Why does glue dry?

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Forum topic by JeffP posted 02-09-2015 08:45 PM 1058 views 0 times favorited 18 replies Add to Favorites Watch
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573 posts in 809 days

02-09-2015 08:45 PM

Topic tags/keywords: question

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?

-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.

18 replies so far

View Mosquito's profile


7924 posts in 1710 days

#1 posted 02-09-2015 08:47 PM

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

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

-- Mos - Twin Cities, MN - -

View dawsonbob's profile


1830 posts in 1173 days

#2 posted 02-09-2015 08:50 PM

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?

-- Mistakes are what pave the road to perfection

View MrUnix's profile


4022 posts in 1617 days

#3 posted 02-09-2015 08:55 PM

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


-- Brad in FL - To be old and wise, you must first be young and stupid

View JeffP's profile


573 posts in 809 days

#4 posted 02-09-2015 09:01 PM

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?

-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.

View Rick M.'s profile

Rick M.

7681 posts in 1798 days

#5 posted 02-09-2015 09:21 PM

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.


View Ghidrah's profile


667 posts in 640 days

#6 posted 02-09-2015 10:36 PM

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.

-- I meant to do that!

View Buckethead's profile


3140 posts in 1286 days

#7 posted 02-09-2015 11:01 PM

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

-- Support woodworking hand models. Buy me a sawstop.

View JeffP's profile


573 posts in 809 days

#8 posted 02-09-2015 11:03 PM


-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.

View Tony_S's profile (online now)


597 posts in 2501 days

#9 posted 02-09-2015 11:57 PM

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….

-- It is the mark of an educated mind to be able to entertain a thought without accepting it. Aristotle

View Tim's profile


3029 posts in 1379 days

#10 posted 02-10-2015 12:25 AM

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.
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.

View Pezking7p's profile


3097 posts in 1069 days

#11 posted 02-10-2015 12:44 AM

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.

-- -Dan

View JeffP's profile


573 posts in 809 days

#12 posted 02-10-2015 01:17 AM

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?

-- Last week I finally got my $*i# together. Unfortunately, it was in my shop, so I will probably never find it again.

View lateralus819's profile


2236 posts in 1307 days

#13 posted 02-10-2015 01:24 AM

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

View Ghidrah's profile


667 posts in 640 days

#14 posted 02-10-2015 01:31 AM

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

-- I meant to do that!

View SCOTSMAN's profile


5839 posts in 3003 days

#15 posted 02-10-2015 12:36 PM

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

-- excuse my typing as I have a form of parkinsons disease

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