Q. What can you tell me about PUR glues for wood? I hear mixed reports, but some items seem like what we need.
A. First, it is important to realize that a properly made glue joint with almost any commonly used woodworking adhesive is stronger than the wood itself. When a joint fails, it is usually because of problems with the wood surface preparation, including moisture content, or with the manufacturing conditions, such as open time, temperature or pressure. So, do not change adhesives because you are seeing failures...first fix what is wrong in your processing. Then, PUR (PolyURethane) adhesives are worth considering.
PUR adhesives are a family of adhesives with three main groups for woodworking:
1) Hot melts (much better than the hot melts of the past, so do not confuse them with the old stuff; cure is by cooling and with moisture).
2) Cold application (room temperature, curing with moisture from the air or wood; excellent in humid environments).
3) Two-part (somewhat similar to epoxy; this formulation does not need moisture to cure, but uses a chemical "hardener" as one of the two parts; not used for wood too often yet).
Why Use PUR?
The PUR adhesives were developed and used in Europe 35 years ago as a response to the demand for structural, "no formaldehyde" adhesives. PURs were and are today available in much of the world. Their complex formulation usually means that they cost more than the adhesives with formaldehyde and more than most PVA adhesives. What are the benefits? (The following comments are generalizations. With so many formulations within the PUR family, it is important to choose the adhesive with the properties you need.)
It is noteworthy that PURs are solvent free. Also, PURs have a long open time (except in humid environments and for some hot melts) and bond well to many materials including porous wood composites.
PURs do not contain water, so they do not have swelling issues at the joint. This can be especially helpful with veneer laminating by avoiding buckling and cracking. Also, with veneer and solid wood, this lack of water along with rapid curing means that the waiting period between gluing and subsequent machining and sanding can be very short.
PURs, when cured, have excellent water resistance, making PURs a good choice for exterior furniture.
Many PURs are not “rock hard” when cured, so that when the wood moves with seasonal changes in MC, the adhesive can give a bit and yet not lose strength.
Squeeze-out from PURs is easy to clean up...this can be a real plus. In addition, a uv indicator can be added to the adhesive (as in other adhesives) to help assure that the excess adhesive has been removed.
On the other hand, it does take a long time (24 hours or more) for the full strength to develop with PUR, especially with very dry wood, as the curing requires moisture...moisture from the wood and air.
I do believe our industry has avoided hot-melts, perhaps because they believed and still believe the old hot-melts were/are terrible for woodworking. That’s no longer true. Hot-melt PURs can be used for structural bonding of wood; that is, a hot-melt PUR joint is very strong.
Hot-melt PURs are solid at room temperature, but when heated to 250 F, flow very well. These hot adhesives, when applied to the wood, then cool and begin to provide good joint strength within 15 seconds to 3 minutes, depending on the formulation. Overall, the fast curing and good initial strength can often eliminate the need for nails and screws.
These are also good gap filling adhesives.
Compared to the old hot-melts of the past, PUR hot-melts involve two different curing mechanisms: first, as the adhesive cools, it solidifies and provides good bonding (as in the past). But then, the adhesive cures further (over the next 24 to 72 hours) by reacting with moisture in the wood and air, thereby providing cross-linking of wood and adhesive molecules and the high strength.
Squeeze out is very easy to clean.
For woodworkers used to PVA adhesives, using a PUR might require a bit of a learning curve. In fact, I suspect that often this is why, from time to time we hear of a person who has problems with PURs. Certainly, some PUR failures are due to poor joint preparation mentioned above, and sometimes, a person uses a cheap, discount, foreign-made PUR. (I have been told that the PUR sold under one label in a foreign country is not the same formulation that we have in the United State.) Another reason is that some adhesive manufacturers use essentially the same name for their PVA and PUR adhesives, so the consumer could possibly get confused and think they had a PUR but did not.
So, look at the PUR benefits and higher cost. It could pay to consider PURs; they are not a cure-all, nor will they replace all PVAs. But they are one useful part of our adhesive arsenal...cold setting and hot-melts.
Also, for any adhesive or chemical that you are using, enter the name of the product and then add MSDS to the search string when doing an internet search. You will get a listing of the documents that indicate risks, exposure guidelines, etc. for that product. For example, for Titebond PUR, which is a good, widely used adhesive (but is not TB 1, 2 or 3, which are pva type), try Google using POLYURETHANE TITEBOND MSDS. The fourth entry down when I do that search has the adhesive's MSDS, reproduced by Rockler, with all the precautions. In spite of the cautions, I do suggest that PUR is often the best choice if and when used properly.
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