Carbon ribbon (tow) and polyurethane glue
 

Couldn't find any info on this searching, so: question for all of you composite gurus...In order to stiffen up a profile I purchased some carbon ribbon to add between the frame and sheeting of the fuse. Due to health concerns I'd rather shy away from CA and epoxy (especially CA) so I was wondering about people's thoughts on using a polyurethane glue (Probond/gorilla glue)....I made a little test piece which is ok, but doesn't seem as strong as an epoxy joint (although i played with it after ~4.5 hours which may not have been long enough).

thoughts or comments? I can use epoxy if that's the concensus (my guess), but think it would be a pain and add a lot of weight in addition to my trying to stay away from it physically...

RE: Carbon ribbon (tow) and polyurethane glue
 

I once worked in a lab testing strengths of various glues (mostly CA and epoxy). This is how I would test a PU and CF bond.

Glue a 1/2 inch wide ribbon to a one inch wide by 1/8 by at least 12 inch long balsa. Let it cure completely.

Now here is the hard part. You will need at build a motorized test stand. You can find small, inexpensive gear motors at some surplus hardware stores, or from McMaster Carr. I will let you, or someone else, figure out how to build yours. I will leave you with a schematic of how it works.

The crank flexes the wood about a quarter of an inch. Let the motor run and count the number of cycles until the bond fails. Do this with a test piece using CF/epoxy, and compare the results.

I am actually planing to build one of these fatigue testers for my self, just don't know when I will get the time.

RE: Carbon ribbon (tow) and polyurethane glue
 

I tried a test piece of carbon tow on the side of a wing spar using Probond. The result was poor. The carbon did not harden and go stiff as it does with epoxy. It had nowhere near the same strength. Use epoxy. Epoxy will not add a lot of weight if applied properly. Apply the epoxy to the ribbon and squeegee it in place, removing any excess resin.

Ed S

RE: Carbon ribbon (tow) and polyurethane glue
 

Ed,

I tore apart my joint and I have to agree that the the mix really doesn't seem to work....so I do it with epoxy and see what I can do to keep it light.

RE: Carbon ribbon (tow) and polyurethane glue
 

Ed,

I have used the ProBond Poly Glues on the CF on sheeted Q500 wings. I have never had a problem with one, the epoxy may be stronger, but the Poly has worked for me.

RE: Carbon ribbon (tow) and polyurethane glue
 

I think what Ed ment, and my little experiment showed me, was that yes, it would work to use a poly glue like I wanted to try, but that the strength of the CF would be greatly reduced from an epoxy bond....

RE: Carbon ribbon (tow) and polyurethane glue
 

Exactly as Z3 says.

An epoxy/carbon composite structure will have strength and stiffness. Sure the poly glue will stick but my sample had very little strength and virtually no stiffness.

Ed S

RE: Carbon ribbon (tow) and polyurethane glue
 

I have to second Dave's thoughs there. I use c.f. tow spars on my pattern wings and use urethane glue with great results. I make sure the c.f is saturated by running an extra bead of glue where they come in contact with the foam and then set the c.f. in the glue on the foam. Then proceed with sheeting in the usual way.

Wayne G.