by Professor Gene Wengert
Q.
I've recently encountered some failed glue joints, and the glue in the failed areas appears sandy. We apply enough glue so there is "squeeze-out' and the moisture content of the wood averages 9%....what's causing the problem?
A.
A grainy or sandy appearance of the glue in a failed glue joint is the result of either a bad batch of glue (too old or frozen perhaps) or the result of very dry wood soaking the water out of the glue before the chemical reactions can occur. Your question includes the MC of 9% (a little too high for many products, but okay for gluing) and the fact that you have squeeze out (a good thing, normally). My guess is that the culprit is old glue. (Note: I assume that you are not using an excessive amount of glue, in which case there is so much glue that the pressure cannot squeeze it out thoroughly and fast enough before it begins to set up. In this case, you would have a very thick glue line.)
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison.
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