Mechanism Of Flame Inhibition Research Articles

A tetra-DOPO derivative as highly efficient flame retardant for epoxy resins

In order to attain highly efficient flame retardant epoxy thermosets without deteriorating the thermo-mechanical performances, a 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO)-derived compound (tetra-DOPO) with different oxidation states of phosphorus and rich aromatic structures was successfully synthesized via addition and Atherton–Todd reactions. The molecular structure of tetra-DOPO was confirmed by the FTIR, 1H- and 31P-NMR spectra. The DMA measurement indicated that the addition of 5.0 wt% of Tetra-DOPO enhanced both the storage modulus and the glass transition temperature of the epoxy composite. The EP composite containing 5.0 wt% of Tetra-DOPO had an LOI of 30.0% and UL-94 V-0 grade, indicating the exceptional fire-retardant efficacy of tetra-DOPO. Additionally, in the cone calorimeter test, the PHRR, THR, and TSP values of EP/tetra-DOPO-5.0 were declined by 25.1%, 18.7%, and 13.9%, respectively, as compared to those of the EP. The reduced mean EHC value suggested the flame inhibition mechanism of tetra-DOPO in the vapor phase by emitting PO• radicals to quench the combustion chain reaction. The flame retardant mechanism of Tetra-DOPO was further clarified by detecting the pyrolytic gas species using TGA-FTIR as well as analyzing the microstructure of the char residues using SEM and Raman spectrometer. The main flame retardant mode of action of Tetra-DOPO was flame inhibition in the vapor phase, while it also played a relatively minor role in the condensed phase by forming an intact and compact char layer containing high graphitized structure.

PET/CLOISITE 15A composite fibers - studies on the structure and the flame inhibition mechanism

The improvement of poly(ethylene terephthalate) (PET) fibers flame retardancy is usually achieved by using antipyrenes, which may be incorporated into polyester molecules during polycondensation or are physically mixed with polymer in the fiber formation process. In this article we present an alternative method to reduce the flammability of PET fibers and fabrics which is analogous to dyeing them with a dispersed dyes in a high temperature bath. We have tested this method many times using various modifiers so far. This time, we applied a commercial organophilized montmorillonite Cloisite®15A (C15A). In the presented work, using Limited Oxygen Index (LOI) flammability tests and the thermogravimetric analysis (TGA) method, the effectiveness of the modification used was demonstrated and its optimal variant was determined. Based on Fourier Transform-Infrared Spectroscopy (FT-IR) studies, the existence of interactions between PET macromolecules and the C15A modifier in the entire temperature range of the oxidative degradation was confirmed. Using the Wide-Angle X-ray Scattering (WAXS) and Small-Angle X-ray Scattering (SAXS) methods, the basic parameters of the nanostructure of the studied fibers were determined, and their nanocomposite nature was confirmed. The most important goal, which was successfully achieved, was to explain the mechanism of flame inhibition by the applied modifier C15A.

Effect of monoammonium phosphate particle size on flame propagation of aluminum dust cloud

The effect of monoammonium phosphate (NH4H2PO4) particles on 5 μm aluminum dust flames is investigated experimentally and computationally. NH4H2PO4 in three particle size is employed to determine the inhibition efficiency on aluminum flame propagation. Flame inhibition mechanism considering both gas and surface chemistry of aluminum particles is developed. Results show that the inhibition effectiveness monotonously increases as NH4H2PO4 particle size is reduced to 25 μm. Flame morphology and flame microstructure change with the addition of different particle size NH4H2PO4. Small NH4H2PO4 particles within the range studied have a greater reduction in average flame propagation compared to the coarser one. Meanwhile, the fine NH4H2PO4 particles almost decompose completely during the penetration of aluminum flame and then undergo a sufficient chemical interaction with the flame. The simulations indicate that the decomposition products of NH4H2PO4 particles obstruct the oxidation of aluminum particles through flame radical consumption. Additionally, the addition of NH4H2PO4 can reduce the vaporization rate and surface reaction rate of aluminum particles.

Flame inhibition of aluminum dust explosion by NaHCO3 and NH4H2PO4

To evaluate the inhibition capacity of NaHCO3 and NH4H2PO4 on the flame in aluminum dust explosions, the inhibition of 5 μm and 30 μm aluminum dust explosions by NaHCO3 and NH4H2PO4 has been studied experimentally, using a high-speed camera for measurement of flame propagation behaviors and a thermocouple for measurement of flame temperatures. The mechanism of flame inhibition is further investigated computationally. It is found that the concentration of NH4H2PO4 required to inhibit the aluminum dust explosion is lower in comparison with NaHCO3. The acceleration and the maximum flame speed significantly decrease and the flame morphology becomes irregular and discrete, as the inhibitor concentration increases. The NH4H2PO4 addition exerts a stronger effect on aluminum flame temperature compared to NaHCO3 addition. The chemical kinetic model indicates that the addition of the inhibitor decreases the concentrations of AlO and O in the reaction zone. This reduction becomes larger with increasing the inerting ratio. Na- and P-containing species promote highly reactive O atoms to recombine and form a stable combustion product O2, which leads to less heat release and a lower flame temperature. For Na-containing compounds, NaO ⇔ Na inhibition cycle is effective to reduce O atoms. NH3 that decomposed by NH4H2PO4 could consume O2 and reduce the flame temperature of aluminum in air. In addition, the gas‒phase chemical effect of inhibitors is significant for aluminum particles in the diffusion-controlled regime.

The Reaction between Sodium Hydroxide and Atomic Hydrogen in Atmospheric and Flame Chemistry.

We report the first direct kinetic study of the gas-phase reaction NaOH + H → Na + H2O, which is central to the chemistry of sodium in the upper atmosphere and in flames. The reaction was studied in a fast flow tube, where NaOH was observed by multiphoton ionization and time-of-flight mass spectrometry, yielding k(NaOH + H, 230-298 K) = (3.8 ± 0.8) × 10-11 cm3 molecule -1 s-1 (at 2σ confidence level), showing no significant temperature dependence over the indicated temperature range and essentially in agreement with previous estimates of the rate constant in hydrogen-rich flames. We show, using theoretical trajectory calculations, that the unexpectedly slow, yet T-independent, rate coefficient for NaOH + H is explained by severe constraints in the angle of attack that H can make on NaOH to produce H2O. This reaction is also central to explaining Na-catalyzed flame inhibition, which has been proposed to occur via the sequence Na + OH (+ M) → NaOH followed by NaOH + H → Na + H2O, thereby effectively recombinating H and OH to H2O. RRKM calculations for the recombination of Na and OH yield k(Na + OH + N2, 300-2400 K) = 2.7 × 10-29 (300/T)1.2 cm6 molecule-2 s-1, in agreement with a previous flash photolysis measurement at 653 K and Na-seeded flame studies in the 1800-2200 K range. These results therefore provide strong evidence to support the mechanism of flame inhibition by Na.

A skeletal mechanism for flame inhibition by trimethylphosphate

On the basis of a multi-step kinetic mechanism for flame inhibition by organophosphorus compounds including more than 200 reactions, a skeletal mechanism for flame inhibition by trimethylphosphate was developed. The mechanism consists of 22 irreversible elementary reactions, involving nine phosphorus-containing species. Selection of the crucial steps was performed by analysing P-element fluxes from species to species and by calculating net reaction rates of phosphorus-involving reactions versus the flames zone. The developed mechanism was validated by comparing the modelling results with the measured and simulated (using the starting initial mechanism) speed and the chemical structure of H2/O2, CH4/O2 and syngas/air flames doped with trimethylphosphate. The mechanism was shown to satisfactorily predict the speed of H2/O2/N2 flames with various dilution ratios, CH4/air and syngas/air flames doped with trimethylphosphate. The skeletal mechanism was also shown to satisfactorily predict the spatial variation of H and OH radicals and the final phosphorus-containing products of the inhibitor combustion. Further reduction of the skeletal mechanism without modification of the rate constants recommended in the starting mechanism was shown to result in noticeable disagreement of the flame speed and structure.

Hydrocarbon flame inhibition by C3H2F3Br (2-BTP)

A kinetic mechanism for hydrocarbon flame inhibition by the potential halon replacement 2-BTP (2-Bromo-3,3,3-trifluoropropene) has been assembled, and is used to study its effects on premixed methane–air flames. Simulations with varying CH4–air stoichiometry and agent loading have been used to understand its flame inhibition mechanism. In particular, the response of lean methane–air flames is examined with addition of 2-BTP, CF3Br, C2HF5, and N2 to illustrate the effect of agent heat release on these flames. The results predict that addition of 2-BTP or C2HF5 can increase the burning velocity of very lean flames, and 2-BTP is less effective for lean flames than for rich. The flame inhibition mechanism of 2-BTP involves the same bromine-species gas-phase catalytic cycle as CF3Br, which drives the flame radicals to equilibrium levels, which can be raised, however, by higher temperatures with added agent (for initially lean flames). Simulations for pure 2-BTP–O2–N2 mixtures predict burning velocities on the order of 1cm/s at 300K initial temperature.

The correlation between carbon monoxide and hydrogen cyanide in fire effluents of flame retarded polymers

This study considers the demonstrated correlation between carbon monoxide and hydrogen cyanide in the special case of fire retarded materials. It shows that the combination of aluminium phosphinate and melamine polyphosphate causes a much smaller increase in the carbon monoxide (CO) and hydrogen cyanide (HCN) yields than the combination of brominated polystyrene and antimony oxide, although both fire retardants inhibit combustion reactions in the gas phase. The formation and destruction mechanisms of CO and HCN are considered. It is shown that both toxicants form early in the flame, and that the OH radical is critical for the destruction of both CO and HCN. Crucially, in the context of the flame inhibition mechanism, this suggests that the phosphorus system reduces the H and O radical concentrations without a corresponding decrease in the OH radical concentration, thus it is an effective gas phase flame retardant which only causes a small increase in the toxic product yields. Conversely, the bromine system reduces the H, O and OH concentrations, and thus increases the fire toxicity, by inhibiting decomposition of CO and HCN. Moreover, while the phosphorus flame retardant is effective as an ignition suppressant at low temperatures, this effect ―switches off‖ at higher flame temperatures, minimising the potential increase in fire toxicity, once the fire develops.

The effect of gas phase flame retardants on fire effluent toxicity

Abstract Standard industry formulations of flame retarded aliphatic polyamides, meeting UL 94 V-0, have been burned under controlled conditions, and the yields of the major asphyxiants, carbon monoxide (CO) and hydrogen cyanide (HCN) have been quantified. Although both the combination of aluminium phosphinate and melamine polyphosphate, and the combination of brominated polystyrene and antimony oxide, inhibit combustion reactions in the gas phase, this study shows that the phosphorus causes a much smaller increase in the CO and HCN yields than antimony-bromine. The mechanisms of CO and HCN generation and destruction are related to the flame inhibition reactions. Both CO and HCN form early in the flame, and the OH radical is critical for their destruction. Crucial, in the context of the flame inhibition mechanism, is the observation that the phosphorus system reduces the H and O radical concentrations without a corresponding decrease in the OH radical concentration; conversely, the bromine system reduces all three of the key radical concentrations, H, O and OH, and thus increases the fire toxicity, by inhibiting decomposition of CO and HCN. Moreover, while the phosphorus flame retardant is effective as an ignition suppressant at lower temperatures (corresponding to early flaming), this is effect “switches off” at high temperatures, minimising the potential increase in fire toxicity, once the fire develops. Since flame retardants are most effective as ignition suppressants, and at the early stages of flaming combustion, while most fire deaths and injuries result from toxic gas inhalation from more developed fires, it is clearly advantageous to have an effective gas phase flame retardant which only causes a small increase in the toxic product yields.

The Generation of a Reduced Mechanism for Flame Inhibition by Phosphorus Containing Compounds Based on Path Flux Analysis Method

In order to analyze the complex chemical kinetic mechanism systematically and find out the redundant species and reactions, a numerical platform for mechanism analysis and simplification is established basing on Path Flux Analysis (PFA). It is used to reduce a detailed mechanism for flame inhibited by phosphorus containing compounds, a reduced mechanism with 65 species and 335 reactions is obtained. The detailed and reduced mechanism are both used to calculate the freely-propagating premix C3H8/air flame with different dimethyl methylphosphonate doped over a wide range of equivalence ratios. The concentration distributions of free radicals and major species are compared, and the results under two different mechanisms agree well. The laminar flame speed obtained by the two mechanisms also matches well, with the maximum relative error introduces as a small value of 1.7%. On the basis of the reduced mechanism validation, the correlativity analysis is conducted between flame speed and free radical concentrations, which can provide information for target species selection in the further mechanism reduction. By analyzing the species and reactions fluxes, the species and reaction paths which contribute the flame inhibition significantly are determined.

A Reduced Mechanism for Flame Inhibition by Phosphorus-containing Compounds Based on Level of Importance Analysis

A reduced mechanism for propane/air combustion and its flame inhibition by phosphorus-containing compounds (PCCs) is constructed with the level of importance (LOI) method. The analysis is performed on solutions of freely propagating premixed flames with detailed chemical kinetics involving 121 species and 682 reactions proposed by Jayaweera et al. For the non-homogeneous reaction-diffusion system, the chemical lifetime of each species is weighted by its diffusion timescale, and the characteristic flame timescale is used to normalize the chemical lifetime. The definition of sensitivity in LOI is extended so that multi-parameters can be used as sensitivity targets. Propane, oxygen, dimethyl methylphosphonate (DMMP), and flame speed are selected to be perturbed for sensitivity analysis, the species with low LOI index are removed, and reactions involving the redundant species are excluded from the mechanism. A skeletal mechanism is obtained, which consists of 57 species and 268 elementary reactions. Calculations for laminar flame speeds, key flame radicals and catalytic cycles using the skeletal mechanism are in good agreement with those by using the detailed mechanism over a wide range of equivalence ratio undoped and doped with DMMP.

Inhibition of atmospheric-pressure H 2/O 2/N 2 flames by trimethylphosphate over range of equivalence ratio

The effect of equivalence ratio (in the range 1.3–3) and dilution of unburnt gases by nitrogen on speed and the structure of premixed atmospheric-pressure H 2/O 2/N 2 flames without additive and doped with 0.04% of trimethylphosphate ((CH 3O) 3PO, TMP) was studied experimentally and by modeling. By comparing modeling and experimental results we validated and refined the mechanism for flame inhibition by organophosphorus compounds. Sensitivity analysis of flame speed to reactions of chain branching and chain termination involving phosphorus-containing species revealed some features of the mechanism for inhibition of hydrogen flames of various equivalence ratio and dilution ratio. It was shown that in hydrogen, in contrast to hydrocarbon flames, reactions of radicals with TMP and organophosphorus products of its destruction play important role. The sensitivity analysis revealed the ratio of rates of chain termination reactions involving phosphorus-containing species to chain branching reaction to increase with rising of equivalence ratio and dilution of unburnt gases. Therefore, the inhibition effectiveness of H 2/O 2/N 2 flames expressed as a relative decrease of burning velocity rises in the range of equivalence ratio from 1.1 to 3 (in contrast to hydrocarbon flames) and with dilution of unburnt gases.

The suitability of halloysite nanotubes as a fire retardant for nylon 6

This work presents the first study on the fire behaviour of halloysite nanotubes–nylon 6 composites. The nylon 6–halloysite composites were prepared at 5–30 wt% of halloysite loadings by a simple melt extrusion process. A range of standard fire tests and characterization techniques were used to assess the efficacy and mechanism by which the halloysite nanotubes inhibited the burning of nylon. We found that for such systems, relatively high concentrations of additive (≥15 wt%) were required to achieve the levels of fire retardant property normally associated with nanoclay (or layered silicate) additives. We proposed that the primary mechanism of flame inhibition for halloysite nanotubes was similar to that of conventional nanoclays; however, the ease of composite preparation is an attractive consideration for further development or study of such systems.

Inhibition of atmospheric lean and rich CH 4/O 2/Ar flames by phosphorus-containing compound

The mechanism of flame inhibition by phosphorus-containing species (PCSs) is known to involve their effect on the recombination of atoms and free radicals in flames. Chemical inhibition of laminar atmospheric methane/oxygen flames by trimethylphosphate over a range of equivalence ratio has been studied experimentally, using the heat flux method for measurement of burning velocity and molecular beam mass spectrometry for measurement of concentration profiles of both stable and labile species, and with numerical modeling using detailed chemical kinetic reaction mechanisms. Concentrations of H and OH in flames with and without the inhibitor were obtained by measurements and modeling. The addition of the inhibitor reduces the maximum concentrations of H and OH (in the reaction zone) in the lean and rich flame. This reduction is much larger in the rich flame than in the lean one. The concentration profiles of PCSs—PO, PO 2, HOPO, HOPO 2, and H 3PO 4 were measured and simulated for rich and lean flames stabilized on a flat burner. According to flame speed measurements for inhibited CH 4/air flames over a range of ϕ, the inhibition effectiveness E i increases in the range of ϕ = 0.8–1.2 and then decreases with a further increase of ϕ. The increase in E i in the range ϕ = 0.7–1.2 is attributed to a change of PCSs composition. The reduction in E i for ϕ > 1.2 can be explained by a decrease in the concentration of active PCSs due to an increase in the concentration of inactive species, such as CH 3PO 2 and other products of incomplete combustion of TMP. The inhibition effectiveness E i versus ϕ correlates with change of H and OH concentration at addition of TMP in flame. Validation of the previously developed model for inhibition by PCSs has shown that in spite of some discrepancies it adequately describes many experimental results.

Numerical study of the inhibition of premixed and diffusion flames by iron pentacarbonyl

Iron pentacarbonyl (Fe(CO) 5) is an extremely efficient flame inhibitor, yet its inhibition mechanism has not been described. The flame-inhibition mechanism of Fe(CO) 5 in premixed and counterflow diffusion flames of methane, oxygen, and nitrogen is investigated. A gas-phase inhibition mechanism involving catalytic removal of H atoms by iron-containing species is presented. For premixed flames, numerical predictions of burning velocity are compared with experimental measurements at three equivalence ratios (0.9, 1.0, and 1.1) and three oxidizer compositions (0.20, 0.21, and 0.24 oxygen mole fraction in nitrogen). For counterflow diffusion flames, numerical predictions of extinction strain rate are compared with experimental results for addition of inhibitor to the air and fuel stream. The numerical predictions agree reasonably well with experimental measurements at low inhibitor mole fraction, but at higher Fe(CO) 5 mole fractions the simulations overpredict inhibition. The overprediction is suggested to be due to condensation of iron-containing compounds since calculated supersaturation ratios for Fe and FeO are significantly higher than unity in some regions of the flames. The results lead to the conclusion that inhibition occurs primarily by homogeneous gas-phase chemistry.

Experimental and computational study of the effect of C2F6 on premixed methane flames

The objective of this work was to specify the mechanism of flame inhibition by compounds that do not contain bromine. Mole fraction profiles have been measured by molecular beam-mass spectrometer technique in a stoichiometric CH 4 /O 2 /Ar low-pressure flame seeded with and without 1% hexafluoroethane (C 2 F 6 ). For both flames, stable and reactive species concentrations have been computed and compared with experimental results in order to validate a detailed mechanism. Extension of the mechanism to atmospheric pressure was carried out by substituting the rate constant of a few reactions by high-pressure values. Mole fraction profiles computed in 1-atm-pressure stoichiometric methane-air flame were in very good agreement with Bechtel's experimental results. Thus validated, the mechanism was used to compute the reduction in flame velocity that results from the addition of increasing amounts of the inhibitor to a methane-oxygen-nitrogen flame. Calculated flame velocities are in very close agreement with experimental values obtained with a premixed flame stabilized on an open tubular burner. This shows that inhibition efficiency of fluorinated compounds can be predicted very accurately.

The mechanism of flame inhibition by sodium salts

A study has been conducted to determine whether the mode of action by the “dry chemical” flame inhibitors, sodium bicarbonate and sodium tartrate, was heterogeneous or homogeneous. The method used was the correlation of the amount of inhibitor vaporized in the flame zone with a measure of the degree of inhibition. For the first, Na atoms were determined by absorption spectroscopy at the end of the reaction zone of partially quenched premixed CH 4 /air flames burning at atmospheric pressure on a flat flame burner. The degree of inhibition was indicated by the extent of the temperature rise of the quenched flame on addition of inhibitor. Tests were conducted on six “siliconized” and size classified salt fractions three each of the two salts. Four of the six powder samples completely evaporated by the end of the reaction zone. The results for all six fractions can be represented by an approximately linear relationship between Na concentration at the end of the reaction zone and the temperature rise on inhibition. It is shown that this correlation is much better than one based on surface area presented to the flame. These results are interpreted as an essentially conclusive proof of the homogeneous mechanism. In addition, measurements of hydroxyl concentrations have shown that addition of inhibitor reduces peak OH concentrations and catalyzes radical recombination. Na atoms are unusually effective in this regard. While a complete mechanism has not been worked out, some discussion is given of the limitations on such a scheme. The existance of dipole-induced dipole stabilized complexes between alkali atoms and water molecules is suggested as a means by which recombination might very effectively be catalyzed.

Flame inhibition by hydrogen halides: Some spectroscopic measurements

The far-ultraviolet absorption spectrum of an air-propane diffusion flame inhibited with hydrogen halides has been studied. Plots of the absorption of light by hydrogen halides as a function of position in the flame and also as a function of the amount of hydrogen halide added to the flame have been obtained. The hydrogen halides are shown to be more stable on the fuel side of the reaction zone than they are on the side. Thermal diffusion is seen to be important in determining the concentration distribution of the beavier hydrogen halides in diffusion flames. The relationship between the concentration distribution of the hydrogen halides in the flame and the flame inhibition mechanism is discussed.

Reaction of H atoms with CH 2Cl 2application to the inhibition of flames

The reaction kinetics of atomic hydrogen with dichloromethane have been investigated in adischarge-flow system in the 298°–460°K temperature range, and at total pressures near 1 torr. A quadrupole mass spectrometer was adapted for the simultaneous determination of atomic hydrogen and stable species concentrations. For the H+CH 2 Cl 2 over-all reaction, the stoichiometry[H] 0 −[H]/([CH 2 Cl 2 ] 0 −[CH 2 Cl 2 ]) determined for various initial [H] 0 /[CH 2 Cl 2 ] 0 ratios always exceeds 4. The products have been identified as HCl, H 2 , CH 4 , C 2 H 2 , and C 2 H 4 . The rate constant of the initial step, H+CH 2 Cl 2 →HCl+CH 2 Cl, was found to be k 1 =1.8×10 −11 exp(−6100/ RT ) cm 3 molecule −1 s −1 from 298° to 460°K. Evidence was found for the occurrence of secondary reactions,H+HCl→Cl+H 2 , Cl+CH 2 Cl 2 →HCl+CHCl 2 . The use of D atoms instead of H atoms gives further information on the reaction mechanism, which is discussed. Reaction (1) may be the inhibiting step in the mechanism of flame inhibition by CH 2 Cl 2 .

On the mechanism of flame inhibition by alkali metal salts

Alkali metal salts, in particular sodium and potassium bicarbonate, are increasingly used as fire-extinguishing powders. Studies have been made in the past of the relative efficiency of alkali-metal salts as flame inhibitors, and the oxalates have been shown to be particularly effective. In the present work it has been found that the high efficiency of the alkali oxalates is due to their ready decomposition in a flame, with the generation of submicron particles of alkali carbonate. Other salts such as alkali ferrocyanides have been found to behave in a similar way. With such salts the surface area required for the extinction of a diffusion flame varies with particle size, there being an optimum size below which decomposition occurs before the flame front is reached and above which little decomposition occurs within the residence time of the particle in the flame. Inhibition is considered to occur via a homogeneous mechanism, involving the volatilization and/or reaction of the initially formed submicron particles to provide gaseous hydroxide as the inhibiting species. In order that the generation of alkali oxide or hydroxide may be facilitated, the anion associated with the alkali metal should be of low acid strength.