Clamping

[adamcherubini]

Way back in the seventies when I was in High School, we used elmer's white glue.  I doubt anything was in clamps more than an hour because there were many classes that needed them.  I never saw a joint fail. 

Now many years later, I always let the glue cure overnight before working it.  I've tried and failed a couple times doing less.

Yeah. All those glues (Elmer’s, titebond etc) develop their bond strength under high clamp pressure. Pull off the clamps too soon, and you lose that pressure and could lose joint strength. I spring joints. So those joints are pulling themselves apart. So I’m sure to leave those clamps on over night.

So time is one factor. Heat and humidity is another effecting cure time (how soon you can take off the clamps). The sort of joint is yet another factor.

[Bill Tindall]

Bottom line. I need more clamps.

Buy Dubuque panel clamps available from Lee Valley and elsewhere.  Industrial use strong, lightweight, easy to use and store.  When we had the lumber business we sold hundreds to the local club members.  Their light weight makes them exceptional for gluing case pieces in addition to the furniture factory panel gluing use they were developed for.  You will use the 2' size most often.

[Bill Tindall]

Yeah. All those glues (Elmer’s, titebond etc) develop their bond strength under high clamp pressure. Pull off the clamps too soon, and you lose that pressure and could lose joint strength. I spring joints. So those joints are pulling themselves apart. So I’m sure to leave those clamps on over night.

So time is one factor. Heat and humidity is another effecting cure time (how soon you can take off the clamps). The sort of joint is yet another factor.

Clamp pressure serves to squeeze the excess glue from the joint.  If the joint gaps, clamp pressure serves the additional function of holding the parts together until the glue dries.   Pressure has no other function.  Bond strength results from loss of water resulting in the coalescing of the glue particles, which is unaffected by clamp pressure.  If the mating surfaces are perfectly made glue can be applied, the surfaces rubbed together to squeeze out excess glue and the result will be as good a bond as if the joint was clamped. We clamp because it is often a more convenient means of squeezing the excess glue from the joint and because the mating surfaces are often not perfect.

[adamcherubini]

Clamp pressure serves to squeeze the excess glue from the joint.  If the joint gaps, clamp pressure serves the additional function of holding the parts together until the glue dries.   Pressure has no other function.  Bond strength results from loss of water resulting in the coalescing of the glue particles, which is unaffected by clamp pressure.  If the mating surfaces are perfectly made glue can be applied, the surfaces rubbed together to squeeze out excess glue and the result will be as good a bond as if the joint was clamped. We clamp because it is often a more convenient means of squeezing the excess glue from the joint and because the mating surfaces are often not perfect.

You probably know something the rest of us don’t Bill. (Not a joke).

I called their engineering dept years ago. The answer I got is all over the Internet if you Google it. I forget the exact response but I recall 100-200psi of bondline pressure to develop full properties. I specifically asked about gap filling properties, and if they had data. Answer I got was dry little strength for unclamped joints.

What I think is true is that pva is super strong and easy to work with. My guess is, none of us really need the full 3000psi strength this glue has to offer.

[Bill Tindall]

You probably know something the rest of us don’t Bill. (Not a joke).

I called their engineering dept years ago. The answer I got is all over the Internet if you Google it. I forget the exact response but I recall 100-200psi of bondline pressure to develop full properties. I specifically asked about gap filling properties, and if they had data. Answer I got was  dry little strength for unclamped joints.

What I think is true is that pva is super strong and easy to work with. My guess is, none of us really need the full 3000psi strength this glue has to offer.

Franklin published a manual for using their adhesives in commercial settings.  Recommended pressure on an edge glued joint ranges from 100 psi for low density (low ending strength) woods to 250 psi for high density (stiff) woods like oak.  This pressure is what is typically required to bring machined joints into contact and squeeze the excess glue out of the joint.  

These glues are about 50% solids +/-.  If there is a glue gap between the boards being joined it will be filled by wet glue.  When 50% solids wet glue dries what will be left is foam with 50% of the space occupied by glue and the rest by air.  This foam will have little strength. Hence, it is necessary to bring boards being joined tightly together with clamp pressure if what is being joined is not perfectly flat. 

For those that can hand plane a flat surface, an experiment to try.  Plane two surfaces.  Add glue and rub to get the excess out.  In a few seconds you will feel the two parts tack together.  Stop rubbing and let the joint cure overnight.  Split it apart the next day and see how strong it was.

BTW, Titebond is not a PVA glue. Franklin calls it an "aliphatic" glue to distinguish it from PVA glue.  This aliphatic glue has improved properties over PVA glue- less creep, more temperature resistance, etc.  I suspect in fact that Titebond is a modified PVA glue, not something entirely different.

[Bill Tindall]

It helps to know how these glues cure.  Titebond and its relatives are made by emulsion polymerization.  The monomer is emulsified in water, a catalyst added and the result is a polymer suspension, called a "latex", of tiny polymer glue balls.   Some other stuff might be added to make the glue balls mushy.  When the glue is applied to wood some glue soaks into the pores and some remains on the surface in excess.  When the joint is mashed together the excess glue is forced out of the joint.  Water in the remaining glue begins to diffuse into the dry wood leaving behind the glue balls, which are mushy, either from water or additives.  If the temperature is high enough (50 degrees or so) the balls coalesce into a film, and essential step in the cure.  Whatever is making the glue mushy then diffuses into the wood, leaving behind a thin layer of whatever polymer the glue is made from.  To be a good glue this remaining polymer must be tough, and ideally stronger than the wood. 

This same process occurs when "latex" paint dries and cures into a film.  Paint has pigment while the glue does not.

[Tankwood]

It helps to know how these glues cure.  Titebond and its relatives are made by emulsion polymerization.  The monomer is emulsified in water, a catalyst added and the result is a polymer suspension, called a "latex", of tiny polymer glue balls.   Some other stuff might be added to make the glue balls mushy.  When the glue is applied to wood some glue soaks into the pores and some remains on the surface in excess.  When the joint is mashed together the excess glue is forced out of the joint.  Water in the remaining glue begins to diffuse into the dry wood leaving behind the glue balls, which are mushy, either from water or additives.  If the temperature is high enough (50 degrees or so) the balls coalesce into a film, and essential step in the cure.  Whatever is making the glue mushy then diffuses into the wood, leaving behind a thin layer of whatever polymer the glue is made from.  To be a good glue this remaining polymer must be tough, and ideally stronger than the wood. 

This same process occurs when "latex" paint dries and cures into a film.  Paint has pigment while the glue does not.

[Tankwood]

Thanks to the original poster for this one..

I learned much , some very knowledgeable members on this site
thanks as well to you fellows that chimed in here..!

Be safe-sm

[adamcherubini]

I’m right again. Bill really does know something the rest of us don’t (maybe more than one thing). Thanks for the explanation, Bill.

What do you use/recommend in unclampable joints? Like hand cut dts, m&t etc? I use hot hide glue. Tried it in tapered dowel joints and it failed. I think I extruded all the glue out with clamp pressure.

[bhh]

Titebond says:

What is the clamping and drying time of Titebond Wood Glues? For most of our wood glues, we recommend clamping an unstressed joint for thirty minutes to an hour. Stressed joints need to be clamped for 24 hours. We recommend not stressing the new joint for at least 24 hours.