Glue failures

Adhesive failure can usually be traced to one of two causes: overly moist wood or defective glue. December 6, 2000

Question
Does anyone have a good estimate of the shelf life of glued panels or turning blanks? We are in the stair business and glue up a lot of tread panels in a large variety of wood species. We also glue up a lot of turning blanks for newels, again in several different species.

We are using a National Casin type 2 PVA glue. We have old glued up panels in the shop now, some over 3 years old with no sign of failure, and other panels less than 3 months old with some failures.

It would help our production flow if we could glue up a projected need for the next 3 months, but sometimes we may not have a request for a particular wood for over a year. The other problem is we’re in Florida, where it’s 90+ degrees and the humidity is about the same.

Forum Responses
Once glued, your blanks shouldn't really have a shelf life. They certainly shouldn't be coming apart after a year or two. It sounds like you might have a moisture problem. If your wood is stored outside and then you bring it into a controlled environment you will have considerable wood movement.

Another consideration is the amount of time that passes between the glue-up and the initial machining. It is important to glue as soon as possible after machining. Otherwise, oils and resins will migrate to the surface and interfere with your gluing process.

Jeff Pitcher, forum technical advisor

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I'm assuming that you're edge gluing the tread panels and face gluing the newel blanks. You didn't say which of these you were having the failures with (maybe it's both). If it's just the face lams, I wouldn't be totally surprised, because face lams have more of a tendency to open up than edge joints. In that case, the MC of your wood is probably the culprit, as Jeff Pitcher suggested in his post. Or, you may be face gluing wood of different thickness together and creating unbalanced laminations, which will add stress to the bond line.

If you're having problems with both the tread panels and the newel blanks on a batch basis, it could be either the MC of the wood or the condition of your machinery when the work was processed. Dull tooling and contaminated machine tables will cause joint failures.

Have you always used a Type II PVA glue, and do you need to use it? Many Type II PVAs skin over and set a bit quicker than non-catalyzed PVAs, and in 90 degree heat, a spread coat can lose its ability to wet a mating surface on assembly pretty fast. As an alternative, you could try an un-catalyzed high solids PVA with an extended open assembly time.

I used to have a shop in South Florida. The most important machine in the shop was the giant fan (an old attic fan in an MDF and hardware cloth enclosure on castors). Most Florida shops have big box fans like that. If you have one, turn it off during glue ups. Airflow will kill a spread in no time.