Vacuum-impregnated tubes

[Swede]

I'm very excited about a possibly new way to make tubes at home. Having gone through the traditional method, it leaves a lot to be desired.

 

In some past projects, I worked with a process known as potting. Potting is the encapsulation of sensitive electronics in resin. As part of the process, the item to be potted is put under a vacuum, which aggressively pulls all air bubbles out of the system. The after the vacuum is released, any voids inside pull resin into them, and the result is a system full of resin, no air bubbles.

 

I decided to try and see if I could vacuum-impregnate kraft paper with a thinned white glue after the tube has been rolled bone dry.

 

http://www.5bears.com/firew/vac01a.jpg

 

The rod is first thoroughly waxed using paraffin or toilet wax. I used the toilet wax and it works well. Calculate how much paper you need, and cut it.

 

http://www.5bears.com/firew/vac02.jpg

 

Start rolling, and after the first circle, run a bead of straight white glue for strength.

 

http://www.5bears.com/firew/vac04.jpg

 

When you are done, glue the last inch or so, again with straight white glue. The first time I tried this, I taped off the last portion, but I think this next method will work better... use three small cable ties to ensure the tube stays compacted.

 

You will find the tube will easily slide on the dowel rod, and that is exactly what you want to see.

 

The next step - UNDER THE VACUUM!

[Swede]

Don't let the term vacuum scare you. The total cost of this setup was IIRC about $75. The vacuum pump was surplus from a place called Surplus Center. The vacuum "jar" is a coffee can, with a fitting soldered on. For a base, you'll need something flat (I used acrylic sheet) and sheet of rubber, which can be found at many plumbing stores. Or, you can use one of those silicone baking sheets found at a grocery store.

 

http://www.5bears.com/firew/vac04b.jpg

 

The "pot" is just a plastic cup filled with 1/4 white glue, 3/4 water, mixed thoroughly.

 

Into the pot goes the dowel plus tube:

 

http://www.5bears.com/firew/vac05.jpg

 

The coffee can is placed over the setup, and the vacuum switched on. It immediately "grabs" the rubber sheet, and the pot + tube goes under vacuum. I added weights just to be sure it sealed well and was stable.

 

http://www.5bears.com/firew/vac06.jpg

 

I've found that about 20 minutes of vacuum will pull the glue throughout the tube, thotoughly saturating all of the paper. Next - pics of the result!

[Swede]

The dowel + tube came out soaked and thoroughly wetted. This is normal and expected for the surface, of course. I found the paper slightly swollen where the cable clamps bit into the tube.

 

http://www.5bears.com/firew/vac07.jpg

 

I cut away the three clamps, and generally smoothed it out. I found I could unroll the saturated paper, so I added a bead of white glue to the very end, and blended it back in for strength.

 

 

http://www.5bears.com/firew/vac08.jpg

 

After sliding the tube off of the waxed dowel, I examined it closely. As far as I could tell, the paper was wetted through. I could see the glue-impregnated paper on the interior of the tube, in the middle, which was the most protected part of the tube. It appears that the glue saturated the paper with no difficulty.

 

http://www.5bears.com/firew/vac09.jpg

 

I'm letting it dry right now. The next step will be to section the tube right in the middle, and see if it is glued throughout. I'm also guessing that I can go with a 50:50 mix of glue + water, rather than 75% water. More glue = more strength.

 

The good part about this process - it's easy! Much, much easier than the traditional method. It is also capable of doing bulk work. If you've got a half dozen or more sections of 1/2" aluminum for dowel pieces, you can vacuum them all at once... no need for a vacuum session for each tube.

 

I'll post more after the tube dries. So far, so good.

[asilentbob]

I would think that the tubes would have enough structural integrity to support themselves vertically without the aluminum rod even when wet, you would just have to be a bit careful.

 

As to how strong the tubes are... perhaps you could test the strength by hydraulically pressing something like sand inside your dried tubes and comparing at what pressure they bust and how badly the are deformed and then do the same test with NEPT, normal hand-rolled, etc tubes of the same ID, OD and height. Perhaps noting weights too.

[tentacles]

An alternative (a cheap one!) to a vacuum pump is a vacuum aspirator. It's a little piece of plastic that uses the "venturi effect" (which doesn't really exist, but nevermind) and water pressure to create fairly strong (28"+) vacuum.

 

I'd think that if you removed the tube from the spindle before impregnating+drying that there would be problems with distortion and shrinkage. Leaving it on until dry would produce very nicely round tubes, etc etc. Some guys on passfire have been using similar setups to impregnate cheapo tubes with various resins to strengthen them up, with varied success. I think the resin/plastic used would be very important. Phenolic would be a great one to try if someone could get some. Rolling a tube out of cloth and then impregnating with resin would make very strong tubes (a composite also known as micarta).

[WarezWally]

Dont people dunk home rolled tubes in waterglass?

[nath0r]

A really good tutorial Swede. I really need to start thinking about making my own tubes, or at least some of them, having to buy them all is costing me a small fortune.

[Swede]

A little follow-up. The tubes look excellent, much better than my hand-rolled traditional tubes, but I'll be the first to admit that I am not skilled at it. My traditional tubes were a bit floppy compared to these.

 

Rather than cable ties, I tried one with cotton string rolled on it in a tight spiral, and the cotton let the glue pass easily, yet kept the tube tight and conforming.

 

There's a lot of room for experimentation. I'm especially interested in trying one with some sort of true resin rather than white glue. Maybe thinned polyurethane? Any other resin types? It has to be air-drying, otherwise you'd get massive waste after the pot sets up.

 

Added: I've got a short tube being vacuumed right now under polyurethane, thinned with MEK. The polyurethane right out of the can seemed thin enough as it was, with a viscosity perhaps a bit thinner than straight white glue, but I added the MEK just in case the p. doesn't migrate as nicely as the water-based mixtures. This one is 100% enveloped in cotton string, and it looks nice in the sense that the string was tightly wound. I'm thinking that if it is left on there, the cotton string will add to the strength of the tube.

[tentacles]

Finishing polyurethane really isn't strong at all. You need something that has strength, ideally some epoxy or polyester. Both of these aren't suitable (except on very small scale) because they cure in a matter of hours once mixed.

 

Polycarbonate might work well, it's strong and can handle shocks well. Acrylic would be a bad choice because it's so brittle.

 

KEEP IN MIND that solvent vapors could be potentially fatal to your vacuum pump!

[Swede]

Hi tentacles, I suspect you are correct about the finishing poly. I'll give it some tests, but visually it looks awesome. Thanks for the suggestions!

 

The polyurethane tube looks great. It's fully saturated, and seems to be bonded as well as it can be. All that remains is to let it dry and test the strength in some fashion.

 

http://www.5bears.com/firew/put01.jpg

 

There's zero doubt the thinned polyurethane penetrated far better than the white glue. I vacuumed it less, and yet the core of the tube is completely wetted, slick as can be, and the tube slid off the rod like it was greased. Polyurethane is good stuff. I don't know if it'll match white glue as a bonding agent, but the potential is there. Maybe thinned polyurethane glue, like gorilla glue, would work better and be stronger. There must be hundreds of resins that are NOT water based that will do a good job. It's just a matter of finding one.

 

A bonus - no H20 means that the paper will not get the "fuzzies" when handled. I noticed on the vacuumed white glue tubes that if you rub it or handle it excessively while wet, the paper is loosened and can come off in patches, as the water has physically weakened the paper a bit before the glue dries. Not so with this stuff. The paper is completely unaffected beyond being bonded. I'd also guess drying time will be faster, and there will be less warping, less swelling, and it'll be waterproof to boot.

 

Some resin is definitely in my rocket tube future, for sure. This is insanely easy. The only trick is the vacuum setup.

[Swede]

Haha, wow, what a mess. I tried polyurethane GLUE (gorilla glue) thinned with acetone. The glue, after mixing, had too many bubbles suspended in it, and when i turned on the vacuum, the pot frothed, overflowed, and dumped 1/2 its contents onto the rubber gasket. And anyone who has worked with that glue knows... it's some nasty stuff.

 

I am seeking suggestions - needed: a 1-part non-aqueous glue/resin to impregnate these tubes, capable of being thinned appropriately. Any more thoughts? If not, I will go back to white glue, which is working well.

[psyco_1322]

Dont people dunk home rolled tubes in waterglass?

I did that once. It wouldnt work very well as rocket tubes and such because it tends to make teh ID shrink do to the coating. But I noticed after awhile that they became cracked all over and white duty crap formed on the surface.

 

You can also heat your tubes slightly and soak then in a thinned epoxy solution for a few minutes and stand the up to dry. This will greatly increase their strength and burn throw resistance. This is coming for a highly experienced rocket builder I met at the 07 PGI convention.

 

An idea I thought about the other day: Why doesnt anyone try to use carbon fiber tubes for motors?

[tentacles]

Psyco: There are folks who do, they call it high powered rocketry. Carbon fiber would be immensely expensive. Fiberglass less so, but that stuff is a bitch to work with without a good process (steam heated mandrel, automated equipment...)

 

As far as something to soak the tubes in, try a polycarbonate solution in possibly toluene or methyl chloride solvent.

 

It IS possible to thermally cure polyester (fiberglass resin) rather than using a catalyst. Aren't the HPR resins thermo-cure, or do they use a catalyst + heat?

 

Here's a tidbit from the phenolic resin entry on wikipedia:

"The properties of phenolic materials make them very well suited to myriad industrial applications. Phenolics are the result of polymerization between resin and a base material that can be paper, glass or cotton. The base material used is dependent upon the intended application of the finished product. Paper phenolics are used in the manufacturing of electrical components such as punch-through boards."

 

Think laminate countertop material. Hard, flexible.

[Sylar]

How about using wheat paste instead of white glue?

[Swede]

Sylar, I'm not experienced with wheat paste. Do you think wheat paste would work better than thinned white glue?

 

Update: The first non-aqueous tube was polyurethane varnish. As Tentacles predicted, it was weak, and not useable. Then I tried thinned polyurethane GLUE aka Gorilla glue. It was a mess - it bubbled over.

 

And it just now struck me as to WHY it did so... the acetone was boiling. DUH! Acetone will not work unless I can modulate the vacuum so as to prevent the boiling. Some solvent BP's

 

Acetone 56C

MEK 79C

Ethanol 78C

 

So perhaps one of the later two. Mineral Spirits may work as well.

 

Anyway, the tube only got about 4 minutes of "vacuum" which really wasn't, as the acetone was vaporizing as quickly as the air was being pulled from the system.

 

During the spill, I put the tube aside and forgot about it. Today, I checked the tube and was amazed - best tube yet, by far. It is as hard as a rock, was completely saturated, and when I cut the end, it was like cutting plastic. Check out the completely even layers, no voids... really almost too much glue. The outside has a fairly thick, shiny layer.

 

http://www.5bears.com/firew/ggtube01.jpg

 

THIS glue could be an excellent method, if the dilution problem can be overcome. I'd prefer alcohol if compatible, as being more benign to my vacuum pump, but I'm not sure if ethanol is compatible with the glue. I may have to dry the ethanol first with anhydrous magnesium sulphate or similar, as moisture is what causes polyu. glue to kick.

[TheSidewinder]

Sylar, I'm REASONABLY certain that wheat paste won't have the strength you need, even using vacuum impregnation.

 

I could be wrong, but I'm certain I've seen posts where it's been said that wheat paste has relatively low strength when dried.

[psyco_1322]

Wheate paste has about the same strength as straight white glue. I seen this one a site the was reporting the results of a college study on adhesives. Deluted white glue seemed to be a bit stronger the both, 50/50 glue/water.

 

I understand the HP guys use stuff like that. Of coarse they would have to be bought premade. You think you would even have to put a sleeve on a CF tube if pressing?

[TheSidewinder]

Hmm... perhaps what I saw was talking about one of the binders (dex, guar, CMC, etc) but I didn't think so.

 

In any case, give it a try Sylar.

 

It's cheap enough that a failure won't cost you more than a few pennies.

 

IIRC you'd want (if not actually need) a sleeve on a CF tube if pressing.

 

If making a cast APCP grain you wouldn't of course.