I've been reading about joining pieces of wood at a 90 degree angle, and the religious war between pocket holes and mortise-and-tenon joints. The thing I find fascinating is that both still suggest the use of wood glue.

I understand how harsher glues work — typically they essentially melt two materials together. Wood glue on the other hand doesn't seem to do this, instead it just bonds the materials somehow, but I can't see that as being a very strong joint. Through what action do wood glue joints get their strength?

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    Chemistry. (Well, maybe physics -- though at the "binding to a surface"/"what actually causes friction at a molecular" level, the distinction sort of blurs.) Not trying to be flippant, but it's going to be hard to give a better answer, I think. This is sort of like asking, "Why is tape sticky?" Good question! But it's very complicated, and has to do with all sorts of interactions at the molecular level. – Katie Kilian Jun 29 '17 at 19:53
  • @CharlieKilian Perhaps I had better take it to chemistry then, but I'll wait and see if we have any chemists in the house here for a little while. :) – Sidney Jun 29 '17 at 20:16
  • This is as much as asking "How do glues work?", because there isn't anything particularly special about the way wood glues work to bond wood, the same basic principles are at play as in all glued joints (just with more of X than Y for a different material). Do you really need to know the chemistry explanation or just be reassured that well-done glued joints are stronger than the wood itself? Because you can trust that this is the case, no ifs ands or buts. – Graphus Jun 29 '17 at 21:49
  • Again, I think this question would benefit from clarity. We can surmise that you mean typical modern PVA glues since you are talking about edge joinery, but even today different woodworkers will use a large array of glue types for different jobs. My glue shelf is nearly as busy as my stain shelf! – jdv Jan 25 '20 at 14:34

Here is a pretty detailed, technical discussion of gluing wood. There are two primary mechanisms of adhesion: valence forces, and interlocking action.

Valence forces are forces of attraction produced by the interactions of atoms, ions, and molecules that exist within and at the surfaces of both adhesive and adherend. Interlocking action, also called mechanical bonding, means surfaces are held together by an adhesive that has penetrated the porous surface while it is liquid, then anchored itself during solidification. Here are a couple of key excerpts:

Mechanical interlocking is probably the primary mechanism by which adhesives adhere to porous structures, such as wood. Effective mechanical interlocking takes place when adhesives penetrate beyond the surface debris and damaged fibers into sound wood two to six cells deep. Deeper penetra- tion into the fine microstructure increases the surface area of contact between adhesive and wood for more effective me- chanical interlocking. The most durable structural bonds to wood are believed to develop not only when an adhesive penetrates deeply into cell cavities, but also when an adhe- sive diffuses into cell walls to make molecular-level contact with the hemicellulosics and cellulosics of wood. If an adhe- sive penetrates deeply enough into sound wood and becomes rigid enough upon curing, the strength of the bond can be expected to exceed the strength of the wood.

There's lots more information in the referenced document if this doesn't answer your question.

  • Might be good to remove the "valence forces" ref, as the document says that this is one of the properties that the ASTM uses to define "adhesion", not that typical wood glues have such properties. – jdv Jan 20 '20 at 15:35

As a chemist, literally every explanation of chemistry in this thread is wrong. PVA is not pv-alchohol, it is pv-acetate. There is absolutely no such thing as "valence forces". Wood fiber is made from a polymer, lignin, and the cells that make up wood fiber are impermeable to water, which is the king of OH groups. It does not penetrate into the cell wall. Wood glue is a polymer, forming long crosslinked chains, and that is where it gets its strength.

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    Interesting. I do not doubt your understanding, but I'm wondering if there is evidence to refute the Vick article in the other answer about penetration "believed" to be one of the reasons for good adherence, not just interlocking action? That article is clear that the current science makes us "believe" that it is both cell-level penetration and interlocking action that is the heart of good adhesion. Has the ASTM (or someone else) refined this over time? – jdv Jan 20 '20 at 15:40
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    Hi, welcome to SE. Stackexchange is not like a regular forum, Answers should ideally not respond to other Answers but be standalone replies to the query in the opening Question. There's quite a lot written about glueing in a woodworking context but most is A, not written by chemists and/or B, dumbed down to make it understandable for any average Joe. Could you do your part to buck that tendency by restructuring this so that it is a more comprehensive Answer to the main query, written from the perspective of modern chemisty and not referring back to old terminology like valence forces? – Graphus Jan 20 '20 at 15:51

Describing the chemistry precisely is useful because it explains why good practice is successful, but perhaps you just want to know what good practice is.

Join long grain to long grain (This explains the geometry of, for example, mortice-and-tenon joints, and why they are better than dowels). Have a smooth surface not a rough one. Have a well-fitting joint (goes together with hand pressure alone, but only just) Allow to set under tight clamping pressure (but not excessively tight, which will squeeze all the glue out) Clean oily woods with mineral spirit.

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    Some good points there Philip, but starving a joint from excessive clamp pressure is actually a persistent myth (I think stemming from the days of protein glues, which in the right circumstances needed no clamp pressure at all). – Graphus Jan 31 '20 at 7:38

Specifically, wood glue matches the functional structure of wood. Woodglue tends to be PVA - polyvinylalcohol- which is made of long chains with lots of alcohol or hydroxy (meaning -OH functionality in chemistry terms) groups hanging regularly off the chain. Wood is a polysaccharide, which is "many sugars". Sugars tend to have 5 of these OH groups arranged to a mainly carbon skeleton ring. The nature of these OH groups is simular to water, H-O-H. These groups can interact very strongly with each other and give water it's very high boiling point for such a simple structure. The OH groups between the polysaccharides and the PVA generate very strong intermolecular bonds, as strong or greater than the polysaccharide chains' interactions making up the wood itself. The porous nature of wood just means this glue can penetrate further. The PVA chains are very long (think spaghetti) so all interact and stay bound to each other and the polysaccharides in equal measure.

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    PVA is not polyvinyalcohol, which rather cuts the legs out from under this explanation. – Graphus Jul 1 '17 at 6:08

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