Preparation And Flame Retardancy Of Bismuth Stannate Nanoparticles And Their Composite Materials

Flexible polyvinyl chloride（PVC）is widely used in industry.However,due to the high content of plasticizers in soft PVC,it is extremely flammable and seriously endangering people’s lives and property.Antimony trioxide（Sb₂O₃）has good flame retardant properties in PVC.However,the toxicity of Sb₂O₃ has become a problem hindering its application.In order to replace Sb₂O₃,this paper prepared bismuth stannate（Bi₂Sn₂O₇,BSO）nanoparticles using a precipitation method,and compared their flame retardant with commercial Sb₂O₃.It was found that BSO has the potential to become a substitute for Sb₂O₃.Secondly,by adjusting the ratio of Sn and Bi in BSO flame retardant,the flame retardant mechanism was futher explored,and the optimal addition ratio with flame retardant performance was seleted.Finally,in order to reduce the cost of flame retardant,BSO-MH composite flame retardant were prepared using magnesium hydroxide（Mg（OH）₂,MH）instead of part of BSO.The main research contents are as follows:1.Preparation of Bi₂Sn₂O₇ nanoparticles and their performance in PVCBSO was prepared using a simple precipitation method and used for the first time as a flame retardant for soft PVC.The transmission electron microscopy（TEM）characterization results showed that the average particle size of the prepared BSO was 20 nm.The flame retardant results showed that the limiting oxygen index（LOI）of pure PVC was 24.8%,and the LOI of PVC composites containing 3 wt.%BSO increased to29.0%,which was very close to 29.5%of PVC/3%Sb₂O₃.In particular,PVC/3%BSO composites pass the UL-94 V-0 rating,while PVC/3%Sb₂O₃ has no rating.In addition,compared with pure PVC,PVC/3%BSO has a 27.4%lower peak heat release rate（PHRR）and a 39.9%lower total smoke generation（TSP）,showing good flame retardant performance.The microstructure and morphology of residual carbon indicate that BSO contributes to the generation of more,more graphitized carbon residue with fewer pores.The good flame retardant properties of BSO are attributed to its catalytic carbonization in the condensed phase and the capture of active free radicals in the gas phase.Finally,the tensile strength and elongation at break of PVC/3%BSO are also slightly better than PVC/3%Sb₂O₃.Therefore,BSO has the potential to replace Sb₂O₃.2.Structural regulation of Bi₂Sn₂O₇ nanoparticles and their flame retardant mechanismIn order to further study the flame retardants mechanism of BSO,compounds with different tin-bismuth contents were prepared by regulating the ratio of Sn and Bi in BSO to study their influence on flame retardant performance.The vertical combustion test found that when the addition ratio of Sn and Bi is greater than or equal to 1:1,the PVC composite does not produce droplets when burning,and can pass the UL-94 V-0 grade.This indicates that Sn promotes the formation of dense and strong carbon residue from PVC.The optical photographs of residual carbon showed that with the introduction of Bi and the decrease of the ratio of Sn:Bi,the thickness of residual carbon would gradually decrease.When the flame retardant is Bi₂O₃,volatile Bi Cl₃will be generated during combustion,which inhibits the formation of the carbon layer,and the thickness of the carbon layer is almost 0.Through the results of flame retardants performance test and the structure and graphitization degree of residual carbon after combustion,the flame retardant mechanism of BSO is as follows:BSO has two flame retardants mechanisms:gas phase and condensed phase.Condensed phase analysis showed that Sn and HCl in BSO formed Sn Cl₂,and Sn Cl₂,as a strong Lewis acid,promoted the early crosslinking of PVC,helped to form more carbon layers,and played a physical barrier.Gas phase analysis showed that,like Sb elements,Bi reacts with HCl to form Bi Cl₃.On the one hand,Bi Cl₃ dilutes the combustible gas;On the other hand,Bi Cl₃ traps free radicals in the combustion zone,preventing flame propagation and reducing flame density.The synergistic flame retardant effect of condensed phase and gas phase makes BSO have good flame retardant performance.3.Preparation of Bi₂Sn₂O₇-Mg（OH）₂ composite flame retardant and its performance in PVCIn order to reduce costs and take advantage of the synergy between MH and BSO,BSO-MH composite flame retardant was prepared by replacing part of BSO with MH.Thermogravimetric analysis showed that the initial decomposition temperature of PVC/B3M1（3:1 mass ratio of BSO and MH）was increased compared to PVC/BSO.The flame retardant performance results showed that 3 wt.%BSO-MH significantly reduced the heat and smoke release of PVC.Compared with pure PVC,the PHRR,total heat release（THR）and TSP of PVC/B3M1 decreased by 34.2%,12.6%and 41.1%,respectively.Moreover,compared with PVC composites containing 3 wt.%BSO,their PHRR decreased by 9.3%and their fire performance index（FPI）increased by 11.9%,showing better flame retardant properties.According to the analysis of char residue structure after combustion and cone calorimetry test parameters,it is found that BSO and MH will produce synergistic flame retardancy.On the one hand,BSO plays the role of condensed phase and gas phase flame retardant;On the other hand,MH will decompose into magnesium oxide and water vapor,which requires a lot of heat consumption and reduces the combustion temperature;Moreover,magnesium oxide can be deposited on the surface of the substrate to achieve the purpose of isolation and protection,thereby effectively reducing the release of heat and smoke.