# Why does glue dry?



## JeffP (Aug 4, 2014)

I know, I know…sounds like a really stupid question.

Epoxy "dries" because of a chemical reaction between the resin and the hardener. Gorilla glue, et. al. hardens due to chemical reaction between water/humidity/damp-wood and the glue.

So my question is, when you put "wood glue" in a joint and then put the pieces together, why does the glue inside the joint dry? Obvious why the glue out at the edges dries…but the glue down inside there…seems like it is bottled up just as though it was still in the plastic bottle.

So again I ask…why does wood-glue (like Titebond for instance) dry when it is inside of a joint?


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## Mosquito (Feb 15, 2012)

Because it wouldn't be a very useful glue if it didn't? lol

Interesting question, I look forward to [real] answers to it


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## dawsonbob (Aug 5, 2013)

At first glance at just the title, I did think it was a silly question. But since you explained, I don't think it's a silly question at all.

I know why the sky is blue, but why DOES glue dry?


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## MrUnix (May 18, 2012)

White/yellow PVA based glue is water based.. that is, water is the solvent used. it evaporates and is also absorbed into the wood.

Cheers,
Brad


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## JeffP (Aug 4, 2014)

So even though the wet glue inside the joint is encapsulated by the drying glue around the edges and the wood itself…you're saying the wood soaks up enough water from the glue for it to "dry".

So a closed grain wood (like white oak) should dry considerably slower than something more open like red oak or poplar?

What about super hard woods like hickory or bubinga?


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## wormil (Nov 19, 2011)

As the water evaporates there is a chemical change in the PVA. This is probably one of those questions where you can keep asking why until you have a 4 year degree in chemistry by the time you get your answer.  Good question though. There are chemists in my local w/w club, maybe I'll ask them.


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## Ghidrah (Jan 20, 2015)

Enclosed or not glue will set over a determined period inside the bottle, I've 86'd more than enough bottles. The vehicle, (water) is drawn into and around the wood, look at a piece of wood under a microscope it's like a fistful of straws. This is why the bond is stronger than the surrounding unaffected wood fiber. The bonding agent then loses it moisture and sets.

If you let your spread glue set in the open for a few mins before clamping it'll set even faster, it almost acts like contact cement.

Manufacturers should provide glue bottles with an internal glue bag to limit air infiltration. The best I can come up with to extend the life of the glue in the bottle is to squeeze it to expel the air then close it and turn it upside down.


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## Buckethead (Apr 14, 2013)

Because when the mommy glue and the daddy glue love each other very, very, very much…..


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## JeffP (Aug 4, 2014)

ROFLMAO


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## Tony_S (Dec 16, 2009)

It's not a stupid question at all.

The water within the glue on the surface evaporates into the air, binding the solids in the glue…the water that is in the glue within the joint first gets absorbed by the cell structure of the wood like a sponge allowing the solids in the glue to bind, the water is then drawn through the cell structure and eventually evaporates into the air.

As the cells absorb the water, they swell, and as the water evaporates, they shrink again. If you hurry the gluing/mill work, sanding process on thicker lumber, say over 1" thick(I'm talking all within a half hour) and then check the glue joint a day later, you can sometimes feel it with your fingers. It's because the work was done while the cells of the wood near the joint were still swollen, then they shrink.

Same idea why it's typically not a good idea to use most PVA adhesives to glue raw veneer's to a substrate. The cells swell, the glue sets, then the cells shrink….and you get cracks.

To get an even better idea of how quickly the absorption can take place, take a raw flat piece of flitch cut veneer and cover one side completely with PVA. Within 20 seconds, you'll have a veneer tube. The cells have swollen up on the glued side faster than the unglued side, causing it to curl. By the time the moisture works it's way to the opposite side of the veneer, glue has been pulled into the cellular structure on the glued side and stays there, eventually drying and not allowing the cellular structure to relax.
Do the same thing with plain water and you'll still get a tube, but as all the water evaporates the veneer will flatten out considerably as the cell structure evens out in size.

Hopefully my blab is understandable….


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## Tim457 (Jan 11, 2013)

I thought PVA cured like any other glue, but you learn something new every day I guess. Everything I found agreed with Tony's explanation. 
http://cameo.mfa.org/wiki/Polyvinyl_acetate
So I guess it does cure after the water is absorbed or evaporated out, but saying it dries is not really all that wrong either.


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## Pezking7p (Nov 17, 2013)

PVA cures like any other glue. There are several components but primarily PolyVinyl Acetate. This is emulsified in water, and the water keeps the PVA molecules far enough apart so that they don't cross link. When the glue starts to dry, the molecules come close enough that they begin to crosslink, which hardens the adhesive. Once crosslinked the adhesive is cured and can no longer be cleaned up with water.

The main difference is that epoxy is (usually) a 2-part adhesive so the reaction doesn't start until you bring parts a&b together. Polyurethane glue needs water to begin the reaction. PVA glues have everything they need to get the reaction started, but the emulsion keeps the chemicals separated.

The reason that PVA glues don't work after they freeze is because the process of freezing breaks the emulsion and the chemicals begin to react while suspended in water. So you really end up with tiny dried glue particles suspended in water.


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## JeffP (Aug 4, 2014)

Thanks all, this has been illuminating.

Still wondering about glue "drying" times with woods that have substantially different cell structure.

Seems that woods that are particularly good for use outdoors, like teak and white oak, would also need substantially longer for PVA glue to fully dry/cure. These woods are said to have a "closed grain structure", which I would think would greatly reduce their tendency to wick the water out of the PVA glue?


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## lateralus819 (Mar 24, 2013)

I quite like Pezkings explanation. Thanks for the info sir!


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## Ghidrah (Jan 20, 2015)

Ipe doesn't take to glue very well, I believe the harder and denser the wood the more resistant it is to gluage.


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## SCOTSMAN (Aug 1, 2008)

Once a chemical change takes place there is no going back .In other words you start out with lets say plaster of paris you mix it with water and eventually it sets.You might think let it dry and grind it down and re-use it.Wrong when the plaster or glue hardens there is a chemical change to it's structure and usually a heat of reaction try holding some plaster as it dries it will soon be too hot to handle.In the glue there is usually an element incorporated to keep it from going hard when it is in the bottle but eventually it too will go rock solid usually by the action of light hence many things stored im dark bottles.If you take plastic cement this is simply plastic powder mixed with a chemical to melt it and keep it fluid.When you apply it the softener evaporates and the plasic reverts to its hardened state.Alistair


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## bondogaposis (Dec 18, 2011)

I googled around until I found this link. I think this quote pretty much sums it up. "How does glue work? Specific adhesion - results from strong molecular attraction between the wood and the glue and for this to occur, the adhesive must penetrate the wood cells and "wet" the fibers. Cohesion - the force which makes the glue stick to itself. This is the solid glue "line" in a joint. It is the strongest when the two pieces to be joined are machined so they mate as closely as possible"


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## Sparks8286 (Apr 28, 2014)

I always figured wood glue dried because the water in it evaporated. That made sense as to why took longer for glue to dry in bigger joints like laminations. It can't dry inside the bottle because the bottle is closed….there's nowhere for the water to evaporate off to.


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## SCOTSMAN (Aug 1, 2008)

Adhesion is the attraction of dissimilar cells, and cohesion is the attraction of similar cells.As in the glue sticks to the wood dissimilar but the glue needs to be cohesive or it will part just as though you used tapwater the cohesion causes the glue to bind it's cellular structure together see Viscosity and meniscosity .Alistair


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