Synthesis, Characterization, and Utilization of a Novel Phosphorus/Nitrogen-Containing Flame Retardant

Synthesis of renewable furan-based phosphate and the superior flame retardancy in biodegradable polylactide

Currently, it has long been considered a challenge to provide sustainable additives for polylactide (PLA) in green way to endow it excellent comprehensive properties. Given the flammability and unsatisfactory crystallization performance of PLA, a furan-based phosphate furfurylamine trimethylphosphate (FATMP) was synthesized from 2-furfurylamine and amino trimethylphosphonic acid by a simple hydration reaction, and the PLA/FATMP composites were prepared by melting blending process. The tensile performance, crystallization behaviors, flame retardancy, and flame-retardant mechanism received special attention. Results showed that the incorporation of only 3 wt% FATMP could indeed increase the LOI value of PLA from 19.8 to 27.3 %, and simultaneously acquired V-0 rating in the vertical burning test owing to the favorable synergistic effect between the vapor phase and the condensed phase. Additionally, the half-crystallization time of PLA was decreased from 12.4 to 5.1 mins with the addition of FATMP, which acted as a nucleating agent. More appealingly, the tensile performance of PLA/FATMP composites was also well maintained. In general, the PLA/FATMP composites we proposed could be promising candidates in application fields where favorable flame retardancy and crystallization ability are required.

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.

Thermal Stabilities and Flame Retardancy of Polyamide 66 Prepared by In Situ Loading of Amino-Functionalized Polyphosphazene Microspheres

The flame-retardant polyamide 66 composites (FR-PA66) were prepared by in situ loading of amino-functionalized polyphosphazene microspheres (HCNP), which were synthesized in the laboratory and confirmed by a Fourier transform infrared spectrometer (FTIR), scanning electron microscope (SEM), and transmission electron microscope (TEM). The thermal stabilities and flame retardancy of FR-PA66 were measured using thermogravimetric analysis (TGA), a thermogravimetric infrared instrument (TG-IR), the limiting oxygen index (LOI), the horizontal and vertical combustion method (UL-94), and a cone calorimeter. The results illustrate that the volatile matter of FR-PA66 mainly contains carbon dioxide, methane₄, and water vapor under heating, accompanied by the char residue raising to 14.1 wt% at 600 °C and the value of the LOI and UL-94 rating reaching 30% and V-0, respectively. Moreover, the addition of HCNP decreases the peak of the heat release rate (pHRR), total heat release (THR), mass loss (ML), and total smoke release (TSR) of FR-PA66 to 373.7 kW/m², 106.7 MJ/m², 92.5 wt%, and 944.8 m²/m², respectively, verifying a significant improvement in the flame retardancy of PA66.

Halogen-free layered double hydroxide-cyclotriphosphazene carboxylate flame retardants: effects of cyclotriphosphazene di, tetra and hexacarboxylate intercalation on layered double hydroxides against the combustible epoxy resin coated on wood substrates

The development of halogen-free flame retardants as environmentally friendly and renewable materials for heat and fire-resistant applications in the field of electronics is important to ensure safety measures. In this regard, we have proposed a simple and halogen-free strategy for the synthesis of flame retardant LDH-PN materials to decrease the fire hazards of epoxy resin (EP), via a co-precipitation reaction between Mg(NO3)2 and Al(NO3)3 and the subsequent incorporation of different cyclotriphosphazene (PN) carboxylate anions. The cyclotriphosphazene-based di, tetra and hexacarboxylate-intercalated layered double hydroxides are designated as LDH-PN-DC, LDH-PN-TC and LDH-PN-HC, respectively. Furthermore, the intercalation of cyclotriphosphazene carboxylate anions into the LDH layers was confirmed by PXRD, FT-IR, TGA, solid-state 31P NMR, nitrogen adsorption and desorption analysis (BET), HR-SEM and XPS. Evaluation of the flame retardant (vertical burning test and limiting oxygen index) properties was demonstrated by formulating the LDH-PN materials with epoxy resin (EP) in different ratios coated on wood substrates to achieve the desired behaviour of the EP/LDH-PN composites. Structure-property analysis reveals that EP/LDH-PN-TC-20 wt% and EP/LDH-PN-HC-20 wt% achieved a V 0 rating in the UL-94 V test and achieved higher LOI values (27.7 vol% for EP/LDH-PN-TC-20 wt% and 29 vol% for EP/LDH-PN-HC-20 wt%) compared to the epoxy-coated wood substrate (23.2 vol%), whereas EP/LDH-PN-DC failed in the vertical burning test for various weight percentages of LDH-PN-DC from 5 wt% to 20 wt% in the composites, with a lower LOI value of 22.1 vol%. Excellent flame retardancy was observed for EP/LDH-PN-TC and EP/LDH-PN-HC due to the presence of more binding sites of carboxylate anions in the LDH layers and less or no spiro groups in cyclotriphosphazene compared to that in EP/LDH-PN-DC. In addition, the synergistic flame retardant effect of the combination of LDH and cyclotriphosphazene on the epoxy resin composites remains very effective in creating a non-volatile protective film on the surface of the wood substrate to shelter it from air, absorb the heat and increase the ignition time, which prevents the supply of oxygen during the combustion process. The results of this study show that the proposed strategy for designing flame-retardant properties represents the state-of-the-art, competent coating of inorganic materials for the protection and functionalization of wood substrates.

A Review on Synthesis, Structural, Flame Retardancy and Dielectric Properties of Hexasubstituted Cyclotriphosphazene

Hexachlorocyclotriphosphazene is a ring compound consisting of an alternating phosphorus and nitrogen atom with two chlorine substituents attached to the phosphorus atom. The six chlorine atoms attached to this cyclo compound can be substituted with any different nucleophile that leads to changes in different chemical and physical properties. The major topics that were investigated in this research are the flame retardancy and dielectric properties of cyclotriphosphazene compounds. Cyclotriphosphazene compounds have high potential to act as a flame retardant, and this compound consists of two active elements attributed to its high flame-retardant character. This compound also demonstrated good ability as a flame retardant due to its low toxicity and less smoke produced. In addition, cyclotriphosphazene compounds were also investigated for their dielectric properties. Cyclotriphosphazene has high potential in the electrical field since it has dielectric properties that can be widely studied in the investigation of any potential application. This review presented literature studies focused on recent research development and studies in the field of cyclotriphosphazene that focused on synthesis, structural, flame retardancy, and dielectric properties of hexachlorocyclotriphosphazene compounds.

Synthesis of Trispirocyclotriphosphazenes with Oxaphosphorine Rings and Their Crystal and Molecular Structures

Novel trispirocyclotriphosphazenes with oxaphosphorine rings (DOP-PZs) were successfully synthesized by an Appel reaction with phosphoramide, which was prepared from ammonia and 10-chloro-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide derived from 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide, generally abbreviated as DOPO. The resulting DOP-PZs were characterized by ¹H, ¹³C{¹H}, and ³¹P{¹H} nuclear magnetic resonance spectroscopy and time-of-flight mass spectrometry and shown to consist of cis-trans isomers. Moreover, the crystal and molecular structures of the DOP-PZs were determined by X-ray diffraction; cis- and trans-DOP-PZs (C₃₆H₂₄N₃O₃P₃, M = 639.49 g/mol) were refined to final R₁ values of 0.0260 and 0.0463, respectively, with the SHELXL refinement package using least-squares minimization. The crystal of cis-DOP-PZ is trigonal in space group R3c and the following cell constants: a = 19.5984(5) Å, c = 13.2754(4) Å, V = 4415.9(3) ų, Z = 6, and Flack parameter = 0.038(8). In contrast, trans-DOP-PZ is monoclinic in space group P2₁/c and the following cell constants: a = 9.98647(18) Å, b = 24.1737(4) Å, c = 12.8472(2) Å, β = 112.649(8)°, V = 2862.26(18) ų, and Z = 4. The molecular structures of these DOP-PZs were compared with those of other trispirocyclotriphosphazenes. In addition, the DOP-PZs showed high thermal stability up to 400 °C, with dielectric constants of 2.76-2.77 and dissipation factors of 0.0017-0.0031 at 10 GHz.

Flexible, Self-Healing, and Fire-Resistant Polymer Electrolytes Fabricated via Photopolymerization for All-Solid-State Lithium Metal Batteries

The cyclophosphazene-based self-healing polymer electrolytes (CPSHPE) is designed and fabricated via the copolymerization of hexa(4-ethyl acrylate phenoxy) cyclotriphosphazene (HCP), (2-(3-(6-methyl-4-oxo-1,4-dihydropyrimidin-2-yl)ureido)ethyl methacrylate) (UPyMA), and poly(ethylene glycol) methyl ether methacrylate (PEGMA) under UV irradiation. The cross-linking structure formed by HCP could effectively enhance the mechanical strength of the polymer electrolyte, and the cyclotriphosphazene as the core is able to improve the flame-retardant properties. Benefiting from the phenyl groups in HCP and the cross-linking structure, the CPSHPE shows high thermal stability (up to 300 °C). On the other hand, the supramolecular network fabricated by the dynamic ureido-pyrimidinone (UPy) dimers endows the polymer electrolyte with good self-healing capability and is expected to improve the reliability of polymer lithium batteries. Moreover, the cells were fabricated with LiFePO₄ (LFP), CPSHPE, and Li anodes show good reversible specific capacity. The CPSHPE could be a promising candidate as the multifunctional polymer electrolyte to improve the safety performance of lithium metal batteries.

The preparation of starch derivatives reacted with urea-phosphoric acid and effects on fire performance of expandable polystyrene foams

A novel bio-based flame retardant, ammonium phosphate starch carbamates (APSC) was synthesized from starch, phosphoric acid and urea to achieve better thermal stability and char formation. Flame retardant expandable polystyrene foams (EPS) were prepared by coating method with APSC. The fire resistance evaluation of EPS composites by limiting oxygen index (LOI), vertical burning (UL-94) and cone calorimeter tests indicated that the addition of 47 wt.% of APSC enhanced the LOI from 17.6%-35.2%, with V-0 rating in UL-94, and decreased the peak heat release rate from 666 kW/m² to 316 kW/m². Moreover, the total smoke production was also sharply decreased from 70 m² of pure EPS to 17 m². By the presence of APSC, the substantial char formation prevented the heat and oxygen transfer, and interrupted the releasing of flammable products, thus protecting the EPS foam from burning.

Study of the epoxy/amine equivalent ratio on thermal properties, cryogenic mechanical properties, and liquid oxygen compatibility of the bisphenol A epoxy resin containing phosphorus

A liquid oxygen-compatible epoxy resin is successfully prepared by changing the epoxy/amine equivalent ratio (SR) of a phosphorus-containing epoxy resin. The liquid oxygen impact test results showed that the modified resin was compatible with liquid oxygen only when the SR was 0.8. The mechanical properties at 90 K showed that the strain energy and impact toughness reached the maximum when the SR was 0.8, which suggested that the reduced rigidity might be beneficial to improve the liquid oxygen compatibility of the polymer. The thermomechanical and thermal results showed that the cross-linking density and thermal stability was proportional to SR. Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy analysis showed that the P=O group in the resin decomposed into phosphoric oxidative solids and P–N intermediates to inhibit the resin from decomposing and contacting with liquid oxygen during impact. Overall, this study provides a new idea for the design of liquid oxygen-compatible epoxy resin.

Synergistic Flame Retardant Effect of an Intumescent Flame Retardant Containing Boron and Magnesium Hydroxide

In this study, to develop an organic/inorganic synergistic flame retardant and to reduce the dosage and cost of flame retardants, organic/inorganic synergistic flame retardants, hexakis(4-boronic acid-phenoxy)-cyclophosphazene (CP-6B), and magnesium hydroxide (MH) were chosen. The flame retardant properties of CP-6B/MH in epoxy resin (EP) were discussed. EP/CP-6B/MH had better flame retardancy and heat resistance compared with EP/CP-6B and EP/MH. A limiting oxygen index of EP/3.0%CP-6B/0.5%MH of 31.9% was achieved, and vertical burning V-0 rating was achieved. Compared with EP, the cone calorimeter dates of EP/CP-6B/MH decreased. CP-6B/MH inhibited combustion and did little to damage mechanical properties. Besides, the flame retardant mechanism was studied by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and pyrolysis-gas chromatography-mass spectrometry. CP-6B/MH exerted good synergistic effects.

Two high-efficient DOPO-based phosphonamidate flame retardants for transparent epoxy resin

In this work, two kinds of 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO)-based phosphonamidate flame retardants named 4,4′-diamino-diphenyl methane (DDM)-DOPO and morpholine (MPL)-DOPO are successfully synthesized by the classic Atherton–Todd reaction, and their chemical structures are characterized by Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR). Epoxy composites containing DDM-DOPO or MPL-DOPO are prepared, which still retain relatively high transparency compared with pure epoxy resin (EP). Its fire behavior is studied in terms of vertical burning test (UL-94), limiting oxygen index (LOI), and cone calorimeter tests. The LOI values can achieve 30% and UL-94 test past V-0 rating for DDM-DOPO-modified epoxy composites with 0.25 wt% phosphorus content. The cone calorimeter results indicate that the modified epoxy composites can release less heat and smoke compared with pure EP. Also, thermal degradation behavior and flame-retardant mechanism of epoxy composites are investigated by thermogravimetric analysis and thermogravimetric analysis/infrared spectrometry. The results indicate that two DOPO-based phosphonamidates show a significant gaseous-phase effect on reducing the release of combustible volatiles.

DOPO-Modified Two-Dimensional Co-Based Metal-Organic Framework: Preparation and Application for Enhancing Fire Safety of Poly(lactic acid)

Co-based metal-organic framework (Co-MOF) nanosheets were successfully synthesized by the organic ligands with Schiff base structure. The laminated structure gives Co-MOF nanosheets a great advantage in the application in the flame retardant field. Meanwhile, -C═N- from Schiff base potentially provides active sites for further modification. In this work, 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) was used to modify Co-MOF (DOPO@Co-MOF) to further enhance its flame retardant efficiency. It is attractive that DOPO has a synergistic effect with Co-MOF on improving fire safety of poly(lactic acid) (PLA). The obvious decrease in the values of peak heat release (27%), peak smoke production (56%), and total CO yield (20%) confirmed the enhanced fire safety of PLA composites. The possible flame retardant mechanism was proposed based on characterization results. Moreover, the addition of DOPO@Co-MOF had a positive influence on the mechanical performance, including tensile properties and impact resistance. This work designed and synthesized two-dimensional MOFs with active groups. As-prepared Co-MOF with expected structure shows a novel direction of preparing MOFs for flame retardant application.

Synthesis of a phosphorus–nitrogen-containing flame retardant and its application in epoxy resin

In this article, a phosphorus–nitrogen-containing flame retardant (DOPO-T) was successfully synthesized by nucleophilic substitution reaction between 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) and cyanuric chloride. The chemical structure of DOPO-T was characterized by Fourier transform infrared spectroscopy, proton nuclear magnetic resonance (NMR) and phosphorous-31 NMR, and elemental analysis. DOPO-T was then blended with diglycidyl ether of bisphenol-A to prepare flame-retardant epoxy resins. Thermal properties, flame retardancy, and combustion behavior of the cured epoxy resins were evaluated by differential scanning calorimetry, thermogravimetric analysis, limited oxygen index (LOI) measurement, UL94 vertical burning test, and cone calorimeter test. The results indicated that the glass transition temperature ( Tg) and temperature at 5% weight loss of epoxy resin (EP)/DOPO-T thermosets were gradually decreased with the increasing content of DOPO-T. DOPO-T catalyzed the decomposition of EP matrix in advance. The flame-retardant performance of EP thermosets was significantly enhanced with the addition of DOPO-T. EP/DOPO-T-0.9 sample had an LOI value of 36.2% and achieved UL94 V-1 rating. In addition, the average of heat release rate, peak of heat release rate, average of effective heat of combustion, and total heat release (THR) of EP/DOPO-T-0.9 sample were decreased by 32%, 48%, 23%, and 31%, respectively, compared with the neat EP sample. Impressively, EP/DOPO-T thermosets acquired excellent flame retardancy under low loading of flame retardant.