Degasing resins

[davidfee]

Does anyone have experience with degasing resins? I have not made the tool yet but, taking the air out of epoxy and silicones (etc.) can be very important. I've seen people use large glass jars and even PVC pipe to make vacuum chambers for the degasing process. You put the mixed resin inside, close it up and slowly pull a vacuum to bring the air bubbles out. Then your surface coat, or cast silicone part, will be more uniform and without air bubbles.

I'll be making such a chamber soon, so I'll post photos when I have it finished.

-David

[Salto]

David,

In a past life working at our National Measurement Laboratry I designed and fabricated many microwave waveguide loads which were tapered assemblies made from carbon powder dispersed in epoxy. The carbon powder was difficult to get mixed in thoroughly, and required heavy mechanical mixing to develop high shear loads to disperse the carbon clumps. This introduced lots of air bubbles which I removed by placing the moulded part in a partial vacuum as it cured.

I used a glass bell jar and slowly pulled the vacuum down. At first the air bubbles just fizzed out of the surface, but then the whole mix appeared to boil, as presumably the higher fractions in the epoxy start to boil off. Strangely, this did not appear to effect the cured properties of the mix. If I left the mixture boiling as it cured I would get a nice uniform foam structure which was good for some applications, but mostly I let it boil for 10 minutes or so then turned off the vacuum. The mixture would then settle down to a nice uniform mix with no trapped bubbles.

A word of warning, don't leave the vacuum running and go away for a coffee break, as the boiling mixture can froth right out of the mould and get into the vacuum line and then to the pump. Don't ask how I know this can happen.

Graham.

[Mighty Mik]

I believe that relatively small batches of solid rocket fuel are mixed in like a 5 gallon bucket which is then covered by a lid that has a gasket seal on it, this has a vacuum source attached. The batch is degassed as best possible, then poured into a mold...at least I THINK. If you search it, it may give you a few ideas.

[raptor22]

Most solid fuel in those quantities (which probably be for experimental rockets) are simply mixed with a mixer (some use kintchenaid mixers) and thrown into a grain mold. The stuff doesn't stick; you just pop a cylinder out and drill a hole in the center or whatever for the core.

--Alex

[berniep1]

I'm no pro when it comes to composites but here's my 2 cents:

I used to use a modified blender (don't laugh) to mix the resin instead of doing it by hand. Instead of the cutting blade I affixed a little prop to pull the mixture down and let the thing run at really slow speed (used a geared motor with an old mechanical speed control) Much better than mixing by hand :) -- a lot less bubblesThrew the thing out at one point since I only use small amounts of resin anyways.
Vacuum is certainly a much better idea...

Resin absolutely bubble-free is great....until you apply it somewhere. I've found that even when I'm really careful the application methods I use always introduce bubbles again.

So I just use a mini-torch (the ones using cigarette lighter gas) and get the tip of the flame to the bubble for a split second or so on the first coats in the mould...bubbles burst instantly. Doesn't affect the quality and gets you an extremly nice surface. Just requires black moulds; otherwise you can't see the bubbles well enough.

Bernie

[ruud zandvliet]

Hi guys,

I always degas my resin because when you do this you will see the amount of air mixed in it.
Especially when making props these one,s i made with vacuum resin injection to make sure that there are now airbubbles in the props.
There is also an additional chemical named "BYK"witch can be used to degass it then degasses just by letting it rest for an couple of minutes.
This is used when we make carbon table blades(2X1Meters,3cm thick)these are made with RTM(vacuum resin injection) and needs 14KG of resin in one go.
These kind off amounts are to big for our glass degassing jar.
the best ruud

[davidfee]

Thank you Ruud, and everyone else for the replies. I have still not made the vacuum jar, but I made a tow wetting machine last night. It needs some adjustments but seems to work well. I will post pictures when it's finished.

-David

[F3X]

Dave,
To answer your question.

Yes, you can degauss surface coat and silicones. You really can't do great job deguassing room temp resins, you can get them to boil under high vacuum but they never really “Break" completely.

If you haven’t tried it yet here is what you need:
A 10” or 12" PVC pipe coupling with a 3/4 to 1" thick Plexiglas lid. A silicone gasket seal around the top (Cut from a flat sheet 1/8 thick.). A base of 1” Plexiglas or 1/2 in aluminum with a silicone gasket. A minimum hose diameter of 3/8. The pump needs to be close to the jar. The vacuum should enter the chamber by a large ball valve so you can regulate the vacuum quickly and easily. Attach a vacuum gauge to the PVC coupling. Just drill and tap the PVC for the gauge and ball valve.

Place a ½ inch of the surface coat or silicone in the chamber in a plastic quart container. Apply vacuum and watch it “boil” to the top of the quart container and shut off the vacuum by closing the ball valve. Keep repeating this until the surface coat breaks and stops the boil. You will be surprised how far just that small amount of surface coat will rise under high vacuum. It’s this reason that you can’t break laminating epoxy, the stuff boils so much you would need a vessel bigger than a 5 gallon drum to degauss a pint of epoxy and a huge pump to do some good.

How far will your pump draw to?

You need to get down to microns of vacuum to do any good and at a high CFM. Just boiling at 25 inches won’t do anything at all.

[Quacker]

Silicones should not be degassed at more than 29" nor for more than 5 minutes. (See the manufacturer for specifics). When an extreme vacuum is applied to RTV (Room Temperature Vulcanizing) silicone, water deliberately present in the silicone and necessary for complete curing will boil off. The result will be an RTV that will never complete the cure cycle and will remain soft and gummy. You might also consider curing resin or silicone under pressure if you have the right gear. After initial evacuation, pressure curing not only reduces any remaining bubbles in size, but can actually drive the bubble into solution similar to carbonated beverages--only because the cured product is solid, you will never hear from the bubble again. You will need to hold the pressure until the product sets.

Best,
Q

[davidfee]

Uhh... wow guys. Some different perspectives here. I work in a UHV research lab, so I have access to all sorts of pumps. However, for this I will be using a Gast diaphragm pump, which I also use for most of my vacuum bagging. I will not be mixing a pint of epoxy at any point in the foreseeable future... I work on a much smaller scale.

I do actually have experience degassing epoxy and silicone; I just don't have a desiccator or other chamber in my shop right now. I thought I'd raise the subject to get some activity in this forum. (Looks like it worked.)

I question the validity of the statement that "boiling at 25 inches won’t do anything at all," since what I am trying to remove is air... not water or epoxy. It does not take extremely low pressures to release air from mixed laminating resin, in my experience. Perhaps you have a different resin system which behaves differently. I'm appreciative of your insight, however, as it stresses the importance of *slowly* applying the vacuum, so that you can control the expansion of the air bubbles.

I'll keep you guys posted...

-David