I agree entirely. Its all to do with surface free energy (or surface
tension).
Teflon has hardly any surface free energy, so nothing sticks to it.
Diamonds have the highest surface free energy of any material, which is why
grease tables are used to extract diamonds from alluvial deposits (the
diamonds stick to the grease while all the other minerals are washed by).
Thus, the idea of tooth adhesion is a myth (except in former Soviet states
were it was held to be true). What abrading does (as you rightly state) is
to increase surface area AND to energise the surface to provide enhanced
surface free energy. The latter effect is short lived as contaminants (eg
atmospheric water) will quickly invade the new high energy sites.
This all sounds abit like a religion, but a chemist would be able to
explain it more authoritatively. What it all means is that, to obtain the
best possible bond, before any surface is coated (paint or epoxy) it should
be first cleaned and second abraded immediately before bonding; without the
cleaning first, contaminants are not removed, they are merely spread around
or rubbed into the nice freshly cut surface.
Duncan McF
On Saturday, October 12, 2002 4:23 AM, Fred Fillinger
[SMTP:fillinger@ameritech.net] wrote:
> McFadyean wrote:
>
> > The improvements in adhesion probably only comes about by the increase
in
> > surface area that the abrading generates. In which case there would be
no
> > benefit in a coarser roughening ....
>
> The article I cited actually goes into extreme depth, and mechanical
> adhesion is affected by many factors that I, at least, never heard of,
> one of which is surface tension. And even solids have surface
> tension. It's true that sanding increases surface area, which
> increases the force necessary to delaminate. More important they say
> is that abrasion breaks the surface tension, and the degree of
> abrasion (if needed at all) need only reduce it below its "critical"
> surface tension, which is also relative to the surface tension of the
> paint being applied. It all has to do with the paint's ability to
> "wet" the surface.
>
> Or so they try to explain, with lots of really big words!
>
> Regards,
> Fred F.
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