thermochromic pigment changes color when warm, and changes back again when cool

Name: yuvaraj

Message: There is a type of dye that, after dyeing, changes its colour: in heat, pink, and in cold temperatures, violet. I just saw a small swatch. My question is what class of dyes is it & how to dye.

What you're talking about is thermochromic pigment (sometimes referred to as thermochromatic). In the case that you saw, a thermochromic ink was used that is blue when cool and colorless when hot, combined with an ordinary pink dye on the same fabric.

I do not believe that this is a true textile dye; that is, the molecule that gives the color does not itself attach to the fiber directly. Instead, it is a pigment, which requires a binder, such as some sort of acrylic, to attach it to the fiber. This means that, in the context of textiles, it is actually a fabric paint, not a dye. The particle sizes of thermochromic pigments are larger than those in other pigment dyes, so it will be necessary to pay attention to the manufacturer's suggestions when applying it.

My experience with a couple of garments that had been commercially colored with Hypercolor thermochromic pigment was one of poor washfastness. The garments did their color change well only for a short time. After a few weeks (of weekly launderings), they faded out and no longer made the color change. However, the thermochromic effect was fun enough to be worth trying even for a short-term effect. I've seen many more non-textile objects with thermochromic ink, such as coffee mugs that change their decoration when filled with a hot liquid. Thermochromic clothing is now available in the US from Dyenamic Infusion, which claims improved washfastness for their line of color-changing shirts.
Thermochromic fabric paint
So far, I have seen only one source for small quantities of do-it-yourself thermochromic fabric paint, in the UK, at Ridgewell Press Smart Textiles. [Update: link to internet archive page.] They do ship internationally, but, if you are not in Europe, you must contact them to find out what the postage will cost. Here are their instructions:
"Using thermochromic fabric paint
The thermochromic paint is already mixed with an acrylic binder so it can be painted or screen printed directly onto the fabric. Choose light coloured fabrics for this work. If you use dark fabrics you will not be able to see the colour change. 

"Dry the fabric, heat seal it with an iron or heat press to fix the dye and the fabric is ready to use. Put a piece of parchment or greaseproof paper on top of the fabric and iron with a hot iron for a short time. 

"Thermochromic dyes are most effective when used over another lighter colour, so you could use the orange on top of yellow, or the blue on top of light green."

On the same page they sell UV-reactive pigments which are colorless indoors, but turn into bright colors in the sunlight. This is a different phenomenon than that of fluorescence, in which a dye or pigment absorbs one wavelength of light (usually invisible ultraviolet) and then emits that same energy as another wavelength. Instead, the UV-reactive pigments actually change their structure to a colored form, under the influence of ultraviolet light, after which the additional visible light which reaches the pigment is reflected so as to produce a color.

There are other sources for thermochromic inks, but these will not be washproof when used on fabric, unless the manufacturer specifically claims that they will. Look for thermochromic plastisol inks that are intended for use in printing t-shirts. These will feel more like plastic on the fabric, like many commercially screen-printed t-shirts, instead of like dye. I wonder whether they might be more resistant to damage in the laundry than the old Hypercolor pigment-dyed shirts. Always wash fabric-painted garments with care, turning them inside out before laundering, and placing them in a net lingerie bag for additional protection against wear. Thermochromic inks should probably, in addition, be washed only in cool water, and drip-dried, not machine-dried, as this might help to extend their working lives.

How does it work? There are two different types of thermochromic inks, one based on liquid crystals and the other based on leucodyes. Liquid crystals are impractical to work with in textiles, so we can ignore them. Leucodyes are chemically part of the vat dye family; the most familiar vat dye is indigo dye. Thermochromic leucodye mixtures are quite different from indigo, however. There are many different dyes that change color when the pH changes. The clever thing about thermochromic inks is that the mixture the dye is placed in causes the pH around the dye ingredient to change when the temperature changes. The leucodye mixtures are microencapsulated in a protective coating: a leucodye that changes color when the pH changes, along with a fatty acid/solvent that will reversibly change state upon exposure to heat, and a weak acid that will change the pH of the solution when the fatty acid is melted. The relatively large pigment particles that result are then mixed with compatible binders and other materials to create the thermochromic ink. Thermochromic inks are very expensive in comparison to ordinary inks, but have many interesting and useful applications. For more information see a 2003 article written by Timothy J. Homola, "Color-Changing Inks: Brighten your bottom line". [Updated link.] Also see Wikipedia's surprisingly well-written article on Thermochromism, as well as one on the Hypercolor line of clothing.

Chapter 10 of Organic photochromic and thermochromic compounds: volume 2 John Crano and Robert Guglielmetti's book, Organic Photochromic and Thermochromic Compounds: Volume 2: Physicochemical Studies, Biological Applications, and Thermochromism, goes into more detail about the chemistry of thermochromic compounds, including "spiroheterocyclic compounds, Schiff bases and related nitrogen-containing molecules, bianthrones and other overcrowded ethenes, and miscellaneous compounds." It's a very expensive book, but perhaps you could find it at the library of a university or other institution.

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Posted: Saturday - December 01, 2007 at 08:00 AM          

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