Q: We are using composite cores now and would like to measure the moisture using an oven test. Does the amount of adhesive affect this test?
A. You do have a good question. Composite products often have about 3 percent of the oven dry weight being the adhesive used. (Actually, more adhesive is used by weight initially, but the water or other vehicle evaporates, leaving the dry adhesive.) The definition of moisture content (MC) in solid wood is the amount of water compared to the oven-dry weight of wood. But, when we do the oven-dry test, the final oven-dry weight has 3 percent more weight due to the 3 percent added by the adhesive. This sounds like an issue, but let’s look at an example.
Consider a small piece of composite material that has 7.0 grams of water, and that weighs 103.0 grams when over-dry. This oven dry weight is 100.0 grams of dry wood and 3.0 grams of adhesive. If we calculate the moisture content using the dry weight of only the wood (7.0 / 100.0) we calculate 7.0 percent MC. If, however, we use the weight of wood and adhesive, then (7.0 /103.0) we calculate 6.8 percent MC. This small difference between the two is not that important in production, so from a practical viewpoint, we overlook or ignore the weight of the adhesive.
Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison.
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