TiO2 coating for electronic filter design 2

[spiraltooth]

I have an electronic air filter that uses charged plates to collect the contaminants and was wondering if they would be easier to wash off with a TiO2 and or Teflon coating in the presence of UV. Would it be necessary to increase the voltage on the plates to maintain the current?

 

[moltenmetal]

You're not going to be able to avoid cleaning your air filter, buddy! TiO2 isn't going to break down any of the inorganic dirt that's there anyway, and you'd need such an enormous amount of artificial UV to degrade the organic fraction that it wouldn't be worthwhile. Blowing it off with compressed air (outside), then washing it with soap and water will be a far more efficient solution.

PTFE's a very good insulator, so I would imagine it would reduce the effectiveness of your electrostatic precipitator considerably. TiO2 is a semiconductor, at least.

 

[spiraltooth]

Thanks Moltenmetal. I had seen the windows promoted as self cleaning and thought I could use the same principle. I take it the windows not being charged wouldn't attract any inorganic matter.

 

[moltenmetal]

The TiO2 coating is claimed to give the windows some "sheeting action" (i.e. reduced interfacial surface tension with water) so that there is less of a tendency for refractory inorganic streaks to form from water impingement/evaporation, but I highly doubt these windows will stay scrupulously clean forever without washing. The greasy film that causes much of the inorganic matter to adhere might be degraded, and that will help- but "dirt resistant" rather than "self-cleaning" might be a more accurate description of these windows. And only on the OUTSIDE of the window will be affected, mind you- the UV portion of the spectrum needed to stimulate the TiO2 photooxidation chemistry is almost totally absorbed by ordinary glasses, not to mention by the TiO2 anatase film that's doing the "cleaning". So throwing out your squeegee and bottle of Windex would be a little premature. Caveat emptor, as usual...

 

[spiraltooth]

Then the greasy film (I had never noticed it was greasy until you mentioned it) which I'm assumeing is organic binds the inorganic matter to the surface in this case a TiO2 coating. I would be satisfied if a simple water rinse would replace scrubbing with soap and water given the coating and UV.

 

[moltenmetal]

The "greasy film" is the theory I've heard. I've got no data which details the composition of exterior grime on windows, but it would be logical to expect that the gooey stuff would be organic. This goo retains windborne particulates, organic or inorganic, and the rest is history. The TiO2 would tend to degrade this film on the outside of the window, reducing the tendency for the rest of the gunk to be attracted.

As far as the tendency for the film to reduce surface tension, I wouldn't expect that to be a permanent property- it would tend to go away over time, and at best it's an idealization rather than a reality. And once water does tend to stay put and evaporate to dryness in place, water streaking and spots of inorganic material would be inevitable. And you won't remove those with a mere water rinse- you'll need to do some scrubbing followed by using a squeegee.

 

[spiraltooth]

I'm less concerned about whether the filter's charged plates are spotless than I would be about my windows as long as I have use of >95% of the filter's collection plate surface after a simple rinse. I'm more concerned about why you think the hydrophilic properties would be degraded. Do you think the coating would spall off?

 

[moltenmetal]

It's my knowledge of chemistry combined with my natural skepticism when dealing with anything anyone tries to "sell" me.

Surface properties are a fickle thing- you're talking about the properties of the top 100 layers of molecules maximum to affect a property like hydrophobicity/hydrophilicity. Anything can happen to a layer that thin. Add to that the fact that the TiO2's surface cleaning feature comes as a result of the photochemical generation of high energy species like hydroxyl radicals, hydrated electrons and "holes" etc., which can do surprising transformative chemistry over the long term. And if the surface chemistry is locally altered, it wouldn't take much to render a small area hydrophobic enough to retain a droplet of water.

As far as making your filter plates easier to clean, forget about TiO2 and UV light. I understand that what's done in commercial (big) electrostatic preciptitators is the use of "rappers" or "hammers" which literally just smack the plates and cause them to vibrate, which causes the heaviest accumulation of solids to fall off into a chute below the plate stack. (I would presume they do this with the flow off...) And from what I understand, as long as the solids on the plates aren't too thick, particles will continue to be attracted so you don't need to wash them scrupulously clean.

 

[spiraltooth]

Thanks very much for your clear explanations and skeptical thinking.