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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