Due to its cost-effectiveness and excellent painting results, powder coating with solvent-free paint is applied in a variety of ways, e.g. when manufacturing household appliances and in automobile production. In this process, a mixture of air and paint particles that are electrically charged by a high voltage is conducted from an electrostatic field onto the specimen that is to be painted. The coated specimen is then heated in a furnace so that the paint particles will melt and form an even lacquer coat.
When the highly flammable powder-paint/air mixture in the paint spray gun flies past the high-voltage electrode, care must be taken to ensure that no potential incendive electric discharges from the electrode create an explosion. In the past, this was achieved by an ignition test of the paint spray gun in an explosive methane/air mixture. Today, the discharge energy is determined throughout the world by an exclusively electrical method according to a suggestion made by Ulrich von Pidoll using a special electrode that is connected to an oscilloscope. The operational reliability of the powder spray gun is guaranteed when its maximum possible discharge energy is lower than the minimum ignition energy (MIE) of all the coating powders that are used in practice.
For this purpose, Dr. Ulrich von Pidoll and Dr. Helmut Krämer of PTB determined the minimum ignition energy of a large number of different coating powders and systematically examined the physical and chemical properties on which the MIE depended. They developed a general formula that permits a sufficiently accurate calculation of the minimum ignition energy of any coating powders. For this work, they were awarded the Helmholtz Prize in the field of "Physical and Chemical Safety Technology" in 1993.
During these experiments, the powder to be examined was air-blown at a pressure of 20 bar via an annular nozzle into a pre-evacuated ignition vessel. A homogeneously dispersed powder cloud developed in this explosion-pressure-proof spherical vessel. Subsequently, an ignition spark of variable energy was generated within the ignition vessel by means of a triple spark gap and the minimum value of the spark energy was measured, i.e. the value which was just sufficient to ignite the powder/air mixture.
An essential problem that had to be solved was that coating powders do not consist of equally sized particles but that they exhibit a particle size distribution. It was therefore necessary to systematically investigate the way in which this distribution has an effect on the measured minimum ignition energy. The experiments showed that two coating powders which only differ with regard to their particle size distribution exhibited equal minimum ignition energies if the specific surface area of the powder particles was equal. A coating powder consisting of medium-sized particles thus has the same minimum ignition energy as a coating powder consisting of large and small particles if both coating powders exhibit the same specific surface area.
The measured minimum ignition energy of a coating powder was inversely proportional to the size of the specific surface area. A coating powder consisting of small particles was therefore easier to ignite than a coating powder of large particles. The chemical composition and the proportion of non-flammable substances also played a major role. The absolutely lowest value for the minimum ignition energy was measured for epoxy clear coats (transparent varnishes) and amounted to 1.7 mJ. The two research scientists suggested using this minimum value for the ignition tests of spraying devices for powder coating. This value as well as the above-mentioned formula are today part of the European standards.
