Synthesis Of Cross-linked Polyphosphazene Modified Magnesium Hydroxide And Their Flame-retarding Application In EVA

Ethylene-vinyl acetate copolymer（EVA）has been widely applied in various industries owing to its excellent insulation and mechanical properties.But the limiting oxygen index（LOI）of EVA is 19-21%,which makes it extremely flammable.Therefore,it is necessary to carry out flame-retardant treatment of EVA.Magnesium hydroxide（MH）,a typical halogen-free,green and environment-friendly inorganic flame retardant additive,is widely used in halogen-free polymer materials because of its flame retardancy,smoke suppression and other functions.However,the expected flame retardant effect can be exerted only by adding more than 60 wt%of MH.In addition,the high interfacial energy and strong hydrophilicity of MH also severely deteriorates the mechanical and processing properties of the material.In recent years,the layer-by-layer assembly（LBL）technology has attracted widespread attention in the field of surface modification of inorganic powder flame retardants due to its simple preparation process and environment-friendly advantages.Based on the above scenario,LBL technology provides a new idea for the preparation of multi-function MH.Based on MH in this paper,two kinds of organic-inorganic hybrid flame retardants,Ni²⁺chelated polyphosphazene modified MH（MH-PZPN-Ni）and boric acid（BA）crosslinked polyphosphazene modified MH（MH-PZPI-BA）were subsequently prepared,and added to EVA to study the flame retardancy and mechanical properties of EVA composites.The flame retardant carbonization mechanism of the two flame retardants were analyzed.The specific work is as follows:Part Ⅰ:Synthesis of MH-PZPN-Ni and its flame retardancy in EVA resin.In this paper,MH was first aminated withγ-aminopropyltriethoxysilane（KH550）,and then poly（cyclotriphosphazene-piperazine）（PZPN）cross-linked coating was formed by cross-linking reaction of hexachlorocyclotriphosphazene（HCCP）with N-aminoethyl piperazine（AEP）on the surface of aminated MH.Finally,MH-PZPN-Ni hybrid flame retardant was prepared by chelating reaction between Ni²⁺and the amino group on the surface of PZPN by LBL process.Its morphology,chemical structure and composition and thermal stability were analyzed by scanning electron microscope（SEM）,transmission electron microscope（TEM）,fourier transform infrared spectroscopy（FT-IR）,X-ray photoelectron spectroscopy（XPS）,and thermogravimetric analysis（TGA）.The results showed that MH-PZPN-Ni had a perfect core-shell structure and good thermal stability.Among them,Ni²⁺and the amino on the surface of PZPN existed in the form of coordination bonds,and the shell thickness of PZPN-Ni was about 44.8 nm,and Ni²⁺was evenly distributed on the surface of MH.MH-PZPN-Ni was applied to the study of flame retardancy of EVA resin.The results showed that the MH-PZPN-Ni achieved a LOI of 30.4%and UL 94 V 0 rating when the amount of MH-PZPN-Ni was 60 wt%.Cone calorimetric test results showed that the peak heat release rate（PHRR）of EVA/MH-PZPN-Ni was reduced by 18.2%compared with that of EVA/MH-PZPN,and EVA/MH-PZPN-Ni had a higher fire performance index（FPI）,a lower fire growth index（FGI）,and a lower CO/CO₂ ratio.Thermogravimetric-mass spectrometry（TG-MS）analysis showed that Ni²⁺promoted the formation of aromatic compounds.In addition,the fractured surfaced analysis of EVA composites showed that MH-PZPN-Ni had good dispersion in EVA matrix.Mechanical tests showed that the tensile strength and elongation at break of EVA/MH-PZPN-Ni were improved by 9.6%and 74.9%compared with those of EVA/MH.Part Ⅱ:Synthesis of MH-PZPI-BA and its flame retardancy in EVA resin.In this paper,MH was first aminated with KH550,and then poly（cyclotriphosphazene-polyethyleneimine）（PZPI）cross-linked coating was formed by cross-linking reaction of HCCP was crosslinked with branched polyethyleneimine（bPEI）on the surface of aminated MH.Finally,MH-PZPI-BA hybrid flame retardant was prepared by crosslinked reaction between BA and the amino group on the surface of PZPI by LBL process.Its morphology,chemical structure and thermal stability were analyzed by SEM,TEM,FT-IR,XPS and TGA.The results showed that the thermal stability of MH-PZPI-BA was good,the thickness of PZPI-BA shell was 47.1 nm,BA evenly distributed on the surface of MH and formed B-N covalent bonds with the amino groups on the surface of PZPI.The MH-PZPI-BA was used to study the flame retardancy of EVA resin.The results showed that the EVA/MH-PZPI-BA composites achieved LOI of 28.6%and UL 94 V 1 rating when the amount of MH-PZPI-BA was 55 wt%.The cone calorimetric test results showed that the PHRR,the peak smoke release rate（PSPR）and the production of CO of EVA/MH-PZPI-BA composites decreased by 39.2%,55.8%and 42.3%,respectively,compared with those of EVA/MH.The char residue was analyzed by FT-IR,XPS,X-ray diffraction（XRD）and TEM.The results showed that the"tortuous path"effect of the boron nitride（BN）of in-situ growth of EVA/MH-PZPI-BA during combustion hindered the transfer of volatile products,and led to the formation of dense char layers in the condensed phase.