Construction Of Halogen-free Flame Retardant Systems For Epoxy Resin And Their Performance

As one kind of thermosetting resins,the epoxy resin has the advantages of good thermal stability,insulation,superior adhesion,good mechanical properties and excellent molding performance,but its flammability brings fire hazard to the epoxy resin materials used in some special fields,so it is necessary to improve its flame retardancy.In recently,due to active reactivity and good flame retardancy,9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide?DOPO?and phosphonitrilic chloride trimer?PCT?which can be used to prepared a variety of flame retardant derivatives,have become a research hotspot of flame retardant modification of epoxy resin.However,DOPO and phosphazene derivatives used as flame retardant modifiers for epoxy resin still have the following shortcomings:1)For obtaining a higher phosphorus content,the addition content of flame retardants was large;2)The transparency and mechanical properties of the cured epoxy resin after flame retardant modification were decreased;3)The flame retardant mechanism of synergistic effect of nano materials and graphene oxide in DOPO derivatives needs further research.In this paper,two kinds of DOPO-based flame retardants containing phosphorus and nitrogen and one kind of flame retardants containing phosphazen structure were designed,and four ways used to improve the flame retardancy of epoxy resin were studied by blending nano silica and graphene oxide.The flame retardancy mechanism of each method and the influence on the mechanical properties of epoxy resin were studied in detail.The main work of this paper was summarized as follows:?1?The p-diaminodiphenylsulfone?DDS?and p-benzaldehyde were used to prepare the imine compound DP by condensation reaction,then the DPD was prepared by the addition reaction of DP and DOPO.The molecular structure of DPD was characterized by ATR-FTIR,NMR and elemental analysis.As a flame retardant additive,DPD was introduced to DGEBA type epoxy resin.Cured by 4,4-diaminodiphenylmethane?DDM?,the EP-DPD were prepared.UL-94and LOI test,TGA and DMA were used to study the flame retardancy,thermal stability and mechanical properties of EP-DPD.The flame retardancy mechanism was studied by SEM,Raman analysis,Pyrolysis Gas Chromatography/Mass Spectrometry?Py-GC/MS?and ATR-FTIR.The results showed that the LOI value of the EP-DPD increased with the increasing content of DPD,and reached 35.9%with an addition of 9 wt%DPD.The average time of continued combustion in UL-94 vertical combustion test decreased significantly to 2.3+1.9s,and passed V-0 grade.The residue char of EP-DPD exhibited an intumescent and continuous morphology with honeycomb inside,and graphitization degree is higher than that of pure EP.DPD advanced the thermal degradation of epoxy resin,lowered the 5%weight loss temperature(T_5%)and the temperature at maximum weight loss rate(T_max),and increased the char residue at high temperature.The phosphoric acid source produced by the degradation of DPD promoted the dehydration and carbonization of epoxy resin matrix,and formed phosphorus char residue.It changed the degradation process of EP-DPD in condensed phase.In gas phase,more carbon dioxide,less combustible gases?such as acetaldehyde and butane?and phosphorus fragments released by the degradation of DPD inhibited the combustion.DPD dispersed in EP-DPD resulted in a nontransparent and yellow appearance.The steric hindrance of DOPO group in the molecule limited the cross-linking,and the tensile strength of EP-DPD decreased.?2?The halogen-free flame retardant curing agent DSD containing phosphorus and nitrogen was prepared from DDS,salicylaldehyde and DOPO.DSD was used together with DDM to prepare EP-DSD resin.UL-94 and LOI tests,TGA and DMA were used to study the flame retardancy,thermal stability and mechanical properties of EP-DSD.The flame retardant mechanism was studied by SEM,Raman analysis,Py-GC/MS and ATR-FTIR.The results showed that LOI value was increased to 35.4%,and passed the V-0 grade of UL-94 test.The char residual was porous and showed a dilated appearance of foamed carbon layer,and the degree of graphitization was higher.The T_5%,T_max and V_maxdecreased gradually with the increase of the content of DSD components,while the char residue CY₇₀₀ increased.Free phosphorus radicals,less combustible gases and phosphoric acid substances were formed in the degradation process of EP-DSD which was beneficial to the dehydration and carbonization of the matrix and the improvement of the flame retardancy.The storage modulus and the loss factor?Tan??of EP-DSD increases gradually,suggested a tight network structure.The tensile strength of EP-DSD enhanced and increased up to 88.83Mpa.?3?DOPCT was prepared by formaldehyde,DOPO and PCT,and used for the preparation of EP-D3-DOPCT together with DSD.UL-94 and LOI test,TGA and DMA were used to study the flame retardancy,thermal stability and mechanical properties of EP-D3-DOPCT.The flame retardant mechanism was studied by SEM,Raman analysis,Py-GC/MS and ATR-FTIR.The results showed that the LOI value was improved to 38.3%at most.The average afterflame burning time is 2.6+1.5s,and passed the V-0 grade of UL-94 vertical test.The residual char was co Mpact,porous and showed a dilated appearance of foamed carbon layer.The degradation of EP-D3-DOPCT resin was advanced.The T_5%decreased under air and nitrogen,and the char residue rate increased.DOPCT changed the thermal degradation process,and the combustible gases such as carbon monoxide,acetaldehyde and butane were significantly reduced,and more nitrogen-containing compounds were produced.The test results of tensile properties showed that the tensile strength of the cured EP-D3-DOPCT decreased but slowly,and the loss of tensile strength was less than 7%.?4?Graphene oxide was successfully prepared by a modified Hummers method,and was used to prepare EP-D3-Si resin together with DSD and nano silica.The flame retardancy,thermal stability and mechanical properties of EP-D3-Si resin were studied by TGA,UL-94 and LOI test.The flame retardant mechanism was studied by SEM,Raman analysis,Py-GC/MS and ATR-FTIR.The results show that there were synergistic effects between nano silica,graphene oxide and DSD.The LOI value of EP-D3-Si was up to 34.1%,and passed the V-0 grade.The char residue of the composite resin showed intumescent morphology,and the char residue yield increased at high temperature.The amount of flammable gases such as acetaldehyde and butane reduced.Silica existed in the char residual,which could block the heat source in the combustion.The loss rate of phosphorus in carbon residue increased,suggested nano materials and graphene oxide enhanced the flame retardancy in gas phase.0.5wt%nano silica can improve the tensile strength of EP-D3-Si,while graphene oxide reduced the tensile strength of EP-D3-Si,but the loss of tensile strength is less than 6%.