Insulating coatings

[arend-jan]

A layered approach may be difficult or impractical, but I find it interesting. Is the consensus that this is done to create a gradient of dielectric constant? What is the advantage of this gradient? Is it to gradually bring the potential down? I'm just thinking out loud here so feel free to shoot.

I'm using a polyurethane based two component paint which is of course not engineered for insulation properties, but it more or less gets the job done. I'd rather have something designed for the task.

[SY]

That would be the theory, but I'm unconvinced of the efficacy. Often, a multilayer system is done just for adhesion purposes (topcoat/bondcoat). Problem with the gradient approach is that the difference in conductivity between the metal and the first layer is something like 10 orders of magnitude, whereas the difference between the insulating layers might be two or three.

[CharlieM]

Hello all,
I've learned some good stuff about materials' insulating properties reading this thread. As a former spray painter experienced with 2-part polyurethanes especially, I would add that one's spraying technique makes a huge difference in helping or mitigating a paint's tendency to run away from sharp edges, such as those around the hole perforations on sheet metal stators. In order to build up the coating thickness over a sharp edge, the first several coats should be misted on quite dry, allowing each coat to setup for 30 minutes or more between mist coats so that solvents in successive coats do not liquify the prior mist coats (or resulting capillary action will pull the paint away from the edge). After several mist coats are applied, allow the coating to setup for at least 60 minutes (it will look rough but you will smooth it out with later coats). The pot life of the 2-part poly will be spent by now so you will need to clean your paint gun and mix a fresh batch of paint to complete the job. Spray successive coats "just wet" with 15 minutes setup time between coats until the desired buildup is achieved. Hope this helps!
Charlie

[arend-jan]

Hi Charlie,

thanks for the tip

I want to try this. When you say 'to mist on', do you mean spray from a larger distance or with a lower paint to air ratio? I'm a spray novice so any guidance you can give would be appreciated. I have a HVLP gun in case you need to know.

[chinsettawong]

What about powder coating - will powder coating be better on the sharp edge?

Wachara C.

[CharlieM]

Powder coating has a proven track record with ESL's and I think it would indeed be preferable-- certainly it would provide a harder, tougher coating with zero entrapped volatiles. Polyurethane isn't cheap either but if you don't have access to powder coating or the cost is prohibitive and you're opting for a sprayed paint coating, it's best to apply the first few coats "dry" for the reasons stated previously. That is; set the fan kinda wide and the fluid flow kinda low such that the the tiny aerosol droplets in the spray strike the surface so sparsely that they do not merge to form a continuous wet coating (if the coating is wet the paint will run away from the sharp edges around the holes). I would apply at least (4) dry-sprayed "tack coats" to serve as a foundation for holding the subsequent wet coats in place over the sharp edges, as needed to buildup the film thickness. Normally you would hold the gun perpendicular to the surface but, since you will be spraying a perforated surface and you want the aerosol droplets to adhere to the edges of the holes, you will need to apply the tack coats with the gun oriented at + or -45 degrees to the panel. Let's say you apply the 1st tack coat moving lengthwise on the panel with the gun angled at +45 degrees, then apply the second coat lengthwise also but reverse the gun angle to -45 degrees so that you hit the other side of the holes. Then apply a 3rd coat moving widthwise on the panel at +45 degrees, followed by a 4th coat moving widthwise also but with the gun angle reversed to -45 degrees-- that way the spray hits all the hole edges from 4 directions for maximum coverage.
Allow the tack coats to completely setup so that they won't re-melt when you come back with wet coats to build up thickness. I used clear polyurethane except added couple of coats of black about midway in the buildup. I would also spray a small test panel and measure it's thickness with a caliper or micrometer before and during spraying until you have at least 12 mils of film thickness on the stators. I found that 12 mils worked OK (about 18 months so far) on my hybrid ESL's, which use 2.5 KV bias voltage and 1/16" stator to diaphragm spacing. However, I did have to redo one panel right off the bat because I either didn't properly smooth over a sharp trim edge and/or I didn't get enough coating thickness on the trimmed edge--- I fired it up and it played wonderfully for about 30 seconds before it flashed over and shorted out the panel and also shut down my beloved Carver power amp (fortunately, just tripped the protection circuitry-- thanks Bob Carver). Anyway, I had a horrendous time stripping and re-coating the stators on that shorted panel-- I thought I would never get all those thousands of holes cleaned out. Lesson learned: Be sure to smooth over the stators' trimmed edges and spend as much attention on coating the edges as the rest of the stators and you won't have a problem.

[Calvin]

Hi,

the second lessons You should have learned is: Test the stator, i.e. the coating before building .
An easy diy friendly test is to use a sheet of aluminium foil for kitchen use) and connect it and the stator to a Bias supply thats voltage can be varied.
Don´t forget to use a current limiting resistor, but rather a small one. You need some energy within the spark. When the spark is of sufficient energy content, it will first make some tickling noise and with increasing energy ´hammer´ a tiny spot into the foil, which allows You to easily find the weak spots in the coating.
Besides You can detect tiny spots within the coating (from dust or bubbles) which stem from the coating process and which You should sand even and recoat.

jauu
Calvin

[CharlieM]

Great advice as usual from Calvin!
I sure wish I had known how to do that test when I built my ESL's. Geeze, Calvin-- you should offer that one up to Roger Sanders in case he ever updates the Cookbook.

[Calvin]

Hi,

of course You should keep in mind, that You operate quite open with potentially lethal voltages/currents. So watch carefully what you do.
Don´t wear anything metallic, eg. watches and keep one hand always in Your pocket and use as weak a Bias supply as possible and a limiting resistor that only allows for very few mA maximum current. A second person as safety supervisor is highly recommended.

jauu
Calvin

ps: When I retire from commercial ESLing I might just write my own book: "Taming the Diva!".
Somehow my investments should pay back, eeh? :-)

[JonasKarud]

Some things that you can buy anywhere are coated with a pretty thick semisoft plastic coting, seemingly PVC. The items seems to be dipped in floating PVC.

I bought a PVC dipped bicycle shopping basket for testing and the results shows that this coating easily could withstand 8 kV@0.3mm thickness.

The basket consists mainly of a metal net with small holes and there's no PVC clogging up the holes.

I wonder how the dipping process is done, anyone heard of it?