Ignitability of Spray Clouds of Organic Solvents, Solvent/Water Mixtures, and Water-Based Paints by Electric Sparks, Arcs, and Open Flames

The ignitability of spray clouds of organic solvents, solvent/water mixtures, and water-based paints by electric discharges and open flames was investigated under worst-case conditions that typically apply to electrostatic paint spraying. More than 200 substances were sprayed with a high-rotation bell spraying device. Rotation speed, substance throughput, and shaping air were optimized in order to achieve an individual ignition optimum for each substance. Organic solvents with flash points between 38 and 150degC showed a minimum ignition energy of about 7 mJ, independent of their flash point. Solvent/water mixtures and water-based paints typically had much higher minimum ignition energies from approximately 75 mJ and upwards. The substances were divided into three groups according to their ignition behavior: (1) not ignitable by an open flame; (2) not ignitable by an electrical discharge of 4 J; and (3) all other substances. Simple formulas were given for water-based paints and solvents, thus allowing an easy classification into these three groups. It is planned to state the necessary safety precautions for each group in future standards on electrostatic paint spraying.