The Behavior of Materials in Fire

This article was updated on the 3rd September 2019.

Lukas Gojda / Shutterstock

Careful measurement of the properties of materials is required for their effective and safe use. Until recently, only standard tests were possible to assess how materials behave in fire, and these only provided a simplistic grading. The data obtained from these tests can only apply to the behavior of the material inside the test apparatus, so it is rarely possible to use such data to calculate the performance of the material in its end use environment.

This situation has improved recently with the development of a range of measuring techniques for fundamental fire properties. This includes  ignitability, flame spread, heat release, smoke production and, toxic gas generation. The new tests provide data that can be used in calculations and mathematical models which allow the designer, regulator or specifier to select effective and safe materials for any end-use environment.

To appreciate the benefits of these developments in testing, it is necessary to understand what characteristics influence materials’ behavior in fire, the way fire grows, and how fire threatens life.

Thermal Inertia

Although the first property that usually comes to mind when considering fire is calorific value, the one which most influences initial fire behavior is usually thermal inertia. This property is closely associated with another important property, comfort.

Materials which are warm to the touch are widely used in buildings and vehicles where contact with skin occurs. If a material is warm to the touch it is because the heat from our skin is transferred to the material and quickly raises its surface temperature. This occurs in materials which have low density, low heat capacity and, low thermal conductivity. Conversely, materials with high density, high heat capacity and, high thermal conductivity tend to feel cold to the touch because heat from the skin is quickly conducted into the body of the material and only has a small effect on surface temperature.

Ignition

A more dramatic effect is observed if human body heat is replaced by an ignition source such as a match flame. It is difficult to heat the surface of materials that are cold to the touch to their ignition temperature, whereas materials that are warm to the touch are quickly raised to their ignition temperature and ignited to rapidly growing fire.

Unfortunately, materials with low thermal inertia are also highly desirable for other valuable applications such as heat insulation. The use of such materials is closely associated with human comfort requirements inside buildings and vehicles and, if not properly controlled, could represent a serious fire risk.

The Behaviour of Polymers in Fire

Some synthetic polymers are popular with manufacturers because they can often be formed into complex shapes using simple molding techniques. This useful feature depends on the materials being thermoplastic. However, what can be formed by heat can also be deformed by heat and this can lead to initially solid materials moving their position in a fire.

Deformation and melting can also lead to increased fire spread. Burning droplets can spread fire downwards and fire growth is substantially accelerated by the formation of pools of hot, melting liquid. This is a particular problem with domestic upholstery where thermoplastic deformation of polyolefin fabrics and the decomposition of polyurethane foams produce flammable liquids which accelerate fire growth.

Growth of Fires

Most people are familiar with the growth of fire in the open through experience with garden bonfires, but their first experience of fire inside a building would probably be in a life-threatening situation. The difference in growth mechanisms between these two types of fire is so great that previous experience does not prepare people for what happens when they encounter a fire inside a building.

The garden bonfire grows progressively as the flames from one item of fuel ignite the next, and heat and smoke rise harmlessly into the atmosphere. Inside a building, the growth of fire follows a different process and is much more dangerous. Better public understanding of the simple processes which control the growth of an indoor fire would greatly improve people’s chances of escaping safely.

Flashover

The fire inside a building initially grows from the first item ignited, just like the garden bonfire. However, the hot smoke this produces is trapped under the ceiling of the room and forms a smoke layer. As the fire progresses, this smoke layer becomes deeper and hotter as more heat is pumped into it from burning below. As the depth and temperature of this layer increase, some of its heat is released as thermal radiation spreads downwards.

If the temperature of the smoke layer rises sufficiently, the intensity of the emitted radiation will become so high that other materials or objects in the room heat to their ignition temperature and ignite virtually simultaneously. This phenomenon is referred to as flashover and produces a massive and immediate change in the size and hazard of the fire, fig. 1.

AZoM - Metals, Ceramics, Polymers and Composites: Flashover in a domestic room, showing how fire spreads and materials combust.

Figure 1. Flashover in a domestic room (shown before and after) occurs at a heat release rate of about 1,000,000 Watts.

Measuring Heat Release Rates

An important parameter in the induction of flashover is the rate of heat release from the initial burning item. In a small room, the heat release rate required for flashover is about 1,000,000 watts. This is a very high temperature, but it has been found that many domestic armchairs can quickly produce rates of heat release above  of this temperature.

Measurements of the rate of heat release in materials can be achieved using oxygen consumption calorimetry. In this method, all combustion products from the burning items are collected by a canopy and passed into a duct in which the volume flow and oxygen concentration are accurately measured. In this way, the amount of oxygen consumed can be measured on a second-by-second basis. For most materials, the heat released for each kilogram of oxygen consumed is 13 megajoules (MJ), plus or minus 10%.

The full-scale fire calorimeter at the Fire Research Station, Borehamwood (fig. 2) has a tall chimney which extracts smoke through natural buoyancy. Most modern calorimeters, however, employ a horizontal duct and a powerful fan capable of extracting about 5 m3.s-1.

AZoM - Metals, Ceramics, Polymers and Composites: full scale fire calorimeter for testing fire resistance of materials.

Figure 2. A full fire scale calorimeter.

While full scale calorimeters determine the heat release from a product or a system, small calorimeters exist which can measure the rate of materials’ heat release on a unit area basis. The cone calorimeter uses a 100 square millimeter sample to determine the heat release of materials. To induce the release of heat, the material needs a heat input, typically an electrical conical heater above the specimen material.

Fire Behaviour of Materials

The difference between the applied heat input and the induced heat output is critical in determining the fire behavior of materials. Many materials ignite at a heat input rate between 15 and 30 kW.m-2, but in developed fires, rates of 50 kW.m-2 are common. In severe cases such as aircraft fires, rates of 100 kW.m-2 may occur.

In these extreme cases, the levels of heat released by materials can be 20 times higher than the applied incident heat levels. This magnification of heat induces fire growth. For this reason, measuring the heat release rate is an important element of characterizing the expected behavior of materials in fire.

Smoke Production

Since both small and large scale calorimeters involve the collection of all of the combusting materials, it is a relatively simple matter to extend measurements of heat release to include the quantitative measurement of visible smoke and a range of toxic gases. Smoke is continuously monitored by an extinction beam photometer (comprised of a light beam and photocell). The optical density this measures can be integrated with the volume flow in the duct to produce a quantitative measure of smoke production rate and total smoke yield.

Smoke production can vary enormously from material to material. For example, burning polystyrene produces approximately 20 times more smoke than an equal mass of wood.

Carbon Monoxide and Other Gas Emissions

The principal toxic gas produced by fires is carbon monoxide. This gas can be monitored continuously by a nondispersive infrared gas analyzer and, as with visible smoke, can be integrated with the volume flow in the duct to provide both production rate and total yield values. Depending on the materials involved, other gases such as oxides of nitrogen, hydrogen chloride and, hydrogen cyanide can all be released by fires.

These gases can all be measured quantitatively using appropriate gas analyzers attached to the calorimeters or by subsequent analysis from grab samples taken periodically throughout the fire’s duration. In addition to these common fire gases, the full spectrum of chemical species present can be monitored by extracting gas samples for subsequent examination using gas chromatography and mass spectrometry.

Fire Inhibitors

Smoke and toxic gases released in fires are frequently the first things that victims encounter and are often responsible for incapacitation, delaying escape. Measuring the production rate and total yield of smoke and toxic gases is therefore of great importance.

Noncombustible materials such as mineral fiber can initially exhibit low thermal inertia without the attendant fire hazard. However, combustible materials can also be made to behave more safely in fire by introducing agents which interfere with the combustion process.

Types of Fire Retardant Materials

Some additives release gaseous species when heated which pass into the flame zone and interfere with the flame chemistry to extinguish the fire. These are referred to as vapor phase fire-retardants. Other additives give protection in their solid-state by inducing the formation of a protective coating inert layer of solid material which forms on the surface of the fuel. This crust or char layer blocks the input of heat to the unburnt fuel below, reducing its burning rate or extinguishing it altogether.

Char layers like this are commonly found on burned wood. Other types of fire retardant additives induce shrinkage of the fuel away from any ignition source or allow heat to evaporate water trapped in a suitable substance such as alumina trihydrate.

Intumescent Coatings

Another common form of treatment involves the addition of an intumescent coating, which swells up when heated to provide a thermally inert and insulating layer. This is usually applied in the form of a paint.

All of these additives can be applied in many ways. They can be added as elements of a polymer chain during the polymerization process or they can be attached to an existing material after the polymerization process. The simplest type of application is the spray-on treatment, but this tends to be easy to take off. The durability of a fire retardant must always be considered by using accelerated aging or washing before fire testing.

Fire retardant chemistry is a complex science and new formulations are continuously being produced by specialist manufacturers.

Several of the techniques described above have been successfully applied to reduce the fire hazard of domestic upholstered furniture. Heat release rates have also been significantly reduced.

Summary

A range of traditional measurement techniques is still widely used throughout the world. The new techniques described here were developed by international collaboration through the International Standards Organization (ISO). Not only do they provide more useful results, but they also offer a route to international harmonization of descriptions of materials’ behavior in fire.

Primary author: Stephen Ames
Source: Materials World, Vol. 1 no. 2 pp. 88-91 February 1993.
This article is under Crown copyright 1992 – Building Research Establishment.

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