic and phosphonic acids and metal salts of these acids. Although these acid activators are quite effective in causing reversible color changes in thermochromic pigments or thermobleaching of the thermochromic dyes, these activators gradually deteriorate the organic dyes used causing an irreversible color change. Similarly, it has been discovered that the use of many of the solvents and compounds commonly used e.g. alcohols, ketones, amino resin, petroleum solvents, etc. to make printing inks are harmful to the thermochromic dye and reduces the shelf life of the thermochromic dyes [3].

Critical Temperature

The critical temperature of a thermochromic dye is the temperature at which the colour starts changing in response to the temperautre stimuli. This is particularly important because the end use application of the thermochromic dyes will depend primarily on the critical temperature. For instance, by selecting one or more of thermochromic dyes whose critical temperature is near human body temperature, for applying on textiles, one could create textiles that would respond to human touch, emotional and psychological response. Similarly, by mixing thermochromic dyes having various critical temperatures and applying it on textiles, a series of color change effects can be brought out at different temperature levels. Moreover, through right choice of critical temperature of thermochromic dyes for special applications like military applications, the required chameleonic effect can be achieved to match with the sorroundings. Likewise a number of applications may be explored by the correct choice and combinations of thermocromic dyes having different critical temperature limits.

Enhancing Light Fastness

Color formers are extensively used for information recording systems or thermochromic systems for textiles, however, their light fastness properties are poor. The improvement of light fastness is, therefore, being sought at the present time. Various kinds of stabilizers have been synthesized to prolong the life of colored species derived from color formers. The retarding effect of these compounds towards photofading has been investigated on cellulose and has been found that the UV absorbers bearing the groups like hydroxyarylbenzotriazoles capable of acting as an amphoteric counter-ion play a very important role in improving the light fastness of colorants for imaging and data recording systems. According to a recent research work conducted by Hironori et al. [4-6] zinc and nickel 5-(2-benzotriazolyl)-2, 4-dihydroxybenzoates, zinc and nickel 3-(2-benzotriazolyl)-2-hydroxy-1-naphthoates, zinc and nickel 2, 4-dihydroxybenzophenone-3-carboxylates and their derivatives can be used as effective stabilizers against the fading of color formers.

Various Thermochromic Dyes: Problems Associated

Numerous inorganic and organic compounds show thermochromism. Inorganic compounds show both reversible and irreversible thermochromism due to phase change or due to change in ligand geometry of the metal complexes. Temperature-indicating paints showing irreversible thermochromism has been used for a long time as a warning of hot spots and as a record of heat history in the electrical and chemical industries. The inorganic compounds have not found favor in textile applications as the color change generally occurs in solution or at high temperature. The ideal thermochromic system for apparel applications should show reversible thermochromism between ambient and body temperature. Reversible thermochromism in the solid state is exhibited by many organic compounds. Sterically hindered ethylene compounds such as bianthrone and dixanthylene also show thermochromism. These compounds are characterized by at least one ethylene group, a number of aromatic rings, and a hetero atom, usually N or O.

The ethylenic bond provides a route for extension of conjugation and places restrictions on possible molecular orientation. As the temperature is increased, the molecule changes to a different stereoisomer, which is colored. These compounds show transition above their melting point ~ 150C, thus are not suitable for textile applications.

Of the various inorganic and organic compounds the specific substances and analogues which exhibit a thermochromic effect include double salts comprising a transition metal, such as cobalt, nickel or manganese, and an aminic amide, such as hexamethylenetetramine. These double salts discolor on releasing water when heated and resume the original color on absorption of moisture when cooled. Other examples are mercury iodide, double complex salts of mercury iodide with other metallic iodides, heavy metal compounds such as lead chromate and ammonium metavanadate, organic compounds such as dixanthylen and bianthrone, some organic dyes and pigments, etc. However, the problems associated with these compounds which limits their usefulness greatly are, high toxicity, being active only in the presence of moisture, no optional selectivity in color change temperature and color change to a similar color and hence indistinguishable.

Thermochromic substances further include cholesteric liquid crystals and mixtures of cholesteric liquid crystals and nematic liquid crystals, but these substances also find greatly limited use because they are low in color density, have no selectivity in color and in color change temperature and are very expensive. Some compositions are also known which undergo a color change (between formation of color and disappearance of color) owing to a suitable change in the ambient temperature and which comprise a normally colorless, electron-donating chromogenic material and an electron-accepting color developer such as attapulgite, kaolin, clay, zinc chloride, a phenol, zinc salicylate, aluminum benzoate or the like to utilize an electron donor-acceptor color forming reaction, these compositions further containing a solvent which is selected from among sparingly volatile solvents such as polyglycols, quaternary ammonium salts, nonionic surfactants and the like, alcohols, esters, ketones and acid amides. Some of the advantages of these compositions are, the chromogenic materials can be prepared for various colors for selective use, the chromogentic material gives a high color density and is, therefore, used in a relatively small amount economically, the composition is applicable directly to articles of any kind, depending on the kind of solvent, the color change temperature can be set over a wide range of low to high temperatures. Because of these advantages, the composition of the type described is thought to be most excellent at present. Nevertheless, these compositions also have some drawbacks such as; 1) the color developers, especially phenols, which serve as one of the essential components, have very low resistance to oxidation in the atmosphere and exposure to sunlight and hence gradually discolor or fail to serve as color developers, with the result that the composition incorporating the color developer seriously deteriorates with lapse of time, 2) the solvent, another essential component, must be used in a very large amount to assure the desired thermochromic effect. This imposes limitation on the color density and the durability of the composition. In fact, the composition has very low resistance to solvents, washing and heat. However, if a reduced amount of solvent is used, a lower desensitizing effect will result, making it difficult to completely eliminate the color to permit the composition to exhibit an objectionable remaining color or to render the composition no longer thermochromic.

Dye Developments

Because of the various drawbacks, the conventional thermochromic materials are used for greatly limited applications only. Therefore, extensive research works have been conducted to overcome the various drawbacks of the thermochromic dyes. In a study, it has been found when 1,2,3-triazole compound is used as a color developer in combination with a weakly basic, sparingly soluble azomethine or carboxylic acid primary amine salt as a solvent, a novel thermochromic composition can be obtained with outstanding properties and free of the drawbacks [7].

Recently Young et al. [8] have synthesized polymethine dyes using bis-imethylaminophenylethylene compound. These dyes can be converted to colorless leuco form by the treatment with alkali. Interestingly, these leuco dyes have showed reversible color formation properties following acid addition. The authors have investigated the feasibility of the use as thermochromic indicator using bisphenol A and 1-hexadecanol. In another study Guoxin et al. [9] have prepared a novel compound of butyl crystal violet lactone (BCVL) by oxidizing leuco butyl crystal violet lactone (LBCVL), obtained by the mixture of N,N-dibutylaniline, p-(dibutylamino) benzaldehyde and methyl-m-(dibutylamino) benzoate. The color of BCVL can change reversibly in some acid or alkali solvents. The result of the dissolution experiment has showed that solubility of BCVL in organic solvent can be improved compared with crystal violet lactone (CVL).

The measurement and correlation of the experimental solubility of a spiroindolinonaphthoxazine (1,3-dihydro - 3,3 - dimethyl-1 - isopropyl-6'-(2,3)-(dihydroindole - 1-yl) spiro [2H-indole-2,3'-3H-naphtho[2,1-b] [1,4] oxazine]) in supercritical carbon dioxide (scCO2) is reported [10]. Solubility results clearly indicate the feasibility of processing this dye using supercritical fluid technologies and processes, for example, supercritical fluid dye impregnation of polymer host materials. .

The inclusion complex properties of the multichromic dye in DMSO with three different types of cyclodextrins, namely α-, β-, and γ-cyclodextrin have been studied by absorption spectra measurements. The first order fading rates have showed the evident decoloration data for interaction of the multichromic dye and cyclodextrins [11].

In an US patent, reversible thermochromic compositions that develop fluorescent color of yellow, yellowish orange, orange, reddish orange, or red with a high color density and high color brightness, yet gives no residual color under non-color-developing conditions, and has remarkably improved light resistance have been disclosed. The reversible thermochromic composition, comprises a solubilized mixture of three components of (a) an electron-donating color-developing organic compound selected from pyridine types, quinazoline types, and bisquinazoline types of compound, (b) an electron-accepting compound for the electron-donating color-developing organic compound, and (c) a compound serving as a reaction medium for causing reversibly an electron exchange reaction between the components (a) and (b) within a specified temperature range [12].

Some of the other notable recent developments are with crystal violet lactone (CVL) which has been prepared by the reaction of N, N-dimethylaniline, p-dimethylanimobenzaldehyde and m-dimethylanimobenzoic acid [13]. Taking alcohols or carboxylic acids as solvents, 15 novel reversible thermochromic complexes can be prepared by means of chelation reaction between electron donor CVL and electron acceptor such as various phenols, aromatic amines, carboxylic acids, and Lewis acids, respectively.

Thermochromic cellulose fibers: Recently Marcin et al. [14] have incorporated the thermochromic pigments in cellulose using Lyocell process. This method is based on spinning fibers from concentrated solvents of cellulose, using dry-wet method in aqueous solidification bath. The cellulose solvent used in this process was N-oxide-N-methylomorpholine (NMMO). The studies have been to check on the features of the fibers obtained from solvents with thermochrome pigmentation, in order to evaluate their practical use. The authors have used 1-10 wt% Chromicolor AQ-INK, Magenta type 27 pigmentation as thermochrome modifier.

Quite interestingly, it is revealed in a patent to change the color of a textile at will using electricity [15]. The textile is manufactured by weaving, embroidering, or otherwise integrating a series of conductive, resistive, and non-conductive fibers into the textile and printing a thermoresponsive colorant on or near the resistive fiber. The pattern and physical configuration of the materials composing the textile determine the visual properties of the textile. Electrical power is supplied to the resistive fiber(s) to change the visual properties of the textile. As the resistive fiber warms, the thermoresponsive colorant is warmed beyond a thermal threshold necessary to effect a color change in the thermoresponsive colorant, thereby creating an electronically controllable, visually dynamic textile.

Applications of Thermochromic Dyes

Thermochromic color changing materials or pigments are generally used in paints, inks, dyes and dyed fabrics. Thermochromic pigments are also used in toys, dolls, recording media and novelties such as seals on beverage mugs that change color when a hot or cold beverage is placed in the mug.

Figure 4: Color change effect of a coffee mug doped with thermochromic pigment

Special Application Requirement for Military - Camouflage Application

Camouflage is an art unintentionally but advantageously employed in nature to conceal by pattern and/or color. Mankind learned basic camouflaging techniques from natural sources long before recorded history in order to conceal human beings, their possessions and even dwellings from human enemies as well as from animals being hunted or possibly hunting human beings. In more recent times, camouflage is commonly employed in sport hunting and in military situations, these differing applications sharing a common intent of concealing people, clothing, armament and other accessories through the use of patterns which merge with a given background. For military applications, the clothes are designed to mimic woodland and desert backgrounds. The patterns used for such military clothing is known as camouflage patterns, which mask the warrior with the background to deceive the opponent. However, the natural background changes markedly with the seasons there being a preponderance of green hues in summer, many of which disappear in winter. To cope with these changes, possibility of using thermochromic dyes which may change color triggered by a significant seasonal temperature change is being examined [16].

In order to provide radiant flash protection, military force requires a class of dyes that normally exhibit the correct visual camouflage print, but which bleach white instantaneously when the flash arrives. For this application thermochromic dyes which absorb the incident radiant energy and get heated to become colorless may be suitable, however, the entire cycle should work in nanoseconds which is the difficult part in it. Research may be initiated in this area to develop such dyes.

Electrical Applications

Attempts have been made to use thermochromic colors as a voltmeter to determine the signal strength of batteries [17]. For example, a thermochromic dye containing layer may be arranged in contact with an electrically conductive layer which extends between positive and negative electrodes of a battery. Heat is generated as the current flows through the conductive layer thus activating the thermochromic dye in the adjacent layer so that a color change is obtained. This principle is explored further to develop overload warnings, to hide or reveal a background for a display, exhibition, advertisement, teaching material, toy, magnetic device, or the like.

Novel Applications

The color changing properties of thermochromic colors are now being well exploited, the number of applications of the colors are increasing. One such new applications is for cleaning i.e. a cleaning composition comprising a thermochromic ingredient that effects a color change at a given temperature or temperature range [18]. Such a composition can be used to provide a signal, i.e., convey information, to a user of the cleaning composition, or a caregiver employing the cleaning composition. Additionally, the cleaning composition can be employed in a substrate, such as a nonwoven. The cleaning compositions can also be used to provide a signal that helps improve cleaning effectiveness and/or safety and/or entertainment value. Furthermore, compositions comprising one or more thermochromic ingredients may be used to signify, or help induce, a given mental state, psychological state, or state of well being.

These thermochromic colors are also used in umbrella that would exhibit different colors under shade and in sunlight (Fig. 5). The canopy of the umbrella is dyed with the color changing dyes which undergo color change when exposed to sunlight [19].

Figure 5: Color changing umbrella

Camouflage Nets

Camouflage nets are meant to conceal military equipments and objects from detection and attack by an enemy. It has a great effect also on the morale of the fighting forces. Historically, camouflage nets were first used during World War I. The earlier nets were made of hemp/cotton twine, garnished with jute strip scrims, and dyed/coated with green and brown colors. These nets blended in with the surroundings and prevented detection by naked eye or by binoculars. These nets had serious limitations, such as poor camouflage properties, premature fading of colors, susceptibility to fungus growth, short life and high water absorption, resulting in a great increase in weight of the net when wet. With the advent of synthetic fibres, the aformentioned problems were solved in combination with color changing dyes for the two types of terrains generally considered for camouflage, green vegetation and desert region. A consideration for this application is that the color and IR reflectance of the net should match with that of the terrain.

Americos: Thermochromic Dyes

As Americos is now focusing on microencapsulation with its special R&D team, it has developed thermochromic colors which are micro-encapsulated pigments that can reversibly change their colors in response to temperature change. Americos has engineered these pigments in such a way that they have various critical temperature limits at which the change can take place. One such critical temperature limits lies close to human body temperature i.e. 31C. This opens up the application of thermochromic pigments in textiles apart from high-tech applications. Americos thermochromic pigment coated textiles can be used for number of applications like on garment to create novel products and promotional items like T-shirts, on fabrics and garments to print company logo / brand name to prevent duplication, on garments which are used for party wear, thermometers and temperature indicators, security printing, etc.

Americos provides a range of thermochromic colours: Americos Red, Americos Magenta, Americos Vermilion, Americos Orange, Americos Yellow, Americos Yellow Green, Americos Charm Green, Americos Sky Blue, Americos Turquoise Blue, Americos Dark Blue, Americos Violet, and Americos Black. Generally, these colors individually become colorless at above the critical limit of 31C and revert to original color when it is cooled down below 31C. Figure 6 depicts the color changing cycle of a fabric printed with Americos Thermochromic Red. Figure 6a shows the actual print on the fabric whose temperature is below 31C, however, while the fabric is pressed with an iron box the color starts fading as can be observed in Figure 6b and hence leading to complete bleaching of color (Fig. 6c). The trial was conducted on January during which the room temperature is well below the critical temperature limit of the thermochromic color and hence on exposure of the pressed fabric to room temperature, the fabric automatically starts cooling down and the print on the fabric reverts back which can be observed from Figure 6d.

Figure 6 Color changing cycle of a fabric printed with Americos Thermochromic Red

The critical temperature intervals are available from 0C to 70C. There are number of advantages one can derive with Americos spectrum of thermochromic colors having different critical temperature intervals. Firstly, one may mix a selected thermochromic pigment with a normal pigment to bring a change of one color to another in spite of color to colorless on exceeding the critical temperature limit. At lower temperature, the color reveals matched shade. When the temperature is increased, the thermochromic pigments start fading to colorless and show color of only the regular pigments. Therefore, the multiple choices of colors of thermochromic pigments and normal pigments can bring about any color change in the visible spectrum. Secondly, one can mix thermochromic colors having various critical temperature limits in order to have color changes at different temperatures rather than having it at one particular temperature. Such mixing of thermochromic colors produce more than two colors like temperature ranges for 3-color variation. Nevertheless, while selecting the thermochromic colors, the desired color effects and temperature levels should be in consideration.

Conclusion

With the progress in science and technology, the fundamental problems associated with the commercial success of thermochromic colors are now being addressed and hence the possibilities of using the color changing behavior of thermochromic colors in textiles are very promising. Americos, with its special R&D efforts have come up with a variety of thermochromic colors having different critical temperature limits which can effectively be used for creating novel color changing properties on textiles not only at a particular temperature limit with one color shift but also at many stages of temperature limits with multiple color change effects in a single piece of fabric or garment.

References

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