MarkD

Is there a real difference between 15 minute + 30 minute epoxies when used for firewalls?

Acording to the packages in the LHS the 30 minute cures the hardest but is more brittle than 15 minute and is best used when there is fairly thick film of glue.

The 15 minute is supposed to be a bit less brittle when cured, and so is better for "areas subjected to extreme vibration."


I would think on clean, well fitting joints either would be acceptable.

Any thoughts?


Mark D.

CafeenMan

Slower epoxy is better. They're comments about brittleness are backward unless it's a brand specific problem. Fast epoxies tend to be more brittle and weaker. I never use anything faster than 30 minute epoxy.

Siefring

Slower cure epoxy is stronger but more brittle. These are different properties. If you take a cured piece of epoxy, hang weights on it, the 5 minute will break with a lighter weight than the 30 minute (everything else being equal). However, the 5 minute will bend and stretch before it breaks. The 30 minute is stiffer/harder and will snap when overloaded. This is what they mean when it says it is more brittle.

Brittleness can be a factor long-term under vibration and flexibility might give an advantage in spreading loads to other pieces. Having knit-picked, I agree, 30 min epoxy is almost always the best choice. The main advantage of 5 min epoxy is speed.

My personal opinion is that the ability to hold up to glow fuel is the most important aspect for firewalls. I just wish I knew which epoxies were best in this aspect.

I agree. It is very easy to undermine the best choice of glue with an oily surface or a big gap in the material. A firewall should have some interlocking structure so that it is not totally dependent on the glue used.

I use 30 min epoxy for gluing the firewall in place and usually reinforce the interior with some small triangle stock. I then paint the front of the firewall with finishing epoxy (or Resin) and try to seal any seams. There may be better (lighter) ways to seal the firewall. I've heard of using brushing lacquer or balsarite for sealing. Both would be lighter, but I have no experience with how they hold up over time.

Carl

CafeenMan

Siefring - Not being a chemist I can't provide any scientific evidence. I would guess the brittleness you speak of comes from slower epoxies curing to a harder state. When I used 5-minute epoxy in the past I noticed it would crumble out of joints several years later. I never noticed any problems at all with slow epoxies. If they're more brittle it has never manifested in structures I've built. But I also don't build up epoxy fillets around joints. Any glue I can see is wasted weight and I remove it.

HighPlains

My Feedback: (1)

I was a big fan of Hobby Poxy 2 with a 45 minute working time and 3 hour cure, and overnight full cure. I'm down to my last box of it since they quit making it 5 or 6 years ago. It is not brittle, and worked well with chopped glass and cabasil.

But I just found an epoxy that looks like it might be about the same glue (it looks like Hobby Poxy) call Power Poxy. It claims 30 minute working time, 3 hour cure, and 12 hour full cure.

Their web site:

http://www.powerpoxy.com/

Click thru the intro, then onto the Pro series. The glue I got is the second from the left in 1 lb package. Haven't tested it yet, but it looks very promising. About $10/lb which is not too bad.

Siefring

Hello Again CafeenMan,

Yes. I agree with all of your recommendations, I just wanted to make the point on what is meant by brittleness.

Most epoxies I've used, I would consider to be brittle, whether they are 5 min or 30 min. If you over stress them they crack. If the 30 min is 50% stronger then the 5 min, a little flexibility is not going to make up for the difference in strength. If the two were very close in strength and you repeatedly flex them below the breaking point, the harder one will crack up first.

I'm not sure about the chemical properties of epoxy, either. It may be that 30 min does not breakdown as easily when soaked in glow fuel. I'm just guessing that the hardness indicates better bonding in the epoxy and and that would mean more fuel proof.


I can't think of any practical cases either.

[:'(]

Best,

Carl

Rodney

My findings are similar to CaffeenMan's, 5 minute is much more brittle than the slower cures and much weaker. I also was a fan for HobbyPoxy epoxy, did not get brittle and was very good. Usually, the improper mixing will be a cause for brittleness too, especially if to much hardner is used. Just remember, even the 5 minute epoxy does not reach full strength for at least 20 hours under normal temperatures.

Siefring

If you just fill a gap with 5 min epoxy, you will have a lot of air mixed in with the epoxy. This will definitely make it weak and seem brittle. If you clamp two items together, squeezing the air out, then let them cure, I don't think you would see as big a difference between the two epoxies.

Carl

fokker20planes

My 2 cents. 5 minute epoxy cures so fast it has little time to soak into the wood you are gluing. Especially plywood. 30 minute or more soaks into the wood better giving you a better bond with the two surfaces. Thats why it is preffered for firewalls and landing gear mounts. I am speaking from experience with a plane that had the landing gear part company on a harder than normal landing. 3o minute epoxy solved the problem.
Tom

Rodney

The primary reason for the difference in strength is not the ability to soak into the surfaces being bonded, it is because the slower epoxies form a longer molecular chain when the cure.

Siefring

I agree with Tom, especially on landing gear. Whenever the joint breaks, it seems the wood pulls apart -- it is not the epoxy breaking. In that case, having a longer soaking time is a big advantage.

Carl

I think I will only use 72 hour epoxy and vacuum bag everything

HighPlains

My Feedback: (1)

My test of brittle is to put a thin skin of epoxy on a thin card and let it cure. Then just bend it. If it breaks, it brittle (most fast epoxies). If it bends and flexes, then it works well for firewalls (hobby poxy II)

waynerr

HighPlains,

I still have a little less than half a portion of HobyPoxy II from about six, or seven years ago. I am starting to use it now and would like to find a replacement for it. It is good stuff. I am wondering, have you had a chance to try the PowerPoxy yet to see if is is, like HobbyPoxy, not as brittle like the other epoxies ? It would be good to find such a glue.

Thank You Kind Person,

waynerr

HighPlains

My Feedback: (1)

It's sitting right beside my keyboard. I'll run a test and post in a day or two. Thanks for reminding me.

waynerr

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Mike James

Another idea...

If you're in a hurry, fast epoxy may seem like a good choice, and it's typically the one that instruction manuals suggest. We all have seen how epoxy eventually becomes weaker and discolors over time. (due to exposure to fuel) A few years ago, I tried something, learned from a pattern flyer, that works quite well...

Don't use epoxy. Use a good grade clear silicone. (or the Zap "Goo" ) Yes, it takes longer to cure. But, it has great vibration damping qualities, and will last a very long time. (at least as long as epoxy) Bonus... Although it holds the firewall or other formers securely in place. it can be cut away with an X-Acto blade, should maintanence of your plane become necessary.

This is not a random suggestion, but a technique I've used on .46 to .91-size planes, that held up quite well. Give it a try sometime, and you may be hooked.

woodys3b

My Feedback: (13)

I will not use anything less than 30 min epoxy. I have had the 5 minute stuff soften up after a couple of years. Never had this happen with the longer cure stuff. I started building planes when all we used was ambroid and white glue. Waiting a little longer is no big deal.

Here are a couple of tips for using epoxy:

1. For fuel proofing areas like the firewall or fuel tank area, thin it to a watery consistancy with denatured ancohol. The kind you get in the hardware store paint section, not the kind you get at the drug store. This reduces the weight you are adding. Brush it on with an acid brush.

2. For cleaning your hands when you are all messy, use vinagar. Works great.

Andy

CafeenMan

Mike - Yours is one of those tips that would be awesome if it works, but it's also the type that I would not want to risk a new plane to. I've seen your work and trust your judgement and advice but this is a real leap of faith for me. I'd want to try it on something non-critical - maybe a landing gear mount or something.

Mike James

Paul,

I was the same way. i first saw this done on a (YS) .61-size Pattern plane with a tuned pipe, and first started experimenting with .45 to .60-size planes of mine. I also did this on planes with YS .91 4-strokes, (and 1/8" AC ply firewalls) and it worked fine.

Just obviously give the silicones their full 24-hour cure, at least, and you'll be fine.

CafeenMan

My firewalls are glued in well before the model is ready to fly so cure time wouldn't be a problem. I think I'll glue up something simple, strap it to my test stand, fire it up and stand back.