Spraying-on a polystyrene-acetone "lacquer" - an ok insulator or no? - Page 1
Forum section: electronics / projects

author: Deactivated-1
author_rank: Frequent Contributor
timestamp: July 22, 2023, 01:43:01 pm
content: I want to try making some polystyrene caps for fun and am thinking about just spraying on a polystyrene dissolved in acetone "lacquer" onto the foil strips, then when it's nearly dry but still soft I'd roll it up, this way it doesnt fracture (hopefully), and probably would even fuse slightly and hold itself together.But will this make an ok insulating layer? I am certainly aware that it's hard to get good solids-coverage with spray "lacquers", so it this a problem with getting a good insulation layer?I want the insulation layer to have a breakdown of at least ~20volts, but it would be nice if I could achieve ~400V breakdown so I can test a few in tube circuits.I've also read that very early film caps were made with nitrocellulose often, is that harder/easier to get a good insulative layer when sprayed-on? do the plasticizers in modern NC lacquers cause problems with the breakdown voltage?

author: TimFox
author_rank: Super Contributor
timestamp: July 22, 2023, 03:30:41 pm
content: Because of its very low service temperature, commercial polystyrene capacitors were never metallized film, always film-foil construction.Of course, the capacitance will vary with the dielectric thickness you achieve and control.Note that polystyrene has a relatively low dielectric constant (2.56), so commercial polystyrene capacitors use a very thin film (0.01 mm minimum), since the insulation strength is high (roughly 20 kV/mm).The dielectric loss for polystyrene is extremely low (D= 0.0001 at 100 MHz).

author: jpanhalt
author_rank: Super Contributor
timestamp: July 22, 2023, 04:56:43 pm
content: Quote from: ELS122 on July 22, 2023, 04:30:17 pmPolystyrene is glassy rather than microcrystalline. I am not sure there would be "interparticle" gaps if it is fully dissolved. My worry would be getting good spreading/wetting on the surface. If you have worked with modern polyurethane paints, you know what I mean by pinholes. Is the surface clean enough to avoid that as the acetone evaporates? Another problem will be getting anything near a uniform layer thickness. The latter may not be important for you.

author: MarkS
author_rank: Unknown
timestamp: July 22, 2023, 05:00:19 pm
content: Acetone will remove nearly all ink and paint and soften or dissolve many plastics. Is this even safe?

author: KE5FX
author_rank: Super Contributor
timestamp: July 22, 2023, 05:47:02 pm
content: Quote from: ELS122 on July 22, 2023, 01:43:01 pmLOL. If you thoughttantalumcaps had an exciting failure mode...

author: Conrad Hoffman
author_rank: Super Contributor
timestamp: July 22, 2023, 08:19:44 pm
content: AFAIK, people make "coil dope" from polystyrene and toluene, not acetone. That dries too fast anyway. Search for "coil dope." I suppose it could be sprayed but it makes me think of hand coating bars for photographic emulsion. Basically a syrupy liquid flows through a slot against the substrate. I'm guessing you're going to have an edge problem; I think they usually use polystyrene film that's wider than the foil. Regardless of the naysayers, give it a try!

author: vk6zgo
author_rank: Super Contributor
timestamp: July 23, 2023, 02:25:43 am
content: Quote from: TimFox on July 22, 2023, 03:30:41 pmSuch construction also made high capacitance value caps larger than even waxed paper capacitors of the same value & voltage ratings, plus (anti-intuitively), the higher values were alsoinductive.We had to replace some 0.47uF metallised paper capacitors in the Vision "Submodulator" of an old Marconi TV transmitters, & were thrilled to receive some really "pretty" 0.47uF "Styroseal" caps.Even though they were bigger, they could be fitted OK, but when we swept the thing, it had a large "notch" in the frequency response.Fitting "scungy old" paper caps fixed the problem!

author: Solder_Junkie
author_rank: Frequent Contributor
timestamp: July 23, 2023, 12:59:09 pm
content: If you examine polystyrene caps, you will see that on many the foils are spot welded at each end, otherwise the inductance of the spiral winding prevents their use at anything higher than a few Hz. Xicon polystyrene caps are made this way.https://www.mouser.co.uk/datasheet/2/449/Elytone_23P_Series-3182363.pdfNote also the outer foil is the one you would normally connect to the ground side of a circuit, if relevant, as then it acts as a shield. If you examine one of these caps closely you will see which wire goes to the outer foil.SJ

author: vk6zgo
author_rank: Super Contributor
timestamp: July 23, 2023, 01:19:23 pm
content: Quote from: Solder_Junkie on July 23, 2023, 12:59:09 pmThese were going to be used as coupling caps in this case.The inductance & capacitance together produced the nice mid frequency notch.In my first job, at an Electronics supplier, we sold quite a number of them back in the early-mid 1960s, as there was a bit of a "fad" for them.None of them were high capacitance values, though.The occasion I was referring to was about 10 years later, & I was surprised to see the 0.47uF polystyrenes.

author: ConKbot
author_rank: Super Contributor
timestamp: July 23, 2023, 01:23:50 pm
content: Quote from: MarkS on July 22, 2023, 05:00:19 pmAcetone is produced in the body in small amounts for some biological processes, so there exists normal biological pathways to remove it.As long as your exposure is sufficiently small, it's fine. For example, if OP is dumping acetone into a polyethylene container full of styrofoam pieces to make the lacquer in a well ventilated area, they are probably fine with the short exposure. Spraying/wiping on the lacquer to make the foil? Should probably have a respirator, because the amount of air you'd need to vent that would probably bring dust and contaminants.This aside from the flammability risk. Obviously open containers full of flammable stuff need the correct respect to not reduce the lab to ashes.

author: MarkS
author_rank: Unknown
timestamp: July 23, 2023, 02:30:56 pm
content: Quote from: ConKbot on July 23, 2023, 01:23:50 pmSorry... Safe for electronics.

author: vk6zgo
author_rank: Super Contributor
timestamp: July 25, 2023, 01:02:31 am
content: Quote from: ELS122 on July 24, 2023, 08:02:22 amThat is why I was surprised by the apparent high inductance of the 0.47uf polystyrenes.We thought maybe it was increased stray capacitance to other nearby objects, but that would normally cause a taper in the HF response, not a "notch".From memory, we did try reversing them, to no avail.As to actually measuring any inductance, we didn't really have anything which could give sensible readings in parallel/series with so much capacitance.A "Q" meter might have done it, but we didn't have one, & besides, were up to our armpits in mods to use the ancient beasties for PAL colour.It was easier to chuck 'em in the "odds & sods" box & use some caps which didn't cause the same problem.

author: Psi
author_rank: Super Contributor
timestamp: July 25, 2023, 01:12:41 am
content: Toluene is used as the solvent in some PCB lacquer/conformal coats, so that should be pretty safe around electronics.Well... So long as you use it 'as-is', and don't tri-nitro itAcetone on the other hand has the nasty habit of trying to dissolve things you don't want dissolved."Why has the bottom fallen out of my plastic container" "Why do I have a gooey plastic mess on my hands" etc..