The discovery of fire is often presented as the most important in the history of man, so much did it condition the development of the genre Homo. By reducing the amount of energy needed to digest food, cooking has notably led to an increase in the brain.
Lmastery of fire seems to have been acquired around 400 years ago even if much older traces of use have been spotted. However, with urbanization, fire has also become a plague if it spreads unchecked. Let's think for example to the great fire of Rome in 64 AD to that of Notre Dame Cathedral in Paris or even to the mega-fires that now ravage many countries.
What is fire?
A fire requires the conjunction of three elements: a fuel, an oxidizer and a heat source, which is called the fire triangle. These elements interact in a complex process involving physical phenomena such as heat transfer and chemical phenomena such as pyrolysis of the fuel source or combustion of pyrolysis products.
Technically, a distinction is made between reaction and resistance to 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 (most often oxygen present in the air). Fire resistance concerns the ability of an element to maintain its load-bearing function and its thermal insulation and gas and smoke tightness properties during a fire. As a combustible material used as a structural element in buildings, wood is concerned by these two aspects which call for specific standards and different tests.
When it comes to firefighting, there are two strategies that are not mutually exclusive. 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.
Many materials, such as most plastics or wood, are inherently 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 change its behavior by disrupting the fire triangle.
Their effects are mainly to delay the appearance of the flame, slow down the speed of its propagation, 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 given application 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 locate the beginnings of fireproofing in antiquity. The Egyptians, around 400 BC. J.-C., used minerals to make certain fabrics resistant to fire such as cotton or linen. Later, during the siege of Piraeus (23 BC), alum solutions 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 551, on the treatment of cotton. In the XNUMXth century, at 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.
There are several families of flame retardants, based on different chemical elements and with various 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 disrupting the combustion reactions taking place in the flame, promoting its extinction and limiting the amount of energy released. This is then referred to as flame inhibition. However, the toxic nature of certain halogenated compounds has led to their banning. Due to the impossibility of easily distinguishing during recycling the brominated molecules authorized from those which are prohibited, it is no longer possible to recycle plastics fireproofed by these flame retardants. Moreover, these molecules lead to the formation of opaque and corrosive smoke during the fire. For all these reasons, this family of flame retardants is now increasingly in the hot seat.
It is mainly replaced by phosphorus flame retardants. 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 carbon-rich and thermally stable residue called “char”. Some particularly effective systems are called intumescent because the char 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 therefore must be incorporated at high rates (up to 65% by mass in outer sheaths for cables) to produce a noticeable effect. Under the effect of temperature, these particles release water in the form of vapor by endothermic decomposition, thus helping to cool the material and dilute the fuels in the flame.
Other chemicals exist, based on nitrogen (melamine), boron (zinc borate) or tin (hydroxystannate) for example. Nanotechnology has also been used for fifteen years in the field of fireproofing. Nanoparticles of the lamellar clay or carbon nanotube type promote the insulating character of the char formed, even at low levels. 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 oil, wood, or coal have in common a composition rich in carbon and hydrogen atoms, 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 ignite the wood in the air. Pyrolysis mainly takes place up to 500°C, after which only 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 impregnation at the heart of the wood, i.e. in the hollow part – called the lumen – of the cells of the wood, by an autoclave process. 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 flame retardant character in the event 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).
Rodolphe Sonnier, Assistant Master of the Schools of Mines, IMT Mines Alès – Mines-Telecom Institute
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