Any experience with EA 1C-LV or other low-ish viscosity epoxies?

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Mark Sato
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Any experience with EA 1C-LV or other low-ish viscosity epoxies?

Post by Mark Sato »

I decided to take my broken single conductor threaded feedthrough and convert it to a 7 conductor part using socket contacts meant for circular connectors, with the intent to use it for low voltage purposes. It's a very tight fit with some clearances being <1mm. I 3D printed a contact locator to hold the contacts in place and planned on potting both ends with epoxy. I purchased Loctite EA 1C for this (and to fix some other chamber issues), but when I went to mix a small batch I found that the viscosity was far too high to be workable for this application.

I see that Loctite also makes a "low viscosity" version of EA 1C, but I'm not sure how much better it would be. Regular EA 1C viscosity is like a paste, and I need something that would run, in part to fill all voids. Does anyone have any experience with it, or perhaps some other suitable non-conductive epoxy for vacuum use?
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Richard Hull
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Re: Any experience with EA 1C-LV or other low-ish viscosity epoxies?

Post by Richard Hull »

Heating most epoxies will almost turn some to water flow levels. It will also boost the cure rate as in most chemical reactions, of which, epoxy is one. This usually demands some sort of mold or stop to control the rapid flow so as to form a finished form. I am sure AI could have told you this.

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Rich Gorski
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Re: Any experience with EA 1C-LV or other low-ish viscosity epoxies?

Post by Rich Gorski »

Mark,

Richard is exactly right about the heating. I've used EA-1C epoxy (Torr seal equivalent) to fabricate/repair many different feedthroughs. EA-1C becomes quite watery when heated to around 50-70C. You can either preheat the epoxy paste say with a heat gun for a few seconds and then apply while its watery. Another approach is to fixture the assembly, apply the paste at room temperature and then place the assembly in an oven for about 45 minutes at 65C (~150F). After 45 minutes the epoxy will be nicely cured and ready to use on the vacuum chamber. Remember that the epoxy will become quite runny in the oven so let gravity do the work to fill the voids. You may also want to use some kind of cover underneath so the epoxy is blocked and doesn't run out onto the bottom of the oven.

I have also found that for intricate feedthroughs and to create a better seal you may want to apply the epoxy a second time. For the second application and using the epoxy at room temperature I would install the feedthrough on the vacuum chamber and start the forepump. Then apply the epoxy. The vacuum will suck the epoxy down into the remaining voids. Just make sure the remaining holes are small enough so the epoxy doesn't get sucked through.

Using this approach I've created many feedthroughs so save on cost or to fabricate something unique. I've also used ceramic tubes with wire through the center to form a feedthrough that can pass through a hole in a vacuum plate. Sealed with the epoxy as described above these ceramic feedthroughs were capable of 10s of kVs of insulation and also sealed well enough to reach into the 10-7 Torr range in a turbo pumped system.

Good luck.
Rich G.
Mark Sato
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Re: Any experience with EA 1C-LV or other low-ish viscosity epoxies?

Post by Mark Sato »

Thanks, Richard & Rich.

I've been so narrowly focused that It didn't even dawn on me to investigate ways of reducing the viscosity of epoxy until this morning, and even then I was thinking maybe chemically. I'm just now reminded of a time I unintentionally found out that heat has that effect on epoxy.

Those are some helpful tips, Rich. I'll heed them when applicable.

One last thing: I have access to one of those silicone casting vac chambers. Is there any sense in outgassing this epoxy before/after pour?
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Dennis P Brown
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Re: Any experience with EA 1C-LV or other low-ish viscosity epoxies?

Post by Dennis P Brown »

If the epoxy is to be under high vacuum for extended times, out gassing is useful. However, outgassing hot epoxy is a bad idea. It will set almost instantiously.
Mark Sato
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Re: Any experience with EA 1C-LV or other low-ish viscosity epoxies?

Post by Mark Sato »

I'd outgas it immediately after pouring it in the feedthrough, so I wouldn't mind if it set up quickly. Would that cause other problems?
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Liam David
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Re: Any experience with EA 1C-LV or other low-ish viscosity epoxies?

Post by Liam David »

EA-1C will work fine for your application and I don't think pre-outgassing is that necessary. We use tons of the stuff for various sealing jobs on the PFRC-2 at PPPL, all air-cured and not separately outgassed. The machine has a plastic (polycarb) vacuum vessel and can still reach low e-7 torr. For a more demanding application, I used some to bond titanium that was subsequently baked to 150C for 12 hours and had I no trouble in the 1e-7 torr range. I used a thermocouple, aluminum foil "oven", and a hot air gun to cure it and to combat the low viscosity I just rotated my part and monitored for drips. Excess was carefully sanded/polished off.

The key to vacuum success is to minimize the surface area exposed to vacuum.

If you're still concerned about outgassing, Accu-Glass sells some higher-spec vacuum-rated epoxy https://www.accuglassproducts.com/fiber ... epoxy-glue (EPO-TEK 353ND) but I think this is overkill in your case unless you plan on doing repeated, high-temperature bakeouts.
Mark Sato
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Re: Any experience with EA 1C-LV or other low-ish viscosity epoxies?

Post by Mark Sato »

My concern was centered around getting any potential air pockets out of the pour, rather than outgassing issues when in use. At any rate my concern was negated by the extremely short working time when heated from an epoxy that already has quite low working time at room temp. With hindsight, I'd probably have gone about building this feedthrough differently.
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As is visible in the images, I attempted to pour the epoxy in both ends, but neither went as I'd hoped. On the vacuum side, it nearly went inside the contact sockets which required me to quickly readjust how I was pouring which resulted in an incomplete pour. The image might make it appear as if there's plenty of space to work with, but I assure you the gaps between the contacts are on the order of <2mm. The other deceiving aspect is that the center hole in the brass housing has a beveled lip - the actual hole is much smaller - the gap between the walls and some of the contacts are less than 1mm. The only way I could possibly fill the center void is by trying to use a syringe, but I suspect that also will work out better in my imagination than in real life.

On the atmosphere side, it looks ok, but I suspect it didn't go anywhere near as deep as intended. If the thing doesn't leak, then I can just quit my bellyaching and call it a day, but that void really irks me.
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Dennis P Brown
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Re: Any experience with EA 1C-LV or other low-ish viscosity epoxies?

Post by Dennis P Brown »

I have done similar type of seals. One issue is proper wetting of the surfaces. The metal being really clean is critical. So using organic cleaning liquids is a a good idea and handle with gloves. I also I also heated my epoxy but then pre-coat all metal surfaces to better ensure proper wetting. After this I then assembled the parts. One issue with copper is the coefficient of thermal expansion (CTE). It is extremely large so the epoxy, when harden, can support micro cracks after temperature cycling. But room temperature cycling should not, I'd think, be any issue. But leakage can occur any time due to this issue so keep that in mind.
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Rich Gorski
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Re: Any experience with EA 1C-LV or other low-ish viscosity epoxies?

Post by Rich Gorski »

Yeah, I see the problem you described.

I didn't realize that you have chosen to use female connectors on the vacuum side and thus the problem of keeping the epoxy from going into the connector holes. I would have chosen male connectors on both the atmosphere and vacuum sides. Then use mating push on female connectors on both sides. No holes on either side to keep free of epoxy.

At this point I would go ahead and see how well it holds vacuum. You might have already succeeded. I think what I would try at this point is to push in the mating connectors and then fill the voids on the vacuum side with epoxy. Then wait like one hour at room temperature so the epoxy has not hardened yet but is solid enough so that you can pull out the connectors without the epoxy filling in the openings. You might also just push in the connectors fill the voids and let it harden. Then you will have permanent flying leads on the vacuum side. Will flying leads on the vacuum side work in your application?

Rich G.
Mark Sato
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Re: Any experience with EA 1C-LV or other low-ish viscosity epoxies?

Post by Mark Sato »

My logic behind opting for female contacts, especially where they would be encased in epoxy is that they would be much less susceptible to damage than male contacts. It's even more so the case when small contact sizes need to be used in order to fit them in a limited space, such as with my feedthrough housing.
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