You see - there are all the "Depends" you could wish for - despite the poor spelling. Only one little extra titbit - urethanes often/usually contain isocyanates that would require/mandate you to provide the operators with artificial respiration equipment, not entirely dissimilar to what Buzz Aldrin had to wear on the moon! I exaggerate just a tad. Graham N On 9 Jun 2008, at 13:53, Douglas O. Pauls wrote: > 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 > Please respond to > TechNet E-Mail Forum <[log in to unmask]>; Please respond to > Mike Johnson <[log in to unmask]> > > > To > [log in to unmask] > cc > > Subject > [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 > > > > > > > *** > The information in this e-mail is confidential and intended solely > for the > individual or entity to whom it is addressed. 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