Thermal analysis of polymer ignition

Gas phase criteria for the onset of flaming combustion of solids in fires are used to locate a critical temperature T_cr in a nonisothermal analysis (TA) experiment that corresponds to the surface temperature of the solid at ignition in a fire test, T_ign. This critical TA temperature occurs at low conversion of solid to gaseous fuel so it is independent of the heating rate in the test or the thermal decomposition reaction model. However, T_cr depends on the thermal properties of the polymer and the conditions of the fire test in which the gas phase criteria were measured. Nonisothermal analysis data in nitrogen and air were obtained for 20 polymers by thermogravimetric analysis and microscale combustion calorimetry. The critical temperatures T_crs obtained from TA experiments compared favorably with analytic results for a simple polymer ignition model and finite element simulations and were in qualitative agreement with ignition temperatures measured in standardized fire tests.     

Effectiveness of pre‐applied wetting agents in prevention of wildland urban interface fires

The purpose of the work described in this paper is to investigate the effectiveness of pre‐wetting structures, dead fuels, and landscaping plants in preventing fire spread from wildland fires to structures. Critical fluxes for fire growth were determined using intermediate‐scale testing for three wetting agents (water, type A foam, and gel) applied to 10 landscaping plants conditioned to 20% moisture, a mulch material, and four external structural materials (vinyl siding, plywood siding, asphalt shingle roofing, and cedar shake roofing). The critical flux for fire growth values was determined at 3‐min heat radiation exposure and simultaneous 300‐mm long flame exposure. Test specimens were exposed to various durations and intensities of drying prior to exposing them to heat radiation. Application of water or foam provided no noticeable protection. Gel was effective in providing protection even after 60 min of laboratory condition drying but was less successful when exposed to fire weather simulating accelerated drying. Some uncertainty is associated with the results of this work because of the variability of landscaping plants and gel wetting agent application uniformity. The intermediate‐scale test results were verified using full‐scale testing. Copyright © 2012 John Wiley & Sons, Ltd.     

Ignition and Combustion of Dust-Gas Suspensions

The ignition and combustion of dust-gas suspensions are considered. It is shown that the ability of these systems to accumulate heat is determined not only by their kinetic and thermal properties but also by the relation between their reaction surface and the heat-removal surface (f). Experimental information on flame temperatures, ignition delays, and flame propagation over gas suspensions is processed using the parameter f, and the postulate on the stimulating role of the developed reaction surface in activating these processes is validated. It is shown that during overall burning, diffusion combustion of particles occurs only for rather small values of f. The ambiguous effect of the parameter f on the ignition and combustion processes leads to the necessity of optimizing the fuel size distribution and concentration for the effective operation of the power devices. The role of the macroparameters of two-phase flames of refractory metals in the synthesis of combustion nanoproducts is analyzed. __________ Translated from Fizika Goreniya i Vzryva, Vol. 41, No. 6, pp. 3–14, November–December, 2005.     

A sample holder for determining material properties

The determination of material properties, such as thermal conductivity, for use in fire models requires precise knowledge of all heat losses. For example, the ignition and subsequent burning of solid samples is sensitive to heat losses from the rear surface of the tested sample. The heat loss depends on the sample holder and its environment. Theoretical predictions of ignition and pyrolysis in standard flammability apparatuses show that the construction of the sample holder has a surprisingly large effect on measured properties especially for thermally thin samples. This makes flammability measurements and inferred thermal properties apparatus dependent. For example, ignition measurements of 6.4 mm (1/4″) plywood samples show a 40% reduction in the critical heat flux for ignition. The back and sides of the sample were sealed by wrapping it with aluminium foil tape. Heat losses from the sample were minimized by placing it on a sample holder having four layers of 3.2 mm (1/8″) thick insulating ceramic paper surrounded by an outer layer of aluminium tape. A detailed mathematical model is developed to fully characterize the thermal response of a sample to a prescribed heat flux in a standard flammability apparatus. The model is used to estimate the residual heat losses not eliminated by the sample holder. Model predictions accurately track the temperature response of blackened brass plates of different thicknesses exposed to several different incident heat fluxes. Brass, of course, has well known thermophysical properties; so, it also provides an excellent means for in situ calibration of the incident heat fluxes. Copyright © 2000 John Wiley & Sons, Ltd.     

Statistics of fires in dwellings involving textiles in the United Kingdom

There is widespread concern at present about the fire risk form the use of modern materials in upholstery and other domestic articles. One of the main characteristics of fires in dwellings which start with the ignition of textiles or furniture is that they have a high casualty rate compared with other kinds of domestic fires. Most of the casualties form such fires occur in fires which start in upholstery or bedding, often accidentally ignited by smokers' materials. Asphyxiations by smoke is the commonest cause of death in ‘textile’ and ‘upholstery’ fires. This paper presents relevant statistical information on fires in dwellings attended by local authority fire brigades which result from the ignition of upholstery, bedding, other textiles or furniture and the casualties which these fires cause.     

Burning plastics: Mechanisms of smoke formation

Quantitative pyrolysis gas chromatography studies have been conducted on three grades of cross-linked polyester resin formulated for improved resistance to surface spread of flame. Pyrolysis temperatures from 723 to 1273 K were used, and these values correspond to ‘surface’ temperatures of materials exposed to heat fluxed of 15–149 kW m^–2 which are typical of conditions experienced in real fires. Comparison is made with earlier macro studies on the evolution of smoke from plastics materials exposed to similar heat fluxes under an inert atmosphere. There is some correlation between the smoke obscuration data obtained in previous macro studies and the yield of volatile aromatic products evolved on pyrolysis. The presence of flame retardants increases the formation of carbon in the pyrolysis residue and decreases the yield of volatile aromatic products isolated. The results are considered in the context of mechanisms of smoke formation.     

A simplified model for predicting the ignition of FRP composites with validation using intermediate‐scale fire experiment data

This study presents a simplified theoretical model to predict the ignition of FRP composites of general thermal thickness (GTT) subjected to one‐sided heating. A simplified GTT heat transfer model to predict the surface temperature of GTT composite panels was developed, and the exposed surface temperature was used as ignition criterion. To validate the GTT model, intermediate scale calorimeter fire tests of E‐glass fiber reinforced polyester composite panels at three heat flux levels were performed to obtain intermediate‐scale fire testing data in a controlled condition with well‐defined thermal boundary conditions. The GTT model was also verified by using results from finite element modeling predictions. This model can be used to estimate the surface temperature increase, time‐to‐ignition, and mass loss of FRP composites for fire safety design and analysis. Copyright © 2013 John Wiley & Sons, Ltd.     

Spontaneous ignition hazards in stockpiles of cellulosic materials: Criteria for safe storage

The ambient temperature at which ignition is brought about in various sizes, shapes and masses of sieved bagasse pith was investigated on a laboratory scale. ΔT-time and ΔT-position profiles were obtained, and critical temperatures are determined to closer than 0.5 K; this is the highest precision for location of the sub-critical/super-critical margin so far reported for any thermal ignition system. Such quality is a pre-requisite for quantitative extrapolation in order to predict the critical sizes for field stock-piles in normal climatic conditions. Scaling rules based on stationary-state, thermal ignition theory are used and comparisons of the results are made with real piles. The predictions are shown to grossly over-estimate the safe size of stock-piles, and the reasons for this discrepancy are discussed. Distinctions between the reactivity in perfectly dry substrates (as is inevitable at the temperatures prevailing in laboratory experiments) and that of natural, wet piles, and the part played by water are important features. A need for supplementary measurements of heat-release rates at temperatures close to those prevailing in stock-piles is stressed. The results are found to be consistent with similar measurements in woodflour and the conclusions are, therefore, applicable to all cellulosic materials at high temperatures.     

Furniture and furnishings in the home—some fire statistics

A statistical study of fires in the United Kingdom involving the ignition of furniture and furnishings is presented. This paper examines the data for one year (1970). The analysis shows that in fires starting in furniture and furnishings the chance of a fatality is over twice that in other domestic fires. The majority of furniture fires involve upholstery or bedding and over 90% were started by smokers' materials, electric appliances, space heating or as the result of the activities of children or suspected arsonists. Eighty-five percent of the fatalities were found in the room of origin of the fire. Eighty per cent were overcome by smoke or toxic gases. Sixty percent of the fatalities were either under 5 or over 65 years of age. Monetary values are assigned for damage, casualties and deaths in fire. These costs can be used to assess the value of fire precautions. With the values taken, the total losses in furniture fires in the home amounted to £19 million in 1970. Life loss accounted for the major part of this sum. The expected annual loss per dwelling as a result of the ignition of furniture is thus only about £1, and is only £3 for all dwelling fires. This low figure suggests an approach of either selective spending on those most at risk (the elderly and handicapped) or by government activity through publicity and education.     

Synthesis and characterization of polyimide/hexagonal boron nitride composite

Polyimide (PI)/hexagonal boron nitride (h‐BN) composites were produced via the thermal imidization procedure from solution mixtures of a polyamicacid, which is prepared from 3,3′,4,4′‐benzophenonetetracarboxylic dianhydride and 3,3′‐diaminodiphenyl sulfone (DADPS) in N‐methyl‐2‐pyrrolidone (NMP), and alkoxysilane functionalized h‐BN. The structure, thermostability, thermal behavior, and surface properties of the resulting materials were characterized by means of Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), scanning electron microscopy (SEM). The thermal characteristics of PI/h‐BN films were better than the pure PIs. The physical and mechanical properties of the films were evaluated by various techniques such as contact angle, chemical resistance, and tensile tests. The flame retardancy of the composite materials was also examined by the limiting oxygen index (LOI). The experiments showed that the LOI values of PIs increased from 32 to 43 for the materials containing hexagonal boron nitride. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Development and application starch films: PBAT with additives for evaluating the shelf life of Tommy Atkins mango in the fresh-cut state

In this study, formulations of cassava starch and poly(butylene adipate-co-terephthalate) flexible films were developed, with glycerol, coconut nanocellulose, annatto, and citric acid in different concentrations, as well as the effectiveness of the selected materials in fresh-cut mangoes storage was evaluated. The tensile strength of the different formulations varied from 1.90 (E4) to 6.65 MPa (E3c), and the strain varied from 206.31 (E1c) to 278.41% (E8); this variation was dependent on the percentage of the polymer matrix incorporated. The a_w values of the formulations ranged from 0.396 (E2) to 0.569 (E3c). The Formulations E4 and E7 (with additives) presented good properties and were selected to condition mangoes. The micrographs of these films showed regions of micropores that can facilitate the diffusion of water from the packaged product to the surface, allowing decreases in moisture and a_w, which is associated with higher color maintenance during fruit storage. E7 presented better barrier properties than E4 (lower values of WVP and water solubility) which may have influenced in a positive way to maintain the stability of the package in the studied period. E7 can be considered as a viable alternative for minimally processed mango storage. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48150.     

Electrostatic Discharge Ignition of Energetic Materials

Electrostatic discharge(ESD) ignition of explosives, pyrotechnics, or propellants is often considered to be mainly dependent upon various physical characteristics of the energetic material. This work shows that ESD ignition of secondary explosives tested in powdered form is primarily dependent upon the chemical characteristics of the energetic material(i.e., the decomposition rate kinetics of the materials). We propose that ignition occurs when a spark raises the temperature of the explosive particles to the point where thermal runaway occurs. ESD sensitivities of a diverse series of explosives were measured using a traveling needle test apparatus with the powders slightly confined by Mylar tape to prevent formation and ignition of a dust cloud. Using global thermal decomposition rate coefficient expressions, two parameters were calculated for each explosive:(1) the critical temperatures according to the Frank-Kamenetskii formula for 20 μm particles of each explosive and(2) the temperatures at which the rate coefficient equaled 10³ s⁻¹. These two sets of data were correlated with the observed ESD sensitivities for 50 percent probability of ignition, E₅₀. Excellent correlations resulted, indicating that for ESD ignition under these conditions the spark discharge is primarily a thermal source.     

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.     

Effect of fiber orientation on viscoelastic properties of polymer matrix composites subjected to thermal cycles

Fibers in polymer composites can be designed in various orientations for their usage in service life. Various fiber orientated polymer composites, which are used in aeroplane and aerospace applications, are frequently subjected to thermal cycles because of the changes in body temperatures at a range of −60 to 150°C during flights. It is an important subject to investigate the visco‐elastic properties of the thermal cycled polymer composite materials which have various fiber orientations during service life. Continuous fiber reinforced composites with a various fiber orientations are subjected to 1,000 thermal cycles between the temperatures of 0 and 100°C. Dynamic mechanic thermal analysis (DMTA) experiments are carried out by TA Q800 type equipment. The changes in glass transition temperature (T_g), storage modulus (E′), loss modulus (E′′) and loss factor (tan δ) are inspected as a function of thermal cycles for different fiber orientations. It was observed that thermal and dynamic mechanical properties of the polymer composites were remarkably changed by thermal cycles. It was also determined that the composites with [45°/−45°]_s fiber orientation presented the lowest dynamic mechanical properties. POLYM. COMPOS., 2010. © 2009 Society of Plastics Engineers     

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.     

Electrokinetic and antibacterial properties of needle like‐TiO2/polyrhodanine core/shell hybrid nanostructures

The aim of this study was to fabricate needle like‐TiO₂/polyrhodanine nanostructures by polymerizing rhodanine monomer on the TiO₂ nanoparticles' surfaces and investigate their antibacterial activities. The structural, thermal, morphological, surface and electrical properties of non‐covalently functionalized nanoparticles were characterized by using FTIR, XPS, elemental analysis, TGA, XRD, SEM‐EDX, TEM, contact angle, and conductivity measurements. Characterization results confirmed the formation of needle like‐TiO₂/polyrhodanine (PRh) core/shell hybrid nanostructures. Alterations on the surface and electrokinetic properties of the materials were characterized by zeta (ζ)‐potential measurements with the presence of various salts and surfactants. The ζ‐potential of needle like‐TiO₂ was observed to increase from ?7.6 mV to +28.4 mV after forming a core/shell needle like‐TiO₂/PRh nanocomposite structure and with the presence of cetyltrimethyl ammonium bromide (CTAB) surfactant. Thereby colloidally more stable dispersions were formed. Antibacterial properties of needle like‐TiO₂/PRh were also tested against Staphylococcus aureus, Klebsiella pneumoniae, and Escherichia coli by various methods and they showed good antibacterial activity. The highest killing efficiency was determined for needle like‐TiO₂/PRh against E. coli by colony‐counting method as 0.95. TEM experiments also showed the immobilizations of the nanoparticles on E. coli and revealed the interactions between E. coli and the nanoparticles. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41554.     

Thermal Decomposition Research on 1,5‐Diazabicyclo[3.1.0] Hexane (DABH) as a Potential Low‐toxic Liquid Hypergolic Propellant

1,5‐Diazabicyclo[3.1.0] hexane (DABH) was found a potential hypergolic liquid propellant. The physical and energetic properties of DABH, 2‐(dimethylamino) ethyl azide (DMAZ), and monomethyl hydrazine (MMH) were compared. The ignition delay time of DABH with nitrogen tetroxide was 1 ms, which was shorter than DMAZ and similar with MMH. The toxicology experiment showed that half lethal dose (LD₅₀) of DABH was 621.0 mg kg⁻¹, which suggested that DABH was promising to be used as low‐toxic liquid propellant. Thermal decomposition experiments showed that the apparent activation energy (E ) was about 66.3 kJ mol⁻¹. The thermal decomposition calculated results from Madhusudanan‐Krishnan‐Ninan integration, Satava‐Sestak integration and Achar differential methods were compared and the pre‐exponential factor were obtained.     

Dependence of the Mechanical Sensitivity on the Fractal Characteristics of Octahydro‐1,3,5,7‐tetranitro‐1,3,5,7‐tetrazocine Particles

Three fabrication methods were used to synthesize HMX powders with different particle sizes and microscopic morphologies. All as‐prepared samples were characterized by laser granularity measurements and scanning electron microscopy (SEM). The mechanical sensitivity and thermal stability of the different HMX powders were characterized using mechanical sensitivity tests and differential scanning calorimetry (DSC). Size distribution data and SEM images were used to find the size fractal dimension (D) and surface fractal dimension (D_s) of HMX samples, which were calculated by the least‐squares method and fractal image processing software (FIPS), respectively. The parameters D and D_s quantize two important properties of HMX particles, namely the complexity of the particle size distribution and the irregularity of the particle surface, which affect the thermal conductivity of the particle group if it is exposed to stimuli such as impact, friction or heating. The fractal dimensions reveal the dependence of the mechanical sensitivity of HMX on the powder size, size distribution and microscopic morphology. The results indicate that the proportion of fine particles in HMX powder increases as the D value increases, which causes decreased impact sensitivity. This occurs because hot spot formation leading to an explosion is more difficult because of the improved thermal conductivity of the particle group. Similarly, the surface roughness of HMX particles increases with an increase in D_s, causing an increase in friction sensitivity because of the excessive accumulation of frictional heat. In addition, thermal analysis results indicate that the maximum thermal decomposition rate of HMX decreases with increasing D and D_s.     

Investigations on the self‐ignition of deposits containing combustibles

Self‐ignition of deposited combustibles is a possible reason for excessive fires occurring on deposits for recycling materials or on waste dumps. Two series of experiments were performed to assess the hazard of self‐ignition: hot storage tests with different homogeneous mixtures of combustible material and inert material and tests with pockets of combustible material embedded in inert matter. In the first test series considerable exothermicity (ΔT>60K) was observed for mass fractions of combustible material as low as 2.5%. In the second series it could be shown that the heat transfer from a pocket of burning material through the inert matter can ignite a second pocket of combustible matter. Based on the experimental data, numerical simulations were performed to predict self‐ignition on real‐scale waste deposits. For a deposit of specific size and shape, the influence of the ambient temperature on the occurrence of self‐ignition has been investigated. Copyright © 2008 John Wiley & Sons, Ltd.