Jean Francois,
Blushing is generally caused by high humidity. It is usually worst in spray
applications, but can become an issue in dipping/brushing applications as
well.
Essentially, as the solvent starts to flash-off, the evaporation causes
cooling (cf. acetone on your hand!), which then causes some of the saturated
moisture vapour to be condensed onto and into the coating. Atomisation
causes more cooling and hence greater problems with blushing.
You could try adding a retarder solvent, but a much better solution would be
to try and reduce your humidity. You could try using a portable
de-humidifier and held-held probe to try and maintain a max RH of about 60%,
but aim for 40%.
Hope this helps,
Best Regards,
Phil Kinner
(Masquerading as Alan Brewin!)
----- Original Message -----
From: Bissonnette, Jean-Francois
To: [log in to unmask]
Sent: 18 June 2001 16:35
Subject: [TN] Conformal coating: white haze
This is a question for all the conformal coating specialists of this
world.
We're experiencing some problems with the application of conformal coating
in high humidity
conditions. Just after the application is finished, some kind of white haze
appears in the coating.
It disappears a little be during curing but some of it stays. The finish of
the coating is then a little
less smooth and shinny than normal.
In order to avoid excess of humidity, the boards are pre-heated for 24h.
The coating used is
acrylic.
Anyone knows what causes this?
Jean-François Bissonnette
Contrôle de la Qualité, Électronique
Quality Control, Electronics
Vapor Canada Inc.
10655 Henri-Bourassa O.
St-Laurent, Qc
H4S 1A1
(514) 335-4200 Ext: 2021
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