Preparation And Properties Of Long-chain Phosphaphenanthrene Synergistic Flame Retardant Ethylene-vinyl Acetat

Polymer materials are widely used in various fields in modern society because of their excellent properties and a wide variety of product forms.Among them,poly（ethylene-co-vinyl acetate）（EVA）as one of the elastomeric materials,made of ethylene and vinyl acetate polymerized in large quantities under high pressure,and it was widely used in adhesives,electrical insulation materials,packaging materials,functional composites and other fields,because of its good physical and chemical properties and processing properties,but because of the flammability and poor fire resistance characteristics of EVA,seriously hindered the scope of its application.In order to further improve the functionality of EVA,especially the flame retardancy,a lot of research has been carried out in academia and industry,but it is still challenging to report studies that substantially improve its flame retardant properties without affecting its mechanical properties.In this paper,we use the synthesized siloxane chain compounds containing phosphorus heterophilic structures——long-chain phosphaphenanthrene flame retardants（DPP）,using the high-temperature catalytic carbon formation properties of silicon-phosphorus flame retardants,combined with the excellent natural physical barrier properties of two-dimensional materials.The flame retardant system was designed and prepared by combining DPP with the two-dimensional materials black phospholene and Ti₃C₂T_x（MXene）in different ways.It is introduced into EVA by physical blending method to achieve the purpose of enhancing the flame retardant properties of EVA without sacrificing its mechanical properties.To provide new ideas and methods for the high performance of EVA.The specific research contents are as follows:（1）Study of DPP flame retardant EVA was carried out.9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide（DOPO）and the Silane coupler KH-560（SCA）were synthesized to obtain long-chain phosphaphenanthrene flame retardants（DPP）by chemical reaction.DPP contains a flame-retardant phosphaferous structure and flexible silane chains,which are melt-blended with EVA to prepare EVA/DPP composites.Compared with pure EVA resin,the EVA/DPP composite achieved V-2vertical combustion test rating,and the ultimate oxygen index（LOI）increased by7.6%（25.9%）to the combustible level,and reduced total heat release（THR）and peak heat release rate（PHRR）of 20.8%and 41.5%,respectively,when DPP is used at 9wt.%.Tensile yield stress and elongation at break were reduced by 51.3%and 36.4%,respectively.（2）A study on the combination of DPP and BP for flame retardant EVA was carried out,in which DPP and two-dimensional（2D）BP were combined by intermolecular hydrogen bonding to achieve uniform dispersion of Si-P flame retardant in EVA matrix.The flame retardant effect of different contents of DPP/BP flame retardants on EVA was studied.The results showed that DPP and BP combined through intermolecular group forces to achieve great flame retardant properties and thermal stability without sacrificing the mechanical properties of EVA.The thickness of BP prepared by mechanical ball milling was about 2.5 nm,and the lateral dimensions are between a few hundred nanometers and a few micrometers.The optimal ratio of DPP to BP was 3:1（DBP）.Compared with pure EVA resin,the EVA/DBP composite with 5 wt.%of DBP achieved UL-94 V-0 vertical combustion rating,9.6%improvement in LOI,and THR and PHRR were 85.0 MJ/m²and 622.2k W/m²,a reduction of 16.1%and 47.5%,respectively.In addition,the tensile yield stress and elongation at break were reduced by only 8.1%and 6.2%,respectively.The high flame retardant efficiency of DBP is due to the synergistic effect between different flame retardant elements,and good dispersion of DBP in EVA and a variety of flame retardant mechanisms,namely dilution of combustible gas concentration,the formation of physical-chemical composite carbon layer and so on.（3）Study on preparation of DPP-MXene and flame retardant EVA in the optimal complex ratio with BP.As a perfect flame retardant system,DMBP combines the advantages of DPP,BP and MXene and is optimized to a certain extent,and the negative impact of flame retardant addition on mechanical properties was overcome.The results of thermogravimetric analysis（TGA）showed that the DMBP synergistic system could significantly improve the thermal stability and carbon formation ability of EVA.Flame retardant performance test results show that the addition of only 3wt.%DMBP in EVA matrix can make EVA/DMBP-3 composite material with good flame retardancy,EVA/DMBP-3 vertical combustion grade reached UL-94 V-0 grade,LOI of 26.7%close to refractory levels,the THR and PHRR compared with pure EVA were reduced by 21.5%and 48.1%,which implies a significant improvement in flame retardant performance.In addition,the tensile yield stress and elongation at break of EVA/DMBP-3 composites were slightly reduced compared to pure EVA,by only16.0%and 4.8%,respectively.The high flame retardant efficiency and carbon formation ability of DMBP can be attributed to the long chain action of DM,making DMBP better perform blocking volatilization and delaying heat and combustible products transfer thanks to its unique"zigzag path"effect,thus reducing the combustion rate.DMBP has both gas phase and condensed phase flame retardant effect at high temperature.