What is special about wood glue?

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Whenever I'm joining wood together, no matter what type of joint, I turn to wood glue. But there are other glues out there, such as epoxies and Gorilla Glue. It seems the alternatives are very strong and capable, but people only turn to them under special circumstances.

What is it about wood glue that makes it the best choice for common joinery?

drs

Posted 2015-04-08T12:02:35.283

Reputation: 1 969

1"Gorilla Glue" used to be synonymous with that company's polyurethane adhesive but they now make every kind of glue (and various tapes!) so it's best to specify. I've seen numerous instances where an asker just says they're using Gorilla glue and responders assumed the polyurethane product only for it to turn out that the OP was using CA/superglue. – Graphus – 2015-04-08T18:01:57.317

@Graphus so its like Kleenex, Band Aids and Aspirin – Matt – 2015-04-08T23:26:36.197

1@Matt, yes it used to be. Now it's the reverse if you think about it, Gorilla glue can't be used as a generic term for polyurethane adhesive. – Graphus – 2015-04-09T10:19:06.630

Answers

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I assume you're talking about PVA glue. I think it just has a lot of desirable properties. Off the top of my head:

  • Water-based and non-toxic - no nasty fumes or health risks. This also means that it is easy to clean off of benches, brushes etc.
  • Dries quickly enough that you don't have to leave parts in clamps for several days, but not so quickly that you have to work too fast with it. This also means that if necessary then you can clamp pieces together, then pull them apart and reposition them if needed within 15 minutes or so. In colder temperatures you may still be able to reposition the parts for an hour or more.
  • Is pre-mixed, single-part (i.e. not a two-part epoxy or powder which requires mixing up in batches to use). This means that there's no real preparation of the glue needed - just pour some out of a bottle and use it for a couple of hours (again depending on temperature).
  • Is at a good viscosity to soak into the wood by the right amount to form a strong joint. Also means you can brush it onto a joint in the correct amounts and it won't just run off over your shop floor.
  • Dries clear - joints do not have obvious glue lines.
  • Is relatively cheap.
  • Is stable enough in different environments (heat, humidity) when cured.
  • Is flexible enough not to be too brittle when cured.
  • Gives no risk of gluing fingers together unlike cyanoacrylate ("super") glues.
  • Versatility - can also be used as a sealer or primer for certain finishes / applications.
  • Has been used reliably for many years.

There are other glues which may be stronger etc. but these are often epoxy or animal glues which take time to mix together and once mixed have a limited working life, otherwise they may be toxic or have nasty fumes, require special preparation or be more expensive.

In summary, PVA is good (strong etc.) enough for most applications whilst also being cheap and easy to work with.

One negative that may be little-known is that PVA joints can "creep" which means that the glue stretches and the joint moves under long term load. For most applications this is not a significant issue though. It's for this reason that animal glues are more useful for things like gluing guitar necks where the joint is under near constant pressure and any movement in the joint would ruin the product or assembly.

There is a good summary of glue types here.

WhatEvil

Posted 2015-04-08T12:02:35.283

Reputation: 2 492

2I would add that it will usually give under stress before the wood does, so one can re-glue the pieces of wood instead of shards of glue stuck to shards of wood. – ewm – 2015-04-08T13:58:42.290

8Actually, in a properly done glue joint, the wood will fail before the glue. – OSU55 – 2015-04-08T14:40:43.357

@OSU55: I suppose that depends what you mean by "properly done". It's certainly possible to glue things in such a way that the glue is stronger than the materials being joined, but that doesn't mean it's always desirable. If an assembly is subjected to so much stress that it can't possibly hold together, a glue joint which gives way while leaving the surrounding material intact may be preferable to one which holds so strongly that the surrounding material breaks instead. – supercat – 2015-04-08T18:38:03.473

1@OSU55 is correct. Wood glue shouldn't fail before the wood does. Depending on the type of wood. – Mike McMahon – 2015-04-09T03:57:46.223

I have to agree with @OSU55 as well. One reason you will get splinters and broken wood pieces (instead of a clean break in the glue) is that the wood sucks up and merges with some of the glue like a sponge. While doing so it also expands a little which makes for tighter joints. That "sponged up" parts are usually the toughest part of the joint a break will almost certainly occur next to that area -> in the wood. – Stoppal – 2016-05-19T10:47:11.690

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Whenever I'm joining wood together, no matter what type of joint, I turn to wood glue.

"Wood glue" isn't actually one thing. It's essentially a marketing term or selling aid more than a description of a specific product.

For a traditionalist the words wood glue would actually call to mind hot hide glue and nothing else. And both epoxy and presumably polyurethane adhesive you also mention are heavily used worldwide to bond wood, in boat building for example, so they could just as easily be called wood glue, even though they're also very good as bonding other materials.

You're referring to what I usually call the conventional wood glues, which are types of PVA (see note bottom). This wasn't developed specifically for wood, it was just found to be particularly suited to it. The simplest type of white PVA sold as wood glue is no different from the white craft glue children use in schools and craftworkers use for cloth and paper projects. It's also used as a bonding aid for plaster, as a bookbinding adhesive and as an additive for concrete.

What is it about wood glue that makes it the best choice for common joinery?

A better question would be: why are PVAs the most common choice? How a person defines best is always going to be somewhat subjective as there's always a component of best for them.

So why are PVAs the most common? I'd have to say it's probably mostly about convenience: it comes at the usable consistency in a bottle, it's easy to apply, easy to spread, doesn't set too quickly, easy to clean up and non-toxic. And of course it's important that it can form very strong joints in wood (the saying "as strong as the wood itself" is not an exaggeration, as destructive testing demonstrates).

While there are still a few adhesives sold for use with wood (such as the Cascamite much favoured by old-school woodworkers in the UK) which can far exceed the strength of PVA, this strength is actually irrelevant in any furniture context since PVA joints can already be as strong or stronger than the material itself if properly formed.

Note: PVA-type glues are by no means one product with a single set of characteristics — there are essentially three classes, Type-I, Type-II and Type-III, with distinct differences (primarily in water resistance).

Graphus

Posted 2015-04-08T12:02:35.283

Reputation: 38 608

1You sir are going to do very well in WW. – Matt – 2015-04-08T18:53:24.197

Easy to clean up: that is a big one. It does not bother you to get on your hands, and WW is a hands-on pastime. The time I tried Gorilla Glue (there was only one) I was in for a nasty surprise, in that it does not come off skin, and removal pre-set requires solvent. – JDługosz – 2015-04-09T00:40:46.017

1PVA strength: I made some test lap joints (3 surface) with everything wrong: not smooth/tight contact, no correct clamping pressure. I did let it set for a few days before stressing. It was still not the point of failure when I got people to do their best to rip it apart. Perhaps Type II is more forgiving about gaps and curing pressure. – JDługosz – 2015-04-09T00:44:38.243

2Your test results are interesting; many tests done on joints that were deliberately made poorly (resulting in a thick and/or uneven glue line) the glue line is the primary (or only) site of failure. As it is probable that the majority of woodworkers don't clamp as hard as they should, and yet there seem to be very few joint failures, it lends support to the idea even sub-par joints can be more than strong enough in many cases. However, I would be careful not to conclude from this that loose joints or improperly prepared joint surfaces are in any way advisable. – Graphus – 2015-04-09T10:12:59.263

4

Well, it works well on wood. I can't use it to fix plastic or ceramic or metal. It's not ultimate or versatile. I guess you often handle materials that it works well on.

You did say you use wood glue whenever you're using wood. So?

There are different substances that can be called wood glue. There are bottled creamy one-part concoctions based on PVA but even then more than one kind: Titebond makes 3 (classic, water resistant and generally improved, waterproof) plus viscus varities for trim (high initial tack). enter image description here

Then there is powder that you mix with water, (Weldwood brand comes to mind) and different kinds of hide glue. I used fish-skin glue to fix a Chinese strinnged instrument, to be authentic.

As for gluing wood with stuff not specifially sold as wood glue, there is epoxy and polyurethane glue and cyanoacrolate glues good for mixed material joints but useful for just wood sometimes: instant bond, no clamping pressure, void filling, etc.

It can also be handy to use a hot-melt glue gun.

So why use only one? Even among the PVA group, if it works OK why do you need more than one? If you had to go beyond the feature set of your stand-by bottle, you would. If you wanted to optimize the joint, you could if you were an enthusiast and didn't mind de-optimizing for ease and convenience.

If I were attaching one piece of trim, I'd note the issues with "regular" glue, but then I'm done. Faced with a large project and remembering that experience, I'd pick up a bottle of "Titebond for trim" when I noticed it on the run to buy supplies. And so your collection grows.

For outdoor chairs, I saw yankee workshop use marine glue from a caulking cylinder dispenser. And that's wood-on-wood.

For occasional and varied uses, you buy what's sold as general-purpose. So instead of asking “why do I always use this?”, you should note: “Isn't it great that there is a general-purpose versitile product that has a long shelf life and works reasonably well on so many tasks?” no surprise that this is what you keep in your garrage.

JDługosz

Posted 2015-04-08T12:02:35.283

Reputation: 269

Welcome to WW. For your formatting I think you should be consistent with your italics. You are using it for quoting, emphasis a lot. Other than that good answer – Matt – 2015-04-08T23:29:12.557

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In a word, effective. Sufficient strength, easy clean up, no fumes, price all make PVA wood glues the choice over epoxy and polyurethane glues because they are overall the most effective. The other glues have their specific uses. This is with respect to furniture joinery, and not structural use such as construction.

OSU55

Posted 2015-04-08T12:02:35.283

Reputation: 976