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


Abstract


Increasing awareness among the consumers of textiles paves the path way for novel developments quite apart from the conventional research focus. The thrust is emphasized on micro and nano technology for special purpose applications like military and medical, and space applications and also for general applications like self-cleaning effects on garments which are expected out of the phenomenon called Lotus Effect. The chromic colors are interesting in that they change the color in response to external stimuli and hence they produce novel effects on fabrics and garments. Of the various chromic colors, thermochromic colors are widely studied and used in textiles for many novel applications. This paper aims at discussing various aspects of thermochromic dyes such as principle, problems associated, recent developments and its various applications. The article also highlights the contributions from Americos in the specialty application using thermochromic dyes.


Introduction


Chromic colors are those which undergo reversible change in color. Generally the color change is based on alteration of the electron states of molecules, especially the π- or d-electron state. This phenomenon is induced by various external stimuli which can alter the electron density of substances. The process of such reversible color change is commonly known as chromism. Chromism is classified by what kind of 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 solvatochromism which depends on the polarity of the solvent. Some of the other known chromisms are ionochromism color change induced by ions, halochromism color change induced by pH, tribochromism color change induced by mechanical friction and piezochromism color change induced by mechanical pressure.


Of these, thermochromic colors are the most common colors used for various applications of which mood ring is a good example. The thermochromic colors are derived fundamentally from liquid crystals and leuco dyes. Due to very high temperature sensitivity, liquid crystals are used in precision applications, however, their color range is limited by their principle of operation. In contrast, leuco dyes allow wider range of colors to be used, but their 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 is warm. The most well-known line of clothing utilizing thermochromics was Hypercolor. The thermometers are often used on the exterior of aquariums, or to obtain a body temperature via the forehead.


Photochromic colors 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 and zinc halides which are inorganic in nature are also extensively used. In particular, silver chloride is extensively used in the manufacture of photochromic lenses for sunglasses, as found in eye-glasses which is one of the most famous reversible photochromic applications. Photochromic dyes are generaly used for making novelty items such as toys, cosmetics, clothing and other industrial applications. Some of the high-tech applications of photochromic colors being investigated with a few commercial successes are as molecular switches in supramolecular chemistry, as data storage in 3D optical data processing.


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