Center for Light and Environmentally-Friendly Structures ZELUBA®

Research project

Improving the weather resistance of fire protection coatings

The performance of a wood coating is highly dependent on the binding agent. Careful selection of the binder can provide control over, for example, the elasticity, the adhesion, the interlocking and the water absorption. The weather resistance of an intumescent fire protection coating depends not only on the binder, but also on the intumescent aggregate mixture. By using durable components to encapsulate sensitive components, the hydrolysis resistance of the aggregate mixtures can be increased. The intumescent process should not be thereby restricted. 

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.

© Fraunhofer WKI
Fig. 1: DSC curves from APP I coated with wax.
© Fraunhofer WKI
Fig. 2: Comparison of the fire performance of the coating with APP coated with the reference formulation.

Through the integration of the endothermic melting peak, it could be determined that with an increasing wax concentration, more energy is required in order to melt the “wax shell” (Fig. 1).

By coating with wax, the water solubility of the APP Type I was considerably lowered. The water solubility is 0.46% with a wax proportion of 2%. When the wax proportion in the hybridizer process is increased to 5%, the water solubility sinks to 0.32%.

In laboratory tests, the best result was achieved by the fire protection coating with 1% wax which was aged in a climatic cabinet. The temperature curve is under the curve of a reference formulation with uncoated APP. The third curve shows the fire performance of a non-climatically stressed formulation (Fig. 2).

The encapsulation of hydrolysis-sensitive materials through the application of wax is a very promising approach towards improving weather resistance. In the coming months, further investigations on this subject will be carried out at the Institute. Furthermore, chemical incompatibilities can be overcome through the manufacture of hybrid material combinations using encapsulation. Chemical incompatibilities, such as the acid sensitivity of carbonates, have led to certain desirable combinations of substances being impossible. As a gas producer, sodium hydrogen carbonate appears to be particularly promising.

Funding

Internationaler Verein für Technische Holzfragen (iVTH) e.V.

(International Association for Technical Issues Related to Wood)