Why Not Use Polyurethane Glue?
In a recent Fine Woodworking article, different glues for bent laminations are discussed, but polyurethane glues are not mentioned. Why is that?
Dennis McGlumphy, via Ask The Experts, None
Polyurethane glues are fine for bent laminations, and they won’t creep since they’re not susceptible to moisture like white and yellow polyvinyl acetates (PVA) glues, but I don’t use them for bent laminations for a variety of reasons.
Polyurethane glues can be a bear to work with as they stick to everything, stain fingers and hands, and require a stronger solvent than water to clean up. Also, they require that you pre-wet your workpieces to increase the wood’s moisture level in order to activate the glue, which can be time consuming during the glue up process.
Finally, polyurethane glue can have potentially disastrous results if you don’t pay attention to its expansion. As the glue cures, it produces a hard foam that can push apart a joint if everything isn’t clamped down tight.
Did I mention the mess?
Urea formaldehyde works better
My favorite adhesive for bent laminations is the resin/powder variety of urea formaldehyde, also called plastic resin glue.
I use a brand called Unibond 800. It dries super hard, which reduces spring back. It won’t creep. It has a long open time (20 minutes or more). It dries fast, anywhere from an hour to three hours depending on how you mix it. That compares to about 12 hours or more drying time with the water/powder variety. Plastic resin glues also clean up with water.
The only downsides to plastic resin glue are the odor and safety. The ingredients used to make the glue can be harmful to if inhaled. It’s important to work in a well-ventilated area and wear a respirator or protective face mask.