Fire protection and polymers: natural flame retardancy

While investigating the limitations of the theory that explains the ignition behavior of a polymer, we discovered something unexpected, a new natural flame retardancy mechanism.

In the UK, a fire is started every 3 min, and over the course of a year, the cost of fire totals approximately £7 billion [1]. It is a major threat, and continues to be the leading cause of property damage worldwide according to the insurance company FM Global. In the modern world, polymer materials are ubiquitous because of its technological, manufacturing and commercial advantages. But they also fuel flames, and are a prime actor in accidental fires. Better understanding of how polymers burn is a necessity if we are to save human lives, protect infrastructure and the environment, and improve businesses.

Ignition is a key process in the initiation and growth of fires. The risk of fire is associated to the ease of igniting the materials present. This is true for the initiating event but also for the subsequent spread. For example, the flames and the hot smoke transfer heat to nearby fuels, igniting these too, thus leading to further growth of the fire.

Pyrolysis is the thermochemical process by which a solid (or liquid) decomposes and produces the gaseous fuels that feed a flame. When a solid fuel is heated it eventually reaches a temperature threshold where it begins to break down chemically (typically around 200 to 300 C). Pyrolysis is similar to gasification but with two key differences, i) pyrolysis is the simultaneous change of chemical composition (e.g. long hydrocarbon chains to shorter chains) and physical phase (i.e. solid or liquid to vapour); and ii) is irreversible. It is an endothermic reaction, meaning that it needs an external supply of heat to continue because the products carry more chemical energy than the original fuel. It does not involve oxidation reactions.

Watch this accelerated video to see the pyrolysis of a block of PMMA, a synthetic polymer used in plexiglass, when it is exposed to a strong source of radiant heat (arriving from the top).

Since World War II, laboratory experiments performed with radiation heat sources have provided a basic understanding of ignition. It has led to what is called the classical ignition theory. This theory allows to calculate the time it takes to ignite a solid fuel when it is exposed to heat. It was developed from experiments conducted at low levels of heat (in the range below ~70 kW/m2). The theory says that the time to ignition decreases with the square root of the incident heat. These calculations have been used extensively in fire science and in fire protection engineering for decades. Although the expression has been altered slightly many times as research developed, it has kept pretty much the same mathematical form (=inverse square root for all heat flux levels).

But in 2006, researches at Worcester Polytechnic Institute conducted experiments at high heat fluxes, up to 200 kW/m2 on a range of polymers (PMMA and wood, for example). Their experimental data could not be predicted correctly by the classical theory. The error at high heat levels was large. Instead of the expected continuous square root behavior, the measurements were diverging from theory with a gradual flattening towards a constant ignition time for heat levels above ~80 kW/m2. The researchers could not explain the phenomena but reported their measurements [2]. This data posed a challenge to the scientific community.

This is an important failure because in large accidental fires, most of the radiative heat arriving to nearby fuel items is above 100 kW/m2. Thus, if this error is not corrected, predictions of the pattern and the rate of spread of a fire will be erroneous. Also, this failure of the ignition theory marks a limitation of our understanding and hinders the development of new fire protection technologies.

Measurements of the time to ignition of PMMA samples found in the scientific literature. The cloud represents experimental uncertainty. Top: Inverse of the square root of the delay time to ignition vs. heat flux (inset: zoom for heat fluxes up to 60 kW/m2); Bottom: Delay time to ignition vs. heat flux. Figure from [3].

So, in 2008 we picked up the challenge and tried to solve this riddle. We conducted a detailed investigation [3] of all the experimental data in the literature for the polymer best studied in fire science: PMMA. Data extended from low to high heat levels, see figure above. We then used a comprehensive numerical model of pyrolysis to revise all the assumptions cast in the classical ignition theory. We interrogated the experimental data using the numerical model to tell us why the failure. We wanted to identify the assumption and the mechanism (or mechanisms) responsible for the unexpected failure at high heat levels. All possible physical and chemical assumptions were systematically studied, one-by-one and combining them. We found it at the end. The classical ignition theory makes a wrong assumption and misses an important mechanism, a physical one, related to radiation heat transfer (and related to optics as well).

The problem is that the classical ignition theory assumes that the radiation is absorbed at the exposed surface of the material. We found that this is a good approximation to all materials at low heat levels, but it is not a valid assumption for many materials at high heat levels [3]. The assumption breaks down at high heat, with PMMA for example, because this material is translucent to some radiation. We could correct the ignition theory by taking into account that a fraction of the radiation penetrates directly in-depth, into the material, such that the surface heats up less. This leads to slower ignition and slower fires. The elusive mechanism is called in-depth radiation absorption.

This discovery was also reached simultaneously and independently by researchers at FM Global [4], although using an analytical approach and a smaller experimental data set. We learn about FM Global's work after presenting our findings at an international conference (BCC 2009 Recent Advances in Flame Retardancy of Polymeric Materials), so we were lucky to be able to cite them too and include their data in our final version published in 2010 [3].

The discovery is important because many polymers are known to exhibit some degree of transparency to radiation. PMMA is just one example, the example for which most fire data exists. Due to in-depth absorption, a material delays ignition because heat reaches directly deep into it thus leading to lower temperatures at the surface and hence taking longer to reach ignition. The work shows that in-depth radiative abortion acts as a natural fire retardant in polymers; it helps to 'cool down' the surface when heated.

This mechanism could be exploited by the plastic industry to design new polymer formulations that favor materials that are transparent to radiant heat and absorb less at the surface but more in-depth. It might help to formulate physical flame retardancy, whereas currently the plastic industry relies mostly on chemical retardants.


References:

[1] An Introduction to Fire Dynamics, 3rd Edition, 2011, by Dougal Drysdal, Wiley. 

[2] Flammability characteristics at applied heat flux levels up to 200 kW/m2, by P Beaulieu and N Dembsey in  Fire and Materials, 32(2), pp. 61-86, 2007

[3] Numerical Investigation of the Ignition Delay Time of a Translucent Solid at High Radiant Heat Fluxes  by N Bal and G Rein in  Combustion and Flame 158, pp. 1109-1116, 2011.

[4] Absorption of thermal energy in PMMA by in-depth radiation by Jiang, De Ris and M.M. Khan, Fire Safety  Journal, 44 (1), pp. 106–112, 2009