Zeolitic imidazolate frameworks-8 modified graphene as a green flame retardant for reducing the fire risk of epoxy resin

Developing flame retardant, smoke suppression and self-healing polyvinyl alcohol composites by dynamic reversible cross-linked chitosan-based macromolecule

With the increasing threat of white pollution to the public health and ecosystem, functional materials driven by green and sustainable biological macromolecule are attracting considerable attention. Inspired by the double-helix structure of DNA, a P-B-N ternary synergistic chitosan-based macromolecule (PBCS) was constructed to prepare flame retardant, smoke suppression and self-healing polyvinyl alcohol composite (PVA@PBCS) via dynamic reversible interactions. The limiting oxygen index value of PVA@PBCS increased from 19.6 % to 28.7 %, whereas the peak heat release rate and total heat release decreased by 47.04 % and 43.37 %, respectively. Besides, the peak smoke production rate and total smoke production of PVA@PBCS also decreased by 45.31 % and 54.98 %. With the presence of borate ester-based covalent and multiple hydrogen bonds, the tensile strength and elongation at break of PVA@PBCS increased by 19.50 % and 16.85 % compared to the control sample, and the healing efficiency for tensile strength and elongation at break was as high as 93.86 % and 90.57 %, respectively. This work developed an eco-friendly and effective scenario for fabricating flame retardant and smoke suppression PVA materials, stimulating the substantial potential of chitosan-based biomacromolecule and dynamic reversible cross-linked tactics in self-healing field.

Designing a MOF-functionalized Nanofibrous Aerogel via Vapor-Phase Synthesis

Designing 3D mechanically robust and high-surface-area substrates for uniform and high-density deposition of metal-organic frameworks (MOFs) provide a promising strategy to enhance surface accessibility and application of these highly functional materials. Nanofibrous aerogel (NFA) with its highly porous self-supported structure composed of interconnected nanofibrous network offers an ideal platform in this regard. Herein, a facile one-pot strategy is introduced, which utilizes direct deposition of MOF on the nanofibrous surface of the NFAs. NFAs are synthesized using electrospun polyacrylonitrile/polyvinylpyrrolidone (PAN/PVP) polymer nanofibers containing zinc acetate (Zn(Ac)2), which are subjected to freeze drying and thermal treatment. The latter converts Zn(Ac)2 to zinc oxide (ZnO), providing the sites for MOF growth while also adding mechanical integrity to the NFAs through cyclization of the PAN. Exposure of the NFA to the vapor-phase of organic ligand, 2-methylimidazole (2-MeIm) enables in situ growth of zeolitic imidazolate framework-8 (ZIF-8) MOF on the NFA. ZIF-8 loading on the NFAs is further improved by more than tenfold by synthesizing ZnO nanorods/protrusions on the nanofibers, which enables more sites for MOF growth. These findings underscore a significant advancement in designing MOF-based hybrid aerogels, offering a streamlined approach for their use in diverse applications, from catalysis to sensing and water purification.

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.

Phosphorylated Metal-Organic Framework for Reducing Fire Hazards of Poly(Methyl Methacrylate)

The low fire safety performance (flame retardant and antistatic properties) of poly(methyl methacrylate) (PMMA) has severely limited practical applications. Here, a phosphorylated Zn-based metal-organic framework (ZIF-8-P) is employed as an effective flame retardant and antistatic agent to reduce the fire risk of PMMA. Encouragingly, the as-prepared PMMA/ZIF-8-P composite demonstrated not merely better mechanical properties (e.g., a rise of ca. 136.9% and 175.0% in the reduced modulus and hardness; a higher storage modulus), but also efficient fire safety properties (e.g., lower surface resistance; a decrease of ca. 73.1% in the peak heat release rate; a lower amount of total pyrolysis products), surpassing those of pure PMMA and a PMMA/ZIF-8 composite without phytic acid modification. Mechanism analysis is conducted to reveal the critical role of catalytic charring, char reinforcing, and the dilution of nonflammable gases from ZIF-8 additives during the combustion and pyrolysis process. Our study paves a promising way to achieve high performance PMMA composites.

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.

Flame-retardant activity of ternary integrated modified boron nitride nanosheets to epoxy resin

In order to improve the fire safety of epoxy resin, ZIF-8 nanoparticle in-situ decorated boron nitride nanosheet (BN-OH/ZIF-8) is fabricated via self-assembly method and then ternary integrated BN-OH/ZIF-8/PA hybrids are prepared through the chemical etching effect of phytic acid. FTIR, XRD, XPS, TEM and TGA measurements are used to characterize the structure and morphology of the nanohybrids. The researches show that BN-OH/ZIF-8/PA not only uniformly distributed in EP matrix, but also improve the thermal stability of EP. The peak heat release rate, peak smoke production rate, total smoke production values, the fire growth index and peak CO production rate obtained from cone test are significantly decreased, demonstrating the reduction of the fire hazards of EP composites containing BN-OH/ZIF-8/PA. The nano barrier effect and catalytic activity of BN-OH/ZIF-8/PA may be conducive to suppress the release of combustible volatile products and heat, facilitate the formation of graphitized carbon layer, and protect matrix from flame damage. The ternary integrated method developed in this study explores a new way to improve the flame retardant properties of EP, thereby promoting its application range.

Comparative Study on the Flame Retardancy and Retarding Mechanism of Rare Earth (La, Ce, and Y)-Based Organic Frameworks on Epoxy Resin

In this work, a series of rare earth-based metal-organic frameworks (RE-MOFs) with the same organic ligand were synthesized and studied as flame retardants on epoxy. Through thermogravimetric analysis, limiting oxide index, UL-94, and cone calorimeter tests, a Y-based MOF (Y-MOF) showed the best flame retardancy compared with a La-based MOF (La-MOF) and Ce-based MOF (Ce-MOF). Further research with Raman, X-ray photoelectron spectroscopy, and theoretical calculation revealed that the reasons for the different flame retardance performances of RE-MOFs resulted from the catalytic carbonizing abilities and the radical-trapping abilities of La, Ce, and Y.

Synergistic Flame Retardant Effect of Barium Phytate and Intumescent Flame Retardant for Epoxy Resin

Recently, widespread concern has been aroused on environmentally friendly materials. In this article, barium phytate (Pa-Ba) was prepared by the reaction of phytic acid with barium carbonate in deionized water, which was used to blend with intumescent flame retardant (IFR) as a flame retardant and was added to epoxy resin (EP). Afterward, the chemical structure and thermal stability of Pa-Ba were characterized by Fourier transform infrared (FTIR) spectroscopy and thermogravimetric analysis (TGA), respectively. On this basis, the flammability and flame retardancy of EP composites were researched. It is shown that EP/14IFR/2Ba composite has the highest limiting oxygen index (LOI) value of 30.7%. Moreover, the peak heat release rate (PHRR) of EP/14IFR/2Ba decreases by 69.13% compared with pure EP. SEM and Raman spectra reveal the carbonization quality of EP/14IFR/2Ba is better than that of other composites. The results prove that Pa-Ba can cooperate with IFR to improve the flame retardancy of EP, reducing the addition amount of IFR in EP, thus expanding the application range of EP. In conclusion, adding Pa-Ba to IFR is a more environmentally friendly and efficient method compared with others.

Nanocarbon-Based Flame Retardant Polymer Nanocomposites

In recent years, nanocarbon materials have attracted the interest of researchers due to their excellent properties. Nanocarbon-based flame retardant polymer composites have enhanced thermal stability and mechanical properties compared with traditional flame retardant composites. In this article, the unique structural features of nanocarbon-based materials and their use in flame retardant polymeric materials are initially introduced. Afterwards, the flame retardant mechanism of nanocarbon materials is described. The main discussions include material components such as graphene, carbon nanotubes, fullerene (in preparing resins), elastomers, plastics, foams, fabrics, and film-matrix materials. Furthermore, the flame retardant properties of carbon nanomaterials and their modified products are summarized. Carbon nanomaterials not only play the role of a flame retardant in composites, but also play an important role in many aspects such as mechanical reinforcement. Finally, the opportunities and challenges for future development of carbon nanomaterials in flame-retardant polymeric materials are briefly discussed.

Waste to resource: preparation of an efficient adsorbent and its sustainable utilization in flame retardant polyurethane composites

In order to realize the comprehensive utilization of industrial solid waste and the treatment of water eutrophication, the flower-like magnesium hydroxide (MH) was synthesized from phosphorus tailings by sulfuric acid hydrolysis and a hydrothermal method and then was modified with a metal organic framework (MOF) to remove the phosphates enriched in water through adsorption. Both MH and MOF-modified MH (MH@MOF) presented good removal performance of phosphates. The phosphate-adsorbed composites (MH-P and MH@MOF-P) were sustainably used as effective flame retardants for thermoplastic polyurethane (TPU) at low loadings by a solution blending method. The cone calorimetry test results showed that MH@MOF-P can significantly reduce the heat release rate (HRR), smoke production rate (SPR), total smoke release (TSR), CO release rate and CO2 release rate of TPU composites, compared with those of neat TPU. The novel strategy proposed in this work is of great significance for resource recycling, environmental governance and improving fire safety of polymer materials.

The effects of DOPO modified Co-based metalorganic framework on flame retardancy, stiffness and thermal stability of epoxy resin

In this work, the effect of a modified metal organic framework material on the fire resistance and mechanical properties of epoxy resin (EP) has been explored. The cobalt based metal organic framework (ZIF-67) was synthesized from an organic ligand with a Schiff base structure. Then DOPO@ZIF-67 was synthesized by modifying ZIF-67 with 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO), and its effect on EP modification was explored. Compared with the pure EP, 4% DOPO@ZIF-67/EP passed the UL94 V-0 level and the ultimate oxygen index (LOI) reached 32.1%. The SEM pictures of carbon residue indicated that DOPO@ZIF-67 formed a more continuous and dense microstructure, which can enhance the thermal barrier and the physical barrier effect. The addition of DOPO@ZIF-67 also can effectively improve the stiffness and damping coefficient of EP composite material. The porous skeleton structure of DOPO@ZIF-67 can endow EP with rigidity and flame-retardant properties. Furthermore, the existence of DOPO made the combination of ZIF-67 with EP better. The results of this study suggest that DOPO@ZIF-67 may be a good additive for modification of the properties of epoxy thermosetting materials.

Recent Progress on Metal-Organic Framework and Its Derivatives as Novel Fire Retardants to Polymeric Materials

High flammability of polymers has become a major issue which has restricted its applications. Recently, highly crystalline materials and metal-organic frameworks (MOFs), which consisted of metal ions and organic linkers, have been intensively employed as novel fire retardants (FRs) for a variety of polymers (MOF/polymer). The MOFs possessed abundant transition metal species, fire-retardant elements and potential carbon source accompanied with the facile tuning of the structure and property, making MOF, its derivatives and MOF hybrids promising for fire retardancy research. The recent progress and strategies to prepare MOF-based FRs are emphasized and summarized. The fire retardancy mechanisms of MOF/polymer composites are explained, which may guide the future design for efficient MOF-based FRs. Finally, the challenges and prospects related to different MOF-based FRs are also discussed and aim to provide a fast and holistic overview, which is beneficial for researchers to quickly get up to speed with the latest development in this field.

Mitigation the release of toxic PH3 and the fire hazard of PA6/AHP composite by MOFs

Aluminum hypophosphite (AHP) is a high-efficiency phosphorus-based flame retardant with high P content, which is widely used in Polyamide 6 (PA6). However, AHP releases phosphine gas (PH₃) at high temperatures, which is highly toxic to human's health and environment. Metal-organic frameworks (MOFs) have porous structure exhibiting high performance in gas adsorption. Therefore, mesoporous iron (III) carboxylate [MIL-100 (Fe)] was synthesized in this work and employed to study the adsorption capacity of toxic PH₃ in PA6/AHP composite during processing. AHP was combined with melamine cyanurate (MCA) and MIL-100 (Fe) followed by blending with PA6 to prepare PA6 composites (PA6/MA and PA6/MAF). PA6/MAF with the weight ratio of 5:5 performed well in inhibiting the release of PH₃ during the processing of composite as well as the accelerated thermal experiment devised by our group. Besides, PA6/MAF (5:5) showed relatively low fire hazard reflected by the reduction of the peak of heat release rate of PA6 composite from 962 to 260 kW/m² compared with that of pure PA6 in the cone calorimeter test, and MIL-100 (Fe) along with MCA also presented synergistic effect in suppressing the emission of carbon monoxide. The subtle selection of MOFs herein has the potential to be used as a promising synergist for hazardous gases released from polymer composites to improve the occupational and fire safety in the society.

A fully bio-based coating made from alginate, chitosan and hydroxyapatite for protecting flexible polyurethane foam from fire

The present study reports the successful synthesis of the flame-retardant and smoke-suppressant flexible polyurethane foam (FPUF) through a fully bio-based coating. Hydroxyapatite (HAP) is added to the solutions containing sodium alginate (SA) and chitosan (CH), respectively, to create negative and positive polyelectrolytes for Layer-by-Layer (LbL) assembly. The influence of the solution concentrations and bilayers numbers deposited on the flame-retardant and mechanical properties of FPUF samples is investigated systematically. Benefitting from the presence of such a fully bio-based coating, the resultant FPUF affords excellent smoke-suppressant and flame-retardant features. In particular, the FPUF coated by 9 bilayers of HAP-SA/HAP-CH exhibits significantly declined peak heat release rate, total release rate and smoke production release by 77.7 %, 56.5 % and 53.8 %, respectively. The compression test verifies the coated FPUFs exhibit lower recovery properties compared with the uncoated one. These results demonstrate that a green and cost-effective strategy is provided for producing flame-retardant, anti-dripping and smoke-suppressant FPUFs.

Preparation of Zeolitic Imidazolate Frameworks and Their Application as Flame Retardant and Smoke Suppression Agent for Rigid Polyurethane Foams

In order to reduce the fire risk of rigid polyurethane foams (RPUF), three kinds of zeolitic imidazolate frameworks (ZIFs) were prepared. The results of Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM and X-ray diffraction (XRD) showed that ZIFs were successfully prepared. The combustion test results showed that the heat and smoke production of the composite containing ZIFs was obviously reduced. In particular, the peak heat release rate (PHRR) of ZIF-8/RPUF decreased from 740.85 kW/m² (Ref. RPUF) to 489.56 kW/m², while the PHRR of ZIF-7/RPUF and ZIF-11/RPUF is 598.39 and 583.36 kW/m², respectively. The addition of ZIFs improved the thermostability of the composite. The T50% of ZIF-8/RPUF, ZIF-7/RPUF and ZIF-11/RPUF increased to 364, 382 and 380 °C, respectively. The maximum light absorption of ZIF-7/RPUF and ZIF-11/RPUF was about 88%, which is higher than that of ZIF-8/RPUF (75%). The results of Raman spectroscopy showed that the I_D/I_G value of Ref. RPUF is 2.96, while the I_D/I_G value of ZIFs/RPUF reduces to less than 2.80. The main mechanism of ZIFs for reducing the fire risk of RPUF was the catalysis and incarbonization of ZIFs during combustion based on the results of thermogravimetric analysis and Raman spectroscopy of char residue.

Novel Phosphorus-Nitrogen-Containing Ionic Liquid Modified Metal-Organic Framework as an Effective Flame Retardant for Epoxy Resin

Metal-organic frameworks (MOFs) have shown great potential in flame retardant applications; however, strategies for fully exploiting the advantages of MOFs in order to further enhance the flame retardant performance are still in high demand. Herein, a novel MOF composite was designed through the generated cooperative role of MOF (NH₂-MIL-101(Al)) and a phosphorus-nitrogen-containing ionic liquid ([DPP-NC₃bim][PMO]). The ionic liquid (IL) was composed of imidazole cation modified with diphenylphosphinic group (DPP) and phosphomolybdic acid (PMoA) anions, which can trap the degrading polymer radicals and reduce the smoke emission. The MOF acts as a porous host and can avoid the agglomeration of ionic liquid. Meanwhile, the -NH₂ groups of NH₂-MIL-101(Al) can increase the compatibility with epoxy resin (EP). The framework is expected to act as an efficient insulating barrier to suppress the flame spread. It was demonstrated that the MOF composite (IL@NH₂-MIL-101(Al)) is able to effectively improve the fire safety of EP at low additions (3 wt. %). The LOI value of EP/IL@NH₂-MIL-101(Al) increased to 29.8%. The cone calorimeter results showed a decreased heat release rate (51.2%), smoke production rate (37.8%), and CO release rate (44.8%) of EP/IL@NH₂-MIL-101(Al) with respect to those of neat EP. This strategy can be extended to design other advanced materials for flame retardant.

Modification of diatomite with melamine coated zeolitic imidazolate framework-8 as an effective flame retardant to enhance flame retardancy and smoke suppression of rigid polyurethane foam

In this work, the core-shell structure (ZIF-8@MA) was prepared first with melamine (MA) coated zeolitic imidazolate framework-8 (ZIF-8), and the ternary composite ZMD containing Si-N-Zn was successfully synthesized with the diatomite modified ZIF-8@MA. Subsequently, the prepared ZMD was added into rigid polyurethane foam (RPUF) to investigate its effect on fire safety of RPUF. The results of cone calorimeter and limiting oxygen index (LOI) tests indicated that ZMD effectively reduced the fire hazard of RPUF. This was because of the physical barrier effect of diatomite, the co-catalyzed char formation in the condensed phase of ZnO produced by the decomposition of ZIF-8 and silica produced by the decomposition of diatomite, and the gas phase effect of MA that enabled RPUF to achieve excellent flame retardancy and smoke suppression. The specific mechanism of flame retardancy and smoke suppression of ZMD for RPUF was also discussed in this study.