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Your Host and Tour Guide:
Paul Oman, MS, MBA - Progressive Epoxy Polymers, Inc.
Member: NACE (National Assoc. of Corrosion Engineers), SSPC (Soc. of Protective Coatings)
"Professionals helping Professionals"
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The following text is from a series of emails I had with one of our epoxy customers. Obviously he's an expert craftsman. His comments are great and his views of gluing and epoxy as a glue (compared to a coating) are insightful.
Back in the good old days when people that built things like furniture and boats and houses actually knew what
they were doing, they would pick wood according to the grain so that the expansion and contraction would all work
together as a system. If it was going to warp, it would warp upwards against gravity or backwards against the cement.
Wood making up a panel was cut so it would warp left/right instead of up/down. Certain joints were glued, others
pinned, others left floating to allow for movement. They knew the difference between a nail or a staple or a screw
or a dowel and how they held or didn't hold. Those days are gone.
Today everything is either made of particle board, plywood or butcherblock so that all those forces cancel out
and they just ASSUME that the shear strength of the glue is sufficient to hold the piece together long enough for
you to get bored with it and throw it in the garbage.
When you see a piece of furniture split at the seams, this is what has happened. The expansion due to moisture
or temperature has exceeded the shear strength of the adhesive and it splits. If the shear strength of the glue
was stronger, it would warp like a pretzel.
The other way to minimizing this is either aging and stabilizing the wood before it's final milling and shaping
(which doesn't appen any more) or using regional wood which has stabilized over time for a particular area (which
also doesn't happen anymore).
Along with matching the hardness of the glue to the material and chemical compatibility of the glue to the surface,
there is also glue joint "design." Simply optimizing the amount of surface area that the glue is grabbing
upon (because these forces are measured in psi) surface area is the easiest to adjust.
Roughing up a surface not only allows the glue to penetrate (in the case of a porous surface) but also increases
the effective surface area by several percent. The more abraded and the cleaner it is, the higher percentage of
the available area the glue can actually adhere to. Preparation is everything.
You know those joints...they break and you can see the glue was only sticking to 50 percent of the area? That happens
on a microscopic level too. If the surface isn't prepped properly or a filler isn't used to fill in the gaps, you
end up with only a percentage of the molecules grabbing the surface. It's like velcro: the harder you push it together
and knead it, the more hooks that grab, and the harder it is to pull apart. This is exactly what is happening at
the joint-glue interface.
Say you have a mortise and tenon joint or some half-assed lap joint that doesn't quite fit (like most joints).
If there is any flex to the adhesive at all the joint will cam sideways (we're only talking a few thousandths of
an inch) and you will have 100% of the compression and 100% of the tension on only two points each. This is the
point where it will fail at. It will break a little more each time until there is only half the bond, then you
end up with more tension at one point than the wood can handle and the whole thing shatters. You've seen a million
of them at antique shows, whole joints shattered with half the part sticking to the leg and the other half sticking
to the original part. These are all from faulty glue joints or the wrong glue.
We don't see it happening in day to day life because this "breakage" may only be a few wood fibers at
a time, a few molecular bonds at a time weakened. It may take a couple years to break, but it WILL break or at
least come loose, 100% guaranteed.
Converse to what one would think, a rigid glue spreads the tension out over the whole area. It essentially fills
in the space between the surfaces and makes the perfect fit. This is why epoxy -- by itself -- is a "bad"
adhesive. It IS rigid but It's meant to glue one thing to another thing, not itself, and is optimal only over a
few thousandths of an inch gap.
Any bigger gap than that and you have to use a solid filler the epoxy can bond to (wood dust or cellulose fibers)
so that what you are essentially doing is creating a "chain" of atomic level insanely strong bonds from
one surface to the other ---- surface-glue-filler-glue-filler-glue-filler-glue-surface....
Even if the surfaces suck and are somewhat dirty you end up with a perfect mechanical match between the two that
is about as stable as a nut on a bolt.
With two heavily abraded surfaces and filler, you will get a joint or bond with epoxy that is not only stronger
than the surfaces, but so strong you could run the joint over with a steam roller and the only thing left would
be the joint.
Epoxies are great glues, the best, for certain things. As far as the soaking in goes, all parts must be abraded
heavily, clean, un-coated, degreased, and the adhesive should be allowed to sit on the surface for a minute or
so before joining them. This is the primary reason why yellow and white glues fail, they are assembled too quickly
and the water all leaches out of the glue leaving
a "starved" bond.
The problem is not that they don't sink in, they are thick and don't bond to "themselves" as well as
something else. A filler must be used. I glue nothing at all without using some kind of filler. The only thing
I use straight epoxy on is porcelain or broken pottery because there the joint is only a few microns "wide."
What's nice about epoxy is that it doesn't cure by evaporation of solvents or absorption of oxygen and moisture
like Polyurethanes and silicone...thus they can be used for large flat surfaces and lap joints.
Carpenters glue and construction adhesive bond to cellulose in the wood, both mechanically and on the atomic level...but
they flex. Great for paneling or a motif glued on a door where there isn't going to be more than a few hundred
psi tension, not a small joint.
Epoxy also bonds mechanically and chemically and atomically to a lot of things, that is why it works so well.
I got in an argument years ago with someone when I explained the prepping I
used when I glue something. The surface is abraded thoroughly with virgin sandpaper or a good sharp cutting tool;
the dust blown off (not vacuumed or wiped); washed with solvent to remove grease; detergent or ammonia and water
to remove water soluble contaminants; water to remove the detergent residue; and then alcohol to remove the water.
(This isn't necessary with wood as the abrading process exposes virgin wood underneath.)
His attitude was that is was total overkill and a waste of his time. (I suppose it's a matter of ethics for me.)
All that being said...
I tend to think that the best use for a flexible adhesive is not when one is dealing with expansions and contractions
-- because the forces involved there are far beyond ANY method of mechanical connection -- but big differences
in the actual hardness of the two surfaces: Tile to wood, metal to wood, cloth to metal, leather to wood, etc.
Again, the thing that is cool about epoxy compared to every other "glue" out there is it doesn't need
air or moisture to cure. As soon as you start dealing with a bigger or sealed in surface where the oxygen can't
get in, most glues won't dry.
Even in the case of something that is subjected to extreme shock, you don't want the glue to be soft enough to
allow the joint to move: just soft enough to not crack from the shock. Maybe shore 60D as opposed to 80-90D (where
a lot of laminating epoxies fall at room temperature).
Find products in our marine catalog - www.epoxyproducts.com/6_nonskid.html
find products in our industrial catalog - www.epoxyproducts.com/b_floor.html
Knowledge is Power - We like informed consumers!
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