The effect of different impact modifiers in halogen-free flame retarded polycarbonate blends - II. Fire behaviour

In this second of a series of two papers, the fire behaviour of halogen-free flame retarded polycarbonate (PC) blends with different impact modifiers was studied. The impact modifiers were acrylonitrile-butadiene-styrene (ABS), a poly(n-butyl acrylate) rubber (PBA) with a poly(methyl methacrylate) (PMMA) shell and two silicone-acrylate rubbers consisting of PBA with different amounts of polydimethylsiloxane (PDMS) and different shell materials (PMMA and styrene-acrylonitrile, SAN). The flame retardant was bisphenol A bis(diphenyl phosphate) (BDP). Flammability was determined by LOI and UL 94. The burning behaviour under forced flaming conditions was studied by cone calorimeter under different external irradiations and by pyrolysis combustion flow calorimeter measurements. The exchange of ABS with the pure acrylate rubber worsened flammability, while similar results were obtained in cone calorimeter measurements. The exchange of ABS with the silicone-acrylate rubbers is promising, particularly with higher amounts of PDMS. In flammability tests strongly enhanced LOI values were obtained and therefore silicone-acrylate rubbers look like promising alternatives for ABS.     

Improving effect of boron carbide on the combustion and thermal oxidation characteristics of amorphous boron

Boron carbide (B₄C) is one of the main products from the primary combustion of boron (B)-based propellants and has a significant influence on the secondary combustion of B. To systematically evaluate its effects on the secondary combustion of B, mixtures of B₄C and B in different mass ratios were prepared. To study the ignition temperatures and combustion flames of the samples, a xenon lamp ignition experimental system and a flame shape test system were designed, respectively. A thermogravimetry–differential scanning calorimetry–Fourier transform infrared spectroscopy combined thermal analysis system was used to study the thermal oxidation characteristics and analyze the gaseous products of the samples. The results indicate that B₄C reduces the heat absorption at the beginning of the ignition, but subsequently prevents the rapid rise of sample temperature. During the stable combustion stage, the maximum flame length under optical density 10⁻⁴ (OD4) filter was 20.4 mm, and the maximum flame length under 580 nm + OD4 filters (represents the combustion of B element) was 16.7 mm. The samples contained a small amount of HBO₂ and H₃BO₃, which led to slight mass loss during the low temperature section of the thermal oxidation process. During the high temperature section, the oxidation of B and B₄C caused considerable mass gain. The gaseous products of the thermal oxidation process include CO₂, CO, and H₂O. In general, the B content of 60% was the most beneficial to decrease the oxidation temperature, increase the combustion intensity, and improve the heat-releasing ability of the samples.

     

Isothermal combustion characteristics of anthracite and spent coffee grounds briquettes

The isothermal combustion characteristics of anthracite and spent coffee grounds briquettes in a bench-scale fixed-bed furnace were examined by using a macro-thermogravimetric analysis approach between 873 and 1173 K. The combustion performances in terms of the conversion rates, flame evolution, pollutant emissions, ash compositions and ash fusion temperatures of two anthracite/spent coffee grounds blend briquettes with blending ratios of 40% and 60% were also compared with those of their individuals. There was only an obvious peak in DTG profile for every sample at each temperature. At 973 K, the average burning rate of the spent coffee grounds was about 8 times higher than that of the anthracite. The average burning rate of 60% anthracite/40% spent coffee grounds blend at 1173 K increased by about 3 times in comparison with that at 873 K. NO₂ emissions for the four samples were much less than other gaseous emissions. The four ash fusion temperatures for the anthracite were higher than 1791 K. The deformation temperature, softening temperature, hemispherical temperature and flow temperature for the spent coffee grounds were 1526, 1626, 1687 and 1791 K. The ash fusion characteristics of the spent coffee grounds were improved by blending the anthracite.

     

Thermal behaviour and the cone calorimetric analysis of the jute fabric treated in different pH condition

In this paper, the effect of pH, i.e. acid and alkali was investigated on thermal stability of ligno-cellulosic polymeric fibrous (jute) material. The jute fabric was subjected to treatment under different pH, namely 4.5, 7, 10, 12, i.e. in acidic, neutral and alkaline conditions followed by drying prior to any thermal and physical characterization. The improvement in the thermal stability of jute to flame was measured in terms of limiting oxygen index value, vertical flammability and temperature profile of burning zone. Likewise thermo-gravimetry, differential scanning calorimetry and cone calorimeter analysis were also used to elucidate the improvement in thermal stability of the treated fabric. The changes in heat release rate, mass loss rate, heat of combustion, smoke production, etc., in the untreated and treated sample were measured in detail in cone calorimeter. Only the alkali-treated jute fabric samples showed profound improvement in thermal stability.

     

电动汽车热泵系统中易燃制冷剂R290的燃烧特性

研究了制冷剂R290在电动汽车热泵空调系统潜在泄漏过程中的燃烧特性,通过实验得到了不同泄漏温度和体积流量下R290的燃烧特性,并与制冷剂R134a进行对比。实验表明:R290在低温下更难点燃,而在高泄漏体积流量下和高泄漏温度下会发生喷射火焰的吹灭现象,且当泄漏温度在30~60℃之间时,泄漏体积流量的增大会提高其燃烧强度。R290制冷剂气体温度越高,其火焰燃烧强度越大,燃烧火焰越细,火焰心高度越低;R290制冷剂气体的体积流量越大,火焰燃烧区域越大,火焰高度越低,热辐射通量越大。     

热辐射通量对沥青改性PMMA燃烧特性的影响

采用本体聚合法制备沥青改性聚甲基丙烯酸甲酯(PMMA)块体,研究了热辐射通量对其热释放速率平均值、质量损失速率平均值、点燃时间、CO及CO_2产率的影响。结果表明,添加沥青的PMMA点燃后300 s内平均热释放速率由569 kW/m~2降低到了525 kW/m~2,沥青使其燃烧更加平稳;沥青改性PMMA的平均热释放速率、质量损失速率和CO产率与热辐射通量成线性递增关系;点燃时间随着辐射通量的增加而呈指数衰减趋势;CO_2产率在热辐射通量低于40 kW/m~2时基本不变,高于40 kW/m~2时呈线性递增。添加沥青之后PMMA能够平稳燃烧使其可以作为锥形量热仪标准物质候选物,得到的燃烧特性与热辐射通量的函数关系式为其不确定度评估提供了依据。     

Combustion of aluminum particles in a high-temperature furnace under various O2/CO2/H2O atmospheres

The study focusing on the combustion of flowing aluminum particles and the properties of condensed phase products has important guiding significance for the practical application of aluminum-based propellants. Based upon an in-house built dynamic combustion experimental system, the dynamic combustion process of aluminum particles and the properties of condensed phase products under different atmospheres were studied in detail. The microstructure, size distribution and active aluminum content of samples were analyzed by field emission scanning electron microscopy, laser particle analyzer and inductively coupled plasma atomic emission spectroscopy. By monitoring the temperature distribution at different points in the furnace, the heat release of the samples at different positions is approximated, and the combustion efficiency is calculated. In the atmosphere containing CO₂, the maximum combustion efficiency can reach the value of 94.41%, followed by that in H₂O atmosphere, which had the value of 81.19%. Finally, under the N₂ containing atmosphere, the combustion is the weakest, and has the value of only 53.91%, confirming that the combustion followed the following descending order: CO₂?>?H₂O?>?N₂. The condensed phase products were mainly composed of agglomerates formed by the aggregation of particles and alumina smoke. It is well known that the reaction of the sample in the furnace not only follows the melt-dispersion mechanism, but also the diffusion mechanism. The high-speed camera captured four typical combustion forms of aluminum particles during flow, which are stable combustion, release of alumina smoke, crushing and extinction. The average burning time of four stages were studied. The two reaction mechanisms occurring under the same reaction conditions are determined by the nature of aluminum particles themselves.

     

Scale effect of porous mesh on the inhibition mechanism of wheat dust flame

The combustible features of wheat dust easily induce a potential hazard in its processing and application. To clearly reveal the effects of porous mesh parameters on the flame propagation of wheat dust, a vertical combustion pipeline together with the data collecting by the high-speed photography and fine thermocouple was built. Results indicate that with the increase in the mesh scale, the dust combustion and peak temperature are intensified first and then decreased with a darker luminescence. The increasing mesh number shows an inhibition effect on both peak temperature and combustion pressure, but an accelerating first and then weakening effect on flame velocity. A smaller particle size contributes to a more complete combustion, causing a higher peak temperature and flame velocity. At the particle mass of 2.5 g, the maximum value of peak temperature, flame velocity and combustion pressure were obtained during the flame propagation.

     

Ignition and burning behaviors of automobile oil in engine compartment

Accidental leakage of automobile oils is of great inclination to initiate pool fires in engine compartment, with threats to induce the flashover of other components and flame penetration into the passenger compartment. This paper presents experimental results of the ignition and burning behaviors of a kind of automobile oils (automatic transmission oil) using a cone calorimeter. Measurements of oil temperature, ignition time, mass loss and heat release rate are performed at different external heat fluxes and initial fuel depths. The comparison between experimental and numerical oil temperature evolutions shows that the variations of the ignition time at different experimental conditions depend on the heat dissipation process inside the liquid phase. The steady mass burning rate is nearly independent of initial fuel depth and has a linear relation with external heat fluxes. In addition, the results indicate an increase in peak heat release rate by a large margin initially, followed by a relatively small margin under thicker initial fuel depths, while its variations are proportional to external heat fluxes. Correlations are also developed to determine the peak heat release rate as a function of the initial fuel depth.

     

Investigation of the influence of pressure on the combustion of Alpagut lignite

This research presents the combustion behavior of lignite under different reaction pressures. Lignite from Alpagut, Çorum of Turkey was combusted in its run off mine (ROM) condition under three different pressure levels of 172, 345, 517 kPa (25, 50, 75 psi). Experiments were done in a fully controlled temperature regime in an isolated combustion tube that operated in coordination with a continuous gas analyzer. Combustion behavior of lignite under different pressures was characterized by effluent gas analysis method. The changes in the amounts of consumed oxygen, evolved carbon oxides as well as variations in the temperature were assessed. The combustion efficiency and effectiveness of lignite was evaluated in terms of thermal features, from the viewpoint of reaction kinetics and by the computation of instantaneous fuel consumption at critical points. It was seen that combustion of lignite tended to turn from a steady profile to a considerably rapid one with increase in pressure, proving to be highly sensitive to the applied pressure level. Also, different levels of pressure resulted in distinctive combustion behavior not only from the view of thermal characteristics, but also in terms of reaction kinetics.     

Preparation and burning behaviors of flame retarding biodegradable poly(lactic acid) nanocomposite based on zinc aluminum layered double hydroxide

A flame retarding biodegradable polylactic acid (PLA) nanocomposite based on flame retardant composites (containing ammonium polyphosphate (APP), pentaerythritol (PER) and melamine cyanurate (MC) by controlling the weight ratio was 2:2:1) and organomodified zinc aluminum layered double hydroxide (Zn-Al-LDH) has been prepared by melt-compounding directly. The morphology and burning behaviour of nanocomposite with 2 wt% Zn-Al-LDH loadings were investigated. The extent of dispersion of LDH was quantified by wide angle X-ray scattering (WAXS) and transmission electron microscopy (TEM), illuminating the good dispersion state for ZnAl-LDH in the PLA matrix. Significant improvements in fire retardant performance were observed for the nanocomposite from microscale combustion calorimeter (MCC) and cone calorimetry (reducing both the heat release rate and the total heat released). It revealed that incorporation of FR and ZnAl-LDH was very efficient in improving the flame retardance of PLA composite.     

Heat release and structural collapse of flexible polyurethane foam

Flexible polyurethane foam used in upholstered furniture remains one of the major fire hazards to date. The heat release rate of burning items made of foam depends strongly on the foam's physical behavior, notably its collapse to a burning liquid that can result in a pool fire. In this contribution, the cone calorimeter was used to study the physical processes and to determine their influence on foam combustion over a range of external heat fluxes. The initial stage of foam collapse can be described as the propagation of a liquid pyrolysis layer through the foam sample. The rate of propagation of the liquid layer was found to depend strongly on the convective heat transfer from the flame, which simultaneously defined and depended on the sample shape. The effective heat of combustion during foam collapse and pool fire was matched to the heat release potential of the components of the foam formulation to deduce which are consumed. The proposed analysis can serve to clarify the mechanism of flame retardant action, as demonstrated for a commercial brominated-phosphorous compound.     

A comprehensive study of the synergistic flame retardant mechanisms of halloysite in intumescent polypropylene

This work aims to evaluate the efficiency of halloysite as synergistic agent in an intumescent PP system based on a coated ammonium polyphosphate (IFR). The first part of the study analyses the thermal stability and fire performance of PP when using the intumescent formulation alone or in combination with the aluminosilicate nanotubes (HNTs). Cone calorimetry reveals that partial substitution of IFR by HNTs (3 wt.%) imparts substantial improvement in flame retardancy with reduced heat release rate and longer burning times. Additionally, a shift from V-1 to V-0 classification is achieved at the UL-94 test with only 1.5 wt.% HNTs. The second part provides a better understanding of the physical and chemical mechanisms of action of HNTs in the intumescent systems. The chemical evolution of the condensed phase during combustion is described by solid state NMR, and in particular using 2D NMR. Results indicate that halloysite speeds up the development of the intumescent shield, but also enhances its mechanical properties by physical reinforcement (i.e. aluminosilicate “skeleton-frame” for the phospho-carbonaceous structure) and/or by chemical interactions with IFR yielding to aluminophosphates. These new chemical species allow thermal stabilization of the char at high temperatures and provide good macro- and micro-structural properties. Both effects increase the mechanical strength of the protective layer during burning ensuring excellent heat and mass transfer limitations between gas and condensed phases.     

Effect of aluminum diethylphosphinate on flame retardant and thermal properties of rigid polyurethane foam composites

Rigid polyurethane foam/aluminum diethylphosphinate (RUPF/ADP) composites were prepared by one-step water-blown method. Furthermore, scanning electron microscope (SEM), thermal conductivity meter, thermogravimetric analysis (TGA), limiting oxygen index, Underwriters Laboratories vertical burning test (UL-94) and microsacle combustion calorimetry were applied to investigate thermal conductivity, thermal stability, flame retardancy and combustion behavior of RPUF/ADP composites. Thermogravimetric analysis–Fourier transform infrared spectroscopy (TG–FTIR) was introduced to investigate gaseous products in degradation process of RPUF/ADP composites, while SEM and X-ray photoelectron spectroscopy were used to research char residue of the composites. It was confirmed that RPUF/ADP composites presented well cell structure with density of 53.1–59.0 kg m^?3 and thermal conductivity of 0.0425–0.0468 W m^?1 K^?1, indicating excellent insulation performance of the composites. Flame retardant test showed that ADP significantly enhanced flame retardancy of RPUF/ADP composites, RPUF/ADP30 passed UL-94 V-1 rating with LOI of 23.0 vol%. MCC test showed that ADP could significantly decrease peak of heat release rate (PHPR) of RPUF/ADP composites. PHPR value of RPUF/ADP20 was decreased to 158 W g^?1, which was 21.8% reduced compared with that of pure RPUF. TG–FTIR test revealed that the addition of ADP promoted the release of CO₂, hydrocarbons and isocyanate compound in first-step degradation of RPUF matrix while inhibited the release of CO in second step degradation. Char residue analysis showed that the addition of ADP promoted polyurethane molecular chain to form aromatic and aromatic heterocyclic structure, enhancing strength and compactness of the char. This work associated a gas–solid flame retardancy mechanism with the incorporation of ADP, which presented an effective strategy for preparation of flame retardant RPUF composites.

     

The effect of inclination angle on fire behaviors of stay cable in an intercepted double-layer cable model

Understanding the fire characteristics of stayed-cable bridge, such as fire growth, during accidental fire is essential to develop prevention strategies for potential damage. This study focuses on the effects of high cable fire that may result from a short circuit or a lightning strike since high flammability heat release characteristics of HDPE sheath causes burning of nearby cables. Fire propagation behaviors on one single cable and between two adjacent cables under working conditions with different inclination angles were obtained. The temperature distribution, drop ignition behaviors and flame spread rate were analyzed. The results show that flame propagation characteristics of stay cables seriously changed as the angle of inclination increases, which explain the fracture sequence of cables to some extent for the Red Stone Bridge fire event. The particle size of molten substance formed by combustion of the upper-layer cables increases, and the ignition position for the under-layer cable gradually moves down as the increase of inclination angle. Moreover, increased inclination angle also resulted in increase in flame height and molten drops flow rate and reduce in the duration of the prosperity stage for cable fire. Although HDPE sheath is a protective device, it does prevent the combustion for inner strands from becoming intense, but once ignited, would become the main fire load, promoting fire development.

     

Effects of a semi‐bio‐based triazine derivative on intumescent flame‐retardant polypropylene

A semi‐bio‐based synergist (N, N′, N″‐1, 3, 5‐triazine‐2, 4, 6‐triyltris‐glycine [TTG]) was prepared by using glycine and cyanuric chloride. The structure of TTG was characterized by ¹H NMR and Fourier transform infrared spectroscopy. The TTG was applied in polypropylene (PP)/intumescent flame‐retardant compounds to improve its flame retardancy. The flame‐retardant properties of PP compounds were evaluated by limiting oxygen index and vertical burning tests (UL‐94). The results showed that 17 wt% intumescent flame‐retardant and 1 wt% TTG makes PP achieve the UL‐94 V‐0 rating without drippings, and the limiting oxygen index value is increased to 29.5 vol%. The thermal degradation behavior and char morphology of PP compounds were investigated by thermogravimetric analysis and scanning electron microscopy. The results indicated that TTG accelerates the formation of char layer, regulates the porous structure of char layer, and enhances its barrier property. Therefore, the temperatures of PP compound after two ignitions during the UL‐94 test are decreased significantly as shown in infrared thermal imaging. In addition, the combustion characteristics of PP compounds were investigated by cone calorimeter. The peak of heat release rate (PHRR) of PP compound is 67% reduced, and the t_PHRR is delayed from 223 to 430 seconds, indicates that the combustion risk of PP compound is reduced.     

Investigation of the flammability of different textile fabrics using micro-scale combustion calorimetry

Evaluating and analyzing the performance of flame retardant (FR) textiles are a critical part of research and development of new FR textiles products by the industry. The testing methods currently used in the industry have significant limitations. Most analytical and testing techniques are not able to measure heat release rate (HRR), the single most important parameter in evaluating the fire hazard of materials. It is difficult to measure HRR of textile fabrics using cone calorimetry because textile fabrics are dimensionally thin samples. The recently developed micro-scale combustion calorimetry (MCC) is able to measure the following flammability parameters for textile using milligram sample sizes: heat release capacity, HRR, temperature at peak heat release rate (PHRR), total heat release and char yield. In this research, we applied MCC to evaluate the flammability of different textile fabrics including cotton, rayon, cellulose acetate, silk, nylon, polyester, polypropylene, acrylic fibers, Nomex and Kevlar. We also studied the cotton fabrics treated with different flame retardants. We found that MCC is able to differentiate small differences in flammability of textile materials treated with flame retardants. We were also be able to calculate the limiting oxygen index (LOI) using the thermal combustion properties of various textile samples measured by the MCC. The calculated LOI data have yielded good agreement with experimental LOI results. Thus, we conclude that MCC is an effective new analytical technique for measuring textile flammability and has great potentials in the research and development of new flame retardants for textiles.     

Burning velocity measurement of HFC-41, HFC-152, and HFC-161 by the spherical-vessel method

The burning velocities of fluoromethane (HFC-41), 1,2-difluoroethane (HFC-152), fluoroethane (HFC-161) and ethane were measured by the spherical-vessel (SV) method at room temperature and at initial pressures of 80-107 kPa over a wide range of HFC/air equivalence ratios (?). The burning velocities were determined from the measured pressure increases by application of a spherical flame model. Schlieren photography was used to directly observe flame propagation behavior in a cylindrical vessel equipped with optical windows. The time evolution of the flame radii derived from the pressure increases agreed with the time evolution observed with the Schlieren technique. The maximum burning velocities of HFC-41, HFC-152, HFC-161 and ethane were 28.3 cm s⁻¹ at ? = 1.01, 30.1 cm s⁻¹ at ? = 1.07, 38.3 cm s⁻¹ at ? = 1.07 and 40.9 cm s⁻¹ at ? = 1.05, respectively. The maximum burning velocities for the HFCs, including previously reported C₁ and C₂ fluoroalkanes, decreased with increasing F-substitution rate (the ratio of the number of F atoms to the sum of the number of H and F atoms). The concentrations of chemical species in the flames were investigated by means of an equilibrium calculation, and the results suggested that the burning velocity was correlated with the concentrations of H and OH radicals that were not deactivated by F radicals in the flame. The results also suggested that the burning velocities were linearly related to the heats of combustion of the C₁ and C₂ fluoroalkanes.     

The pressure effect study on the burning rate of ammonium nitrate-HTPB-based propellant with the influence catalysts

The burning rate of AN–HTPB-based propellant catalysed with chromium salt has been studied using conventional strand burner under the various pressure range, i.e. from atmospheric pressure to 6.897 MPa and verified with Piobert law, i.e. r = aP ⁿ . At atmospheric pressure, the burning rate AN–HTPB propellant was being accelerated with the chromium-based catalysts used. In case of lead chromate-catalysed system, burning rate was observed 2.655 times higher than burning rate (r = 0.200 mm s⁻¹) of virgin AN–HTPB propellant sample. However, the Copper chromate-catalysed propellant burned with slower rate (r = 0.160 mm s⁻¹) than the virgin AN–HTPB propellant sample. The burning rate of all catalysed propellant samples are found to be the pressure sensitive and accelerated higher with rise of pressure. The highest burning rate (r = 2.422 mm s⁻¹) was recorded with ammonium dichromate and lowest (r = 1.40 mm s⁻¹) with lead chromate-catalysed propellant sample with the rise of pressure up to 6.897 MPa at different pressures. A linear relationship was observed between the burning rate and pressure rise which followed the Piobert law, i.e. r = aP ⁿ . The pressure index (n) values of AN–HTPB-based samples were calculated higher when catalysed with ammonium dichromate, Copper Chromate, Cr₂O₃, Potassium dichromate (n = 0.525, 0.555, 0.429, and 0.408 respectively) and lower (n = 0.226) with lead chromate compared to virgin sample (n = 0.405). Higher value indicates the positive effect on accelerating the burning rate with catalyst at higher pressure ranges.     

Experimental study on combustion characteristics of diesel–hydrogen dual-fuel engine

As a clean and sustainable energy source, hydrogen is widely considered as an engine fuel by top researchers. In view of the fact that the uneven fuel mixture of diesel fuel deteriorated the combustion and emissions process, it is expected to adopt diesel and hydrogen dual-fuel combustion technology to optimize combustion and heat release of diesel engine. In this study, experiments are carried out on a diesel engine and the combustion characteristics of the engine with different hydrogen ratios (RH) are compared. It has been found that hydrogen addition is conducive to accelerate the heat release rate and improve the thermal efficiency. Specifically, compared with pure diesel conditions, the peak pressure increased by 7.7% and the cumulative heat release rate increased by 3.7% under the condition of RH of 20%. Moreover, although the effect on the ignition delay period is not clear, the higher RH brings about earlier heat release center and more cumulative heat release while enhancing the heat release of premixed combustion reducing the diffusion combustion and post-combustion.