Epoxy Bond Failure Between MDF and Paper-Backed Veneer

      What could be causing birch veneer to delaminate from MDF when applied with epoxy? November 15, 2010

Question
I seem to be in a pickle again. A few days ago I laid up 2 sheets of 3/8" MDF with birdseye maple (face) and flat sawn maple (back), using West epoxy as my adhesive and a vac bag as my clamp. The veneer is paper backed. Today I started cutting the sheets into cab door panels and discovered that I could pull the bird veneer off the MDF, in a single piece, without much effort. The flatsawn veneer was more tenacious, but I was still able to pull much of it off. There is no lifting at the edges of the veneer nor are there any bubbles in the panel. Except for being able to pull it off, everything looks fine. I double coated the MDF with the epoxy mix and single coated the veneer paper. I used a foam roller for uniform coverage. Shop temp was around 60 and my vac was set at about 10" Hg. (5 psi +/-). I had good vacuum and an overnight cure before removing from the bag. I did this twice over a two day period and the results are the same for both sheets. I've done this many times and never had an issue, but I've never used MDF as my substrate.

I lightly sanded the MDF with 120 and blew it off prior to applying the epoxy. After pulling a piece off a scrap panel I can see what appears to be wood/paper fibers adhered to both the MDF and the veneer paper. A light scraping of either produces a small pile of wood/paper fibers. Any ideas of what went wrong here? Any thoughts would be greatly appreciated.

Forum Responses
(Adhesive Forum)
From contributor N:
Have you used the west system for stuff like this before/I use epoxy from US composites and it does not achieve full hardness for several days.



From contributor J:
Could temperature be a factor? Did you do the gluing in an unusually cold area, or turn the heat off during curing?


From contributor J:
A couple things come to mind, mostly anecdotal. I've really tried to like paperback veneer, but after having the same problem happen on two different jobs, I've given up and will never use it again. In my case, the paper delaminated from the veneer, which it seems was about four atoms thick. You mentioned you saw fibrous material stuck to both parts, so don't eliminate that possibility just yet. Second, are you certain you mixed your epoxy thoroughly? What's the remainder look like on the roller or in the pan? Hard and crunchy, or soft and chewy? Third, did you use a caul on the top? Or just bleeder mesh? Or just a bag, for that matter? Even though epoxy is a good space-filler, you've got highly absorbent material on the substrate and the veneer backer. It may have been starved.


From contributor A:
MDF is like a sponge. West System epoxy is thin. I assume you did not add any filler to your mix. The epoxy soaked into the MDF and starved the glue joint.


From contributor H:
The MDF has very little fiber to pull off. Not a lot of internal strength. I would have gone to a white glue instead of epoxy so it would get more into the MDF fiber.


From the original questioner:
Thanks for all the replies. This forum is truly awesome. First off, I have used west epoxy for this application, although not with an MDF substrate. I've probably used a couple of hundred gallons of the stuff within the last 20 years or so. I can only count two failures in that time. One about ten years ago traced to a bad pump that wasn't metering the catalyst properly, and one a few months ago that I'm still pondering (probably a lack of thickener in the mix). Add this one to the list. I've used epoxy in very cold environs and had perfectly acceptable results, but a full cure can take some time. 60 degrees is an ideal temp for working with epoxies, long open time with good curing conditions.

As far as mixing goes, yes that can be an issue. I had a friend mixing for me and he too has quite a bit of experience so I'm doubtful that that is it. All of the stir sticks, rollers and pots are covered with thoroughly cured material. Nothing even slightly soft, and they were put outside where it was cold (30's) for several nights. I too have some suspicions regarding the paperback veneer, but I've yet to have it delaminate. When using epoxy it really does eliminate bleed-through, which would be nearly impossible to remove entirely.

In my bag I used cauls on both faces of the layup and a breather fabric on top. The layup looks really good, it just doesn't stick very well. Upon really close inspection today, with good light and a magnifying lens, it looks as though the bond between veneer paper and the top few atoms of the MDF are good. There is a very thin but uniform layer of fibers stuck to the veneer paper. It would seem that a couple of mils of the fiberboard just peeled off. Is this possible? My first suspicion was, and maybe still is, a starved joint. I double coated the MDF because the first coat did seem to soak right in, so maybe a second coat still didn't do it. I used over 1/2 gallon of adhesive to lay up two sides of two sheets (128 sq/ft). Seems like it ought to be enough. Both panels, done on two different days, were put in the bag with the flat sawn veneer (the cheap stuff) on the bottom and the bird (the really expensive stuff) on top. It is only the bird (the really expensive stuff) that is coming off.



From contributor Y:
Why not buy your Birdseye maple from Certainlywoods? It comes rotary cut in a good sheet size. You then won't have a problem with the paper. If you want a backing so you don't get bleed through of glue then ask for it to be put on a veneer backing. Sometimes MDF can have a high amount of wax in it which will then affect anything trying to stick to it.


From contributor H:
I have to agree with contributor Y. I think your problem is in the MDF, not the veneer. The epoxy may not soak in enough to get good fiber pull. That is why a little fiber is seen in the glue line instead of a lot.


From contributor G:
We once had a situation when we were finishing a batch of doors with 1/4" MDF panels after the first coat was applied it seemed like a waxy layer on the finish in patches. We were told it could be the board was very fresh and the finish was reacting with the board. The finish would not set up in these patches and we had to wash the surface with thinner and then reseal. Maybe something like this is causing your problems?


From contributor F:
I'm not much of an expert in these things, but I think the general consensus is correct: The joint was starved due to the MDF soaking up the epoxy, and the vacuum pressure causing the MDF to "super sponge", essentially increasing the rate of diffusion of the epoxy through the material.

It seems that the best approach is to rough up the MDF and coat it once with epoxy to essentially saturate the first several millimeters of the MDF, and allow it to dry. Thus inhibiting a further sponging deep into the material. Since MDF is so very absorbent, you should be able to follow standard procedure with a second coat of epoxy, veneer and vacuum bag and achieve a very good result.



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