thinning epoxy for antennas? - Page 1
Forum section: electronics / general

author: coppercone2
author_rank: Super Contributor
timestamp: November 27, 2019, 08:16:36 am
content: I have standard JB weld brand clear epoxy. I want to coat a single turn ferrite rod antenna with it (~12mH), but I want the glue layer to be thin.What is a good solvent to dilute it with to get a sleek layer on it? I figured xylene or alcohol. But at what proportion (my mass), I am scared it might not cure properly, flake off, stay gummy, etc. I wanted it at least to cure properly.

author: rs20
author_rank: Super Contributor
timestamp: November 27, 2019, 08:27:46 am
content: You fears of unintended consequences are backed up by at least one random internet result:https://epoxycraft.com/westsystem/thinning-epoxy-best-practice-reducing-viscosity-west-system-epoxy/A suggestion therein would be that you could preheat your antenna, which will heat the epoxy and therefore reduce its viscosity. However, I think starting with a high viscosity epoxy like JB weld would be too much of a gap to bridge. Also, consider buying specific low-viscosity epoxy.

author: coppercone2
author_rank: Super Contributor
timestamp: November 27, 2019, 08:38:50 am
content: as usual, stupid project and costI said it before, I hate keeping lots of epoxy around. The perfect epoxy for this (jewelry cast) died because I used it like 3 times in 3 years. I know the same thing is going to happen. Lost 80% of 20$... very bad investmentmaybe someone has some confident information... these 'little fun' one day projects stop being fun when they are wasteful IMO. Only glue I managed to keep for long time is the new epoxy containers from loctite (and the old ultra thin CA type glues). I don't mind buying a few Oz of that stuff.. but epoxy just hurts... and then I feel compelled to do crazy shit with epoxy just to use it up

author: coppercone2
author_rank: Super Contributor
timestamp: November 27, 2019, 08:46:35 am
content: there is tax.. and there is glue tax..

author: Ian.M
author_rank: Super Contributor
timestamp: November 27, 2019, 10:01:55 am
content: How do you get 12 mH out of a single turn, even on a ferrite core? I think you mean 'single layer'.The traditional coating for high performance RF coils is polystyrene dope. One brand is 'Q dope', or you can DIY it with polystyrene (either shavings or chopped up expanded polystyrene) dissolved in an appropriate solvent.Another possibility would be well thinned polyurethane varnish.N.B. superglue isn't recommended as a coil dope as it attacks many types of magnet wire insulation, with a risk of shorted turns.

author: Berni
author_rank: Super Contributor
timestamp: November 27, 2019, 10:24:06 am
content: Varnish or any kind of clear paint will do just fine. They can be thinned down as much as you like with the appropriate solvent.

author: Cerebus
author_rank: Super Contributor
timestamp: November 27, 2019, 03:55:27 pm
content: Plastidip?

author: duak
author_rank: Super Contributor
timestamp: November 27, 2019, 07:40:03 pm
content: I've used isopropanol to thin epoxy in about a 1 to 3 ratio. I use an eyedropper to get to the right consistency. Isopropanol doesn't seem to affect the reaction but it can take a lot longer to set up. You might want to add a bit more hardener to make sure.

author: rs20
author_rank: Super Contributor
timestamp: November 27, 2019, 11:36:51 pm
content: Quote from: duak on November 27, 2019, 07:40:03 pmThis suggestionseemsto betray a complete lack of understanding of the chemistry of epoxy resins. The hardener isn't a magic substance that imbues "hardness", it's a co-reactant that cross links the resin. The provided mix ratio is (very nearly) stoichiometric. The extra/excess hardener would have no free sites to react with; so it would fail to react, fail to have any useful contribution, and just end up being a contaminant in the final product.However, chemistry is a complicated and reality is messy; I'm happy to be corrected. Is the hardener volatile?

author: Ian.M
author_rank: Super Contributor
timestamp: November 28, 2019, 12:04:57 am
content: 'Epoxy' is the generic term for a wide range of resins with considerable differences in their polymerization reaction chemistry. Seehttps://en.wikipedia.org/wiki/Epoxyfor an overview. Some epoxy systems have hardeners that are degraded by moisture, so if you are using Isopropanol as a dilutant and it isn't absolutely dry, a slight excess of hardener may result in slightly less degradation of the cured resin's mechanical properties, but in my experience, usually excess hardener itself results in significantly poorer strength, and excessive softness of the cured resin.I'd also be extremely wary of deviating from the recommended mix ratio or adding polar dilutants for RF work as the odds are the dielectric losses will be significantly higher than for the same epoxy system, mixed without dilutants according to the manufacturer's recommendations.

author: Mr.B
author_rank: Unknown
timestamp: November 28, 2019, 12:12:19 am
content: I just use common nail varnish for this type of application.Dirt cheap.If you don't want to use clear you can choose any colour of the rainbow.

author: SilverSolder
author_rank: Super Contributor
timestamp: November 28, 2019, 12:47:28 am
content: Maybe it can be covered with goop? That stuff thins well...https://youtu.be/wdML79G45OQ

author: coppercone2
author_rank: Super Contributor
timestamp: November 28, 2019, 07:02:40 am
content: ended up gambling and mixing with denatured alcohol. It ends up kinda rubbery and you need to do a oven cure on it, so long the layer you deposit is thin it cures good, but if there are bumps because of excess epoxy accumulating on the bottom these are a bit weak.and tear off fairly easily.However, the coating does seem appropriate enough. If you are very careful to get rid of excess before it gets too gummy to remove (and comes off leaving 'nasty' texture) it will work fine IMO. Feel that it will protect the coil during installation and preliminary testing from any mishaps before its securely mounted and protected in shield.The casting resin from jewelry store is harder/better, but I do wonder if this has advantages in terms of mechanical compliance if its used as a power element.. (will it put less strain on the wire in the long run then the hard coating?)

author: coppercone2
author_rank: Super Contributor
timestamp: June 07, 2020, 11:51:58 pm
content: update : failed

author: Cerebus
author_rank: Super Contributor
timestamp: June 08, 2020, 12:06:07 am
content: So what was the failure mode?

author: coppercone2
author_rank: Super Contributor
timestamp: June 08, 2020, 12:20:32 am
content: it peels like sun burnt skin, project sat in storage pending analysis of the cure

author: coppercone2
author_rank: Super Contributor
timestamp: June 08, 2020, 12:24:26 am
content: basically what happens is, if you have a thin layer, it seems to cure properly. However on the underside of the coil there was some accumulation over time due to excess glue, and these spots stayed rubbery, and peel off easily, and you are left with moon craters.It's unsightly and it will require a touch up. I assume the excess solvent can't diffuse from more then a very thin layer, so the epoxy reaction in the thicker spots is not had reliably, even after 7 months. I did not notice these spots because this is held by large brackets that are wide.Not to say normal varnish does not do this, stalagmites form on commercial transformers as well after dip, and they usually snap off, this kind of rips off in a rubbery manner though.

author: Gregg
author_rank: Super Contributor
timestamp: June 08, 2020, 06:03:39 pm
content: There are a number of marine grade epoxies but many are not UV tolerant and need to be painted or otherwise protected.Source of epoxy at reasonable prices:https://www.epoxyproducts.com/this is a terrible websiteThinning epoxy can be done with benzyl alcohol and it also makes the epoxy more flexible but less strong and it doesn't gas off. It is the same solvent marketed for thinning epoxy by the above seller but much cheaper elsewhere.Some lacquer thinners can be used to thin some epoxies.I wouldd suggest a good 2 part epoxy paint or a 2 part polyurethane paint; these are often available as aircraft paint.

author: coppercone2
author_rank: Super Contributor
timestamp: June 08, 2020, 09:00:51 pm
content: how do you feel about q-dope?

author: Gregg
author_rank: Super Contributor
timestamp: June 09, 2020, 12:32:22 am
content: I feel q-dope is overpriced clear nail polish. It basically is a lacquer meaning it hardens by solvent evaporating. The MSDS states it has MEK for solvent. MSDS are often good clues to suitability of product.A lot of missing details may help narrow the recommendations.How exactly are you constructing your antenna? Is it something that is flexible like magnet wire wound around a fiberglass fishing pole? Will it be exposed to UV sunlight? Do you want to just adhere the wire to whatever it is wound on or do you also want to put a protective coating over everything? Are the windings close together so that it is difficult to get your adhesive to penetrate between them? How long do you want this to remain functional?For a quick and dirty antenna, I would just adhere the wire every few turns with a drop of UV curing adhesive and when it was all done spray it with some Krylon Chrystal Clear from a rattle can (available cheap at Wallmart).

author: coppercone2
author_rank: Super Contributor
timestamp: June 09, 2020, 12:51:23 am
content: the uv cure idea is interesting.There is also a formula on youtube on how to make Q-dope, which you can do, right out of the anarchists cook book lolWhy do you think solvent cure adhesives are bad for coils? Just fragility? (this is very well mounted mechanically). It's a giant ferrite rod antenna for low frequencies (my first attempt at narrow band low frequency antenna, to see how it compares to the high bandwidth active loop).

author: Gregg
author_rank: Super Contributor
timestamp: June 09, 2020, 01:47:14 am
content: I’m not stating that solvent based adhesives are bad for coils; that is if the insulation is impermeable to the solvent. Use whatever works for you. If the solvent breaks down the dielectric strength of the coil it may be hard to discern the actual problem. I was merely stating that q-dope is basically the same thing as clear nail polish which is solvent based. I wasn’t aware of the major parameters for the initial question, just some proposed solutions.For an antenna wound on a stiff ferrite core, epoxy is probably very suitable if you find the right epoxy that you can work with. You could pot the whole coil in thin slow cure epoxy and vacuum degass it if you wanted.I mentioned the UV cure adhesive as a device to keep the wire from unwinding a bit at a time as you were winding it.

author: Ian.M
author_rank: Super Contributor
timestamp: June 09, 2020, 09:16:36 am
content: On the one hand, as mentioned in the Q-dope video in reply #20, the dielectric constant of most epoxies is a lot higher than that of polystyrene, and also the losses are higher, so epoxy coating the coil will result in higher stray capacitance and reduced Q. Also both the dielectric constant and losses of epoxy are highly temperature sensitive, so you may experience problems with frequency stability.On the other hand, most nail varnishes are nitrocellulose based with a witches brew of additives to improve adhesion, toughness and other cosmetically desirable properties. The effects of the additives on its electrical properties are unknown so you'll need to test its suitability for high Q RF circuits, e.g. by painting some on a PTFE sheet then when dry, see if it melts, burns or significantly heats up in a microwave oven (with a beaker of water as a load for the magnetron).On the gripping hand, polystyrene is well known to be a low loss dielectric, and is easy to dissolve in many solvents, with a long history of use as a coil 'dope' so its only a matter of picking a solvent that's compatible with the insulating varnish on your magnet wire, and being patient enough to let the polystyrene dope dry properly, then, ideally, baking out residual solvent in a cool oven.