Article

High-temperature thermal decomposition of triphenyl phosphate vapor in an inert medium: Flow reactor pyrolysis, quantum chemical calculations, and kinetic modeling

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Abstract

Improving the fire resistance of polymeric materials is an important research problem, which is solved using various flame retardants. Organophosphorus compounds are among the most effective and environmentally friendly flame retardants. This paper reports an experimental, theoretical, and kinetic modeling study of the conversion of triphenyl phosphate (TPP) during thermal decomposition in an inert medium, i.e., under conditions typical of the flame zone near the polymer surface. Pyrolysis of TPP vapor was examined in a thermal reactor under argon flow at a pressure of 1 atm. The temperature dependence of the composition of TPP pyrolysis products leaving the thermal reactor was investigated by molecular beam mass spectrometry in the temperature range of 50 0-130 0 K. The geometry of all structures on the potential energy surfaces of TPP and primary and secondary decomposition products of TPP was optimized using density functional theory (DFT) (ωB97XD) with the 6-31G(d) basis set. The kinetic rate constants of the thermal decomposition reactions of TPP were calculated using the Rice-Ramsperger-Kassel-Marcus theory with the master kinetic equation (RRKM-ME) implemented in the MESS code, and thermochemical parameters were obtained for TPP and primary and secondary decomposition products of TPP in the temperature range of 20 0-60 0 0 K. A detailed chemical kinetic mechanism for TPP pyrolysis was developed by combining the primary TPP decomposition reaction pathways with the rate parameters derived from the theoretical calculations and submechanisms available in the literature for the conversion of the phenyl and phenoxy radicals and phosphorus containing products. The proposed kinetic mechanism quantitatively reproduces the measured temperature-resolved TPP mole fraction profile at the reactor outlet. The mechanism also provides a good fit to the experimentally observed trends in the conversion of major phosphorus-containing intermediates detected in this work.

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... Organophosphorus compounds (OPCs) such as trimethyl phosphate (TMP), trimethyl phosphite (TMPI), dimethyl methyl phosphonate (DMMP), triphenylphosphate (TPP) [1] and diisopropyl methyl phosphonate (DIMP) have attracted the most interest in fire [2] safety as combustion inhibitors and flame retardants [3] due to their high flame retardancy efficiency [4][5][6][7][8][9][10][11], low toxicity, excellent thermal stability [12], high environmental safety, low cost [13]. and potential as green alternatives to traditional halogenated compounds [14]. ...
... Recent studies have used quantum calculations and rate kinetics to investigate the combustion characteristics, thermodynamic properties, and reaction pathways of organophosphorus compounds such as triethyl phosphate (TEP) [15], diisopropyl methyl phosphonate (DIMP) [16] and triphenyl phosphate (TPP) [1]. Werner and Cool [17] studied the roles of H and OH radicals in the initiation reactions of varied flame conditions for DMMP to explain the decomposition of OPCs. ...
... Reaction rates are sensitive to these factors, and slight adjustments can result in differing rate constant values. 41 Rigorous approaches, such as experimental validation, benchmarking against dependable data, and employing highlevel quantum chemistry calculations, allow the creation and optimization of reaction processes, guaranteeing safety, efficiency, and meticulous predictions across diverse applications. 42 Theoretical calculations predict reaction barriers and product distributions, uniting empirical observations with comprehension. ...
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The impact of the chemical structure of four different aryl bisphosphates on the flame retardancy of bisphenol A polycarbonate/acrylonitrile–butadiene–styrene blends (PC/ABS) was investigated. The impact of the bridging unit was studied, by comparing bisphenol A bis(diphenyl phosphate) BDP with biphenyl bis(diphenyl phosphate) BBDP and hydroquinone bis(diphenyl phosphate) HDP; as well as the influence of an aromatic substitution by comparing BBDP with biphenyl bis (di-2,6-xylyl phosphate) BBXP. The blends were investigated in terms of pyrolysis (thermogravimetry TG, TG coupled with Fourier transformed infrared spectroscopy (FTIR) and mass spectrometry (MS)) and fire performance (cone calorimeter, LOI, UL 94). The decomposition temperature of the flame retardant is a main parameter enabling a condensed phase interaction with PC decomposition products. The phosphate esters reacting with phenolic groups during pyrolysis were shown to increase cross-linking and reduce the hydrolysis/alcoholysis of the carbonate group. Variation of the aromatic substitution with the use of biphenyl bis (di-2,6-xylyl phosphate) led to reduced performance, highlighting the importance of the reactivity of the flame retardant with the decomposing PC.
Article
Combustion of polypropylene (PP) in a high-temperature, low-oxygen oxidizer enriched with H2O and CO2 in stagnation-point flow was studied numerically to explore fundamental characteristics of polymer incineration under the condition of high-temperature air combustion (HiTAC). The detailed chemistry of propene as a decomposition gas was used for the calculation. Two typical gas radiation models, i.e., the optically thin model (OTM) and the statistical narrow-band (SNB) model, were employed to clarify the effect of gas radiation on PP combustion as well as the validity of the use of these models under HiTAC conditions because the H2O and CO2 included in the burnt gas recirculated in HiTAC furnaces are highly radiative species. Results showed that, under HiTAC conditions, calculations using OTM overpredicts regression rates compared with those using SNB, indicating that OTM is not suitable for use with polymer combustion under HiTAC conditions, while the differences of these gas radiation models were slight when ordinary air at high temperature was used as an oxidizer. It was also shown that when SNB was used, CO2 enrichment in the oxidizer hardly enhanced the regression because CO2 near the PP surface behaves as a barrier against the radiative heat flux. The most effective condition to increase the regression rate is to maintain a higher concentration of H2O in the high-temperature oxidizer so as to enhance the gas radiation to the PP surface.
Article
A review of the results of experimental studies of the destruction chemistry of organophosphorus compounds modeling sarin in hydrogen-oxygen rarefied flames is presented. These studies were performed at the Institute of Chemical Kinetics and Combustion of the Siberian Division of the Russian Academy of Sciences by soft ionization probe molecular beam mass spectrometry. A method is described that allows one to identify almost all starting, intermediate (including atoms and free radicals), and final organophosphorus compounds and to measure concentration profiles in flames. The destruction products of organophosphorus compounds—dimethyl methylphosphonate and trimethylphosphate—are identified in various zones of an H2/O2/Ar flame. Mass peak intensities proportional to the concentrations of the indicated products are measured. The inhibition and promotion phenomena of the flames are discovered and studied. A chemical mechanism for the destruction of organophosphorus compounds in the flames is proposed. The results obtained are important for understanding the processes involved in the incineration of chemical warfare agents and munitions and other toxic and hazardous substances, for optimization of this technology, and also for understanding the inhibition and promotion mechanisms of flames.
Article
The thermal degradation of polycarbonate/triphenylphosphate (PC/TPP) and PC/resocinolbis(diphenylphosphate) (PC/RDP) in air has been studied using TGA/FTIR and GC/MS. In PC/phosphate blends, the phosphate stabilizes the carbonate group of polycarbonate from alcoholysis between the alcohol products of polycarbonate degradation and the carbonate linkage. Thus, the evolution of bisphenol A, which is mainly produced via hydrolysis/alcoholysis of the carbonate linkage, is significantly reduced, while, the evolution of various alkylphenols and diarylcarbonates increases. The bonds that are broken first in the thermal degradation of both the carbonate and isopropylidene linkages of polycarbonate are the weakest bonds in each, when a phosphate is present. Triphenylphosphate and resocinolbis(diphenyl-phosphate), even though they exhibit a significant difference in their volatilization temperature, appear to play a similar role in the degradation pathway of polycarbonate.
Article
A method for systematic evaluation of the Lennard-Jones parameters for polycyclic aromatic hydrocarbon (PAH) compounds is presented, in which correlations for these parameters are derived using a group contribution technique for critical temperatures and pressures and the Tee-Gotoh-Stewart correlations of corresponding states. The Lennard-Jones self-collision diameters and well depths of 29 PAHs were estimated using this approach and are shown to correlate with the molecular weights of aromatics. The gaseous binary diffusion coefficients of aromatics in common gases were calculated with Chapman-Enskog equation using the estimated Lennard-Jones parameters and were found to compare well with the available experimental data and the predictions of one of the most reliable empirical approximations. The effect of ordinary diffusion of PAH species on their predicted concentration profiles in a 20-torr laminar premixed acetylene flame demonstrated computationally.
Article
An additive of triphenylphosphine oxide (Ph3PO), which is a widely used flame retardant, was shown to inhibit a CH4/O-2/N-2 flame by decreasing concentration of active flame species H and OH.
Article
The effects of three nitrogen additives (urea, guanidine carbonate, and melamine formaldehyde) on the flame retardant action of cotton cellulose treated with tributyl phosphate (TBP) were investigated in this research. The limiting oxygen index (LOI) of treated cotton cellulose clearly revealed the synergistic interactions of TBP and nitrogen compounds. The Kissinger method was used to evaluate the kinetics of thermal decomposition on treated cellulose. The results show that adding nitrogen additives increases the activation energy at a higher degree of degradation, thus indicating better thermal stability at higher temperatures. Scanning electron microscope pictures of chars formed after a LOI test show the formation of protective polymeric coatings on char surfaces. Evaluating char surfaces using attenuated total reflectance-Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy suggests that these coatings are composed of species containing phosphorus–nitrogen–oxygen. Possible chemical interactions of phosphorus and nitrogen compounds during the burning process and the formation of a protective coating could be the reason for the observed synergism. Potential reaction pathways contributing to the formation of this protective polymeric coating have also been proposed.
Article
Variations of Gaussian-3 (G3) theory are presented in which the quadratic configuration interaction (QCISD(T)) energy calculation is replaced by a coupled cluster (CCSD(T)) energy calculation. This modification is made to four G3 methods that have been previously introduced. The replacement of the QCISD(T) energy by the CCSD(T) energy results in little change in the accuracy of the methods as assessed on the G2/97 test set, although the maximum deviations decrease slightly. These new G3 methods based on the coupled cluster technique are a useful alternative to the G3 methods based on quadratic configuration interaction.
Article
The chemical and thermal structure of a Mache-Hebra burner stabilized premixed rich CH4/O2/N2 flame with additives of vapors of triphenylphosphine oxide [(C6H5)3PO], hexabromocyclododecane (C12H18Br6), and ethyl bromide (C2H5Br) was studied experimentally using molecular beam mass spectrometry (MBMS) and a microthermocouple method. The concentration profiles of stable and active species, including atoms and free radicals, and flame temperature pro.les were determined at a pressure of 1 atm. A comparison of the experimental and modeling results on the flame structure shows that MBMS is a suitable method for studying the structure of flames stabilized on a Mache-Hebra burner under near-adiabatic conditions. The relative flame inhibition effectiveness of the added compounds is estimated from changes in the peak concentrations of H and OH radicals in the flame and from changes in the flame propagation velocity. The results of the investigation suggest that place of action of the examined flame retardants is the gas phase.
Article
There is much interest in the combustion mechanism of organophosphorus compounds (OPCs) due to their role as potential halon replacements in fire suppression. A continuing investigation of the inhibition activity of organophosphorus compounds under a range of equivalence ratios was performed experimentally and computationally, as measured by the burning velocity. Updates to a previous mechanism were made by the addition and modification of reactions in the mechanism for a more complete description of the inhibition reactions. Reaction pathways for HOPO2 + H and HOPO + H are analyzed using the BAC-G2 approach. A new reaction pathway for HOPO2 + H = PO2 + H2O has been identified which results in a higher rate constant than that reported in the literature. In this work, the laminar flame speed is measured experimentally and calculated numerically for a premixed propane/air flame at 1 atm, under a range of equivalence ratios, undoped and doped with dimethyl methylphosphonate (DMMP). A detailed investigation of the catalytic cycles involved in the recombination of key flame radicals is made for two equivalence ratios, fuel lean and fuel rich. From this, the importance of different catalytic cycles involved in the lean versus rich case is discussed. The chemical kinetic model indicates that the HOPO2 ⇔ PO2 inhibition cycle is more important in the lean flame than the rich. The OPCs are similarly effective across the range, demonstrating the robustness of OPCs as flame suppressants. In addition, it is shown that the phosphorus compounds are most active in the high-temperature region of the flame. This may, in part, explain their high level of inhibition effectiveness.
Article
We report re-optimization of a recently proposed long-range corrected (LC) hybrid density functional [J.-D. Chai and M. Head-Gordon, J. Chem. Phys., 2008, 128, 084106] to include empirical atom–atom dispersion corrections. The resulting functional, oB97X-D yields satisfactory accuracy for thermochemistry, kinetics, and non-covalent interactions. Tests show that for non-covalent systems, oB97X-D shows slight improvement over other empirical dispersion-corrected density functionals, while for covalent systems and kinetics it performs noticeably better. Relative to our previous functionals, such as oB97X, the new functional is significantly superior for non-bonded interactions, and very similar in performance for bonded interactions.
Article
A variation of Gaussian-3 (G3) theory is presented in which the geometries and zero-point energies are obtained from B3LYP density functional theory [B3LYP/6-31G(d)] instead of geometries from second-order perturbation theory [MP2(FU)/6-31G(d)] and zero-point energies from Hartree–Fock theory [HF/6-31G(d)]. This variation, referred to as G3//B3LYP, is assessed on 299 energies (enthalpies of formation, ionization potentials, electron affinities, proton affinities) from the G2/97 test set [J. Chem. Phys. 109, 42 (1998)]. The G3//B3LYP average absolute deviation from experiment for the 299 energies is 0.99 kcal/mol compared to 1.01 kcal/mol for G3 theory. Generally, the results from the two methods are similar, with some exceptions. G3//B3LYP theory gives significantly improved results for several cases for which MP2 theory is deficient for optimized geometries, such as CN and O2+. However, G3//B3LYP does poorly for ionization potentials that involve a Jahn–Teller distortion in the cation (CH4+, BF3+, BCl3+) because of the B3LYP/6-31G(d) geometries. The G3(MP2) method is also modified to use B3LYP/6-31G(d) geometries and zero-point energies. This variation, referred to as G3(MP2)//B3LYP, has an average absolute deviation of 1.25 kcal/mol compared to 1.30 kcal/mol for G3(MP2) theory. Thus, use of density functional geometries and zero-point energies in G3 and G3(MP2) theories is a useful alternative to MP2 geometries and HF zero-point energies. © 1999 American Institute of Physics.
Article
The master equation for a thermal unimolecular reaction in gases at low pressures is formulated. Steady‐state solutions are derived in analytical form with an exponential model of collisional transition probabilities, (i) for vibrational energy transfer in molecules with variable densities of states, and (ii) for combined rotational and vibrational energy transfer in molecules with variable heights of the centrifugal barriers. Other models of transition probabilities are treated numerically. The diffusion limit of energy transfer is discussed. In all cases, the nonequilibrium populations of excited states and the weak collision efficiency factors βc are calculated.
Article
The flame retardancy mechanisms of three aryl phosphates, triphenyl phosphate (TPP), resorcinol bis(diphenyl phosphate) (RDP) and bisphenol A bis(diphenyl phosphate) (BDP), in a polycarbonate/acrylonitrile–butadiene–styrene (PC/ABS) blend are investigated and compared. Further, the influence of polytetrafluorethylene (PTFE) on viscosity and thermal decomposition is discussed in the systems PC/ABS and PC/ABS + BDP. Mechanisms are proposed based on the results of various methods. Thermogravimetric analysis, Fourier transform infrared spectroscopy and kinetics are used to study the pyrolysis. The fire behaviour is studied by means of cone calorimeter measurements at different heat fluxes and the flammability is specified by limiting oxygen index (LOI) and UL 94. Rheology measurements are used to illuminate the changed dripping behaviour due to PTFE. TPP shows only a gas phase action. RDP shows mainly a gas phase action and some condensed phase action. BDP shows a crucial condensed phase action in addition to a gas phase action. TPP and RDP are somewhat superior in terms of flammability (LOI), whereas BDP shows superior performance in forced flaming combustion (cone calorimeter). Synergistic effects between PTFE and BDP are found. Copyright © 2007 Society of Chemical Industry
Article
The chemical and thermal structure of a premixed rich CH4/air/N2 flame (ϕ=1.18±0.02) that contains either triphenylphosphine oxide [(C6H5)3PO] or hexabromocyclododecane [C12H18Br6] and that is stabilized on a Mache–Hebra burner was studied experimentally using molecular beam mass spectrometry (MBMS) and the microthermocouple technique. Compounds such as hexabromocyclododecane (HBCD) and triphenylphosphine oxide (TPPO) are representative flame-retardant additives that are added to polymers to reduce the flammability of the base polymer. Both compounds provide flame retardation in the gas phase by the production of active species that effectively scavenge key combustion radicals to shut down the combustion process. The MBMS method was used to determine the concentration profiles of stable and active species directly in the flame, which includes atoms as well as free radicals. Thin thermocouples were employed to determine temperature profiles in a flame stabilized on a Mache–Hebra burner at a pressure of 1 atm. A comparison of the experimental data and simulation results for the flame structure shows that MBMS is suitable for studying the structure of flames that are close to freely propagating conditions. The relative effectiveness of flame inhibition by the compounds tested was estimated from changes in the peak concentrations of H and OH radicals in the flame and from changes in the estimated flame velocity.
Article
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, PO2, HOPO, HOPO2, and H3PO4 were measured and simulated for rich and lean flames stabilized on a flat burner. According to flame speed measurements for inhibited CH4/air flames over a range of ϕ, the inhibition effectiveness Ei increases in the range of ϕ = 0.8–1.2 and then decreases with a further increase of ϕ. The increase in Ei in the range ϕ = 0.7–1.2 is attributed to a change of PCSs composition. The reduction in Ei 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 CH3PO2 and other products of incomplete combustion of TMP. The inhibition effectiveness Ei 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.
Article
The fire retardant efficiency of melamine (MA) and triphenyl phosphate (TPP) in poly(butylene terephthalate) (PBT) was studied by the limiting oxygen index (LOI) and the UL94 test. On adding 10 wt. % MA and 20 wt. % TPP, LOI increased from 20.9 to 26.6 and the UL94 V-0 rating was achieved. SEM and DSC analyses show that the fire retardants are compatible with PBT and facilitate crystallization of PBT. The occurrence of an interaction between MA + TPP and PBT was elucidated by TGA, dynamic FTIR, and pyrolysis/GC/MS. MA + TPP changes the degradation path of PBT and modifies the compositions of the gas and condensed-phase products.
Article
This paper presents a generalized pyrolysis model that can be used to simulate the gasification of a variety of combustible solids encountered in fires. The model, Gpyro, can be applied to noncharring polymers, charring solids, intumescent coatings, and smolder in porous media. Temperature, species, and pressure distributions inside a thermally stimulated solid are determined by solving conservation equations for the gaseous and condensed phases. Diffusion of species from the ambient into the solid is calculated with a convective–diffusive solver, providing the capability to calculate the flux and composition of volatiles escaping from the solid. To aid in determining the required material properties, Gpyro is coupled to a genetic algorithm that can be used to estimate the model input parameters from bench-scale fire tests or thermogravimetric (TG) analysis. Model calculations are compared to experimental data for the thermo-oxidative decomposition of a noncharring solid (PMMA), thermal pyrolysis of a charring solid (white pine), gasification and swelling of an intumescent coating, and smolder in polyurethane foam. Agreement between model calculations and experimental data is favorable, especially when one considers the complexity of the problems simulated.
Book
Until recent years knowledge of chemical processing was descriptive and qualitative. In 1810 modern chemical theory was born and process description became quantitative. Then about 1900 the quantitative engineering approach was developed, first for physical changes, called the Unit Operations, and somewhat later for chemical operations. This we call the American approach. In 1957 European chemical engineers brought together the design of chemical and their related physical operations under the name of Chemical Reaction Engineering, or CRE. This approach and name received practically universal acceptance. Today the methods of CRE are widely used in the processing of biochemical and all sorts of other systems, This talk wanders through this development.