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Most people around here probably know I use hot hide glue for pretty much everything. The exception is neck laminations, and finger boards which I use West System epoxy. I'm getting ready to try a laminated neck block. I'm probably going to use polyurethane glue for it, since I understand it doesn't creep, and isn't affected by heat. That got me to thinking about using it in other areas on stuff that should never need to be dis assembled. Like gluing the neck and tail blocks in, maybe joining plates, and maybe even kerfed linings. I currently use hot hide glue for all this stuff. Am I missing something, do I not understand polyurethane glue, or am I just plain crazy? I also use hot hide glue to laminate my upper transverse brace with carbon fiber. I understand lots of people use polyurethane glue to laminate braces too.


Again, feel free to tell me I'm crazy, it won't be the first time

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Woody, there certainly isn't any issues with the strength of polyurethane although there may be other issues:

1). The stuff expands like crazy. There really is no way to keep it from doing this. It foams everywhere and is extremely sticky. There is a very small window between total sticky mess and way too hard to clean up nicely as it is drying. On kerfed linings, the kerfs would largely fill with glue as it expanded - very ugly. On other parts (braces etc..) cleanliness is an issue. I find polyurethane to be the most time consuming and least likely glue to achieve a clean look afterward

2). If you get any on your fingers it turns them black for days. There are things that they say will cut the stuff and clean it off. I have yet to see any of these work.

3). There may be tonal considerations. This glue is not brittle and hard. It is flexible and hard. Although this may be a small consideration, it could affect tone in areas like the soundboard.

On the upside, I would think you could use it quite successfully on neck laminations although you still wouldn't escape #1 and #2 above.
Also, if you ever dropped your guitar in a river or lake, you could be assured of zero de-lamination on the glue line

Mario Proulx has been using PU glue on his neck laminations for a few years now. Nice thing with neck laminations is that it's easy to clean off once you final size the blank after glue up. Any other area would be a nightmare to get clean IMO.

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Woody,
There is an article in the latest issue of guitar maker(ASIA) it talks about the hardness of glues . Poly is listed check it out.

Brent

Woody, I have been using polyurethane glue on my heel build up. I started doing this since I use a dovetail joint and have concerns of delamination during a reset. I also use it when I laminate a neck block for the same reasons.

I would not recommend using it for linings or joing plates for the same reasons Randolph listed. If you decide to try poly, I would recomend gloves, since it is nearly impossible to remove from your hands!

Chuck

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PU is also quite 'slippery' so be sure to think through your clamping arrangements -a set-up that seems quite stable 'dry' will be much different with the PU in there. It's worse than epoxy in that regard.

PU is a great adhesive in the right applications-not gap filling, though.

Also, some PU adhesives seem to be less 'foamy' than others, so it might be worth checking different brands.

Cheers
John

The thing about urethane, whether it be foam, rigid, flexible, or adhesive is that it is sensitive to atmospheric moisture.

In a production environment, the purity of urethanes and their catalysts are controllable but a number of steps must be taken to do so. We had to maintain temperature with a microprocessor, keep the containers airtight, and keep a layer of nitrogen on the material. It's a pain...and I didn't design anything around the use of urethanes unless I HAD to. Sometimes I had to because there were no other suitable materials...but for gluing wood there are much more predictable adhesives.

A comment was made above about how urethanes expand and foam. That's most likely due to the components being exposed too much to a damp atmosphere. Unless the material was designed to expand or foam and has received some type of blowing agent to create that effect, fresh material won't expand or foam unless one or both of the components has been contaminated by water.

http://www.bjbenterprises.com/category.asp?category=3

That's a link to one of the suppliers we used. Note that all these urethane could be considered adhesives. Having such a wide range of properties makes it hard to pick sometimes but I think a good glue for instruments would a fairly high durometer hardness, a longish open time, and not be too viscous. Soft urethanes will likely not be good vibration conductors - even if applied as a thin film.

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StuMusic

Stuart-
I thought woodworking PU adhesives (all one-part as far as I know, so no separate catalyst) required moisture for curing?
Most of the directions suggest light dampening one of the wood surfaces if the wood is dry.

So I don't understand your 'contaminated by moisture' comment. It sounds like the 'industrial-strength' PU adhesives are something different?

Cheers
John

I only know what I have read, but apparently all PU's are not created equally. Some are non-foaming and get the moisture required for curing from the wood being glued. No additional moisture is necessary.

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Yeah....the word "contaminated" wasn't accurate. It's the amount of hydrogen present that is the issue.

Water is a part of the curing process with urethanes. Isocyanates react with water (or any molecule with hydrogen in it). The trick with urethanes is to control how that reaction occurs. With industrial urethanes it is understood that they have to be controlled from the beginning of the urethane manufacturing process to the end of the industrial application.

Glues that are sold off the shelf as one part systems are partially dependant upon gathering hydrogen from the atmosphere to cure. Curing polymers this way can be "iffy". When conditions vary too much you will get differing results. One of the by-products of a reaction between isocynanates and hydrogen is carbon dioxide. This is what causes the undue (or by design) foaming and expansion of glues.

I guess what I'm trying to say is that the hydrogen required for the proper, contolled cure of urethanes is contained and contolled within the components of two part systems. If a two-part system is properly stored it will be more predictable than a one part system for this reason; a two part system doesn't react too strongly to hydrogen in the air when it's curing. It is designed to be a self contained chemical reaction. One part systems are less stable because they require an outside source of hydrogen. Both systems suffer from being improperly stored.

Epoxy is more better.

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I read Emerson on the can. A foolish consistency is the hobgoblin of little minds...true...but a consistent reading of Emerson has its uses nevertheless.

StuMusic

Thanks for the explanation, Stuart.

I know a lot of woodworkers who use the calking tubed polyurethane glues such as liquid nails for laminating in cabinet work and furniture.

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When I was at PRS, I did a little exploration into CNC sanding. Specifically, I was looking at possibly using the CNC machine with a sanding drum mechanism. One of the issues I encountered was that traditional wood glues "melt" onto a sanding drum...and that Polyurethane glues don't. They sand really nicely...just turning into dust instead of depositing a "glue line" onto the drum. So, that's one more difference.

The only place I saw it used in production was for gluing up Cocobolo acoustic backs. We could have used Epoxy though I'm sure as well....the acoustic guy they brought in was partial to PU glue for some reason.

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