Troubleshooting Polyurethane Glue Failure

Problems with poly glue bond. Is it a problem with the glue, or the glue-up methods? April 22, 2014

We had this happen twice in the last two weeks. We glued up some white oak 4/4 with polyurethane glue and once it was unclamped it fell apart and the glue was hard and very brittle. The wood is in the 6-8% range and we did wet the edges of the boards. They were clamped for 4-5 hours. The squeeze out was hard when unclamped. The last one that did it was foamed out and you could see a hairline crack in the foamed out poly before it was unclamped. Edges are right off the straight line and tight. The glue is maybe 6 months old. Could it be bad? We have glued other stuff recently and it did not fail. We are going to throw out the glue. Hate to because it's almost 3/4 of a gallon. Says it's good for 1 year It's been open for 6 months, but seems good, not chunky or separating at all. Temp is around 60.

Forum Responses
(Adhesive Forum)
From contributor J:
Poly glues (like epoxies) don't particularly benefit from smooth, jointed surfaces or tight clamping. It's quite possible to squeeze almost all of it out of a 3/4" joint with 1" pipe clamps.

As a rule, any glue that is suspect should be tossed immediately into the dumpster (it's just not worth another fiasco). Before you do that, though, you might perform the non-clamped end-grain test (which polys are famous for). If it holds fast and furious, you will have your answer.

As far as I am concerned, the only reason to use this messy stuff in the first place is the lower temperature range and the bonding of wood and non-wood surfaces. My hands and tools all end up looking like the gorilla on the label.

And, poly glues are not any stronger than "old yeller" (no matter what it says on the label).

From Jeff Pitcher, forum technical advisor:
Contributor J, I agree with everything you said. Well put!

From contributor M:
I used poly for about a year on outdoor projects (outdoor kitchens was a niche I found during that time). This was before Tite Bond Type 3 came out. I hated every minute of that crap. Sticky, messy, impossible to clean up, foamy ugly glue lines and going home with brown fingers were all part of the experience. I used it because I thought I was giving the client some extra assurance for these outdoor kitchens that are totally exposed.

When I read the specs for Titebond Type 3, I never bought another bottle of poly again. I still occasionally use it on cement to wood or stone joints.

I would guess that you either over clamped or over wetted the edges and caused it to all foam out. I would wet the edge with a sponge and glue it a couple minutes later.

From contributor A:
Poly is weaker than most other glues. Especially when gap filling. It's obvious that foam is not strong.