Ammonium polyphosphates (APP) are applied as flame retardant additives. Thermal decomposition results in the release of ammonia. Polyphosphoric acid forms an insulating, glass-like surface and leads to a reduction in the oxygen concentration. Furthermore, carbon and hydrogen-rich material in intumescent fire protection coatings is dehydrated by the released inorganic acids. APP exist in six crystal forms. The crystal forms I and II are commercially available. They differ from one another in the chain length. APP I, with a lower chain length, demonstrates a higher level of water solubility and a lower thermal stability than APP II. APP I is widely used in the paint industry. The water solubility of APP II can be considerably reduced by applying a coating. APP II are commercially available with coatings from, for example, silanes, melamine and various resins. The hydrolysis-sensitive APP types Phase I are not available in coated qualities. APP Type I should therefore be encapsulated using nanoparticles, in order to significantly reduce the hydrolysis sensitivity. A wide range of substances has been subjected to screening tests regarding their suitability as shell particles (e.g. waxes, TiO2, SiO2).
A suitable binder must demonstrate good compatibility towards a high pigment volume concentration as well as towards salts. Five commercial acrylates were examined. According to the technical data sheets, the tested systems had a solids content of 40-50%, a pH value of 7-9, a minimum film-binding temperature of 0-25°C and a water absorption of 5-15%.
During encapsulation of the aggregate mixtures, a core particle (1-500µm) is enclosed by a shell particle (0.1-50µm). The application of the shell particle to a core particle is carried out solely through mechanical mechanisms in accordance with the hybridizer principle. For this, APP I is ground and sieved. The fraction from 20 to 50µm is coated with a wax (5µm) with a lower melting temperature (82°C). The wax is incorporated in concentrations of 1 to 5%. The quality of the encapsulation is then determined through DSC analysis and water solubility tests.
Coated APP and other intumescent aggregates with a lower water solubility were incorporated into an optimised water-based recipe. These recipes were then applied to metal and plywood sheets measuring 19cm x 19cm.
Following a drying period of one week, a topcoat was applied. The test specimens were subsequently aged for a period of one week at 20°C and 98% relative humidity. The fire behaviour was tested using WKI methods under laboratory conditions.