Paints & Coatings Resource Center

Ask the Expert Question-and-Answer Archive

by Ron Joseph

August, 2007

RTV Spray Particle Size

Q: My company runs an airless paint spray booth where we coat a silicone rubber called RTV dissolved in Xylene onto industrial equipment.  I am modifying the existing air permit for the paint spray booth which has an attached carbon adsorber and particulate prefilter (the prefilter is first and the carbon second).  One of the difficult points that I must prove is the efficiency of the prefilter.  I know form practical experience that greater than 99% of all solids are removed by the prefilter, however, the filter design is based on solid particle size, and the testing only goes up to 10 microns, and the manufacturer’s stated efficiency at that size is 83%.

The nozzle is a Nordstrom flanged cross-cut nozzle 0.012 / 20 (equivalent diameter = 0.0155 inches)

Because our application sprays a polymer, I am fairly certain that the particle size of the RTV droplets is greater than 10 microns, but I can not measure this because the polymer will trash any measuring equipment I use.

Is there any calculation or published literature, perhaps from the automotive industry, or military coatings, that may give me a more refined range for particle size for this application.  This information is important to completing the permit application.

Any assistance you could give would be appreciated.

A: EPA Method 319 “Determination of Filtration Efficiency for Paint Overspray is the recognized method that filter manufacturers are required to perform when evaluating the filtration efficiency for the Aerospace NESHAP regulation. I would imagine that your state agency will accept this method since it was specifically devised for this purpose.  Rather than use paints this method uses both a liquid and powder “challenge”.  “The liquid test aerosol is reagent grade, 98 percent pure, oleic acid. The solid test aerosol is (potassium chloride) KCl aerosolized from a solution of 20 percent KCl in water. In addition to the test aerosol, a calibration aerosol of monodisperse polystyrene latex (PSL) spheres are used to verify the calibration of the (Optical Particle Counter) OPC.”

Your state agency might allow you to use the RTV silicone rubber as the challenge. Perhaps you can retain the services of one of the test labs that perform this efficiency test.  Your filter manufacturer should be able to track down a lab that performs this test.

Perhaps a simpler method is to place a glass panel on the front face of the pre-filter and capture the RTV particles. Measure the diameter of the largest particles to confirm that they exceed 10 microns.  If you need assistance in doing this, please get back to me as I might be able to get the work done for you.


Ron Joseph

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