Well,
There you have it folks, take your choice, but welding
seems the winner!
I will still consider adhesive/rivets as a footwell
floorpan solution, but not for any serious load
bearing usage.
BTW, since the bond is aluminum friendly, and I know
that aluminized steel is available, would that resolve
the problem? Or is the AL-steel bond itself also weak?
Thanks for sharing your expertise.
Carter Shore
--- "R.D. Waid" <msp01091@isd.net> wrote:
>
> Listers,
> I thought I might weigh in on this subject (epoxy vs
> welding), as I have
> formulated structural epoxy adhesives for 3M
> (aerospace and automotive) for 13
> years or so. The main reason that structural bonding
> in aerospace applications
> has been successful is the chemistry of aluminum and
> aluminum alloys. Before
> aluminum is structurally bonded, it is subjected to
> chemical etching and
> anodization in phosporic acid which produces a
> thick, porous oxide. This oxide
> is ideally suited for being wet out by epoxy-based
> adhesives. The etching
> process is beyond the capabilities of the average
> garage mechanic, if he were
> working on an aluminum structure. I was involved in
> experimental progam with
> Ford, where we built an all-aluminum Taurus/Sable.
> Here, they purchased
> specially pre-treated aluminum from Alcan, but still
> spot welded it (very
> difficult with aluminum, again because of the
> nonconductive oxide present) due
> to concerns with the load bearing of the joints over
> time.
>
> Steel does not give the option of "structural"
> anodization-the oxide formed on
> steel does not adhere, but instead oxidizes to red
> rust. We had programs for
> structural steel bonding at 3M at various times, but
> never found anything other
> than sandblasting followed by a chromate-filled
> epoxy-based primer that did
> much. However, bonds tested under sustained loads
> always failed miserably
> compared to comparable aluminum bonds. Epoxies
> developed for the auto industry
> today are weld-bondable, meaning that the automakers
> spot weld right through the
> bonded joints to prevent catastrophic failure.
> Typically, they are slopping the
> epoxy right on oily panels to act as a
> semi-structural sealer rather than a
> primary bonding medium. Finally, all of the
> higher-performing epoxies I have
> described are thermally cured materials, requiring
> bonding cycles of at least 30
> minutes at a minimum of 250degrees F. Uniform
> bondline heating of structures as
> big as a car are hard to do unless you are going
> into a paint curing oven or
> large autoclave.
>
> I think a riveted floor with epoxy can be done, but
> you won't really be getting
> much from the epoxy without scrupulous preparation
> and curing, it will just be
> an expensive sealer. Despite my access to various
> high performanse adhesive
> materials, I plan on having my floors welded.
>
> Bob Waid
> 71 Mark IV, FK4081L
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