Mike,
As others have said, the two don't mix.  I have tried to clean silicones 
from spray equipment and you can never get it down enough such that it 
does not cause fisheyes and other coating problems.  Save yourself a whole 
lot of problems and simply get new equipment.  If you cannot go this 
direction, then I would suggest that you use naphtha as a cleaning 
solvent.  It seems to be more effective than other solvents on silicone 
residues.  Then follow the naphtha cleaning with alcohol cleaning.  You 
would probably need to clean your exhaust stacks too.  Replacing might be 
easier and cheaper in the long run.

You did not say what kind of silicone you were going from (solvent vs 100% 
solids) and what kind of urethane you are going to (solvent-borne (notice 
the proper spelling) vs. two part reactive).  Where you may not have had 
an environmental exhaust permit issue before, you may have one now if you 
are going to a VOC-based solvent urethane.  Check with your local EPA or 
DNR.

Someone else brought up the dreaded "O word" - OSHA.  Check what personal 
protective equipment you will need.   Many of the urethanes have higher 
health impact ratings than silicones. 

Urethanes and silicones have dramatically different properties in terms of 
chemical resistance, thermal performance and rigidity.  Have you checked 
with the designers to determine if there is any impact to form, fit or 
function that this change represents?

Urethanes and silicones flow differently, have you figured in the time to 
retrain your operators on the new methods? 

Urethanes and silicones look and feel different in their finished form. 
Are you ready for the change in workmanship standards?  Have you accounted 
for the re-training time for your inspectors?

Urethanes and silicones have completely rework and repair procedures. 

What does the end user think about the change?

Are you ready for the culture change?  The material switch is relatively 
easy.  It's changing the culture that is a cast iron *^%*#^(*#.

Doug Pauls




Mike Johnson <[log in to unmask]> 
Sent by: TechNet <[log in to unmask]>
06/06/2008 10:04 AM
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[TN] Changing conformal coating from silicone to urethane...






We are considering changing our pcb conformal coating process from a 
silicone base material to a urethane base material.  Other than a thorough 
cleaning of our machine, replacing material delivery lines, complete 
disassembly and cleaning of nozzles, ordering new nozzles to meet 
viscosity requirements of new material if necessary, are there other 
concerns we should be addressing?

Mike






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