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.

Does it fill gaps?

Again, some manufacturers advertise on their bottle that their polyurethane glue "expands to fill gaps" and "expands as it dries," which suggests that the glue plugs gaps much like a two-part epoxy. Unfortunately, this is not the case.

While it’s true that polyurethane glues expand as they cure (the squeeze-out actually foams up like aerosol insulation), glue experts agree that polyurethanes cannot fill a gap larger than 1 or 2 millimeters. After a millimeter or two the foam has no strength, so it is merely a cosmetic filler.

"If you look at the squeeze-out there are a lot of air bubbles in there," says Barry Brewer, marketing manager for the DIY channel for Loctite Corp., the manufacturers of Wood Wizard. "Those bubbles are in any of your gaps, so you’re not going to get strength from those bubbles. On small gaps, polys fill a gap and make it cosmetically appealing. It’s great for the woodworker at home like me. I don’t have a planer or a jointer so it’s sometimes difficult to get the absolute perfect joint.

"But it will not fill a gap like epoxy," he says.

Zimmerman with Franklin International agrees. "We don’t make an epoxy at Franklin," he says. "But I tell people that if they are going to repair a wobbly chair, use epoxy because it fills gaps."

Can you stain it?

Every polyurethane glue manufacturer advertises that their glue accepts stain better than yellow glue. This, we thought, was going to be a great feature of the glue. But we’ve had mixed results with polyurethane glue in our shop at Popular Woodworking.

For example, we used polyurethane glue to assemble the case of the maple Chippendale Secretary featured a few years ago. As the polyurethane glue cured, it squeezed out a bit, which is completely normal. We scraped it out of the corners using chisels, cabinet scrapers and sandpaper. We thought we had removed all of the glue. But when we colored the wood with an aniline dye, the areas where the glue squeezed out wouldn’t accept stain. In all honesty, however, we’ve stained other projects built using polyurethane glue without encountering this problem.

The glue experts we talked to say that a couple things might have happened here. Because the glue squeeze-out was in corners, it might have been especially difficult for us to clean out all the squeeze-out and we missed some. Singer, the founder of Gorilla Glue, says it’s also possible that the glue sealed the pores of the maple, which is a very tight-grained wood, and prevented the stain from penetrating the wood.

The bottom line, experts say, is that polyurethane glue is much more stainable than yellow glue because it sits on top of the wood and doesn’t seal the pores of the wood the same way that yellow glue does.

"Polyurethane glue is not foolproof," says Zimmerman with Franklin International. "But it eliminates the first 90 percent or more of problems you might have with staining."

So what should you do if you have this problem? Singer recommends using a rag with some lacquer thinner on it to clean up the squeeze-out before it cures. Lacquer thinner thins polyurethane glue and allows you to wipe it up more easily.

Zimmerman says this process is sound, but adds that you should make sure you wipe up the glue using short "rolling" strokes instead of snowplowing your way across the board, which pushes glue into the pores. Another way to keep this from happening is to apply masking tape where your squeeze-out is going to occur. When the glue cures, simply pull the tape up.

Is it waterproof? Does it really stick to almost everything?

Manufacturers say polyurethane glues have excelled in these areas so well that some they are looking to challenge epoxies in the boat-building and home-repair markets. But recent federal government studies of the glue have concluded it is water-resistant, and not waterproof.

"Really boat building is a bigger market that poly glues are just tapping into," says Brewer with Loctite. "We’ve done a lot of testing on this product and can say it is completely waterproof. We don’t even have an epoxy in our line that we claim that with."

Elmer’s Products, which makes ProBond Polyurethane Glue, has started going after the home-repair market by selling its product in drug stores in 2-ounce sizes for $3.99, and at that price it has been "flying" off the shelves, according to Elmer’s officials.

"That’s a great way of getting people to try this product," says Mitch Kon, vice president for marketing at Elmer’s. "We’re positioning this product as not necessarily a stronger product but as a more universal glue. It will bond almost anything to anything."

In fact, at the Elmer’s offices in Columbus, Ohio, employees are proud to show off their ProBond sculpture, which is a towering pile of different kinds of materials -- all stuck together using their polyurethane glue.

"It’s a single-component glue that’s a replacement for epoxy -- not a replacement for yellow glue or our Probond," Kon says. "It’s great for that situation where you want to glue a piece of baseboard back into place. You don’t think you should use yellow glue, so what do you use? Polyurethane glue will do the job."

Are poly glues here to stay?

Glue manufacturers all say that the public’s interest in their products has been increasing steadily since polyurethane glues were introduced in the states. But according to Brewer at Loctite Corp., not everyone is likely to survive.

He says his company estimates the wood glue market is about a $30 million market per year. Only a small sliver of that is made up of professionals or serious hobbyists -- people who are most likely to use polyurethane glues. Most of the glue market is made up of people who need glue for occasional use in their homes.

"There is some growth potential for the product," Brewer says. "But there was a lot of dust raised in the first year these were introduced, so we’ll see. Frankly, there just isn’t room on the shelf for all these guys."

So now all you need to do is decide if polyurethane glues have a place in your heart. Here in our shop we’ve found a permanent place for the polyurethane adhesives in our glue cabinet -- right next to our large supply of good old-fashioned yellow glue. PW