AV - Urethane Glue

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(The following is NOT correct with regard to the 'manufacturer's instructions,' at least with regard to 'Gorilla Glue.' See the footnote as to why.)


When applied according to the manufacturer's instructions, all modern-day adhesives, including one-part urethanes, provide a bond that is stronger than the softwoods typically used for airplane construction.

Homebuilders often compare urethane unfavorably to epoxies but on examination most of those unfavorable comparisons are due to the builder's failure to follow the manufacturers recommendations with regard to clamping pressure.

Urethane glues expand as they cure. The expansion of urethane adhesives is by design, providing the mechanism by which the glue is forced into the micro-structure of the wood. The extent of that infusion -- and the ultimate strength of the bond -- depends upon a significant amount of clamping pressure to ensure the expansion forces the adhesive into the wood rather than simply pushing the joint apart.

To ensure the best bond (ie, typically >80% wood-shear failure in the standard FPL test [spec is 75%]) I've found urethanes need about the same amount of clamping pressure as adhesives containing water, such as Plastic Resin or resorcinol, (Ed.Note: ie, 70 to 100 psi) for about the same amount of time (ie, typically 24 hours). If urethane is applied in the same manner as epoxies, most of which do not require significant clamping pressure, you'll end up comparing apples to oranges.

Although urethanes have been used in Europe since the 1970's they are relatively new in the United States. For an American source of quantified data on the strength of urethane glues, see:

http://www.fpl.fs.fed.us/documnts/pdf1998/vick98b.pdf

For homebuilders, especially those who have developed a sensitivity to epoxies, urethane glue is Good Stuff but some brands do better than others (as shown by the FPL tests, even though all exceeded miniminum shearing requirements). If you haven't tried urethane glues you can learn a lot from a few experiments.

On the practical side, once you've opened a container of urethane glue it is going to set-up, sooner or later, due to the air introduced into the container. (On this particular planet all air contains some amount of moisture, and since the water acts only as a catalyst, given enough time the moisture in a teaspoon of air will harden a whole pint of glue.)

The manufacturers tell you to squeeze all the air out of the container before capping, which is okay after the job is done but impractical during the course of a big lay-up (I've used it on built-up spars). The container is an excellent air pump, swallowing a fresh gulp of air each time you squeeze out a little glue.

Bottom line is that using urethane adhesive calls for some subtle changes in your gluing tactics and strategy, such as buying mostly small containers, reserving larger ones for bigger jobs. Also, store any container which has been opened upside down. You can't prevent some air from getting inside but when stored upside down the hardening will take place on the bottom of the glue, allowing you to get the maximum usable amount from any container. Another tactic is to store your upside-down container (simply stand it in a tin can) in a refrigerator between use.

As with any adhesive, be sure to spread it properly. Urethane needs only a very thin layer (on both surfaces). Allowing a few minutes of open-time before mating the parts ensures enough moisture for the cure.

I still use resorcinol for props, Plastic Resin when cost (or 'Certified Repair') is a factor and epoxies when I can't provide adequate clamping pressure, but I find myself using urethanes for about half my gluing chores.

-R.S.Hoover

PS -- Urethane is a near-perfect adhesive, more than willing to bond to you, your clothes and anything you happen to touch. Lacquer thinner will remove it if it hasn't cured. Accidental drops of glue onto concrete, metal or wood can be chipped or sanded away but if it gets onto fabric, it's there to stay... which lead to some interesting experiments using urethane glue and urethane varnish(!) on fiberglas... or even an old bed sheet. Very handy for making a quick fairing.

PPS -- Urethane glue and a pneumatic brad-driver has become my favorite method of assembling mock-ups, jigs and fixtures.

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Footnote:

November 2006

After posting the above a fellow builder who's opinion I respect wrote to tell me that with 'Gorilla Glue' he'd followed the manufacturer's instructions religiously and the results were unacceptable even for household repair work. I've since completed a series of tests using 'Gorilla Glue' (brand name), a product of Denmark available from a variety of retailers including Harbor Freight.

I did 122 tests, all tolled. Test coupons were 1-1/2" wide to 3/4" wide. Woods used were Sitka spruce, Western Hemlock and Douglas Fir. Moisture content of the wood varied from 10% to 15%, as measured by a contact meter.

The instructions on the bottle of Gorilla Glue (and on their web site: www.gorillaglue.com ) say to apply glue to only one surface, as opposed to the Forest Products Laboratory who applied it to both. Gorilla Glue says the work need be clamped for only three to four hours to achieve 90% strength, and that the open-time could be up to twenty minutes.

I used two clamping pressures; approximately 45 psi and approximately 75 psi. 'Approximately' because I used spring clamps and did not calibrate them individually.

The tests were done over a period of about two weeks, determined by spare time and the temperature of the area where I did the work, which was when the temp was between 75 and 80 degrees Fahrenheit. Relative humidity varied between 35% and 80%.

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Results:

All of the test samples in which glue was applied to only a single surface failed to meet the minimum shearing test, typically showing <10% wood-shear. Those clamped with more pressure did slightly better but the difference was not significant.

All of the test samples in which glue was applied to both surfaces passed the FPL shear-test. Indeed, the shear line in a couple of 1.5" DF coupons clamped with 75psi was well away from the glue line.

When clamped for only 3-1/2 hours (ie, splitting the difference in the recommended "3 to 4" hours, none of the coupons passed the shear test. Samples in which glue was applied to both surfaces and subjected to heavy clamping did best but even then the wood-shear was never more than about 25% (the accepted minimum for airplane joints is 75%).

The Open Working Time of 'up to 20 minutes' appears highly optimistic , wildly so with regard to how much glue is needed to make a good joint ("Min. Coverage 1/2oz. per square foot")

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The tests were done for my own education. Although I tried to do things in an orderly fashion, keeping notes and taking a few photos, the tests should not be taken as definitive. Except by me :-)

My general impression is that when applied according to the FPL tests (ie, applied to both surfaces then clamped for 24 hours) Gorilla Glue is no worse than other urethane glues when used to join softwoods common to aircraft construction. That includes Excel One, PL and Elmer's Ultimate Glue. But neither is it any better. In my opinion, the instructions that come with the glue do not reflect reality and should be ignored in favor of the method described by the Forest Products Laboratory.

-R.S.Hoover