Re: [ATM] glued aluminium mirror cell update

["Andreas Derwahl" <andreas@duda-derwahl.de>]

Hi all,

@ Shane

thanks for sharing your expertise with glued connections.

> I don't see (on your web site/description) that you used sandpaper on the
> surfaces of the aluminium prior to bonding two pieces together with
> epoxy.

I should've mentioned, I did sand the aluminium -  first dry, then degreased
with acetone, then sanded lightly with epoxy on the sandpaper to keep the
freshly exposed aluminium under an epoxy layer to mininise oxidation. I
didn't find a big difference in strenght, though,  compared to just sanding,
degreasing and then glueing.

> A vacuum chamber filled with nitrogen or...

Being an industrial chemist I am used to inert techniques but currently I
don't have access to a glove box or argon (which can be used to keep away
air by just filling a bucket with it, as it is heavier than air and keeps
oxygen out, at least for some time).

> ....  Can the epoxy take that kind of heat (356F!/180C)?

I used the epoxy as the instructions indicated for maximum strength. When
cured at 180 centigrades the connection should give maximum strength and
hold 3000 N per cm^2. This value decreases the lower the curing temperature
is and at room temp it comes down to 1200 N per cm^2. The instructions
didn't say anything about a specific curing schedule so I just chucked it in
the oven for 7 mins and presto, the thing was rock solid. I didn't do all
connections in one go, though, so it could be an issue that repeated heating
to 180 centigrades for some of the connections wasn't ideal.

I did some initial tests as to how strong the connections are as I was
worried, that the small crossection when glueing end to end - as I did - was
not sufficient.
It certainly looked like it was strong enough (tested it with ca. 15 Kg
pulling perpendicular to the glued connection and it took it), but obviously
it wasn't.

> My guess is that your problem lies somewhere within these three main
issues:
> - you overheated the epoxy.
> - you heated it far too rapidly.
> - the bond joint needs different preparation.

What about the small crossection? I know, using glue in an end to end
connection is pushing it a bit. In those applications you metioned (boat
building, space travel etc.) I assume the actual bonding surface is much
bigger? As far as I know flexure in the cell wouldn't be a big issue, as
long as it always flexes in the same way and the same amount.




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