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Posted for Guy Buchanan <bnn@abac.com>:

Some thoughts on bonding:

Wood is porous and composites are not. That's why comparisons between the two
don't really work. Breaking into wood with coarse sandpaper opens more grain
for adhesive flow. (Unless you're gluing to end grain.)

If you model the sanded surface cross section as a sawtooth, it can be shown
that a fine sawtooth has exactly the same surface area as a coarse sawtooth.
The only requirement is that the sawtooth be coarse enough for the adhesive
to penetrate and not bridge due to surface tension. (Depends on viscosity,
filler, yada, yada, yada, but you've found the empirical answer in about
220/320.)

In a composite all of the strength is in the fibers. If you cut the fibers
while sanding you're back to the strength of the resin. If you sand a
laminate to a coarse sawtooth surface, the fibers are cut and the peaks are
connected to the substrate with resin. Though the adhesive has a lot of
surface to bond to, the peaks have the same "root" area as a perfectly flat,
un-sanded, panel.  

Thus when you test the surface you simply pull all the peaks off the
substrate.  (Some fiber enters and exits the peaks so the strength is quite a
bit better than bonding to an un-sanded panel.) If you sand to a fine
sawtooth surface you get a fiber "fray" (looking at SEM photographs,) of
fibers that are not necessarily cut by every sawtooth. These provide a
stronger bond surface. The relation is between grit size and fiber size.
(Again, it's been found to be about 220/320 for most composites.)


Guy Buchanan
BUCHANAN & NEWCOMER


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