Thermochromism in Ink Chemistry
Thermochromism
Thermochromism refers to the phenomenon of color changes by the agency of heat. Obviously, the color changes are made possible by the temperature-induced chemical or physical changes of materials incorporated into the inks.
Sometimes, the color change occurring at a temperature is permanent, and at other times the original color can be regained on cooling.
Accordingly, we have an irreversible or reversible thermochromic system. The required chemistry can be adopted based on the end use. That means one can select an irreversible thermochromic system when a certain temperature crossing is to be monitored and a reversible system when the actual temperature range is to be monitored. The color change may be achieved with a single chemical material or a mixture of them through physical or chemical changes [3,4,5].
In fact, thermochromism is a special case of the phenomenon called "chromotropism," which refers to the changes in color caused by external influences. To this category belongs the phenomenon of "piezochromism," which is the change in color caused by pressure. If the color change is due to the frictional force, it is referred to as "tribochromism."
The color changes observed when certain materials are ground in a mortar come under the purview of this class, though the possibility of color change emanating from a reduction in the particle size during grinding should be ruled out.
Similarly, the color change shown in different solvents is called as "solvatochromism." Added to this is the input from the branch of photochemistry called photochromism that represents light induced color transitions. Other areas such as "electrochromism" (color changes caused by electricity) are also emerging.
Applications
There are many applications where the temperature at which a certain change occurs is required to be registered. For example, it may be necessary to ensure that a delicate material or foodstuff is maintained at a
Thermochromism refers to the phenomenon of color changes by the agency of heat. Obviously, the color changes are made possible by the temperature-induced chemical or physical changes of materials incorporated into the inks.
Sometimes, the color change occurring at a temperature is permanent, and at other times the original color can be regained on cooling.
Accordingly, we have an irreversible or reversible thermochromic system. The required chemistry can be adopted based on the end use. That means one can select an irreversible thermochromic system when a certain temperature crossing is to be monitored and a reversible system when the actual temperature range is to be monitored. The color change may be achieved with a single chemical material or a mixture of them through physical or chemical changes [3,4,5].
In fact, thermochromism is a special case of the phenomenon called "chromotropism," which refers to the changes in color caused by external influences. To this category belongs the phenomenon of "piezochromism," which is the change in color caused by pressure. If the color change is due to the frictional force, it is referred to as "tribochromism."
The color changes observed when certain materials are ground in a mortar come under the purview of this class, though the possibility of color change emanating from a reduction in the particle size during grinding should be ruled out.
Similarly, the color change shown in different solvents is called as "solvatochromism." Added to this is the input from the branch of photochemistry called photochromism that represents light induced color transitions. Other areas such as "electrochromism" (color changes caused by electricity) are also emerging.
Applications
There are many applications where the temperature at which a certain change occurs is required to be registered. For example, it may be necessary to ensure that a delicate material or foodstuff is maintained at a
References: (1.) L. A. Ford, "Chemical Magic," 2nd edition, Dover Publications, Inc., New York, 1993. (2.) J (3.) J. H. Day, Chemical Reviews, 63, (1963) 65. (4.) J (5.) J. H. Day, "Kirk-Othmer 's Encyclopedia of Chemical Technology," Vol.6, Wiley-Interscience, New York, 1979, p.129. (6.) C (7.) R. Foster, "Organic Charge-Transfer Complexes," Academic Press, London, 1969. (8.) H (9.) R. C. Teitelbaum et al., Journal of American Chemical Society, 100 (1978) 3215 (10.) C (11.) J. G. Hughes, Journal of Chemical Education, 75 (1998) 57. (12.) K (13.) K. Nassau, "The Physics and Chemistry of Color," John Wiley & Sons, New York, 1983, pp. 77,109. (14.) S (15.) P. J. Collins, "Kirk-Othmer Concise Encyclopedia of Chemical Technology," 4th edn. John-Wiley & Sons, Inc., New York, 1999. (16.) D (17.) Joy T. Kunjappu, Ink World, August, 1998, p.32. (18.) Joy T