I have run into PUR several times recently and it seems to be working very well. Can you give a brief overview of this adhesive?
PUR is an abbreviation for PolyURethane and includes several distinctly different adhesive types. Among the types seen in wood-related applications are:
1. PUR Hot Melt. This adhesive is typically applied from a heated cartridge or roller coater. It is applied hot (about 250癋 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.
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
Comment from contributor A:
I have been using Pur glue (gorilla glue) on oily woods such as IPE, cocobolo, vera, and others. I get very good results with fresh machining, dampening both sides and gluing up. I have done a lot of tests and this method gave me joints where the wood fails before the glue does.
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.