Polyurethane (PUR) adhesives
1. PUR Hot Melt. This adhesive is typically applied from a heated cartridge or roller coater. It is applied hot (about 250°F is typical) and sets quickly as it cools. In this respect, it is like a conventional hot melt adhesive. It achieves its ultimate strength over the next few days by reacting with moisture in the air and substrates to form a significantly stronger bond than a conventional hot melt. PUR hot melts are used for general assembly and lamination of panels. Franklin's ReacTITE and HiPURformer adhesives are examples of reactive (PURHM) hot melts.
2. Liquid Polyurethane. These products come in a liquid form, are applied at room temperature, and cure by moisture in the air and substrates. They normally offer excellent water resistance. They are suitable for general assembly including finger joints. Franklin's Polyurethane Wood Glue is an example of a liquid polyurethane.
3. 2-part Polyurethane. These are not seen as frequently. They may be used in place of epoxy resins in many cases. Unlike the PUR Hot Melt and liquid polyurethanes, the 2-part systems do not depend on moisture for cure, relying on the hardener. They can be used to bond two non-porous substrates. Franklin's Urethane Repair Kit is an example of a 2-part.
Reactive Polyurethanes have been used in Europe for over twenty years. Their introduction in the U.S. was about 8 years ago and their acceptance is growing steadily. I believe Europe developed them as a reaction to the strict controls placed on formaldehyde based products. While they perform well in many applications, the cost still tends to be significantly higher than traditional adhesives. The key market for liquid PURs is the structural market (Glu-Lam, I-beams etc.). Products such as Ashland's IsoSet and Franklin's 2-part are contenders here, but to date, they haven't been able to meet all of the ANSI and ASTM specifications necessary to enter this market. I'm sure this will happen in time.
In the general woodworking market, there are often times the higher price for the Hot Melt PURs can be easily justified. I've seen many operations more than triple production in an area and save labor costs at the same time by switching to PUR technology (often a better bond result as well). At the retail level, I believe liquid PURs are being oversold as a sort of "cure-all". I've seen many fine woodworkers pay $15 to $20 per pint for an adhesive that in reality is more difficult to work with and won't result in a better bond than that achieved with a good PVA. There are times when a liquid PUR can be a great choice. Exterior furniture, some aspects of boat building, and gluing some non-porous materials are often excellent applications for PURs.
Jeff Pitcher, forum technical advisor
I am a small cabinet manufacturer and use PUR in my production, not for increased bond strength, but for speed. Jet Weld sets up in seconds and Excell Express in about 15 minutes. My primary goal is to get products out of the clamps. From that standpoint, this adhesive is a real plus for me. The down side is cost, but some of that is offset by less labor.
From the original questioner:
In one operation I observed, the speed was a benefit, but clean-up of squeeze out was the real benefit. Where there had been squeeze out, it was easily removed and the cleaned up area stained and finished like the rest of the wood. I know that squeeze out of most adhesives can seriously affect subsequent staining and finishing.
The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).
Comment from contributor A:
Comment from contributor B:
In the discussion of the use of polyurethane adhesives, many complain about the price of the product or the fact that they already have a PVA adhesive available on the shelf, so why would they want to purchase another adhesive?
One of the reasons to consider using a polyurethane adhesive as opposed to a PVA is the longer open time before the glue starts to cure. Remember, the curing agent for polyurethane is water, and if you are using the adhesive to bond wood which has been kiln dried, the wood will not have enough moisture content to properly activate the adhesive for a full strength cure. Therefore, the time spent applying the adhesive to the wood will not be considered as part of the cure time (in the absence of high a humidity environment). If you are working in an area with high humidity, the adhesive can start to slowly activate by drawing moisture from the air.
A second reason to consider using polyurethane adhesive as an alternative is the fact that, obviously, it is not an adhesive which contains water. Therefore, when it is applied to a veneer, it will not cause the veneer to expand and curl. This causes many problems getting it to lie down properly while putting the cauls in place. Even worse is the subsequent tendency for the veneer to shrink back to its original size as the moisture evaporates from the veneer. This has lead to cracking of the veneer, either before or after the application of the finish and delivery to the customer.
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