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Thread: Experience in face gluing phenolic plywood?

  1. #1
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    Experience in face gluing phenolic plywood?

    Just wondering if anybody has experience of gluing smooth faced birch phenolic ply face to face - it seems that standard woodworking adhesives like Titebond don't wet it properly and fall off. A web search brings up suggestions that polyurethanes work, but there's not much detail.

    The chemist at a local adhesives outfit is recommending their general 30min open time general purpose liquid polyurethane structural adhesive (the stuff in cartridges is filled to make a paste) - they say they have other clients successfully bonding phenolic ply in trailers and cold stores with it. The recommendation is to sand it with 80G, then vacuum and wash the surface with cellulose thinners. A light spray of finely misted water on one surface is recommended as it's a moisture curing polyurethane - then apply the glue to one surface and assemble.

    The project is a set of extension tables for my machines with top and bottom skins in 18mm phenolic birch ply, with strips of 24mm birch ply on about 175mm centres forming a honeycomb core.

    Plan A was to rout 1mm off the phenolic along the bond lines using the router table, and to then use a woodworking glue. That's still an option. Trouble is one set of panels is quite large at 970x760, and the perimeter glue lines are 100mm wide. I'm a bit wary that I may not be able to very precisely maintain the depth of cut over that sort of area, and that it would risk causing an out of flat assembly. Plus it's quite a large job, and would need lots of hold downs etc made.

    ian

  2. #2
    Make a dado at each location and glue in the cross pieces, just as you suggested. Polyurethane works, or so I'm told, but why bother with that? It will be stronger this way too.

  3. #3
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    I'd love to John as the saw has a nice big table, but don't have a dado cutter yet. It's on the buy list in a month or so...

    ian

  4. #4
    I was thinking just use a router bit. I sold my dado stack. I just use a straight edge and a router bit. I made up a "template" that is a strip of wood exactly as wide as the distance from the center of the bit to the edge of the router. I put it down where I want to rout, stick a straight edge next to it, and clamp down the straight edge. To keep the straight edge from squirming around, I have a little bit of PSA sandpaper on one side of the straight edge

  5. #5
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    That's good to know John. I even have a clamp on aluminium guide rail that would do the job, but have never tried long and wide (110mm) cuts like that before. What made the me cautious was the question of whether or not I could hold a constant dept of cut relying on the router base - especially since it would require multiple passes with the 50mm dia cutter I have. The thought was that gluing the phenolic face would (a) avoid any risk in that respect, and (b) save time.

    How do you find the depth control works out in that sort of cut?

    A shallow dado would be great to locate the parts while the glue is wet though - although a few panel pins on the inner faces of the panels would do it too.

    I have to confess that shortly after posting this morning I had myself so convinced to try it that I ordered some polyurethane adhesive. One plan might be to glue up a few test pieces tomorrow, and make a decision to go routing or not depending on how they perform...

    ian
    Last edited by ian maybury; 04-02-2012 at 10:03 AM.

  6. #6
    I have glued a lot of phenolics using epoxy.

    Scuffing it up by sanding first is a good idea.

  7. #7
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    That's interesting Carl. My initial thought was epoxy - basically because epoxy and phenolic resins are related (phenol is a precursor I think used in epoxy manufacturing), and because the two resins are often blended in epoxy phenolic coatings to deliver various blends of chemical and heat resistance, wetting ability and physical properties.

    There seem to be mixed views about how well epoxies work on phenolic plywood. I don't have enough chemistry to understand why (or even if that's true), but it seems like maybe the issue is that both epoxy phenolics and epoxies can have fairly widely varying properties depending on how they are formulated. The cure system used with epoxies apparently can make a big difference to their wetting ability, which might explain why some seem to work on phenolic, and others not so well.

    It's an interesting but pretty mind boggling field these polymers - in that there's some overlap between their chemistries. (e.g. between phenolics, formaldehydes, epoxies, acrylics and even polyurethanes) It seems almost to be possible to say that some/all of them evolved as developments of the underlying chemistry of earlier resins.

    ian

  8. #8
    Well, I dont know the composition of phenolic ' plywood', but it very well may be like the others - in that there are different types and grades and methods. I have used linen, paper, and woven fabric based phenolics. As well as G10 which is more of an epoxy base than phenolic.

    If you want the engineering, look up the masterbond epoxies. Highly designed and tested for many different applications.

    But without an engineering spec on your material combined with a validated spec on the adhesive, it's all just using some judgment and trying it. Personally, Im a big fan of the simple 5 min epoxy that you get at the craft store ( although for a large surface you should use a longer set time ).

  9. #9
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    I don't know the chemistry either, but I will tell you that you can source Phenolic Baltic Birch Ply with only one phenolic face...then you can glue two sheet back to back. Or, if making a thicker sandwich glue in one or more sheets of ordinary BB between single face Phenolic ply.

    On a related note, it took me a long time to find a local (relatively) source for Phenolic Ply. It was a builders yard in Deptford, NJ that happens to be surrounded by concrete manufacturers/contractors. Once I did locate the stuff, I was shocked to find a wide range of products...6'x8' sheets in a variety of metric thicknesses, 8'x8' sheets in 3/4" (metric equivalent), single faced, flexible core for bending. It was also much cheaper than the small chunks they sell at Woodcraft. I think WC sells 2'x4'x3/4" for $63. I bought a 6'x8'x3/4" for $98. They took it off the stack, checked for any dinged edges, got the next sheet down, cross cut it for me and helped me load it. It is heavy stuff!!! One issue was that I did not get much of a choice of colors (phenolic surface). I was hping for a very light surface so as not to absorb precious shop lumens, but had to settle for Candy apple red. Looks pretty good as an out feed table behind the black and red SS PCS.
    Last edited by Matt Kestenbaum; 04-03-2012 at 10:30 AM.

  10. #10
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    Instead of phenolic-faced plywood, you could use Formica on conventional plywood. The phenolic layer is exactly the same plastic as Formica (and all its competitors -- the generic name is high pressure laminate) except it is thinner. The advantage of the formica route is that you get to do anything you like about the plywood, and then you surface it with the formica. The back side of formica glues quite nicely with yellow carpentets glue, if you have a vacuum press to ensure good contact while the glue cures. If you don't have a press, contact cement works.

  11. #11
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    Jamie,

    Again I don't know the chemistry, but the phenolic coating on my out feed table is transparent (I can see the wood through it)...it looks like BB that has a film on it. The surface is actually glassy. Also, it doesn't chip at all around the edges. My experience with formica has not been as positive.

  12. #12
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    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
    Last edited by ian maybury; 04-03-2012 at 7:00 PM.

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