Theory of Adhesives?

[Existential Angst]

Awl --

Previously (on RW), I asked about coatings, waterproofing plywood.

Now, in an interior environment (moisture not an issue), I'm curious about laminating an approx. 4' x 6' area of plywood with some other thin-ish, or even not so thin-ish material -- from formica laminates, to .030 SS or Alum sheet, to 1/4" rubberized tile, linoleum.

I recall carpenters using gobs of white elmers glue for large-contact wood areas, in building mezzanines in lofts, etc. Then there is elmer's carpenter's/wood glue, mastics for tile (what's a mastic, inyway??), epoxies, a zillion other adhesives.

The general Q is: when to use what for what?

For example, in the .030 SS/Alum I will place over 1/2" ply, I would like to be able to just roll on some suitable adhesive over the full 4x6 area, and press them together, proly via weight plates. What would good choices be? How thick?

Interestingly, an increase in thickness from ..5 to .532 will generate about a 13% increase in stiffness, since stiffness is proportional to the square of thickness, but ONLY if there is no slippage between the two layers. Thus, the requirement for full-surface bonding, via adhesive.

Ackshooly, that 13% inc in stiffness presumes similar materials. I measured that 1/4" alum plate is as stiff or stiffer than decent 1/2" ply, so the increase in stiffness is likely to be a bit more than 13%.

Inyway, I'd like an idea of how to gauge the type of adhesive to application. Bang fer the buck is always good, as well.

Along these lines, I see all kinds of two-part epoxies, specialized for wood, metal, ceramics/glass, etc. Afaict, they all work the same on everything, with perhaps the exception of things like nylon, delrin, other "greasy" plastics. Any opinions on this?

[Ed Huntress]

Proportional to the *cube* of thickness.

Correct.

Laminating materials with different Young's modulus is not generally additive.

If you're laminating on the neutral axis, the situation is not demanding. White glue works fine for two layers of softwood, of equal thickness. Shear loads are low.

When the lamination occurs off the neutral axis you have bigger shear loads on the glue line, and they can be quite high.

Aluminum is a bugger to bond to if you need high strength. Commercially, they use PAA -- phosphoric acid anodizing -- and you actually bond to the anodizing, not the aluminum. I've described the "scratch-in" method of getting a good bond to epoxy several times here. That would work, but it's not practical for a big surface. It's good for small things.

When I have a question like that I get on the phone with 3M, Loctite, or one of the other big guys, and keep bugging them until I get to an engineer who knows what he's talking about. Ask about self-etching adhesives and adhesive primers for aluminum. I've heard of them, but I don't know of any specifics.

There are a blue million epoxy formulations. The common one that would be reasonable in your application is an ordinary room-temperature-cure (RTC), phenol-cured epoxy. In other words, boatbuilding epoxy. You probably have a West Marine Supply (not WEST System) somewhere near Yonkers. We have them in NJ. They carry common brands.

Epoxy is anti-thixotropic. In other words, it drools, unless it's modified for surfacing.

When you price it, you'll probably realize that you're better off buying overlaid plywood in the first place. If you want to paint it, MDO (medium-density-overlay) plywood is fairly cheap and very smooth. Any lumber supply that serves custom cabinet builders should have it. Don't get it wet or you'll be sorry.

There are many other overlays available, including at least two thickness of melamine (Formica, etc.). If print-through of the grain is an issue, make sure it's either a thick overlay (MDO is thick enough), that it's a hard material (like melamine), or it has a hardwood top veneer under the overlay. Now you're getting into more costly stuff.

Good luck.

[BobF]

When I worked on diesel locomotive equipment, we used to use various types of caulk to glue all the parts inside various chassis, such as transformers and filter caps in power supplies. We did this because the vibration would break everything loose - you would find large parts rolling around inside the chassis if you didn't!

We started with official GE RTV sealant but eventually just bought household silicon bath caulk at the hardware store because it was cheaper!! When you needed to replace something it was easy to cut the stuff with a knife and pull it off. We even used it on high voltage strobe lights.

We used that shit to glue everything!

[Tim]

For 030 aluminum over 1/2" plywood the aluminum is going to carry most of the tensile load from bending, which means that (a) the shear load at the aluminum to plywood joint is going to be tremendous, and (b) the assembly is going to be approximately as stiff as a 3/16" plate of aluminum (because the cube root of 9/16 cubed minus 1/2 cubed is about equal to

3/8).

At least until the glue joint, or the underlying plywood, gives way. Then it'll be a gawdaful mess.

[Ed Huntress]

Yeah, as long as it's on the tension side....

It doesn't sound like a good idea, does it? I wonder what kind of load EA is thinking about, that he's concerned about bending stiffness.

[CaveLamb]

No, it actually sounds like a very Bad Idea (tm).

Advice to OP, pick ONE material and use enough of it to handle the load.

And, unless you can match Boeing's aluminum bonding technology, don't.

[Tim]

The plywood would keep the aluminum on the compression side from buckling, for a while. Its kinda like an aluminum honeycomb between solid sheet, with the plywood instead of the honeycomb.

If "light and stiff" are what he's looking for, it'd certainly deliver. But when it let go it'd let go with a bang, and there's all sorts of wrong with trying to bond wood to metal.

[Existential Angst]

Christ, idn't ANYTHING simple, anymore??? Sheeesh.....

[dadiOH]

Mastic is a pasty material. Like linoleum paste...Liquid Nails...etc. __________________

most of the things you mentioned, contact cement. _________________

[dcaster]

Without knowing what you are using the plywood and aluminum for it is a little hard to provide good advise. But I suspect you could use contact cement that is used to laminate formica on to particle board. Be sure to clean the aluminum with sandpaper just before applying the contact cement on the aluminum.

You put contact cement on both the aluminum and the plywood and let dry until not tacky. Then cover the plywood with newspaper and then put on the aluminum sheet. Get it aligned and then pull out the newspaper and press on the aluminum sheet.

Dan

[J. Clarke]

Not if you're looking for "light and stiff".

If you want something light, stiff, and cheap and easy to make from readily available materials, use pink foam insulation and bond 1/8" plywood to each side with a structural-rated adhesive labelled for use with foam (some structural adhesives use solvents that will dissolve foam).

Home Depot and Lowes should have everything you need for small panels, you may have to go to real lumber yard to get 1/8" ply in big panels.

The ply gives you the tension and compression members, the foam carries the shear between them, and the foam takes glue well.

If you're going to need to put screws into it or bolts through it, cut out the foam and put some solid wood blocks where the screws or bolts will go.

For a neat job you can also put solid wood around the edges--pieces milled from the spruce 2x3s that Home Depot sells work well for this and are very light.

Don't trust anybody's life to it without making up a sample and testing to destruction.

[Jim Wilkins]

The temperature coefficients of expansion, for one.

jsw

[Ed Huntress]

That shouldn't be a big issue indoors. With wood, the big issue is expansion and shrinkage from changes in humidity, even indoors, between seasons. If you put an impermeable material on just one side, you're likely to have some serious warping between seasons.

[Robatoy]

Yes. Many opinions. Just make sure that when laminating dissimilar materials, to do both sides of the substrate, a 'balance' sheet. Trying to get away without one is foolish. In case of laminate, just a cheap discontinued colour will suffice. Also, look into Wilsonart 3000 adhesive. Water based, will stick anything to anything (other than greasy plastics). Lots of open time and a decent roller which will allow your body weight to do the work is all you need. Wilsonart 3000 is NOT contact cement but works a bit like it. One side of the joint has to have minimal porosity.

[Tim]

Contact cement would, I think, work well for 030 aluminum to one side of

1/4 plywood, where the wood would have a chance to flex. It may work on 1/2" plywood if the bond isn't heavily stressed. But I think you'd overcome the strength of the bond if you tried it on both sides, and then loaded things heavily.

'course, a quick look at yield strengths of Al and contact cement, plus some calculations, would tell you what's what.

[Existential Angst]

emailed them about.

So it seems!!!

[Existential Angst]

Yes. Many opinions. Just make sure that when laminating dissimilar materials, to do both sides of the substrate, a 'balance' sheet. ==========================================

A very inneresting idea, altho in this case it will likely add substantially to costs. But mebbe a workable compromise is to laminate both sides, but not with identical materials. Proly an ongoing experiment. But I do hear the demands of symmetry.... :)

Trying to get away without one is foolish. In case of laminate, just a cheap discontinued colour will suffice. ===========================================

In non-stress situations, like countertops, one-sided lamination seems OK, but mebbe even there, two-sided lamination might help longevity (warping, etc)..

Also, look into Wilsonart 3000 adhesive. Water based, will stick anything to anything (other than greasy plastics). Lots of open time and a decent roller which will allow your body weight to do the work is all you need. Wilsonart 3000 is NOT contact cement but works a bit like it. One side of the joint has to have minimal porosity. ==============================================

Will look into Wilsonart.

What does "contact cement" connote, adhesive-wise? Is it a specific genre, or are there a variety of types?

[Existential Angst]

You're saying the bond would be less stressed with thinner ply than thicker?? That seems counter-intuitive!

But I think you'd

Iow, you advise against Robatoy's advice of laminating both sides? Reason?

4 ft x 6 ft is a lot of contact area. There will be some flex, but not like bending anything into a U. I'm thinking over a 4-6 ft dimension, occasional flex of mebbe an inch or two.

[Ed Huntress]

There are a several types. They have great relative peel and cleavage strength, but they yield a bit in shear. You want that with a countertop or you'd have warping problems.

EA, you've asked a question that could go around in circles for a long time. Why don't you just tell us what it is you're trying to accomplish? That will narrow it down.

As for being complicated, back in '78 or '79 I wrote a 16-page Special Report for _American Machinist_, titled "Adhesives in Metalworking," or something like that. I took four months to research it and I travelled all over the country, visiting engineering companies and suppliers who work at the high-performance end of the business. My report had so many elements in it, it was hard to keep it organized. And it just scratched the surface of the subject.

You're right, it is a very complicated subject. If you've ever read the reports on adhesives from the USDA Forest Products Laboratory, you know they're long -- and excellent. And that's just for wood and wood composites. When you get metal and plastics involved, the complexity compounds.

That is, if you need high performance. The gummy glues, like contact cement and consumer-grade moisture-cure polyurethanes, solve a hundred different problems very neatly. But that's because most applications only require one or two types of strength -- in their case, it's mostly peel and cleavage, which are the toughest kinds of strength to get, usually, when you're bonding dissimilar materials. Contact cement is not a strong adhesive in engineering terms. But it may well be the strongest, and the best, for your application.

So tell us what the application is.

[Ed Huntress]

Tim is right.