Synergistic Effect of Multifunctional Layered Double Hydroxide-Based Hybrids and Modified Phosphagen with an Active Amino Group for Enhancing the Smoke Suppression and Flame Retardancy of Epoxy

To improve the fire hazard of epoxy resin (EP), phosphomolybdate (PMoA), as a classical Keggin cluster, was successfully intercalated into Mg, Al, and Zn layered double hydrotalcite (LDH) by the reconstruction method, and it was denoted as MgAlZn-LDH-PMoA. The structure and morphology of MgAlZn-LDH-PMoA were characterized by X-ray diffraction and Fourier transform infrared spectroscopy. Subsequently, hexa(4-aminophenoxy)cyclotriphosphazene (HACP) was prepared and characterized as a high-performance organic flame retardant, which is rich in flame elements phosphorus and nitrogen. The synergistic effects of MgAlZn-LDH-PMoA and HACP on the fire safety of EP composites loaded with different amounts of flame retardant hybrids were studied in detail. Thermogravimetric analysis showed that the char residue of these EP composites increased significantly. Compared with the EP matrix filled with only MgAlZn-LDH-PMoA or HACP, the incorporation of MgAlZn-LDH-PMoA and HACP had a synergistic effect on promoting char formation of EP composites. Particularly, the char yield of EP7 is as high as 29.0%. Furthermore, the synergistic effects of incorporation of MgAlZn-LDH-PMoA with HACP were investigated using the cone calorimeter combustion tests. The results showed that the total heat release and peak heat release rate of the EP composites remarkably declined by 35.2 and 50.9%, respectively, with a loading of 7 wt % hybrid flame retardant. Moreover, the hybrid flame retardants also showed an obvious inhibitory effect on the total smoke production and the release of toxic CO gas. The detailed analysis of the residual char indicated that the main mechanism for improving the flame retardancy and smoke suppression performance is due to both the catalytic carbonization of MgAlZn-LDH-PMoA and phosphoric acid compounds and physical barrier function of the char layer. In addition, the molybdenum oxides produced from [PMo₁₂O₄₀]^3- during combustion can not only increase the yield and compactness of the char layer but also reduce the release of CO through a redox reaction, which has important application value to reduce the fire hazard.

Synthesis of Layered Double Hydroxides with Phosphate Tailings and Its Effect on Flame Retardancy of Epoxy Resin

In this work, phosphate tailings (PTs) were used as raw materials for the preparation of Ca-Mg-Al layered double hydroxides (LDHs-1) and Ca-Mg-Al-Fe layered double hydroxides (LDHs-2) by co-precipitation method. The as-prepared samples were characterized by FT-IR, SEM, XRD, and XPS and applied as a flame retardant to improve the fire safety of epoxy resin (EP). The results showed that both LDHs-1 and LDHs-2 exhibited layered structure and high crystallinity. Compared with neat EP, the value of limiting oxygen index (LOI) increased from 25.8 to 29.3 and 29.9 with 8 wt% content of LDHs-1 and LDHs-2, respectively. The flame retardant properties of the composite material were characterized by cone calorimeter (CC), and the results showed that the peak value of the smoke production rate (SPR) decreased more than 45% and 74%, total smoke production (TSP) reduced nearly 64% and 85% with the addition of LDHs-1 and LDHs-2. Meanwhile, the value of the total heat release (THR) reduced more than 28% and 63%. The conversion from LDHs to layered double oxide (LDO) might be conducive to the fire safety of EP. Moreover, the transformation of Fe-OH to Fe-O could promote the early cross-linking of polymer. In summary, LDHs-2 could significantly improve the carbonization process of EP and suppress the smoke released during the combustion process.

Synthesis of a highly efficient flame retardant containing triazine and pentaerythritol phosphate groups and its intumescent flame retardancy on epoxy resin

A novel phosphorus-nitrogen-containing flame retardant (DOPT) has been successfully synthesized via the substitution reaction of cyanuric chloride, pentaerythritol phosphate and 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide. The chemical structure of DOPT was confirmed by 1H, 31P and 13C nuclear magnetic resonance, Fourier transform infrared spectroscopy and elemental analysis. Then, flame retardants were added to epoxy resin to prepare epoxy resin composites by pouring method. Thermal properties, flame retardancy, and combustion behavior of epoxy resin composites were evaluated by thermogravimetric analysis, vertical burning, limiting oxygen index and cone calorimeter test. Thermogravimetric analysis test showed that the carbon residue rate of DOPT at 800°C reached 52.53%, which indicated that the introduction of high-efficiency char-forming agent triazine and pentaerythritol phosphate groups could significantly improve its char-forming performance and thermal stability. The epoxy resin composite achieved vertical burning V-0 grade and the limiting oxygen index value reached 35.5% when 7 wt% DOPT was incorporated. Furthermore, the cone calorimeter test results manifested that the addition of DOPT stimulated degradation of the epoxy resin matrix during the combustion process and accelerated the formation of an expanded and dense carbon layer. Additionally, the incombustible gas produced during the decomposition of DOPT had played a flame-retardant effect in the gas phase. Hence, compared with neat epoxy resin, the total heat release and total smoke production of the EP-7 wt% DOPT composite decreased by 14.0% and 25.3%, respectively. Moreover, owing to the excellent compatibility and the strong interface effect between DOPT and epoxy resin, the addition of DOPT also enhanced the mechanical and fire resistance properties of the epoxy resin composite. Therefore, it is proposed that DOPT could be exploited as an economical and high-efficiency flame retardant, and it has considerable prospects in flame retardant epoxy resin composites.

Preparation of a Novel Organic Phosphonic Acid Intercalated Phosphate Tailings Based Hydrotalcite and Its Application in Enhancing Fire Safety for Epoxy Resin

Phosphate tailings (PTs) are solid waste, which is produced by phosphate flotation. In this work, PTs were used as raw materials for the preparation of diethylenetriamine pentamethronic acid (DTPMP) intercalated trimetal (Ca-Mg-Al) layered double hydroxides (TM-DTPMP LDHs) by co-precipitation method. TM-DTPMP LDHs were characterized by X-ray diffraction, fourier-transform infrared spectroscopy, scanning electron microscopy, differential thermal gravimetric analysis, X-ray photoelectron spectroscopy and applied as a flame retardant to improve the fire safety of epoxy resin (EP). The results showed that the composite materials exhibited obvious layered structure. After intercalation, layer spacing increased from 0.783 to 1.78 Å. When the amount of TM-DTPMP LDH in EP was 8%, the limitted oxygen index of the composite material increased from the original 19.2% to 30.2%. In addition, Cone calorimeter (CC) and Raman spectrum results indicated that with the addition of TM-DTPMP LDHs, the value of heat release rate peak (pHRR) and total heat release (THR) were reduced by more than 43% and 60%, while the value of smoke formation rate (pSPR) and the total smoke production (TSP) decreased nearly 64% and 83%, respectively. The significant reduction in the release of combustion heat and harmful smoke during EP combustion may be attributed to the synergistic flame-retardant effect between hydrotalcite and DTPMP. This work exhibited great potential for the green recycling of PTs and the enhancement of the fire safety of EP.

Preparation of a novel functionalized magnesium-based curing agent as an intrinsic flame retardant for epoxy resin

In this study, a novel organic-inorganic hybrid flame retardant 10-(1,4-dicarboxylic acid magnesium salt)-9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DMMH) was synthesized via neutralization and addition reaction of maleic acid, magnesium hydroxide and 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and subsequently used in an intrinsic flame retardant epoxy resin. The chemical structure and morphology of DMMH were analyzed by Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Further, the prepared DMMH was combined with ammonium polyphosphate (APP) to form an intumescent flame retardant system. The thermal stability and flame retardance were evaluated by thermogravimetric analysis (TG), UL-94 vertical burning test, limiting oxygen index (LOI) and cone calorimetry. It was observed that the addition of 1.7% DMMH and 5.3% APP led EP-7 to acquire UL-94 V-0 rating, with the limiting oxygen index of 26.0%. As compared with pure EP, the peak heat release rate, total heat release, smoke production rate and total smoke production of the sample was noted to decrease by 54.5%, 35.1%, 43.6% and 38.1%, respectively. In addition, the introduction of DMMH did not negatively impact the mechanical properties of the epoxy resin.

Rheological and Electrical Study of a Composite Material Based on an Epoxy Polymer Containing Cyclotriphosphazene

In this work, we have studied, formulated, prepared, and characterized the rheological and electrical behavior of a composite material based on an epoxy resin Diglycidyl Ether of Bisphenol A (DGEBA) reinforced with hexaglycidyl cyclotriphosphazene (HGCP). The epoxy system was cured with 4,4’-methylene dianiline (MDA). DGEBA-HGCP-MDA epoxy composite materials with reinforced HGCP which varied from 5% to 10% by weight were prepared by mixing in the molten state. The morphology was evaluated by SEM. The rheological behavior was studied using small deformation rheology. The electrical characterization was carried out with a frequency variation range from 1 Hz to 100 KHz at room temperature. These measurements revealed that the rheological and electrical behaviors strongly depend on the quantity of HGCP in the DGEBA matrix. The linear viscoelastic properties study reveals that the modulus of elasticity G’ is dependent on the amount of HGCP present in the epoxy resin DGEBA. The capacitance-frequency measurements suggest a distribution of localized states in the band gap of the blends.

Study on the Synergetic Fire-Retardant Effect of Nano-Sb₂O₃ in PBT Matrix

Nano-Sb₂O₃ has excellent synergistic flame-retardant effects. It can effectively improve the comprehensive physical and mechanical properties of composites, reduce the use of flame retardants, save resources, and protect the environment. In this work, nanocomposites specimens were prepared by the melt-blending method. The thermal stability, mechanical properties, and flame retardancy of a nano-Sb₂O₃–brominated epoxy resin (BEO)–poly(butylene terephthalate) (PBT) composite were analyzed, using TGA and differential scanning calorimetry (DSC), coupled with EDX analysis, tensile testing, cone calorimeter tests, as well as scanning electron microscopy (SEM) and flammability tests (limiting oxygen index (LOI), UL94). SEM observations showed that the nano-Sb₂O₃ particles were homogeneously distributed within the PBT matrix, and the thermal stability of PBT was improved. Moreover, the degree of crystallinity and the tensile strength were improved, as a result of the superior dispersion and interfacial interactions between nano-Sb₂O₃ and PBT. At the same time, the limiting oxygen index and flame-retardant grade were increased as the nano-Sb₂O₃ content increased. The results from the cone calorimeter test showed that the peak heat release rate (PHRR), total heat release rate (THR), peak carbon dioxide production (PCO₂P), and peak carbon monoxide production (PCOP) of the nanocomposites were obviously reduced, compared to those of the neat PBT matrix. Meanwhile, the SEM–energy dispersive spectrometry (EDX) analysis of the residues indicated that a higher amount of C element was left, thus the charring layer of the nanocomposites was compact. This showed that nano-Sb₂O₃ could promote the degradation and charring of the PBT matrix, improving thermal stability and flame retardation.