Hi guys, thanks for the thoughts. The phenolic one side deal would certainly have been a good solution for me (presuming that it stays flat), but I bought a couple of sheets with the smooth phenolic facing on both sides a few months ago. For sure there may be variations that influence outcomes between brands/manufacturers, even if it's only the use or not of release agents etc.
The stuff I have is a very nice quality birch ply, the phenolic coating is an opaque and smooth dark brown. The phenolic seems to be the phenolic resin impregnated paper variety.
I've just finished testing some samples with the locally sourced liquid PU adhesive mentioned above - the adhesive is fairly low viscosity (like thin syrup), a single part moisture curing type with an open time of about 1/2 hour and it's cured although not fully hardened in about 2 1/2 hours. It looks quite similar to the Franklin Titebond or Gorilla glue polyurethanes.
The results were very conclusive, but of course may or may not be generally applicable.
The test pieces (see photo below) are 18mm birch ply, some plain, and some phenolic coated. Moving left from the RHS the pairs (top and bottom) go (1) phenolic to phenolic as received (no preparation), (2) phenolic to birch ply as received (no preparation), (3) phenolic to phenolic wiped clean before bonding with a cellulose thinners soaked clean rag, (4) phenolic to phenolic sanded with 80G paper and then wiped with a cellulose thinners soaked clean rag, and (5) phenolic to birch ply sanded with 80G paper and wiped with a cellulose thinners soaked clean rag.
Pieces were misted with water from a plant spray misting bottle on one surface, and the glue applied to the other - then lightly clamped. It spreads and runs very easily, but the water mist is apparently essential to getting a decent cure speed on kiln dried wood.
Testing was done with one end of the resulting lap joint held in a vise, while the other was hit by steadily increasing force starting from a moderate tap with a standard carpenter's hammer. This as well as delivering an impact involves quite a high level of peel which tends to be the worst case situation for glue joints.
(1) and (2) (unprepared phenolic and birch ply) failed on first moderate tap, leaving parts of the adhesive film on both surfaces. Not a good bond, although with some strength - a decent push with the heel of ones hand would have broken the joint. There was no damage to the phenolic coating. Both were very similar.
(3) (solvent wiped phenolic) took quite a decent blow to break, and tore out small patches of the phenolic coating. Not easily broken by hand as above. Reasonably strong bond.
(4) and (5) (80G sanded and solvent wiped) took a very hard whack to break, and pulled out a layer over more or less the entire surface comprised of the phenolic coating and the top lamination of the ply. The ply failed through the birch rather than along a bond line. Would have taken a painful level of force applied by hand as above to break. As strong a bond as the ply is capable of, and both phenolic to phenolic and phenolic to birch were very similar.
pu test samples 3-4-03.jpg
This is a one off fairly crude test, and it's possible as above that it may not be representative - that it may not fully apply to all makes of phenolic ply and/or PU adhesive.
This test however seems to suggest that liquid polyurethane can bond the stuff very effectively face to face, but that surface preparation is important. The weakness of the bond on unprepared phenolic makes this clear.
Even just wiping the surface with a solvent like cellulose thinners delivers a very decent improvement in bond strength.
That said a pretty thorough sanding with 80G paper before the solvent wipe improves the bond strength so that in even this fairly unfavourable test the bond is stronger than birch ply. This joint should be very strong in shear.
Based on this I'm going to make up my extension tables using PU adhesive on sanded and solvent washed bond lines - it seems like it should be plenty strong.
ian