Liquid Flame Research Articles

Synthesis of a novel DOPO-based ionic liquid flame retardant and its application in epoxy resin

In contrast to conventional flame retardants, this study presents the development of a reactive flame retardant that significantly enhances both flame resistance and mechanical properties.

A high-performance flame retardant liquid synthesized on the basis of DOPO hydrolysis products avoids the defects of DOPO

It was first proposed to use 2-(2-hydroxyphenyl) phenyl phosphonic acid (HPPA), which is hydrolysis product of DOPO, has a structure more similar to EP, to replace DOPO in the preparation of flame retardant, and finally synthesized a highly efficient liquid flame retardant DP based on HPPA, the theoretical phosphorus content of DP is up to 17.5wt%, so EP can achieve excellent flame retardant/inhibit combustion effect with a small amount addition of DP. For example, the LOI of EP/DP6.0 is increased to 41.2 % and reaches the V-0 rate of UL-94, the PHRR and THR are reduced by 52.4 % and 45.2 % respectively, in addition, the addition of DP in small quantities does not destroy the overall performance of EP, maintains and improves the excellent mechanical and thermal properties of epoxy resin as a whole, and DP has excellent curing promoting effects, meeting the processing and use requirement of high performance EP. It is better than DOPO overall and has good application prospects.

Synthesis of P/Si liquid flame retardant via the covalent modification and in-situ dispersion strategies for improving comprehensive performance of epoxy resin

Maintaining the balance between thermal stability, smoke suppression and mechanical properties of flame retardant epoxy resin (EP) systems has been very challenging in industry and academia. Herein, we prepared a P/Si flame retardant fluid (KHDOPO) to modify expandable graphite (EG) through covalent modification strategy for flame retardant EP. The comprehensive performance of EP/KEG was further enhanced via the in-situ dispersion strategy. The covalent modification strategy provided a synergistic P/Si flame retardant effect for EG. The in-situ dispersion strategy further improved the dispersion properties of KEG in EP. Surprisingly, EP/KEG exhibits a UL-94 V-0 rating, limit oxygen index (LOI) value of 30.9 % and total smoke production (TSP) value of 15.5 m2 with an ultralow loading of 4 wt% KEG. Besides, EP/KEG maintains superior thermal stability and mechanical properties due to ultra-low addition and better dispersion of KEG.

A DOPO-eugenol linear siloxane as a highly effective liquid flame retardant also imparting improved mechanical properties to an epoxy amine network

A liquid bio-derived eugenol-containing phosphorus silicone hybrid molecule (EGDS-DOPO) that endows excellent flame-retardancy and enhanced mechanical performance to an epoxy amine network at low levels of addition was successfully synthesized and characterised. The EGDS-DOPO significantly improved the flame retardancy requiring only 5 wt% of EGDS-DOPO to achieve a limiting oxygen index (LOI) of 34.9 % and a UL 94 V-0 rating. Cone calorimetry measurements reveal a reduction of the peak heat release rate, the total heat release, and total smoke production by 62.2 %, 34.0 % and 47.5 %, respectively when EGDS-DOPO concentration was 20 wt%, compared to the unmodified epoxy network. Viscosity remained low throughout cure, and despite some retardation of reactivity, the epoxy amine resin remains fast curing and suitable for liquid moulding composite fabrication technologies. Moreover, the modified EGDS-DOPO epoxy networks display a glass transition temperature (Tg) between 218.5 °C and 174.1 °C, a hydrophobic surface with water contact angle of 97.5°–106.6°, a low moisture ingress of 2.35 %–2.91 %, and improved overall mechanical properties in flexure (maximum 39.8 % increase in flexural strength and 27.0 % in flexural modulus), compression (maximum 24.5 % and 35.0 % increase in compressive strength and modulus, respectively), and fracture toughness (maximum 28.0 %).

Promoting microalgal biofilm formation by crushed oyster shell-hydroxyapatite layer on micropatterned aluminum coating for heavy metal ions removal

Microalgal biomass has shown inspiring potential for the heavy metal removal from wastewater, and forming microalgal biofilm is one of the sustainable methods for the microalgal biomass production. Here we report the formation of microalgal biofilm by accelerated colonization of typical algae Chlorella on thermal sprayed aluminum (Al) coatings with biologically modified surfaces. Micro-patterning surface treatment of the Al coatings promotes the attachment of Chlorella from 6.31 % to 17.51 %. Further enhanced algae attachment is achieved through liquid flame spraying a bioactive crushed oyster shell-hydroxyapatite (CaCO3-HA) composite top layer on the micropatterned coating, reaching 46.03–49.62 % of Chlorella attachment ratio after soaking in Chlorella suspension for 5 days. The rapidly formed microalgal biofilm shows an adsorption ratio of 95.43 % and 85.23 % for low concentration Zn2+ and Cu2+ in artificial seawater respectively within 3 days. Quick interaction has been realized between heavy metal ions and the negatively-charged extracellular polymeric substances (EPS) matrix existing in the biofilm. Fourier transform infrared spectroscopy (FTIR) results indicate that both carboxyl and phosphoryl groups of biofilms are crucial in the adsorption of Cu2+ and the adsorption of Zn2+ is due to the hydroxyl and phosphate groups. Meanwhile, the biofilm could act as a barrier to protect Chlorella against the attack of the heavy metal ions with relatively low concentrations in aqueous solution. The route of quick cultivating microalgal biofilm on marine structures through constructing biological layer on their surfaces would give insight into developing new techniques for removing low concentration heavy metal ions from water for environmental bioremediation.

Microsecond-Scale Transient Thermal Sensing Enabled by Flexible Mo1-xWxS2 Alloys.

Real-time thermal sensing through flexible temperature sensors in extreme environments is critically essential for precisely monitoring chemical reactions, propellant combustions, and metallurgy processes. However, despite their low response speed, most existing thermal sensors and related sensing materials will degrade or even lose their sensing performances at either high or low temperatures. Achieving a microsecond response time over an ultrawide temperature range remains challenging. Here, we design a flexible temperature sensor that employs ultrathin and consecutive Mo1-x W x S2 alloy films constructed via inkjet printing and a thermal annealing strategy. The sensing elements exhibit a broad work range (20 to 823 K on polyimide and 1,073 K on flexible mica) and a record-low response time (about 30 μs). These properties enable the sensors to detect instantaneous temperature variations induced by contact with liquid nitrogen, water droplets, and flames. Furthermore, a thermal sensing array offers the spatial mapping of arbitrary shapes, heat conduction, and cold traces even under bending deformation. This approach paves the way for designing unique sensitive materials and flexible sensors for transient sensing under harsh conditions.

Few-layer NbTe2 nanosheets-based saturable absorber for generating 176 fs pulse at 1 μm in ultrafast fiber laser

Ultrafast fiber lasers play an increasingly vital role in diverse fields of science and industry. In this study, a novel NbTe2 nanosheet-based saturable absorber (SA) was prepared by using liquid phase exfoliation and flame brush technology for the first time. Using a twin balance measurement system, the nonlinear optical absorption property was checked. It was found that the fabricated NbTe2 nanosheets-based SA has excellent saturable absorption properties with a large modulation depth of 5.3 %, a very high saturation intensity of 1.11 GW/cm2, and a low non-saturation loss of 63.6 %. After inserting the NbTe2 nanosheets-based SA into a ring-cavity ytterbium-doped fiber laser (YDFL), a stable soliton mode-locking state was achieved. A 176-fs ultrafast soliton pulse train was successfully generated at 1029.76 nm with the repetition rater of 3.097 MHz. Thanks to the unique and very high saturation intensity of NbTe2 nanosheets, the proposed YDFL system achieved a maximum peak power of 96.3 kW and a single pulse energy of 14.09 nJ under 370 mW pump power. These findings indicate that NbTe2 nanosheets have great potential for generating ultra-short optical pulses with higher peak power in mode-locked fiber lasers.

A DOPO modified cyclosiloxane reactive diluent as an effective flame retardant for a high-performance epoxy network

A novel phosphorus-siloxane containing liquid flame retardant epoxy resin was synthesized and incorporated into an epoxy amine network, maintaining low viscosity and rapid cure characteristics, good mechanical and thermal properties, and affording excellent flame retardancy at low concentration. The synthesized flame retardant incorporated a tetra-functional glycidyl ether cyclosiloxane substructure which was substituted with two DOPO segments to obtain a liquid bi-functional flame-retardant epoxy resin (BGTS-DOPO). At only 10 wt% BGTS-DOPO, the modified epoxy amine resin had a viscosity of less than 210 mPa s at 90 °C, gelled rapidly at 150 °C, (9.3 min at 150 °C), exhibited a Tg of 198.7 (extrapolated onset of E’) and 233.0 °C (tan δ max), and possessed improved flexural strength (4.5 %), modulus (4.1 %), strain (7.7 %) and fracture toughness (26.1 %). The self-extinguishing behaviour easily achieved a UL-94 V0 rating, exhibited a maximum limiting oxygen index of 36.7 %, while cone calorimetry produced a 24 % improvement in combustion behaviour of the carbon fibre composite. Presented here therefore is a new highly fire-retardant epoxy resin system that exhibited fast cure, low viscosity and excellent mechanical and thermal properties.

S-Triazine phosphonates as flame retardants for polyurethane and polyisocyanurate rigid foams

The liquid flame retardant tris(2‑chloro-isopropyl) phosphate (TCPP) is widely used in polyurethane and polyisocyanurate rigid foams. However, TCPP is currently under assessment by the European Chemicals Agency ECHA and National Toxicology Program NTP, USA. For this reason, there is a high demand for new flame retardants with similarly good processing and application properties. This paper examines the influence of low-molecular liquid s-triazine phosphonates on the foaming behaviour and flame retardancy of polyurethane and polyisocyanurate rigid foams for building applications. The flame retardant investigations consisted of thermogravimetric analysis of the components and flame retardancy tests performed on the foams: limiting oxygen index, EN 13501 class E, and cone calorimetry. The newly developed class of s-triazine phosphonates are promising flame-retardant candidates with good processing and application properties.

Binary-Network structured PI@SiO2 nanofibrous composite aerogels with temperature invariant superelasticity for thermal insulation

The extreme temperatures encountered in aerospace pose formidable challenges to the performance of elastic materials in spacecraft and related apparatus. Traditional organic insulation materials face hindrance due to inadequate fire resistance, while inorganic insulation materials are often brittle. Herein, we designed binary-network composite aerogels with dual-crosslinked PI nanofibers as the scaffold and uniformly distributed silica nanoparticles within the anisotropic aerogel matrix. Owing to the dual-crosslinked PI nanofiber network, the resulting PI@SiO2 aerogel can withstand 1000 cycles of radial fatigue testing under 50 % compressive or buckling strains, maintaining structural stability across a wide angular frequency range of 0.1–100 rad/s. DMA testing shows that PI@SiO2 aerogel can endure 100,000 fatigue cycles from 25 to 300 °C, with storage modulus and loss modulus, and damping ratio, indicating robust long-term performance over a wide temperature spectrum. Even with liquid nitrogen (−196 °C) and butane torch flames (1100 °C), PI@SiO2 aerogel preserves its superelasticity through repeated compressions. Moreover, the composite aerogel exhibits excellent thermal insulation, with low thermal conductivity (27.2 mW m−1 K−1), and reachs the top flame-retardant level (UL94-V0). This work not only establishes a novel pathway for constructing polymer-based materials with temperature-invariant superelasticity but also holds great promise for extensive applications in ongoing and near-future aerospace exploration.

Effect of synthesized lignin-based flame retardant liquid on the flame retardancy and mechanical properties of cotton textiles

The flammability of cotton (Gossypium hirsutum) is a significant concern for technical applications, prompting ongoing research into solutions to mitigate this risk. Traditional flame-retardant methods utilizing acid-based approaches are complex and can negatively affect the mechanical properties of textiles. To address these challenges, this study focuses on developing a liquid bio-based flame retardant (LBF) utilizing a lignin-silica-based liquid (LSL) extracted from rice husk (RH) (Oryza sativa ssp. japonica) and 9, 10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO). A one-pot dip-coating technique is utilized to treat cotton with the LSL, and the flammability and mechanical properties of the treated cotton are subsequently tested. Fourier-transform infrared spectroscopy (FTIR) confirms the covalent bond formation of the LSL with DOPO and the hydrogen bond formation of the LBF with cotton. Scanning electron microscopy (SEM) confirms the uniformity of the coating. The experimental results demonstrate that the treated cotton exhibits self-extinguishing behavior during a vertical burning test (VBT), with a 78% reduction in peak heat release and a 65% reduction in total heat released during pyrolysis combustion flow calorimetry. Surprisingly, the treatment also improves the tensile behavior of the cotton by 21.7% and thermal stability by producing a protective char layer that accounted for 36.9% of the final residue. This study provides a promising approach for improving the flame resistance and mechanical strength of cotton for technical applications.

Silver nanoparticle coatings with adjustable extinction spectra produced with liquid flame spray, and their role in photocatalytic enhancement of TiO2

Silver nanoparticles deposits were produced with liquid flame spray (LFS) on glass and TiO2 substrates to study their optical response and photocatalytic enhancement. The correlation between extinction spectrum of the nanoparticle coating and the LFS process parameters was studied. The spectra consisted of two partly overlapping peaks: one centered in the UV region and the other in the visible light region. The visible light peak redshifted as either the silver mass concentration in the precursor solution or the precursor solution feed rate was increased, which also correlated with growing primary particle size. However, simultaneous correlation with photocatalytic activity of the decorated TiO2 surfaces was not observed, which was attributed to particle sintering on the surface. Instead, the photocatalytic activity was seen to change as the surface coverage of silver nanoparticles was varied. When the surface coverage was raised from ∼10 % to roughly 30 %, the activity, and then decreased as the loading was further raised. The increase was assumed to originate from plasmonic activation, and the decrease was attributed to the excessive amount of silver either blocking reactive area of the TiO2 or absorbing/scattering too much of the incoming light, which hindered the photocatalytic activity.

Effects of Gas Layer Thickness on Capillary Interactions at Superhydrophobic Surfaces.

Strongly attractive forces act between superhydrophobic surfaces across water due to the formation of a bridging gas capillary. Upon separation, the attraction can range up to tens of micrometers as the gas capillary grows, while gas molecules accumulate in the capillary. We argue that most of these molecules come from the pre-existing gaseous layer found at and within the superhydrophobic coating. In this study, we investigate how the capillary size and the resulting capillary forces are affected by the thickness of the gaseous layer. To this end, we prepared superhydrophobic coatings with different thicknesses by utilizing different numbers of coating cycles of a liquid flame spraying technique. Laser scanning confocal microscopy confirmed an increase in gas layer thickness with an increasing number of coating cycles. Force measurements between such coatings and a hydrophobic colloidal probe revealed attractive forces caused by bridging gas capillaries, and both the capillary size and the range of attraction increased with increasing thickness of the pre-existing gas layer. Hence, our data suggest that the amount of available gas at and in the superhydrophobic coating determines the force range and capillary growth.

Fast-facile synthesis of single-phase nanocrystalline β-Ca2P2O7 powder by an aerosol synthesis method

Recently, β-phase of calcium pyrophosphate has gained wide attention for potential in different applications. In this communication, we report for the first time an ultra-fast, cost-effective, and facile one-step synthesis of single-phase β-CPP powder without post-heat treatment by applying liquid flame spray (LFS). LFS is a scalable, continuous, and adjustable one-step flame-based aerosol synthesis method. The effect of Ca/P ratio on structural properties was investigated. β-Ca2P2O7 could be successfully synthesized in spherical nano-and micron-sized particles, as well as plate-like particles. The synthesized single-phase powder exhibited high thermal stability and relatively high specific surface area with different (nano)porous structures.

Topology characteristics of liquid ammonia swirl spray flame

The utilization of liquid ammonia in gas turbines can reduce energy loss and start-up time. However, the flash boiling phenomenon and the high latent heat of liquid ammonia make the spray flame difficult to stabilize. Increasing the preheated air temperature or adding a small amount of hydrogen as a piloted fuel are considered as effective methods to enhance the stability. To understand the flame topological structure, simultaneous Mie scattering and planar laser-induced fluorescence of OH (OH-PLIF) techniques were used to visualize the liquid ammonia spray structure and flame region information. Results show that the liquid ammonia swirl spray flame exhibits the flame topological structure of distinct zoning characteristics, including the droplet zone, the mixing zone, and the flame zone. Increasing the preheated air temperature accelerates the evaporation of liquid ammonia, leading to an increase in the local equivalence ratio and radial flame splitting. At lower air temperature conditions, increasing the hydrogen blending ratio has minimal impact on the flame topological structure. However, at higher temperature conditions, hydrogen blending significantly promotes reaction intensity upstream and reduces the flame lift-off height, which makes the mixing zone smaller. In general, to achieve a better flame stability effect, the two factors need to be reasonably matched, which has important reference value for the development of liquid ammonia fueled gas turbine combustors.

Deep convolutional autoencoders for the time–space reconstruction of liquid rocket engine flames

The integration of high-fidelity numerical simulations into the rocket engine design-cycle promises to cut costs in an ever more competitive launch market. Although still being intractable in the optimization of most of combustion devices, their potential can be harnessed via appropriate reduced order models (ROMs). In recent years, significant progress has been made in the ROM literature through the adoption of non-linear methods, coming from the realm of deep learning. These methods promise to deliver where conventional linear techniques, such as Proper Orthogonal Decomposition (POD), fall short due to their inherent qualities.In the current work, we carry out a first attempt to develop purely data-driven emulators for a shear-coaxial liquid rocket engine injector. In particular, a Convolutional Neural Network autoencoder (CNN-AE) with a Multi-Layer Perceptron (MLP) latent dynamics encoder architecture is considered. The main objective is to assess the capacity of autoencoders based models to reconstruct, in time and space, the instantaneous temperature field of a non-canonical, complex, reactive, flow configuration. The data used is made of Large Eddy Simulations (LES) of three distinct, real-life inspired, injector configurations.The autoencoder based models show superior reconstruction capabilities compared to POD with similar latent-space dimension. Moreover, the time–space reconstructed fields preserve the spectral content of the snapshots, with only a small loss in gain at high frequencies. These results serve as proof-of-concept for further developments on surrogate models of liquid rocket engine injectors.

Achieving high flame stability with low NO And Zero N2O and NH3 emissions during liquid ammonia spray combustion with gas turbine combustors

Liquid ammonia spray direct injection is more desirable than gaseous ammonia injection in large scale gas turbine combustors because it enables a larger volumetric energy density and allows a reduction in cost and size of the fuel supply system. However, previous studies have shown that the large evaporative cooling effects of liquid ammonia droplets vaporization impair flame stabilization, and inhibit efficient control of emissions such as NO, N2O and unburned fuels. This article reports the development and testing of a novel two-stage gas turbine combustor for pure liquid ammonia spray, which achieves approximately zero N2O and unburned fuel emissions with NO emissions as low as 282 ppmv (@16% O2) in a 50 kW micro gas turbine combustor test rig. The combustor design focused on optimization of the secondary injection holes to prevent the dilution of the primary zone (PZ) by air injected into the secondary zone; improving mixture formation using a multi-nozzle twin-fluid atomizer; and elongation of the combustor length to allow sufficient fluid residence time for N2O reduction as well as droplets vaporization and combustion. It was found that the mitigation of PZ dilution resulted in a significant increase in the combustion temperature thereby encouraging efficient combustion of the liquid droplets. On the other hand, the multi-nozzle twin fluid atomizer resulted in a more uniform combustor wall temperature, a more efficient combustion and lower NOx emissions than a single-nozzle pressure swirl atomizer. It is considered that the multi-nozzle injector allowed for an improved droplets dispersion, which is necessary to mitigate large local heat extraction from the flame. These features of the combustor enabled a substantial improvement in liquid ammonia spray flame stability and a better simultaneous control of NO, N2O and unburned fuel emissions never reported before.

Phosphorus-containing reactive compounds to prepare fire-resistant vinyl resin for composites: Effects of flame retardant structures on properties and mechanisms

The traditional flame-retardant method for carbon fiber reinforced vinyl ester resin composites is to add a large amount of solid flame retardants, which not only leads to the deterioration of the material's mechanical properties, but also increases the resin viscosity and affects the molding process of composites. In addressing this issue, this work adopts a liquid reactive flame retardant strategy and synthesizes six novel liquid phosphorus-containing reactive compounds as flame retardants. A comparative study is conducted on the effects of the structure of compounds (valence and group of phosphorus and number of reaction groups) on the properties of materials. The results demonstrate significantly improved flame retardancy and toughness, as well as unaffected viscosity and strength of the resin. When the phosphorus content does not exceed 3 wt%, the limiting oxygen index (LOI) of the resin can reach over 30% and UL-94 passes the V-0 rating. The structure of compounds significantly affects their flame retardancy mechanism. Especially, compounds rich in P-O-C structure have more outstanding condensed phase flame retardancy (promoting charring) and are more favorable for improving the LOI; Compounds rich in P-Ph and P-C structures have higher thermal stability and mainly exert gas phase flame retardancy, which is more beneficial for improving the self-extinguishing properties of materials. Furthermore, a synergistic flame retardant effect is observed between carbon fiber and compounds rich in P-Ph.

Experimental investigation on the combustion characteristics and thermoacoustic instability of liquid spray flame with blended fuel

This study experimentally investigates the combustion characteristics and thermoacoustic instability of two fuels, ethanol and RP-3, at five different blending ratios. The flame images, root temperature of blended fuel flame and pollutant emissions were photographed and tested. The experimental results indicate that high-pressure pulsation amplitude and airflow disturbance can enhance droplets’ evaporation and combustion processes, resulting in a reduction in flame length. Lower evaporation temperature ensures a higher temperature of the flame root while increasing the blend ratio of RP-3 and increasing the global equivalence ratio can increase the temperature level inside the combustion chamber. The root temperature of ethanol flame can reach over 1400 K. Fuel characteristics and burner structure can influence the coupling between pressure pulsation and flame heat release pulsation, leading to differences in the amplitude of combustion instability. The shorter the outlet length of the burner, the wider the stability range of the flame. Moreover, the increase in total burner length results in a shift of the oscillation dominant frequency towards lower frequencies. Emission characteristic curves reveal that high-pressure pulsation amplitude can improve the combustion efficiency of blended RP-3 fuel but are unfavorable for the complete combustion of ethanol. Meanwhile, under the dilution effect of the airflow, the emissions of NOx and CO at the outlet gradually decrease as the airflow increases.

Numerical study on spherical flame propagation in dispersed liquid ammonia droplets

Directly using liquid ammonia in combustion devices can simplify the fuel supply system and reduce equipment costs. However, the detailed ignition and propagation mechanisms of liquid ammonia flames have not been fully understood. This study aims to investigate the ignition and propagation of liquid ammonia flames numerically under various conditions. Spherically expanding liquid ammonia flames are studied using detailed chemistry. The effects of initial temperatures, droplet diameters, equivalence ratios, and heat fraction are investigated. The results indicate that the rapid flash boiling of liquid ammonia droplets leads to a strong local heat loss, hindering the ignition and propagation of the flame. Preheating can significantly increase the reaction rate, which compensates for the heat loss caused by phase change, allowing the flame kernel to ignite and propagate outwards. While droplet-flame interaction is present but not significant due to the rapid evaporation of liquid ammonia. The effect of droplet on the flame surface curvature is only distinguishable at conditions of large diameter. Equivalence ratio and ammonia heat fraction ratio change the amount of liquid ammonia. The more liquid ammonia, the more heat is absorbed by the phase change process, making the flame more difficult to be ignited.