Degreasing aluminum

It isn't just degreasing you have to do, to get the maximum strength bond, you have to remove the oxide layer, too. And that has to be done with the surface under something, either wet with epoxy or with solvent. What you need to use to get oil and grease off depends on what's on there to start with. When doing motor home trim on the powder-coating line, the pieces used to have to go through a power washer with a proprietary detergent, motor oil was used on the jigs to bend them up. Then they had to go through an oven to remove the water. There's a lot of things that will remove oil and grease, a lot fewer that don't leave residue. Acetone and methyl-ethyl ketone will do a job on lightly soiled surfaces, you may have to use a degreaser and then a solvent wash to remove the degreaser residue, depending on what the parts are contaiminated with. And a lot depends on what sort of job you're doing. Is this a hobby project and one off, or is it for a long run of parts in production? You may have to have fume hoods, a washing station or other setups to satisfy the OSHA-type folks if it's an industrial operation. And have to pay attention to MSDS, too. You're opening yourself up to a LOT of legal liability if you don't train the workers when using any sort of chemicals.

Stan

If it is clean stock you could use alcohol or acetone. Use a dip tank and drip rack. Just don't allow that clean aluminum to set for more than a few minutes prior to bonding. Aluminum oxides form REAL fast. What I would likely do is use the epoxy as a wetting solution and something like scotch-brite to roughen the surface prior to the final bonding.

Dawn and water with a toothbrush works as well as any solvent.

When I tried to degrease various sheets of metal to pass the water test (water runs of in a sheet, does not bead up) I found that acetone by itself never did a complete job. Usually I had to do a 2-stage procedure: Acetone first, then Zep Industrial Strength Cleaner (that has 2-butoxyethanol, same as many other cleaners and degreasers including Simple Green, and sodium silicates). This would pass the test no problem.

However, the cleaner the stuff gets the quicker it oxydizes - sometimes in front of your very eyes.

There is something about doing the final wash in tap water: It seems to promote oxydation. I managed to avoid it to some extent by doing the final washes in distilled water. At least that is my personal impression. I have not found any literature reference to this phenomenon.

It's for small-scale industrial production...maybe ten assemblies per month.

I have a feeling that it might be a better idea to have the machine shop send the parts to an anodizer before we get them. They can deal with acid washing, etc. I was trying to avoid anodizing for better thermal conductivity, but the alternative (having to setup a full- blown chemical operation in a corner somewhere) just isn't going to happen. My guess is that we can wipe down the anodized parts with de- natured alcohol just to clean the surface and then proceed with epoxy bonding and we'll be just fine.

-Martin

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Acetone will leave a residue, Iso alcohol will not. They work real well in conjunction. Acetone first, wait 5 min, then alcohol. Aluminum oxidizes within seconds, I've been told.

Possibly due to substantial aeration of the water as it comes out of the tap, vs. minimal aeration of distilled water being poured from a jug.

Distillation would remove dissolved air. My shop sink faucet has a hose adapter instead of an aerator and doesn't cause problems.

CLIP

No kidding! For years, I thought acetone left no "evidence" of use... Then I tried to prep a window that was tinted on the opposite side for a decal and found this really annoying oily substance all over the window afterwards.

We use acetone to prep tapers before installation but I'm betting the combo-approach is better.

Now, I wonder why that is. Basic chemistry tells us that acetone should evaporate completely leaving no residue. Could it be that it is not an acetone residue at all but residue of such oils and fat that were there before acetone was applied, dissolved in acetone and the acetone just evaporated too quickly? The speed of acetone evaporation is noticeable, just feel the cooling of the area (or the rag) where the acetone was applied.

That was my initial thought... But we tried it on the glass after it was totally clean (Windex clean anyway) and it still left the same film whether we used a clean rag or paper towel.

Something in the acetone leaves what looks like an oily film but apparently it does not affect products mounted onto Jacobs or Morse tapers as once I load them, they are often close to impossible to unload - and I like it that way.

Acetone itself does not leave a film. However, most of the acetone on the commercial market is recycled from high-volume industrial uses, where it picks up oil. The recycling can occur at different levels of quality, and the ordinary stuff usually contains some oil.

I learned this when I worked in a fiberglass shop at Ranger Yachts. We had a "commercially pure" grade we used for work where there was going to be some bonding or painting after the cleaning. I don't know how the grades are differentiated today but you can be sure, or you could be sure back when I worked at Ranger, that any acetone you buy in a hardware store or from a general supplier contains some oil.

-- Ed Huntress

Generally, I buy it a gallon at a time from Home Depot when I go in.

When I'm in a rush and want to pay through the nose for shipping, I use McMaster.

Thanks for the background. Very useful.

It looks like you got lots of good advice here, but the point that aluminum forms a weak oxide layer almost immediately is an overriding one for some applications. It sounds like your application only loads the epoxy in shear, which is a lot easier than any job that loads it in peel or cleavage.

You won't get a strong bond to aluminum just by cleaning it; but it may be strong enough. The mechanical bond you get from ordinary roughness of the material actually is pretty good when the joint is loaded only in shear. High-performance applications use PAA (phosphoric acid anodizing) to get a strong mechanical bond; PAA gets a tight grip on the parent metal and it has lots of surface roughness for mechanical bonding. Someone else mentioned the scratch-in method, were you sand the aluminum through a wet layer of epoxy. This can work very well -- roughly the same as PAA -- and the bond, they tell me, is chemical as well as mechanical. I have some experience with this, using it to bond yacht hardware, and it's been very reliable for me, even in cleavage joints. But that doesn't sound practical with slots.

As for cleaning, see my note to Joe regarding the impurity of ordinary commercial acetone. A pure, oil-free detergent may work as well for your job and it will solve a lot of safety problems -- but that's only a guess. Be aware that detergents usually are alkaline and they will attack aluminum. The result is that you'll have an especially fast development of the oxide layer, but it doesn't have to degrade your joint, since you'll probably have only a mechanical bond to begin with -- one that works by cogging the epoxy against the roughness of the underlying aluminum.

The chemistry of detergents is complex these days, and I wouldn't try to sort them out on my own. I would call a big epoxy supplier and ask them about detergents that they've had success with. The perfumes, emollients, bleaches, and other crap in retail brands of detergents may bite you in the back. The reason I think they should know is that they use detergents to remove the amine blush on epoxy layups, and they have to be able to get a good bond after washing, when putting on the next layer of epoxy/cloth or whatever. The guys at WEST System have always been very helpful and accommodating to me.

Good luck.

-- Ed Huntress

Thanks, this and other posts have been very helpful. I'll research PAA. Is this a process that most anodizing shops can apply or is it specialized?

Per my prior post, I am now thinking that I might go for an anodized finish rather than raw aluminum. This would have the anodizing shop deal with post-machining/stamping cleanup. We could then simply wipe the parts clean with alcohol and proceed to bonding. My guess is that anodizing won't be seriously harmful to thermal conductivity. This is a heatsink application for electronics.

-M

I've never had it done, but you'll find plenty of shops that offer it. My understanding is that it's quite common.

If you know who Ted Mooney is (an expert old timer I've come to respect; I used to write the Finishing column for American Machinist, and I wrote a lot about adhesive bonding to metal, but it was decades ago), you'll want to see his brief comments about it:

The comments in that thread go a little overboard for your application, but they will give you some perspective. None of this stuff is new, BTW. I researched and wrote about PAA roughly 30 years ago.

I don't know about the insulating properties of standard thicknesses of PAA. Anodized layers are basically aluminum oxide, which, at that crystalline level, is considered to be a ceramic. But my vague recollection is that its thermal conductivity is high relative to other ceramics.

I would be wary of it, if you need really good thermal conductivity. But the layer of epoxy itself may be much more insulating, making the thermal insulation of the anodized layer moot. It's not something I've studied.

PAA is very effective and reliable but it may be more than you need. Not knowing the strength requirement or the specific loading on the joint, I can't give you any specific advice about it. All I can tell you is that it's the standard in industry for bonding to aluminum, especially in aircraft.

Note the comments about applying a primer within two hours. That's something to watch for, but my recollection is that it's only relevant to very high-performance applications. It may have something to do with the hygroscopic property of anodizing. The chemistry of this is over my head, but anodizing of many types, maybe all types, chemically absorbs and binds water. It's hard to imagine that your bonding job is going to be very sensitive to this, because the cogging bond is probably all you need.

-- Ed Huntress

"Ed Huntress" fired this volley in news:48a43d44$0$20907$ snipped-for-privacy@cv.net:

A freshly anodized surface forms the essence of a molecular sieve. There are tiny, wormhole-like tunnels all through the surface of the oxide layer. Those that are sized appropriately will entrap water molecules.

It's not exactly hygroscopic in the sense of attracting moisture by adding to water of crystalization or by dissolution (like salt or sugar in humid air), but any moisture that arrives is captured mechanically, so it amounts to the same thing at a practical level.

LLoyd

Aha. Thanks, Lloyd.

-- Ed Huntress

So in theory using a "clean" acetone should avoid this problem. By that I mean stuff like nail polish remover or medical plaster remover. Having said that I wonder how clean they really are.