When is a stronger glue warranted?

[echnidna]

As basically all modern ww glues are stronger than the timber, what is the advantage of using a stronger glue? ;) ;)

[bitingmidge]

None. (That's just my view though, and I'm not a chemist!)

Providing the joint is a proper fit to suit the limits of the glue.

Then you start looking at creep, waterproofing, flexibility etc, but strength is a non issue IF it's stronger than the timber.

One of the interesting failures that crops up from time to time is the "foaming" polyurethenes which are said to be gap filling. Trouble is that they aren't all that strong as gap filling agents, the bond strength reduces remarkably, and I've seen a few failures (in small boats) as a result.

If you need a couple of mm tolerance, you'll need fortified epoxy!

Cheers,

P :D

[Richardwoodhead]

Mainly in gap filling joins (M&T, etc) where there may be a little too much movement / slop. Plus I've found white PVA's usually end up creeping & ridging after a few years. I've been using epoxy (West System) for the last 5 years and never had a problem. But some PVA's I've used have started opening up. I'm not a chemist, but that's been my experience.

[journeyman Mick]

Peace of mind - that's what a better glue will give you. In the wet tropics I've seen PVAs let go just from the atmospheric moisture (also some lesser quality plys from the 50s and 60s). So if it's got any stress on it at all and/or it's a paying job then it gets glued with epoxy.

Mick

[brit_in_oz]

I am a chemist that makes adhesives for a living, yes PU adhesives do foam, becasue they emit gas as they cure. Different materials emit different amounts of gas depending on the amount of free isocyanate. The higher the amount the quicker they cure, but (usually) the more they gas.

We market a very good rigid PU for decking and yellow tongue flooring, but if its misused or not clamped properly you can run into problems, because as stated above you get a bubbly film that is weak.

PVA's are very varied in performance, if you use a D3 or higher classed PVA the *should* be ok even in humid environments, but yes epoxy is a more water resistant glue. OPut of all the glues they are probably in most vases the strongest because they penetrate the wood fibre to a deeper level than others, which is why you get fibre not adhesive failure