Photochromic pigments change color in sunlight

Name: Chris

—ADVERTISEMENTS—

Syuzi Pakhchyan's book

Fashioning Technology:
A DIY Intro to Smart Crafting
includes instructions for using photochromic screenprinting ink to make light-darkened window blinds

Heinz Durr's book
Photochromism: Molecules and Systems
provides detailed technical and chemical information about photochromic dyes and pigments.

Country or region: U.S.A.

Message: I have bought T-Shirts in the Caribbean that are white until they are in the sun. Then the colors come out. I'd like to make my own. Where can I find these markers? Or is it a paint?

Since you want a color that disappears entirely to white whenever it's in the shade, I think what you're looking for are photochromic pigments.

Most dyes you see described as "UV-reactive" are actually fluorescent, with an added brilliance that shows up under ultraviolet light, such as a blacklight. I don't think that's what you're looking for, because fluorescent dyes are not white under regular light. A blacklight just makes fluorescent dyes shine much more brightly, almost as though they had a light source in them, usually (though not always) in the same color as the one you see without the blacklight. Photochromic pigments are much rarer and harder to find than ordinary fluorescent dyes and pigments, and they are much more expensive.

I wrote about these pigments several years ago, in a paragraph on a page entitled "Thermochromic pigment changes color when warm, and changes back when cool", in the December 01, 2007 entry in my All About Hand Dyeing Q&A blog:

"...UV-reactive pigments...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."

Photochromic pigments then return to their original color shortly after being returned to low-ultraviolet conditions. I haven't seen these pigments being sold in marker form, but you can buy photochromic screen-printing ink, or you can buy the photochromic pigments in order to mix your own color-changing fabric paint.

MUTR (Middlesex University Teaching Resources) in the UK sells the photochromic pigments. As they describe them (the picture at the left is an example from their website),

"These pigments normally have a pale, off-white appearance but in sunlight or UV light they instantly change to a bright, vivid colour. The pigments revert to their pale colour when away from sunlight or UV light.

"The active photochromic dye is contained in micron-size microcapsules dispersed in water to make up the pigment, which has the consistency of thick cream. If the pigment is mixed with a binder such as our acrylic base, it can be permanently applied to almost any surface. The more dilute the pigment, the less dramatic the colour change; the mixing proportions are therefore a matter for trial and error but a 50:50 mix is a good starting point.

"The practical applications of the pigments are almost limitless. Uses range from UV warning systems to ‘smart graphics’ in publications and packaging and garment design. Methods of application include brush painting, block printing, stencilling, screen printing etc."

The pigments are rather costly, compared to non-changing dyes and fabric paints, and it's possible that they will not endure many launderings. A three-gram syringe of pigment, which is made using microencapsulated photochromic dyes, costs about £2, or about 3 US dollars (plus significant shipping costs). It should be mixed with an equal quantity of a colorless clear acrylic fabric paint binder, such as Jacquard Products' Neopaque Clear Extender; this binder will serve to 'glue' the pigment particles to the outside of the fibers in the fabric. You would probably need at least ten or twenty of these syringes in order to make enough fabric paint to cover an entire t-shirt. This means that it makes more sense to use this material to make smaller designs on a shirt that is otherwise dyed conventionally, rather than using it for coloring the entire surface of the shirt; since paints made from photochromic pigments are transparent, be sure to apply them only to white sections of fabric, or to colors selected for how well they will combine with the photochromic colors. The syringes of photochromic pigment are available in four colors: magenta, blue, orange, and purple.

Some screen-printing ink suppliers carry specialty inks, including screen-printing ink that contains photochromic pigments. Note that screen-printing ink, unlike dye, tends to leave a very noticeable change in the feeling of the fabric. Apollo Colours, in the UK, sells photochromic inks in red, blue and yellow. As they describe their products, "Prints produced using these inks will change from colourless to coloured when exposed to light. They can be supplied in several ink types for plastic, metal and paper. This product is available in standard and a long afterglow version." The long afterglow version seems like it would be more fun.

Look for photochromic pigments from screen-printing suppliers in the form of Photochromic Plastisol Inks. One source for these in the US is the Union Ink Company, who list it among their specialty inks. They write, "Photochromic Plastisol Inks are almost colorless when viewed indoors but when viewed outside or under a source of intense UV light they quickly acquire a rich, bright color. This enables you to provide your customers with prints that display one design indoors and a dramatically changed design indoors." The only drawback to ordering from a large-scale screen-printing supplier is likely to be the large size of a minimum order. You should contact screen printing suppliers to ask about minimum order sizes and prices for their photochromic plastisol inks. They might be able to refer you to a retailer who sells their products in smaller quantities. Photochromic plastisol inks are said cost about four times as much as other plastisol inks.

Photochromic pigments are also available in automotive paint, not suitable for use on fabrics, largely because it would be extremely stiff and scratchy; see Stardust Colors. You can also purchase photochromic embroidery thread! There are many other photochromic products available from Solar Active International.

Color-changing pigments are a fascinating novelty, but they tend to be less useful for clothing than other colorants, due to a limited lifespan when subjected to repeated laundering. Pigment-dyed photochromic shirts will not remain colorful nearly as long as a shirt that has been permanently dyed in non-changing color using a fiber reactive dye, such as Procion MX dye.

A different but related phenomenon is sunprinting, in which the sun is used to make a one-time-only change, in order to use light to create designs on fabric. Sunprinting can be done with transparent fabric paints, with light-sensitive vat dyes, or with cyanotype printing using the same principle as old-fashioned blueprints. For more on this subject, please see my page, "How to Dye and Paint Fabric with Light".

(Please help support this web site. Thank you.)

Posted: Wednesday - September 08, 2010 at 08:11 AM

Follow this blog on twitter here.
Tweet

getBlogName(); > getCategoryName("<$EntryCategoryFolder$>"); >

Photochromic pigments change color in sunlight

> >