Improving fire safety of epoxy filled with graphene hybrid incorporated with zeolitic imidazolate framework/layered double hydroxide

Composites Filled with Metal Organic Frameworks and Their Derivatives: Recent Developments in Flame Retardants

Polymer matrix is vulnerable to fire hazards and needs to add flame retardants to enhance its performance and make its application scenarios more extensive. At this stage, it is more necessary to add multiple flame-retardant elements and build a multi-component synergistic system. Metal organic frameworks (MOFs) have been studied for nearly three decades since their introduction. MOFs are known for their structural advantages but have only been applied to flame-retardant polymers for a relatively short period of time. In this paper, we review the development of MOFs utilized as flame retardants and analyze the flame-retardant mechanisms in the gas phase and condensed phase from the original MOF materials, modified MOF composites, and MOF-derived composites as flame retardants, respectively. The effects of carbon-based materials, phosphorus-based materials, nitrogen-based materials, and biomass on the flame-retardant properties of polymers are discussed in the context of MOFs. The construction of MOF multi-structured flame retardants is also introduced, and a variety of MOF-based flame retardants with different morphologies are shown to broaden the ideas for subsequent research.

Organic-Inorganic Modification of Magnesium Borate Rod by Layered Double Hydroxide and 3-Aminopropyltriethoxysilane and Its Effect on the Properties of Epoxy Resin

To alleviate the safety hazards associated with the use of epoxy resin (EP), a multifunctional filler was designed. This study firstly combines the superior mechanical properties of magnesium borate rods (MBR) with the excellent smoke suppression and flame-retardant characteristics of layered double hydroxide (LDH). H₂PO₄^- intercalated LDH (LDHP) was coated on the MBR surface to obtain inorganic composite particles MBR@LDHP. Subsequently, MBR@LDHP was modified with 3-aminopropyltriethoxysilane (APES) to obtain organic-inorganic composite particles MBR@LDHP-APES. Eventually, the hybrid particles were added to EP to prepare the composite materials. Thereafter, the morphology, composition, and structure of MBR@LDHP-APES were characterized utilizing scanning electron microscopy (SEM), Fourier transform infrared (FTIR), and X-ray diffraction (XRD). The results indicated the successful preparation of MBR@LDHP-APES, after which we investigated the effects of MBR@LDHP-APES on the smoke suppression, flame retardancy, and mechanical characteristics of EP. As observed, the EP composites containing 7.5 wt% MBR@LDHP-APES exhibited superior smoke suppression and flame retardancy abilities. The limiting oxygen index reached 33.5%, which is 36.73% greater than pure EP, and the lowest values of total heat and smoke release were observed for the composite materials. In addition, the mechanical properties test revealed that MBR@LDHP-APES considerably enhanced the tensile strength as well as the flexural strength of the composites. Furthermore, mechanistic studies suggested that the barrier effect of MBR, endothermic decomposition of LDHP, and the synergistic effect of LDHP and APES contributed essentially to the smoke suppression and flame-retardant properties of the material. The findings of this research point to a potential method for enhancing the EP's ability to suppress smoke and flames while enhancing its mechanical properties.

Core-Shell ZIF67@ZIF8 Modified with Phytic Acid as an Effective Flame Retardant for Improving the Fire Safety of Epoxy Resins

Despite many important industrial applications, epoxy resin (EP) suffers from high flammability and toxicity emission, extremely hampering their applications. To circumvent the problem, core-shell structured ZIF67@ZIF8 is successfully synthesized and further functionalized with phytic acid (PA) to obtain PA-ZIF67@ZIF8 hybrids. Then, it is used as an efficient flame retardant to reduce the fire risk of EP. The fire test results show a significant reduction in heat and smoke production. Compared with EP, incorporating 5.0 wt % PA-ZIF67@ZIF8 into EP, the peak heat release rate, total heat release, and peak carbon monoxide production are dramatically reduced by 42.2, 33.0, and 41.5%, respectively. Moreover, the EP/PA-ZIF67@ZIF8 composites achieve the UL-94 V-0 rating and the limiting oxygen index increases by 29.3%. These superior fire safety properties are mainly attributed to the excellent dispersion and the catalytic effect of metal oxide and phosphorus-containing compounds. This work provides an efficient strategy for preparing a promising ZIF-based flame retardant for enhancing flame retardancy and smoke toxicity suppression of EP.

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.

Hydrothermal Synthesis of Lanthanum-Doped MgAl-Layered Double Hydroxide/Graphene Oxide Hybrid and Its Application as Flame Retardant for Thermoplastic Polyurethane

A novel lanthanum-doped MgAl-layered double hydroxide/graphene oxide hybrid (La LDH/GO) with a La3+/Al3+ molar ratio of 0.05 was successfully synthesized by the hydrothermal method. The structure and morphology of as-prepared samples were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and transmission electron microscopy (TEM). Then, La LDH/GO was added into thermoplastic polyurethane (TPU) to investigate its effect on flame retardancy, smoke suppression, and thermal stability of TPU composites. The cone calorimeter test (CCT) results indicated that the peak heat release rate (PHRR) and peak smoke production rate (PSPR) values of TPU with La LDH/GO decreased by 33.1% and 51% compared with neat TPU, respectively. Therefore, La LDH/GO can play a good role in flame retardancy and smoke suppression of TPU matrix during combustion. In the meantime, La LDH/GO could improve the char yield of TPU composites, which is attributed to the interaction between the physical barrier effect of GO and the catalytic effect of 0.05 La LDH.

Design of Hierarchical NiCo-LDH@PZS Hollow Dodecahedron Architecture and Application in High-Performance Epoxy Resin with Excellent Fire Safety

Developing advanced performance epoxy (EP) resin with low flammability and light smoke has been an increasing focus of its research. Especially, it is crucial to reduce the emission of smoke and toxic gases generated during the burning of EP, so that it meets the green and safe industrial requirement. Therefore, a 3D NiCo-LDH@PZS hollow dodecahedral structure was designed and synthesized by using the ZIF-67 as both the precursor and an in situ sacrificial template and the amino group-containing polyphosphazene (PZS) as interfacial compatibilizer and flame retardant cooperative. The release behaviors of heat, smoke, and poisonous gases were carefully investigated. More precisely, the EP/NiCo-LDH@PZS4.0 is endowed with a decrease of 30.9% and 11.2% of the peak heat release rate and the total heat release, respectively. The emissions of smoke and poisonous gases including nitric oxides, aromatic compounds, carbonyl compounds, oxycarbide, and hydrocarbons are much less as well. Especially, the maximum release concentrations of HCN of EP/NiCo-LDH4.0 are reduced by 87.8%. With regard to styrene, methane, and ethane, the maximum release concentrations of EP/NiCo-LDH@PZS4.0 are reduced by 85.9%, 90.6%, and 93.1%, respectively. The total yield of CO and CO₂ and the consumption of O₂ of EP/NiCo-LDH@PZS4.0 are also reduced by 64.5%, 32.4%, and 33.6%. The fractional effective dose, an index of toxicity smoke, of EP/NiCo-LDH@PZS4.0 is reduced by 20.4%. The DMA tests were performed to study the mechanical properties of EP composites, and the storage modulus and Tg of EP composites are increased with the incorporation of NiCo-LDH@PZS. The possible mechanism of flame retardant was proposed based on the analysis of the condensed and gas phases of EP composites.