Which is stronger?
 

Not speed related, at least not exclusively, which is stronger, CA, Epoxy, or Elmer's Wood Glue in tensile strength, anyone know?

RE: Which is stronger?
 

I believe it's epoxy...but I mainly wanted to chime in to say that ductility [being pliable] is real important too.

RE: Which is stronger?
 

They are all stronger than the wood.

RE: Which is stronger?
 

Definately epoxy. There's a reason that when building a kit, the instructions say use epoxy for things like firewalls, landing gear mount pads, and vertical/horizontal stabs. I've tried using carpenters glue on small planes on some of these areas, and had problems with it coming apart on me. I glued it back with epoxy, no prob. Same for a field box drawer I have. It kept come apart, I tried everything but epoxy (since it would require a lot and it's not too cheap these days) and finally used it the last time. The thing is rock solid now, and shows no signs of splitting up. But, you gotta use 30 minute, it's the strongest, and actually has time to seep into the wood forming a better bond than 5 minute.

RE: Which is stronger?
 

CA's only real benefit is that it is fast and convenient. Normal CA's are inherently brittle, and therefore the "toughness" of the joint is suspect in many cases. You have to use your smarts when deciding if CA is suitable for what is at hand. It has no solvents so the volume applied is the volume you end up with (almost), except that again it is brittle and fillets are of dubious structural value.

Wood glue can make tremendously strong wood-wood joints, but they must be tight fitting and you have no filleting ability. It does have a solvent so there is shrinkage.

Epoxies have the benefit of curing through an addition reaction, there is no shrinkage to speak of, and the structural properties of epoxy make it suitable for forming fillets. Pure unfilled epoxy systems (most of what we use on models) are inferior to a epoxy adhesives with structural fillers, but again the epoxy itself has enough in terms of physical properties to lend a fair job when used for fillets/filling gaps, being as tough as most substrates we use. The bonds have excellent shear and peel strength to most woods, and because you can form fillets you can greatly reduce the localized stresses at the joint by forming small fillets.

The tensile strength alone (not the only important property) of the cured/dried resins themselves are usually greater than the wood substrates. It is the application and the substrate materials that determine the best choice - it's all about transferring loads without creating high local stress [see CP's comment about being pliable]. But in short, for high stress joints of the kind predominantly used in models, epoxies will give the toughest end job. And they tolerate a wide variety of substrate materials so are excellent for mixed material bonds too.

MJD

RE: Which is stronger?
 

I use thick and thin CA almost exclusively to build my planes with great results. I do keep some laminating epoxy around tor fuel proofing, fiber glassing and preparing surfaces for painting but not for joints. Some preliminary test (by da rock in another forum) showed that CA, which is already very light, lost around 75% more weight as it cured. Epoxy loses almost none and not sure about other glues.

RE: Which is stronger?
 

75%? Several tests I did in the chem lab about three years showed that thin CA lost in the ballpark of 1.0%. CA homopolymerizes - reacts with itself - when triggered by substrate surface chemistry or outside influence (kickers or whatnot).

Epoxy cures by an addition reaction and loses next to nothing. Wood glues lose a lot as they shed water, and they shrink as a result. Cements like Ambroid are solvent based and lose the solvent mass and volume over time, same as plastic cements which are essentially just the base resin dissolved in an appropriate solvent.

MJD

RE: Which is stronger?
 

p.s. my first Demon motor mount was assembled entirely with CA, but fuel proofed with epoxy, and has bounced off the dirt more than once without incident after bad launches with skinny short props. CA used wisely makes for pretty darn strong structures. It's just that there are times and places it is not appropriate. CA dissolves in nitromethane.

MJD

RE: Which is stronger?
 

Have you ever used rocket powder with Ca?
Its like tiny xrystals that is used with thin ca. Excellent gap filler for instant work. A quick puff of accerator and job done!!
Can be drilled and sanded. Good for grp work too.

If nitro methane eats ca then does the nitro in the fuel destroy the ca acting as fuel proofing?

Ive found using thick ca and accelerator the skin of the thick ca hardens but leves a soft center like a confectionary candy sweet! Maybe you got to let thick slow set or I had a bad bottle.? Never used since.

Just recently being trying gorrila glue. Expands well. wouldnt use it instead of epoxy in stress areas but has a use. Water activates the ape inside.

RE: Which is stronger?
 

The test we did were with medium and thick CA. Forum Moderator da rock had the scales and weighed the glue before and after. My only personal test was a drop of thick CA that reduced in size by over 75% after it cured. My scales work in grams, not grains.

RE: Which is stronger?
 

Interesting that CA can evaporate that much. I can't remember the last time I laid a medium thick glue joint and didn't Zip Kick it, though.
Zip Kicking must at least double the brittleness?.

MK1...I used to do a lot of flying with planes powered with 40% nitro fuel and unprotected CA would eventually dissolve. Stuff like CA'd hinges would come undone.

RE: Which is stronger?
 

Yes, over time raw nitromethane will eat away at CA used at fuelproofing. It doesn't evaporate in an instant, but NM is most definitely a solvent for cured CA. It just means be aware of raw fuel soakage areas. Not a panic issue, just an awareness one.

Gorilla glue rocks for skinning wings BTW. Because it foams and fills little voids, it fills the surface voids and makes for a bondline of great integrity. Like anything of this nature, the right amount is right, too much or too little you reap what you sow. But in theory, you can skin a wing with equal integrity or better, with less weight than epoxy.

It is essentially a mixed two-part system, with a polyol and a 'blocked' isocyanate curative that is activated by the presence of moisture. The same moisture reacts with the NCO curative to generate CO2 that cuases the foaming, while sufficient NCO is left to cure with the polyol to take care of the structural bond. If you are interested in those details.. :) Similar chemistry to rocket propellant binders, with added features.

MJD

RE: Which is stronger?
 

I wouldn't try skinning an important project with GG unless there was a little bit of vaccuum.

RE: Which is stronger?
 

This is the rocket powder. Right up your street mike!

RE: Which is stronger?
 

I use the Ape for every joint I would've used epoxy for, including firewalls. No failures or disappointments.

RE: Which is stronger?
 

I'd tend to make the same comment about epoxy - with the foaming adhesive it expands and works its way into the substrates whereas epoxy is where it is unless squeezed somewhere. I did some small wings with 1/32" sheeting and a vapour thin coating of GG-equivalent and weights, and they worked out very well. They both work well, I think in theory a better and lighter job is possible with foaming PU - doing it is another matter. It is certainly beyond the strength of either substrate. And vacuum is a good idea regardless.

Hey mkI - can't these guys spell?

MJD

RE: Which is stronger?
 

I guess the 'C' got stuck some where else.

You ever used it?

RE: Which is stronger?
 

No, have not seen it in any LHS.

RE: Which is stronger?
 

could be unique to uk then?

RE: Which is stronger?
 

I saw a video about glue strength once and the result was wood glue by far. Tribond III was the strongest. I dont think epoxy did that well. This was a wood on wood test.

RE: Which is stronger?
 

In the right situation wood glues do a phenomenal job. But for poor fitting joints (truly common in model building) and applications requiring fillets and whatnot, epoxy makes up for a lot.

MJD