Evaluating ignition data using the flux time product

A protocol based on the flux time product (FTP)¹ is used to analyze ignition data obtained from the Cone Calorimeter under an impressed flux in the range 20–70 KWm^?2 for different orientations and modes of ignition for conditioned cellulosic materials. The mean, maximum and minimum ignition times are depicted graphically by orientation and mode of ignition. Flux time products, FTP indices, critical irradiances and estimates of the convective heat loss associated with a change in specimen orientation are derived using the mean time-to-ignition data. It is demonstrated that consideration of the thermal thickness of a specimen may not be necessary when the proposed FTP methodology is utilized to determine valid correlations between the time-to-ignition and the incident radiant flux.     

Ignition and combustion of low‐density polyimide foam

The ignition, flaming and smoldering combustion of low‐density polyimide foam have been studied using a cone calorimeter. Low‐density polyimide foam exhibits a high ignition resistance. The minimum heat flux for the ignition of flaming combustion ranges from 48 to 54 kW/m². This minimum heat flux also indicates the heat flux for transition from smoldering to flaming combustion. The flaming combustion results show that the heat release rate of low‐density polyimide foam is very low even at a high incident heat flux of 75 kW/m². The smoldering combustion results show that the smoldering of low‐density polyimide foam becomes significant when the incident heat flux is greater than 30 kW/m². The smoldering combustion of low‐density polyimide foam cannot be self‐sustaining when the external heat source is removed. Copyright © 2003 John Wiley & Sons, Ltd.     

Comparison of cone and OSU calorimetric techniques to assess the flammability behaviour of fabrics used for aircraft interiors

Two test methods for measuring the heat release rate, HRR have been compared on fabric composites used for aircraft interior materials as side‐wall panels. These methods are based on the principles of direct measurement of the convective and radiant heat by thermopiles using an Ohio State University (OSU) calorimeter, and oxygen consumption using a cone calorimeter. It has been observed when tested by standard procedures, cone results at 35 kW/m² incident heat flux do not correlate with OSU results at the same heat flux. This is because in the cone calorimeter, the sample is mounted horizontally whereas the OSU calorimetric method requires vertical sampling with exposure to a vertical radiant panel. A further difference between the two techniques is the ignition source—in the cone it is spark ignition, whereas in the OSU it is flame ignition; hence, samples in the OSU calorimeter ignite more easily compared to those in the cone under the same incident heat fluxes. However, in this paper we demonstrate that cone calorimetric exposure at 50 kW/m² heat flux gives similar peak heat release results as the 35 kW/m² heat flux of OSU calorimeter, but significantly different average and total heat release values over a 2 min period. The performance differences associated with these two techniques are also discussed. Moreover, the effects of structure, i.e. type of fibres used in warp/weft direction and design of fabric are also analysed with respect to heat release behaviour and their correlation discussed. Copyright © 2005 John Wiley & Sons, Ltd.     

Experimental study on the radiative ignition of silicones

Silicones comprise a wide variety of materials such as fluids, elastomers, resins, and foams. This paper reports the ignitability of some typical silicones under various external radiant heat fluxes. The ignitability of silicones was studied using a cone calorimeter under radiant heat flux levels of 0.5–60 kW m⁻². The time to ignition of the silicones was found to be proportional to a power of the incident heat flux that varies from −1.33 to −2.84. For silicone fluids, viscosity (or molecular size) is the key variable in controlling the ignitability. For silicone elastomers, the fillers play an important role in controlling the ignitability, especially at incident heat fluxes lower than 35 kW m⁻². The ignitability of silicone resins depends on the chemical structure of the resins: the pure trifunctional resin has the lowest ignitability. The ignitability of the silicone foams having the same density depends on the foam thickness, especially at incident heat fluxes lower than 30 kW m⁻². © 1998 John Wiley & Sons, Ltd.     

Ignition performance of new and used motor vehicle upholstery fabrics

This paper examines the standards for fire safety in transport systems and in particular the test method for the flammability of materials within passenger compartments of motor vehicles. The paper compares data from ignition tests conducted in the cone calorimeter and the FIST apparatus with tests conducted using the FMVSS 302 horizontal flame spread apparatus. Ten materials were selected as representative of those used as seat coverings of private and commercial passenger vehicles. The time to ignition of new and used materials subject to exposure heat fluxes between 20 kW/m² and 40 kW/m² was measured. The results from the ignition tests were analysed using thermally thick and thermally thin theoretical models. The critical heat flux for sustained piloted ignition was determined from the time to ignition data using the thermally thin approach. Derived ignition temperatures from both the thermally thick and thermally thin methods were compared with measurements using a thermocouple attached to the back surface of materials in selected tests. The flame spread rates in the FMVSS 302 apparatus were determined and a comparison was made between the performance of the materials in the flame spread apparatus, the cone calorimeter and the FIST. The results suggests that a critical heat flux criterion could be used to provide an equivalent pass/fail performance requirement to that specified by the horizontal flame spread test although further testing is needed to support this. Copyright © 2004 John Wiley & Sons, Ltd.     

Controlled-atmosphere cone calorimeter studies of silicones

A controlled-atmosphere cone calorimeter was used to investigate the burning of a silicone fluid and two silicone elastomers. The silicone materials were tested at 50 kW/m² incident heat flux in environments containing 15–30% oxygen. The test results were compared with a high molecular weight hydrocarbon fluid and an ethylene propylene rubber in terms of time to ignition, peak heat release rate and total heat released, carbon monoxide yield and carbon monoxide production rate, and smoke production and smoke production rate. The data from this study show that when materials burn in oxygen-enriched, normal, and vitiated atmospheres, silicone-based materials have a comparatively low peak heat release rate, total heat released, average CO production rate, and average smoke production rate as compared with organic-based materials. The smoke production and smoke production rate of silicone elastomers can be significantly reduced by adding appropriate smoke suppressants and additives. © 1997 John Wiley & Sons, Ltd.     

Flame spread measurements on wood products using the ASTM E 1321 LIFT apparatus and a reduced scale adaptation of the cone calorimeter

Flame spread experiments were conducted in an ASTM E 1321 lateral ignition and flame transport (LIFT) apparatus and a reduced scale ignition and flame spread test (RIFT) adaptation of the cone calorimeter. Wood‐based products were tested and a flame spread model was applied to the results to obtain the flame spread parameter and the minimum heat flux required for flame spread. The materials used were plywood, medium density fibreboard, hardboard, two‐particle board products, Melamine (Melteca) covered products with two types of wood substrate along with New Zealand grown Rimu, Beech, Macrocarpa and Radiata Pine. The RIFT gave comparable results to the LIFT for several of the materials investigated. There appeared to be an effective limit on suitable materials that can be successfully tested in the RIFT to those that have a minimum flux for flame spread of less than 7kW/m². This limitation was due to the rapid decay of the heat flux profile along the sample and the lower resolution dictated by the smaller size of the RIFT apparatus. It was found that the limit on the minimum heat flux for flame spread was approximately equivalent to a minimum ignition flux of 18kW/m². Copyright © 2009 John Wiley & Sons, Ltd.     

Application of cone calorimeter for the assessment of class of flame retardants for polypropylene

The study presented addresses the fire behaviour of polypropylene compounded with six classes of flame retardants. The application of cone calorimetry for the assessment of the thermal characteristics of the tested materials and their comparison with thermogravimetry are the central point of this research. This study only presents data for 25 kW/m² of incident heat flux exposure and includes five tests for polypropylene with no additives and five tests for polypropylene with flame retardants based on triglycidylisocyanurate and lignin. The data collected include the rate of heat release, mass loss rate, char yield, time to ignition and time of total combustion. Results represent meaningful comparison between the behaviour of the materials under simulated fire conditions, using the cone calorimeter, and in the slow dynamic environment utilized in thermogravimetric analysis. © 1998 John Wiley & Sons Ltd.     

Flammability of GRP for use in ship superstructures

The burning characteristics of glass-reinforced panels with an isophthalic polyester resin, the same resin with an inorganic flame retardant, two differing vinylester resins or a resole phenolic as the matrix were tested at a range of incident heat flux values using a cone calorimeter. The phenolic composite was superior at all levels showing a longer ignition time, reduced heat output, less contribution to a low-level sustained fire (25 kWm^?2) and lower smoke yield.     

Laser‐Induced Deflagration of Unconfined HMX – The Effect of Energetic Binders

The effect of three energetic binders [poly(3‐methyl‐3‐nitratomethyloxetane (polyNIMMO), polyglycidyl nitrate (polyGLYN) and an energetic polyphosphazene (PPZ‐E) – all at 10%] on the unconfined laser‐induced deflagration of cyclotetramethylene tetranitramine, commonly known as High Melting point Explosive (HMX) by a near IR (NIR) diode laser (801 nm) has been examined. Hydroxyl terminated polybutadiene (HTPB) and PPZ (the precursor to PPZ‐E – before nitration) were used as reference materials. The formulations required the addition of an optical sensitizer – carbon black (CB) – for ignition. At the designated threshold flux density of 2.3 kW cm⁻², a minimum of ∼1 wt.‐% CB was needed for the reliable ignition of unbound HMX and its formulations with polyGLYN, PPZ‐E and PPZ. Under similar conditions HMX/polyNIMMO and HMX/HTPB required 3% CB. Ignition maps (ignition time versus laser flux density) have been constructed for the five formulations. Comparison of ignition times and ignition energy densities for HMX and HMX/polyGLYN showed this binder to have only a marginal effect. In contrast, HTPB, PPZ and PPZ‐E all retarded HMX ignition at the threshold flux density, but showed negligible effect at higher flux densities. As PPZ and PPZ‐E produced both similar delays in the ignition time and similar increases in the flame development times (10–90%) at the threshold flux density, the inhibition of the HMX ignition by these PPZs appears to be largely independent of the polymer energy content. Such characteristics could be useful for high performance and insensitive energetic formulations. PolyNIMMO (3% CB) increased the ignition time of HMX only slightly at 2.3 kW cm⁻². However, at this threshold flux level the HMX flame development times with polyNIMMO or HTPB were much longer than that for the unbound material; this effect is attributed to the enhanced CB content.     

Radiative Ignition of Fine‐Ammonium Perchlorate Composite Propellants

Radiative ignition of quasi‐homogeneous mixtures of ammonium perchlorate (AP) and hydroxyterminated polybutadiene (HTPB) binder has been investigated experimentally. Solid propellants consisting of fine AP (2 μm) and HTPB binder (~ 76/24% by mass) were ignited by CO₂ laser radiation. The lower boundary of a go/no‐go ignition map (minimum ignition time vs. heat flux) was obtained. Opacity was varied by adding carbon black up to 1% by mass. Ignition times ranged from 0.78 s to 0.076 s for incident fluxes ranging from 60 W/cm² to 400 W/cm². It was found that AP and HTPB are sufficiently strongly absorbing of 10.6 μm CO₂ laser radiation (absorption coefficient ≈250 cm⁻¹) so that the addition of carbon black in amounts typical of catalysts or opacitymodifying agents (up to 1%) would have only a small influence on radiative ignition times at 10.6 μm. A simple theoretical analysis indicated that the ignition time‐flux data are consistent with in‐depth absorption effects. Furthermore, this analysis showed that the assumption of surface absorption is not appropriate, even for this relatively opaque system. For broadband visible/near‐infrared radiation, such as from burning metal/oxide particle systems, the effects of in‐depth absorption would probably be even stronger.     

Study of Ignition of High-Energy Materials with Boron and Aluminum and Titanium Diborides

This paper describes the ignition of high-energy materials (HEMs) on the basis of ammonium perchlorate and ammonium nitrate and an energetic binder, containing the powders of Al (base composition), B, AlB₂, AlB₁₂, and TiB₂, upon initiation of the process by a CO2 laser in the heat flux density range of 90–200 W/cm². The ignition delay time and surface temperature of the reaction layer during the heating and ignition of HEMs in air are determined. It is obtained that the complete replacement of a micron-sized aluminum powder by amorphous boron in the composition of HEMs significantly reduces the ignition delay time of the sample (by 2.2–2.8 times) with the same heat flux density, and this occurs due to the high chemical activity of and difference between the mechanisms of oxidation of boron particles. The use of aluminum diboride in HEMs reduces the ignition delay time by 1.7–2.2 times in comparison with the base composition. The ignition delay time of the HEM sample with titanium diboride decreases slightly (by 10–25%) relative to the ignition delay time of the base composition.     

On the thermal ignition of combustible materials

Formulas are derived for the time to achieve the ignition temperature as a function of the incident heat flux and the various thermophysical material parameters for thermally thick, thermally thin and thermally intermediate solid combustibles. Predictions are compared with recent experimental data for various natural wood species and wood products, and to previous data for wood and thermoplastics. The correlations are excellent when (1) the physical parameters used as the axes of the plots are chosen consistent with those of the theoretical formulas and (2) the experiments and the materials do not violate any of the restrictions imposed by the theory. From these plots it is easy to estimate the minimum heat flux for ignition, which is of great importance both in practice and for making theoretical predictions.     

Spectral Acquisition and Calibration Techniques for the Measurement of Radiative Flux Incident upon Propellant

A technique for the absolute calibration of a time‐resolved spectrographic system has been developed at QinetiQ, specifically designed to be relevant to spectral acquisition from within the interior of translucent gun‐propellant samples. The technique has shown itself to be particularly useful in the realm of propellant ignition as it allows for the precise determination of the moment that propellant combustion processes begin, as well as measuring the incident radiative flux leading up to ignition. Scope exists to extend its use for high‐pressure measurements of the incident radiative flux during both conventional propellant burn and high‐powered electrothermal‐chemical (ETC) discharges. This paper sets out to describe both the technique and some of the pitfalls encountered during the development of the technique. The use of this technique in some of the experimental work performed at QinetiQ, including the results of measurements that compare the incident radiative flux with propellant ignition during both ETC discharges and conventional gunpowder burn, have been published separately; references for this experimental work are given in this paper.     

Flammability characteristics at applied heat flux levels up to 200 kW/m2

This paper documents the first of the two interrelated studies that were conducted to more fundamentally understand the scalability of flame heat flux, the motivation being that it has been reported that flame heat flux back to the burning surface in bench‐scale experiments is not the same as for large‐scale fires. The key aspect was the use of real scale applied heat flux up to 200kW/m² which is well beyond that typically considered in contemporary testing. The main conclusions are that decomposition kinetics needs to be included in the study of ignition and the energy balance for steady burning is too simplistic to represent the physics occurring. An unexpected non‐linear trend is observed in the typical plotting methods currently used in fire protection engineering for ignition and mass loss flux data for several materials tested and this non‐linearity is a true material response. Using measured temperature profiles in the condensed phase shows that viewing ignition as an inert material process is inaccurate at predicting the surface temperature at higher heat fluxes. The steady burning temperature profiles appear to be invariant with applied heat flux. This possible inaccuracy was investigated by obtaining the heat of gasification via the ‘typical technique’ using the mass loss flux data and comparing it to the commonly considered ‘fundamental’ value obtained from differential scanning calorimetry measurements. This comparison suggests that the ‘typical technique’ energy balance is too simplified to represent the physics occurring for any range of applied heat flux. Observed bubbling and melting phenomena provide a possible direction of study. Copyright © 2007 John Wiley & Sons, Ltd.     

The effects of geometry and ignition mode on ignition times obtained using a cone calorimeter and ISO ignitability apparatus

Three timber-based materials were investigated by subjecting them to a constant uniform heat flux in the range 20–70 kW m^?2 using a Cone Calorimeter and the ISO Ignitability Apparatus. The specimens were examined in the vertical and horizontal orientations in the Cone Calorimeter using gas flame pilot, spark pilot and spontaneous methods of ignition. They were also studied using the ISO Ignitability Apparatus in the normal and inverted horizontal orientations using gas flame pilot and spontaneous methods of ignition. The results obtained are compared by specimen, orientation and mode of ignition.     

Further variations of smoke density with heat flux

The heat flux in the NBS smoke chamber has been varied from 1.0 to 5.0 W cm^?2 in 0.5 W cm^?2 steps for some fifteen natural and synthetic materials. These experiments confirm the results given in previous papers for a smaller heat flux range, namely that smoke density varies ove4r the temperature/heat flux range. The extended range studied ensured that a much higher proportion of the materials reached a heat flux at which ignition occurred. As before at this and higher heat fluxes, the experiments were repeated in the flaming mode to ensure better reproducibility. With a few exceptions the general shape of the specific optical density/heat flux plot was similar, with a steep rise to a peak and then a corresponding drop to a low value at high fluxes. At 5 W cm^?2the drop had not been reached in some cases, but it is assumed to occur later by inference from earlier work on a smaller-scale apparatus at high temperatures. Complex materials can give unusual plots during the transition from pyrolysis to combustion. This is true of certain polymers containing fire retardant when the pyrolysis of the additive can have a separate effect.     

Ignition of Combustible Forest Materials by a Radiant Energy Flux

The paper reports results of experimental ignition of litter layers consisting of needles of cedar, pine, and fir-tree, birch leaves, lichen (Cladonia), and moss (Pleurozium shreberi). It is established that the moss is ignited faster than the other combustible forest materials. It is shown that with equal moisture contents, the ignition times of needle litter from different trees are identical within the experimental error, and for litter of birch leaves, the ignition time is shorter than that for litter of coniferous trees. This difference is found to be due to differences in the interaction of the radiant flux with litter layers of needles and leaves. Minimum values of the ignition heat pulses for needle and leaf litter layers are estimated for various heat-flux densities. These values tend to a minimum for a heat-flux density of 0.5–0.8 MW/m².     

Effects of variations in incident heat flux when using cone calorimeter test data for prediction of full‐scale heat release rates of polyurethane foam

The development of methods to predict full‐scale fire behaviour using small‐scale test data is of great interest to the fire community. This study evaluated the ability of one model, originally developed during the European Combustion Behaviour of Upholstered Furniture (CBUF) project, to predict heat release rates. Polyurethane foam specimens were tested in the furniture calorimeter using both centre and edge ignition locations. Input data were obtained using cone calorimeter tests and infrared video‐based flame area measurements. Two particular issues were investigated: how variations in incident heat flux in cone calorimeter tests impact heat release rate predictions, and the ability of the model to predict results for different foam thicknesses. Heat release rate predictions showed good agreement with experimental results, particularly during the growth phase of the fire. The model was more successful in predicting results for edge ignition tests than for centre ignition tests and in predicting results for thinner foams. Results indicated that because of sensitivity of the burning behaviour to foam specimen geometry and ignition location, a single incident heat flux could not be specified for generating input for the CBUF model. Potential methods to determine appropriate cone calorimeter input for various geometries and ignition locations are discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

Evidence for the Hypothesis of Ignition of Propellants by Metallic Vapour Deposition

This paper examines the experimental evidence to support the hypothesis of ignition of energetic material by the process of metallic vapour deposition. The hypothesis has been presented previously, and this is the continuation of the work. The hypothesis has been developed at QinetiQ to explain certain measurements of incident radiative flux during electrothermal‐chemical (ETC) experimentation, which show no correlation with the ignition event. Indeed, measured levels of radiation have been so low that radiative energy transfer for ETC plasma ignition could be said to be negligible. Measurement of the thickness of a metal layer resulting from dropwise vapour condensation gives good correlation to the flux requirements for ignition.