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Determination of the flammability properties of polymeric materials: A novel method
Polymer Degradation and Stability 96 (2011) 314e319
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Determination of the flammability properties of polymeric materials: A novel method R. Carvel*, T. Steinhaus, G. Rein, J.L. Torero BRE Centre for Fire Safety Engineering, University of Edinburgh, Edinburgh, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2010 Accepted 12 August 2010 Available online 20 August 2010
Standard flammability tests, like the Cone Calorimeter, were developed several decades ago and provided sufficient flammability data for the purposes of the time. However, recent pyrolysis models have revealed the limitations of the standard test in providing adequate data for current flammability analysis and modelling. This paper reviews the assumptions in the standard test and proposes a novel sample holder for the cone calorimeter which incorporates a large block of aluminium at the rear face of the sample under test. This allows the heat losses at the rear face of the sample to be measured precisely and enables more accurate calculation of the material flammability properties. Tests of PA6 and a nano-composite of PA6 & Cloisite 30B, carried out using the standard and new sample holders, are presented and discussed. The peak of high heat release rate observed in standard tests of PA6 is not observed using the novel sample holder, where the burning behaviour of PA6 and the nano-composite material are largely similar. The implications of these observations are discussed. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Cone calorimeter Flammability Heat transfer Nano-composite Fire retardant
1. Introduction Computer fire models such as computational fluid dynamics (CFD) codes are rapidly developing in complexity and capability. Advances in modelling techniques and computer processing speed, as well as an improved scientific understanding of parts of the burning process have enabled these models to be a useful and versatile tool for the fire safety engineer. CFD models are primarily fluid flow models used, in fire applications, to calculate smoke movements and thermal transfer in a given scenario. Until recently, these models were generally used to predict the effects of a prescribed fire. That is, the properties of the fire in terms of heat release rate and species production rate(s) were imposed on the simulation by the user. In recent years a number of pyrolysis models have been developed, aiming to predict the production rates of gaseous fuels from solids subjected to external heat fluxes (e.g., [1e3]). Currently, there is considerable effort being made to combine CFD models with pyrolysis models in order to obtain the capability of predicting fire behaviour from material properties and an adequate description of the environment. For example, a pyrolysis model has been included
* Corresponding author. E-mail address: [email protected] (R. Carvel). 0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.08.010
in the current version of the Fire Dynamics Simulator (FDS) CFD code developed by NIST [4]. One thing that is clear from the development of these pyrolysis models is that current flammability test methods are not able to provide all the input properties required for modelling [5]. The development of test methods and analysis techniques for material flammability properties has not advanced sufficiently to cope with the detailed needs associated with CFD pyrolysis models. Current techniques, for example the ISO 5660 [6] using the Cone Calorimeter were developed in the 1980s and gave both the required parameters and an acceptable level of accuracy for the techniques in use at the time (i.e., Zone Models [7]). However, the limited detail provided by these techniques, based on several assumptions, is not sufficient for current or future applications. An important failing of current test methods regards the definition of the thermal boundary condition at the rear face of the sample in tests such as the cone calorimeter. If the results of a test were to be incorporated to a Zone Model, the aim would be to obtain a realistic Heat Release Rate, thus insulation at the back end would have been deemed as a conservative yet realistic boundary condition [8]. If the test results were to be used to establish the input parameters or to validate a pyrolysis model intended for a CFD code, then a well defined boundary condition is a better choice, as will be discussed. When used to model a realistic situation, the pyrolysis model will incorporate the realistic boundary condition. This approach will allow such models to be used with
R. Carvel et al. / Polymer Degradation and Stability 96 (2011) 314e319
test data to extract more accurate material properties than current methods allow. This paper will review the existing standard test method and analysis, highlighting some of its limitations, and will present a novel approach to the analysis of cone calorimeter data which is able to provide an accurate definition of the rear face boundary condition in the cone test. This novel approach also raises some issues regarding the interpretation of flammability tests carried out in the standard cone calorimeter test. These are also discussed. 2. Standard flammability tests The most common standard flammability apparatus is the cone calorimeter, which is used to assess flammability in ISO 5660 [6], ASTM E 1354 [9], NFPA 264 [10], BS 476 Part 15 [11] and other standards. The main part of the apparatus is shown in Fig. 1. In the standard cone tests, the 100 mm 100 mm sample (generally wrapped in aluminium foil, leaving only the top face exposed) is placed in an insulated sample holder on a load-cell, a short distance beneath a conical radiant heater. This heater subjects the exposed face of the sample to a nominally constant (in time) and uniform (across the surface) heat flux. Provided the external flux is large enough, the sample will eventually begin to pyrolyse and release flammable gases. Once sufficient gases have been released to produce a flammable gas/air mixture in the volume above the surface, the gas is ignited by means of an electrical spark, positioned slightly above the sample surface. The time between the initial exposure of the sample to the external heat flux and the establishment of a persistent flame at the surface is taken to be the ‘ignition delay time’ or ‘time to ignition’, (tig). Air, pyrolysis and combustion gases are drawn away from the cone through a hood and duct positioned above it. Various sensors in the duct record temperature, pressure, velocity, opacity and gas concentrations (oxygen, carbon monoxide and carbon dioxide). From these, the heat release rate (HRR; Q_ ) can be calculated. As the _ is also recorded by the load-cell, the heat of mass loss ðmÞ combustion (DHc) of the sample may be estimated. Calculation of flammability parameters using the cone calorimeter involves carrying out repeated tests of a sample material across a range of heat fluxes. Plotting the inverse of t0.5 ig against heat flux yields a straight line across most of the range of fluxes (the linearity breaks down as the heat flux tends towards the critical heat flux for ignition, as discussed in more detail in [12]). The standard flammability analysis uses the critical heat flux for ignition (experimentally assessed) to estimate the ignition temperature
Extract duct Radiant heater
Sample Aluminium foil Insulation Sample holder
Spark igniter
Load cell Fig. 1. Schematic of cone calorimeter.
315
for the material (Tig) and with the ignition temperature and the gradient of the ignition delay time line calculates the ‘thermal inertia’ of the material under test [13]. The method is described in the standards (e.g. [6]) and is not discussed here. The thermal inertia of the material is not a single property, but is the lumped property krc, where k is the thermal conductivity of the material, r is its density and c is its heat capacity (at constant pressure). Of course, none of k, r or c are constant with temperature, so the thermal inertia estimated in the standard manner is the average value across the temperature range from ambient to Tig. The standard method contains a number of other assumptions which may be appropriate for basic flammability testing (i.e., for ranking of materials) or for input data for Zone Models, but which do not allow assessment of material properties for using in today’s detailed numerical models. The principal assumption in the standard flammability test is the ‘inert solid’ assumption. That is, it is assumed that the solid under test remains physically unchanged and chemically unreactive until the moment of ignition. Of course, this is never the case, as the pyrolysis process must have been ongoing for some time before ignition, such that a flammable gas/air mixture could be generated at the pilot spark location. This has been studied in detail by Dakka et al. [14] and Beaulieu & Dembsey [15] using poly (methyl-methacrylate) (PMMA). They demonstrate that the difference between the pyrolysis delay time and the ignition delay time may be as much as 100% in some instances. Furthermore, as the assessment of tig requires the generation of a flammable gas/air mixture at the pilot location, it is also dependent on the flow conditions around the sample, thus the properties predicted in this manner are dependent on the setup of the apparatus, not merely the material itself. Another standard assumption which may be valid for some, but certainly not all, materials is that there is no in-depth absorption of radiation; all of the heat flux is assumed to be absorbed at the surface. This assumption considerably simplifies the analysis, but has not been adequately justified in the literature. The relative importance of in-depth radiation and pyrolysis chemistry has been studied by Bal and Rein [16]. Finally, the standard analysis simplifies the boundary conditions of the sample by assuming the material is thermally-thick (that is, the thermal wave does not reach the rear face of the sample before ignition occurs), thus may be considered to be a semi-infinite solid, and by fixing simple convective and radiative heat losses at the exposed face of the material the necessary boundary conditions for the analysis are obtained. The heat transfer coefficients commonly cited are 15e25 Wm2 K1 for the convective loss, while the total convective and radiative loss is often taken to be about 45 Wm2 K1 [12]. It is only by making this assumption that the rear face boundary condition can be neglected. While the semi-infinite solid assumption can be considered valid for ignition studies for many materials of low and medium thermal conductivity, it has long been shown that it breaks down once the material starts burning. Many studies have reported “heat feedback” effects from the back end insulation that result in a drastic increase in the burning rate as the heat wave approaches the insulation. Recently these effects have been studied to establish the role of the “heat feedback” in the interpretation of cone calorimetry data [17,18]. While these studies manage to describe the phenomenon [17] and then model it [18], the objective remains the interpretation and prediction of the test behaviour. Here, a different approach is taken, it is recognized that while quantifiable under idealized circumstances, the “heat feedback” effect is an artefact of the difference in thermal conductivity between the sample and the insulation backing and is very difficult to precisely quantify it with complex (i.e., realistic) materials. Thus,
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a precise boundary condition cannot be established in the standard test. By replacing the back end of the sample with a high conductivity inert material, a methodology to define the boundary condition is proposed.
1400 50kW/m2 flux Standard test
PA6 (no nano, no FR) test 1
1200
PA6 (no nano, no FR) test 2 PA6 (with nano, no FR) test 1
1000
3. Materials under test This work was carried out as part of a larger project studying a variety of polymeric materials, some containing intumescent and nano-composite fire retardant additives. The work was partially carried out as part of the PREDFIRE-NANO project (see, e.g. [19]). The wider context of the project is not relevant here, so only two materials will be discussed. For the purposes of this paper only results for polyamide-6 (PA6; Nylon 6), with and without a nanofiller additive, will be presented. The two materials under test are presented in Table 1, along with their flammability properties as estimated using the standard cone calorimeter test. The nano-filler (NC) material used was Cloisite 30B. All samples tested were 100 mm 100 mm 6 mm thick and had been stored at room temperature, in a dry location for several weeks before they were tested. A detailed description of the materials and their properties is given by Samyn et al. [20]. Graphs of the heat release rate of both materials are given in Fig. 2, for standard burning tests in the cone calorimeter, with an external flux of 50 kW m2. Here, the differences in burning behaviour between the two materials, as highlighted in the literature by Kashiwagi et al. [21], are clearly seen. Two runs of each sample are presented, which demonstrate the reproducibility of some features of burning behaviour. If peak HRR in the standard cone test is taken to be a measure of fire retardancy, clearly the nano-filler material has a retardant effect, even though the time to ignition is slightly shorter than for pure PA6 and the initial burning ‘plateau’ is broadly similar in terms of HRR for the first two minutes of burning. 4. Modified flammability test One of the limitations of the standard test analysis, as described above, is that it neglects the effects associated to the back boundary of the material, simply concentrating on Q_ . If DHc was to be _ emphasized, then (by dividing Q_ by m) this effect could be accounted for, but this is not standard practice. If a material is to be studied in detail and pyrolysis rates are to be generated, then the standard approach becomes limited. To overcome this limitation, a novel sample holder was developed which incorporated a large block of aluminium (100 mm 100 mm w20 mm thick) at the rear face of the material. Fig. 3 contrasts the expected in-depth temperature profiles of samples in the standard test (Fig. 3(a)) and the modified test (Fig. 3(b)). The in-depth temperature profiles were confirmed experimentally for tests using the novel sample holder, see Fig. 4. If the heat transfer problem was to be solved for the standard test, then it would be necessary to solve for the entire ensemble of the sample and insulation, accounting for contact resistance at the interface. Given that the material properties and dimensions of the sample change continuously (as do the properties of the insulation
Table 1 The materials under test and their flammability properties derived from a standard analysis. Material
PA6 % (by wt.)
nano-filler %
krc W2 s m4 K2
Tig C
q_ 00crit kW m2
PA6 PA6 þ NC
100 95
0 5
7.98 105 4.83 105
465 462
20.0 19.8
HRR (kW/m2)
PA6 (with nano, no FR) test 2 800
600
400
200
0 0
100
200
300
400
500
600
Time (s)
Fig. 2. Heat release rate graphs for pure PA6 and PA6 þ NC, exposed to an external flux of 50 kW m2, in a standard cone test.
and the contact resistance), this problem becomes a very complex one that is full of uncertainties. In contrast, if the heat conductivity of the heat sink (aluminium block) is much larger than that of the insulation and sample, then by embedding thermocouples into this block, the temperature changes of the block and, hence, the heat transfer to the block during a test could be estimated. Using this approach, the pyrolysis model can concentrate on solving the necessary equations in the sample, contact resistance can be neglected and the heat loss ðq_ 00loss Þ can be obtained from the measurements as follows:
q_ 00loss ¼
1 dT mcp A dt
where m is the mass of the aluminium block, cp the specific heat capacity of Aluminium, A the contact surface area and dT/dt is obtained experimentally. Fig. 4 shows clearly the lumped nature of the aluminium block, thus demonstrating the accuracy of the equation above. The evolving temperature gradients shown in this figure will be the result of solving the energy equation in the sample. The solution of the energy equation can include all the different terms associated with any specific pyrolysis model. Fig. 5 shows the HRR data and aluminium block temperatures for two tests using samples of PA6 þ NC, subjected to external fluxes of 50 and 30 kW m2. It is clear from the data that the thermally-thick/infinite solid assumption is not valid at 30 kW m2 flux as the temperature of the block had risen significantly before ignition. That is, there were considerable heat losses at the rear face of the sample significantly before ignition occurred. However, in the 50 kW m2 case, the temperature of the block did not rise above ambient before ignition, so the thermally-thick assumption may be valid in this instance. Given that the heat capacity of aluminium (0.90 J g1 K1) and the mass of the block (692 g) are both known, it is simple to estimate the heat losses at the rear face of the block during periods of quasi-linear temperature rise of the block. For example, in Fig. 5, the temperature rise of the block in the 50 kW m2 test, between 150 s and 450 s, is approximately 0.24 K s1. Thus the total flux to the block can be calculated using:
q_ ¼
1 dT 0:24 692 0:90 kW mcp ¼ z15 2 A dt 0:1 0:1 m
The 15 kW m2, which is the heat lost to the backing, is almost a third of the incident heat flux. Similarly, in the 30 kW m2 example given, the loss to the backing is reasonably constant at almost 9 kW m2 from about 250 s to 600 s.
R. Carvel et al. / Polymer Degradation and Stability 96 (2011) 314e319
Temperature
317
Temperature Sample Insulation Aluminium Block
a
b
Fig. 3. Schematic of temperature profiles in (a) the standard configuration with insulation backing, and (b) the novel configuration with heat sink (aluminium block).
The modified sample holder allows a more precise estimation of heat transfer in the sample to be developed which, together with a more detailed model of material degradation and burning, will allow the optimisation of the model with experimental data to predict various flammability properties more accurately than the standard test. This is discussed in the following section. During testing of the various samples using the modified sample holder in the cone calorimeter, an interesting observation was made. It was observed that the burning behaviour of the pure PA6 samples and the samples containing the Cloisite nano-filler in the novel sample holder were very similar, see Fig. 6. Specifically, the high peak HRR which was observed using the pure polymer material (and not with the nano-composite) prior to burnout in the standard test, was not observed using the novel sample holder. The differences between the test results for pure PA6 using the standard insulated sample holder and the novel heat loss sample holder are highlighted further in Fig. 7. The results shown in Fig. 7 are consistent with experimental observations made by Schartel et al. [17] who carried out similar tests with a copper plate behind the sample to investigate the influence of the backing material. From these observations it is clear that the period of peak heat release rate, observed in the standard test shortly before burnout, is not a property of the material itself, but is an artefact of the test method. Schartel proposed that the burnout peak is not observed in the standard tests with nano-composite materials because the layer of nanofiller, deposited as the material burns, attenuates the burning in the later part of the test.
0
Further verification of these observations is provided numerically by Wasan et al. [18]. Their results clearly show the influence of the back-face boundary condition on the magnitude of the burnout peak (see Fig. 7 in [18]); as the boundary condition is progressively changed from the assumption of no heat loss, the burnout peak diminishes, and vanishes altogether when the thermal conductivity of the backing is modelled as being about 20 Wm2 K1. The attenuation of the burning proposed by Schartel presumably also occurs in the modified test, using the novel sample holder, but the influence of this effect in the modified test is negligible. No significant fire retardant effect is observed when comparing burning behaviour in tests using pure PA6 with tests using PA6 þ NC in the novel sample holder, see Fig. 6. Indeed, the material containing the nano-filler actually burns with a slightly higher HRR than the pure material in the novel sample holder for most of the burn time as the heat losses are lower. Despite these observations, the magnitude of the burnout peak in standard cone calorimeter tests is still routinely used by some as a measure of flammability, and the apparent reduction of this peak by the inclusion of nano-filler materials is still used by some as an indication of a fire retardancy effect. 5. Discussion The intention of introducing the aluminium block at the rear face of the sample is not specifically to question the validity of the burnout peak as a measure of flammability. The primary function of the block in this study was to introduce an improved boundary condition into the test, so that experimental data could be better fitted to numerical models. Data from standard fire tests are useful for ranking materials, but if the intention of testing is to identify the thermal and pyrolysis
-5 400
HRR (kW/m ) / Temperature (C)
Depth (mm)
-10
-15 After 50s After 100s After 150s After 200s After 250s Sample / Block interface
-20
-25
-30
PA6+NC 50 kW/m2 Temp Al block 50 kW/m2
300
PA6+NC 30 kW/m2 Temp Al block 30 kW/m2
200
100
-35 0
50
100
150
200
250
300
350
Temperature (C)
0 0
250
500
750
1000
Time (s)
Fig. 4. Measured temperatures at three depths in a 12 mm thick sample of PA6 (no additive) and at three depths in the aluminium block; at various times (before ignition) following exposure to a 30 kW m2 radiant heat flux.
Fig. 5. Heat release rate graphs and temperature rise at the rear of the sample, in the modified cone test, for PA6 þ NC at 30 and 50 kW m2.
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R. Carvel et al. / Polymer Degradation and Stability 96 (2011) 314e319 500 50kW/m2 flux Modified test
PA6 (no nano, no FR) test 1 PA6 (no nano, no FR) test 2
400
PA6 (with nano, no FR) test 1 HRR (kW/m2)
PA6 (with nano, no FR) test 2 300
200
100
0 0
100
200
300
400
500
600
Time (s)
Fig. 6. Heat release rates for tests of pure PA6 and PA6 þ NC in the cone calorimeter using the novel sample holder, with a heat flux of 50 kW m2.
thermal properties of the aluminium will also vary during the test, they vary in a known manner which the model can account for. One application of the novel sample holder has been demonstrated by Steinhaus [22] using the model of flaming ignition of solid fuels presented by Torero [12]. He used an optimised set of flammability parameters from cone testing (using the novel sample holder) to predict the time to ignition of another sample of the same material under a completely different heating regime (the heat flux increased with time, rather than being constant) in a different apparatus (the Fire Propagation Apparatus (FPA)). Thus, it has been demonstrated in principle that it is possible to extract a ‘working set’ of parameters from tests using the novel sample holder in the cone calorimeter, which can be used to predict ignition and burning in a different ‘real world’ scenario. Development to extend the work to predict large scale burning is ongoing. Further details are given in the literature and need not be presented here. 6. Conclusions
properties of materials, then it is immensely valuable to know the conditions at the rear face of the sample under test in the cone calorimeter. In the standard test it is impossible to ascertain the position of the thermal wave as it progresses through the material during the test. Using the aluminium block enables the arrival of the thermal wave at the rear face to be measured accurately, thus providing thermal models with more information and allowing a more accurate prediction of the thermal properties of the material. Even if it was possible to introduce enough thermocouples at the interface between the sample material and the insulation and within the insulation to experimentally reproduce the temperature gradients in the standard test, there would still be the problem of assessing the conductivity of the insulation when heated and contaminated by sample traces. The practical problems involved in this process are completely circumvented when using the aluminium block. Furthermore, the block provides a rigid surface to which the sample may be fixed (e.g. using a spot of glue), whereas the boundary between a standard sample and insulation cannot be guaranteed in this way, introducing the problem of contact resistance and an unknown thermal transfer process. The thermal properties of the insulation itself will vary with time and temperature (during the test) and with repeated use (from test to test), due to moisture and degradation of the material itself. While the
A modification of the sample holder in the cone calorimeter test has been presented. This novel method introduces a well defined and measurable heat loss boundary condition at the rear face of the sample under test. The test methodology developed enables the use of data from cone calorimeter tests for validation of pyrolysis models and can be used to obtain material properties for use in such pyrolysis models. Use of the novel method has demonstrated that the peak of heat release rate, observed shortly before burnout for some polymeric materials in standard cone calorimeter tests is not a material property, but is an artefact of the test method. The magnitude (or absence) of this peak has, thus, been demonstrated to have little or no value as a measure of flammability or fire retardancy. Acknowledgements The authors would like to thank FM Global for funding the studentship of Thomas Steinhaus. Thanks also to all the partners in the PREDFIRE-NANO project, especially Dr Alberto Fina at Politecnico de Torino, Prof Richard Hull & Dr Anna Stec at the University of Central Lancashire and Prof Michael Delichatsios & Dr Jianping Zhang at the University of Ulster. The PREDFIRE-NANO project was financially supported by the European Union under Grant No. 013998 in the context of Sixth Framework Programme.
1400 50kW/m2 flux PA6 no nano
References
Standard test 1
1200
Standard test 2 Novel test 1
1000 HRR (kW/m2)
Novel test 2 800
600
400
200
0 0
100
200
300
400
500
600
Time (s)
Fig. 7. Heat release rates for pure PA6 subjected to an external flux of 50 kW m2 in the cone calorimeter, using the standard (insulated) and novel (heat loss) sample holders.
[1] Di Blasi C. Modeling and simulation of combustion processes of charring and non-charring solid fuels. Progress in Energy and Combustion Science 1993;19:71e104. [2] Moghtaderi B. The state-of-the-art in pyrolysis modeling of lignocellulosic solid fuels. Fire and Materials 2006;30:1e34. [3] Stoliarov SI, Crowley S, Lyon RE, Linteris GT. Prediction of the burning rates of non-charring polymers. Combustion and Flame 2009;156:1068e83. [4] Fire Dynamics Simulator. Software, user guide and technical reference guide (four volumes) available for download from, http://fire.nist.gov/fds/. [5] Lautenberger C, Fernandez-Pello C. Generalized pyrolysis model for combustible solids. Fire Safety Journal 2009;44:819e39. [6] ISO 5660e1:1993 Fire tests e reaction to fire e part 1: rate of heat release from building products (cone calorimeter method). [7] W.D Walton, D.J. Carpenter & C.B. Wood "Zone computer fire models for enclosures" chapter 3e7 of SFPE handbook of fire protection engineering (4th ed.). Society of Fire Protection Engineers, Bethesda, Maryland, USA. pp. 3-222 e 3-228 [8] Babrauskas V, Grayson SJ. Heat release in fires. Spon Press; 1992. ISBN: 978e1851667949. [9] ASTME-1354-03. Standard test method for heat and visible smoke release rates for materials and products using an oxygen consumption calorimeter. Philadelphia: American Society for Testing and Materials; 2003.
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Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Determination of the flammability properties of polymeric materials: A novel method R. Carvel*, T. Steinhaus, G. Rein, J.L. Torero BRE Centre for Fire Safety Engineering, University of Edinburgh, Edinburgh, UK
a r t i c l e i n f o
a b s t r a c t
Article history: Received 2 February 2010 Accepted 12 August 2010 Available online 20 August 2010
Standard flammability tests, like the Cone Calorimeter, were developed several decades ago and provided sufficient flammability data for the purposes of the time. However, recent pyrolysis models have revealed the limitations of the standard test in providing adequate data for current flammability analysis and modelling. This paper reviews the assumptions in the standard test and proposes a novel sample holder for the cone calorimeter which incorporates a large block of aluminium at the rear face of the sample under test. This allows the heat losses at the rear face of the sample to be measured precisely and enables more accurate calculation of the material flammability properties. Tests of PA6 and a nano-composite of PA6 & Cloisite 30B, carried out using the standard and new sample holders, are presented and discussed. The peak of high heat release rate observed in standard tests of PA6 is not observed using the novel sample holder, where the burning behaviour of PA6 and the nano-composite material are largely similar. The implications of these observations are discussed. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: Cone calorimeter Flammability Heat transfer Nano-composite Fire retardant
1. Introduction Computer fire models such as computational fluid dynamics (CFD) codes are rapidly developing in complexity and capability. Advances in modelling techniques and computer processing speed, as well as an improved scientific understanding of parts of the burning process have enabled these models to be a useful and versatile tool for the fire safety engineer. CFD models are primarily fluid flow models used, in fire applications, to calculate smoke movements and thermal transfer in a given scenario. Until recently, these models were generally used to predict the effects of a prescribed fire. That is, the properties of the fire in terms of heat release rate and species production rate(s) were imposed on the simulation by the user. In recent years a number of pyrolysis models have been developed, aiming to predict the production rates of gaseous fuels from solids subjected to external heat fluxes (e.g., [1e3]). Currently, there is considerable effort being made to combine CFD models with pyrolysis models in order to obtain the capability of predicting fire behaviour from material properties and an adequate description of the environment. For example, a pyrolysis model has been included
* Corresponding author. E-mail address: [email protected] (R. Carvel). 0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.08.010
in the current version of the Fire Dynamics Simulator (FDS) CFD code developed by NIST [4]. One thing that is clear from the development of these pyrolysis models is that current flammability test methods are not able to provide all the input properties required for modelling [5]. The development of test methods and analysis techniques for material flammability properties has not advanced sufficiently to cope with the detailed needs associated with CFD pyrolysis models. Current techniques, for example the ISO 5660 [6] using the Cone Calorimeter were developed in the 1980s and gave both the required parameters and an acceptable level of accuracy for the techniques in use at the time (i.e., Zone Models [7]). However, the limited detail provided by these techniques, based on several assumptions, is not sufficient for current or future applications. An important failing of current test methods regards the definition of the thermal boundary condition at the rear face of the sample in tests such as the cone calorimeter. If the results of a test were to be incorporated to a Zone Model, the aim would be to obtain a realistic Heat Release Rate, thus insulation at the back end would have been deemed as a conservative yet realistic boundary condition [8]. If the test results were to be used to establish the input parameters or to validate a pyrolysis model intended for a CFD code, then a well defined boundary condition is a better choice, as will be discussed. When used to model a realistic situation, the pyrolysis model will incorporate the realistic boundary condition. This approach will allow such models to be used with
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test data to extract more accurate material properties than current methods allow. This paper will review the existing standard test method and analysis, highlighting some of its limitations, and will present a novel approach to the analysis of cone calorimeter data which is able to provide an accurate definition of the rear face boundary condition in the cone test. This novel approach also raises some issues regarding the interpretation of flammability tests carried out in the standard cone calorimeter test. These are also discussed. 2. Standard flammability tests The most common standard flammability apparatus is the cone calorimeter, which is used to assess flammability in ISO 5660 [6], ASTM E 1354 [9], NFPA 264 [10], BS 476 Part 15 [11] and other standards. The main part of the apparatus is shown in Fig. 1. In the standard cone tests, the 100 mm 100 mm sample (generally wrapped in aluminium foil, leaving only the top face exposed) is placed in an insulated sample holder on a load-cell, a short distance beneath a conical radiant heater. This heater subjects the exposed face of the sample to a nominally constant (in time) and uniform (across the surface) heat flux. Provided the external flux is large enough, the sample will eventually begin to pyrolyse and release flammable gases. Once sufficient gases have been released to produce a flammable gas/air mixture in the volume above the surface, the gas is ignited by means of an electrical spark, positioned slightly above the sample surface. The time between the initial exposure of the sample to the external heat flux and the establishment of a persistent flame at the surface is taken to be the ‘ignition delay time’ or ‘time to ignition’, (tig). Air, pyrolysis and combustion gases are drawn away from the cone through a hood and duct positioned above it. Various sensors in the duct record temperature, pressure, velocity, opacity and gas concentrations (oxygen, carbon monoxide and carbon dioxide). From these, the heat release rate (HRR; Q_ ) can be calculated. As the _ is also recorded by the load-cell, the heat of mass loss ðmÞ combustion (DHc) of the sample may be estimated. Calculation of flammability parameters using the cone calorimeter involves carrying out repeated tests of a sample material across a range of heat fluxes. Plotting the inverse of t0.5 ig against heat flux yields a straight line across most of the range of fluxes (the linearity breaks down as the heat flux tends towards the critical heat flux for ignition, as discussed in more detail in [12]). The standard flammability analysis uses the critical heat flux for ignition (experimentally assessed) to estimate the ignition temperature
Extract duct Radiant heater
Sample Aluminium foil Insulation Sample holder
Spark igniter
Load cell Fig. 1. Schematic of cone calorimeter.
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for the material (Tig) and with the ignition temperature and the gradient of the ignition delay time line calculates the ‘thermal inertia’ of the material under test [13]. The method is described in the standards (e.g. [6]) and is not discussed here. The thermal inertia of the material is not a single property, but is the lumped property krc, where k is the thermal conductivity of the material, r is its density and c is its heat capacity (at constant pressure). Of course, none of k, r or c are constant with temperature, so the thermal inertia estimated in the standard manner is the average value across the temperature range from ambient to Tig. The standard method contains a number of other assumptions which may be appropriate for basic flammability testing (i.e., for ranking of materials) or for input data for Zone Models, but which do not allow assessment of material properties for using in today’s detailed numerical models. The principal assumption in the standard flammability test is the ‘inert solid’ assumption. That is, it is assumed that the solid under test remains physically unchanged and chemically unreactive until the moment of ignition. Of course, this is never the case, as the pyrolysis process must have been ongoing for some time before ignition, such that a flammable gas/air mixture could be generated at the pilot spark location. This has been studied in detail by Dakka et al. [14] and Beaulieu & Dembsey [15] using poly (methyl-methacrylate) (PMMA). They demonstrate that the difference between the pyrolysis delay time and the ignition delay time may be as much as 100% in some instances. Furthermore, as the assessment of tig requires the generation of a flammable gas/air mixture at the pilot location, it is also dependent on the flow conditions around the sample, thus the properties predicted in this manner are dependent on the setup of the apparatus, not merely the material itself. Another standard assumption which may be valid for some, but certainly not all, materials is that there is no in-depth absorption of radiation; all of the heat flux is assumed to be absorbed at the surface. This assumption considerably simplifies the analysis, but has not been adequately justified in the literature. The relative importance of in-depth radiation and pyrolysis chemistry has been studied by Bal and Rein [16]. Finally, the standard analysis simplifies the boundary conditions of the sample by assuming the material is thermally-thick (that is, the thermal wave does not reach the rear face of the sample before ignition occurs), thus may be considered to be a semi-infinite solid, and by fixing simple convective and radiative heat losses at the exposed face of the material the necessary boundary conditions for the analysis are obtained. The heat transfer coefficients commonly cited are 15e25 Wm2 K1 for the convective loss, while the total convective and radiative loss is often taken to be about 45 Wm2 K1 [12]. It is only by making this assumption that the rear face boundary condition can be neglected. While the semi-infinite solid assumption can be considered valid for ignition studies for many materials of low and medium thermal conductivity, it has long been shown that it breaks down once the material starts burning. Many studies have reported “heat feedback” effects from the back end insulation that result in a drastic increase in the burning rate as the heat wave approaches the insulation. Recently these effects have been studied to establish the role of the “heat feedback” in the interpretation of cone calorimetry data [17,18]. While these studies manage to describe the phenomenon [17] and then model it [18], the objective remains the interpretation and prediction of the test behaviour. Here, a different approach is taken, it is recognized that while quantifiable under idealized circumstances, the “heat feedback” effect is an artefact of the difference in thermal conductivity between the sample and the insulation backing and is very difficult to precisely quantify it with complex (i.e., realistic) materials. Thus,
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a precise boundary condition cannot be established in the standard test. By replacing the back end of the sample with a high conductivity inert material, a methodology to define the boundary condition is proposed.
1400 50kW/m2 flux Standard test
PA6 (no nano, no FR) test 1
1200
PA6 (no nano, no FR) test 2 PA6 (with nano, no FR) test 1
1000
3. Materials under test This work was carried out as part of a larger project studying a variety of polymeric materials, some containing intumescent and nano-composite fire retardant additives. The work was partially carried out as part of the PREDFIRE-NANO project (see, e.g. [19]). The wider context of the project is not relevant here, so only two materials will be discussed. For the purposes of this paper only results for polyamide-6 (PA6; Nylon 6), with and without a nanofiller additive, will be presented. The two materials under test are presented in Table 1, along with their flammability properties as estimated using the standard cone calorimeter test. The nano-filler (NC) material used was Cloisite 30B. All samples tested were 100 mm 100 mm 6 mm thick and had been stored at room temperature, in a dry location for several weeks before they were tested. A detailed description of the materials and their properties is given by Samyn et al. [20]. Graphs of the heat release rate of both materials are given in Fig. 2, for standard burning tests in the cone calorimeter, with an external flux of 50 kW m2. Here, the differences in burning behaviour between the two materials, as highlighted in the literature by Kashiwagi et al. [21], are clearly seen. Two runs of each sample are presented, which demonstrate the reproducibility of some features of burning behaviour. If peak HRR in the standard cone test is taken to be a measure of fire retardancy, clearly the nano-filler material has a retardant effect, even though the time to ignition is slightly shorter than for pure PA6 and the initial burning ‘plateau’ is broadly similar in terms of HRR for the first two minutes of burning. 4. Modified flammability test One of the limitations of the standard test analysis, as described above, is that it neglects the effects associated to the back boundary of the material, simply concentrating on Q_ . If DHc was to be _ emphasized, then (by dividing Q_ by m) this effect could be accounted for, but this is not standard practice. If a material is to be studied in detail and pyrolysis rates are to be generated, then the standard approach becomes limited. To overcome this limitation, a novel sample holder was developed which incorporated a large block of aluminium (100 mm 100 mm w20 mm thick) at the rear face of the material. Fig. 3 contrasts the expected in-depth temperature profiles of samples in the standard test (Fig. 3(a)) and the modified test (Fig. 3(b)). The in-depth temperature profiles were confirmed experimentally for tests using the novel sample holder, see Fig. 4. If the heat transfer problem was to be solved for the standard test, then it would be necessary to solve for the entire ensemble of the sample and insulation, accounting for contact resistance at the interface. Given that the material properties and dimensions of the sample change continuously (as do the properties of the insulation
Table 1 The materials under test and their flammability properties derived from a standard analysis. Material
PA6 % (by wt.)
nano-filler %
krc W2 s m4 K2
Tig C
q_ 00crit kW m2
PA6 PA6 þ NC
100 95
0 5
7.98 105 4.83 105
465 462
20.0 19.8
HRR (kW/m2)
PA6 (with nano, no FR) test 2 800
600
400
200
0 0
100
200
300
400
500
600
Time (s)
Fig. 2. Heat release rate graphs for pure PA6 and PA6 þ NC, exposed to an external flux of 50 kW m2, in a standard cone test.
and the contact resistance), this problem becomes a very complex one that is full of uncertainties. In contrast, if the heat conductivity of the heat sink (aluminium block) is much larger than that of the insulation and sample, then by embedding thermocouples into this block, the temperature changes of the block and, hence, the heat transfer to the block during a test could be estimated. Using this approach, the pyrolysis model can concentrate on solving the necessary equations in the sample, contact resistance can be neglected and the heat loss ðq_ 00loss Þ can be obtained from the measurements as follows:
q_ 00loss ¼
1 dT mcp A dt
where m is the mass of the aluminium block, cp the specific heat capacity of Aluminium, A the contact surface area and dT/dt is obtained experimentally. Fig. 4 shows clearly the lumped nature of the aluminium block, thus demonstrating the accuracy of the equation above. The evolving temperature gradients shown in this figure will be the result of solving the energy equation in the sample. The solution of the energy equation can include all the different terms associated with any specific pyrolysis model. Fig. 5 shows the HRR data and aluminium block temperatures for two tests using samples of PA6 þ NC, subjected to external fluxes of 50 and 30 kW m2. It is clear from the data that the thermally-thick/infinite solid assumption is not valid at 30 kW m2 flux as the temperature of the block had risen significantly before ignition. That is, there were considerable heat losses at the rear face of the sample significantly before ignition occurred. However, in the 50 kW m2 case, the temperature of the block did not rise above ambient before ignition, so the thermally-thick assumption may be valid in this instance. Given that the heat capacity of aluminium (0.90 J g1 K1) and the mass of the block (692 g) are both known, it is simple to estimate the heat losses at the rear face of the block during periods of quasi-linear temperature rise of the block. For example, in Fig. 5, the temperature rise of the block in the 50 kW m2 test, between 150 s and 450 s, is approximately 0.24 K s1. Thus the total flux to the block can be calculated using:
q_ ¼
1 dT 0:24 692 0:90 kW mcp ¼ z15 2 A dt 0:1 0:1 m
The 15 kW m2, which is the heat lost to the backing, is almost a third of the incident heat flux. Similarly, in the 30 kW m2 example given, the loss to the backing is reasonably constant at almost 9 kW m2 from about 250 s to 600 s.
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Temperature
317
Temperature Sample Insulation Aluminium Block
a
b
Fig. 3. Schematic of temperature profiles in (a) the standard configuration with insulation backing, and (b) the novel configuration with heat sink (aluminium block).
The modified sample holder allows a more precise estimation of heat transfer in the sample to be developed which, together with a more detailed model of material degradation and burning, will allow the optimisation of the model with experimental data to predict various flammability properties more accurately than the standard test. This is discussed in the following section. During testing of the various samples using the modified sample holder in the cone calorimeter, an interesting observation was made. It was observed that the burning behaviour of the pure PA6 samples and the samples containing the Cloisite nano-filler in the novel sample holder were very similar, see Fig. 6. Specifically, the high peak HRR which was observed using the pure polymer material (and not with the nano-composite) prior to burnout in the standard test, was not observed using the novel sample holder. The differences between the test results for pure PA6 using the standard insulated sample holder and the novel heat loss sample holder are highlighted further in Fig. 7. The results shown in Fig. 7 are consistent with experimental observations made by Schartel et al. [17] who carried out similar tests with a copper plate behind the sample to investigate the influence of the backing material. From these observations it is clear that the period of peak heat release rate, observed in the standard test shortly before burnout, is not a property of the material itself, but is an artefact of the test method. Schartel proposed that the burnout peak is not observed in the standard tests with nano-composite materials because the layer of nanofiller, deposited as the material burns, attenuates the burning in the later part of the test.
0
Further verification of these observations is provided numerically by Wasan et al. [18]. Their results clearly show the influence of the back-face boundary condition on the magnitude of the burnout peak (see Fig. 7 in [18]); as the boundary condition is progressively changed from the assumption of no heat loss, the burnout peak diminishes, and vanishes altogether when the thermal conductivity of the backing is modelled as being about 20 Wm2 K1. The attenuation of the burning proposed by Schartel presumably also occurs in the modified test, using the novel sample holder, but the influence of this effect in the modified test is negligible. No significant fire retardant effect is observed when comparing burning behaviour in tests using pure PA6 with tests using PA6 þ NC in the novel sample holder, see Fig. 6. Indeed, the material containing the nano-filler actually burns with a slightly higher HRR than the pure material in the novel sample holder for most of the burn time as the heat losses are lower. Despite these observations, the magnitude of the burnout peak in standard cone calorimeter tests is still routinely used by some as a measure of flammability, and the apparent reduction of this peak by the inclusion of nano-filler materials is still used by some as an indication of a fire retardancy effect. 5. Discussion The intention of introducing the aluminium block at the rear face of the sample is not specifically to question the validity of the burnout peak as a measure of flammability. The primary function of the block in this study was to introduce an improved boundary condition into the test, so that experimental data could be better fitted to numerical models. Data from standard fire tests are useful for ranking materials, but if the intention of testing is to identify the thermal and pyrolysis
-5 400
HRR (kW/m ) / Temperature (C)
Depth (mm)
-10
-15 After 50s After 100s After 150s After 200s After 250s Sample / Block interface
-20
-25
-30
PA6+NC 50 kW/m2 Temp Al block 50 kW/m2
300
PA6+NC 30 kW/m2 Temp Al block 30 kW/m2
200
100
-35 0
50
100
150
200
250
300
350
Temperature (C)
0 0
250
500
750
1000
Time (s)
Fig. 4. Measured temperatures at three depths in a 12 mm thick sample of PA6 (no additive) and at three depths in the aluminium block; at various times (before ignition) following exposure to a 30 kW m2 radiant heat flux.
Fig. 5. Heat release rate graphs and temperature rise at the rear of the sample, in the modified cone test, for PA6 þ NC at 30 and 50 kW m2.
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R. Carvel et al. / Polymer Degradation and Stability 96 (2011) 314e319 500 50kW/m2 flux Modified test
PA6 (no nano, no FR) test 1 PA6 (no nano, no FR) test 2
400
PA6 (with nano, no FR) test 1 HRR (kW/m2)
PA6 (with nano, no FR) test 2 300
200
100
0 0
100
200
300
400
500
600
Time (s)
Fig. 6. Heat release rates for tests of pure PA6 and PA6 þ NC in the cone calorimeter using the novel sample holder, with a heat flux of 50 kW m2.
thermal properties of the aluminium will also vary during the test, they vary in a known manner which the model can account for. One application of the novel sample holder has been demonstrated by Steinhaus [22] using the model of flaming ignition of solid fuels presented by Torero [12]. He used an optimised set of flammability parameters from cone testing (using the novel sample holder) to predict the time to ignition of another sample of the same material under a completely different heating regime (the heat flux increased with time, rather than being constant) in a different apparatus (the Fire Propagation Apparatus (FPA)). Thus, it has been demonstrated in principle that it is possible to extract a ‘working set’ of parameters from tests using the novel sample holder in the cone calorimeter, which can be used to predict ignition and burning in a different ‘real world’ scenario. Development to extend the work to predict large scale burning is ongoing. Further details are given in the literature and need not be presented here. 6. Conclusions
properties of materials, then it is immensely valuable to know the conditions at the rear face of the sample under test in the cone calorimeter. In the standard test it is impossible to ascertain the position of the thermal wave as it progresses through the material during the test. Using the aluminium block enables the arrival of the thermal wave at the rear face to be measured accurately, thus providing thermal models with more information and allowing a more accurate prediction of the thermal properties of the material. Even if it was possible to introduce enough thermocouples at the interface between the sample material and the insulation and within the insulation to experimentally reproduce the temperature gradients in the standard test, there would still be the problem of assessing the conductivity of the insulation when heated and contaminated by sample traces. The practical problems involved in this process are completely circumvented when using the aluminium block. Furthermore, the block provides a rigid surface to which the sample may be fixed (e.g. using a spot of glue), whereas the boundary between a standard sample and insulation cannot be guaranteed in this way, introducing the problem of contact resistance and an unknown thermal transfer process. The thermal properties of the insulation itself will vary with time and temperature (during the test) and with repeated use (from test to test), due to moisture and degradation of the material itself. While the
A modification of the sample holder in the cone calorimeter test has been presented. This novel method introduces a well defined and measurable heat loss boundary condition at the rear face of the sample under test. The test methodology developed enables the use of data from cone calorimeter tests for validation of pyrolysis models and can be used to obtain material properties for use in such pyrolysis models. Use of the novel method has demonstrated that the peak of heat release rate, observed shortly before burnout for some polymeric materials in standard cone calorimeter tests is not a material property, but is an artefact of the test method. The magnitude (or absence) of this peak has, thus, been demonstrated to have little or no value as a measure of flammability or fire retardancy. Acknowledgements The authors would like to thank FM Global for funding the studentship of Thomas Steinhaus. Thanks also to all the partners in the PREDFIRE-NANO project, especially Dr Alberto Fina at Politecnico de Torino, Prof Richard Hull & Dr Anna Stec at the University of Central Lancashire and Prof Michael Delichatsios & Dr Jianping Zhang at the University of Ulster. The PREDFIRE-NANO project was financially supported by the European Union under Grant No. 013998 in the context of Sixth Framework Programme.
1400 50kW/m2 flux PA6 no nano
References
Standard test 1
1200
Standard test 2 Novel test 1
1000 HRR (kW/m2)
Novel test 2 800
600
400
200
0 0
100
200
300
400
500
600
Time (s)
Fig. 7. Heat release rates for pure PA6 subjected to an external flux of 50 kW m2 in the cone calorimeter, using the standard (insulated) and novel (heat loss) sample holders.
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