Simulated Effects of Physical Parameters on Heat Transfer of Intumescent Fire-Retardant Polypropylene during Burning

Physical properties of intumescent materials are important parameters as input data in modeling the combustion behavior of intumescent materials in a fire. In this paper, effects of important physical properties on heat transfer of intumescent materials during burning are simulated based on a combustion model of intumescent fire-retardant polypropylene (IFR-PP) materials. Physical properties selected are thermal conductivity of virgin material and char layer, specific heat capacity of virgin material, density of virgin material, surface emissivity of virgin material and char layer, and intumescent temperature. Predicted temperature curves at a location 9 mm from the bottom of the IFR-PP material at an incident heat flux of 50 kW/m² are shown for the varied physical parameters values. The results show that these varied parameter values can affect the heat transfer of materials remarkably.     

Synergistic Effect of Organic Vermiculite on the Flame Retardancy and Thermal Stability of Intumescent Polypropylene Composites

Organic vermiculite (OVMT) prepared from vermiculite (VMT), with high aspect ratio and orderly arranged platelets intercalated by octadecyl trimethyl ammonum bromide (OTAB), was used as a synergistic agent on the flame retardancy of a polypropylene/intumescent flame retardant (PP/IFR) system. The flammability and thermal stability of PP/IFR/OVMT composites were investigated by limiting oxygen index (LOI), UL-94 testing, cone calorimetry tests, and thermogravometric analysis. The results of LOI and UL-94 testing showed that low loading of OVMT improved the flame retardancy and retarded dripping for PP/IFR composites. OVMT, with 1% loading, increased the char residue of PP/IFR composites and could act as an effective additive for improvement in flame retardancy, which was confirmed by the cone data. The char layer morphological structures observed by scanning electron microscopy (SEM) showed that OVMT with 1% loading can promote formation of a continuous and compact intumescent char layer. Raman spectroscopy results indicated that the OVMT or its pyrolytic products led to a decrease in size of the carbonaceous micro-domain during combustion, resulting in formation of more compact charred layers. Thus, OVMT with 1% loading showed a synergistic effect with IFR in the combustion of the PP/IFR composites.     

Effect of Polyborosiloxane on the Flame Retardancy and Thermal Degradation of Intumescent Flame Retardant Polypropylene

A novel synergistic flame retardant agent containing boron and silicon, namely polyborosiloxane (PBSil), was prepared via the condensation reaction of boric acid (BA), tetraethoxysilane (TEOS), and octamethyl cyclotetrasiloxane (OMCTS). The obtained PBSil was then combined with an intumescent flame retardant (IFR) to flame retard polypropylene (PP), and the effects of PBSil on the flame retardancy and thermal degradation of the PP/IFR composite were investigated. It was found that PBSil could improve the compatibility between the IFR and the PP matrix, thereby improving the mechanical properties of the composite. Compared with zinc borate, zeolite, and nano-silica, PBSil showed much better flame retardancy and smoke suppression in the PP/IFR composite. When the content of PBSil was 3.0 wt%, the limiting oxygen index (LOI) value of the flame retardant PP was increased from 29.0% to 35.0%, and the UL-94 rating was improved from V-1 to V-0 rating. Simultaneously, the heat release rate (HRR) and smoke production rate (SPR) of the composite were decreased dramatically. The thermogravimetric (TG) analysis, Fourier transform infrared (FTIR), and thermogravimetry-Fourier transform infrared spectrometry (TG-FTIR) results showed that, PBSil could enhance the thermostability of the IFR, and promote the char formation. Furthermore, the compactness and thermostability of the intumescent char were significantly improved, contributing to the improvement of the flame retardancy of the composite.     

Study on Heat Transfer in Intumescent Fire Retardant Epoxy Coatings During Combustion

The heat transfer process and combustion behavior of intumescent fire retardant epoxy (IFR-EP) coatings were studied using a cone calorimeter and multi-channel data acquisition instrument. The results showed that the combustion process of the coating and its heat transfer process were closely connected and interacted. Compared with the intumescent char layer of IFR-EP coating B, that used expandable graphite (EG) as a synergistic additive, the char layer of IFR-EP coating A, that used Sb₂O₃ and chlorinated paraffin (CP) as synergistic additives, was more effective to hinder the transfer of external heat to the interior of the sample. Various other factors, such as the content of fire retardants, can also remarkably affect the heat transfer process of the coatings. Two typical char layer physical structure models, a uniform-cellular char physical structure and a nonuniform-crack char physical structure, are presented based on our investigations of the char layers, with each being most appropriate for one of the two types of samples.     

Synergistic Effect of Phosphorus-Containing Montmorillonite with Intumescent Flame Retardant in Polypropylene

Phosphorus-containing montmorillonite (P-MMT) was successfully prepared via intercalating resorcinol bis(diphenyl phosphate) (RDP) into montmorillonite (MMT) layers, and was utilized as a synergistic agent in the polypropylene/melamine pyrophosphate/pentaerythritol (PP/MPP/PER) intumescent flame retardant (IFR) system. The synergistic effect of P-MMT and IFR was investigated by dynamic mechanical analysis (DMA), thermogravimetry (TG), limiting oxygen index (LOI), UL-94 test, cone calorimeter test (CCT), and scanning electron microscopy (SEM). It was found that P-MMT could significantly improve the thermostability and flame retardancy of the PP/IFR composite. When 2.0 wt% P-MMT replaced the same amount of IFR in the composite, both the onset decomposition temperature (T _onset) and the maximum-rate decomposition temperature (T _max) of the PP/IFR composite were increased by more than 14°C. Meanwhile, the LOI value was increased from 29.5% to 32.5%, the UL-94 rating was enhanced from V-1 to V-0, and the heat release rate (HRR), total heat release (THR), and mass lose rate (MLR) were decreased dramatically, which proved that P-MMT had a good synergistic effect with IFR in flame retardant PP.     

Experimental Study on Heat Transfer of Intumescent Fire Retardant Polypropylene Materials

Intumescent fire retardant polypropylene (IFR-PP) materials, primarily comprising polypropylene, ammonium polyphosphate (APP), pentaerythritol (PER), and polyamide-6, were prepared. The burning behavior of IFR-PP materials was measured using a cone calorimeter. The heat transfer processes for IFR-PP materials were studied using a cone calorimeter and a multi-channel data acquisition instrument. The temperature distribution results inside IFR-PP materials indicated that the temperature measured near the top surface of the sample was much higher than the temperature at far from the top surface. The external temperature field measurement results for IFR-PP samples showed that the temperature rose gradually with time before the intumescent char was formed. Then the temperature values of the sample decreased sharply due to the formation of char cap and accumulating gases. The heat transfer processes of IFR-PP materials were also affected by the difference in formulations such as changes in the contents of APP and PER.     

高超声速再入体表面热解烧蚀效应数值模拟

表面防热材料热解与烧蚀效应研究在高超声速飞行器总体设计中具有重要应用价值。以热解烧蚀效应对飞行器目标特性及通信性能影响的预测评估为背景, 从化学非平衡气体动力学方程及固体热传导方程出发, 建立了气-固交界面上热解烧蚀壁面边界条件的一般形式及热物理化学模型, 发展了高超声速再入体绕流流场与表面材料内部温度场耦合求解的数值模拟方法, 并对计算模型和数值方法的可靠性进行了验证分析。在此基础上针对复杂外形再入体及表面硅基防热材料, 开展了典型再入条件下再入体绕流及尾流流场的数值模拟, 重点分析了表面材料热解烧蚀效应对流场等离子体分布的影响。研究表明: 在表面材料中不含碱金属杂质的情况下, 热解与烧蚀效应对流场中等离子体分布影响较小, 而在含有微量碱金属杂质的情况下, 热解与烧蚀效应对流场中等离子体分布及化学组分分布具有很大影响, 由此对再入目标特性与电磁通信性能带来的影响不容忽视。      

Analysis of conditions for determining the thermophysical characteristics of energetic materials by the laser pulse method

An analysis of conditions for determining the thermophysical characteristics of energetic materials by the laser pulse method is performed. Based on the results of numerical solution of the heat conduction problems for a sample of a material irradiated with a short heating laser pulse corresponding to actual experimental conditions, the time dependences on the sample surface temperature are determined. The heat pulse duration required to determine the thermophysical properties of materials and the ignition delay time are compared. It is shown that the determination of the thermophysical characteristics of a typical energetic material by the laser pulse method is possible at pulse durations of no longer than 0.24 s.     

The Thermal Stability and Flammability of Expandable Graphite-Filled Polypropylene/Thermoplastic Polyurethane Blends

Polypropylene/thermoplastic polyurethane (PP/TPU) blends filled with two different particle sizes (45 and 150 μm) of expandable graphite (EG) were prepared by melt blending. Thermogravimetric analysis (TGA) was carried out to explain the effect of EG on the thermal stability of PP/TPU blends. In addition, the fire behavior of PP/TPU and PP/TPU/EG was investigated by a cone calorimeter. The char morphology and carbonation of the above systems were also characterized. The experimental results indicated that intumescent EG significantly enhanced the thermal stability and fire resistance of these blends. With the smaller particle size of EG, the thermal stability and flame retardancy were improved. The results from TGA and cone calorimeter demonstrated that the addition of EG could retard the degradation of the polymer materials above the temperature of 500°C by promoting the formation of a compact char layer. This char layer prevented further degradation of the polymer matrix and protected it effectively from heat penetrating inside, resulting in lower weight loss rate and better flame-retarded performances.     

基于拓展分离变量法的层合材料瞬态传热分析

层合材料各层热物理参数不同,难以用常规的分离变量法求解.针对此问题对常规分离变量法进行了拓展,将层合材料受热时的温度场在时间域上分成微小时间段,在每个微小时间段内层合材料交界处的温度可认为随时间正比变化,并假设比例系数,此时在微小时间段内对各层分别利用分离变量法单独求得解析解,根据交界处温度相等能量连续的关系可求出比例系数,进而求出该微小时间段内的温度场,通过循环求解可得整个时间段内的温度场.之后,利用拓展的分离变量法对常用层合隔热材料瞬态传热进行了分析,通过与有限元方法计算的结果比较,验证了本文方法的正确性,分析了隔热材料类型、厚度,材料表面对流换热系数,空气温度等参数对隔热效果的影响.拓展分离变量法利用解析的方式求解了层合材料瞬态传热问题,物理意义比常规的数值方法明确,计算效率也较高.     

Effect of pyrolysis conditions on gasification reactivity of woody biomass-derived char

The effect of pyrolysis conditions on char reactivity has been studied using Raman spectroscopy. This paper reports on the relationship between the properties of biomass char and the gasification rate. The gasification kinetics of biomass char have been revealed by measuring the rate of weight loss during its reaction with CO₂ as a function of temperature. First-order kinetic rate constants are determined by fitting the weight loss data using a random pore model. The relationship between the char structure and CO₂ gasification reactivity was investigated in the range of 15–600 °C/min at a constant pyrolysis pressure (0.1 MPa), and 0.1–3.0 MPa at a constant heating rate (15 °C/min). The experimental results reveal that the reactivity of biomass char is determined by the pyrolysis condition. The CO₂ gasification rates in char generated at 0.1 MPa exhibited approximately twice the values as compared to those obtained at 3 MPa. This is because the uniformity of the carbonaceous structure increases with the pyrolysis pressure. The uniformity of carbonaceous structures would affect the CO₂ gasification reactivity, and the decreasing uniformity would lead to the progression of cavities on the char surface during the CO₂ gasification process. The gasification rate of biomass char increases with the heating rate at pyrolysis. This is due to the coarseness (surface morphology) of biomass char and rough texture, which increases with the heating rate.     

Effect of the thermophysical properties of the material of a local energy source on conditions and characteristics of ignition of metallized composite propellants

Solid-phase ignition of metallized composite propellants by a single particle heated to a high temperature under conditions of an ideal thermal contact has been numerically studied. The effect of the thermophysical properties of the material of a local energy source on the conditions and characteristics of ignition of composite propellants has been analyzed. It has been found that sources with a high heat storage capacity exhibit shorter ignition delay times for metallized propellants (by 10–60%) and lower initial temperatures required to initiate the combustion process (by 170 K). In addition, it has been found that the presence of particles of metals (boron, aluminum, magnesium, lithium) in the propellant composition leads to an increase in the effective thermal conductivity of the propellant. The cumulative effect of the thermophysical properties of the materials of the “particle heated to a high temperature–metallized composite propellant” system leads to an increase in the ignition delay times (by 25–65%) and the heat penetration depth of the near-surface layer of the propellant (by 25–40%) at the time of combustion initiation compared with metal-free compounds.     

Pyrolysis of corrugated cardboard in inert and oxidative environments

The thermal decomposition of corrugated cardboard has been studied in inert and oxidative (non-flaming) atmospheres under a range of radiant heat fluxes relevant to fire conditions in warehouse storage applications. Experiments were performed in a Fire Propagation Apparatus (FPA) on double-wall corrugated cardboard at heat flux levels of 20, 60, and 100 kW/m². Pyrolysis data comprised of gasification rates and surface temperatures were collected for tests carried out in ambient atmospheres consisting of 100% N₂ as well as 2%, 6%, 8%, 10%, and 14% (mol) O₂ in balance nitrogen. It is shown that the presence of oxygen has an appreciable effect at all heat flux levels; however, it is most prevalent at low heat fluxes. Analyses are presented in an effort to gain further understanding of char oxidation processes. Results show that the maximum heat evolved in oxidative environments is relatively constant and similar for all conditions tested. Furthermore this heat release rate is found to be comparatively small relative to the high radiant fluxes tested; this explains the experimentally observed behavior. This study provides a comprehensive dataset that may be used in conjunction with approaches recently adopted in the fire community in which optimization procedures are employed to generate material properties for pyrolysis models used in CFD fire simulations.     

Heterogeneous fixation of N2: Investigation of a novel mechanism for formation of NO

Formation of NO initiated by heterogeneous fixation of N₂ during pyrolysis is investigated experimentally and theoretically. The experiments were conducted with beech wood as well as with the pure biomass components cellulose, xylan, and lignin. The NO formation during char oxidation was recorded as function of pyrolysis atmosphere (N₂ or Ar), pyrolysis temperature (700–1050 °C), and oxidizing atmosphere (O₂ in N₂ or Ar). The results confirm earlier reports that biomass char may be enriched in N during pyrolysis at 900 °C and above. The N-uptake involves re-capture of N-volatiles as well as uptake of N₂. During char oxidation, the captured N is partly oxidized to NO, resulting in increased NO formation. The NO yield from oxidation of beech wood char made in N₂ increases with pyrolysis temperature, and is about a factor of two higher at 1050 °C than the corresponding yield from chars made in Ar. The experiments with pure materials show that the lignin char has the strongest ability to form NO from uptake of N₂, while xylan char forms only small amounts of NO from N₂. Density Functional Theory (DFT) calculations on model chars have revealed a number of chemisorption sites for N₂, many of which are weakly bound and therefore expected to have a short half-life at the higher pyrolysis temperatures. However, the chemisorption of N₂ across a single ring of the armchair surface was found to have an activation energy of 344 ± 30 kJ mol⁻¹ and form a stable, exothermic product with cyano groups. This demonstrates that at least one channel exists for the high-temperature incorporation of N₂ into a char which could give rise to the observed increase in NO release in subsequent char oxidation.     

Mathematical modelling of thermophysical processes at peat firing and smoldering

A new statement and the numerical solution of the problem of the peat layer firing as a result of the effect of the surface fire is given on the basis of a mathematical model of a porous reacting medium. The original reagent smoldering at moderate temperatures (T ₁ ≤ 750 K) is found to be determined by the processes of heat and mass exchange with the forest fire source, peat drying and pyrolysis, reaction of the carbon oxide oxidation, thermophysical characteristics, and the peat height as well as by the thickness of the water layer under the peat layer.     

影响热虹吸管运行性能的参数分析

依据热力学第二定律,在分析热管运行过程的基础上,建立了热管工质循环过程的T-S熵分析模型,对热管稳态运行时的几何参数和工质流体热物性参数进行分析,推导得出了热管设计和成功运行的基本条件,热管的结构参数组必须大于工质的热物性参数和封装材料组,二者相容才能建立热管的热力循环。用试验结果验证了此结论。所推导的方程对于一定结构的热管提供了一个设计和运行的指导准则,并且与传热率无关,不管热流多大热管结构参数必须和材料相容。     

铝纳米颗粒的热物性及相变行为的分子动力学模拟

采用分子动力学方法模拟了纳米金属铝在粒径为0.8-3.2 nm 时的熔点、密度和声子热导率的变化, 研究了粒径为1.6 nm的铝纳米颗粒的密度、比热和声子热导率随温度的变化. 采用原子嵌入势较好地模拟了纳米金属铝的热物性及相变行为, 根据能量-温度曲线和比热容-温度曲线对铝纳米颗粒的相变温度进行了研究, 并利用表面能理论、尺寸效应理论对铝纳米颗粒熔点的变化进行了分析. 随着纳米粒径的不断增大, 铝纳米颗粒的熔点呈递增状态, 当粒径在2.2-3.2 nm时, 熔点的增幅减缓, 但仍处于递增趋势. 随着纳米粒径的增大, 铝纳米颗粒的密度呈单调递减, 热导率则呈线性单调递增, 且热导率的变化情况符合声子理论. 随着温度的升高, 粒径为1.6 nm的铝纳米颗粒的密度、热导率均减小. 该模拟从微观原子角度对纳米材料的热物性进行了研究, 对设计基于铝纳米颗粒的相变材料具有指导意义.     

Synthesis and Application of a Novel Charring Agent Poly(p-ethylene terephthalamide)

A novel charring agent poly(p-ethylene terephthalamide) (PETA) was synthesized by using terephthaloyl chloride and ethylenediamine through solution polycondensation at low temperature. Poly(p-ethylene terephthalamide) was used together with ammonium polyphosphate (APP) to prepare a novel intumescent flame retardant (IFR) for acrylonitrile–butadiene–styrene (ABS). The thermal degradation behavior and flame retardancy were investigated by thermogravimetric analysis and limiting oxygen index (LOI) tests, and the morphology and structures of residues generated in different conditions were investigated by scanning electron microscopy and Fourier transform infrared spectra. The results showed that PETA could be effective as a charring agent, the flame retardancy of ABS and the weight of residues improved greatly with the addition of IFR. When the content of APP was 25 wt% and PETA was 12.5 wt%, the LOI value of IFR–ABS system was found to be 33, and class V-0 of UL-94 test was passed. The microstructures observed by scanning electron microscope indicated that the charring agent (PETA) can promote formation of uniform and compact intumescent charred layers in IFR–ABS system after burning.     

金属盐对生物质热解特性影响试验研究

基于深入了解生物质热解行为的目的,在自行研制的热解机理试验台上系统研究了金属盐对生物质热解的影响规律。试验结果表明,钾离子对生物油中的一些大分子量组分发生重聚反应生成焦炭和小分子气体产物具有强烈的催化作用,从而降低了热解生物油产量而得到更多的焦炭和气体产物。相比钾离子而言,钙离子对焦炭生成的促进作用更为强烈;镁离子对白松热解的影响远没有钾离子和钙离子明显。     

Model of workpiece erosion for electrical discharge machining process

The previously published step-wise model (SWM) of cold electrodes erosion of electric arc heaters (EAHs) was modified for the calculation of workpiece removal rate (WRR) in electrical discharge machining (EDM) process. Modified model applies both relations the step-wise erosion model and the point heat source erosion model and takes into account the discharge current, the discharge pulse/pause time and thermophysical properties of machined material. The results of calculations show a reasonable agreement with data obtained experimentally by different authors about different materials and conditions.