Material fire properties and predictions for thermoplastics

Ignition and burning rate data are developed for nylon 6/6, polyethylene, polypropylene and black polycast PMMA in a cone calorimeter heating assembly. The objective is to examine a testing protocol that leads to the prediction of ignition and burning rate for thermoplastics from cone calorimeter data. The procedure consists of determining material properties, i.e. thermal inertia, specific heat, thermal conductivity, ignition temperature, heat of gasification and flame heat flux from cone data, and utilizing these properties in a model to predict the time to ignition and transient burning rate. The procedure is based on the incident flame heat flux being constant in the cone calorimeter which occurs for flames above the top of the cone heater. A constant net flame heat flux of approximately 20 kW/m² for nylon 6/6, 19 kW/m² for polyethylene, 11 kW/mP² for polypropylene and 28 kW/m² for black PMMA is obtained for irradiation levels ranging from 0 to 90 kW/m². The burning rate model is shown to yield good accuracy in comparison to measured transient burning in the cone assembly.     

热塑性高分子材料强制点燃过程模拟与分析

采用锥形量热仪对典型热塑性高分子材料——聚甲基丙烯酸甲酯(PMMA)在不同外部入射热通量下进行了强制点燃的实验研究。在气相反应和固相反应动力学及传递过程分析的基础上，建立了热塑性高分子材料强制点燃过程的数学模型，通过数值分析的方法将点燃数据相关联，得到PMMA强制点燃的点燃时间与临界表面温度的表达式，并计算了PMMA不同外部入射热通量下强制点燃时间及点燃的临界表面温度。模拟计算结果与实验结果的比较表明，二者基本吻合。采用所得到的数据关联式对强制点燃过程的影响因素进行了定量分析。     

Radiant smoldering ignition of plywood

This paper investigates the role of self-heating in the smoldering ignition of 18 mm (three-quarter inch) thick maple plywood exposed to radiant heat fluxes between 6 and 15 kW/m² in the cone calorimeter for up to 8 h. The minimum heat flux for smoldering ignition was experimentally determined to be 7.5 kW/m². This compares favorably to predictions made using classical self-heating theory. The role of self-heating was explored via temperature measurements distributed within the specimens. Elevated subsurface temperature profiles indicated self-heating was an important ignition factor resulting in ignition at depth with smolder propagation to the surface and into the material. The ignition depth was shown to be a function of the heat flux with the depth moving towards the surface as the heat flux increased.     

Contribution to flashover modelling: Development of a validated numerical model for ignition of non-contiguous wood samples

A computational model of flashover is presented that closely follows the experimental setup at CNRS-ENSMA-Poitiers. A propane burner with thermal power of 55 kW is used as a primary source of fire and square beech wood samples (30 mm×30 mm×5 mm) as fire spread targets. The computational model describes the wood pyrolysis with a progress variable. Using the conservation of heat fluxes at the solid–gas interface, the thermal diffusion in the wood samples is coupled with the convective and the radiative heat transfer in the ambient gas phase. The incoming heat flux at the upper surface of the wood samples reaches values between 20 and 30 kW/m². With the ignition and subsequent combustion of the pyrolysis volatiles, the heat flux increases by approx. 12 kW/m². The results show that the ignition of the wood samples is triggered at an approx. surface temperature of 650 K. Due to large local variations in incident heat flux, significant differences in the ignition times of the wood samples are observed. The comparison of the calculated and the experimentally measured temperature shows a good agreement for the first wood sample and the model predicts the ignition time very well. But for the second and the third wood samples the model overpredicts the temperature, which leads to a premature ignition of these wood samples.     

Flammability assessment of fire-retarded Nordic Spruce wood using thermogravimetric analyses and cone calorimetry

Experimental techniques such as the cone calorimeter, representing realistic fire conditions, and the thermogravimetric analyser (TGA) combined with evolved gas analysis (EGA) can be used to determine flammability and degradation properties of materials. The desire is to correlate the flammability properties measured in the cone calorimeter for samples of size 100 mm×100 mm with those measured or deduced from TGA combined with EGA for milligram samples. Such an achievement will allow the design of fire-safe materials by quickly assessing (a) the fire safety of materials in their earliest milligram formulation and (b) the dependence of their flammability properties on the molecular structure of the material. In the present study, a cone calorimeter and TGA investigation is conducted for commercial Nordic Spruce wood impregnated by mono-ammonium phosphate (fire retardant, FR) through a vacuum pressure method. Experiments in the cone calorimeter with increasing FR concentrations indicated that (a) the char yields increased, (b) the apparent ignition temperature increased, (c) time to piloted ignition increased, (d) the total amount of heat released was reduced, (e) the peak heat release rate was reduced and (f) the carbon monoxide and smoke yields increased especially before ignition occurred. By comparison, char yields also increased with FR content in the TGA degradation experiments in nitrogen. The increase in the char yield with FR content explains quantitatively the decrease in the heat release in the cone calorimeter. By contrast, the onset temperatures measured in TGA decreased, whereas the ignition temperature deduced in the cone calorimeter increased with FR content. This difference is attributed to reduced yield of levoglucosan (reported in recent literature using TGA/EGA) with increased char yield as well as to the presence of phosphorous containing moieties in the volatiles, which both can quench piloted ignition. Finally, the TGA measurements showed that the FR concentrations decreased for milligram samples at different distances from the surface of the wood used in the cone calorimeter measurements. The variation of FR retardant with depth needs to be considered when using TGA data to interpret cone calorimeter measurements and the fire performance of the FR wood in approval tests such as the single burning item (SBI).     

Ignition,Heat Release Rate and Suppression of Elastomeric Materials

The focus of this paper is to determine flammability characteristics of rubber materials that are common to vehicle tires, conveyor belts, and electrical power cable insulation and to compare the thermal magnitude of cargo quantities of these materials to other fuels that are publicly transported. Although a literature review was performed, very little data was found on this topic. Standard flammability test procedures were used to measure the critical flux for ignition, critical ignition temperature, and heat release rates (HRR) of rubber compounds common to tire tread materials and conveyor belt covers. Both the intermediate scale calorimeter: ISO 14696, ASTM E-1623 (ICAL) and the cone calorimeter: ISO E-5660, ASTM 1354 (Cone) provided the bulk of the data. Critical ignition flux and vertical flame spread data for rubber based electrical insulations were determined using a radiant panel from a modified ASTM flame spread apparatus: ASTM E-162. thermogravimetric analysis was also used to evaluate thermal decomposition progression of selected test materials. Further, suppression tests were conducted on tire piles to evaluate agents to extinguish and control tire fires. Also, the HRR of the tire piles were measured and compared to work performed by others. Results confirm that the area heat release rate of rubber materials is directly proportional to exposure flux intensity. The critical exposure flux for ignition of a variety of rubber-based materials is approximately 20 kW/m² to 30 kW/m² and the critical temperature for piloted and non-piloted ignition were independent of exposure intensity at ~400°C and ~600°C respectively. In large quantities, rubber tire loads have total HRR comparable to the heat released from similar areas of liquid hydrocarbon spills.     

Fire thermal boundary condition measurement using a hybrid heat flux gage

New experimental methods have been developed using a hybrid heat flux gage to quantify the thermal boundary condition to a surface exposed to fire. The hybrid heat flux gage is capable of measuring the net heat flux and exposure heat flux at gage temperatures up to 1000 °C without the need for water cooling. Using these heat fluxes at elevated surface temperatures, new methods were developed to quantify the convective heat transfer coefficient and adiabatic surface temperature. In addition, a procedure is presented for determining the convective and radiative heat flux components when the gas temperature is measured close to the gage surface. Techniques were validated in a series of experiments performed in a cone calorimeter at different heat fluxes. Cold surface heat fluxes from the hybrid heat flux gage were within 5% of heat fluxes measured using a water-cooled Schmidt-Boelter gage. Temporal adiabatic surface temperature measurements from the hybrid gage compared well with steady-state plate thermometer measurements.     

铺地材料临界辐射通量计算与试验验证

介绍临界辐射通量的测量方法及原理。利用锥形量热仪测得试样的点燃时间和所受初始热辐射强度,拟合出两者存在的一次函数关系。对铺地材料临界辐射通量进行近似计算,利用锥形量热仪进行试验验证,给出误差分析。     

锥形量热计研究木材临界辐射能流和点燃温度

锥形量热计不但可以直接测量出材料的某些燃烧特性数据（如热释放速率、质量损失率、点燃时间等），而且可以根据所测得的数据间接得出材料的其它燃烧特性数据。笔者介绍了一种从锥形量热计测得的数据推导临界辐射能流和点燃温度的方法。     

Determination of the convective heat transfer coefficient in three-dimensional inverse heat conduction problems

A thin steel plate probe (24 mm wide×24 mm long×3 mm thick) surrounded by insulation except for the exposed surface was constructed. Using the temperature measurements of the steel plate and the heat flux measured by a Gardon gauge and with the assistance of numerical calculations, two methods based on three-dimensional inverse conduction problems are developed to determine the convective heat transfer coefficient in fire experiments. The first one is based on optimisation of the predictions of the temperature of the steel plate using different heat transfer coefficients, while the second adopts a “predictor–corrector” method to determine the instantaneous heat transfer coefficient. Validation of both methods is accomplished by performing experiments in the cone calorimeter at a known constant heat flux. Subsequently, the two methods are applied to experiments in enclosures to examine the sensitivities of the two methods.     

Smoke and heat release and ignitability as measures of fire hazard from burning of carpet tiles

A series of 22 commercial carpet tiles, covering the range of backings found in the marketplace, and with the same face material (nylon) was chosen for fire testing. All the carpets were tested in the cone calorimeter rate of heat release apparatus. They were also all tested in the NBS smoke density chamber, in the flaming mode. A selection of samples was further tested using the flooring radiant panel. A preliminary investigation was made to choose the optimum radiant incident flux to be used, which was determined to be 25 kW/m².

It was found that the carpets showed a wide range of fire performance, including ranges of peak rate of heat release and of time to ignition of c. 3 and of smoke factor of c. 8. It was not found possible to correlate the results of the NBS smoke chamber or radiant panel tests with any of the results obtained from the cone calorimeter. A classification scheme was proposed to determine fire performance of carpets, based on the ratio of time to ignition (in seconds) and peak rate of heat release (in kW/m²). According to this scheme, four categories of fire performance would be expected:

4.

     

基于CONE的超高温耐火电缆火灾危险性分析

利用锥形量热仪对超高温耐火电缆在不同辐射功率下的点燃时间（TTI）、热释放速率（HRR）、质量损失速率（MLR）和燃烧残余物进行了研究。研究表明,随着辐射功率增加,耐火电缆的TTI逐渐缩短,HRR和MLR逐渐增大,火灾危险性逐渐增加。超高温耐火电缆在35 kW/m2和50 kW/m2辐射功率下火灾性能指数相比于25 kW/m2分别增加了44.4%和176.5%,火灾增长指数分别增加了30.4%和83.0%。结合理论分析可以得出,耐火电缆的临界辐射功率为3.61 kW/m2、零辐射平均热释放速率为36.5 kW/m2,表现出较低的火灾危险性。     

低辐射强度下古建筑木构件材料的燃烧特性

为研究古建筑木构件材料在火灾下的燃烧特性,使用锥型量热仪对古木材试样的点燃时间、临界辐射强度、热释放速率、燃烧气体中CO2体积分数等燃烧特性进行试验研究。试验发现3种不同厚度试样的临界辐射强度分别为8.81,9.40,10.55 kW/m2。试样厚度增加会使古木材点燃时间延长,临界辐射强度增大,材料的热释放速率所形成的双峰值曲线的峰值降低。燃烧气体中CO2体积分数曲线与热释放速率所绘制出的曲线的形态一致,同一厚度的试样在不同辐射强度下其燃烧特性呈现出显著区别。与新木材相关燃烧特性参数对比,劣化后的古木材临界热辐射强度降低、点燃时间减小,5 min内平均热释放速率较高。研究表明,古建筑木构件材料燃烧特性有显著变化,可为古建筑火灾研究提供参考。     

设备尺度对可燃材料失重速率的影响

文章分别利用锥型量热计和火灾早期特性实验台对柞木的失重速度进行了实验测量。研究结果表明：尺度效应的作用非常明显。相同辐射能量条件下，相比于火灾早期特性试验台实验，锥型量热计实验中试样容易着火，而且过程中火焰稳定，火陷持续时间较长。     

Burning rate and flame heat flux for PMMA in a cone calorimeter

Ignition and burning rate data are developed for thick (25 mm) black Polycast PMMA in a cone calorimeter heating assembly. The objective is to establish a testing protocol that will lead to the prediction of ignition and burning rate from cone data. This is done for a thermoplastic like PMMA. The incident flame heat flux, for irradiation levels of 0–75 kW/m², is found to be approximately 37 kW/m² for black PMMA. Its constancy is shown due to the geometry of the cone flame. Also, this flame is shown to be nearly transparent for cone irradiance (>90%). The heat of gasification of the black PMMA used is found to be approximately 2.8 kJ/g; higher than values reported for other PMMA samples. This is believed to be due to differences in molecular structure or pigmentation of the PMMA tested. A burning rate model is demonstrated to yield good accuracy in comparison to measured transient values. An exact solution is found for constant heat flux conditions.     

Absorption of thermal energy in PMMA by in-depth radiation

An experimental technique is developed to quantify the absorption of thermal energy in black PMMA (Polycast) by in-depth radiation in semi-transparent media. In-depth heating occurs when non-reflected incident heat flux enters the solid without first being absorbed at the exposed surface. Transient conduction due to temperature gradients occurs within the solid in response to this in-depth absorption. An analytical model is developed for predicting time to ignition for such in-depth heating situations. Using the measured absorption coefficient, κ, the analytical prediction for time to ignition is found to be in excellent agreement with data from experiments of Saito and Delichatsios.     

Measuring incident radiant heat flux using the plate thermometer

This paper shows that the plate thermometer as described in the fire resistance test standards ISO 834-1 and EN 1363-1 can be used for measuring incident radiant flux under ambient conditions as an alternative to water cooled total flux heat metres (HFMs). Measurements with a plate thermometer mounted in the cone calorimeter and exposed to different heat flux levels were analysed as well as simultaneous measurements with total HFMs and plate thermometers in large scale tests. It is shown how the incident radiant flux to a target can be derived from measurements with total HFMs and plate thermometers, respectively, and how well these two methods match. The plate thermometer is therefore deemed to be a practical alternative for measuring thermal conditions including incident radiant heat flux particularly under field conditions. It is, however, recommended that the plate thermometer should be modified when used under ambient conditions to reduce errors.     

Evaluating the influence of varied fire-retardant surface coatings on post-heat flexural properties of glass/epoxy composites

This paper investigates the influence of char-forming, flame-inhibiting and intumescent surface coatings on the post-heat flexural behavior of glass fibre-reinforced epoxy composites exposed to one-sided radiant heating. The fire performance of glass/epoxy laminates surface-coated with varied formulations was evaluated using a cone calorimeter under various external heat fluxes (25, 50 and 65 kW/m²). Self-sustained ignition was significantly delayed for fire-protected laminates relative to the control. The peak heat release rates and the time to reach this event were significantly reduced in thermally-protected laminates. In addition to the chemical nature of flame retardant chemicals present in the coatings, the time to reach the peak heat release rate and the magnitude of the total heat release were influenced by the quantity and morphology of the char formed by each type of surface coating. The post-heat flexural performance of heat-damaged laminates was determined via three-point bending. There was a significant improvement in the post-heat flexural moduli retention in thermally-protected glass/epoxy laminates. This may be due to the flame retardation efficacy of the coatings and the ability of the charred structures to reduce the heat transfer into the substrate thereby protecting the underlying composite and its mechanical integrity.     

An experimental evaluation of critical surface temperature as a criterion for piloted ignition of solid fuels

With a view to developing a simple engineering method for the prediction of piloted ignition, the validity of the critical surface temperature criterion for piloted ignition is examined experimentally for seven thermoplastic materials. The results indicate that the surface temperature at piloted ignition for each material studied varies by ±15 K or less. As such, the surface temperature criterion appears to be suitable for engineering calculations.

Analysis of time-surface temperature histories shows that the radiant heat source temperature has a significant effect on the material heating over the range of source temperatures utilized (700–1050 K). The variations in material heating are of sufficient magnitude to cause changes in ignition times by a factor of two or more for different heat sources present in typical fire scenarios.

Our current level of understanding of piloted ignition is shown to be insufficient to support extrapolation procedures to determine the minimum incident radiant flux required for piloted ignition. An experimental approach to determination of the minimum radiant flux required for piloted ignition is demonstrated to be feasible.     

Fire Performance Properties of Solid Wood and Lignocellulose-Plastic Composite Deck Boards

Traditionally fire performance properties of externally located deck boards have been characterized by their flame spread index (FSI) as determined by UL 723/ASTM E 84. In this test a nominal 0.6 m wide by 7.3 m long array of deck boards is exposed to an approximately 90 kW ignition source fire for 10 min in the Steiner Tunnel. More recently the University of California Forest Products Laboratory developed a new fire test protocol based the principles of oxygen consumption calorimetry, California SFM 12-7A-4, Part A: Under-Deck Flame Test. This protocol addresses the potential ignition of a deck from underneath as may occur during a wildfire. In this protocol a nominal 0.44 square meter deck-system of deck boards mechanically fastened to wood joists is subjected to an 80 kW ignition source fire for 3 min. For this study the fire performance characteristics of more than thirty-five deck board types were evaluated by the above two methods and by a smaller-scale material-based test, ASTM E 1354 cone calorimeter. Deck boards were selected to represent a range of materials (untreated wood, lignocellulose-polymer composite), structures (solid, voided, microcellular foam), and cross-sectional profiles (width, thickness, presence of hidden fastener system longitudinal edge grooves). The results from this study were used to: 1. Develop correlations for deck boards between the material-based cone calorimeter tests and system-based under-deck tests. 2. Develop correlations for deck boards between the small-scale cone calorimeter tests and large-scale Steiner Tunnel tests. 3. Estimate the significance of ASTM D 2898 Method A accelerated weathering on fire performance.