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Woodworker's Gazette
Gazette Archive 4/29/98

Stuck On You; A Cure?:
by Chuck Ring

One of the most enduring complaints about polyurethane adhesive is that it tends to cure inside the container. While I usually use all of the contents of any container I have on hand before it starts to cure in the bottle, others have experienced a great deal of frustration with this aspect of polyurethane adhesive. The culprit for this "curing in the container" tendency is the water vapor or moisture contained in the air that has entered the bottle; either through the spout or through the walls of the container. Keep in mind that water or moisture is the "catalyst" which sets the curing process for polyurethane adhesive on course.

Manufacturers, and users of the adhesives have suggested expelling the air from the bottle after each use, storing the bottle upside down and, some even suggest storing the adhesive in a refrigerator. I have used both of the first two methods and both methods seemed to work well, but I was left with a very malformed container over a period of time.

Recently, I happened to read a post on rec.woodworking which offered a possible solution to the problem. The author of the post was Paul T. Radovanic, a woodworker who uses polyurethane adhesive to assemble some his projects. Mr. Radovanic wrote that he had just begun to add approximately 1/4" of acetone to the container just prior to capping the container for storage. Mr. Radovanic related that up to the time of his post, he had favorable results with his method.

As you may know, acetone; along with denatured alcohol and mineral spirits is a solvent for polyurethane adhesives. They are very useful in cleaning up excess adhesive from joints before the adhesive has cured as well as, removing the adhesive from the skin and other surfaces. My first reaction to Mr. Radovanic's method was to think that the acetone or either one of the other two solvents would degrade the adhesive and eventually render it useless, but since I've found Mr. Radovanic to always offer reasoned advice on rec.woodworking, I decided to write to one of the manufacturer's to obtain their opinion on the method.

I wrote to Mr. Dan Crandall of the Gorilla Group (distributors of Gorilla Glue) who forwarded my query to Gorilla's manufacturer in Denmark. The manufacturer's representative replied that there should be no problem with the addition of the solvent, but he would recommend using mineral spirits over the other two solvents. The representative reasoned that mineral spirits was less volatile and would therefore be somewhat safer in use.

The representative explained that the solvents all have an attribute which he referred to as "steam pressure". This term was "Americanized" as vapor pressure by a reader of a post I made, but as the representative explained, it simply means that "....all solvents have a certain pressure in gas form depending on temperature, When the "atmosphere" in the bottle has absorbed as much solvent gas as it can at a given temperature, then the atmosphere is saturated." I believe the "saturated atmosphere" then keeps moisture from meeting or mixing with the adhesive in the container and prevents the curing process from starting in the bottle. The representative cautioned that bottles or containers with added solvent should be kept out of close proximity to heat sources and out of direct sunlight because of the volatility of the solvents.

With the above information, I decided to conduct some tests using mineral spirits and a half used container of polyurethane adhesive which I had set aside some months past. It was just happenstance that I had mislaid this container and I found that a cured crust had developed over the main body of the adhesive. I further found that the adhesive, once the crust was penetrated, was stiff and would not flow well from the spout of the container. To start the test, I poured about 1/4" of mineral spirits into the container and let it set for about 12 hours.

The next step was to prepare several samples of 6/4 yellow poplar for joining with biscuits. I chose biscuit assembly because I am able to saw the cured assemblies apart at the center of the joint and observe whether or not the biscuit has completely filled the biscuit slots from side to side and also observe any visible glue line. After preparing the sample parts, I then poured the mineral spirits out of the container and proceeded to assemble three test assemblies. Each slot was completely coated with adhesive and the biscuits were dunked in a container of water just before they were inserted in the slots. The samples were then clamped and allowed to cure for approximately 24 hours.

The next step in the testing process involved sawing across the joints so that I could observe 1/2 of the assembly along the length of the biscuit halves. In each case with each sample, the biscuit slot was complete filled side to side and the glue line was barely visible. I then attempted to pry the biscuits from the slots using an awl and found that the biscuits came apart in very small fragments, often taking a thin portion of the slot wall with the fragments.

Over the course of approximately two weeks this complete process has been repeated eight times and each time the results were the same. While the adhesive seems to flow well, there does not appear to be any degrading of the adhesive and there has been no curing of the adhesive in the container. I intend to continue with a further series of tests as before, but I don't expect the results to differ. If I find they do, I will report the differences. I might add that Mr. Radovanic has advised me that he continues to use acetone as before and his results have continued to be as before.

If any reader finds results different from those I have presented, I would appreciate hearing of the details. I can be reached via e-mail at cring@concentric.net

C.E. "Chuck" Ring

April 28, 1998

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