Non-paint powder coating

[dshepherd]

I have an oddball application in which I'm trying to apply a decently uniform ~1.5 mil powder coating to a small, somewhat convex plastic object....let's say it's roughly the size and shape of the inside of a portable CD player case. Cure temperature won't be a problem because the coating I want to apply isn't a powder paint, it's a phosphor powder, and it doesn't need curing. There will be a conductive coating (either a sputtered-on ITO coat, the Electro-Prep clear conductive paint, or a silver-bearing sprayed-on paint), so I shouldn't have a problem getting a good ground on the product. The powder I want to use has particles in the 30 micron range, and I'm wondering if I can use the Caswell hobbyist gun (or other powder coating gun) to apply it. Is this particle size too large for a powder coating gun? Are there any other reasons I might be wasting my time? In other words, is it worth trying?

Doug

[DALE]

What holds your coating after it's applied? You mentioned it required no curing. It should spray ok but seems a little large on the particle size but I see no problems passing that through the gun. Could you elaborite on the purpose of this coating a little bit, what are you trying to achieve?

[dshepherd]

It's a bit of a longshot that I can get it to work, and a little time-consuming to explain the whole concept, but if you're willing to read I'm willing to type! I want to see if it's possible to make the translucent plastic case of a small device (like the portable CD player) glow electroluminescently. There are a number of people who make flexible, flat electroluminescent (EL) panels, and they can easily be bent into a cylinder, but they won't conform to a 3-D surface. I'm investigating the possibility of applying the necessary coatings to my product instead of to the flexible plastic film.

To make an EL 'lamp' requires a sandwich consisting of at least four layers: 1) an inner conductive layer, 2) the phosphor itself, 3) an electrical insulator, and 4) the outer conductor. (Of course one or both of the conductive layers needs to be transparent to allow the light to pass through, but clear conductors are available.) The phosphor glows when ~100 volts AC at 400-6000 Hz is applied across the conductive layers. I'm an EE so I can easily build an inverter to drive it electrically, but laying down the sandwich gets me pretty far out of my element.

The two conductive layers are pretty easy, and the insulator shouldn't be too bad, there are commercially available spray-on coatings available that should work, it's applying the phosphor in a fairly even coating that's the right thickness that gets tricky. I was thinking that I'd try blowing it on with a powder gun, using the already-applied conductor layer as a ground, then encapsulate the particles with a very thin 'fog' coating of an electrically insulating clear coat (i.e. a conformal spray coat). A few more coats of the insulator would be applied when the fog coat had flashed off, then the second conductor layer would be sprayed on.

Again, I realize it's going to be difficult to get it to work, but even if it has splotches, dark and bright spots, or just doesn't put out much light, I still think it would be a VERY cool effect. What do you think?

Doug

[DALE]

ok I think it will work for your aplication but I wonder what will happen you try to apply wet coat over it. The powder will try to suspend in the liquid some at the surface and some below it does it have to be a continous layer. With this method you might get breaks in the layer. Just a though you never know for sure I've seen other things work before that I doubted.

[dshepherd]

I had the same thought, that's why I was thinking it would be best to go with a very thin first coat to fix the particles in place (and not blow them around too much), let that dry or at least flash off, then a couple more thicker coats on top to form a nice unbroken insulator layer. If it'll work, or if it's necessary, I don't know, I'm just guessing at this point.

Doug

[non-stick]

I'm not really sure what I can add to this process but I'll try.

The Caswell gun will spray 30 micron I feel. I'm sure something that grainy would need a little bit of practice for a uniform coat, but by no means would it be impossible. Let me ask you this,though. In regards to step #3, have you tried using a liquid such as milk of magnesia and letting it dry? I ask this because that "coating" is used industrially as an insulator AND reflective coating on the insides of aerospace lights and the like. I know it sounds a bit un-kosher but if you took a common bottle of maalox and sprayed it you'd find this to be true. Even more to your advantage is that when it dries ( force cured in a convection oven at 120F for 10-15 minutes) you'll notice that it's ver porous and acts as a natural primer for your next layer. Just some random thoughts for you to toy with.... Russ

[dshepherd]

Wow! It's amazing what you learn from posting on a forum like this. I looked up MoM online and it seems that it's magnesium hydroxide. I haven't yet seen anything about its electrical insulating properties but I'll keep looking.