Study on atomization mechanism in spray coating of organic paint mixed with high-pressure carbon dioxide as a diluting solvent

The atomization phenomenon in spray coating of organic paint mixed with high-pressure carbon dioxide as a diluting solvent was investigated. Industrial organic paint for automotive body coating was used without any diluting organic solvent. Under high-pressure conditions, CO₂ was continuously mixed with the organic paint to reduce its viscosity to spray. The atomization phenomenon was investigated regarding the effects of high-pressure spraying and CO₂ vaporization from droplets. The experimental conditions were varied such as the temperature, pressure, GtP (gas to paint ratio: CO₂ gas flow rate [g/min]/paint flow rate [g/min]). The average droplet sizes under the CO₂ spray at the 150 mm from nozzle (actual spray distance for automotive manufacturing) were measured by using a laser droplet size analyzer. The average droplet size was strongly affected by the fluid conditions (temperature and GtP), and the sizes were varied from 12 to 113 µm under conditions of 20–60 °C and 8–16 MPa. The minimum size of average droplet was 12 µm at 60 °C, 14 MPa, and 0.5 of GtP. The moderate heating (40–60 °C), high pressure (12–16 MPa) and two-phase (CO₂-saturated paint and high pressure CO₂) fluid spraying under high GtP condition in the nozzle are important factors to produce small and homogeneous droplet.