Inconsistent epoxy wood bonds

Another interesting question is how much this reported behaviour of epoxy varies from that of other adhesives used in similar applications/test regimes... If for example 40% of bond strength lost is common to all adhesives, or, for that matter the remaining bond strength still exceeds that of alternatives then is there actually an issue at all? Its just another facet of materials behaviour that you build into your design.

 

The results of "bond strength tests" should be read with a clear understanding of wood strength. It doesn't matter an awful lot if some of the bonds fail around a third of dry wood strength when the wood is saturated, since the wood strength itself is going to be reduced by a similar amount. Take a look at the values in the table.

We know that a boat's structure is weaker when wet, the question is - can it still do its job? Saturated wood is typical of the working condition of traditionally-built boats with their heavy frames and thick planks, held together with fasteners.

Epoxy is far more than a substitute for screws and nails. It comes into its own as part of an entire regime of modern construction methods and materials. This produces efficient, lightweight boats - if it doesn't the extra cost of materials such as epoxy, glass and marine plywood is largely wasted. Small boats made in this manner become light enough to remove from the water after each use, larger boats are dimensionally stable enough to remain leakproof after repeated drying out cycles with the strength-to-weight ratio to safely travel at highway speeds on trailers. In the largest sizes an inner and outer skin of impermeable epoxy and glass keeps the wood core dry; even if it fails at that job it renders the weakening of the saturated wood core largely irrelevant provided it is still protected from rot. Such a boat can safely remain afloat for years.

 

experiences and theories

Water degradation of adhesive bonds, some notes -

I noted strength degradation of Titebond III joints when wetted thoroughly. It wasn't a scientific or even a deliberate test so I didn't note moisture content or measure before and after strengths. Just several test samples got soaked unintentionally. What I did note was, the glue lost a considerable amount if its bond strength when soaked, the glue seemed to soften, but apparently regained all or most of its strength when the wet bonds were dried again.

My theory of Titebond III is, the bond is a mechanical bond relying on keying of the adhesive into the wood grain and when the adhesive softens it can pull out, but if not stressed to failure the glue returns to its former rigidity when dried and bond strength substantially returns. The theory may be total BS of course, but that Titebond III recovers to a considerable extent when dried is definite.

My understanding about epoxy is, the bond is at least partly chemical with the resin bonding directly with the lignin and therefore bond strength is less affected by water.

Wood strength does deteriorate to some degree, that is presumably worse in air-dried wood rather than kiln dried wood where the lignin experiences a permanent chemical change due to process heat. That being the case an epoxy-built boat made from kiln-dried wood should deteriorate less than one built with air-dried wood in which the wood itself retains more of its strength when saturated with water.

It would be nice if someone more informed than myself would support these theories of mine or shoot them down in flames. One should welcome all learning experiences no matter how embarrassing . . .