Preparation Of Bismuth-based Flame Retardant And Its Properties In Flexible Poly(Vinyl Chloride)

In recent years,two-dimensional layered materials have been extensively studied in the field of flame retardancy due to their ability to block the transmission of thermal insulation during the burning of polymers.Bismuth oxychloride?BiOCl?nanosheets have two-dimensional layered structure,and bismuth and antimony elements have similar chemical properties in the same family.It is expected that BiOCl can replace antimony oxide?Sb₂O₃?as a new green flame retardant in PVC.In this paper,several flame retardants of BiOCl nanosheets were prepared,and their flame retardant properties and flame retardant mechanism in PVC were studied.First,the BiOCl nanosheets were prepared by hydrothermal method,and the BiOCl nanosheets were modified with silane coupling agent?KH570?.Flame retardant performance of KH570-BiOCl was superior to commercially Sb₂O₃ in PVC matrix.Secondly,in order to solve the problem of instability of the formed char layer during the combustion in PVC/KH570-BiOCl composite,BiOCl@CS-PA composite flame retardant was prepared by introducing biomass phytic acid?PA?with high phosphorus content to further improve PVC composites of flame retardancy performance.Finally,based on the previous research in our group,Fe and Sn elements were introduced into the BiOCl nanosheets by doping to improve the flame retardant performance of BiOCl by using synergistic effects.The main research contents and results are displayed as following:1.Preparation of Bismuth Oxychloride Nanosheets and Study on the Performance in PVCBiOCl nanosheets were prepared by a hydrothermal method.We adopted silane coupling agent?KH570?to simply modify BiOCl nanosheets in order to improve dispersibilityand interfacial adhesion in PVC matrix.The as-prepared BiOCl nanosheets were confirmed by X-ray powder diffractometer?XRD?and scanning electron microscopy?SEM?,which showed that the morphology of BiOCl nanosheets with nanometer to micrometer and thickness was about30-40 nm.The KH570-BiOCl nanosheets were prepared which were confirmed by infrared?IR?and thermogravimetric?TG?.And the morphology of KH570-BiOCl nanosheets didn't change.The cross-section after tensile test was observed by SEM.The introduction of silane coupling agent can improve the dispersibility of BiOCl nanosheets in the PVC matrix.The flame retardant properties of the PVC composites preliminarily tested by the limitting oxygen index?LOI?.With 3wt.% of PVC/KH570-BiOCl composite increased from 25.1% to 28.7% and which blonged to the nonflammable materials.In addition,compared with LOI values of unmodified BiOCl?28.2%?nanosheets and Sb₂O₃?29.7%?,KH570 helps to improve the LOI value of BiOCl nanosheets,and LOI value of PVC/KH570-BiOCl is close to Sb₂O₃.The cone calorimetry test?CCT?showed that the KH570-BiOCl nanosheets can further reduce the peak heat release rate?PHRR?and the total smoke production?TSP?of PVC.Compared with PVC/Sb₂O₃ composite,PVC/BiOCl and PVC/KH570-BiOCl composites exhibited lower PHRR,peak smoke release?PSRR?and TSP,and showing excellent flame retardancy.Through thermogravimetric-infrared?TG-IR?,the introduction of KH570-BiOCl nanosheets effectively inhibited the release of organic volatiles,and can promote the dehalogenation reaction of PVC and the formation of residue char.Furthermore,KH570-BiOCl nanosheets can promote the graphitization degree of char layers by the Raman analysis.2.Preparation of Chitosan and Phytic Acid Coated Bismuth Oxychloride Composite FlameRetardant and Study on the Performance in PVC While KH570-BiOCl can enhance the flame retardancy of the PVC,we found that the char residue of the PVC/KH570-BiOCl composite was unstable.In order to address this issue,we adopted phytic acid?PA?with high phosphorus-containing biomass,and chitosan?CS?as bridging agent and carbon-forming agent,coated on the surface of BiOCl nanosheets for enhancing the stability of the char residue.The XPS ? EDS and SEM results showed that compared to the uncoated BiOCl nanosheets,the coated BiOCl nanosheets showed characteristic peaks of P and N elements and the corresponding surface is rough and there is obviously a layer of sediment,which indicated that the CS-PA was coated on the BiOCl surface and formed BiOCl@CS-PA composite flame retardant.When 3 wt.% of BiOCl@CS-PA was imparted into PVC matrix,the resultant PVC composite can pass V-0 rating in the UL-94 test.Additionally,CCT demonstrated that 3wt.% of BiOCl@CS-PA reduced the first PHRR and inhibited the pronounced PHRR of PVC matrix in comparison with the BiOCl nanosheets?between 300?400 s?,indicating further improvement in flame retardancy of PVC/BiOCl@CS-PA composite with the help of PA in the condensed phase.Microstructure and morphology of the char residue of PVC/BiOCl@CS-PA were analyzed to reveal the flame retardant mechanism.The results showed that PVC/BiOCl@CS-PA composite formed integrated and continuous char layer wih higher degree of graphitization,which indicated that the joint action of PA and CS improves the quality of char layer,and contributes to improving the flame retardancy in condensed-phase.The tensile test results showed that the tensile strength and elongation at break of PVC/BiOCl@CS-PA composite were significantly improved compared with PVC/BiOCl.3.Preparation of Iron and Tin Metal Ion Doped Bismuth Oxychloride Nanosheets and Study on the Performance in PVCIron-doped?Fe-BiOCl?and tin-doped?Sn-BiOCl?nanosheets were prepared by hydrothermal method.XRD and SEM results showed that the as-prepared Fe-BiOCl and Sn-BiOCl had the characteristic diffraction peaks of BiOCl and no extra peaks;the size of as-prepared nanosheets is in the range of 100-200 nm,and the thickness was around 10 nm.TG results showed that the introduction of Fe and Sn elements improved the initial thermal decomposition temperature and significantly increased the amount of residual char at 800?C.The flame retardant tests results showed that: compared with PVC/BiOCl,when 3 wt.% of Fe-BiOCl and Sn-BiOCl were incorporate into PVC,the LOI values increased from 28.5% of PVC to 31.5%and 30.1% respectively.Both samples can pass V-0 rating in UL-94 test.PVC/Fe-BiOCl composite had lower PHRR,THR,and TSP,indicating that PVC/Fe-BiOCl composite possessed better flame retardant performance than PVC/Sn-BiOCl composite.The analysis of the condensed phase results showed that the introduction of Fe and Sn elements formed a double condensed phase synergistic effect.Analysis of the gas phase results showed that both Fe and Sn elements not only promoted the early dehalogenation and carbonization of PVC,but also significantly reduced the release of organic hydrocarbons and aromatic compounds during the pyrolysis process.The tensile test results showed the as-prepared flame retardant incorporated into PVC matrix improved the tensile strength of PVC composites to varying degrees.The increased tensile modulus of PVC composites indicated the increased rigidity.Compared with pure PVC,the addition of Fe-BiOCl improved the elongation at break of PVC composites.