A theoretical basis for flammability properties

Theoretical formulations are presented for the fire growth processes under external radiant heating. They included ignition, burning and energy release rate, and flame spread. The behaviour of these processes with external heating is described along with the critical conditions that limit them. These include the critical heat fluxes for ignition, flame spread and burning rate. It is shown how these processes and their critical conditions depend on a limited number of properties measurable by a number of standard test methods. The properties include heat of combustion, the heat of gasification, ignition temperature and the thermal properties of the material. Alternatively, the properties could be related to parameters easily found from data; namely: (1) the critical heat flux (CHF) for ignition; (2) the slope of the energy release rate with externally imposed flux, defined as heat release parameter (HRP); and (3) the ignition parameter, defined as thermal response parameter (TRP). It is further shown that the flame heat flux differences between small laminar flame ignition sources and larger turbulent flames can affect flame spread due to heat flux and ignition length factors. Finally, it is found that the critical energy release rates theoretically needed for ignition, sustained burning, and turbulent upward flame spread are roughly 13, 52, and 100 kW/m², respectively, and independent of material properties. Copyright © 2005 John Wiley & Sons, Ltd.     

Fire performance studies on glass-reinforced plastic laminates

Polyester, epoxy and phenolic glass-reinforced laminates were compared in terms of their fire properties in the cone calorimeter. The presence of a gel coat on the polyester and epoxy products had some effect, mainly in the FR grades. The FR polyester with a brominated flame retardant showed a marked decrease in rate of heat release and in rate of smoke production. Aluminium hydroxide (commonly called alumina trihydrate of ATH) caused a significant delay in ignition time for the epoxy product and some reduction in rate of heat release but rate of smoke release was still relatively high. Phenolic laminates showed a long ignition time and relatively low rate of heat release (RHR). The peak RHR was further reduced by the presence of aluminium hydroxide and by changing the type of glass reinforcement. The influence of thickness was explored for a series of phenolic laminates in both cone calorimeter and the International Maritime Organization (IMO) surface flammability test. Ignition delay time was significantly affected and the cone calorimeter results showed that the shape of the RHR curve was also changed as the thickness increased. However, the heat release measurements in the IMO test were not sensitive enough to pick up the differences in this property. The type of substrate under the laminate significantly altered the burning rate but this may have been because they were not glued or mechanically fixed.     

Correlations between pyrolysis combustion flow calorimetry and conventional flammability tests with halogen‐free flame retardant polyolefin compounds

Seven halogen‐free flame retardant (FR) compounds were evaluated using pyrolysis combustion flow calorimetry (PCFC) and cone calorimetry. Performance of wires coated with the compounds was evaluated using industry standard flame tests. The results suggest that time to peak heat release rate (PHRR) and total heat released (THR) in cone calorimetry (and THR and temperature at PHRR in PCFC) be given more attention in FR compound evaluation. Results were analyzed using flame spread theory. As predicted, the lateral flame spread velocity was independent of PHRR and heat release capacity. However, no angular dependence of flame spread velocity was observed. Thus, the thermal theory of ignition and flame spread, which assumes that ignition at the flame front occurs at a particular flame and ignition temperature, provides little insight into the performance of the compounds. However, results are consistent with a heat release rate greater than about 66kW/m² during flame propagation for sustained ignition of insulated wires containing mineral fillers, in agreement with a critical heat release rate criterion for burning. Mineral fillers can reduce heat release rate below the threshold value by lowering the flaming combustion efficiency and fuel content. A rapid screening procedure using PCFC is suggested by logistic regression of the binary (burn/no‐burn) results. Copyright © 2008 John Wiley & Sons, Ltd.     

Comparison of three test methods for measuring rate of heat release

Three test methods for measuring the rate of heat release, RHR, have been compared on the same building materials. The test methods are the OSU-box modified for O₂-consumption, the STFI open arrangement and the NBS cone calorimeter. All methods are based on the oxygen-consumption technique. Radiation intensities up to 50 k Wm^?2 have been applied. Thirteen different materials were tested. The results obtained using the three methods seem to be quite similar in spite of the different equipment used. The minor deviations observed in the results seem to be systematic and may be due to the different testing arrangements. There is a good correlation between results, e.g. total heat release up to 1min after ignition, obtained by the different methods. Additional information about the time to ignition is obtained simultaneously. Heat-release rates in the vertical and horizontal orientations are also compared in the NBS cone calorimeter.     

Radiative combustion of pyrolyzing fuel in a cylindrical combustor

Investigation on ignition and flame propagation of pyrolyzing fuel in a cylindrical combustor is accomplished. The pyrolyzing fuel of cylindrical shape is concentrically located in a combustor sustained at high temperature. Due to gravity, the buoyancy motion is inevitably incurred in the combustor and this affects the flame initiation and propagation behavior. The radiative heat transfer due to absorption gas plays an important role since it absorbs and emits radiative energy. Numerical studies have been performed over various parameters relevant to gas radiation as well as overheat ratios. The ignition criterion is decided on experimental basis. For the case of relatively small overheat ratio, the gas absorption reduces the flow intensity by the far-reaching effect of radiation. Visible blue flame hence does not evolve. For a high overheat ratio, the strong visible blue flame is generated and self-propagates. The gas radiation makes the considerable effect on ignition delay and flame thickness as well as thermal and reactive nature of visible blue flame.     

Biodiesel combustion in an optical HSDI diesel engine under low load premixed combustion conditions

An optically accessible single-cylinder high-speed direct-injection (HSDI) diesel engine was used to investigate the spray and combustion processes for biodiesel blends under different injection strategies. The experimental results indicated that the heat release rate was dominated by a premixed combustion pattern and the heat release rate peak became smaller with injection timing retardation. The ignition and heat release rate peak occurred later with increasing biodiesel content. Fuel impingement on the wall was observed for all test conditions. The liquid penetration became longer and the fuel impingement was stronger with the increase of biodiesel content. Early and late injection timings result in lower flame luminosity due to improved mixing with longer ignition delay. For all the injection timings, lower soot luminosity was seen for biodiesel blends than pure diesel fuel. Furthermore, NO_x emissions were dramatically reduced for premixed combustion mode with retarded post-TDC injection strategies.     

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.     

Thermal stability and flammability of polyamide 66/montmorillonite nanocomposites

Exfoliated nanocomposites based on polyamide 66 (PA66) and montmorillonite (MMT) were prepared and their thermal stability and combustion behaviour were investigated by using thermal gravity analysis and cone calorimeter. The nanocomposites exhibit higher thermal stability and good flame retardancy. The catalytic decomposition effect of MMT and the barrier effect of layer silicates are presented directly in isothermal oxidation experiment. The initial heat release rate plots show that the addition of MMT can accelerate the ignition of PA66 matrix. A ceramic-like char forms in the surface of the nanocomposites during burning. It is characterized by attenuated total reflection infrared spectra and scanning electron microscopy.     

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.     

Flame retardant synergism of GUP and boric acid by cone calorimetry

Wood was treated with a new composite flame retardant (FRW), with its components guanyl urea phosphate (GUP) and boric acid (BA) to impart flame retardance. The flame retarding behavior of these samples was valued by cone calorimeter. The flammability parameters, including rate of heat release (RHR), total heat release (THR), effective heat of combustion (EHC), total mass loss (TML) and mass loss rate (MLR), yield of CO, smoke production rate (SPR), and specific extinction area (SEA) were recorded simultaneously. By analyzing these data, it was concluded that most combustion parameters of wood decreased by the treatment, especially for the FRW treatment, considerably decreased while the date for wood treated with GUP or boric acid decreased much less for the similar upload, which indicated a synergistic effect of flame retardance and suppressing smoke between GUP and boric acid in FRW, which has not been reported in other researches until now. Meanwhile, the probable flame retardation mechanism was proposed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 5522–5527, 2006     

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.     

Flammability testing of pure and flame retardant-treated cotton fabrics

A Controlled-atomosphere cone calorimeter was used to investigate the burning of pure and flame retardant-treated cotton fabrics. The condensed-phase flame retardants used were Morguard (containing ammonium dihydrogen phosphate and diammonium hydrogen phoisphate) and Nochar (containing ammonium sulfate and a sodium salt). The fabrics were tested at 25 kW m^?2 incident heat flux in environments containing 15–30% oxygen. The flame retardants increased the time to ignition, residue yield, and CO and CO₂ yields. The flame retardants decreased the peak and average mass loss rates, the peak and average heat release rates, the effective heat of combustion at peak heat release rate, and the propensity to flashover. The effect of oxygen concentration on the burning of pure and flame retardant-treated cotton fabrics has also been investigated. The flame retardants had better performance when the treated fabrics burned in the lower oxyge concentrations. The result of this study indicate that the controlled-atmosphere cone calorimeter is a good tool for studying the effect of flame retardant and oxygen concentration on the burning of materials.     

Heat propagation in thermally conductive polymers of PA6 and hexagonal boron nitride

Thermally conductive polymers offer new possibilities for the heat dissipation in electric and electronic components, for example, by a three‐dimensional shaping of the heat sinks. To face safety regulations, improved fire performance of those components is required. In contrast to unfilled polymers, those materials exhibit an entirely different thermal behavior. To investigate the flammability, a phosphorus flame retardant was incorporated into thermally conductive composites of polyamide 6 and hexagonal boron nitride. The flame retardant decreased the thermal conductivity only slightly. However, the burning behavior changed significantly, due to a different heat propagation, which was investigated using a thermographic camera. An optimum content of hexagonal boron nitride for a sufficient thermal conductivity and fire performance was found between 20 and 30 vol%. The improvement of the fire performance was due to a faster heat release out of the pyrolysis zone and an earlier decomposition of the flame retardant. For higher contents of hexagonal boron nitride, the heat was spread faster within the part, promoting an earlier ignition and increasing the decomposition rate of the flame retardant.     

The Accumulation of Flammable Gases in Cone Calorimeter Tests is Responsible for Flash Ignition of Wool and Cotton Samples

In this paper a possible explanation is presented for the differences found between the fire behaviour of materials in small-scale cone calorimeter tests and the large-scale furniture calorimeter. The results obtained with cone calorimeter/FTIR equipment at 35 kW m⁻² will show that the early flash ignitions of typical materials like cotton and wool are due to the liberation of flammable gases during the pyrolysis phase and the typical ignition situation on the cone calorimeter, that is, the presence of a sparking igniter above the sample. This fast flash ignition and the early heat release behaviour on the cone calorimeter may be in contradiction to the early fire growth in other fire tests where the ignition conditions are clearly different from pyrolysis circumstances, that is, ignition via a burning newspaper, match, gas flame, etc.     

Combustion characteristics of halogen‐free flame‐retarded polyethylene containing magnesium hydroxide and some synergists

Halogen‐free flame‐retarded polyethylene materials have been prepared by using magnesium hydroxide (MH) as a flame retardant combined with red phosphorous (RP) and expandable graphite (EG) as synergists. The effects of these additives on the combustion behavior of the filled linear low density polyethylene (LLDPE), such as a limiting oxygen index (LOI), the rate of heat release (RHR), the specific extinction area (SEA), etc., have been studied by the LOI determination and the cone calorimeter test. The results show that RP and EG are good synergists for improving the flame retardancy of LLDPE/MH formulations. In addition, a suitable amount of ethylene and vinyl acetate copolymer (EVA) added in the formulations can increase the LOI values while promoting the char formation and showing almost no effect on the RHR and SEA values. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 206–214, 2001     

乳胶泡沫引火位置对火蔓延特性的影响

利用自行搭建的小尺寸实验平台,开展了对不同点火位置的乳胶泡沫材料燃烧过程的对比实验,通过对火蔓延过程中的部分重要参数（如最大火焰高度、火蔓延速度和蔓延过程中样品表面温度变化等）的测定,分析了点火位置不同时,乳胶泡沫材料的火蔓延特性。结果表明：边缘点火和中间点火条件下,最大火焰高度分别为397和491 mm,火蔓延速度分别为1.8和0.97 mm·s^-1;边缘点火时的乳胶泡沫材料表面火蔓延过程中的温度低于中间点火情况下。     

Survey of fire testing of electrical cables

The National Electrical Code (NEC) is the document which regulates electrical cables in the United States. It addresses two fire properties only for which it requires testing: flame spread and smoke obscuration. Thus, a hierarchy of tests exists which cables need to pass to be allowed in occupancies regulated by the NEC. On a flame spread basis they are, in increasing severity: UL VW-1, UL 1581, UL 1666 and UL 910. For smoke obscuration only one test is mentioned in the NEC: UL 910. The /LS category (limited smoke) introduced in NEC '90, as a voluntary label, will probably be addressed in NEC '93 under the new standard UL 1685, a modification of UL 1581. Rate of heat release is measured for cables only for R&D purposes. However, it is by now well established that rate of heat release is the one most important fire property to assess fire hazard. Cable flame spread tests (except for VW-1) have all been modified, in non-standard ways, to measure rate of heat release, which gives much more useful results than char-length determinations. Moreover, small-scale RHR test instruments (cone calorimeter, OSU calorimeter) have also been used extensively to test cables. The results of such tests have been correlated with those of UL 1581 (and of similar cable tray tests) in several cases, with excellent outcome. Work is underway to develop algorithms to predict largescale cable test results from small-scale compound tests. This area of research is very promising, and, once completed, would decrease product development costs considerably and allow faster introduction of advanced materials into the market. However, such work can only be completed by using rate of heat release techniques in both large- and small-scale tests.     

Preparation and smoke suppression mechanism of flame-retardant PBS composites with BNNS@DOPA

Currently, the flame-retardant modification of polybutylene succinate (PBS) is mainly focused on improving flame-retardant efficiency, ignoring the negative impact of the smoke produced by combustion on the human respiratory tract. To address this problem, PBS composites were prepared by melt blending method in this study. The effect of boron nitride-grafted DOPO flame retardant (BNNS@DOPA) on flame retardancy and smoke suppression of PBS composites was investigated. Incorporating 3% BNNS@DOPA into PBS composites results in a 90% improvement in thermal conductivity. This resulted in a reduction of the peak heat release rate, total heat release rate, and actual smoke rate to 453.7 kW m⁻², 86.3 MJ m⁻², and 1035.9 m², respectively, compared with pure PBS. The latter indicated a decrease of 34.0%, 37.6%, and 51.2%, respectively. Furthermore, the ignition time was extended by 45 s and the limiting oxygen index value increased by 12.5%. This functionalization approach presents a new way to study PBS flame retardancy improvement, consequently boosting its application in fire safety for polymer materials.     

Fire risks of burning asphalt

Eyewitnesses describe burning pavement surfaces in extreme fire scenarios. However, it was believed that the pavement plays a negligible role in comparison to other items feeding such an extreme fire at the same time. The asphalt mixtures used differ widely, thus raising the question as to whether this conclusion holds for all kinds of such materials. Three different kinds of asphalt mixtures were investigated with the aim of benchmarking the fire risks. Cone calorimeter tests are performed at an irradiance of 70kWm^?2. All three investigated asphalts burn in extreme fire scenarios. The fire response (fire load, time to ignition, maximum heat release rate and smoke production) is quite different and varies by factors of up to 10 when compared to each other. The fire load per mass is always very low due to the high content of inert minerals, whereas the effective heat of combustion of the volatiles is quite typical of non‐flame retarded organics. The heat release rate and fire growth indices are strongly dependent on the fire residue and thus the kind of mineral filler used. Comparing with polymeric materials, the investigated Mastic Asphalt and Stone Mastic Asphalt may be called intrinsically flame resistant, whereas the investigated Special Asphalt showed a pronouncedly greater fire risk with respect to causing fire growth and smoke. Thus the question is raised as to whether the use of certain kinds of asphalts in tunnels must be reconsidered. Apart from the binder used, the study also indicates varying the kind of aggregate as a possible route to eliminate the problem. Copyright © 2010 John Wiley & Sons, Ltd.     

点火位置对乳胶泡沫水平火蔓延规律的影响

为研究点火位置对乳胶泡沫材料水平方向火蔓延规律的影响。搭建小尺寸实验平台,在距离材料中心点0（x1）、3.54 cm（x2）、7.08 cm（x3）、10.62 cm（x4）、14.16 cm（x5）、17.70 cm（x6）位置处点火,研究了试样表面温度、质量损失、火焰高度、火蔓延速度等特性参数的变化规律。结果表明,随着点火位置由材料中心点向边缘点移动,平均火蔓延速度分别为0.24、0.23、0.19、0.31、0.42、0.51 cm·s^-1,呈现先减小后增大的规律;x3点火位置时的平均火焰高度较低,燃烧时间较长,平均质量损失速率较低,主要与火蔓延过程中的热量传递方式有关。研究结果显示了乳胶泡沫的火蔓延过程,得到了点火位置对火蔓延的影响规律。