Ultra-low phosphorus loading to achieve the superior flame retardancy of epoxy resin

The Effect of Flame-Retardant Additives DDM-DOPO and Graphene on Flame Propagation over Glass-Fiber-Reinforced Epoxy Resin under the Influence of External Thermal Radiation

The flammability of various materials used in industry is an important issue in the modern world. This work is devoted to the study of the effect of flame retardants, graphene and DDM-DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide-4,4'-diamino-diphenyl methane), on the flammability of glass-fiber-reinforced epoxy resin (GFRER). Samples were made without additives and with additives of fire retardants: graphene and DDM-DOPO in various proportions. To study the flammability of the samples, standard flammability tests were carried out, such as thermogravimetric analysis, the limiting oxygen index (LOI) test, and cone calorimetry. In addition, in order to test the effectiveness of fire retardants under real fire conditions, for the first time, the thermal structure of downward flame propagation over GFRER composites was measured using thin thermocouples. For the first time, the measured thermal structure of the flame was compared with the results of numerical simulations of flame propagation over GFRER.

A Superior Two-Dimensional Phosphorus Flame Retardant: Few-Layer Black Phosphorus

The usage of flame retardants in flammable polymers has been an effective way to protect both lives and material goods from accidental fires. Phosphorus flame retardants have the potential to be follow-on flame retardants after halogenated variants, because of their low toxicity, high efficiency and compatibility. Recently, the emerging allotrope of phosphorus, two-dimensional black phosphorus, as a flame retardant has been developed. To further understand its performance in flame-retardant efficiency among phosphorus flame retardants, in this work, we built model materials to compare the flame-retardant performances of few-layer black phosphorus, red phosphorus nanoparticles, and triphenyl phosphate as flame-retardant additives in cellulose and polyacrylonitrile. Aside from the superior flame retardancy in polyacrylonitrile, few-layer black phosphorus in cellulose showed the superior flame-retardant efficiency in self-extinguishing, ~1.8 and ~4.4 times that of red phosphorus nanoparticles and triphenyl phosphate with similar lateral size and mass load (2.5~4.8 wt%), respectively. The char layer in cellulose coated with the few-layer black phosphorus after combustion was more continuous and smoother than that with red phosphorus nanoparticles, triphenyl phosphate and blank, and the amount of residues of cellulose coated with the few-layer black phosphorus in thermogravimetric analysis were 10 wt%, 14 wt% and 14 wt% more than that with red phosphorus nanoparticles, triphenyl phosphate and blank, respectively. In addition, although exothermic reactions, the combustion enthalpy changes in the few-layer black phosphorus (-127.1 kJ mol^-1) are one third of that of red phosphorus nanoparticles (-381.3 kJ mol^-1). Based on a joint thermodynamic, spectroscopic, and microscopic analysis, the superior flame retardancy of the few-layer black phosphorus was attributed to superior combustion reaction suppression from the two-dimensional structure and thermal nature of the few-layer black phosphorus.

Biobased Epoxy Composites Reinforced with Acetylated Corn Straw

Corn straw/epoxy resin composites (CS/ECs) and maleic anhydride acetylated CS/ECs (MA-CS/ECs) were prepared through dry mixing and high-temperature curing. Corn straw is a kind of abundant, eco-friendly, and low-cost biomass material. Unmodified and modified corn straws were characterized using Fourier transform infrared spectroscopy, scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). The interfacial affinity of the composite was testified by the contact angle. The results of XPS and SEM demonstrated that maleic anhydride had been successfully bonded onto the structure of corn straw. Corn straw particle-reinforced epoxy resin composites were prepared using a casting and molding process. Results showed that the MA-CS/EC had better impact and flexural resistance than the unmodified corn straw/epoxy resin composites when the corn straw addition was 15 wt %. The result of the contact angle showed that the interfacial compatibility between composites is also stronger than that of CS/EC.

Amino Phenyl Copper Phosphate-Bridged Reactive Phosphaphenanthrene to Intensify Fire Safety of Epoxy Resins

To improve the compatibility between flame retardant and epoxy resin (EP) matrix, amino phenyl copper phosphate-9, 10-dihydro-9-oxygen-10-phospha-phenanthrene-10-oxide (CuPPA-DOPO) is synthesized through surface grafting, which is blended with EP matrix to prepare EP/CuPPA-DOPO composites. The amorphous structure of CuPPA-DOPO is characterized by X-ray diffraction and Fourier-transform infrared spectroscopy. Scanning electron microscope (SEM) images indicate that the agglomeration of hybrids is improved, resisting the intense intermolecular attractions on account of the acting force between CuPPA and DOPO. The results of thermal analysis show that CuPPA-DOPO can promote the premature decomposition of EP and increase the residual amount of EP composites. It is worth mentioning that EP/6 wt% CuPPA-DOPO composites reach UL-94 V-1 level and limiting oxygen index (LOI) of 32.6%. Meanwhile, their peak heat release rate (PHRR), peak smoke production release (PSPR) and CO₂ production (CO₂P) are decreased by 52.5%, 26.1% and 41.4%, respectively, compared with those of EP. The inhibition effect of CuPPA-DOPO on the combustion of EP may be due to the release of phosphorus and ammonia free radicals, as well as the catalytic charring ability of metal oxides and phosphorus phases.

Mechanisms of the Action of Fire-Retardants on Reducing the Flammability of Certain Classes of Polymers and Glass-Reinforced Plastics Based on the Study of Their Combustion

In the present review, using an integrated approach based on the experimental and theoretical study of the processes of thermal decomposition and combustion of practically important polymers, such as polymethyl methacrylate, polyethylene, and glass-fiber-reinforced epoxy resin, the features of the mechanism for reducing the combustibility of these materials with phosphorus-containing flame-retardants (FR), as well as graphene, are identified. A set of original experimental methods was developed and applied that make it possible to study the kinetics of thermal decomposition and the thermal and chemical structure of the flames of the studied materials, including those with FR additives, as well as to measure the flame propagation velocity, the mass burning rate, and the heat fluxes from the flame on the surface of a material. Numerical models were developed and tested to describe the key parameters of the flames of the studied polymeric materials. An analysis of the experimental and numerical simulation data presented showed that the main effect of phosphorus-containing fire-retardants on reducing the combustibility of these materials is associated with the inhibition of combustion processes in the gas phase, and the effect of adding graphene manifests itself in both gas and condensed phases.

Enhanced fire-proofing performance and crystallizability of bio-based poly(L-lactic acid): Dual functions of a Schiff base-containing synergistic flame retardant

Poly(L-lactic acid) (PLA) is a kind of important bio-macromolecule which can be prepared via fermentation of starch of maize and sweet potato. Flammability and extremely poor crystallizability limited its wide application. In this work, a novel Schiff base derivate (CP) was synthesized and, combined with ammonium polyphosphate (APP) as a synergistic flame retardant and nucleating agent to investigate its effects on LOI, UL-94 rating, thermal stability, combustion behavior and crystallizability of PLA. With loading of 5%CP/10%APP, PLA showed a significantly enhanced LOI and passed V-0 fire-safety rating with self-extinguish effect. PLA/5%CP/10%APP presented the lowest pHRR, THR and TSR, and highest char residue yield, FPI and FRI in cone calorimetry test, indicating an excellent flame retardancy effect, enhanced fire safety and longer escaping time in the fire. A continuous, compact and thick char layer structure formed as a protective barrier in combustion process, to enhance heat-insulating and oxygen resistance property, thermal stability and smoke-suppressing capacity of PLA. Flame retardancy mechanism was proposed and discussed based on comprehensive and in-depth characterization techniques. Also, 5%CP/10%APP presented a good nucleation effect to enormously increase crystallizability and shorten crystallization time of PLA.

The Influence of Flame Retardants on Combustion of Glass Fiber-Reinforced Epoxy Resin

For the first time, next to the flammability tests (LOI, UL-94 HB, VBB, TGA), experimental tests and computer simulation have been conducted on the flame spread and combustion of glass fiber-reinforced epoxy resins (GFRER) with 6% graphene and 6% DDM-DOPO flame-retardant additives. The downward rates of flame spread (ROS) in opposed flow with oxidizer and the upward ROS along GFRER composites have been first measured as well as the distribution of temperature over the combustion surface of the composites with flame-retardant additives and without them. The LOI and UL-94 HB tests showed a reduction in the flammability of GFRER when flame retardants were added and predicted a higher effectiveness of DDM-DOPO compared to graphene. Adding DDM-DOPO resulted in increasing the rate of formation of the volatile pyrolysis products and their yield, indicating, together with the other data obtained, the gas phase mechanism of the flame retardant’s action. Adding graphene resulted in an increase in the soot release on the burning surface and an increase in the amount of non-volatile pyrolysis products on the burning surface, reducing the amount of fuel that participated in the oxidation reactions in the gas phase. The developed numerical combustion model for GFRER with a DDM-DOPO additive, based on the action of DDM-DOPO as a flame retardant acting in the gas phase, satisfactorily predicts the effect of this flame retardant on the reduction in downward ROS over GFRER for 45–50% oxygen concentrations. The developed model for GFRER with graphene additive, based on a reduction in the amount of fuel and increase in the amount of incombustible volatile pyrolysis products when graphene is added, predicts with good accuracy downward ROS over GFRER depending on oxygen concentration.

Facile Construction of Polypyrrole Microencapsulated Melamine-Coated Ammonium Polyphosphate to Simultaneously Reduce Flammability and Smoke Release of Epoxy Resin

A unique mono-component intumescent flame retardant, named PPy-MAPP, of which melamine-coated ammonium polyphosphate (MAPP) was microencapsulated by polypyrrole (PPy), was synthesized and carefully characterized. The obtained PPy-MAPP was applied to epoxy resin (EP) for obtaining flame-retarded EP composites. The results show that PPy-MAPP imparts better flame retardancy and smoke suppression properties to EP compared to the same addition of MAPP. The EP composite with 15 wt% PPy-MAPP easily passes the UL94 V-0 rating and achieves an LOI value of 42.4%, accompanied by a 61.9% reduction in total heat release (THR) and a 73.9% reduction in total smoke production (TSP) when compared with pure EP. The char residue analysis shows that PPy-MAPP can promote a generation of more phosphorus-rich structures in the condensed phase that improve the integrity and intumescence of char against fire. The mechanical test indicates that PPy-MAPP has a less negative effect on the tensile strength and elastic modulus of epoxy resin due to the good compatibility between PPy-MAPP and the EP matrix, as supported by differential scanning calorimetry (DSC) analyses. In this paper, these attractive features of PPy-MAPP provide a new strategy to prepare satisfactory flame retardant and super flame retarding EP composites.

High performance flame-retardant organic-inorganic hybrid epoxy composites with POSS and DOPO-based co-curing agent

Polyhedral oligomeric silsesquioxane (POSS) and a highly effective 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-based flame retardant co-curing agent (D-bp) were chemically introduced into the 4,4'-diaminodiphenyl methane (DDM)/diglycidyl ether of bisphenol A (DGEBA) epoxy system to create organic-inorganic hybrid epoxy composites with simultaneously improved flame retardancy and mechanical properties. The results revealed that POSS/D-bp/DGEBA hybrid composites exhibited excellent comprehensive performance, in which the V-0 criterion of the UL-94 test was passed and the peak of heat release rate (P-HRR) was significantly decreased from 939 to 371 kW m-2 when the phosphorus content was only 0.25 wt%. The glass transition temperature (T g) increased by 16.2 °C and obvious improvement in the mechanical properties was also evidenced.

Experimental and Numerical Study of Downward Flame Spread over Glass-Fiber-Reinforced Epoxy Resin

For the first time, a comprehensive study of downward flame spread over glass-fiber-reinforced epoxy resin (GFRER) slabs in oxidizer flow has been carried out experimentally and numerically. Microthermocouples were used to measure the temperature profiles on the solid fuel’s surface and in the flame, and a video camera was used to measure the rate of flame spread (ROS). The ROS was found to be linearly dependent on the oxygen concentration, to be inversely proportional to the slab thickness and not to depend on the direction of the flame spread over the slab. The absence of the influence of the forced oxidizing flow velocity and the weak influence of the GFRER pyrolysis kinetics on the ROS were observed. For the first time, a numerical model of flame spread over reinforced material with thermal conductivity anisotropy was developed on the basis of a coupled ‘gas–solid’ heat and mass transfer model, using modifications of the OpenFOAM open-source code. The sensitivity analysis of the model showed that the thermal conductivity in the normal direction to the GFRER surface had a much greater effect on the ROS than the thermal conductivity along the direction of flame propagation. The numerical results show good agreement with the experimental data on the dependences of the ROS on oxygen concentration, slab thickness and the N₂/O₂ mixture flow velocity, as well as temperature distributions on the fuel surface, the maximum flame temperatures and the flame zone length.

Discotic Liquid Crystal Epoxy Resins Integrating Intrinsic High Thermal Conductivity and Intrinsic Flame Retardancy

The integration of intrinsic thermal conductivity and intrinsic flame retardancy of epoxy resins shows wider application prospects in electricals and electronics. Discotic liquid crystal epoxy (D-LCE) is synthesized from pyrocatechol, 2-allyloxyethanol, and 3-chloroperoxybenzoic acid. P/Si synergistic flame-retardant co-curing agent (DOPO-POSS, DP) is synthesized from p-hydroxybenzaldehyde, 9, 10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO), and amino terminated polysilsesquioxane (POSS). Finally, D-LCE is cured within liquid crystal range with 4, 4'-diaminodiphenyl methane (DDM) and DP, to obtain intrinsic highly thermal conductive/flame-retardant epoxy resins (D-LCER_DP ). D-LCER_DP-10.0 (10.0 wt% DP) synchronously possesses excellent intrinsic thermal conductivity and intrinsic flame retardancy, with thermal conductivity coefficient in vertical and parallel direction (λ_⊥ and λ_∥ ) of 0.34 and 1.30 W m^-1 K^-1 , much higher than that of general bisphenol A epoxy resin (E-51, λ_⊥ of 0.19 W m^-1 K^-1 , λ_∥ of 0.65 W m^-1 K^-1 ). The limiting oxygen index (LOI) value of D-LCER_DP-10.0 reaches 31.1, also better than those of E-51 (19.8) and D-LCER (21.3).

A Study on Circular Economy Material Using Fish Scales as a Natural Flame Retardant and the Properties of Its Composite Materials

Fish scales (FSs) are fishery wastes that can cause environmental pollution. This study aimed to solve this environmental problem. FSs were used as a flame retardant for polymer materials, making them valuable. Fish scales were combined with a commercial flame retardant, ammonium polyphosphate (APP), through synergistic effects to reduce the amount of commercial flame retardant. The use of FSs conforms to the concept of a circular economy and lowers costs by reducing the consumption of APP. Thermogravimetric analysis (TGA), integral procedural decomposition temperature (IPDT), pyrolysis kinetics, limiting oxygen index (LOI), the Underwriters Laboratories 94 (UL94) flammability test, scanning election microscopy, Raman spectroscopy, and energy-dispersive X-ray spectroscopy were used to determine the thermal properties, flame retardant properties, flame retardant mechanism, char morphology, and composition of the composites. The TGA results indicated that the addition of 40% flame retardant raised the char residue from 16.45 wt.% (pure EP) to 36.07 wt.%; IPDT from 685.6 °C (pure EP) to 1143.1°C; LOI from 21% (pure EP) to 30%; and UL94 classification from fail (pure EP) to V-0. These results suggest an increase in char residue, which indicates better protection of the polymer matrix material. The improvements in IPDT, LOI, and UL94 classification, which indicate greater thermal stability, lower flammability (from flammable to fireproof), and higher flammability rating (from fail to V-0), respectively, suggest that the composite material has favorable thermal properties and is less inflammable.

α-Aminophosphonate Derivatives for Enhanced Flame Retardant Properties in Epoxy Resin

This work demonstrates the introduction of various α-aminophosphonate compounds to an epoxy resin system, thereby improving flame retardance properties. The α-aminophosphonate scaffold allows for covalent incorporation (via the secondary amine) of the compounds into the polymer network. This work explores the synergistic effect of phosphorus and halogens (such as fluorine) to improve flame retardancy. The compounds were all prepared and isolated in analytical purity and in good yield (95%). Epoxy samples were prepared, individually incorporating each compound. Thermogravimetric analysis showed an increased char yield, indicating an improved thermal resistance (with respect to the control sample). Limiting oxygen index for the control polymer was 28.0% ± 0.31% and it increased to 34.6% ± 0.33% for the fluorinated derivative.

Synthesis of a Novel Flame Retardant Containing Phosphorus, Nitrogen, and Silicon and Its Application in Epoxy Resin

A novel flame retardant (TDA) containing phosphorus, nitrogen, and silicon was synthesized successfully via a controllable ring-opening addition reaction between 1,3,5-triglycidyl isocyanurate, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, and 3-aminopropyltriethoxysilane, and TDA was then blended with diglycidyl ether of bisphenol A to prepare flame-retardant epoxy resins (EPs). The chemical structure and components of TDA were confirmed by Fourier transform infrared (FTIR) spectra, ³¹P nuclear magnetic resonance, and X-ray photoelectron spectroscopy. Thermogravimetric analysis results indicated that after the introduction of TDA, cured EP maintained good thermal stability with a minimum initial decomposition temperature of 337.6 °C, and the char yields of a EP/TDA-5 sample significantly increased by 76.2% compared with that of the neat EP thermoset. Additionally, with the addition of 25.0 wt % TDA (1.05 wt % phosphorus loading), the limited oxygen index value of cured EP increased from 22.5% of pure EP to 33.4%, and vertical burning V-0 rating was easily achieved. Meanwhile, after the incorporation of TDA, the total heat release and total smoke production of the EP/TDA-5 sample obviously reduced by 28.9 and 27.7% in the cone calorimeter test, respectively. Flame-retardant performances and flame-retardant mechanisms were further analyzed by scanning electron microscopy, FTIR, energy-dispersive spectrometry, and pyrolysis gas chromatography/mass spectrometry. The results reveal that the synergistic effect of phosphorus, nitrogen, and silicon plays an excellent flame-retardant role in both gaseous and condensed phases. In addition, the mechanical and dynamic mechanical properties of cured EP thermosets are well maintained rather than destroyed. All the results demonstrate that TDA endows epoxy resin with excellent flame retardancy and possesses great promise in the industrial field.

Synthesis of chitosan-based flame retardant and its fire resistance in epoxy resin

A novel flame retardant CCD was synthesized by the condensation between cinnamalde and chitosan, followed by the addition reaction with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO), and CCD was named from the initials of the three raw materials. The intermediate product CC and the target product CCD was then characterized by infrared spectrometry and elemental analysis. The flame retardancy of EP thermosets modified by CCD was dramatically improved. Epoxy resin (EP) with 10wt% CCD passed vertical burning (UL-94) V-0 rating and possessed limited oxygen index (LOI) value of 31.6 %. Cone calorimeter test exhibited the introduction of 3.5g CCD into 35g EP decreased the total heat release by 38.8 % and decreased the total smoke product by 72.0 %. XPS, FTIR, SEM and Raman tests were proceeded to determine the char residue for EP/10 % CCD thermoset, and the results showed that char residue for EP/10 % CCD thermoset possessed dense and compact structure which played a positive effect in blocking the exchange of heat and gas.

Synthesis of multielement phosphazene derivative and the study on flame-retardant properties of epoxy resin

Herein, we have successfully synthesized phosphorus/nitrogen/silicon tri-elements compound phosphazene derivative hexa-[4-( N-(3-(triethoxysilyl)propyl)acetamide)phenoxy]cyclotriphosphazene (HNTPC) from hexachlorotriphosphazenitrile, methyl 4-hydroxybenzoate, and 3-triethoxysilylpropylamine, and it was used as an additive flame retardant in epoxy resin (EP). Then, the thermal stability and flame retardancy of the composite (HNTPC/EP) were tested. Thermogravimetric analysis showed that the presence of HNTPC made EP matrix decompose at a relatively low temperature, thus promoted the formation of a stable coke layer and protected the matrix from fire. Therefore, the amount of carbon residue was markedly increased at 800°C, indicating an outstanding condensed phase flame-retardant effect. Furthermore, various combustion test data manifested that the addition of HNTPC could significantly improve the flame-retardant performance of EP. In addition, the sample could pass the vertical burning tests (UL-94) V-1 grade when the addition amount was 10% and the limiting oxygen index value was 32.6%, the peak heat release rate and total heat release rate decreased by 40.0% and 21.5%, respectively. Besides, the results of scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy also showed that HNTPC can promote the formation of carbon layer and improved the flame-retardant property of EP. Finally, the condensed phase and gas phase synergistic flame-retardant mechanism of HNTPC was proposed.

Thermal Stability and Flame Retardancy Properties of Epoxy Resin Modified with Functionalized Graphene Oxide Containing Phosphorus and Silicon Elements

Phosphorus- and silicon-modified graphene oxide was prepared to improve the thermal stability and flame retardancy properties of epoxy resin. 9,10-Dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and vinyltriethoxysilane (VTES) were successfully grafted onto the surface of graphene oxide (GO) through solvothermal synthesis and hydrolysis-condensation reaction, respectively. Subsequently, the functionalized graphene oxide grafted by DOPO and VTES (DOPO-VTES-GO) was incorporated into the epoxy resin by the solution blending method. The effect of DOPO-VTES-GO on the thermal stability and flame-retardant properties of epoxy resin was systematically studied. Thermogravimetric analysis showed that the thermal stability and char residue yield of DOPO-VTES-GO/epoxy were increased obviously compared with those of pure epoxy resin and DOPO-GO/epoxy. Cone calorimeter test results showed that DOPO-VTES-GO/epoxy had better flame retardancy than pure epoxy resin and DOPO-GO/epoxy on reducing the peak of heat release rate, total heat release, and total smoke production. Furthermore, the char residue after the cone calorimeter tests was investigated by scanning electron microscopy-energy-dispersive X-ray spectrometry, Raman spectroscopy, and Fourier transform infrared measurements. These results demonstrated that the DOPO-VTES-GO can enhance the graphitization degree of char residues and promote the formation of the thermally stable char. In addition, the mechanism of flame retardancy was proposed, and DOPO-VTES-GO exerts the synergistic effect mainly by means of catalytic charring in the condensed phase and capturing hydroxyl or hydrogen radicals from thermal decomposition of epoxy resin in the gas phase. This work provides novel insights into the preparation of phosphorus-silicon-graphene oxide ternary synergistic flame retardants for thermosetting polymer materials.