Thread: mechanical strength of epoxy/ply epoxy 'lamination'?

Most epoxy formulators have tested their products, some with different fillers and on different substrates. The problem with your initial questions are they cover a huge range of possibilities and goo formulators can't do all the possible variations.

Secondly, your question of strength is an engineer's nightmare. What kind of strength are you speaking? Elasticity?, Tensile?, Compression?, Elongation?, Hardness?, Point of yield?, Sheer and peel?, The list is extensive so you might want to narrow it down a bit.

Most marine grade laminating epoxy will have a tensile strength of 11,000 to 12,000 PSI. Making a peanut butter mixture with silica and wood flour can reduce this to 7,000 to 9,000 PSI. This is mostly because the novice user puts a lot of bubbles into the mix, but also the nature of the filler materials used. For example a mix of mostly phenolic spheres will reduce tensile strength to about 1,500 PSI if in a peanut butter consistency.

In fact, it's best to use just the amount of filler you need for the task. Most applications don't need a non-sagging mixture (this is what peanut butter is), so the resin/filler ratio is much higher, which improves the strength of the mixture.

The same can be said of hardness. Straight cured resin will be in the 80 to 85 bracket (Shore D). Lets call it 83. A peanut butter mixture of milled fibers reduces this by 10%, a mixture of wood flour/silica by 20%, and micro balloons as much as 50%.

Everything else is affected too, elongation, modulus of rupture, etc.

When applying goo over a substrate the same rules apply. You'll be mostly interested in peel and sheer qualities, but other physical properties will be helpful too.

I'm not sure what you're trying to do with the last question, but if you just paint a surface with epoxy, you've done nothing more then paint it. There's no stabilization of the wood beneath it. "Letting it breath" is the worst thing you can do. For epoxy to work, any part receiving it must be completely coated, especially end grain with enough epoxy (three coats usually) so no moisture can commute through the coating. Only this, nothing less, will stabilize wood. Moisture content is meaningless in this regard, but not unimportant. It's most desirable for the moisture content to be below 15%, preferably below 12%, before epoxy is applied. The reason for this should be obvious, moisture levels over this amount will cause internal stresses to occur with temperature changes, which will threaten the coating (at least).


So, to answer this question, yes, the "face laminate" will peel from the substrate, because the wood isn't stable and moisture content cycling, will eventually cause the fibers at the epoxy penetration level to fail, probably in rolling sheer.

Thanks PAR,
I already understand the tech data, and I have seen that all the technical data is engineering 'vague' outside of a narrow range of uses.
Hence my questions.

I'll break the second question down a little further:

I assume that boat repairs are done when the timbers have not reached a true hygroscopic stabilisation, due to the environment that the boat has come from, as well as the environment that the boat yard is in (e.g. wet).

in that case, what is the consequence of applying epoxy over timber that has not reached stabilisation, from the perspective of:
a) encapsulating moisture within timber, but not being able to achieve a true seal over all the timber surfaces, and
b) encapsulating wet (+/- 22%mc) stable timber, but allowing a moisture egress point with a view to drying a large 'glue laminated' beam?

Encapsulation without a true seal isn't encapsulation and you're just using it as really expensive paint.

Again, moisture content over optimum levels, will cause internal stresses within the piece from moisture and wood movement. These will shear the coating at the very least and likely promote areas within the piece for rot. Coating the outside of a piece and leaving the end grain exposed (for example) will surly prompt rot, because the moisture has a limited avenue of escape. With moisture moving in and out of this piece, even if it's routes are limited, the piece will change dimension.