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After the country’s major glue manufacturers introduced their polyurethane glues at the Chicago Hardware Show four years ago, we came back to our shop here at Popular Woodworking excited about trying this new “wonder” product. The polyurethane promise was enticing. We were told that it’s stronger than yellow glue, it’s waterproof, it fills gaps, it’s stainable and it will bond almost any two materials together. The downsides were that it is more expensive, can be messy and it has a shelf life of about a year after the bottle is opened.

After 18 months of use in our shop we were pretty impressed with some of the properties of polyurethane glue. But we weren’t sure that the adhesive was living up to all of the hype we read in the catalogs, advertisements and heard from friends. So we talked to the glue experts about their products to see if they could help us separate the science from the stuff you might hear from your woodworking buddies.

First, a little background. It would be a mistake to call polyurethane glues “new.” Polyurethanes have been popular in Europe for decades and were first imported to the United States and Canada about six years ago by the Gorilla Group (which sells Gorilla Glue) and AmBel Corp. (which sells Excel). Pretty soon the big glue manufacturers took notice and introduced their own lines of polyurethane glue. So now we have six polyurethane adhesives to choose from — though glue-industry insiders suggest that the polyurethane market is crowded and that number might soon drop. But that’s another story. Here, in a nutshell, is what you need to know about polyurethane glue.

Is it stronger?

Some polyurethane glues advertise themselves as “super strong” and “the toughest glue on planet Earth.” Other polyurethane glues don’t make any claims about glue strength on the bottle. What gives? Frankly, glue manufacturers seem divided on this issue.

Dale Zimmerman, a technical specialist with Franklin International, which manufactures Titebond Polyurethane Glue, says that his company’s tests don’t show that polyurethane glue is any stronger than yellow glue. Here’s how they tested the adhesives: They glued together 1″-square blocks of hard maple using a long grain-to-long grain joint — the strongest type of glue joint for wood. Then they attempted to break the blocks apart by testing the joint’s shear strength, which basically means they tried to break the bond by pushing one block up while pushing the other block down.

And what did they find? Zimmerman says that the joints made by the polyurethane glue failed around 3,510 pounds per square inch (psi) of pressure. Franklin’s Titebond, a popular yellow glue, failed at 3,600 psi. And Titebond II, their exterior-grade outdoor glue, failed at 3,750 psi.

“Polyurethane glues stick well and hold odd materials, but they generally aren’t stronger than yellow glues,” Zimmerman says. “Yellow glue makes a bond that is stronger than the wood. So while the polyurethane glue might actually be stronger, it doesn’t matter because the wood will always fail first. It’s not a stronger joint.”

Other glue manufacturers disagree. Mark Singer, the founder of Gorilla Glue, says that polyurethane glue is actually stronger than yellow glue when you use it in types of joints other than the one that Franklin International tested. Franklin, he says, used a long grain-to-long grain joint, and that’s a joint where traditional yellow glues already excel. Singer says that the real strength of polyurethane glues is in an end grain-to-end grain joint or an end grain-to-long grain joint, which are two joints where yellow glues have always been lacking.

“Shear strength is not polyurethane glue’s strongest feature,” he says. “In end grain-to-end grain the stuff is incredible. It far outperforms (yellow glues) in end grain. If you coat both surfaces with polyurethane glue, I’ve seen it (the glue) migrate 2″ into wood.”

Singer says his company hasn’t actually tested this joint scientifically, but he says he’s seen tests from Europe and in the United States that confirm this statement.

Zimmerman at Franklin says that his company hasn’t tested polyurethanes in this manner either, but it would make sense that polyurethane glue would be stronger in a joint with end grain.

“End grain is like a bunch of soda straws,” Zimmerman says. “So they suck up yellow glues, which contract when they dry, and this makes a poor joint.” That’s not the case with polyurethane glues, which expand as they cure, preventing the end-grain joint from becoming glue-starved and weaker.

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