Paint Stripping / Hydroxyl Radicals 2

[PBEautoguy]

I would like to develop a system to strip paint from a surface using Hydroxyl Radicals. I have been able to do this with H2O2 and UV light, but the process is too slow. Any ideas out there?

 

[moltenmetal]

I suggest giving up. To remove any quantity (mass) of material with hydroxyl radicals, you will truly be waiting a LONG time. They're generated in water to remove comparatively small masses of material, but that's a totally different situation than trying to remove material from a surface.

Hydroxyl radicals recombine with a reaction rate constant on the order of of 10^-8 L/mol s, which gives them a half-life for recombination into hydrogen peroxide on the order of milliseconds or less. You can't make them in one place and then transport them somewhere to do oxidation. Unless you're forming the radicals immediately adjacent to the thing you intend to oxidize, you won't be doing anything but wasting energy re-making peroxide or ozone or whatever other molecule you're using to make the radicals from.

Surfaces which are self-cleaning of organic deposits can be made using films of anatase TiO2, but there the loadings are minimal and the radicals don't have to travel at all to reach the stuff they're supposed to oxidize.

 

[PBEautoguy]

Thank you for your reply. The paint isn't "removed" as such. The molecular bond to the substrate is broken somehow and the paint literally falls off in one piece. I have done this in about 5 days with the part completely submerged. I have also done it using a continuous spray onto the part. I guess there is no faster way to accomplish this, but if anyone has an idea, I would love to here it.

 

[Latexman]

I've seen the paint stripped off cars by spraying "oven cleaner" (i.e. caustic - OH radicals) on them, letting it sit (I don't know how long - overnight?), and then scrapped off. It may take a few times to get down to the metal.

Something like that?

Good luck,
Latexman

 

[PBEautoguy]

Yes. Something like that. Overnight would be fast enough, but we are trying to get away from the scraping. I will do some experimenting. With the H2O2 and UV Light the paint just popped off cleanly and in one piece. That would be the ultimate solution. I very much appreciate your input. Thank you.

 

[moltenmetal]

*OH radicals are NOT the same thing as OH- ions which you find in oven cleaner. *OH has an unpaired electron, is charge neutral and is a vicious oxidant. It's what you get when you remove a hydrogen ATOM, with its electron, from water. OH- is what you get when you remove a proton, i.e. an H+ ion, from water.

OK- so you don't actually want to oxidize away the paint per se- you merely want to embrittle it so it's easy to remove. I'm not quite sure whether you're oxidizing the substrate or merely embrittling the paint. You may find, depending on the paint and the substrate, that intense UV light works just as effectively or better without the peroxide as it does with it. Whatever the radicals themselves are doing, they're doing it very near the paint's surface, not in the body fo the paint, as they don't survive long enough to have time to diffuse.

The peroxide is absorbing most of the UV light and generating some *OH radicals, provided your UV is short enough in wavelength (the 254 nm light from a low pressure mercury arc lamp is a little too long in wavelength to be strongly absorbed by hydrogen peroxide, which has a UV absorption peak in the 200-240 nm range). The light from a medium pressure mercury arc lamp is a little richer in this short wavelength light and hence is more effective.

 

[coatingengineer]

Hi.
There are, you know, three ways to remove paint: thermally, chemically, and mechanically. Depending on your requirements, you may be constrained by all three. If I assume safety, the environmental aspect, and substrate and surrounding surface preservation must be considered, have you considered grit blast with baking soda? In the late eighties, early nineties, I was involved with the developers of a paint stripping system for Church & Dwight, manufacturers of Arm & Hammer baking soda. Sodium bicarbonate works.

William Gunnar

http://www.IndustrialCoatingsWorld.com