Degassing epoxy

How about an epoxy mixer that doesn't add any bubbles to begin with.

AN:

I followed your link. The machine looks pricey. Have you ever used one? I'm looking for something to "de-bubbelize" thickened and pigmented polyester resin.

Regards,

EH

Casting or laminating resin?

(sorry, hadn't been following closely)

I have not used one or priced it, But I saw the video a while ago and thought it would be very useful. These are made by several places, search for centrifugal mixer.

Here is an article describing it. Looks like it would not be hard to make if you were so inclined. My current method for removing bubbles from mixed epoxy is to spread it out very thin on something smooth with a spatula, wait for the bubbles to pop, then scrape it back onto the spatula for application.

RL:

I'm using casting resin in a surface application to simulate shell in fossil restoration. Aside from the wax already in the polyester, thixotropic powdered glass is added so the material doesn't obey gravity in creeping off the curvature of the specimens. I usually add a single colored pigment or composition of them in the mix for a base color, catalyze that and pour it on the specimen. If--and that "if" occurs often--there is detailed color pattern in the adjacent shell, I will "paint" different pigment patterns into the pour before the catalyst has a chance to set. Once set, the repair gets sanded and polished. The difference between the shell and the original material can sometimes be very difficult to discern when all is done.

My problem appears to be the thixotropic material trapping air. Bubbles form three ways: in the mixing process; when air is trapped in specimen surface irregularities on resin application; and when additional colorant is daubed in once the resin is in place. The last wave of bubbles constitute the smallest number and, I think, are probably just something you have to understandably live with.

Glass works better than microballoons as a thickening agent but because of the amount of material necessary to keep the liquid from creeping off the rock, the viscosity of the mix prevents bubbles from easily migrating out.

A vaccuum pump and chamber recently arrived and next time out, they will get a trial. But other approaches and thoughts are always welcome.

Regards,

EH

AN:

Thank you very much for the above. I'll check it out when I'm newly awake.

If you'll forgive me for referring you to what I wrote to Richard Lamb above in detail, the short story of my primary problem seems to be the thickening agent trapping bubbles. I've tried compressing the resin as you suggest but the "lumpiness" of the material both retains bubbles well and gathers some more when I try to scrape it into a pourable mass.

Regards,

EH

Edward,

Thanks for the details.

I haven't used much casting resin. Way back in once-upon-a-time time when I was putting ships in bottles...

There are other fillers as well - talc for instance, wood flour, colloidal silica (cabosil), cotton flox, etc.

I've had some "interesting" results using glass in epoxy. They seem to help contain the exothermic heat in the cup and cause a run away situation. It rises like bread and overflows the mixing cup!

But back to your issue...

I'd almost suggest that microbaloons are the wrong material for what your work entails.

Perhaps a quick experiment with other stuff might be useful as I've not had much problem with it developing bubbles in the mix.

West 406 (blended? colloidal silica) is good stuff, but it's available form other sources cheaper.

Aircraft Spruce or Wicks Aircraft both carry it.

For my serious work I often mix silica and cotton. Silica to thicken the resin and cotton to make it hard as a rock. Balancing those two can produce a wide range of physical properties.

For what it's worth, cotton can be "dyed" before mixing. It has possibilities...

The other question with composites is always - "how big"? Smaller batches may show more variation in properties, but are more controllable.

If you don't really need the transparency's of casting resin, perhaps an epoxy? When fresh most epoxies are pretty much clear (well, not ALL). West, for instance, is really clear when fresh, nut turns dark as the catalyst ages.

Wish I could see what you are up to. It sounds interesting.

Richard

A CLUE!

Ed, take a fine strainer and run the filler through it before mixing. Use your fingers to rub it through the strainer. (and wear a mask!)

It Should NOT be lumpy!

Most likely the air making the bubbles is mechanically trapped in the lumps.

Spread it out as thin as possible in your vacuum chamber (ie, a bunch of petri dishes would be much better than a beaker, say) and give it some time under vacuum. My personal analysis of the best way to do this has always proved more complex to implement than has been worth it anywhere I've needed to do this - that would be to pour the epoxy from one container to another (or directly into the mold in the case of molded parts) while under vacuum, and thus gets into mechanical seals and or electrical apparatus to facilitate pouring while under vacuum. Can be done, just more money and bother than has been desired to expend on this detail.

In the late 1960s I worked as a summer hire at RCA in Burlington, MA, in the Plastics Lab, where were were potting modules for the US space program. These modules were in fact going to the Moon, and many did.

The modules were cordwoods, which have a pair of glass-epoxy circuit boards with axial-lead components strung between and soldered to the boards. One board had solder terminals and the other board was potted with alumina-loaded epoxy.

Degassing was done twice. First, the just-mixed epoxy was degassed in a vacuum chamber. The mold was then filled, and the entire mold with cordwood and uncured epoxy was again degassed under vacuum.

I assume that the centrifugal mixer had not been invented, because we did not have one, and it would have been a great improvement on the manual mixing we used.

Joe Gwinn

RL:

"Turgidity" is a word that would have been much better used. Another unmentioned reason for using a thickener like powdered glass (or cabosil which has so far come in a near second) is that it allows pigments to appear opaque in very thin sections (@ 3/32" or

"Ecnerwal" wrote in message news: snipped-for-privacy@news.eternal-september.org...

E:

Vacuum degassing the resin in a flat dish would be possible. Any hint at the level of vacuum best used?

Because the fossil substrate that takes the pour is very large, the prospect of building a sizeable air-tight chamber is not one preferably contemplated.

Regards,

EH

I've not de-gassed epoxy but have done so with a silicon rubber mould making material, it seems to work very well and the material is fairly thick. Be aware that the mix can bubble up to 3 or 4 times the volume when the bubbles start to come out initially so make sure you have sufficient volume to contain that in your container. In my case I got a cheap polycarbonate bell jar from a lab supply house and initially used a venturi vacuum generator which would produce about 90% vacuum, I've subsequently acquired an electric vacuum pump which is more efficient than using large quantities of compressed air to produce the vacumm.

I'd expect that to do it (obviously you may have to experiment a bit). Remove the vacuum before the resin sets, so that air pressure collapses the remaining bubbles.

If you'd pumped it down properly, there shouldn't _be_ any remaining bubbles. What would be in them?

Thanks, Rich

On a dial gauge, 0 (or 30, if it's reading inches of mercury). You may wish to pause briefly on the way down for some of the larger bubble/foam events before that point. Not really an application that calls for thermocouple or ionization gauges, though you might get to the point that a TC gauge would register. Basically whatever your mechanical vacuum pump can manage, and there should be no call for a diff pump or cryo pump (though I have diff pumped epoxy, it was mostly to play with the disused diff pump that happened to be attached to the chamber we generally used for degassing epoxy, normally without ever turning that on.)

I expect your application as described will be fine with the mix degassed. We did a lot of fairly fussy stuff without actually degassing the final cast, just the mixed resin.

I've used vacuum degassing for polyurethane rubber casting and it works well, but when I tried to degas epoxy it was so viscous the air bubbles could not migrate out. I was trying to do it in the mold, did not think of spreading it out in a very thin layer first. I use a mechanical vacuum pump that reaches 0 or 30 inches of Hg (depending on how the gauge scale is set up; actually about 30 mtorr on a thermocouple gauge) but I think anything under about 1 torr is enough (for polyurethane, anyway). The other way to go is to apply about 100 psi of air pressure after you paint on or pour the epoxy, and leave it under pressure until it cures. That compresses the bubbles so they are basically invisible. You just have to come up with the pressure vessel :-). There are usually a few vacuum degassing setups for sale on ebay, made from a large stock pot or bain marie pot and a flat acrylic lid with an oring and connection to the pump and vent valve. I don't think you gave the max size you need to deal with but if it would fit into a 10 or 12" id pot the ebay units would be a simple, cheap way to go. I was going to make a pressure vessel from 6" black iron pipe with caps threaded on each end, but the vacuum worked so well for what I was doing that I never bothered.

----- Regards, Carl Ijames

CJ:

The specimens at hand are large, in the range of 20-26" in diameter and about 12" high, resembling a large snail shell. All surfaces are curved, which mandates the thixotropic additive to keep the resin from traveling.

Obviously, this application is unusual and it seems that the next step is to test some of the ideas you and others have been so kind to outline. Thanks.

Regards,

EH