All about polyurethane glues

      Pros, cons and various applications of polyurethane adhesives. November 7, 2000

I have been reading about the newer polyurethane glues on the market. They seem like the ideal glue for woodworking projects. Anyone had any negative experiences with these glues?

Forum Responses
There are two kinds of reactive polyurethanes available to the general woodworking market. The first, a reactive polyurethane in hot melt form, is used in a wide range of applications. It is especially suited for assembly projects as well as edgebanding and profile wrapping.

The other reactive polyurethane on the market is one part liquid. (I'm guessing this is the product you are referring to.) It's available under a variety of labels such as "Gorilla Glue", "Excel", "PL Premium" or "RPA". This kind of adhesive works very well in a wide range of applications but, as with all adhesives, there are both positive and negative sides to it. On the positive side, reactive polyurethanes are strong, water resistant and have the ability to bond to a wide range of substrates. On the negative side, they are expensive (relative to PVA), difficult to clean up and are often oversold to people who don't need them. I normally tell customers that they are well suited to exterior applications like sign building and outdoor furniture construction. For general indoor assembly like cabinets or furniture you're wasting your money if you use a reactive polyurethane.

Jeff Pitcher, forum technical advisor

I disagree. I use Gorilla glue whenever I have a larger glueup requiring a longer open time than PVA. I'm not sure what the previous respondent means by cleanup- I keep mineral spirits handy for quick cleanup of wet drips, and I find dried polyurethane joints easier to cleanup than PVAs. He is right-on in saying that PURs are oversold to the public, but they're an excellent OPTION to us "real McCoys". Cost is a relatively minor consideration in my shop.

You are right in saying that mineral spirits will clean up PURs. However, once it's dried on your hands, it's difficult to remove. I've had a lot of calls from people who have gotten it on their hands and have trouble getting it off. As for the longer open time, there are many different formulations of PVA that will give you a longer open time. Certainly anyone in business needs to consider product cost. Yes, adhesive is a minor component, but a cost none the less.

Jeff Pitcher, forum technical advisor

I would agree about the "over sold" aspect of these one comonent PURs. Testing at the USDA Forest Products Lab shows them to have great dry strength. However, the water proof aspect of the advertising fell way short. I would advise against using them in any application that requires wet strength.

I have been working with PUR 1 component as well as 2 component. 1 component usually foams a lot as it is using moisture as a catalyst. 2 component usually doesn't take any less pressing time than 1 component but foams less and bonds better. Pot life is an issue but with low RH and good weather, you should be able to get relatively long pot life. On the other hand, pressing time might be slightly prolonged.

This is much better than PVA if you need to bond primed metal sheets, wooden panels etc.

Price will be higher but do consider the application first. With proper planning, lesser wastage, optimum glue spread, the price will not be extremely high.

We make large wooden bowls for the food service industry. Our decision to use "Gorilla Glue" was based on a simple test. A bowl assembled with the test glue had to survive 30 cycles in a commercial dishwasher at 180 degrees with no joint movement or failure. It also had to be foodsafe. Polyurethane is the only glue that passed. The secret to a good bond is to mist the glue surfaces with water 5 minutes before application to open the wood pores and to use sufficient clamp pressure to force the glue into the wood. A joint that's not tight will allow bubbles to form resulting in a weak bond. 500 bowls later, we've had 0 rejects and only a few returns.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor A:
Yes, monocomponent PUR adhesives are practical. They are moisture resistant, fast curing, have optical gap filling properties (the cured foam in a joint line has minimal mechanical strength, it is only cosmetics). But on the downside they are not UV proof (sunlight), and uncured adhesive is a carcinogen (PUR contains Diphenylmethan-4,4'-diiosocyanate). So, be careful with it.

Comment from contributor J:
I used Gorilla Glue recently to repair an outside concrete garden bench that had a chipped corner. The repair was excellent; however I did get the water/glue on both of my hands and tried many things to remove it. First I tried paint thinner, then acetone, then lava soap and water and then I finally resorted to Mentholatum and every bit of the stickiness vanished. It also helped my old damaged hands heal too.

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