Stealing fire from combustible material

Fire-retardant coatings could be one option for providing enhanced protection to buildings during a wildfire. A study by the Insurance Institute for Business and Home Safety, South Carolina (US), showed that weathering exposure reduced the effectiveness of fire protection of intumescent coatings, but the weathered coatings did not act as additional fuel. Pic courtesy: Insurance Institute for Business & Home Safety

With wood construction on the rise in recent years – ranging from sustainable wooden apartment buildings, libraries and schools – regulations regarding wood construction, including fire safety, have been updated over the years.

According to Tikkurila, a leading Finland-based paint company, wood can be used as a construction material without fire protection treatment. However, depending on the site and customer requirements, treatment can make the building safer.

This extra protection enables a wider use in schools, kindergartens and assisted-living facilities (old age homes); offices, hotels and libraries; storage facilities and extra floors with wooden frames.

Fire retardants slow the ignition of wood and the spread of fire, while also creating less smoke than untreated wood. This provides valuable extra time for people to escape and for emergency services to reach them, while reducing the effects of smoke inhalation.

Intumescent coatings

When exposed to heat, fire-retardant material expands and creates an insulating foam layer on wood. The porous foam layer insulates the wood from the flames, preventing the wooden surface from heating up rapidly or catching fire.

This is known as intumescent technology: fire protection based on the paint surface expanding when exposed to heat. Compared to salt-based fire-retardant products, surfaces treated with intumescent technology can better withstand moisture, UV light, and weather.

In Europe and much of North America, construction materials are labelled with the following markings (with the exception of flooring):

A1: materials that do not contribute to fire at all (non-combustible materials)

A2: materials with extremely limited contribution to fire

B: materials with very limited contribution to fire

C: materials with limited contribution to fire

D: materials with acceptable contribution to fire

E: materials with acceptable fire behaviour

S: materials with no defined type of fire behaviour

S1: very low smoke emission

S2: low smoke emission

S3: smoke emission does not meet S1 or S2 requirements

D0: no flaming droplets or particles

D1: flaming droplets or particles burn out fast

D2: production of flaming droplets or particles does not meet D0 or D1 requirements

Emission norms

If a fire-protected surface needs to be abrasion resistant, it should be finished with a top lacquer or paint. For claddings, a topcoat should always be used to make sure the fire protection system can withstand exposure to the elements.

Indoors, a fire-retardant lacquer alone is enough for wood surfaces that are not subject to wear, such as ceilings.

Most fire-retarding coating products are developed, and primarily intended, for industrial painting. In addition to vacuum application, spraying, and curtain coating, the products can be applied onto small surfaces with brushes or rollers.

Most modern fire-retardant products contain no ingredients that are hazardous to humans. And since they are waterborne, they cause less emission and are environment-friendly and safe to use.

However, there are very different building materials that can also burn and cause a fire to spread, especially office materials (including stationery) and plastics. The inflammations progress relatively fast and the smoke development is comparatively strong.

Raw materials

What raw materials do manufacturers of fire protection coatings use? According to Clariant AG, a Swiss speciality chemicals company, 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.

But the intumescent effect needs further ingredients, which have to be dosed into the colours exactly.

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. They deactivate high energetic radicals and slow down the chain reaction in the fire scenario.

The disadvantage is that in case of fire, hydrogen halide (hydrochloric acid) is formed, as well as other toxic gases. Therefore, halogenated fire protection agents are being increasingly forbidden. This applies especially to PU base materials.

Oxygen consumption

Modern methods additionally generate various physical test data such as combustion energies or oxygen consumption. The latter is measured using a cone calorimeter.

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.

There are also tests for textiles and plastics. 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 each material are very different, and fire-retardant coatings tailored to the respective application.

There is a general trend towards water-borne formulations. 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.

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