Strength of aluminium and epoxy bond deteriorating

[andrew700andrew]

Hi, I'm using epoxy to increase the torque strength of a joint between two lenghts of aluminium box section (currently held by a single machine screw), where lightness is critical. I'm aware that an oxide layer forms on aluminium as soon as it is exposed and that in order for the bond to be stronger I should try and apply the epoxy ASAP after having prepared the surface. At the moment I'm preparing it by sanding the aluminium with wet and dry paper to remove the oxide layer, then quickly applying epoxy and placing the other length of box section ontop and securing it with the machine screw. The joining area is only 2.25cm^2.

The way I have done is plenty strong enough for my needs, however, I tested it only 12hrs after I applied the (15 minute) epoxy and I'm worried that the bond strength may weaken over time, due to the aluminium oxide layer reforming (read this somewhere). But I can't see why this would be the case since the epoxy will prevent any oxygen reaching the aluminium. So I ask, is this even true or will the bond remain a similar strength however long I leave it? Thanks.

 

[Burnerman]

I think we are into aircraft structure technology here, but personally I think your methods will be fine - particularly if your structure doesn't come into contact with corrosive liquids such as brine.
More specialised adhesives are available from the Loctite group, together with their preparation requirements if you need them.
Maybe a slightly more abrasive key to your components could help but without testing to destruction it's impossible to say.
John

 

[Dave54]

Just to add that I'd normally degrease with "pure" 99% iso propyl alcohol or something after keying. I generally find the "standard set" epoxy is better than the "fast" stuff.
I'd have a talk to the technical people at the supplier, or as John says talk to Loctite for knowhow about a specific application.

 

[Burnerman]

Yep, bang on about the degreasing, Dave.
An aircraft engineer pal of mine reckons that a lot of air frame corrosion comes from leaking urine
Oh, great
John

 

[Dave54]

Yep, bang on about the degreasing, Dave.
An aircraft engineer pal of mine reckons that a lot of air frame corrosion comes from leaking urine
Oh, great
John

So either the waste tanks are leaking, or the passengers are getting nervous?
Neither gives me great confidence!

 

[andrew700andrew]

Ok, thanks all for replying.

I'll give the alcohol a go, as well as using a coarser wet and dry paper.

Don't worry, it's only for a DIY Quadcopter drone frame so it's not got onboard lavatories yet!

 

[Dave54]

You going to use it as a camera or video platform? Let us know how you get on.

 

[andrew700andrew]

You going to use it as a camera or video platform? Let us know how you get on.

It's part of a team project with the aim of using it for areal surveying so we will have a camera on it, controlled remotely by a Raspberry pi computer. It should be done in March ( although I'm a tad doubtful at the moment)!

 

[andrew700andrew]

I asked the manufacturer (Evo-stik / Bostik) and they emailed me this:

“We normally advocate abrading the surface, as you suggested, with Emery paper or similar.
This must be followed by degreasing with a non oily solvent on a clean cloth or plain white kitchen paper.

One suitable solvent which is readily available is Methylated Spirit.
Do not use White Spirit, as this leaves an oily residue behind, which impairs adhesion.

For best results, ensure that the aluminium and the adhesive are at room temperature, when you assemble the bond.
Provided that the bond is not exposed to extremes of temperature, or any aggressive chemicals, we would expect the bond to be permanent. “

 

[Dave54]

Sounds an interesting project. As a photographer I'd be interested in seeing the kit and the results.
I've used meths myself in the past and it's seemed to work OK, but it's worth noting that it does leave some residue when it evaporates, apparently there is something put in there as well as the purple dye which is left. I usually use pure isopropyl alcohol. Useful for all sorts of stuff, and available off eBay. Watch out that it is pure though. 99.9%. Some of it is 70% and has distilled water added.