Preparation Of Layered Double Hydroxide Microcapsules And Their Study On Flame Retardant And Smoke Suppression Properties Of TPU

Thermoplastic polyurethane（TPU）is widely used in cable,footwear,construction,automotive and other industries because it can meet the diverse demands of modern technologies.However,TPU itself is flammable and releases a large amount of black smoke and toxic gas during combustion,which poses a great threat to human life and property safety.Therefore,it is necessary to add flame retardants to TPU to improve its flame retardancy.Three-dimensional（3D）hollow layered double hydroxide（LDH）microcapsules have great potential in the field of flame retardancy due to their excellent dispersibility.In addition,the 3D hollow LDH microcapsules can be synergized with other flame retardants to further improve the flame retardant properties of TPU.In this paper,3D hollow NiFeCe-LDH/NiFeTb-LDH and NiCo-LDH microcapsules were synthesized by selective etching using MIL-88A and ZIF-67 as precursors and in situ sacrificial templates,respectively,and the three 3D hollow LDH microcapsules were hybridized with molybdenum disulfide（MoS₂）to construct 3D hollow LDH/MoS₂hybrid materials.The composition,structure and thermal stability of the synthesized samples were analyzed by X-ray diffraction（XRD）,Fourier transform infrared spectroscopy（FTIR）,scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,energy scattering spectroscopy（EDS）and thermogravimetric analysis（TGA）.Then,TPU composites filled with MoS₂,3D hollow LDH and 3D hollow LDH/MoS₂（2 wt%addition of flame retardants）were prepared by melt blending method,and the combustion behavior,thermal stability and fire safety of TPU composites were evaluated via cone calorimeter test（CCT）and Thermogravimetric-Fourier Transform Infrared Spectroscopy（TG-FTIR）.The XRD and FTIR results showed that NiFeCe-LDH,NiFeTb-LDH and NiCo-LDH all have typical characteristic peaks and absorption peaks of LDH.The SEM and TEM results revealed that NiFeCe-LDH,NiFeTb-LDH and NiCo-LDH all maintain the morphology of the precursors and are hollow structures.In addition,these three LDH and MoS₂also successfully synthesized 3D hollow LDH/MoS₂hybrid materials.The TGA results indicated that the thermal stability of the 3D hollow LDH/MoS₂hybrid materials is higher than that of LDH.The CCT test results showed that the flame retardant and smoke suppression effects of TPU/NiFeCe-LDH/MoS₂,TPU/NiFeTb-LDH/MoS₂and TPU/NiCo-LDH/MoS₂composites are better than that of single LDH or MoS₂filled TPU composites.Compared with pure TPU,the peak heat release rate（PHRR）values of TPU/NiFeCe-LDH/MoS₂,TPU/NiFeTb-LDH/MoS₂and TPU/NiCo-LDH/MoS₂decreased by 48.8%,50.9%and 42.9%respectively,peak smoke release rate（PSPR）values decreased by 55.8%,60.2%and 55.7%respectively,and the total smoke release（TSP）values decreased by 39.8%,35.8%and 33.3%respectively.The TG-FTIR results demonstrated that these three 3D hollow LDH/MoS₂hybrid materials can improve the thermal stability,carbon formation rate and fire safety of TPU composites.The SEM and XRD results of the carbon residues showed that the carbon residues of the LDH/MoS₂filled TPU composites are denser,and the characteristic peaks of graphite crystallites and metal oxides appear in the XRD patterns of the carbon residues.Metal oxides are not only excellent smoke suppressants but also can promote the formation of carbon layers and improve the degree of graphitization of carbon layers.According to the analysis of three 3D hollow LDH/MoS₂hybrid materials,NiFeCe-LDH/MoS₂with better flame-retardant and smoke-suppression properties were selected to compound with zinc borate（ZB）to prepare flame-retardant TPU composites.The CCT results indicated that ZB as a synergist can significantly improve the flame retardant and smoke suppression properties of TPU composites.Compared with the TPU composites containing only Ni Fe-Ce-LDH/MoS₂,the PHRR,PSPR and TSP values of TPU4（containing 2 wt%Ni Fe-Ce-LDH/MoS₂and 2 wt%ZB）decreased by 24.1%,38.0%and 41.9%,respectively.The TGA results showed that the thermal stability and carbon residues of TPU4 were significantly improved compared with NiFeCe-LDH/MoS₂filled TPU composites.The results of carbon residues analysis demonstrated that TPU4 formed expanded and dense carbon layers after combustion.At the same time,the B₂O₃produced by the thermal decomposition of ZB can cover the surface of the carbon residue to protect the unburned substrate.