The long history of mankind’s struggle against fire

The discovery of fire is often presented as the most important in the history of Man, as it has conditioned the development of the genre Homo. By reducing the amount ofenergy necessary for the digestion of food, cooking has in particular led to an increase in the brain. mastery of fire seems to have been acquired about 400,000 years ago, even if much older traces of use have been identified. However, with urbanization, fire has also become a plague if it spreads unchecked. Consider, for example, the great fire of rome in 64 AD to that of the Notre-Dame de Paris cathedral or even to mega fires which are now ravaging many countries.

What is fire?

A fire requires the conjunction of three elements: a combustiblea oxidizer and a source of heat, the so-called triangle of fire. These elements interact in a complex process in which phenomena occur. physical such as heat transfers and chemical phenomena such as pyrolysis from the fuel source or the combustion pyrolysis products.

Technically, we distinguish between reaction and resistance fire. Reaction to fire concerns combustible materials, which are likely to release heat during their decomposition under the effect of temperature and in the presence of an oxidizer (usuallyoxygen present in theair). Fire resistance is concerned with the ability of an element to maintain its load-bearing function and its properties ofthermal insulation and D’sealing to gas and smoke during the fire. Inasmuch as material fuel used as a structural element in buildings, wood is concerned by these two aspects which require specific standards and different tests.

In matter firefighting, there are two non-exclusive strategies. The first provides for the use of so-called active devices in the event of a fire: fire extinguishers, smoke detectors or automatic water extinguishers. The second is to use materials that will contribute as little as possible to the spread of the fire.

fireproofing

Many materials, such as most plastics or wood are intrinsically highly combustible, and it is necessary to incorporate additives called flame retardants, which, incorporated into or on the surface of a combustible material, are intended to modify its behavior by disturbing the fire triangle.

Their effects are mainly to delay the appearance of the flame, to slow down the speed propagation of the latter, reduce the release of heat and the power of the fire, limit the opacity of the smoke and its toxicity. All these effects are assessed through standardized reaction to fire tests. They lead to classifications which determine the potential use of the material in a application given according to the regulations.

There is no universal flame retardant. A fireproofing system must be adapted to the material it aims to protect, taking into account in particular its decomposition process. Furthermore, the choice of a flame retardant is also guided by the manufacturing process of the material and must not significantly alter the expected functional properties.

Archaeologists place the beginnings of fireproofing in antiquity. The Egyptians, around 400 BC. J.-C., used minerals to make certain fabrics fire resistant such as cotton or linen. Later, during the siege of Piraeus (23 BC), solutions ofalum were used to make the wooden ramparts fire resistant. It was then necessary to wait until June 18, 1735 for the Englishman Obadiah Wyld to file the first patent, patent number 551on the treatment of cotton. In the 19th centuryat the request of the King of France, Louis XVIII, an effective system had to be found to prevent fires in Parisian theaters lit by candles. Joseph Louis Gay-Lussac then filed a patent on the use of a mixture of ammonium phosphate, ammonium chloride and borax for the fireproofing of curtains in theatres.

flame retardants

There are several families of flame retardants, based on chemical elements different and with different modes of action. Historically, the halogenated molecules containing chlorine or bromine, have been widely used because of their effectiveness, even in small quantities. These molecules act by disturbing the combustion reactions taking place in the flame, favoring its extinction and limiting the amount of energy released. We then speak ofinhibition of flame. However, the toxic nature of certain halogenated compounds has led to their banning. Due to the impossibility of easily distinguishing when recycling them brominated molecules authorized from those which are prohibited, it is no longer possible to recycle plastics fireproofed by these flame retardants. Furthermore, these molecules lead to the formation of opaque and corrosive fumes during a fire. For all these reasons, this family of flame retardants is now increasingly in the hot seat.

It is mainly replaced by flame retardants phosphorus. These are of a very wide variety and, therefore, they can act according to different modes of action. However, the main mode of action remains the promotion of a residual layer on the surface of the fuel protecting the sound part of the material. The strategy consists of disrupting the pyrolysis reactions (decomposition of the material under the action of heat) and promoting the formation of a residue rich in carbon and thermally stable called “char”. Some particularly effective systems are called intumescent because the tank forms an expanded, insulating and very protective layer. This type of intumescent system is used in particular in coatings to protect metal elements or wood.

We can also mention metal hydroxides, which are inexpensive but proportionally less effective and which must therefore be incorporated at high rates (up to 65% in mass in outer cable ducts) to produce a noticeable effect. Under the effect of temperature, these particles release water in the form of vapor by decomposition endothermicthus helping to cool the material and dilute the combustibles in the flame.

others chemicals exist, based on thenitrogen (melamine), the boron (borate of zinc) or thetin (hydroxystannate) for example. The nanotechnology have also been used for fifteen years in the field of fireproofing. The nanoparticles of the clay type lamellar Where carbon nanotubes promote the insulating character of the char formed, even at low rates. But they are insufficient on their own to provide overall protection for the material.

And the wood?

In general, materials of organic origin (from the living world) such as oilwood, or coal have in common a composition rich in atoms carbon andhydrogen, likely to be oxidized. They are therefore combustible. Wood is a material with a complex structure with an elementary chemical composition consisting half of carbon (50%), oxygen (44%), and a small amount of hydrogen (6%).

Not very dense, the wood has a natural ability to char, ie a protective layer of char forms between the healthy wood and the flames. During its combustion, the wood will first lose water to become completely dry at 120°C. Then its structure gradually breaks down with increasing temperature. Its constituents are relatively stable up to 250°C, the temperature above which a release of smoke is observed. At 320°C, the quantity of gas is such that it can ensure theinflammation wood in the air. Pyrolysis mainly takes place up to 500°C, after which only the charcoal (char) remains, which can slowly decompose by oxidation. If the char layer slows down the pyrolysis of the sound underlying wood, its mechanical resistance is on the other hand negligible. As pyrolysis progresses, the useful section of a wooden structural element is therefore reduced and so is its bearing capacity.

The flame retardants used for the fireproofing of wood belong to the families mentioned above (phosphorus, boron, nitrogen, metal hydroxides). However, unlike plastics, it is not possible to integrate these additives during the manufacture of wood. Fireproofing therefore takes place in two forms: the deposition of a surface coating (paint, varnish) and theimpregnation in the heart of the wood, i.e. in the hollow part – called lumen – wood cells, by a process in autoclave. This involves filling all of the lumens by first degassing under vacuum and then forcing the penetration of the flame retardant by overpressure. This more complex solution makes it possible to avoid a deterioration of the character fire retardant in case of surface defects. In the case of a coating, if it is altered, it can no longer play its fireproof role and leaves the wood unprotected in the event of a fire.

This article was co-written with Clément Lacoste (IMT – Mines Alès), Laurent Ferry (IMT – Mines Alès) and Henri Vahabi (University of Lorraine).

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