Fire retardant coatings: “Preventing the fire from spreading
When do we use which fire protection coating?
Klaus Bender: There are very different coatings, depending on what you want to achieve. If a fire is to be prevented from developing, a coating is used that prevents the inflammation of building materials. This concerns combustible undergrounds, thus above all wood. This should then protect against a source of ignition such as a short circuit or a discarded cigarette butt. Here we have a comparatively low requirement level, which concerns the temperature effect.
We find another situation when it burns and a fire spreads by causing building materials, office materials and plastics to burn. This has become a big issue in our homes, as we find a lot of plastics here today. The inflammations progress relatively fast and the smoke development is comparatively strong. Such a fire then spreads through openings into other rooms and we quickly have to deal with a major fire, such as the Grenfell Tower in London a few years ago.
If you want to protect the entire building from collapse in such a case, you need stronger weapons. Then intumescent coatings are used to protect the steel structures in particular. These coatings work by foaming in the event of a fire and forming a protective foam that provides very good insulation against the high temperatures.
And what raw materials do the manufacturers of fire protection coatings use?
Bender: In the first case, when the paint is simply to be protected from ignition, ammonium polyphosphate or aluminium trihydrate (ATH) can be used. The hydrates form water vapour and can thus lower the temperature of the incipient fire. Ammonium polyphosphates also form a carbon layer on the surface and react with the ingredients of the paint, shielding the building material from the oxygen supply and preventing the fire from spreading further.
Klaus Bender is head of Technical Marketing Intumescent Coatings at Clariant. He will also lecture a seminar on the basics of flame retardant coatings for formulators and other coatings professionals.
Intumescent coatings are designed to provide passive fire protection. For comparison: Active fire protection would be a sprinkler system. Passive fire protection, on the other hand, is simply present and acts reactively in the event of a fire without having to be activated.
Here we also find ammonium polyphosphates as the main component to a large extent. This intumescent effect then needs further ingredients, which have to be dosed into the colours exactly.
Halogenated fire retardants are also still used. What is the current situation here?
Bender: Halogenated fire retardants, whether chlorinated or brominated, still play a relatively important role, especially in plastics. In the event of a fire, the halogens are split off and act as radical scavengers, i.e. they deactivate high energetic radicals, e.g. hydroxyl radicals and hence slow down the chain reaction in the fire scenario.
The disadvantage is that in case of fire hydrogen halide, i.e. hydrochloric acid, is formed as well as other toxic gases. Therefore, the halogenated fire protection agents are more and more under observation and partly already forbidden. This applies especially to PU base materials, where TCPP may no longer be used in the future. But there are alternatives that can be used. Here at Clariant, for example, we offer phosphorus-based fire retardants that have the same effect.
Which test methods are currently used for fire protection coatings?
Bender: The tests that have been developed to simulate the fire are as varied as the fire itself. This starts with small flames with attractants, where one tries to ignite a small surface. Here you usually only get a simple result, which then means passed or failed.
Modern methods additionally generate various physical test data such as combustion energies or oxygen consumption. The latter is measured using a cone calorimeter, which we also use here at Clariant, where a comparatively small sample of 10 x 10 cm is used.
The oxygen consumption is also required in the European SBI test (Single Burning Item), which is now the standard building material classification test for the whole of Europe. The old test standards and the old test methods still exist. In Germany, for example, this would be the “Brandschacht-Test”, in England the Radiant Panel Test or in France the Epiradiateur, who basically all want the same thing, namely the assessment of a building material with regard to its burning capacity. These tests were actually taken off the market in order to obtain a uniform test for the whole of Europe.
How do the tests work from the perspective of a paint manufacturer?
Bender: Normally, one uses a reference substrate from which one knows the fire behaviour, for example a chipboard. The varnish is applied to this and the coated and uncoated test specimens are compared. And, of course, the coated sample must not perform worse than the uncoated one.
It is difficult to test a paint film yourself, because it would have to be so thick that the results are no longer realistic. There are also tests for textiles and plastics, for example. In the case of plastics, for example, a glowing wire is used, which is then placed on the surface and you can see whether it starts to burn or whether something drips off burning. The characteristics of the tests are therefore very different and tailored to the respective application.
There are standards that are globally available and recognised, for example in the plastics or automotive sectors. But there are also many that are only applied locally, for example in Russia, South Korea or Japan.
What trends are they observing in the field of fire protection coatings?
Bender: We see a general trend towards water-borne formulations. Of course, we also have to provide answers in the area of fire retardants. Ammonium polyphosphate, for example, can also be used in aqueous systems, but there are already special requirements, such as viscosity behaviour, which is different for aqueous formulations than for solvent-based ones.
Where water-borne systems cannot be used, there is a tendency towards 2K-systems which, for example, have very good properties under harsh conditions. High demands have to be met, for example, in terms of durability.
The interview was conducted by Jan Gesthuizen
At the European Coatings Seminar Flame retardant coatings on 22 May in Amsterdam you will discover the underlying chemistries of fire-retardant coatings. This seminar provides a comprehensive overview of current European fire standards and laws.