High strength Dielectric Coatings, fact or fiction
There does appear to be threads in this forum which speaks to this topic, and I would be naive to even think as much. however they are several years old and there may have been other products which may have been introduced to the market since then with great dielectric properties. In my recent quest for knowledge I have learned there are many products currently on the market which appears to have properties which looks promising as a Stator Insulator. These products have data sheets on the net, and have arc resistance rated any where between 800v per mil to 3000v per mil. My question is that am I being naive to think that these products are something special, or are they nothing more than re-badged resins, alkyds and polyurethanes etc. As I have about 16 panels to spray I obviously would like to buy the right product, and not make a career of this. so far we have used a product called Hysol, very expensive paint used in the aerospace industries for electrical insulation....failed. we then sprayed automotive clear...you would have thought sprayed metal flake on it when I performed an arc test. I did a small controlled test on a piece of brass I used one layer of primer, one and three light coats of Helmans spar polyureathane "containing alkyds". which simply worked better as an insulator than what I have previously used.
Below are some products I have researched which seems to be fairly easy to obtain, and according to their data sheet have properties that seem perfect for ESL's stator coatings. I would like to hear some opinions!
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Aervoe 403 Insulating Epoxy - www.tooldex.com
Caswell Inc. - Glyptal
CRC 18411 Seal Coat Urethane Coatings - www.tooldex.com
In my researches I have found that the clears always worked the best so far.
Because there is no pigment at all in them.
It seems to me that both that both black or white conduct,But by how much of a difference I am not sure.
This is why I built such a high voltage variable supply as it has taken me a while to do so.
But stay away from anything that has a white pigment, normaly this is titanium dioxide.
Although it has a exteremly high resistance in some forms it still conducts and at high voltages it is like a high value resistor.
I have tried one of those spray bombs claiming 1800v per mil or better and it seemed to fail in my application.
However it did work okay but not at the 10kv or so for a 5mil thickness that I was expecting.
But this is before I had any means of measuring such high voltages directly and did any controlled tests.
I have had Krylon's engineers find out for me if their conformal coating is the same as their clear acrylic enamel spray and the conclusion was that it appears to be but he could not give me a straight answer as yes or no.
The confromal coating is rated at at 1500v to 2000v per mil exact numbers I don't remember But I can compile and post this data at a later date.
The main ingredient is n-butyl acetate this is common in all of the coatings I have research so far and is the main ingredient in the clears.
There is one other one that is used as well (but the name escapes me at the moment) that is used to create the paint coating.
When it comes to epoxy coatings they use an additive that makes the coating harder and more durable,what this compound is I don't know it remains their secret as it is proprietary information.
I can tell you this in my original search of coatings I tried the rustoelum appliance epoxy enamel paint and this did seem to work the best of the ones that I had tried at the time.
But I remind you that these all had white pigment in them as well and it is what I had used on my first panels.
And was before I had found out the white is/or could be worse than black.
I still have those very same panels and I will get some voltage stress measurements soon.
The black pigment is carbon but it is contained in its own little plastic shell persay so this is something I plan to test sometime,But to avoid any issues of such I stick with clears that have no pigment.
Primers have talc in them as a filler and this may be a good thing as it is a great insulator and has a high dielectric constant.
This may be a good thing to consider as it will help to raise the capacitance of the panel as well as insulating it.
In my discussion with Roger Sanders he had told me that what ever coating that I do choose to use make sure that you pick the one that has the highest dielectric constant than/over volts per mil first.
This brings me to polyurethene as it has a slightly higher dielectric constant than acrylics do.
Dielectric constant is not to be confused with dielectric breakdown at volts per mil.
Glyptal seems to claim the highest breakdown factor I have found so far of 2000v to 3000v or so per mil.
I have not been able to test it, as the stuff I had was in the form as Coil Dope and was all dried up when I found my little bottle of it in 2003.
Just as it had been stated that the volts per mil advertised ratings seems more of a sales pitch to me (please take this statement lightly).
They may very well do what they claim for the first mil of thickness but the factor of volts per mil gets less and less as the coating thickness gets greater.
As well as its rating is for a flat surface as mentioned.
When you start talking about sharp edges and thin wires(as I use) as a stator material this factor is derated quite considerably.
I have found this out first hand the my little panels using window screen as a stator material.
My best ones were powder coated But I have had issues on getting them done correctly and lately I have had to coat the with clear acrylic to get them were I want them to be.
As after time, flaws have develop from corrosion as well as micro cracks in the coating but resealing them with clear acrylic worked like a charm.
This is all I have for now on my findings for decent stator coatings,But more on this later as I have to go now. :(
I'm surprised that no one has tried shellac yet. Clear shellac can be bought in spray cans now and I would think it would be an excallent insulator.
yes I was looking for shellac back in 2003 but it wasn't very available back then as it has become just recently.
Shellac has very good characteristics for high voltage use.
Polyurathene has been my and many others favorite for years for tesla coils and stuff.
Spray cans of insulating paint or even regular enamel paint are a bit costly as it takes alot of them to get a good thickness.
Although I have seen some as low as 2$ to 3$ in bulk mail order from the internet.
But I can tell you that I have gone through at least 20 cans on just my few small panels.
I just picked up a cheap 6oz gravity feed sprayer for for $13 at Harbor Freight and this will allow me to spray some common polyurathene straight out of the bulk can with maybe a little thinning.
The same stuff in a spray bomb is close to $8 a can now and that is much to costly as it is quite thinned and you will yield much less material than in bulk.
One good deal that I have found is the 2x clear acrylic an a can and is slightly cheaper than the normal stuff at about $5 a can.
I have been stockpiling some of this for my next build and I have used it to reinforce a couple of my older paper 10" cones and it worked quite well.
I used alot less to do the job as it is quite thick.
I forget to mention in my last post that I have some screen already coated with high talc primer and needs to be coated with some clear to be voltage tested for my next build of some smaller panels.
This build will be to test the new coating method to see if a primer should be used and to check the stability of a 11.5" wide diagphram.
I have also noticed the harbor freight does also now carry powder coating powder in small quanities and I'm devising a method to powder coat my screen stators without having to buy a powder coating gun system as there really isn't much to them.
I'm am thinking of a vat dip method or something.
My little panels now have a high performance coating that consists of two layers of powder coating and a good heavy coat (about 6 to 8 normaly wetted coats) of clear spray can acrylic.
It withholds all of about 7kv with the grounded test probe touching the screen without arc through at all.
The edges were the toughest to seal as the cut wires are quite sharp.
I had originaly used clear silicone sealant to seal them.
Then I redid them I used some old clear nail polish that had been given to me by a friend and drops of super glue as well.
Now I have to repair a rip in one of them so I will be venturing down that path one more time and this time I am thinking epoxy, as it is much thicker than the others.
some of really good dielectrics would be polyimides, fluoropolymers and silicones.
I doubt you can get polyimide based laquer (enamel, pain etc.) due to toxicity of solvent but fluoropolymers can be used for powder coating and are soluble in ketones. Silicones are not as good as the other two but are hydrophobic...
All three of materials are pretty stable to partial/barrier discharge, ozone, etc.
Do you really need high strength dielectric - may be it's just a quality of paint job? Almost any paint dried has voids, that's why you may need to put layer after layer after layer...
Common wisdom is to have semiconductive paint with high permittivity, stable against ozone...
Jer, Thanks for detailing your experiences with insulating coatings, It sounds like we are on a parallel quest. I too am now in possession of the gravity feed sprayer. The economic of buying many cans quickly add up!
In a quick test to insulate a piece of brass, using automotive primer worked significantly better than spraying epoxy appliance painted directly on to the metal test piece. It still needed to be sprayed with urethane however it took only three coats to achieve a 3kv barrier.
As you and others here have mentioned, it takes multiple thin coats of substrate to achieve an optimal conformal coat which is directly proportional to the proposed bias voltage.
Alexberg, do you have experience using shellac, I would be curious to hear it, the last time i used shellac was high school applying a finish to my "flying v" ... (I thought it was essentially superseded by modern more durable top coats?
I have a 16 15"x48" panels to spray, which essentially translates to 32 surfaces, and 1,920 sq feet of spraying.... I'm going to be busy for the holidays.
Jer, Getting back to your powder coating thought...I believe I know exactly where your going with your thoughts...My issue would be, in building an oven to bake my panels in? Im still coming up with ideas!
For know I will likely buy a gallon of the Helmsman spar polyureathane, thin it a bit and apply multiple thin evenly/multiple direction applied thing coats.
Charlie, I did hear your suggestion, however my buddy who helped me with the clear coat were sprayed at his house, with polyurethane it only needs to be thinned and I can spray it here over the course of a week!!
Regards to all,
P.S. Charlie/Jer Ill try to attached a link of some photos of my HV boxes using XLR connectors (we ended up braiding 14g multistrand wire),...it looks cool too!
Very Good,Alexburg,I wish that I understood plasics as well as you do ! :)
One of the best covering for wire I have found is Kynar as it is a polymide I believe.
I was going to try a test panel using it but it was at a time when the prices of wire started to go through the roof.
I had tested a piece of wire wrap wire covered with it and it withheld my 10kv (at least) test with absolutely no sign of leakage,breakdown or arc through.
Pretty amazing stuff for such a thin covering.
I Agree with you on as for as having a limited amount of an insulating factor as well.
This makes a lot of sense as I remember someone mentioning that they had tried a stator that had used a nylon or maybe it was teflon coating and that it work very good and that it worked so well that the stator started to hold a charge of its own.
This eventualy decreased the output of the panel and I would imagine causing other issues as well.
I have not yet built a panel with no stator coating yet but I plan to do so as a control for the next series of tests.
I do know this that even though I my stator coating could handle every thing that my bias supply could give it with out arcing it also withstood approximately 25kv p-p across the .15" gap between the two panels and the thing was quite loud.
Until a Breakdown had occured on the edge of the panel due to a sharp piont of wire that was not properly insulated and the plastic frame had ignited but it kept playing and I played it in this condition for nearly 10minutes until I decided to stop.
It did seem to get louder as well when pushed to the higher levels of voltages even with the breakdown of the air in the gap.
Although the sound quality was down considerably from all of the arcing and sparking as well as the clipping of the diagphram to the stators.
The voltages were much higher than (probably double) I had ever pushed them to before.
In fact they were so high that the air in the gap of the diagphram and the stators were breaking down with the familar purple glow as mentioned.
And very small arcs of discharge when the diagphram got close enough to a stator on some bass peaks but never arced through to the other stator.
Except when the edge failure occured but never in the middle of it.
When the failure occured it arced around the outside edge of the panel and this distance is actualy a greater path length than the inside edge that is blocked by the overhang of the diagphram frame.
Upon disassembly of the panel I found no holes burned in the mylar that I could find using my best and biggest magnifying glasses.
And during the voltage stress test to find were the breakdown had occured my findings were that there was no leakage or damge to the coating except for the edge that had failed.
So I repaired the edge and they are ready to go again except that this is the same stator that now has a rip in it from a small speaker cabinet falling on it so I never got to retest it after I repaired it.
But it did pass the voltage stress test after I repaired it.
My bias voltage was confirmed at 7.5kv and was monitored with my scope during these tests.
As I had said there was a little corona at this point but no solid arcing.
And it sat there idling quietly.
My painted stator panel was subjected to the same test and failed as it could not get to those level of voltages and had burned a few small micro holes in the mylar as well.
This is when I found out upon disassembley of those panels that the white paint conducts at a certain point of high voltage regardless of its thickness.
It was at this point last year in about September that I had stopped in order rearrange my sound room and audio equipment.
One other paint that I have been considering is the clear stuff that has a Matte finish in polyurathene or clear acrylic.
Most of my research has indicated that the additive used to create this finish is Silicon Dioxide and is way it is so durable as a scratch resistant as well.
I used to get Paint and Powder Magazine and there has been some research on using glass as an additive for insulative coatings and this is something that may have some relevance as well.
I have a can of matte clear that was bought by mistake but It may be a good mistake and we will find out soon.
There has been some dicussions on whether or not a coating can or can't do something to the efficiency of the panel.
It seems to me it is all about the actual forces that exist in the gap more than the actual voltage applied to the stator.
The voltages in the gap is limited to the breakdown factor of the air itself.
But I am sure that this is already realized as it has been discussed many times over.
Yes,Jerry,I too have been contemplating on building an oven as well.
You can get heating elements rather cheaply or salvage some from some old ovens.
And possibly taking two ovens and cut the ends off and fasten the two halves together.
Or maybe something like belt drive type of oven like a pizza oven only without the big fan (as it might blow the coating off) and use just radiant heat from some heat lamps or something.
The possibility's are endless.
My first quest is going to be building the applicator so that there is little waste that is why I am think about the dip method as i think this will also allow the most amount of material applied as well as consistant thickness.
I had asked my coater to do a third coat and he wouldn't do it.
He didn't really explain why,he just said I can't.
Later on I found out it is because that once the coating gets to a certain thickness the new material doesn't or won't stick to the part.
I thinking that this is a function of the applicator voltage myself but I will figure this out when I start to venture down that road.
Alexberg, it is possible the quality of the painting has much to be desired, since I am the painter. It looks good but arcs plenty!
Ultimately I will likely spray these with multiple coats of polyurethane through a spray gun!
Jer, I too have thought a bout the pizza oven, and the two halves of an oven to form one!!
I also considered using using a gun safe, and old oven parts with a simple thermostat. I think that powder coating would be the best however it really is hard to find someone who would understand in order to get it right. When I try to explain it to them as to why I need it coated multiple times, they look at me with their heads tilted like the victor record label dog...LOL!
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