Synthesis Of Nitrogen Containing Phosphorus Sulfur Flame Retardant And Study On Flame Retardant Modification Of Epoxy Resin

After curing with curing agents,epoxy resin（EP）is endowed with excellent properties that are different from other thermosetting resins,and is widely used in aerospace,coatings,electronic devices,and other fields.However,the defect of highly flammable epoxy resin limits its application in a wider range of fields.Therefore,it is necessary to study the flame retardant modification of epoxy resin.Single element flame retardants are often difficult to achieve excellent flame retardancy in both gas phase and condensed phase,while multi-element flame retardants often exhibit higher flame retardancy efficiency through synergistic action.In this paper,three synergistic flame retardants containing N/P/S were synthesized from9,10-dihydro-9-oxa-10-phosphazephenanthrene-10-sulfide（DOPS）,melamine,itaconic acid（ITA）,and graphite phase carbon nitride（g-C₃N₄）.The structures of the three flame retardants were characterized by FTIR,¹H NMR,¹³C CP/MAS NMR,and ³¹P NMR.Three flame retardant epoxy resin composites were prepared by adding them to epoxy resin/4,4’-diaminodiphenylmethane（DDM）.The thermal stability,flame retardancy,combustion behavior,mechanical properties,and flame retardant mechanism of flame retardant EP were investigated through various tests.The specific research content is as follows:Applying DOPS-Me to the EP/DDM system,the results of thermogravimetric analysis（TGA）showed that DOPS-Me could improve the carbonization ability of EP,reduce the thermal degradation rate of EP,and improve its thermal stability.When 10 wt%DOPS-Me was added,the spline passed the vertical combustion（UL-94）V-0 level,and the limiting oxygen index（LOI）value increased from 15.7%to 25.7%.At the same time,the peak heat release rate（PK-HRR）,total heat release（THR）,and total smoke release（TSP）of EP/10 wt%DOPS-Me were reduced by 58.9%,21.9%,and 17.7%compared to pure EP,respectively.The results of scanning electron microscopy（SEM）,X-ray photoelectron spectroscopy（XPS）,Raman spectroscopy（Raman）,and thermogravimetric infrared spectroscopy（TG-FTIR）indicate that the dilution effect and quenching effect of DOPS-Me in the gas phase simultaneously promote the dehydration of EP matrix in the condensed phase to form a dense carbon layer rich in phosphorus and sulfur.Flame retardant modification of EP with DOPS-CN was conducted.The test found that due to the large number of rigid groups in DOPS-CN and the increase in the addition amount,the compatibility became poor,and the mechanical properties of the composite decreased.When 10%DOPS-CN is added,the UL-94 level of EP composite material reaches V-0 level,and the LOI value reaches 28.7%.The initial decomposition temperature(T_5%)of the composite system decreased,the carbon residue increased,and the thermal stability of the matrix was enhanced.The SPR,TSP,PK-HRR,and THR of the EP/DOPS-CN system are significantly reduced,and the dense carbon layer formed by combustion can play a good shielding role.Raman and XPS studied the carbon layer and found that the enrichment of phosphorus and sulfur elements on the surface of the carbon layer enabled the composite to exert good flame retardancy in the condensed phase.TG-FTIR testing showed that the flame retardance of DOPS-Me at the vapor phase end mainly came from the quenching effect of phosphorus containing free radicals and the dilution effect of flame retardant gases.The nitrogen rich group g-C₃N₄combines with DOPS-ITA to form a phosphate flame retardant DOPS-ITA-CN containing N/P/S.Add DOPS-ITA-CN to the EP/DDM system.The results showed that the thermal degradation rate of DOPS-ITA-CN modified samples was slowed down,the carbonization ability was enhanced,and the EP matrix was endowed with good flame retardancy.After adding 10%DOPS-ITA-CN,EP composites passed UL-94 V-0 grade and had a 25.9%LOI value.DOPS-ITA-CN significantly inhibits the generation of heat and the release of smoke from the EP substrate.SEM,Raman,and XPS tests on the carbon layer showed that the graphitization degree of the carbon layer increased significantly after adding DOPS-ITA-CN,and the phosphorus and sulfur content in the carbon layer increased,making the material have a good heat insulation and smoke suppression effect.