Epoxy resin (EP) is an important thermosetting material. Because of its excellent chemical resistance and stability, good optical transmittance and adhesion, it is widely used in adhesives, coatings and advanced nanocomposites. However, EP has the characteristics of high flammability and low decomposition temperature, so that it will generate a large amount of CO, CO2 and toxic fumes during the combustion process, which seriously limits the application field of EP. In order to improve the flame retardant performance of EP, expand its application fields, and add efficient flame retardants to it, it has become one of the important directions to improve the flame retardant performance of EP matrix. In recent years, green and environmentally friendly halogen-free flame retardants have been proven to significantly improve the fire safety performance of polymer matrices. They have the characteristics of low smoke generation and no toxic and harmful gas generation, and have attracted more and more attention in the field of flame retardants.
However, while improving fire safety performance, the addition of green halogen-free flame retardants will seriously reduce the transparency of EP, limiting the application of EP in certain special fields, such as solar pavement, light-emitting diodes, multi-purpose coatings, etc. Therefore, while improving the safety performance of EP fire, the research of green halogen-free flame retardant materials that can still maintain its transparency is a difficult and research hotspot in the field. Nanomaterials of different sizes have been shown to significantly improve the flame retardant properties of polymers. Basic magnesium borate (MBH) as an attractive new halogen-free flame retardant has low cost, environmental friendliness and excellent comprehensive performance, which has application and research value in the field of flame retardant.
In order to effectively improve the thermal stability and fire safety of EP and maintain high optical transmittance, the team of Professor Liu Zhiqi of the Qinghai Salt Lake Institute prepared MBH and added 1 wt% ~ 10 wt% of MBH to the epoxy resin to prepare EP /MBH composite material. It is shown that due to the similar refractive index of MBH and EP, the optical transmittance of the composite material in the 800-400 nm wavelength range is still greater than 70% when the MBH content is 10 wt%. The addition of MBH increases the amount of residual carbon in EP/MBH composite materials and reaches a flame retardant level. Compared with EP, the flame retardant parameters of EP/MBH materials have decreased significantly, and EP/5MBH composite materials are the most prominent. Their peak heat release rate (pHRR), smoke release rate peak (pSPR), CO release rate peak (pCOP), the peak CO2 release rate (pCO2P) and the fire spread index (FIGRA) are reduced by 30% to 50%. The flame retardant mechanism research was indicated that MBH flame retardant EP is a solid phase flame retardant mechanism. MBH decomposes into magnesium borate in the flame retardant process, played a role of condensed phase flame retardant, and prevented the spread of heat and combustible gas. For details, please refer to "Salt Lake Research", Issue 4, 2019-"Research Highlights" of the Salt Lake Chemical Special Issue: pages 1-8.
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(A) Photographs and (b) Optical transmittance curves of EP and EP/MBH composite materials
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