Hi Mike

 

Are the angle brackets you talk about aluminum? There is definitely a problem bonding aluminum if it isn't prepped right.  Aluminum oxidizes rapidly and while you might get a great bond to the oxide, the oxide doesn't have a good bond to the aluminum.  Adhesive is also critical, and Hysol is one of the few appropriate adhesives. And as you said, the prepregs are difficult to bond to and easy to do incorrectly.  I also use a technique for nonstructural bonds in aluminum that works pretty well by first sanding the aluminum prior to bonding, then mix the epoxy and wetsand the aluminum again with wet sandpaper so that only clean aluminum is exposed.  Then I wipe it down to get rid of the excess and contaminated adhesive and bond it in with some more epoxy.  It works well but the bonds are not reliable enough to use for critical structures.  When it really counts, use bonding epoxy and CAA or PAA (chromic acid anodized or phosphoric acid anodized) treated aluminum, cleaning with de-ionized water and technical grade acetone. 

Speaking of which, it costs more but I like to use technical grade acetone for the critical bonds.  A wipe with the Jasco from the hardware store is OK I think for getting the gross amounts off but I'd rather do the last wipe with a higher grade of purity than the constituted 'tone.  If you keep it as your last wipe, a gallon would probably do the whole plane.  You can get it from chemical supply stores.  And as a former scientist, one thing you learn is that you never put the rag against the bottle and tip the bottle since it washes contaminates from the rag back into the bottle.  I would disagree with the idea of exciting the molecules by sanding though.  What you want is a good clean unreacted surface for your bonds, not one that has been left open and linked to other miscellaneous bits of whatever.  Many people in the industry say that a surface that has just had the peel ply removed is perfect for bonding, then others have said that they prefer a light sanding even if it was just released.  What I suspect is that it's fine after the peel ply is removed but it is my old training earlier that makes me want to sand it anyway.  I'm sure someone has the data comparing it but I haven't come across it.

 

The next posting I do is going to have the data I collected on Jeffco vs Hysol and WEST epoxy.

 

Dan