Color-Changing Materials Named After Ömer Edip Kuzugüdenli

Color-Changing Substances

Paints are used for many different purposes on the materials they coat: protection, aesthetics, emphasis, attention-drawing or warning. In coloring applications, new concepts and practices are developing at an increasing rate. One of these is the color-changing property arising from preference or necessity. For example, what if a white shirt worn by an office worker turned rosy purple when the collar button came undone and the jacket was removed? Or if tools being used became hot enough to burn the hand and changed color to red as a warning? Some people might even want their home's color to change with the seasons. In fact, all of these can be realized to a certain extent today and are expected to become more widespread in the future. Many physical or chemical principles can be mentioned that could impart color-changing properties to materials. This article will address a few of them. Every material has an expansion coefficient that differs more or less from others. This effect can also be seen in daily life. For example, a glass with a small expansion coefficient is not affected by thermal shock (pouring hot water into it), whereas a glass with a large coefficient will crack. The operation of many thermometers is based on this different expansion principle. Using materials with different expansion coefficients, products that undergo heat-dependent color and pattern changes will find many application areas not only for decorative but also for functional purposes. In woven products, due to the different expansion of interlocking components under heat, some colors can be hidden and others revealed by slipping, thus providing color and pattern differences[1]. To give a simplified example: let a weave be created with yellow, with limited expansion, horizontal parallel thin strips, and perpendicular to these, with high expansion coefficient, blue strips. Let the surface of this structure, with each strip group fixed at one end, be painted red. When temperature changes, the lengths of these strips will also change, so over time the red color will be hidden while yellow and blue colors will begin to appear. In addition to the material's own expansion properties, if the end-fastening and color arrangements are changed, the visual variety will also increase. By programming, factors such as the color, expansion coefficient, elasticity and fastening ratios of each material used can be planned to create very different patterns (Figures 1 and 2). As a result, very different appearances can be obtained from the mixture of color tones used.

Figure 1. Every material expands differently under the effect of temperature.

Figure 2. Using materials that expand differently, many different colors and patterns dependent on ambient temperature can be obtained. Liquids with very high boiling points can take days, weeks or even months to evaporate. These types of liquids, called fixatives in perfumes, when mixed with pleasant-smelling substances, extend the effect duration of those substances for as long as the fixative remains. Similar effects can be achieved by mixing such liquids with color-giving substances. That is, the duration of color retention on a surface can be limited. If several color layers are applied, changing color effects will be observed. For example, if a blue coating is first applied over a white base and then a yellow coating, yellow will be seen first, then as its effect diminishes and the blue underneath begins to show, green will appear, then blue, light blue, and finally the white color at the base (Figure 3). Figure 3. By limiting paint retention time on the surface, the color underneath can be revealed. Color changes based on chemical transformations generally offer broader possibilities. Such changes occur as a result of chemical transformations that color-giving substances undergo. These transformations can arise from many causes over time such as structural degradation, air, moisture, heat and light. Copper sulfate, which is colorless when dry, gradually becomes its more commonly known blue color over time due to the moisture in the air. The compounds found on litmus paper change color depending on the acidity and alkalinity of the solution they are immersed in, and this is utilized in pH measurement based on the reaction type. Optical brighteners from detergent additives remain on the laundry after washing and convert absorbed ultraviolet light into bluish-white light, preventing light soiling from being noticed. The number of such unidirectional or reversible examples in daily life and industrial products is quite considerable. Some materials have temporary or permanent color-changing properties dependent on temperature or light [2-6]. For example, the transformation of silver with copper or chlorine into structures of different colors depending on heat and light is used in the production of light-sensitive eyeglasses and car windows. AgCl = Ag0 + Cl0 or Ag+ + Cu+ = Ag0 + Cu++ The tendency of these equations is directed to the right by strong light, and the increase in product amount causes darkening of color (Figure 4). In this reversible reaction, when light intensity decreases, the amount of reactants will increase and as the proportion of products decreases, color lightening or transparency will be observed. Figure 4. In sunglasses lenses, the product of the reaction occurring under sunlight darkens the color of the lens.   Photographic film image recording occurs similarly but as a result of irreversible reactions. Depending on how equilibrium is established in a chemical reaction, in addition to color formation, failure to establish equilibrium can produce very interesting results. Due to failure to reach equilibrium, as a result of oscillation in the forward and reverse directions of the reaction, continuous color change (oscillating color) is observed. In these events, called chemical clock or oscillation reactions, where stable equilibrium cannot be achieved, periodic changes occur in the concentration of one or more of the reaction components, and accordingly color changes are observed. Belousov-Zhabotinsky, Briggs-Rauscher, Bray-Liebhafsky and iodine clock reactions can be cited as examples of such reactions. Among very interesting laboratory experiments, this phenomenon is still among the subjects whose secrets are being solved. Adapting the interesting color properties of these reactions to paint materials is not very suitable due to limitations such as practicality and safety [7-10]. The number of color-changing methods exemplified above can be increased. While applications are currently limited, environments and products that change color according to the properties gained or the surroundings in which they are found are expected to become much more widespread in the future.

Assoc. Prof. Dr. Ömer Edip Kuzugüdenli - Chair of Industrial Chemistry Department / Faculty of Science - Erciyes University

References

1. Kuzugudenli, O.E., Tacar, Z. andKuzugudenli, S., "Development of ColorandPatternChangingTextilesDuetoDifferentExpansionswithChangingHeat", UTIB VII. International R&D BrokerageEvent, Bursa, April 26-29, 2015.

2. Shreve, R.N. andBrink, J.A., ChemicalProcessIndustries, 4th ed., McGrawhillInc., New York, USA, 1976.

3. Jones, M. M., Netterville, J. T., Johnston, D. O. andWood, J. L., Chemistry, Man andSociety, 2nd ed., SaundersCo., Philadelphia, USA, 1976.

4. Adamson, A.W. andGast, A.P., PhysicalChemistry of Surfaces, 6th ed., John WileyandSons, Inc., New York, 1997.

5. Serway, R.A. andBeichner, R.J., PhysicsforScientists, 5th ed., Saunders, N.Y., 2000.

6. Atkins, P.W, PhysicalChemistry, 6th ed., Oxford UniversityPress, N.Y., 1998.

7. Pellitero, M.A., Lamsfus, C.A. andJavierBorge, J., "TheBelousov−ZhabotinskiiReaction: ImprovingtheOregonator -Model withtheArrheniusEquation", J of Chemical, Education, Nov. 2012.

8. Prypsztejn, H.E., Douglas R. Mulford, D.R.andDougStratton, D., "ChemiluminescentOscillatingDemonstrations: TheChemicalBuoy, theLightingWave, andtheGhostlyCylinder W", J of ChemicalEducation, Jan 2005.

9. Matsugo, S. andKanamori, K., "Chemicaloscillation of vanadiumcomplexes: Simple andaperiodicsystems,"CoordinationChemistryReviews, No. 255, 2011.

10. Lefelhocz, J.F., TheColorBlindTraffitLight - An undergraduatekineticsexperimentusing an oscillatingreaction, Vol. 49, No. 5, 1972.