David,

Why do you want to change? Silicones are generally more expensive materials
and typically require a greater thickness/mass to achieve the same
protection performance as an Acrylic.

As a general rule of thumb, if you were to look at the molecular level you
would find that an Acrylic or Urethane will have molecules the size of a
tennis ball. Silicones and Epoxies would be the size of basketballs.
Considering that all coating breathe and permit air, which may contain water
vapour, to migrate in and out of the film - then it follows that you should
require more coating with the silicone so you have a double disadvantage.

Furthermore, silicones generally swell in the presence of hydrocarbon
solvents. So your thought of poor performance in the presence of fuel oil
would probably be correct.

However, how much fuel oil? Vapour, mist or spray? Maybe a urethane would be
better.

If it is the environment that concerns you (rightly so!) then consider
coatings made up from the new LOC (VOC exempt) type solvents.

If you are proposing to use the same spray line for acrylics and silicones,
you will almost certainly create problems, specifically de-wetting. You
really need to look at dedicated feed lines for each and keep them that way!

Regards, Graham Naisbitt
__________________________________________________________________________

[log in to unmask]

Concoat Ltd
Alasan House, Albany Park
Camberley GU15 2PL UK                        http://www.concoat.co.uk

-----Original Message-----
From: David Paluck <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: 11 June 1998 17:28
Subject: [TN] silicone conformal coatings


>Hello.  We are considering a change in conformal coatings used on our
>PCB's.  Currently in use are a silicone elastoplastic resin flow coating
>and an acrylic spray coating.  We would like to switch to an automated
>coating process using a low-VOC silicone elastoplastic resin and eliminate
>the use of acrylics.  However, we expect to meet some resistance from our
>customers, especially those in the paint industry, who think a silicone
>coating will not perform as well in resisting paint and/or fuel oils.
>Also, is there a possibility of silicone causing flaws in paint? (e.g.
>coating with silicone resin a circuit board used in a paint sprayer.)  Any
>feedback would be appreciated.
>
>Erin Macy
>Electronic Systems, Inc.
>
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