Behaviour of Polymers In Fires

This example illustrates the range of oxygen concentrations and the kind of temperature that can occur in a real fire, but this is only an illustration. There are no standard conditions for real fires, since each fire is a unique physico-chemical event. 

With the increasing use of polymers in both the home and the workplace, there seems to have been a change in the nature of fires. Fire brigades now report fires that are shorter and more intense than previously there is also much more smoke and significantly greater amounts of toxic gases. All of these arise from the nature of the polymers being used in everyday life. 

Thermosets and thermoplastics behave differently from each other in fires. Thermosets do not melt when heated but may well undergo further crosslinking. The presence of such additional crosslinks hinders movement of any volatile degradation products through the polymer matrix. Hence the combustion zone tends to be starved of fuel and for this reason thermosets tend to be relatively non-flammable. 

Thermoplastics, on the other hand, melt when heated and this assists the volatiles to move through the material. Thus fuelling of the combustion zone is readily achieved and, for this reason, many thermoplastics are highly combustible unless treated appropriately. 

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Drysdale, D.D. Fundamentals of the fire behaviour of cellular polymers. In Fire and Cellular Polymers, Buist, J.M., Grayson, S.J., Woolley, W.D., Eds. Elsevier Applied Science London, U.K., 1986 pp. 61-75. 

The fire behaviour of polymer foams is largely dependent on their exposure to air and is dominated by the characteristic low thermal inertia which permits the surface to respond very rapidly to any imposed heat flux and consequently ignition maximum rates of burning can be achieved very quickly. Approaches toward reducing the flammability of polymer systems, in general, can be grouped in several categories [25,142,155-176] ... 

The major improvement sought in polymers in terms of their fire behaviour is reduction of flammability. For certain applications, however, reduction in smoke evolution is sought but these two aims tend to be mutually incompatible. Reduction in flammability is brought about by making the combustion process less efficient. A penalty for inefficient combustion is increased smoke production. Similarly a reduction in smoke evolution may be achieved by increasing the efficiency of any accidental combustion that is, by increasing the flammability. 

To assess suitability of plastic storage containers for distribution of silver nitrate, behaviour under fire exposure conditions of various polymers in contact with the salt was examined. All polymers tested burned vigorously. 

Polymer clay nanocomposites have, for some time now, been the subject of extensive research into improving the properties of various matrices and clay types. It has been shown repeatedly that with the addition of organically modified clay to a polymer matrix, either in-situ (1) or by melt compounding (2), exfoliation of the clay platelets leads to vast improvements in fire retardation (2), gas barrier (4) and mechanical properties (5, 6) of nanocomposite materials, without significant increases in density or brittleness (7). There have been some studies on the effect of clay modification and melt processing conditions on the exfoliation in these nanocomposites as well as various studies focusing on their crystallisation behaviour (7-10). Polyamide-6 (PA-6)/montmorillonite (MMT) nanocomposites are the most widely studied polymer/clay system, however a systematic study relating the structure of the clay modification cation to the properties of the composite has yet to be reported. 

Polymers in their raw state are usually technically unsatisfactory in one respect or another, such as their stability to light or heat, or their processability, or flammability, or colour, or opacity, or antistatic characteristics, etc., and they simply could not be used in commercial applications successfully without the incorporation of one or more additives [2] to modify behaviour. The additives are often present at very low concentrations (0.1-3 parts per hundred of resin, by weight) and are called stabilizers, UV absorbers, viscosity modifiers, lubricants, fire retardants, pigments, etc. Fillers may be present at 50 or even 150 parts per hundred of resin, by weight. Thermosetting resins tend to have fewer additives of the kind... 

Bond properties at elevated temperatures. The bond, which relies heavily on the mechanical (shear) properties of the polymer matrix or adhesive, can be expected to be severely reduced at temperatures exceeding the glass transition temperature, Tg, of the matrix or the adhesive. Essentially no information is currently available on the specific behaviour of the bond between unprotected externally bonded FRP materials and concrete or masonry at high temperature. For example, in the case of insulated FRP systems, it is not clear exactly how long the bond between the externally bonded FRPs and the substrate can be maintained during a fire. 

Price has reported the use of pulsed laser pyrolysis to generate conditions, analogous to those behind the flame front in a real fire, at the surface of a polymer sample. The temporal behaviour of the species escaping from the reaction zone created in this way at the polymer surface is monitored by the very fast scanning time-of-flight mass spectrometer (ToF-MS). The ToF-MS is able to provide a complete mass spectral analysis every 25 ps. This identifies the species involves and indicates their relative concentrations. 

There are several ways to change the fire behaviour of a polymeric material especially at the different phases of combustion. However, there is no single best way to improve the fire behaviour of a certain polymer/polymer product because the needed performance depends on the requirements in the intended end-use. Hence, the same product may need to be optimised in two or more ways if it is to be used in different applications. 

The polymer may be modified significantly by the use of fire-retardant additives, which may act as fillers, plasticisers, and so on [27]. The combustion behaviour of fillers is discussed in Section 7.6.3.6... 

D. Price, F. Gao, G. J. Milnes, E. B., C. I. Lindsay, and P. T. McGrail, Laser pyrolysis/time-of-flight mass spectrometry smdies pertinent to the behaviour of flame-retarded polymers in real fire situations. Polymer Degradation and Stability, 64 (1999), 403-10. 

The need for a building material with high strength and durability afforded the development of Polymer Mortars (PM). PM show, in general, high mechanical, chemical and durability properties however, they also exhibit great sensitivity to high temperatures and creep phenomena, and mainly, deficient behaviour under fire (Ribeiro et al., 2004, 2008 Tavares et al., 2002). 

Kandola, B.K. Nazar6, S. Horrocks, A.R. Thermal degradation behaviour of flame retardant unsaturated polyester resins incorporating functionalised nanoclays, in M. Le Bras, C.A. Wilkie, S. Bourbigot, S. Duquesne, and C. Jama, Eds., Fire Retardancy of Polymers New Applications of Mineral Fillers. Royal Society of Chemistry, London, 2005, pp. 147-160. 

Typical applications that are ideal for TG-DSC are temperature stability, decomposition behaviour, drying and firing processes, transition and reaction temperatures, melting and crystallisation processes. Redfern [290] has reviewed single sample simultaneous thermal analysis, i.e. TG-DSC and TG-DTA smdies of polymers, and has reported TG-DSC of an uncured polyimide resin in which a more accurate determination of the quantitative measurement... 

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