The intumescent paint works in the following way: when the temperature is increased, the structure of colour, with the help of catalysts such as phosphoric acid, expands and forms the carbon foam which expands in a ratio of 1:50. A typical layer 1 mm in thickness thus expands to a thickness of 50 mm. The usual dry film thicknesses depend on the individual requirements and may range from between 100 to 5000 microns. As the carbon foam is a good insulator, the speed of heat energy transfer to the steel beam is reduced and, as a result, the time in which a dangerous overheating occurs is significantly extended.
Regarding the limitations when using the fire stopping paint, we may take into consideration their sensitivity to the application conditions, for example when the rust is not sufficiently sand-blasted from the beams, when the paint is applied at too low a temperature (at temperatures below 10 °C the application is practically no longer possible), in rain or at dew point or when an incompatible basic top coat is used. They shall be applied by using the technology of airless spraying or manually by using rollers. For higher levels of fire resistance several layers are required. The application time might be a limitation as it should be taken into consideration that the intermediate drying of layers is required (usually 24 hours). Intumescent paints have a limited guaranteed service life and are relatively sensitive to abrasion and impacts. The fire resistance of intumescent paints may range from R 30 to R 180. It is the most advisable to use R 60 as the maximum. Above that value the number of layers and the thickness are irrational and also actual reliability in case of a real fire decreases as the layers fall off. Furthermore, it is necessary to mention the particular feature at testing the fire stopping paints, namely, the tests performed at opened profiles I, H and U are not valid on hollow profiles and vice versa. It should be checked previously if the manufacturer has one or the other test or both at the same time. In order to be allowed for use in the EU, the intumescent fire stopping paints should be tested according to EN 13381-8, their combustibility should be classified according to EN 13501-1 and they should be classified for resistance class according to EN 13501-2. They should also have the European Technical Assessment (ETA) according to ETAG 018, the certificate of constancy of the product (COC), which all together permits issuing the CE mark, or any other proofs in accordance with the Regulation on Construction Products.