Urethane Wood Glue
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Water Resistance
The first biggest advantage of urethane glue is that many of them are waterproof. Urethane glues are the best choice for wood projects with an outdoor application like patio furniture or outdoor project. If you want a glue that is totally waterproof, Make sure the label says Type I adhesive. Type II glues are also called waterproof but the test not as difficult. Both of these tests are conducted using 6 inch by 6 inch birch laminates glued together to make three-ply plywood. The test for Type I is clearly more stringent than Type II, and involves boiling the glue bonds and testing the specimens while they are wet. Type I testing involves cutting the 6 inch by 6 inch assemblies into 1 inch by 3 inch specimens, boiling them for 4 hours, then baking the specimens in a 145°F oven for 20 hours. They are boiled for an additional 4 hours, then immediately cooled using running water. The specimens are sheared while wet, and the bonds must pass certain strength and wood failure requirements to pass the Type I specification. Type II testing involves cutting the 6 inch by 6 inch assemblies into 2 inch by 5 inch specimens, soaking them for 4 hours, then baking the specimens in a 120°F oven for 19 hours. This is repeated for a total of three cycles, and the bonds must not delaminate to pass the Type II specification.
Curing Process
The curing of urethane wood glues is very different than yellow wood glue. Unlike yellow glue that dries in air, urethane uses moisture to support a chemical reaction that changes the Adhesive from liquid to solid. The glue can uses moisture in the air or wood. Many users dampen their wood before using urethanes to ensure that there is enough moisture to get a full cure. This is especially critical if you live a dry climate.
Urethane takes about 10 - 30 minute to cure where yellow glue can dry in 1 ? 3 minutes.
Expansion
The polyurethane glues expand--up to 4:1 and that helps fill crack and crevices. It is excellent for a closed joint like a dove tails, but not on an open joint like your miter joint. The expansion can push a apart a miter joint where the expansion helps fill dove-tail joint. The expansion also creates over flow that must be removed by wiping, sanding other removal processes. Urethane glue is less gummy when dry so does not clog up a belt sander like yellow wood does.
Cost and Shelf life
Glues like Sideband and Gorilla Glue cost about 70% more than yellow or white glues. Urethane glues also have a more limited shelf life. Sideband has a one-year shelf life per the manufacturers datasheet. One the glue is open, moisture can get inside the container and slowly cure the glue in the bottle.
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