Question
I have seen how a once smooth PVA glueline joint can change with the seasons to really show itself right through finish. One solution is to use a plastic resin glue, which seems to "freeze" the two edges tight. Do any of you bondsmen know what other options are available? I am hoping to learn of some that are easier to use.
Forum Responses
(Adhesive Forum)
From contributor T:
Try polyurethane mastic, such as PL Premium construction adhesive. I don't mean polyurethane glue such as Gorilla Glue. There is a difference. PL Premium expands slightly which fills the joint, Gorilla Glue foams like crazy.
This glue comes in a caulking cartridge so just squeeze out a bead and clamp. It does need 3-4 hours to cure before stressing. I won't use anything else for tabletops where glue lines need to stay flat. Sometimes end grain orientation, quarter sawn next to flat sawn can cause surface levels to change.
I think water-based finishes are going to change the way we do woodwork. I am very glad I switched from solvent based finishing. Yellow and white glues can become re-dissolved by water-base and they aren't rigid, and can creep. Not good enough for finer work, but great for general woodwork and carpentry.
All adhesives have a place in a wood shop. Have you tried the new PowerGrab mastic for instant grab? It is different again and only good for stuff like glue blocks, small trim, parts where our grandfathers used hide glue. It's designed for sticking up crown moulding without nails and it works in my house! It’s a fantastic time saver.
There are lots of gimmicky products out there but these two adhesives are good ones. I'm not sure, but I think the PUR hot melt system is the same as PL Premium, just faster. I buy the large cartridges to save money. For a large shop I'd look into boxes of the large cartridges.
Over time, I have used many different glues, so the results I speak of are from experience, not theory. Plain carpenters glue of any brand will retain a tiny degree of flexibility, though I cannot say for how long after the initial glue up. I know it is at least a year.
This is not only evident in butt joints, but by the greater degree of spring-back one sees with bent laminations. Plastic resin cures rock hard, so spring-back after clamp release is very slight in comparison.
When plastic resin is used for butted edge panel joints, it really does limit the two sides in their tendency to swell at different rates, as they seem to lock together very tight.
Water based adhesives work excellent in solid wood and veneer conditions when the glue is allowed to organize uniformly on a molecular level. Pressure control allows the polymerization to be complete, instead of having isolated pockets of bond and crystallization in areas where the polymer hasn't received its negative ionic charge from the introduction of the catalyst. Adding heat can not only speed up clamp time, but also assist in uniformly accelerating the atomic particles in the polymer to give a more complete glue process, reducing what is known as creep.
Water based polymers work, plastic resins work when the right conditions occurred at glue up, try a vacuum press and heat blanket, and test the results for yourself. The nice thing about plastic resin is that it's stiff and impervious to moisture transfer. That also is its downfall. It’s more brittle than water based polymers. It all starts with the right products - try West Systems, or Unibond 800 for starters and the Unibond Blocker for filling spread joints.
Note also that the original question involved the change in a joint with seasons. The adhesives are not hygroscopic, compared to wood and it is well know (not a guess) that the RH changes with the seasons, causing moisture changes in wood which causes size changes.
The role of pressurization to polymerize an adhesive is interesting. I have never seen this comment before. Certainly, pressure is needed to put the two pieces of wood close to each other and to squeeze out excessive adhesive and to push the adhesive into the very small nooks and crannies. I look forward to hearing how pressure causes polymerization of PVA and similar adhesives.
Perhaps the three items mentioned above mentioned in practical terms are actually what you are referring to when you talk about negative ionic reception. In any case, note that these adhesives are not catalyzed, so I am not sure what you mean about "introduction of the catalyst."
I am also surprised that you imply plastic resins are better than PVAs when both make a joint stronger than the wood itself and both have no significant creep in furniture, cabinet and similar items. Further, PVA’s are plastic resins. Plastic resin adhesives are not, as a group, impervious to water. Nor are they more brittle.
Incidentally, Unibond 800 is a two part urea resin adhesive. The catalyst is formaldehyde, which is often avoided in furniture and cabinet production. The glue is a gap filling (implying that high, uniform pressure is not needed) and is rigid when dry.
West Systems is know for their two part epoxy adhesives, which are usually too expensive for wood and this higher expense must be offset by high benefits, which again for furniture and cabinets does not happen.
