Fire hazards and some common polymers

Polymer Degradation and Stability 67 (2000) 383±396

Review paper

Fire hazards and some common polymers D.J. Irvine, J.A. McCluskey, I.M. Robinson* ICI Acrylics, Room T235, PO Box 90, Wilton Centre, Middlesbrough, Cleveland TS90 8JE, UK Received 18 May 1999; accepted 27 June 1999

Abstract The annual ®re statistics for the UK shows the pattern of injury and loss due to ®re. Drawing on the current knowledge of ®re safety engineering, the types of hazard commonly encountered in ®res are described, including thermal parameters, smoke and toxic gas evolution. Fire tests based on established Standard tests measure only a limited range of material behaviours to a ®re. Using the growing discipline of ®re science, a better description of the intrinsic ®re hazards for each material can be made. In a series of model calculations, some of the more commonly encountered polymers used in the construction industry are compared in their fundamental behaviour to ®re using a ®re science approach. From this, a simple ranking of material behaviour across a range of constant external heat ¯uxes can be made for each major ®re hazard. The inclusion of data necessary for ®re safety engineering to the growing number of polymer property databases is a readily available means to help promote ®re safety through good engineering practice. # 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction Accidental ®res have been a problem for as long as human history. In our modern environment we are surrounded by a wide range of highly combustible materials, which under the right conditions readily ignite and burn vigorously. These materials include both natural (e.g. cellulosics) and man made polymers. Fire is a continuous threat to life and property. The human cost is ®nancially incalculable. The direct costs from loss of property, the cost of ®ghting ®re and treating those injured can be measured and it ran to some 450 million pounds per annum [1] in the early 1990s, with indirect costs estimated at adding a further 190 million pounds to the bill. This gives a total annual loss in the order of 0.25% of UK GDP. The annual ®re statistics for the UK [2] reveal the detailed pattern of injury and loss. The number of accidental ®res in buildings (both dwellings and other buildings) has shown a slight reduction over a 10 year period, with the numbers currently running at about 76,000 per annum. In Fig. 1, the annual trend in ®re fatalities by cause shows the extent of human su€ering. The number of fatalities has fallen during the past 10 years, due partly to improvements in ®re detection (e.g. smoke detectors) and in the ability to * Corresponding author. E-mail address: [email protected] (I.M. Robinson).

®ght ®res. Looking at Fig. 1, it is clear that the majority of casualties result from being overcome by the combination of smoke/toxic gases, with burns caused by exposure to excess heat also a signi®cant contribution. These are the primary ®re hazards which kill and injure so many people. In this paper a review of a typical pattern of ®re growth and the evolution of ®re hazards is made. Building on this, the concept of escape criteria and their relation to the primary ®re hazards are introduced, which leads to general statements on tenability in a ®re. The role of ®re testing is discussed including an appraisal of the methodology behind generic types of ®re test, the in¯uences that test conditions can have on outcomes and the practical signi®cance of such tests. Using the principles of ®re science, the constitutive equations controlling the primary ®re hazards are outlined. From material property data published in the technical literature a comparison of ®re behaviour for a range of common polymers, such as polystyrene, polypropylene, polyethylene, polymethylmethacrylate, polycarbonate and unplasticised polyvinylchloride can be made using the constitutive equations of ®re hazard. Some general conclusions are o€ered on the relative performance of these polymers from the perspective of their ability to generate heat, smoke, toxic gases and irritant compounds. The use of ®re science may improve our ability to predict the behaviour of di€erent materials

0141-3910/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved. PII: S0141-3910(99)00127-5

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Fig. 1. Annual total number of ®re fatalities in the UK (1985±1995) by cause of death (Crown Copyright).

across a range of possible ®re scenarios. It may also help to prevent poor materials selection made on the basis of arbitrary ®re tests, where the conditions of the ®re test can be far removed from a real ®re situation. 2. The pattern of ®re growth and evolution of ®re hazards The growth of a ®re in a compartment may be illustrated by referring to the variation of average temperature following ignition. Fig. 2 shows a schematic diagram for the increase in the average temperature in a burning room as a function of time. The temperature± time curve shows distinct phases, starting with ignition and early growth, to the fully developed stage, leading through to the eventual decay of the ®re. These stages are found in real ®res. Each stage of the process is now described, with respect to the evolution of ®re hazards and approximate conditions. 2.1. Ignition All ®res start with an ignition event, where a source of heat comes into contact with a fuel in the presence of oxygen. This initiates a ¯ow of ¯ammable degradation products which react with oxygen from the air to produce a ¯ame and heat. Some of the heat is transferred back to the surface of the fuel maintaining the ¯ow of ¯ammable volatile degradation products. Fire is a

dynamic process, so at any time there is an equilibrium between the rate of heat transfer back to the fuel, heat losses from the fuel and the production of degradation products. The ®re may go out, smoulder for a long time before developing, or become vigorous relatively quickly, depending on circumstances. 2.2. Growth stage As the ®re grows. three primary hazards are generated which are heat, smoke and toxic gases (chie¯y carbon monoxide, CO and possibly other gases such as hydrogen cyanide, HCN), and other irritant biproducts from incomplete combustion (chie¯y hydrochloric acid, HCl and simple organic species such as acrolein). The causal relationships controlling the production of the primary ®re hazards are shown in Fig. 3 which is based on the CRISP (Comparison of Risk Indices by Simulation Procedures) model [3]. The various relationships controlling the production of each type of hazard are shown; solid arrows indicate positive correlation (i.e. an increase in x produces an increase in y) and dotted arrows indicate a negative correlation (i.e. an increase in x produces a decrease in y). Thus after ignition of a fuel, a release of heat raises the temperature and heat ¯ux to the fuel, which in turn causes further pyrolysis and combustion, in the process consuming more oxygen, and liberating more heat, as well as smoke and toxic gases. The smoke results in a reduction in visibility (an

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385

Fig. 2. Schematic representation of a ®re vs time, from ignition and growth, ¯ashover, fully developed and eventual decay. Each stage is marked by the dotted line.

Fig. 3. Schematic diagram based on the CRISP model[3] showing the generation of the main types of hazard in a ®re and their relationship to escape criteria. The black arrows show a positive correlation (x causes an increase in y) between variables; the grey arrows show a negative correlation (x causes a decrease in y) between variables.

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increase in optical density) and the toxic gases are steadily evolved. Providing there is a ready supply of fuel and adequate ventilation, the fuel cycle can be self sustaining, as Fig. 3 demonstrates. It can readily be seen that the rate of evolution of these primary hazards is a key factor in the growth of the ®re, which is determined by factors such as the local heat ¯uxes, materials involved and oxygen levels. In the ®re `growth stage' under the right conditions, the ®re may develop exponentially with time and the increasing levels of heat, temperature, smoke and toxic gases all a€ect the ability to escape. This is discussed in more detail below with reference to tenability, or escape criteria for each of the primary ®re hazards. 2.3. Flashover Eventually an unchecked ®re will grow rapidly, accompanied by an increase in heat and temperature resulting in ignition of nearby materials by heat transfer. Flashover is reached when all combustible materials in the compartment start to burn. A precise de®nition of this stage is not strictly possible; typically it corresponds to temperatures around 500±600 C at ceiling level, and radiant heat ¯uxes in excess of 20 kW/m2 at ¯oor level. The exact rate of production of the primary ®re hazards depends upon the ®re conditions, including the supply of oxygen, the materials being burnt and their con®guration. 2.4. Fully developed ®re At this stage, the generation of heat reaches a peak and the temperatures may reach around 1000 C, or higher if there is sucient ventilation. The ®re may spread to other parts of the building and may be severe enough to induce structural damage. Even if the ®re remains localised, smoke and toxic gases may spread far beyond the original source of the ®re, causing dangerous conditions. Conditions such as ventilation (hence oxygen levels) control the production of smoke and toxic gases and the overall pattern of combustion. 2.5. Decay Eventually the temperature begins to fall as the fuel is consumed. Flaming ®nally stops and all that remains in the compartment are glowing embers. 3. Escape criteria and the primary ®re hazards People may try to escape from a ®re as it grows within a compartment and generates further hazards around the building. The ability to escape from the ®re involves many factors, including being able to move to a position

of safety along a route through the building which has locally tenable conditions for people to continue to move and survive. If escape is to occur, the total time required to make an e€ective escape from the growing ®re must be less than the time for `untenable conditions' (when escape becomes impossible), tu to be reached [4] tp ‡ ta ‡ trs < tu

…1†

where tp is the time from the start of the ®re until it is detected by a potential victim, ta is the time taken to begin e€ective escape activity and trs is the time taken to reach a position of relative safety. If untenable conditions are reached, any persons attempting escape along any point on the particular path in the building under consideration will be trapped and is likely to become a casualty. Thus the rate at which untenable conditions are reached is crucial for escape. The time to reach untenability, tu needs to be as long as possible and can be a€ected by the materials in the compartment. The time for detection, tp needs to be as short as possible, hence the improved chance of escape through the use of smoke detectors, which activate at an early stage of a ®re. The time taken to reach a position of relative safety, trs depends on factors to do with people (e.g. age, mobility, state of consciousness etc), building geometry and clear escape routes. The ®re statistics indicate that about 80% of deaths occur in dwellings, with most people killed in the room of ®re origin (about 60% of the total). These are mostly con®ned to living rooms or bedrooms with upholstery or bedding being the items ®rst ignited. A signi®cant number of fatalities were estimated to be asleep, intoxicated or both when the ®re broke out. These conditions, of course, dangerously increase the time for an individual to detect ®re and e€ect a speedy escape. A smaller percentage of injuries and fatalities originate from ®res in other types of building, such as shops, clubs, hotels and hospitals. The various untenability limits (or escape criteria) have been described in detail elsewhere [5], and have been developed principally for modelling escape in various ®re scenarios. They serve as a guideline for dangerous local conditions in a ®re and are listed in Table 1 for the main classes of ®re hazard: heat, smoke density (visual impairment), toxic gases and hypoxia (lack of oxygen). Fig. 3 shows the link between the fuel cycle, the generation of the ®re hazards, through their intermediate processes, to ®nally the escape criteria for each class of hazard. Numerous studies have shown that the ®rst most likely encountered tenability criteria in a real ®re is that due to smoke evolution. Studies have shown that a high percentage of people (80%) who turn back from a smoke ®lled environment do so once the visibility starts to decrease to 10m or less, with a smaller percentage doing so at shorter distances (2m), which translates into

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Table 1 List of tenability criteriaa Units

Value for parameter without causing incapacitation

Commentsb



kW/m2 C  C

<1.0 <120 <60

Severe pain and onset of burns if exceeded Severe pain and onset of burns if exceeded Severe pain and onset of burns if exceeded

Smoke density Visibility

m

>10

Optical density

OD/m

<0.13

Extinction coecient

mÿ1

<0.3

Suggested by Rasbash [20]. Other limits are also discussed in the literature [6] Suggested by Rasbash [20]. Other limits are also discussed in the literature [6] Suggested by Rasbash [20]. Other limits are also discussed in the literature [6]

% COHb (metabolised CO)

< 34% COHb equivalent to <26,500 ppm/min cumulative exposure

HCN

ppm

complex, generally <150

HCl Acorolein

ppm ppm

<100 (severe irritancy) <5.5 (severe irritancy)

Most common cause of death Present in the combustion of all materials containing carbon Exposure dose at a ®xed time to reach incapacitation, expressed as concentration in ppm  10^(4.479ÿ1.036* log time mins) Note concentration/exposure time is cumulative. See Ref. [5] for details on procedures for calculating this Present in combustion of materials containing nitrogen Concentration in ppm to reach incapacitation  exp^ (5.396ÿ0.023 *times mins). Note concentration/exposure time is cumulative. See Ref. [5] for details on procedures for calculating this Present in decomposition/combustion of PVC [5] Present in combusion from a number of polymeric materials [5]

Hypoxia O2

ppm

>100,000

If less, rapid loss of physical/mental ability [5]

Hazard type Heat Radiative heat ¯ux Convective temperature Conductive temperature

Toxic gases CO

a b

The criteria listed give the values necessary for escape from a ®re. All criteria must be met for escape along a particular route. Data taken from Refs [5, 6 20].

an optical density level of about 0.13 m as the tenability limit [6]. Smoke is a major hazard in buildings, as it slows the rate of escape, thus increasing the exposure levels to the lethal conditions created by heat and toxic gases. The production of irritant compounds such as HCl and Acrolein can also add signi®cantly to the smoke hazard, again by slowing the rate of escape and increasing the exposure to the other primary ®re hazards. Thermal exposure can result in injury or death through burns (skin, respiratory tract) or by causing heat stroke. Some general tenability limits are listed in Table 1 and discussed at length elsewhere [5]. The quantity of toxic gases (CO and HCN), required to produce incapacitation is shown in Fig. 4, and are given as an exposure dose or concentration as a function of time, expressed in units of ppm per minute, for people engaged in light activity. There is an exposure level to each toxic gas which results in loss of consciousness with a higher dose resulting in death. These levels have been established by animal experimentation. The biological e€ects of receiving a fraction of this dose under a given time interval remains with a potential victim, as the toxic e€ects cannot be easily or quickly

metabolised out. Consequently the 1 min levels of exposure dose for CO and HCN are approximately 30,000 ppm / min and 240 ppm / min respectively. This means receiving a dose of 3000 ppm of CO per minute would likely result in a loss of consciousness after 10 min, whereas a dose of 15,000 ppm of CO per minute would likely result in a loss of consciousness in 2 min. The exposure dose relationship is more complex for HCN as Fig. 4 shows, and is detailed in Table 1. The e€ect of simultaneous exposure to CO and HCN is likely to be additive, rather than synergistic, as the main narcosis mechanism of each toxic gas is to remove the ability of blood to transport oxygen around the body. Comprehensive post mortem studies have shown that the majority of fatalities (50 to 80%) in ®res had excessive levels of carboxyhaemoglobin, consistent with exposure to high levels of CO [7±9]. The full details behind all these toxicity concepts are discussed fully elsewhere [5]. Even if an escape is made out of an immediate area on ®re, the resulting movement of smoke with growing levels of CO and other toxic gases or irritant compounds can cause injury or death far from the origin of the ®re and the high temperatures encountered there.

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Fig. 4. Relationships between concentration of a toxic gas (in ppm) and the resulting time to incapacitation (in mins) for carbon monoxide, CO, and hydrogen cyanide, HCN. Note the cumulative dose received (concentration level vs length of time exposed) determines if incapacitation is reached during a ®re. See Ref. [5] for further details.

In summary, the following observations of tenability and ®re hazards can be made:

4. Testing Ð methodology, in¯uences of test conditions and practical signi®cance

1. The rate of evolution of each type of ®re hazard is a key concern; an increased rate of ®re hazard production reduces the time to untenable conditions, hence directly a€ecting the chances of escaping a ®re. 2. Smoke or radiant heat ¯ux are often the ®rst tenability criteria reached during a well ventilated ®re [8] in the room of origin, followed by the toxic gas criteria. Untenable conditions can rapidly develop in a room on ®re within a few minutes under the right circumstances. In poorly ventilated ®res, the pattern described above can be di€erent. 3. Up to ¯ashover, ®re is contained in the room of origin; post ¯ashover the ®re may spread to other parts of the building (this occurs in about 13% of ®res in the UK). 4. The evolution of smoke, toxic gases and irritant compounds can a€ect the rate of escape far from the origin of the ®re, increasing the exposure levels to the lethal conditions created by heat and toxic gases. 5. Carbon monoxide is the principal toxic gas and is the largest recorded cause of death in ®res, as established by various post mortem studies [6±8].

Given this picture of the main life threatening criteria of heat, smoke, and toxic gases/irritants in a ®re, together with the measured pattern of casualties, it is pertinent to ask what role individual materials have to play in a ®re and how their ®re performance may be quanti®ed compared to each other. These straightforward questions are dicult to answer, due to the complexity of ®re, which is a transient process and highly interactive with its environment. A simple example of this can be made by listing the factors which a€ect the rate of ¯ame spread across a surface [10]. The list includes the duration and level of exposure to an imposed heat ¯ux, the composition of the atmosphere, the surface orientation, direction of ¯ame propagation, air velocity, surface roughness, geometry, sample thickness, continuity and proximity to local heat sinks. In addition various material parameters also have a major contribution. These include the speci®c heat capacity, thermal conductivity, density, ®re point temperature and the di€erence between heat ¯ux due to the ¯ame and heat losses from the burning material. This complex matrix of material properties, environmental conditions, sample geometry and con®guration within a compartment can give greatly di€erent levels of ¯ame spread,

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depending upon the exact test conditions used. It should be borne in mind that in a real ®re, many of these variables are constantly changing (such as temperature, imposed heat ¯ux, oxygen levels etc) unlike the ®xed conditions used in some tests, so the rate of ¯ame spread will also change. There are a variety of tests which have been developed to ascertain the manner in which materials will behave in ®res [10,11]. These are principally small scale standard tests (such as BS476 pt 6 [12]) which allow materials to be ranked in a somewhat arbitrary order according to their performance in the test, full scale tests (which examine the response of a full scale construction to a given initial ®re condition) and ®nally newer tests which examine a materials `reaction to ®re' (such as the cone calorimeter [13]) which provides ®re data capable of rational assessment. It is undoubtedly true that the behaviour of a material in a small scale standard test is sensitive to the exact conditions of the test. These often bear no relation at all to the end use con®guration, and conclusions can be drawn from these tests on the suitability of materials usage, which in real life can lead to potentially dangerous situations. As an alternative strategy, ®re science seeks to measure real property data and develop the causal understanding of the phenomena of ®re, so that modelling a wide range of potential ®re scenarios becomes possible, which then matches the real usage of materials. The myriad picture of standard ®re tests, developed on a national basis has been summarised by Troitzsch [12]. The de®nitions of acceptable behaviour to ®re from both materials and products is not universally agreed. What may constitute `acceptable behaviour' by one test method may be deemed unacceptable in another. A well referenced paper by Emmons [14] has shown that the ranking of materials found by many common national ®re test methods shows no correlation with each other, even to the extent that the `best' material by one test appears to be the `worst' in another test. An extra dimension to the development and use of conventional standard ®re tests are the specifying community, ranging from trade associations, national agencies and international standards committees, each employing tests of a di€erent type and scale for their own reasons. Small scale tests will continue to be used to develop new materials and products in a cost e€ective manner. Full end use tests of ®re behaviour is an alternative, but expensive means of resolving the choices between complex material constructions and designs [15]. Another more ¯exible approach uses reaction to ®re' tests allowing the determination of key material characteristics involved in a ®re. The data can be coupled with elements of ®re protection engineering as a method of linking materials and designs for end use applications on a rational basis. This ®nal approach compares with that for mechanical integrity of an article or structure, where a range of well established test methods, measure

389

properties which can be used to calculate end use behaviour, according to the rules of materials science and mechanical engineering. The growing discipline of ®re science and its practical application in ®re safety engineering aims to bring the understanding of ®re to an equivalent level of certainty in complex situations. 5. Constitutive equations controlling the rate of generation of the primary ®re hazards The primary ®re hazards from materials can be assessed by their heat, smoke and toxic gas evolution with time, and is an important method of judging the relative merits of materials in a ®re situation. Fire science o€ers some methods in resolving these e€ects based on a fundamental understanding of the ®re phenomena. Excellent texts [10,16] give the comprehensive detail behind the subject; only brief highlights are made here. Given that the tenability criteria described earlier set the time limit for escape during a ®re, it is possible to de®ne important material properties behind each of these criteria and their contribution to the overall ®re. These are: 1. 2. 3. 4. 5.

the time to ignition; the rate of mass loss; the rate of heat released to the ®re environment; the rate of smoke released to the ®re environment; the rate toxic gases and irritants are released to the ®re environment.

Simple expressions for the above criteria are given below which reduce complex behaviours detailed elsewhere [10,16], to the fundamental material properties, whenever possible. The symbol '' is used to mean per unit area, and the symbol  is used to mean rate of  change of y with time, hence y'' means rate of change of y with time per unit area (a ¯ux). In the discussion below, the relationships have been determined under well ventilated conditions, with air¯ow rates consistent with obtaining reproducible ignition behaviour, and thermally thick samples under a constant external heat ¯ux. These are the conditions which exist in a cone calorimeter. 5.1. The time to ignition For a material in a cone calorimeter, under the conditions outlined above, ignition can be achieved if an  external heat ¯ux Q''e greater than a certain critical heat ¯ux, CHF, is applied for a time interval tign. The details of the process are complex and described fully in reference [16]. When the heat ¯ux is applied, heat is conducted into the interior of the material, the rate of which in¯uences the time it takes to ignite. The slope of  1 a plot of tign.ÿ 2 vs Q''e yields the `thermal response parameter', TRP, which is given by:-

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ÿ
p TRP ˆ Tign ÿ Ta kCp

…2†

where Tign = ignition temperature (K) Ta = ambient temperature (K) k = thermal conductivity (kW/m-K) p = density (g/m3) Cp = speci®c heat capacity(kJ/g-K) TRP = Thermal response parameter (kW-s1/2/m2) It should be noted that the values for the thermal response parameter as determined by cone calorimetry depend upon sample thickness, applied heat ¯ux, insulation e€ects etc. [16±18] and may be di€erent from the values which are directly calculated from a knowledge of the material properties listed above as measured by other techniques. At heat ¯uxes greater than the critical heat ¯ux, CHF (kW/m2), the time to ignition under constant external heat ¯ux Q''e can be given by: tign

ÿ
2  Tign ÿ Ta kCp …TRP†2   2    2 4 Q''e ÿ CHF 4 Q''e ÿ CHF

…3†

Thus, the time to ignition depends on the critical heat ¯ux, the thermal inertia (the product kp Cp) and the ignition temperature. This equation explains why foams are readily ignited compared to solids; the time to ignition scales directly to the density. Thus, a foam with a density0.05% of the bulk solid will ignite in approximately 0.05 of the time to ignition for the solid, despite the foam and solid being chemically identical. The rate of ¯ame spread in a material depends on many variables as discussed earlier, but broadly scales inversely with the time to ignition, so raising the time to ignition reduces the rate of ¯ame spread, hence the growth of a developing ®re. It should be stressed that Eq. (2) is not strictly valid as the external heat ¯ux approaches the critical heat ¯ux as extra thermal loss mechanisms come into play. Also the thermal response parameter depends upon sample thickness; the TRP reduces as the thickness reduces [16]. 5.2. The rate of mass loss 

The rate of mass loss, m'', is given by      m'' ˆ Q''F ‡ Q''E ÿ Q''L =Lv 

…4† 

where Q''F is the heat ¯ux from the ¯ame, Q''E is the external heat ¯ux, and the heat ¯ux loss is Q''L. This

indicates that the rate of mass loss depends upon the imposed heat ¯ux, as expected and upon the following material parameters; heat ¯ux from ¯ame, heat ¯ux loss and the latent heat of gasi®cation, LV. For a material burning by itself in the open, with no thermal feedback from the environment, the external heat ¯ux is zero and the rate of mass loss is driven by the di€erence between the heat ¯ux from the ¯ame less the heat ¯ux loss, divided by the latent heat of gasi®cation. For a material either in the presence of another burning object, or subject to reradiation from trapped heat within the compartment in which the material is burning, the external heat ¯ux is greater than zero, and the rate of mass loss is higher according to Eq. (3). Thus, materials burning in enclosed compartments will tend to burn faster than in the open. The rate of mass loss ultimately determines the rate of heat release, rate of smoke and toxic gas production, and is consequently key to the ®re hazards that burning materials produce. 5.3. The rate of heat release to the ®re environment 

The rate of heat release per unit surface area, Q''c,  depends on the rate of mass loss, m'', the heat of combustion,
Hc, and the ¯ame combustion eciency, . The rate of heat release per unit area is given by 



Q''c ˆ m''


Hc

…5†

where the rate of mass loss, m'' is given in Eq. (4), the ¯ame combustion eciency is c (with a value less than 1) and the heat of combustion is
Hc . The rate of heat release depends strongly upon the ratio of two key materials parameters found in Eqs. (4) and (5), namely the heat of combustion divided by the latent heat of gasi®cation,
Hc /Lv. This term has been called the combustibility ratio [19] and provides a means of ranking materials according to their burning behaviour, which broadly matches the consensus view of their behaviour. This can be experimentally measured by cone calorimetry [13]. The rate of heat release is arguably the most important ®re parameter, since it controls the rate of growth in the ®re, including heat, and ultimately the amount of smoke and toxic gas generated. Trapped heat in the compartment produced by the ®re is reradiated back, this further increases the  external heat ¯ux, Q''E, which in turn accelerates the rate of heat release, smoke and toxic gas production. This process directly a€ects the time to reach untenable conditions after which escape from the evolving ®re situation is no longer possible. 5.4. The rate of smoke release to the ®re environment The amount of smoke produced in a ®re is a complex subject and detailed descriptions of the processes

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involved can be found in Refs [10,16,20]. Smoke results from incomplete combustion and relates to chemical composition and to the local burning environment, especially the temperature and oxygen levels. As shown earlier, these are likely to be changing during a real ®re, so the rate of smoke production will also change according to these conditions. Tewarson [16] presents a means of correlating smoke production with the level of oxygen present (the lower the oxygen level present, the higher the amount of smoke produced). A method of comparing materials by the amount of smoke production is the total % conversion to smoke by mass, Ys, (in units of g/g) which is readily measurable. The rate of  smoke production per unit area, R''s, relates to the rate of mass loss, multiplied by the % conversion to smoke by mass. 



RS ˆ m'':YS

…6†

This implies materials with high conversion rates of smoke YS, but lower rates of mass loss, will produce lower overall rates of production of smoke, which may reduce the smoke hazard compared to a material with lower conversion rates of smoke but higher rates of mass loss. This is typically the choice a ¯ame retarded polymer presents to a potential user compared to the behaviour of the original polymer, as the ¯ame retardants generally lower the rate of mass loss at a given heat ¯ux but raise the % conversion of the fuel to smoke. 5.5. The rate of toxic gases and irritants released to the ®re environment Like smoke production, the amount of toxic gases and irritants evolved are dependent on the conditions of burning and the chemical composition of the fuel. All polymers will produce some carbon monoxide when they burn. Those containing nitrogen will produce various nitrogen containing products, particularly if the air supply is restricted. These may include nitriles, HCN and nitrogen oxide; the amounts of each depend on the conditions of burning. Common irritants found in real ®res such as HCl are produced by PVC undergoing decomposition; a range of polymers also produce acrolein. Tewarson [16] presents a means of correlating CO production with the level of oxygen present (the lower the oxygen level present, the higher the amount of CO produced). A method of comparing materials by the amount of toxic gases and irritants production is the total % conversion to these species by mass (e.g. YCO for yield of CO in units of g/g) which is measured under standard test conditions. The exact yields depend upon the prevailing oxygen levels de®ned by the temperature and equivalence ratio [16]. Toxic gas and the production of irritants will also relate to the rate of mass loss as the

391

following example for rate of carbon monoxide production per unit area R''CO shows. 



R''co ˆ m'':Yco

…7†

A similar argument exists for ®re retarded polymers with respect to the rate of toxic gas production as was presented in the case of smoke generation, with ¯ame retardants generally increasing the % conversion to CO but reducing the rate of mass loss. It should be recognised that in a real ®re situation, the external heat ¯ux is likely to be constantly changing as a ®re in a compartment develops, due to reradiation e€ects and other materials becoming ignited, resulting in changing levels of heat release rate, smoke and toxic gases. Also polymers are often used in situations where, for example, they can be described as being thermally thin, or located close to heat sinks, so Eqs. (2±7) under these circumstances may not strictly be true for all cases of interest and a more detailed analysis is required. Nonetheless, they serve as an important means of comparing behaviour. 6. Comparing materials behaviour by the constitutive equations of rate of generation of the primary ®re hazards With the relationships [Eqs. (2±7)] outlined in the previous section in place, it is possible to compare performance between polymers using the primary ®re hazards criteria. The material properties necessary for this are presented in Table 2 for some common polymers used in the construction industry, which includes polystyrene PS, polypropylene PP, polyethylene PE, polymethyl methacrylate PMMA, polycarbonate PC, and unplasticised polyvinyl chloride uPVC. It should be noted that the ®re properties for the polymers listed are generic only, as no details are available on the individual grades, their molecular weight or the level of additives present (e.g. ¯ame retardants, ®llers or plasticisers), all of which may alter the ®re behaviour of each type of polymer. We believe that the data in Table 2 re¯ect the behaviour of the homopolymers, without the presence of additives. Also, the data in Table 2 were obtained under ¯aming, well ventilated conditions; Tewarson details the changes to these data under ventilation controlled combustion [16]. The calculations assume the ®re to be well ventilated and a number of simplifying assumptions regarding behaviour are made. These include thermally thick conditions, constant external heat ¯ux at each point calculated for each ®re hazard, instantaneous steady state burning conditions, and Eq. (3) being valid at external heat ¯uxes as low as 20 kW/m2 etc. These assumptions allow the construction of a family of ®re hazard response diagrams, using Eqs. (2±7) and

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Table 2 Various material parameters used to calculate the time to ignition, the rate of heat release, and the rates of smoke, CO and HCL evolutiona Variable

Symbol

Units

PS

PP

PE

PMMA PC

Time to ignition Thermal conductivity Density Heat capacity Thermal inertia Ignition temperature Thermal response parameter Critical heat ¯ux

k  Cp kCp Tign TRP CHF

W/m.K 0.11 kg/m3 1100 kJ/kg.K 1.2 W2.s/m4k2 145.2 K 603 kW sÿ1=2/m2 162 kW/m2 13

Rate of heat release Heat of combustion Latent heat of gasi®cation Combustability ratio Heat ¯ux from ¯ame Heat loss via radiation Combustion eciency


Hc Lv
H c/Lv  Q ''F Q''L 

kJ/g kJ/g none kW/m2 kw/m2 none

uPVC

0.24 940 1.9 428.6 588 193 15

0.44 970 2.3 981.6 548 321 15

0.19 1184 1.42 319.4 555 274 11

39.85 1.76 22.64 75 13 0.4

43.31 2.03 21.33 67 15 0.6

43.28 2.32 18.66 61 15 0.6

24.89 1.62 15.36 57 11 0.7

29.72 2.07 14.36 52 11 0.4

16.43 2.4 6.85 50 15 0.36

0.17 0.16 1200 1400 1.26 1.05 257 235.2 773 663 420 406 15 15

Comments/references Table 1.2 p3 [10] Table 1.2 p3 [10] Table 1.2 p3 [10] Calculated from above Table 1.7.4 p1±112 [16] Table 3.4.2 p3±58 [16] Table 3.4.2 p3±58 [16] Table 1.5.3, p1±83 [16] Table 5.8 p173 [10] Calculated from above Table 3.4.5 p3±70 [16] Table 3.4.4 p3±7 [16] Table 5.12 p180 [10]

Smoke generation Yield of smoke (¯aming conditions Ys

g/g

0.15

0.1

0.06

0.02

0.12

0.17

Toxic gas + irritants generation Yield of CO (¯aming conditions) Yield of HCl (¯aming conditions)

g/g g/g

0.06 ±

0.024 ±

0.024 ±

0.01 ±

0.03 ±

0.384 Table 3.4.11 p3±78 [16] 0.54 Table 3.4.20 p3±104 [16]

a

YCO YHCL

Table 2.15.1 p2±218 [16]

The parameters have been measured under well-ventilated, thermally-thick conditions using cone calorimetry.

the data in Table 2 as a basis for the calculations. The derived times to ignition are shown in Fig. 5 using Eqs. (2,3), the rates of heat release are shown in Fig. 6 using Eqs. (4,5), the rates of smoke release are shown in Fig. 7 using Eqs. (4,6) and ®nally the rates of CO and HCl release are shown in Fig. (8) using Eqs. (4,7). These ®gures demonstrate the importance of the external heat ¯ux [Eq. (4)] as it directly a€ects the rate of mass loss, hence production of the major ®re hazards from each polymer. The case where the external heat ¯ux is zero represents the behaviour when the material is burning by itself in the open. The calculations at higher external heat ¯uxes, represent the case where another burning object, or reradiation from trapped heat within the compartment is contributing to the material combustion. In the discussions below, the ®rst listed polymer shows the highest level of hazard in the category, the last polymer the least across the range of constant external heat ¯uxes used to calculate the response. By category the rankings are: 6.1. Time to ignition (s) over a range of constant external heat ¯uxes The polymers rank in the following order PS > PP > PMMA > PE > PC  PVC The typical response for the time to ignition for each of the di€erent polymers subjected to the range of constant

external heat ¯uxes can be seen in Fig. 5. At low external heat ¯ux, the time to ignition rises up to an increasing level, reaching e€ectively in®nite time at the critical heat ¯ux, CHF. These are listed in Table 2 for each polymer. As the external heat ¯ux is increased, the time to ignition falls, until at 100 kW/m2 , the time to ignition is no greater than about 20 s. As a means of comparison between polymers, Fig. 5 demonstrates that PS will ignite in 60 s after exposure to 30 kW/m2 external heat ¯ux, whereas uPVC requires about 60 kW/m2 external heat ¯ux to ignite in 60 s, with the other polymers somewhere between the two. Alternatively comparing ignition time behaviours at the same external heat ¯ux of 60 kW/m2, PS will ignite in approximately 9 s and uPVC after 60 s, again with the other polymers somewhere between the two as shown in the diagram. 6.2. Rate of heat release (kW/m2) over a range of constant external heat ¯uxes The polymers rank in the following order PP > PE > PMMA > PS > PC > uPVC The typical response for the rate of heat release across the range of constant external heat ¯uxes can be seen in Fig. 6. The values rise in a linear manner with increasing external heat ¯ux from the limit where the ¯ux is zero (representing the steady state conditions reached for each polymer in the open under well ventilated conditions), to

D.J. Irvine et al. / Polymer Degradation and Stability 67 (2000) 383±396

393

Fig. 5. Time to ignition in seconds across a range of constant external heat ¯uxes under thermally thick, well ventilated conditions for a series of common polymers.

Fig. 6. Rate of heat release across a range of constant external heat ¯uxes for a series of common polymers, under thermally thick, well ventilated conditions. The external heat ¯uxes range from zero (corresponding to burning in the open) to 100 kW/m2 (corresponding to a fully developed compartment ®re). The data for PC and uPVC have been trended back beneath their critical heat ¯ux to indicate they are unlikely to support combustion at low external heat ¯uxes.

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Fig. 7. Rate of smoke release rate across a range of constant external heat ¯uxes for a series of common polymers, under thermally thick, well ventilated conditions. The external heat ¯uxes range from zero (corresponding to burning in the open) to 100 kW/m2 (corresponding to a fully developed compartment ®re). The data for PC and uPVC have been trended back beneath their critical heat ¯ux to indicate they are unlikely to support combustion at low external heat ¯uxes.

the highest calculated external heat ¯ux at 100 kW/m2. The data for uPVC and PC have been extrapolated from 15 kW/m2 back to zero external heat ¯ux, and are unlikely to continue to burn as they approach zero external heat ¯ux. At zero external heat ¯ux, the steady state conditions reached for the rate of heat release range from 666 kW/ m2 for PP to 86 kW/m2 for uPVC, with the other polymers between the two limits. At the highest external heat ¯ux considered (100 kW/m2) the heat release rate for PP has increased to 1946 kW/m2, whereas uPVC generates 333 kW/m2, again with the other polymers somewhere between the two. 6.3. Rate of smoke release (g/m2 s) over a range of constant external heat ¯uxes The polymers rank in the following order PS > uPVC > PC > PP > PE > PMMA

increasing external heat ¯ux from the limit where the ¯ux is zero (representing the steady state conditions reached for each polymer in the open under well ventilated conditions), to the highest calculated external heat ¯ux at 100 kW/m2. The data for uPVC and PC have been extrapolated from 15 kW/m2 back to zero external heat ¯ux for the reasons outlined above. At zero external heat ¯ux, the steady state conditions reached for the rate of smoke production range from 5.8 g/m2 s for PS to 0.6 g/m2 s for PMMA, with the other polymers between the two limits. At the highest external heat ¯ux considered (100 kW/m2 ) the rate of smoke produced for PS has increased to 15.1 g/m2 s, whereas PMMA only generates 2.0 g/m2 s, again with the other polymers somewhere between the two. 6.4. Rate of toxic gas and irritant compounds released (g/m2 s) over a range of constant external heat ¯uxes The polymers rank in the following order PS > uPVC > PC > PP > PE > PMMA

The typical response for the rate of smoke production across the range of constant external heat ¯uxes can be seen in Fig. 7. The values rise in a linear manner with

…CO-main toxic gas† uPVC only (HCl-irritant compound)

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395

Fig. 8. Rate of toxic gas (CO) and irritant (HCl) release rate across a range of constant external heat ¯uxes for a series of common polymers, under thermally thick, well ventilated conditions. The external heat ¯uxes range from zero (corresponding to burning in the open) to 100 kW/m2 (corresponding to a fully developed compartment ®re). The data for PC and uPVC have been trended back beneath their critical heat ¯ux to indicate they are unlikely to support combustion at low external heat ¯uxes.

It should be noted that the CO yield is very sensitive to burning conditions, further details of the dependencies are given elsewhere [21,22], so these rankings apply only to the ideal conditions described earlier. The typical response for the rate of toxic gas (CO) and irritant (HCl) production across the range of constant external heat ¯uxes can be seen in Fig. 8. Again the values rise in a linear manner with increasing external heat ¯ux from the limit where the ¯ux is zero (representing the steady state conditions reached for each polymer in the open under well ventilated conditions), to the highest calculated external heat ¯ux at 100 kW/m2. The data for uPVC and PC have been extrapolated from 15 kW/m2 back to zero external heat ¯ux for the reasons outlined earlier. At zero external heat ¯ux, the steady-state conditions reached for the rate of toxic gas (CO) production range from 2.1 g/m2s for PS to 0.3 g/m2s for PMMA, with the other polymers between the two limits. At the highest external heat ¯ux considered (100 kW/m2) the rate of toxic gas (CO) produced for PS has increased to 5.5 g/ m2s, whereas PMMA generates 0.9 g/m2s, again with the other polymers somewhere between the two. In addition to toxic CO production, uPVC also produces

an irritant, HCl, as a by-product of combustion. The values for these two hazards for uPVC at zero heat ¯ux are 0.9 g/m2s (CO), 4.2 g/m2s (HCl) and at 100 kW/m2 they are 3.5 g/m2s (CO) and 16.3 g/m2s (HCl). 7. Conclusions Some general conclusions can be made from these observations of the primary ®re hazards. Coming to the individual polymers, the rankings o€er a few simple statements. Both uPVC and PC are relatively good on time to ignition and rate of heat release and relatively poor on the smoke and toxic gas criteria. The latter is especially true for uPVC, given the added hazard from the production of irritant HCl. Both PS and PP are very poor in terms of time to ignition. They also have relatively unfavourable heat of release rates. In addition PS is poor in terms of smoke and toxic gas production. PMMA is relatively poor on rate of heat release, intermediate on time to ignition and relatively good on smoke and toxic gas criteria. In order to obtain a balanced view of the overall ®re hazard for a given material, it is instructive to compare

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the three primary ®re hazards in conjunction with each other. The simple statements on polymer rankings with external heat ¯uxes have limited value, as they only apply to the modelled situations, with the caveats outlined earlier. The purpose of presenting the data in Table 2 and in Figs. (5±8) is to demonstrate that the means of calculating likely results from a range of ®re scenarios is possible using ®re science and ®re safety engineering principles, thus allowing a more rational basis for design with materials, if the necessary property measurements are made. Given the multitude of ®re scenarios possible, it is dicult to do this in a systematic way in this paper. Using a ®re safety engineering approach may help prevent poor materials selection made on the basis of arbitrary standards based ®re tests, where the conditions of the ®re test can be far removed from the test of a real ®re. As mentioned earlier caution should be placed on the data presented in Table 2. Unlike the physical property information found in established comparative databases for polymers, such as computer aided materials preselection by uniform standards (CAMPUS) [23], there appears to be less information on the exact grades and compositions of polymers studied by a variety of authors in the ®re science area. It is known that polymer variables, such as changes in molecular weight, levels of incorporated copolymers, additives and ®llers (such as plasticisers, ®re retardants, glass ®bres etc) can a€ect the thermal inertia, heat release rates, smoke and toxic gas production so the comparisons between materials o€ered in this paper are qualitative only. The more comprehensive databases, such as CAMPUS, only comment on polymer ¯ammability behaviour using standard based ignition tests, such as UL94 [12] and the glow wire test [12]. As this paper has shown, the hazards presented by burning polymers are more complex than these simple tests measure. Addition of the type of data presented in Table 2, grade by grade, polymer by polymer, to the various database initiatives becoming widely available, would greatly aid proper materials selection based on ®re safety engineering principles and consequently help public safety with the ever present risk of ®re.

Acknowledgements The authors wish to thank Professor D. Drysdale, University of Edinburgh, for the helpful comments and suggestions made to the paper References [1] Wilmot T. United Nations Fire Statistics Survey. World Fire Statistics Centre Bulletin 7, 1989: Geneva. [2] Goddard G. Summary ®re statistics United Kingdom. 1995, ISSN 0143 6384. [3] Phillips WGB. SFPE handbook of ®re protection engineering. NFPA, 1995 [Section 5/Chapter 1, p. 5±6]. [4] Marchant EW. 5th Int Fire Protection Symp, Karlsruhe, 1976. [5] Purser DA. SFPE handbook of ®re protection engineering. NFPA, 1995 [Section 2/Chapter 8, p. 2±85]. [6] Bryan JL. SFPE handbook of ®re protection engineering. NFPA, 1995 [Section 3/Chapter 12, p. 3±259]. [7] Halpin B, et al. Fire fatality study. Int. Symp. on Toxicity and Physiology of Combustion Products, University of Utah 22±26 March 1976. p. 11. [8] Harland WA, Andeson RA. Causes of death in ®res, Toxic gases from burning plastics, QMC Industrial Research, 6±7 January 1982. p. 15/1. [9] Kracklauer J. Flame-retardant polymeric materials. Plenum Press, 1982. p. 285. [Chapter 9]. [10] Drysdale D. An introduction to ®re dynamics. 2nd ed. J Wiley, 1999 [Chapters 6 & 7]. [11] Test methods, speci®cations and standards, vol. 2, ®re safety aspects of polymeric materials. Technomic, 1979. [12] Troitzsch J. International plastics ¯ammability handbook. Hanser, 1983. [13] Janssens M. SFPE handbook of ®re protection engineering, NFPA 1995 [Section 3/Chapter 2, p. 3±16]. [14] Emmons HW. Fire abstracts and reviews 1968;10(2):133. [15] Punderson JO. Fire and materials 1981;5(1):41. [16] Tewarson A. SFPE handbook of ®re protection engineering. NFPA, 1995 [Section 3/Chapter 4, p. 3±53]. [17] Drysdale D. An introduction to ®re dynamics. 2nd ed. J Wiley, 1999 [Chapter 6, Section 6.3]. [18] Hopkins D, Quintiere JG. Material ®re properties and predictions for thermoplastics. Fire Safety Journal 1996;26:241. [19] Rasbash DJ. 5th Int Fire Protection Symp, Karlsruhe, 1976. [20] Rasbash DJ, Drysdale D. Fundamentals of smoke production. Toxic gases from burning plastics. QMC Industrial Research 6±7 January 1982, p. 5/1. [21] Drysdale D. An introduction to ®re dynamics. 2nd ed. J Wiley, 1999. p. 300. [Chapter 9]. [22] Tewarson A. SFPE handbook of ®re protection engineering, NFPA. 1995 [Section3/Chapter 4, p.3±91,92]. [23] CAMPUS is described fully via Internet at http://www.campusplastics.com