Flame Inhibition Research Articles

Study on the inhibition of explosion and combustion of coal dust based on the structure of core-shell microencapsulated polyurethane

In order to study the effect of core-shell microencapsulated ammonium polyphosphate modified materials on the suppression of explosion and combustion of different metamorphic coal, a core-shell microencapsulated ammonium polyphosphate explosion inhibitor was prepared using in-situ polymerization method. Through experiments on flame suppression and overpressure suppression during explosive combustion, the inhibitory effect of core-shell microencapsulated ammonium polyphosphate explosion suppressant on coal dust explosion was studied, revealing the coupling inhibition mechanism of thermal propagation isolation obstruction effect and group reaction reduction of key atoms. The experimental results show that when 60 wt% core-shell microencapsulated ammonium polyphosphate explosion suppressant is added, the suppression effect on the explosion and combustion of different metamorphic coal is significant, and the flame propagation and pressure rise of coal dust explosion can be basically achieved. By analyzing the substances before and after the explosion through characterization experiments, the inhibitory mechanism of core-shell microencapsulated ammonium polyphosphate explosion suppressant on different metamorphic coal was obtained. Expanding the application field of microcapsule materials provides important methods and theoretical basis for developing core shell microcapsule flame retardant and explosion suppression materials to prevent coal dust explosion disasters.

Effect of silver inhibition on the ceramic foam as flame suppression

Aluminium dust explosions pose significant safety and economic challenges in various industrial processes. Due to this, the current research explores an innovative approach by inhibiting the silver nanoparticles (Ag NPs) to ceramic porous form substrate as a flame suppressant in order to mitigate the risks associated with these explosions. The antimicrobial and non-toxic qualities of silver are also attractive to be applied in medical and food technology. However, the interfacial adhesion between the metallic (nanosilver) and non-metallic (silica-based-ceramic) is still vaguely studied due to the mechanical and surface energy mismatch between the organic surface and the inorganic layers. From this study, the physicochemical and mechanical properties of the silver-coated ceramic foam were analyzed using X-ray diffraction, field emission scanning electron microscopy with energy dispersive X-ray, thermogravimetric analysis, and compression test. From the mechanical testing, it was found that the percentage increase of maximum load for silver-ceramic foam from the original ceramic foam was about 60%. The results indicate that silver-coated foam has a better compressive strength of 0.93 MPa as compared to 0.58 MPa by the original ceramic. The inhibition effect of Ag NPs powder on the explosion pressure evolution and flame spread mechanism of aluminium powder at different concentrations and particle sizes was tested using the Hartmann experimental system.

Suppression effect of nanocomposite inhibitor on Methane-Coal dust explosion flame propagation

To enrich the methane–coal dust explosion inhibition theory at different methane concentrations, the influence of the potassium oxalate-montmorillonite (OA-Mt) nanocomposite inhibitor on the flame propagation characteristics of 5%, 7% and 9% methane–coal dust explosion was studied experimentally and its explosion inhibition mechanism was revealed. Results showed that the optimum K2C2O4 loading concentration for explosion flame inhibition was 80%. The flame brightness was weakened, the flame instantaneous peak velocity vm was decreased, and the flame average velocity inhibition ratio φ was enhanced, proving that OA-Mt could be adopted as an excellent inhibitor for the methane–coal dust explosion. Additionally, OA-Mt had the most significant inhibition on 7% methane–coal explosion. Finally, an inhibition physical model of OA-Mt on methane–coal dust flame zones was established and the suppression mechanism was discussed. The physical-dominated inhibition effect in the flame preheating zone and the chemical-dominated inhibition effect in the combustion reaction zone co-mitigated the methane–coal flame propagation.

Enhancing properties of jute/starch bio-composite material through incorporation of magnesium carbonate hydroxide pentahydrate: A sustainable approach

This research aims to develop an environment-friendly composite material that possesses enhanced fire retardant (FR), thermal, as well as mechanical characteristics. The aim has been accomplished with the development of a jute/thermoplastic starch (TPS) based bio-composite. The fire retardancy and thermal stability of the jute/TPS composite were enhanced by the incorporation of magnesium carbonate hydroxide pentahydrate (MCHPH). Upon exposure to heat or fire, the MCHPH particles decompose in a two-stage process to yield water vapors and a char layer of MgO and CO2, which restrict access to oxygen and result in flame suppression. Moreover, the main contribution of the article is the improvement of mechanical properties simultaneously with the enhancement in the fire retardant and thermal properties, which have rarely been reported in the literature. The enhancement of mechanical properties is supported by the compatibility of MCHPH particles with jute/TPS. All the composites were developed with a constant 40 % jute fiber content, while the MCHPH concentration varied from 0 % to 9 % by weight. The tensile strength of TPS was enhanced by 595 % with the reinforcement of jute fiber and MCHPH nano-filler. Compatibility between TPS, jute, and MCHPH was discovered through Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM) was used to study the fractured surfaces of the composites. Thermo-gravimetric analysis (TGA) revealed a decrease in the weight loss of the MCHPH-filled jute/TPS composites at high temperatures. The vertical burning test also revealed that the composites met the requirements for a V-0 rating. The heat release rate of the composites was reduced by 36 % after the addition of MCHPH, as measured by cone calorimetry test. The biodegradability test confirmed the eco-friendly nature of the composites by demonstrating significant weight loss in soil over a 4-weeks period. Thus, the present study provided the basis for the development of a novel green composite with commendable gains in flame resistance, thermal stability, and mechanical robustness.

Hexaphenoxy cyclotriphosphazene/boron nitride high-efficiency charring system enhancing the flame retardancy and thermal conductivity of polycarbonate

Polycarbonate (PC), as an engineering plastic, has excellent mechanical properties and electrical insulation properties. Nevertheless, with the rapid spread of 5 G applications, there is an increasing demand for functional PC materials with both thermal conductivity and flame retardant properties. In this study, the flame retardant thermal conductivity composite system of boron nitride (BN) and hexaphenoxy cyclotriphosphazene (HPCP) was constructed to study the simultaneous improvement of the flame retardant and thermal conductivity of PC. The results reveal that the inclusion of HPCP and BN in PC can significantly enhance its flame retardancy. This enhancement is evidenced by prolonged ignition time, reduced heat release rate and decreased total smoke production. In particular, 20BN/3HPCP/PC composite shows the best flame retardant effect. Through the investigation of flame retardant mechanisms, it has been found that the network structure formed by the BN sheets with phosphorus-containing products of HPCP decomposition can effectively filter and adsorb pyrolysis debris and increase the residence time of burning fragments in the pyrolysis zone promoting the pyrolysis products further fully burned to reduce the smoke caused by incomplete combustion. The phosphoric acid substances after the decomposing of HPCP can promote PC matrix to dehydration and carbonization to form the phosphorus-containing residues, which adheres on the BN network structure to form hardness char layers in the condensed phase to play an excellent role in blocking heat and combustible flue gas, which has a positive effect on reducing heat release rate and improving flame suppression. In terms of improving thermal conductivity, the infrared thermal imaging experiment further confirmed that BN sheets can increase the thermal conductivity of PC composites by more than 4 times, and the layer and layer stacks structure of BN sheets can endow an efficient thermal conductivity path for PC matrix to improve the thermal conductivity. Therefore, this study is meaningful practical exploration for developing the PC composites with excellent thermal conductivity and flame retardancy.

Inhibition of Laminar Premixed Flames of Dimethyl Ether by a Phosphorus-Containing Compound

ABSTRACT The present paper reports experimental and modeling results on the laminar burning velocity and structure of stoichiometric and fuel-rich premixed laminar flames of DME with and without TMP additives. The spatial variations of the mole fractions of H, OH, PO, PO2, HOPO, HOPO2 and some intermediate hydrocarbons in the one-dimensional burner-stabilized flames with various equivalence ratios are measured by the flame-sampling molecular beam mass spectrometry. The novel measurement data for the DME flames are used to validate a kinetic mechanism available in the literature for flame inhibition by organophosphorus compounds. The TMP inhibition effectiveness of DME flames has been determined and revealed to be lower than that of CH4/air flame. The speeds of CH4/air and DME/air flames were found to be sensitive to the same reactions involving phosphorus compounds, while the sensitivity of the methane flame speed to inhibition reactions is higher than that of DME/air flame. The observed differences were explained by the higher concentration of the chain carriers H, OH, and O in the DME flame. Based on the results obtained, we conclude that the phosphorus-containing inhibitors are promising candidates for reducing the flammability and ensuring the fire safety of DME mixtures with air.

Inhibition of hydrogen-air mixtures by using chemical vapor additives

Various approaches are being used for the minimization of risk related to hydrogen hazards. The present study highlights the suppression of hydrogen-air explosion using additives (bromoethane and bromopropane) through experimental and theoretical methods. The experiments were carried out with different equivalence ratios (Ф = 0.5–2) and additive contents (3–15%) at 298 K and 1 bar initial conditions. The results indicate the influence of additives on the maximum pressure-rise, minimum ignition energy and burning velocity. Theoretical study was conducted using CHEMKIN and determined the sensitivity analysis, species profile and reaction path analysis. After the explosion, pressure increases at lower additive fractions for fuel-lean and stoichiometric mixtures and decreases at higher additive contents. In fuel-rich mixtures, pressure rapidly decreases with the increasing additive contents. In the presence of additives, the relative pressure-rise rate decreases, while the time to maximum pressure-rise and ignition energy increase are irrespective of Ф. Moreover, the maximum declination of burning velocity reflects at Φ = 2 and 3% of either additive reduces the burning rate almost by half. The sensitivity coefficient decreases for the flame-promoting reactions and increases for the flame-inhibiting reactions. In such reactions, the inhibiting radicals capture the free radicals and cease the flame propagation, resulting in flame suppression. The reactions R788 (C2H5Br + OH = C2H4Br + H2O) and R834 (C3H7Br + OH = C3H6Br + H2O) have a negative reaction sensitivity coefficient that reflects the effect of suppression. The enhancement of the mole fraction of hydrogen bromide (HBr) with additive content signifies flame suppression. The effect of additives on the explosion characteristics indicates that bromopropane is more effective than bromoethane in minimizing hydrogen hazards.

Experimental investigation of hydrogen jet flame inhibition by nitrogen jet

With the rapid development of hydrogen energy and the increase of the risk of hydrogen jet fires, a reliable fire protection technology is required to extinguish hydrogen jet flame efficiently. An experimental investigation was carried out to study the inhibition of a hydrogen jet flame by a nitrogen jet. The impact angle, nozzle diameter and nozzle position of the nitrogen jet were the variables. The flame characteristic parameters were analyzed, including the flame length, deflection height and deflection angle. The results show that the inhibited flame length was always positively correlated with the vertical distance between the nozzles. The increase of the impact angle had a significant inhibitory effect on the flame length. Besides, the explanatory factor was used to describe the influence of various initial release conditions on flame inhibition and was calculated by the combination of the explanatory variables, such as impact angle, impact stroke and impact height. The flame length reduction ratio, deflection height reduction ratio and dimensionless deflection angle are positively correlated with the explanatory factor. It means that the jet flame is more easily extinguished under a larger explanatory factor. The critical parameter of the explanatory factor for extinguishing the hydrogen jet flame was verified experimentally.

Near-extinction transient behaviors of counterflow nonpremixed flames in a porous spherical burner at very low stretch rate

Near extinction transient behaviors of CH4/N2 versus O2/N2 counterflow nonpremixed flames were investigated by adopting a porous spherical burner with a large radius of curvature to achieve a very low buoyant stretch rate. By varying the oxygen concentrations in the oxidizer stream, four regimes were identified by three limits of sooting, instability, and extinction. The sooting limit was sensitive to flame temperature, and was characterized by two indexes of fuel injection velocity uF and oxygen concentration XO2. When decreasing uF, the sooting limit was different from high stretch flames because of appreciable heat loss toward the burner surface. In the instability regime, periodic holes were observed whose frequency decreased with XO2. Periodic holes were also found to occur with N2 dilution in the fuel stream, which were more regular with larger frequencies compared with the periodic holes in the diluted oxidizer streams. The edge propagation behaviors for periodic holes in diluted fuel streams were analyzed. The statistical average of the propagation speed of advancing edge (hole closing) Se decreased with increasing fuel concentration gradient, and the ratio of Se with the stoichiometric laminar burning velocity SLst of Se/SLst was larger than unity. Also, the stretch rate significantly affected the propagation speed of advancing edge flames. Determination of the propagation speed of the retreating edge (hole opening) had intrinsic difficulty in estimating the transient radial flow velocity, however, the approximated |Se/SLst| was larger than unity and decreased with the fuel mole fraction. Nitrogen dilution in the oxidizer stream was proven to be more effective than in the fuel stream for flame suppression of methane. This study could enrich the understanding of very low-stretch nonpremixed flames in a counterflow burner and provide information regarding low-stretch flame suppression, which may be relevant to microgravity flame behaviors.

Properties and flame retardancy of polylactide composites incorporating tricresyl phosphate and modified microcrystalline cellulose from oil palm empty fruit bunch waste

One of the major environmental issues that have an impact on humans, animals, and their surroundings is plastic garbage. The use of biodegradable polymers in place of traditional plastics is one of the best solutions to this significant issue. The bio-circular-green (BCG) economic model is supported by the use of microcrystalline cellulose (MCC) as a bio-filler for polylactide (PLA) composites, which may also help to address the issue of improper plastic waste management. This study explores the chemical modification of MCC derived from oil palm empty fruit bunch waste (OPMC). Maleic anhydride-modified OPMC (MAMC) is successfully synthesized by a solvent-free and low temperature heating procedure. MAMC and tricresyl phosphate (TCP) were used as additives in PLA composites which were processed by melt extrusion and compression molding. Characterization studies confirmed the successful modification of MAMC and indicated that TCP played a crucial role as an effective plasticizer and flame retardant for PLA. All PLA/TCP composites showed significantly improved toughness and delayed ignition. The appropriate TCP level was 10 phr. The incorporation of TCP and MAMC resulted in a synergistic enhancement of impact strength and maintained excellent flame inhibition. Moreover, the thermal stability of the PLA composites increased with increments of MAMC.

Enhancement and suppression of counterflow diffusion flame by HFC-125

Efficient fire extinguishing is a safety prerequisite for protecting the environment from fire damage. Understanding the effect of fire extinguishing agent on flame in complex environment is basic work for efficient fire extinguishing. We investigate the impact of HFC-125 on the extinguishment of propane-air highly stretched diffusion flames. Flame thickness is measured using the proportional method. Parameters such as dimensionless flame stand-off distance and flame stretch rate are calculated. Low concentrations of HFC-125 initially increase the flame thickness, but as the concentration increases further, the flame thickness decreases and the flame suppression effect of HFC-125 intensifies. HFC-125 reduces the amount of carbon soot generated and accelerates the transition from yellow to blue flame. Within a specific concentration range, N2 has a minimal effect on flame structural parameters compared to HFC-125. High concentration of N2 forms a high bright blue flame, while HFC-125 reacts with the flame to form a blue-violet flame. Compared to HFC-125, HFC-227ea can significantly reduce flame thickness and increase dimensionless flame stand-off distance, exhibiting a notable suppression effect. Furthermore, high concentrations of HFC-125 substantially reduce the critical flame stretch rate and provide a lower extinguishing concentration in counterflow diffusion flames than in the cup burner.

Study on the inhibition mechanism of green suppressants zinc borate and zinc silicate for oil shale based on flame propagation experiment and thermodynamic analysis

The Green materials zinc borate (ZB) and zinc silicate (ZS) were adopted as suppressants. Hartmann tube was used to study the flame inhibition effect of oil shale explosion. It showed that the flame front length became shorter and the propagation velocity and acceleration gradually decreased by increasing suppressants. When 30 wt% ZB was added, the complete inhibition of oil shale explosion flame could be achieved, while the flame inhibition effect of ZS was relatively weak. Flynn-Wall-Ozawa and Starink methods were used to study the activation energy of oil shale, which showed that the activation energy increased significantly after adding the suppressant. By characterization tests, the microstructure changes of oil shale before and after the explosion were analyzed, and the mechanism of the explosion inhibition difference between two suppressants was revealed. ZS played a physical inhibiting role by absorbing heat, reducing temperature and diluting the concentration of combustible gases. While ZB not only had the physical inhibition role of absorbing heat and diluting the concentration of gases, but also had a covering physical inhibition role. Simultaneously, ZB also had the chemical effect. It played a highly effective physical and chemical synergistic inhibition role in the deflagration of oil shale.

Development of a novel subgrid combustion model for buoyant turbulent diffusion flames

A novel approach is presented to incorporate finite rate reactions in modeling of turbulent diffusion flame, with the existence of unresolved flamelets inside the CFD grid cells taken into account. This approach, called SCM (subgrid combustion model), is applied in large eddy simulations of the UMD line burner flames. With the global kinetic parameters fitted for the measurement data and detailed simulations of autoignition in stoichiometric methane-air mixture, as well as for the extinction strain rate in the opposed non-premixed jets, the SCM model is shown to replicate the mean temperature profiles in unsuppressed flame with mean total heat release of 50 kW. For flame suppression via dilution of the oxidizer flow by nitrogen, the lift-off and extinction of non-anchored flame are predicted in agreement with the experimental data. The SCM is designed as a tool to allow for the finite rate kinetics effects in large eddy simulations of buoyant turbulent diffusion flames, with the focus on weakly turbulent regions, in which applicability of conventional combustion models is limited.

Effect of metal coating on physicochemical properties of ceramic foam for flame suppression application

Combustible dust may result in a catastrophic dust explosion with much larger accident consequences than gas explosions. In this study, porous materials are explored as a promising solution for mitigating explosions. This study focussed on the investigation of aluminium powder explosion in the influence of ceramic foam, with and without coating with non-precious nickel nanopowders. The microscopic properties and structural characteristics of the ceramic foam were analyzed using X-ray spectroscopy, X-ray diffractometry, and scanning electron microscopy. The findings revealed that the nickel-coated ceramic foams are non-uniform and almost spherical in shape, with obvious agglomerations of metallic-silica present on the strut structure. Compared to non-coated ceramic, it can be said that nickel-coated ceramic foams demonstrate a significant reduction in maximum overpressure by approximately 10%. This suggests that the interconnected micro-network structure of the nickel-ceramic walls aids in quenching the flame of the aluminium dust explosion and hence, suppressing the overpressure.

A highly thermostable flame retardant for poly(ethylene terephthalate) and its fabric with satisfactory comprehensive flame retardancy

The charring enhancement of polymers at high temperatures is essential for improving their anti-dripping, flame inhibition and smoke suppression performance. In this work, a highly thermostable and charring phosphorus-nitrogen (P/N)-based flame retardant (HMCP), which can boost the char-forming of poly(ethylene terephthalate) (PET) was synthesized. Amazingly, melt-dripping of PET was completely suppressed as 4 wt% of HMCP was added. For PET composite with 8 wt% of HMCP (PET/HMCP8), its limiting oxygen index increased to 33.6% and the flame self-extinguished within 5 s. Besides, the after-flame time and total smoke production of PET/HMCP8 were reduced by 96.1% and 39.5% respectively compared with pure PET. The enhanced fire safety of PET/HMCP composites was attributed to the barrier effect and the dilution effect produced by the synergism of P/N-based heterocycle and benzimidazole groups. Additionally, PET fabric woven with the filaments prepared by the melt spinning of PET/HBCP4 presented pleasing fire safety and mechanical properties.

Aromatization of residue caused by group aggregation of hyperbranched ester macromolecule capped with benzene rings improved flame retardancy of epoxy resin

To investigate the effect of the distribution state of phosphophenanthrene groups within the branched structure on the flame retardant efficiency, the hyperbranched ester macromolecules Bz-DQMC-n (n = 1, 2, 3) capped with benzene rings were synthesized. The aggregation effect of phosphaphenanthrene groups in Bz-DQMC-n enhanced the flame retardancy of epoxy resins (EPs). At the same phosphorus contents of EPs, the Bz-DQMC-n/EP passed the V-0 rating, except for 2-(6-oxid-6H-dibenzo[c,e][1,2]oxaphosphorin-6-yl)-1,4-benzenediol (DOPO-HQ)/EP without aggregated phosphaphenanthrene groups, which only reached the V-1 rating in UL 94 test. The Bz-DQMC-2 showed a better flame inhibition effect. The limiting oxygen index (LOI) value of the Bz-DQMC-2/EP reached to 39.3%, and the peak heat release rate (pk-HRR) value was 563 kW/m2, which decreased by 62.5% compared to neat EP. In addition, the total heat release (THR), and average effective heat of combustion (av-EHC) values of EP with Bz-DQMC-2 were also lower than that with Bz-DQMC-1 and Bz-DQMC-3 in the cone calorimetry test. The mechanism demonstrated that the aggregated phosphophenanthrene groups and benzene rings in Bz-DQMC-2 jointly promoted the formation of high-quality residue with more aromatic structures. Meanwhile, the concentrated release feature caused by group aggregation slightly enhanced the quenching effect in the gas phase. The aggregation effect of phosphophenanthrene groups in specific structures provides guidance for the development of high-efficiency DOPO-based flame retardants.

Numerical investigation of the flame suppression mechanism of porous muzzle brake

An excellent flame suppression effect can be achieved using a novel porous brake. To understand the flame-suppression mechanism of a porous brake, combustion using a muzzle brake is investigated. A set of internal ballistic equations is employed to provide accurate velocity and pressure for a projectile moving to the muzzle. The multispecies transport Navier–Stokes equations, which incorporate complex chemical reactions, are solved by coupling a real gas equation of state, the Soave–Redlich–Kwong model, and a detailed chemical reaction kinetic model. The development of muzzle flow with a chemical reaction is simulated, and the interaction between chemical reactions with the muzzle flow field is numerically calculated to explain the muzzle combustion mechanism with a porous brake. The underlying mechanism is analyzed in detail. The results demonstrate that, first, the gas is fully expanded in the brake, leading to a reduction in pressure and temperature at the muzzle, thereby reducing the initial flame. In addition, the shock wave weakens due to the expansion and separation process, leading to a reduction in the mixture of gas and air, ultimately resulting in a reduction in the intermediate and secondary flames.

Research on the Rule of Explosion Shock Wave Propagation in Multi-Stage Cavity Energy-Absorbing Structures.

The propagation laws of explosion shock waves and flames in various chambers were explored through a self-built large-scale gas explosion experimental system. The propagation process of shock waves inside the cavity was explored through numerical simulation using Ansys Fluent, and an extended study was conducted on the wave attenuation effect of multiple cavities connected in a series. The findings show that the cavity's length and diameter influenced the weakening impact of shock waves and explosive flames. By creating a reverse shock wave through complicated superposition, the cavity's shock wave weakening mechanism worked. By suppressing detonation creation inside the cavity, the explosive flame was weakened by the cavity's design. The multi-stage cavity exhibited sound-weakening effects on both shock waves and explosive flames, and an expression was established for the relationship between the suppression rate of shock force and the number of cavities. Diffusion cavities 35, 55, 58, and 85 successfully suppressed explosive flames. The multi-stage cavity efficiently reduced the explosion shock wave. The flame suppression rate of the 58-35 diffusion cavity explosion was 93.38%, whereas it was 97.31% for the 58-35-55 cavity explosion. In engineering practice, employing the 58-58 cavity is advised due to the construction area, construction cost, and wave attenuation impact.

Characterisation of flame retarded recycled PET foams produced by batch foaming

CO2-assisted batch foaming was used to manufacture low-density (ρ = 200–350 kg/m3) microcellular foams from bottle-grade recycled poly(ethylene terephthalate) (rPET) in flame retarded form. The foamability of the PET regrind was enhanced using a reactive chain extender while the flame retardant properties of the rPET foams were improved by incorporating 6% aluminium-tris-(diethylphosphinate) flame retardant (FR). The effects of adding 1% natural montmorillonite (MMT) and polytetrafluoroethylene (PTFE) powder as potential cell nucleating agent and flame retardant synergist were also investigated. As a result of FR addition, the foam structure became less uniform, but the presence of MMT was found to improve uniformity of cellular size and distribution. The applied FR mainly acts in the gas phase as flame inhibitor, but also promotes charring of the foams, as revealed by thermogravimetric analysis (TGA) and pyrolysis combustion flow calorimetry (PCFC). The fine microcellular structure and high cell density allow homogeneous distribution of FR additives, and thus only moderate increase in flammability was observed for the high-porosity (>75%) foams compared to the corresponding bulk materials, as characterized by similar limited oxygen index (LOI) values. In cone calorimeter tests, for the flame retarded foams a 50% reduction in peak heat release rate (PHRR) and a 30% reduction in total heat release (THR) values were measured compared to the FR-free reference. The PTFE addition to the FR formulation was found to increase the time-to-ignition (TTI), reduce PHRR and effective heat of combustion (EHC) while increase charring. The mechanical performance of the flame retarded rPET foams was found to be primarily determined by the apparent density and less affected by the presence of FR additives. Due to strain-induced crystallization occurring during cell growth, the rPET foams are highly crystalline (χ > 25%) which leads to increased thermomechanical resistance compared to unfoamed references.

Effects of different metal ions in phosphonitrile-modified organometallic complexes on flame retardancy of epoxy resin

Two organometallic (Zn and Fe) coordination compounds, modified by phosphonitrile, were prepared by solvothermal method. The introduction of the two organometallic coordination compounds significantly improved the flame retardancy, smoke suppression and thermal stability of epoxy resins (EP). Especially, zinc-containing coordination compounds exhibited higher efficiency than iron-containing ones in flame-retardant EP. LOI values of Zn-PDH/EP and Fe-PDH/EP were further enhanced and reached up to 29.8% and 28.9%, respectively. Organometallic coordination compounds modified by phosphonitrile mainly exerted the flame-retardant action in condense phase by the combined effect of the catalysis charring effect of metal ions and flame-retardant effect of phosphonitrile, and thus a high-temperature stable graphitized carbon layer with P- and N-containing cross-linking structures formed. This high-quality carbon layer was a vital factor in improving the heat/smoke suppression properties of EP thermosets. Further, due to the weaker catalytic char-forming effect of iron ions and the formation of fewer cross-linked structures, the strength and barrier properties of the Fe-PDH/EP char layer were relatively lower than those of Zn-PDH/EP. Consequently, Zn coordination compounds presented higher efficiency in suppressing heat/smoke release of EP matrix than Fe hybrids. Finally, the gas-phase function of organometallic coordination compounds should not be overlooked due to releasing phosphorus-oxygen free radicals and N-containing fragments during pyrolysis, exhibiting flame inhibition and dilution effect in gas phase, especially for 6%Zn-PDH/EP.