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September 2003

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From:
Brian Ellis <[log in to unmask]>
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TechNet E-Mail Forum.
Date:
Fri, 5 Sep 2003 10:02:19 +0300
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Steve

In a former life, I did an experiment of casting a diaphragm of
identical size and thickness, as far as I could within empirical errors,
of a few conformal coating formulations (one each single component PU,
one two component PU, one two component flexibilised epoxy, one acrylic
and one two-component silicone). Thess diaphragms separated the two
compartments of a plastic box (actually two Tupperware boxes bolted
together with an intermediate machined holder for the diaphragms). I put
water in one compartment (below the level of the hole) and a weighed new
silica gel pack in the other. I left it for a given length of time (I
think it was a week, but am no longer sure, as it was nearly 40 years
ago) and weighed the pack. The silicone coating was very much worse than
any of the others for humidity permeability, but I forget the details.
The single-component PU was much better, but next in pecking order. The
rest had all very little diffusion across the diaphragm in the time
scale of the experiment. I thought I had reported the details of this in
my book, but could not find them today. Must have been somewhere else.

A word of warning: this experiment was done decades ago. It needs
repeating with modern coatings.

We also did some shock transmission experiments with different coatings
(the application was for portable equipment that was expected to be
subjected to severe shocks under all climatic conditions). I did this by
successively casting layers to form a block about 5 cm cube (to ensure
full curing), dropping a large bearing ball from 30 cm onto the top of
the cube, held rigidly, by the sides in a jig and measuring the
displacement at the bottom with a differential transformer system
connected to an oscilloscope. The silicone was, by far, the worst for
transmitting the shock, both by displacement and acceleration. I think
the 2-component PU was best, if my memory is still OK.

Same caveat.

We concluded that no silicones would be used on any of our assemblies.

Brian

Steve Gregory wrote:
> Hi All!
>
> I know when you look up properties for Silicone conformal coating, it
> usually states that it has good moisture resistance, but is it really
> that good?
>
> I've heard that it is somewhat permeable to moisture, that if you were
> worried about moisture, and the assembly were going to see the outside
> environment, you would be better off with an acrylic, or urethane coating.
>
> Any thoughts?
>
> Thanks!
>
> -Steve Gregory----------------------------------------------------
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