Zeolitic imidazolate framework-8 was coated with silica and investigated as a flame retardant to improve the flame retardancy and smoke suppression of epoxy resin

Abstract

A material (ZIF-8@SiO 2 ) with a nuclear shell structure was synthesized to improve the flame retardancy and smoke suppression of epoxy resin (EP) through a synergetic catalytic effect. Zeolitic imidazolate framework-8 (ZIF-8) was synthesized and its surface was coated with SiO 2 by hydrolyzing tetraethyl orthosilicate. Core-shell structured ZIF-8@SiO 2 was obtained. Then ZIF-8@SiO 2 was added to EP to explore its effect of flame retardancy and smoke suppression on the EP composite. The results of a cone calorimeter test showed that the peak heat release rate, total heat release, smoke production rate and total smoke production of the material were decreased by 75.9%, 38.9%, 51.1% and 39.8%, respectively, after 2 wt% ZIF-8@SiO 2 was added to EP. Besides, through the analysis of char residue, the mechanism of ZIF-8@SiO 2 /EP's flame retardancy and smoke suppression was determined to be due to the physical barrier effect of SiO 2 and the co-effect of SiO 2 and ZnO decomposition by ZIF-8. IntroductionMetal-organic frameworks (MOFs) are porous materials comprising metal ions and organic bridging ligands.1 ZIF-8 is a metal-organic framework material with a rhombic dodecahedral structure and is synthesized in organic solvent with zinc ions and 2-methylimidazolate.2 Because ZIF-8 has a high specic surface area, high thermal stability and unique pore structure, it is widely applied in adsorption, catalysis, separation, gas storage and drug delivery.3-8 Due to the diversity of nanometer ZIF-8 materials, their new functions are being explored. Therefore, the application of ZIF-8 as a ame retardant has also attracted attention. For instance, Shi et al. prepared ZIF-8/PLA composite materials, and their research indicated that the oxygen index of the composite could reach 26.0% when 3 wt% ZIF-8 was added. The application of ZIF-8 as a new ame retardant is worthy of further exploration.In recent years, the application of silicon-containing compounds in ame retardants has been investigated by many researchers because they not only improve the ame retardant performance, but also form environment-friendly ame retardants.10 The combustion of silicon-containing polymer composites may produce SiO 2 which is easy to migrate and accumulate to the polymer surface, and generate Si-O-Si networks inside the char layer structure and ceramic-like protective material on the substrate surface to prevent the polymer's further burning and protect internal polymer. In addition, some compounds containing Si-C structure are also produced, which enhance the thermal stability of composite materials and increase the strength of the char layer.11-13 Meanwhile, SiO 2 decomposed by silicon-containing compounds is a solid acid with more acidic sites, which can catalyze the degradation of polymers and have a good co-effect of ame retardancy with metal oxides. 14 According to existing literature, 15 the use of SiO 2 coated brucite as a ame retardant has a better effect of ame retardancy compared with the simple physical mixing of SiO 2 and brucit...

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