Polyurethanes, polymeric compounds that are close cousins of epoxies, were first discovered in 1937 by the German chemist Dr. Otto Bayer. They are commonly encountered as foams, for example, as insulating foam for mattresses and molded foam used in surfboards, but they have also been used as adhesives throughout much of the world since the 1950s.
Polyurethane glue entered the American market until 1994 when the Gorilla Glue company began selling polyurethane glues. (For many Americans, Gorilla Glue is the more familiar name for polyurethane than its chemical one.) Over the last two decades, it has become one of the most common go-to construction adhesives that contractors depend upon for all sorts of jobs. Popular brands include Titebond and Loctite.
Similar to cyanoacrylates, polyurethane adhesives require a chemical reaction in order to cure, and that reaction is set off by moisture.
“If you’re gluing concrete to wood, no problem,” explains builder and instructor Jordan Smith. “Concrete has moisture; wood’s got moisture.” When one of the substances is non-porous, however, some extra moisture is required. “Make sure that you moisten one side of your joint using just a little spritz bottle,” Smith says, “then apply glue to the other side and stick them together tight.” If you are using polyurethane across large surfaces, like drywall panels, you can spread the foam into place using trowels and other hand tools.
While the application of polyurethane as an adhesive is not especially complicated (though it can be a little messier than some other glues), first you need to know its advantages and disadvantages when compared to wood glue, epoxies, and other options.
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At a minimum you will want to assure that the surface is free of dirt and debris, but oils can also interfere with polyurethane’s bonding process. Cleaning the surface with soap and water can eliminate oils.
A rougher surface can help polyurethane, as well as other glues, achieve a tighter bond and more holding power. Sand blasting metals or using sandpaper on other surfaces will help achieve a tighter bond.
Consider the surface you are working with and whether a polyurethane with a higher or lower viscosity will make the project easier. As a rule, glues with higher viscosities work better with vertical surfaces, when the glue running may be an issue.
The foam produced by some polyurethane adhesives when they are curing can push the objects being glued apart. Properly clamping the two surfaces together can minimize this risk.
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