Theory of Adhesives?

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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?
--
EA



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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.
--
Ed Huntress


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On Mon, 11 Jul 2011 23:23:50 -0400, Ed Huntress wrote:

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.
--
Tim Wescott
Control system and signal processing consulting
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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.
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Ed Huntress

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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.
--

Richard Lamb
http://www.home.earthlink.net/~cavelamb
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On Tue, 12 Jul 2011 01:06:07 -0400, Ed Huntress wrote:

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.
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Tim Wescott
Control system and signal processing consulting
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Christ, idn't ANYTHING simple, anymore??? Sheeesh.....
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EA




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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
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says...

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.
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The temperature coefficients of expansion, for one.
jsw
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wrote:

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.
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Ed Huntress



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Dayum....... :)

It appears ahm gonna need it!! Unless some of the simple solutions turn out. Also, effing .030 alum is not cheap!! 1/8" would cost on the order of $150, while 1/32" costs $75!!!!! Talk about non-linear pricing -- holy shit.....
.030 SS is of course worse: $150 for the piece, altho I didn't compare this to 1/8". I suspect about as non-linear. Heh, and the 1/2" ply costs.... $20???? LOL
Appreciate the overview.
Oh, I asked elsewhere, but what does "contact cement" specifically connote? Is it a genre, like "two-part epoxy" or white glues? A few people have recommended it. Just wondering if I should be looking at a specific type.
--
EA


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Rubber-based. Usually Neoprene family. Solvent-type can be a wide variety of Neoprene-type rubbers. Water-based typically is polychloroproprene. Don't ask me what the hell that is, except that it's related to Neoprene.

It refers to glues that you usually apply to both surfaces, wait a bit, and then press them together. You'll need a slip sheet of slick drafting paper or something similar. Brown kraft paper will work. If you haven't done it, look it up on YouTube and save yourself some grief.
The adhesive is somewhat air-impeded, like laminating polyester resin, so the surface of each side is ready to stick when most of the solvent evaporates.
But it's rubber, in the end.

Solvent-based usually is stronger, but don't count on that anymore. 3M supplies some data on their formulations.
If you're counting on that bonded stiffness, and the panel stiffness that results from making the structure thicker, that you were talking about earlier, you won't get a lot of it with contact cement. But I think you're chasing your tail on that issue, anyway. Again, it's a question of what your application is.
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Ed Huntress


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wrote:

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!
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Existential Angst wrote:

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

http://www.thistothat.com /
For most of the things you mentioned, contact cement. _________________
--

dadiOH
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On Tue, 12 Jul 2011 07:13:44 -0400, dadiOH wrote:

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.
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Tim Wescott
Control system and signal processing consulting
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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.
--
EA





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Tim is right.

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Because there's less distance to the neutral axis? If I'm right, do I get a prize??
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No, because the force is a product of the volume of the material that's trying to stretch or shrink, and thicker material exerts more force.

When you're right, we'll consider it. d8-)
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Ed Huntress


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