Flame retardation and char formation mechanism of intumescent flame retarded polypropylene composites containing melamine phosphate and pentaerythritol phosphate

The flame retardation of polypropylene (PP) composites containing melamine phosphate (MP) and pentaerythritol phosphate (PEPA) was characterized by cone calorimeter. The formation mechanism of the char obtained from the combustion of the composites after cone calorimeter testing was studied by scanning electron microscopy (SEM), Fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Raman diffusion. Results demonstrated that the PP composite containing MP and PEPA showed good flame retardancy. It had been found that the intumescent char could be principally divided into three parts, i.e. outer, middle and inner char layer, according to their different structures and components.     

Influence of caged bicyclic phosphate and CaCO3 nanoparticles on char-forming property of PU rigid foams

Char-forming property of PU rigid foams, which can be assessed by char residue (%) when PU is burned at certain temperature, was studied by TG and DTG. The results showed that pure PU rigid foam had low char residue of only 17%, while 33% of char residue was achieved when PU rigid foam was modified by adding 8 wt% of 1-oxo-2,6,7-trioxa-1-phosphabicyclo[2,2,2] octane (PEPA), which is a caged bicyclic phosphate. The experiment results of FTIR and XPS showed that the PEPA modified PU rigid foam could be dehydrogenated and dehydrated at temperature between 380 and 450 °C, resulting in the increase of char residue of PU rigid foam. Further study also revealed that the addition of CaCO₃ nanoparticles could enhance the char stability when the PEPA modified PU rigid foam was being burned. The mechanism was investigated and it was found that the enhanced char stability could be attributed to the limited permeation of oxygen caused by the formation of calcium phosphate and calcium pyrophosphate by the reaction of PEPA and CaCO₃ at high temperature, which were covered on or buried in the char layer.     

Effects of melamine phosphate on the thermal decomposition and combustion behavior of reconstituted tobacco sheet

In this paper, first the MP-modified reconstituted tobacco sheet (RTS) was prepared by a paper-making process. Thermogravimetric analysis coupled to Fourier transform infrared spectrometer (TG-FTIR) had been used to investigate the influences of melamine phosphate (MP) on the thermal decomposition and the formation of evolved volatile products of RTS. TG-FTIR results illustrated that the incorporation of MP into RTS could retard the thermal decomposition of the major components of RTS and meanwhile lead to the formation of more thermally stable char. Moreover, the main gases released during the pyrolysis of RTS and MP-modified RTS were H₂O, CO₂, CO, NH₃, carbonyl compounds, alcohols, phenols, alkanes, and alkenes. The presence of MP changed the formation of evolved volatile products of RTS obviously. The effects of MP on the combustion behavior of RTS were studied by micro-scale combustion calorimetry and cone calorimetry. Results demonstrated that the formation of combustible gases was mainly determined by the thermal decomposition stage occurred in the temperature range of 150–600 °C. The incorporation of MP into RTS influenced the release of fuel gases and the char formation in the process of the thermal decomposition of RTS, and eventually retarded the flammability and combustibility of RTS.     

Reaction of melamine phosphate with pentaerythritol and its products for flame retardation of polypropylene

Due to being halogen‐free, non‐toxic, non‐erosive and environmentally friendly, melamine‐based flame retardants are attracting more and more attention. As a melamine‐based intumescent flame retardant, in this study the melamine salt of pentaerythritol phosphate (MPP) was prepared from melamine phosphate (MP) and pentaerythritol (PER). The reaction of MP with PER was then systematically investigated. The reaction product MPP was utilized to flame‐retard polypropylene (PP). FT‐IR, TGA and DSC were used to characterize MPP and also to investigate the reaction of MP and PER in depth. The experimental results show that MPP has good thermal stability and matched decomposition temperature with that of PP, making it suitable for flame retarding of PP. Also, MPP is melting‐blendable due to its softening during the heating process. The structure of MPP at a MP:PER molar ratio of 2.0 was confirmed as the same to that of the product synthesized from phosphorus oxychloride, pentaerythritol and melamine. The reaction of MP with PER was greatly influenced by the MP:PER proportion, reaction temperature and reaction time, rather than the physical state of PER, and the reaction mechanism of MP with PER was proposed. The prepared flame‐retarded polypropylene composite with 35 wt% intumescent flame‐retardant MPP has a flame retarding level of 3.2 mm UL 94 V‐0, tensile strength 27.0 MPa, Young's modulus 2442 MPa and Izod notched impact strength 3.8 kJ/m². Copyright © 2007 John Wiley & Sons, Ltd.     

Activation of acyl phosphate monoesters by lanthanide ions: enhanced reactivity of benzoyl methyl phosphate

Acyl phosphate monoesters are intermediates in many biochemical acylation reactions, such as those involving aminoacyl adenylates. Benzoyl methyl phosphate, a typical acyl phosphate monoester, is slowly hydrolyzed in neutral solutions but reacts rapidly with amines. Since biochemical processes of acyl phosphate monoesters involve accelerated reactions with oxygen-centered nucleophiles, we sought catalysts for hydrolysis and methanolysis of benzoyl methyl phosphate to mimic the biochemical outcome. Lanthanide ions are particularly effective catalysts, accelerating reactions much more than comparable levels of magnesium ion. Detailed kinetic analysis of the hydrolysis reactions reveals formation of a 1:1 complex, followed by rapid reaction with a nucleophile. The hydroxide-dependent hydrolysis rate in the europium complex is about 10(5) times that of free substrate with hydroxide. A mechanism that accounts for the data and observed behavior involves bidentate coordination of the metal ion by the acyl phosphate through phosphate and carbonyl oxygens, lowering the energy of the tetrahedral addition intermediate and the associated transition states. The dependence of the metal ion catalyzed process on the concentration of hydroxide ion is consistent with coordinated hydroxide acting as a nucleophile. The reaction of benzoyl methyl phosphate with methanol to form methyl benzoate and methyl phosphate is 30 000 times more rapid in the presence of 0.0001 M lanthanum triflate (in the absence of the metal ion k(obs) = 2.1 x 10(-7) s(-1), at 25 degrees C). Thus, the combination of acyl phosphate esters and lanthanide salts appears to be a promising method for biomimetic acylation of hydroxyl groups.     

Effect of nitrogen additives on flame retardant action of tributyl phosphate: Phosphorus-nitrogen synergism

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.     

Investigation of thermal degradation characteristics of polyamide-6 containing melamine or melamine cyanurate via direct pyrolysis mass spectrometry

In this work the changes in thermal degradation characteristics of polyamide 6 (PA6) in the presence of melamine (Me) or melamine cyanurate (MC) were investigated systematically via direct pyrolysis mass spectrometry. Though thermal stability of PA6 was not affected by the presence of flame retardants, the changes in the products and in their distributions were detected. The reaction of carbonyl groups of PA6 with amine groups of melamine was the main cause for the changes in the product distribution. In the presence of melamine cyanurate, new products due to the reaction of cyanic acid generated by the decomposition of cyanurate, with the amine groups of PA6 were detected. Significant increases in the evolution temperatures of melamine and melamine cyanurate in the presence of PA6 were attributed to intermolecular interactions, most probably by H-bonding, with PA6.     

Reactions of bis(2,4-dinitrophenyl) phosphate with hydroxylamine

For dephosphorylation of bis(2,4-dinitrophenyl) phosphate (BDNPP) by hydroxylamine in water, pH region 4-12, the observed first-order rate constant, k(obs), initially increases as a function of pH, but is pH-independent between pH 7.2 and pH 10. The initial BDNPP cleavage by nonionic NH(2)OH (<0.2 M) involves attack by the OH group and follows first-order kinetics, but the overall initial reaction of BDNPP liberates ca. 1.7 mol of 2,4-dinitrophenoxide ion (DNP). This initial reaction generates a short-lived O-phosphorylated hydroxylamine, 2, followed by three possible reactions: (1) reaction of 2 with hydroxylamine, generating 2,4-dinitrophenyl phosphate (DNPP, 3), which subsequently forms DNP; (2) intramolecular displacement of the second DNP group and rapid decomposition of the cyclic intermediate to form phosphonohydroxylamine and eventually inorganic phosphate; (3) a novel rearrangement with intramolecular aromatic nucleophilic substitution involving a cyclic intermediate and migration of the 2,4-dinitrophenyl group from O to N. Values of k(obs) increase modestly with pH > 10, the reaction is biphasic, and the yield of DNP increases. An increase in [NH(2)OH] also increases the yield of DNP, due largely to accelerated hydrolysis of DNPP.     

Catalytic action of phospho-tungstic acid in the synthesis of melamine salts of pentaerythritol phosphate and their synergistic effects in flame retarded polypropylene

A solid acid, phospho-tungstic acid (PTA), has been used to catalyze the pentaerythritol-melamine phosphate (PER-MP) reaction to synthesize intumescent flame retardant, melamine salt of pentaerythritol phosphate (MPP) used in flame retardant polypropylene (PP). This novel and environmentally friendly synthesis technology well solves the problems of conventional preparation methods. PTA plays a double-role: on one hand, it remarkably enhances the conversion of the above reaction and decreases the reaction temperature; on the other hand, it acts as an effective synergist with MPP and greatly improves the flame retardancy; accordingly, no additional process is needed to remove PTA after the reaction, and the products of the catalyzed reaction were directly incorporated with PP to prepare high-performance flame retardant materials. The catalytic and synergistic effects of PTA, as well as the flame retardancy and mechanical properties of the corresponding flame retardant PP were investigated.     

Altered mechanisms of reactions of phosphate esters bridging a dinuclear metal center

Kinetic isotope effects in the nucleophile and leaving group were obtained for the reaction of p-nitrophenyl phosphate monoester coordinated to a dinuclear Co(III) complex. The metal complex of the p-nitrophenyl phosphate monoester was found to hydrolyze by a single-step concerted mechanism, with significant nucleophilic participation in the transition state. By contrast, the hydrolysis of uncomplexed p-nitrophenyl phosphate occurs by a very loose transition state with little bond formation to the nucleophile. Previously, the metal complex of the diester methyl-p-nitrophenyl phosphate was found to hydrolyze via a two-step addition-elimination mechanism, in contrast to the concerted hydrolysis mechanism followed by uncomplexed diesters with the p-nitrophenolate leaving group. These results show that coordination to a metal complex can significantly alter the mechanism of phosphoryl transfer.     

Study of thermal properties of intumescent additive

Bicyclic compounds containing phosphorus on their skeleton such as 2,4,6-trioxa-1-phosphabicyclo[2,2,2]octane-4-methanol phosphate (PEPA) having three active ingredients required for intumescence have been synthesized. The structural characterization of PEPA was carried out by FT-IR, ¹H and ¹³C NMR. The thermal behaviour of the material was studied using TGA, TGA–MS and pyrolysis GC–MS. Thermogravimetric analysis reveals that PEPA undergoes several stages of degradation with a char of about 12% at 800 °C. The TGA–MS studies indicate that the material degrades with the liberation of water, formaldehyde, alkene and alcohols as the major degradation products. Pyrolysis GC–MS results reveal that PEPA isomerizes in the acidic medium. PEPA and/or isomers of PEPA react with formaldehyde, one of the degradation products, to form cross-linked structure and cyclic products with the elimination of water molecule. The thermal degradation mechanisms for PEPA are presented and discussed.     

Synergistic effects between iron–graphene and melamine salt of pentaerythritol phosphate on flame retardant thermoplastic polyurethane

A comparison of melamine salt of pentaerythritol phosphate (MPP), and a synergistic agents, iron–graphene (IG) was performed in thermoplastic polyurethane (TPU) by masterbatch‐melt blending on thermal and flame retardant properties. The flame retardant properties of TPU composites were characterized by limiting oxygen index (LOI), UL 94 and cone calorimeter test (CCT). The CCT results revealed that IG can significantly enhance flame retardant properties of MPP in TPU. The peak heat release rate of neat TPU and flame retardant TPU/MPP composites decreased from 2192.6 and 226.7 to 187.2 kW/m² compared with that of TPU containing 0.25 wt% IG. The thermal stability and thermal decomposition of TPU composites were characterized by thermogravimetric analysis (TGA) and thermogravimetric/Fourier infrared spectrum analysis (TG‐IR). The results indicated IG and MPP can improve the thermal stability of TPU. The formation of thermal conductive network by IG can promote the decomposition of MPP into nonflammable melt, which can play the role of heat barrier and restrict the diffusion of fuels into combustion zone and access of oxygen to the unburned fuels. Copyright © 2016 John Wiley & Sons, Ltd.     

Intramolecular general acid catalysis of the hydrolysis of 2-(2'-imidazolium)phenyl phosphate, and bond length-reactivity correlations for reactions of phosphate monoester monoanions

Rate constants for the hydrolysis of 2-(2'-imidazolium)phenyl hydrogen phosphate (IMPP) in water at pH<6 indicate that activation by the imidazolium moiety disappears with the deprotonation of the phosphate group, and the reaction involves the hydrogen-bonding of the imidazolium NH with the aryl oxygen leaving group. The reaction should involve a near-planar conformation of the imidazolium and the phenyl groups in the activated complex, which favors proton-transfer. The crystal structure of IMPP was solved, and a bond length-reactivity correlation for reactions of phosphate monoester monoanions is described.     

Copper(II) complexes of novel N-alkylated derivatives of cis,cis-1,3,5-triaminocyclohexane. 2. Metal-promoted phosphate diester hydrolysis

Aqueous copper(II) N,N',N' '-trimethyl-cis,cis-1,3,5-triaminocyclohexane (Cu(tach-Me(3))(2+)(aq)) promotes the hydrolysis of activated phosphate diesters in aqueous medium at pH 7.2. This complex is selective for cleavage of the phosphate diester sodium bis(p-nitrophenyl) phosphate (BNPP), the rate of hydrolysis of the monoester disodium p-nitrophenyl phosphate being 1000 times slower. The observed rate acceleration of BNPP hydrolysis is slightly greater than that observed for other Cu(II) complexes, such as [Cu([9]aneN(3))Cl(2)] ([9]aneN(3) identical with 1,4,7-triazacyclononane). The rate of hydrolysis is first-order in phosphate ester at low ester concentration and second-order in [Cu(tach-Me(3))](2+)(aq), suggesting the involvement of two metal complexes in the mechanism of substrate hydrolysis. The reaction exhibits saturation kinetics with respect to BNPP concentration according to a modified Michaelis-Menten mechanism: 2CuL + S <==> LCu-S-CuL --> 2CuL + products (K(M) = 12.3 +/- 1.8 mM(2), k(cat) = (4.0 +/- 0.4) x 10(-)(4) s(-1), 50 degrees C) where CuL (triple bond) [Cu(tach-Me(3))](2+), S (triple bond) BNPP, and LCu-S-CuL is a substrate-bridged dinuclear complex. EPR data indicate that the dicopper complex is formed only in the presence of BNPP; the active LCu-S-CuL intermediate species then slowly decays to products, regenerating monomeric CuL.     

Thermal degradation of pentaerythritol phosphate alcohol

Intumescent material, 2,6,7-trioxa-1-phosphabicyclo-[2,2,2]-octane-4-methanol phosphate (PEPA), is synthesized and characterized using FTIR, ¹HNMR and ¹³CNMR. The degradation properties of PEPA are studied by employing TG and TG?CMS technique. The activation energies for the degradation process of PEPA are calculated by using TG curves obtained from multiple heating rates (Friedman, Kissinger?CAkahira?CSunose and Flynn?CWall?COzawa methods). The degradation that is occurring in the temperature region 307?C366?°C has the highest activation energy. Eventhough the calculated activation energies for the degradation differ depending on the approximation method employed, the trend in variation of activation energy for degradation is similar. Single ion monitoring technique proved the evolution of H₂O, CO/C₂H₄, HCHO, C₂H₅OH/HCOOH and trace amounts of C₂H₇O₃P and C₄H₉O₄P from the degrading PEPA. The thermal conductivity and stability of the char formed during the TG analysis are also discussed.     

Theoretical studies on the hydrolysis of phosphate diesters in the gas phase,solution, and RNase A

Density functional theory, polarizable continuum models and semiempirical hybrid quantum mechanical/molecular mechanical (QM/MM) calculations were applied to the hydrolysis of phosphate diesters in the gas phase, in solution, and in the enzyme RNase A. Neutralization of the negative charge of the pentacovalent phosphorane intermediates provides a substantial stabilization of the transition‐state structures in the gas phase. Inclusion of solvent effects on the phosphate/phosphorane species was critical to reproducing the trends in reactivity observed experimentally. Finally, the catalytic mechanism for the hydrolysis of uridine 2′,3′‐cyclic phosphate by RNase A was studied by QM/MM calculations. Our results suggest that the rate‐limiting transition state of the reaction corresponds to the approach of a water molecule to the phosphate and its activation by His119. Thus, His119 acts as a generalized base for the reaction. The water attack leads to a pentacovalent phosphorane transition state of formal charge ?2; this excess of negative charge in the transition state is stabilized by a number of positively charged residues including His12 and Lys41. In the second stage of the reaction, the phosphorane is converted into products. This part of the reaction proceeds without a detectable barrier, and it is facilitated by a proton transfer from Lys41 to the departing O_2′. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Infrared study of UV-irradiated tungsten trioxide powders containing adsorbed dimethyl methyl phosphonate and trimethyl phosphate

The photodecomposition of dimethyl methylphosphonate (DMMP) and trimethyl phosphate (TMP) adsorbed on monoclinic WO₃ powders when irradiated by ultraviolet light (UV) in air, oxygen, and under evacuation was investigated using infrared spectroscopy (IR). The IR spectra show that DMMP decomposes into methyl phosphonate upon exposure to 254 nm UV for 2 h at room temperature in air. The same decomposition of DMMP occurs only at temperatures above 300°C without UV illumination. TMP differs from DMMP in that the photodecomposition product is not the same as the decomposition product obtained by heating above 300°C. Thermal decomposition leads to formation of a phosphate on the surface, whereas photodecomposition leads to the same adsorbed methyl phosphonate as found for the thermal or photodecomposition of DMMP. Since TMP does not contain a P-CH₃ bond, the formation of a methyl phosphonate on the surface after UV illumination involves a mechanism where CH₃ groups migrate from the methoxy group to the phosphorous central atom. No decomposition is observed at room temperature when DMMP or TMP adsorbed on WO₃ is irradiated under vacuum or in nitrogen atmosphere. Therefore, the photodecomposition of either DMMP or TMP adsorbed on WO₃ at room temperature does not involve a reaction with the lattice oxygen but rather a reaction with the oxygen radicals produced by the decomposition of ozone.     

An altered mechanism of hydrolysis for a metal-complexed phosphate diester

Isotope effects in the nucleophile and in the leaving group were measured to gain information about the mechanism and transition state of the hydrolysis of methyl p-nitrophenyl phosphate complexed to a dinuclear cobalt complex. The complexed diester undergoes hydrolysis about 1011 times faster than the corresponding uncomplexed diester. The kinetic isotope effects indicate that this rate acceleration is accompanied by a change in mechanism. A large inverse 18O isotope effect in the bridging hydroxide nucleophile (0.937 +/- 0.002) suggests that nucleophilic attack occurs before the rate-determining step. Large isotope effects in the nitrophenyl leaving group (18Olg = 1.029 +/- 0.002, 15N = 1.0026 +/- 0.0002) indicate significant fission of the P-O ester bond in the transition state of the rate-determining step. The data indicate that in contrast to uncomplexed diesters, which undergo hydrolysis by a concerted mechanism, the reaction of the complexed diester likely proceeds via an addition-elimination mechanism. The rate-limiting step is expulsion of the p-nitrophenyl leaving group from the intermediate, which proceeds by a late transition state with extensive bond fission to the leaving group. This represents a substantial change in mechanism from the hydrolysis of uncomplexed aryl phosphate diesters.     

A binary catalytic system based on mixed monolayers of a phospholipid and amphiphilic bis(Zn2+-cyclen)

Herein, we report on the study of the properties of mixed Langmuir monolayers composed of a synthetic amphiphilic receptor, alkylated bis-cyclic zinc complex of 1,4,7,10-tetraazacyclododecane, and a lipid, distearoyl phosphatidylcholine. The kinetics of a hydrolysis of a model substrate, bis(p-nitrophenyl) phosphate in individual and mixed monolayers of the amphiphilic receptor was studied by using fiber-optical absorption/reflection spectroscopy. The hydrolysis of the organic phosphate in these planar systems proceeded by a two-stage mechanism. This mechanism comprises substrate adsorption on the monolayer via a reaction of the zero order with respect to the adsorbate followed by the pseudo-second-order reaction of the hydrolytic decomposition of the substrate. Unlike the reaction in molecular and colloidal solutions, the process in the monolayer results in the complete decomposition of the model substrate into nitrophenol and phosphate anion. The amphiphilic receptor is directly involved in this reaction to yield a stable complex with the phosphate anion as a resulting product of hydrolysis. An increase in the receptor affinity for the phosphate anion is, most likely, due to the effect of the interface on the strength of the coordination bonds in an intermediate product and the receptor-phosphate complex. Immobilization of the receptor within the lipid matrix increases the rate of substrate decomposition in the monolayer by almost an order of magnitude. We suggested an explanation for the observed effect of the lipid matrix on the catalytic properties of the amphiphilic metallocomplex.