Daffodil International University
Faculty of Engineering => Textile Engineering => Topic started by: nawshin farzana on July 22, 2014, 05:37:17 AM
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Chromic Materials
Chromic materials are the general term referring to materials which radiate the color, erase the color or just change it because its induction caused by the external stimuli, as "chromic" is a suffix that means color. It can be classified depend on the Stimuli. Out of this the first four chromic materials are important and has potential to cater the market
. Photochromic: external stimuli energy is light.
. Thermochromic: external stimuli energy is heat.
. Ionochromic: external stimuli energy is pH value
. Electrochromic: external stimuli energy is electricity.
. Piezorochromic: external stimuli energy is pressure
. Solvatechromic: external stimuli energy is liquid.
. Carsolchromic: external stimuli energy is electron beam.
1 Photochromic :
In this kind of chromism the color change is due to the intensity of the light(UV radiation also). The photochromic dyes interact with the electromagnetic radiation in the near UV (300-400nm),Visible(400-700nm) and near IR(700-1500nm) to produce verity of effects, which is reversible when the radiation is withdrawn. Photochromism is of Two types. Positive and Negative. In Positive Photochromism the colorless substance converted in to colored object when exposed in to the light due to Uni-molecular reaction system. Bi molecular reaction system is called Negative Photochromism i.e. from colored to colorless. the transformation is between two states that have different absorption spectra. It may be induced in one or both the direction by electromagnetic radiation. This process is a non destructive process., but side reactions may occur. Oxidation is the major cause for the degradation of the Photochromism. Main class of Photochromism is Spiropyrans. It is used in Optical switching data and Imaging system rather then the textile applications.
2 Thermochromic:
Thermally induced reversible color change occur in the thermochromism. A large variety of substrates such as Organic ,Inorganic Orgonomatallic and Macro molecular systems exhibit this phenomena. Mercury Iodide salts like Ag2 HgI4 shows color change from yellow to orange at 51C.This is due to the reason that each compound can under go phase change at particular temperature .Majority of thermochromic systems are unacceptable simply because of the change in the color requires large amount of energy due to involvement of inter molecular transformation. Using verity of liquid crystals ,it is possible to achive significant changes in the appearance over the narrow temperature range(5-15C) and to detect small variation in the temperature(≤1C).The thermochromic dyes used extensively in the printing of Textiles, Micro encapsulation ,coating or dope dyeing .
3 Ionochromic dye:
These chromic materials are sensitive to pH. Widely used these classes dyes are Phthalides, Triarylmethans and Fluorans. In analytical chemistry these dyes are used extensively. There are no commercial application of these dyes in textiles but direct thermal printing can be used. In this process substrate contain both the color former and acid co reactant in a single layer. simply by heating the surface of the paper with a thermal head causes the components to react and to produce color.
4 Electochromic dye:
The material that change color upon the application of Voltage are called electrochromes. This is due to oxidation and reduction process with in the electochromic material. This are of three types. First type, the coloring species remain in the solution. In the second type the reactants are in solution but the colored product is of solid. In the third type is both reactant and the color is in form of solid e.g. composite Film. Most available electochromic dyes are of inorganic oxides such as cobalt oxide, nickel oxide, molybdenum trioxide. A research is going on in MIT,USA to use thin film composite electrode material with layer by layer assembly technique, to identify whether electrochemical cell is fully charged or discharged by using color change. The most important commercial application of the electrochemic dye in the textile is of US-Military IR camouflage material (Dynam IR) .
5 Solvantochromic dye:
The Solvantochromism is a reversible variation of the electronic spectroscopic properties (absorption and emission)of a chemical species, induced by the solvents. It is one of the oldest chromism have been described as long as ago 1878.This is used as probes for application in polymer characterization. Where they can be used to look for localized polar features at the molecular level.
Chromic dyes contain highly specialized components that require extraordinary careful manufacturing technique and has great potential for both fashion and higher end market.