*8.2.1 As a disperse dyes in printing*

Photochromic dyes can be used as a disperse dyes which are insoluble in water. The photochromic dyes are disperse with dispersing agent and wetting agent of anionic nature. The dye dispersion is milled on a roller mill by using ceramic balls in glass jar. The dye dispersion is mixed with sodium alginate thickener solution to get printing paste. The polyester or nylon fabric can be printed with printing paste. The fabric is dried at 100 °C and cured at 140 °C for 5 minute. After reduction clearing treatment, printed samples were soaped with nonionic detergent and finally neutralization is done.

### **8.3 Application of thermochromic materials**

Photochromic materials have applications in both textiles and non textiles. In the textile field new fashionable colors in T-shirts, hand bags and caps are produced by dyeing with photochromic dyes. There is application of photo colorable textured yarn in knitting, weaving and embroidery. Polypropylene threads are produced by mass coloration by adding photochromic dyes in melt polymer solution, which on exposure to U.V. light produces different colors. Photochromic colorants are also used in developing camouflage patterns for military protective clothing. The patterns change their colors on exposure to sunlight and match with surrounding environment. Photochromic microencapsulated fabrics are produced which change their color on exposure to sun light. In non textile field photochromic materials are used in ophthalmics, surface coating applications and dye lasers.

developed by U.V. irradiation varies with photochromic colorant classes. It was revealed that in selected spirooxazine colorants the degree of photo coloration increases with initial washing and subsequently decreases. In case of napthopyrans, the degree of photo coloration decreases continuously with successive number of washings. In case of printing the washing fastness more depends on binder quality.

The conventional method of exposing the sample to accelerated fading instruments (Xenotest or MBTF) is not applicable to photochromic colorants. In photochromic colorants due to dynamic color change properties, for light fastness measurement a normalized value of color value to be calculated. The normalized value is defined as the

ΔEo = color difference before and after U.V. Irradiation without exposing to

Thermochromism may be defined as the reversible change in the color of compound due to temperature change [47]. The phenomenon of thermochromism may occur even in small temperature interval. The thermochromism can be depicted as

The following type of materials can exhibit thermochromism properties [48].

It may occur as a result of equilibrium between molecular species such as acid – base, keto-enol and different crystal structure. The organic thermochromic mate-

*Reversible color change from species a to B due to thermochromism. Here A = colored or colorless. B = colored or*

thermochromic compounds show sharp color change due to temperature variation. The different organic thermochromic compounds may be classified as follows.

Some organic materials when pass from crystalline solid to isotropic liquid state, they form stable intermediate phases (mesophase). Transitions between phases are

rials have application in fibers, optics and optical sensors. The organic

degree of photo coloration after a particular time of exposure on the xenotest instumemt to the fraction of initial degree of photo coloration i.e., ΔE/ΔEo.

**8.6 Light fastness/photostability**

*Photochromic Dyes for Smart Textiles*

*DOI: http://dx.doi.org/10.5772/intechopen.96055*

xenotest instrument.

**9. Thermochromism**

shown in **Figure 10**.

• Polymers

• Sol gels

**Figure 10.**

*colorless.*

**91**

• Organic compounds

• Inorganic compounds

**10. Organic compounds**

**10.1 Liquid crystals**

#### **8.4 Color measurement**

Due to reversible color changing properties of photochromic dyes, it is very difficult to measure color value of the shade produced due to photochromic effect. For measuring the color value, it is essential to control several parameters such as temperature and time interval between U.V. irradiation of sample and measurement. A.F. Little et al. [45] developed a technology to measure the color value of photochromic textiles using independent U.V. irradiation with traditional spectrophotometer. The temperature of sample measuring cabinet was controlled by localized air heating system. The temperature of sample cabinet is maintained at 24 °C and time interval of 30 sec. is kept between irradiation and measurement of sample which can be seen in **Figure 9**.

#### **8.5 Washing fastness test**

Due to dynamic color change properties of photochromic dyes, it is difficult to measure the fastness properties. The traditional assessment method of color fastness using gray scale standards [46] are not appropriate, therefore it is measured by comparative test method. In this method we measure the color difference of sample before and after wash and compare with color difference before U.V. irradiation and after 1 min. Exposure to U.V. irradiation. The level of photo coloration

#### *Photochromic Dyes for Smart Textiles DOI: http://dx.doi.org/10.5772/intechopen.96055*

developed by U.V. irradiation varies with photochromic colorant classes. It was revealed that in selected spirooxazine colorants the degree of photo coloration increases with initial washing and subsequently decreases. In case of napthopyrans, the degree of photo coloration decreases continuously with successive number of washings. In case of printing the washing fastness more depends on binder quality.

#### **8.6 Light fastness/photostability**

**8.3 Application of thermochromic materials**

*Dyes and Pigments - Novel Applications and Waste Treatment*

**8.4 Color measurement**

which can be seen in **Figure 9**.

**8.5 Washing fastness test**

**Figure 9.**

**90**

used in ophthalmics, surface coating applications and dye lasers.

Photochromic materials have applications in both textiles and non textiles. In the textile field new fashionable colors in T-shirts, hand bags and caps are produced by dyeing with photochromic dyes. There is application of photo colorable textured yarn in knitting, weaving and embroidery. Polypropylene threads are produced by mass coloration by adding photochromic dyes in melt polymer solution, which on exposure to U.V. light produces different colors. Photochromic colorants are also used in developing camouflage patterns for military protective clothing. The patterns change their colors on exposure to sunlight and match with surrounding environment. Photochromic microencapsulated fabrics are produced which change their color on exposure to sun light. In non textile field photochromic materials are

Due to reversible color changing properties of photochromic dyes, it is very difficult to measure color value of the shade produced due to photochromic effect. For measuring the color value, it is essential to control several parameters such as temperature and time interval between U.V. irradiation of sample and measurement. A.F. Little et al. [45] developed a technology to measure the color value of photochromic textiles using independent U.V. irradiation with traditional spectrophotometer. The temperature of sample measuring cabinet was controlled by localized air heating system. The temperature of sample cabinet is maintained at 24 °C and time interval of 30 sec. is kept between irradiation and measurement of sample

Due to dynamic color change properties of photochromic dyes, it is difficult to measure the fastness properties. The traditional assessment method of color fastness using gray scale standards [46] are not appropriate, therefore it is measured by comparative test method. In this method we measure the color difference of sample before and after wash and compare with color difference before U.V. irradiation and after 1 min. Exposure to U.V. irradiation. The level of photo coloration

*Color measurement (color bleaching) of photochromic dyes fabric samples in 30 sec. Time interval.*

The conventional method of exposing the sample to accelerated fading instruments (Xenotest or MBTF) is not applicable to photochromic colorants. In photochromic colorants due to dynamic color change properties, for light fastness measurement a normalized value of color value to be calculated. The normalized value is defined as the degree of photo coloration after a particular time of exposure on the xenotest instumemt to the fraction of initial degree of photo coloration i.e., ΔE/ΔEo.

ΔEo = color difference before and after U.V. Irradiation without exposing to xenotest instrument.

#### **9. Thermochromism**

Thermochromism may be defined as the reversible change in the color of compound due to temperature change [47]. The phenomenon of thermochromism may occur even in small temperature interval. The thermochromism can be depicted as shown in **Figure 10**.

The following type of materials can exhibit thermochromism properties [48].


#### **Figure 10.**

*Reversible color change from species a to B due to thermochromism. Here A = colored or colorless. B = colored or colorless.*

#### **10. Organic compounds**

It may occur as a result of equilibrium between molecular species such as acid – base, keto-enol and different crystal structure. The organic thermochromic materials have application in fibers, optics and optical sensors. The organic thermochromic compounds show sharp color change due to temperature variation. The different organic thermochromic compounds may be classified as follows.

#### **10.1 Liquid crystals**

Some organic materials when pass from crystalline solid to isotropic liquid state, they form stable intermediate phases (mesophase). Transitions between phases are

**12. Microencapsulation**

*Photochromic Dyes for Smart Textiles*

*DOI: http://dx.doi.org/10.5772/intechopen.96055*

narrow color ranges.

**13.1 By exhaust method**

• Cationic agent 5–8% (owf)

• Acrylic Binder 10–15%

thermochromic colorant occur.

• Cationic agent 10–15%

• Acrylic binder 20%

**93**

**13.2 Continuous method (In solution)**

• Thermochromic pigment 50%

• Nonionic dispersing and leveling agent 10–15%

• Thermochromic Pigments 10–15% (owf)

derivatives.

In reversible thermochromic compounds a colorless dye precursor and color developer both are dissolved in hydrophobic non volatile organic solvent and resulted solution is encapsulated [53]. On heating, melting of organic solvent occurs and there is appearance of color in thermochromic compound. On cooling solvent solidify and system comes to original color. Microencapsulation has some advantages that it protect sensitive coloring agents from external environment and allows several thermochromic colorants to be combined together and produces several

The organic solvents used in microencapsulation are alcohols, hydrocarbons,

Thoroughly pretreated fabric taken in water keeping M; L ratio 1:20 and we add cationic agent at temperature of 60 °C and run in the aqueous media for 15 min to get positive charge on fabric. After treatment fresh water is taken in the bath and thermochromic pigments are added, temperature is maintained at 70 °C run fabric for 10–15 min. During dyeing non ionic dispersing agent and leveling agent are added. Finally acrylic binder is added and we run the fabric at 70 C for 15 min.The fabric sample is soaped and washed. In microencapsulated fabric the melting temperature of solvent control the temperature at which decolonization/colorization of

ester, ketones, thiols and alcohol –acrylonitrile mixture. The important thermochromic colorants are N-acyl leuco-methylene blue derivatives, fluoran dyes and diphenylmethane compounds. There are large number of compounds work as color developers such as phenol derivatives specifically bisphenol A and bisphenol B. In some recent work there are use of 1,2,3 triazoles such as 1,2,3 benzotriazole, dibenzotriazole, thioureas and 4 hydroxy coumarin

**13. Application of thermochromic pigments on textiles**

• Non ionic leveling and dispersing agent 10–15% (owf)

**Figure 11.** *Chiral molecules in cholesteric mesophases form.*

brought either by influence of temperature or solvent respectively [49]. The cholesteric (chiral nematic) are the most important type of liquid crystals for thermochromic systems. Here molecules are arranged in helical form. The reflection of light by liquid crystals are influenced by temperature. The pitch of the helical arrangement of the molecules decides the wavelength of reflected light [50]. The liquid crystals are microencapsulated to get the thermochromic effect. They are applied to the fabric with the help of binder (**Figure 11**).

#### **10.2 Stereoisomerism**

Organic molecules which posses stereoisomerism, show thermochromism especially ethylenes. When change in temperature takes place, molecules of these compounds switch between different stereoisomers. Generally the required temperature for thermochromism to takes place is more than 150 C. So they can not be used for textiles application [51].

In another case, the crystal violet lactone is pH dependent rather than temperature dependent. At pH above 4.0 crystal violet lactone is colorless and below 4.0 pH, it is violet.

#### *10.2.1 Rearrangement*

Due to molecular rearrangement of organic compounds more conjugated structure resulted and formation of new chromophores take place. Such type of molecular arrangement occur due to temperature variation, change in polarity of the solvent or pH of the solution.

#### **11. Inorganic thermochromic system**

Thermochromic behavior are exhibited by solid or liquid inorganic molecules. In such type of molecules thermochromic properties are due to following mechanism [52],


These compounds show theromochromic properties at high temperature (150<sup>0</sup> C), therefore they are not suitable for textile application.
