Super-bright glow-in-the-dark paints Discussion with pictures about a new type of paint that glows in the dark and is ten times brighter than normal phosphorescent paints

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  Three things always bring a smile to the faces of children, and adults who still have the joy of youth: soap bubbles drifting on a breeze, helium-filled balloons, and things that glow in the dark. Of these three, the last is the most mysterious and regrettably, many times disappointing because whatever it is that is glowing, isn't glowing very brightly... but not any more!

  All of the glow-in-the-dark paints available in hobby and party-supply stores use zinc sulfide-based phosphorescent pigments in an acrylic medium. These paints were developed in the 1970s and glow with a weak, yellow-green light. Occasionally, colored paints can be found but these are even dimmer because the colored pigments added to create the color block and absorb most of the light. Also, the acrylic medium is so thick it's difficult to get an even coat. The result is a weak, uneven glow. BUT, then someone discovered a new formulation!

  Industrial-strength glow-in-the-dark paints use strontium (the non-radioactive type) based pigments that glow ten times brighter and last ten times longer than zinc-sulfide pigments. At least that's what the advertisements claimed when I discovered them during an Internet search. It sounded too good to be true so I ordered small bottles of six different colors and tested them against some of the regular paint obtained from the craft section of a Jo-Ann's Fabric store. These high-end paints cost $20.00 per ounce or about eight times the cheap stuff and were developed in the 1990s. Is it worth the cost? Rather than simply state the results of my tests I thought it would be more entertaining to take pictures of the tests, post them on this page and let visitors judge for themselves. Here they are:

  The first test compares the brightness of several different thicknesses of paint using regular zinc-sulfide paint (bottom three squares) versus the super-phosphorescent paint (top squares). The bottom right square is one coat, the middle two coats, and the left square three coats. The brightness of the regular paint in the lower left square (three coats) is one-third the brightness of the super-bright paint in the upper right square (one coat). This shows that the strontium-based paint is vastly brighter than the zinc-sulfide. Is it ten-times brighter? It was hard to tell when I did this test because the light from the top row was so strong it washed out the glow from the bottom row. This picture doesn't due justice to how bright the strontium paint is glowing. A two-inch-by-two-inch square glows as brightly as a .03 watt electroluminescent night light. This glow paint casts shadows in a dark room. You can read easily with it. No, it's not like a flashlight but compared to normal glow-in-the-dark paints, this stuff will knock your socks off... which you'd be able to find because it glows brightly enough for you to see them... unless they ended up under the bed... or you were standing on them... or... oh, well... you get the idea.

  The second test was to see the effect of backing material on the brightness. The right-hand rectangle was painted on flat black paper. The middle rectangle was on the shiny side of aluminum foil, and the left-hand rectangle on white poster board. I had expected that the black backing would be dimmer but was surprised by how much darker it turned out to be. I thought the foil backing would be the brightest but it was slightly dimmer than the white-backed rectangle. If you paint any of these paints on to poster board, be sure to paint on the shiny side of the board. If the paint is on the dull side, it will be absorbed into the cardboard, make the cardboard translucent, and weaken the glow because light will escape out through it. Also, poster board may look opaque but it isn't, a lot of light leaks out through the back and is wasted. The glow's brightness can be increased slightly by giving the poster board a coat of one-coat-covers-everything type gloss white paint. Even though the paint is many times thinner than the poster board, it is much more opaque and will reflect more light back out through the glow-in-the-dark paint.

  Here are the six colors I purchased. From left-to-right: yellow-green, orange, turquois, violet (looks more like purple in real life than the blue shown here), blue, and red. The red is dim because it was only available in a colored pigment, which means that much of the radiated light is absorbed by the coloring. Orange was very disappointing. I had expected that it would have been brighter because orange is close to yellow. In truth, it looked more like brown. I should point out that with the exception of red, these paints all glow in the color seen. They aren't glowing with white light that is colored by pigments in the medium.

  I did a time-to-charge test. Glow-in-the-dark paints work by absorbing white light and them emitting it slowly in one color. They are sort of like rechargeable batteries: before you get anything out of them you have to put something in. And just like a battery, it takes time to charge the paint up. Exposing samples for ten minutes, five minutes, two minutes, one minute and thirty seconds generated glows that all looked to be equally bright, but two hours later the sample that had charged for ten minutes was twice as bright as the one that only charged for thirty seconds. This test only used a single heavy coat of paint. A sample with many coats might take longer to become fully charged.

  Another interesting test compared the charging times for strontium and zinc-sulfide paints. For charging periods under twenty seconds, the zinc-sulfide glowed brighter than the strontium paint. At twenty seconds they were equal and after that the strontium paint's brightness greatly out-glowed the zinc-sulfide.

  Speaking about how long the the paints glow, the strontium paint is clearly visible eight hours later. The zinc-sulfide paint faded to oblivion in fifteen minutes.

  One catch to using the strontium-based paints is that the pigment is much heavier than the lacquer medium used to carry it. This means that the pigment settles quickly and regular stirring in necessary. I found that I had to stir the bottle every five minutes to keep it even. Also, after sitting on a shelf for a day, the pigment packs down in the bottom of the jar so densely that it's difficult to getting it broken up and back into suspension. It helps to set the bottle upside down for ten minutes to let the pigment start falling away from the bottom before stirring. That way there is less chance of some of the pigment crystals being damaged from being dug out of the bottom of the bottle.

  My next project involved attempting to increase the brightness of the glow by increasing the amount of glowing pigment in the paint. Typically, only thirty percent of the paint (by weight - it looks much less than this by volume) is pigment, a fine granular material, that glows. It would seem that if there was more pigment and less binder, the paint should glow brighter. Also, using a super-super phosphorescent paint (still based on strontium but with a larger grain size, which is supposed to make it glow twelve percent brighter) should also help. My first attempt used ninety-five percent, by weight, of pigment and only five percent binder. This mixture formed a very dry paste similar in texture to a very dry pie dough. It was so dry that it had to be pressed into place. It did glow slightly brighter and the glow lasted longer than the ready-mixed paint, but not enough to make up for the hassle of the mixing and pressing. Also, the mix was so dry that some pigment crystals were damaged. These didn't glow and they made the surface look as if it had a scattering of dark grains on it. It would seem that a mixture of sixty percent pigment and forty percent binder is optimal.

  I found that the type of paints I used were extremely slow drying: it took days. When I tried speeding things up by placing a sample in the sun on a cool day, the paint formed a network of small bubbles just under the surface. These bubbles dulled the glow. When the paint finally dries, it is still very flexible, whereas the cheap acrylic paint hardened to a layer that cracks when bent.

  So, do super-phosphorescent paints work enough better than the hobby-shop stuff to warrant the additional cost? In my book, yes. They are so much brighter and last so much longer that there simply isn't any comparison. Also, the medium used by the company that supplied my paints was smoother and more flowing than the hobby-store acrylic paint so that after being brushed on, it flowed to an even thickness. One problem I ran into while trying to paint very thick coats is that the pigment would settle to the bottom of the layer of paint just like it does in the bottle. This results in a thick layer of dry medium on top of the pigment, with the result that the glow is dimmed. The solution to this would be to increase the ratio of pigment to medium.

 
How Phosphorescent Paints Work:

Fluorescent paints are paints that glow only when irradiated by an energy source. A good example is the phosphor lining on the inside of a fluorescent tube. Current flowing through mercury vapor inside the tube creates ultraviolet rays that strike the phosphor. These ultraviolet rays knock the electrons around the phosphor atoms into higher orbits. The electrons immediately fall back down releasing the energy they absorbed from the ultraviolet rays as visible light.

Phosphorescent paints function in much the same way, except that once a light ray bumps an electron into a higher or more energetic orbit the electron gets stuck there. It's rather like a ball getting stuck in one of the traps in a pinball machine. It stays put until the plunger underneath the trap pushes it out so it can drop down to the bottom of the table. The temporary entrapment is called a metastable state. In the case of phosphorescent paints, what nudges the electrons out of the energy trough that has them trapped is random thermal fluctuations in the crystal structure of the pigment. This is why phosphorescent paints glow weaker but longer when cold and brighter but shorter when hot. An easy way to demonstrate this is to get some phosphorescent glowing and press it with your thumb for a few seconds. The area under the thumb will be heated and when the thumb is removed, will glow slightly brighter than the surrounding cool area.

The bright zone in the center, overexposed to white, is where
I placed my thumb to warm an area and make it glow brighter.

The color of the light released from a phosphorescent paint depends on the difference between the energy levels of the trough the electron gets trapped in and the ground state it falls to after being bumped out of the trough. If this energy difference is high, the color will be bluish. Mid-range energy differences show up as green or yellow-green, and low energy differences as orange or red. This is simply because a blue photon has more energy than a red photon.

In 2000 new zinc-based phosphorescent paints were developed that radiate red and orange light instead of the traditional yellow-green. Additionally, they are twice as bright as the older zinc sulfide paint.

 
Links:

I purchased my super glow-in-the-dark paints from:

Shannon Luminous Materials, Inc.,
304 A North Townsend Street
Santa Ana, CA 92703
USA

Phone: 714-550-9931 or 800-543-4485

URL: http://www.blacklite.com

  Both gentlemen I spoke to at Shannon were very patient and helpful and while their company is geared more for industrial quantities, were more than happy to supply small quantities at reasonable prices.

Great New Link!!! I found the following site to be very interesting and informative: http://www.glowinc.com. They also seem to have very good prices.

 

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