Synthesis Of Novel Bridged-cyclotriphosphazene Derivatives And Their Flame-retardant Application In Epoxy Resins

Epoxy resins have been recognized as a kind of high performance thermoset resins,owing to its excellent properties such as outstanding mechanical properties,low dielectric constant,good resistant to chemicals,solvents and moisture.But the high flammability of epoxy resins make it difficult to meet safety standards and limit its broader applications.Although the traditional halogenated flame retardants for epoxy resins have been well developed to meet the considerable secure requirements in most areas,the possible health and environmental hazards make them replaced by halogen-free ones gradually.Currently,cyclotriphosphazene derivatives have been widely studied due to their non-halogen,high contents of P/N and excellent flame retardant effects.Meanwhile,the characteristic substitution chemistry of cyclotriphosphazene makes it possible to prepare abundant reactive and additive flame retardants for EP.In this work,four novel bridged-cyclotriphosphazene derivatives were designed and synthesized by introducing different bridging structures and substituent groups on cyclotriphosphazene.The properties and flame retardant mechanisms of modified EP thermosets were discussed comprehensively and systematically.The main contents are summarized as follows:1. As intermediates,bisphenol S bridged chlorocyclotriphosphazene?BPS-BCP?and bisphenol A bridged chlorocyclotriphosphazene?BPA-BCP?,were synthesized from hexachlorocyclotriphosphazene,bisphenol-S and bisphenol-A.The chemical structure of BPS-BCP and BPA-BCP was characterized via Fourier transform infrared spectroscopy?FTIR?,¹H NMR,³¹P NMR,ESI-MS and X-ray diffraction analysis.Furthermore,BPS-BCP and BPA-BCP reacted with aniline to complete the full substitution of chloride atoms in its phosphazene ring to obtain two reactive bridged aniline-cyclotriphosphazenes,bisphenol S bridged penta?anilino?cyclotriphosphazene?BPS-BPP?andbisphenolAbridged penta?anilino?cyclotriphosphazene?BPA-BPP?.Additionally,BPS-BCP and BPA-BCP reacted with phenol to complete the full substitution of chloride atoms in its phosphazene ring to obtain twoadditivebridgedphenoxy-cyclotriphosphazenes,bisphenolSbridged penta?phenoxy?cyclotriphosphazene?BPS-BHP?andbisphenolAbridged penta?phenoxy?cyclotriphosphazene?BPA-BHP?.The chemical structure of BPS-BPP,BPA-BPP,BPS-BHP and BPA-BHP was characterized via FTIR,¹H NMR,³¹P NMR and ESI-MS.2.BPS-BPP and BPA-BPP were mixed in different proportions with DGEBA/DDM system in order to prepare a series of flame retardant epoxy resins?EP?,respectively.The differential scanning calorimeter?DSC?results indicated that the glass transition temperatures?T_g?of EPs increases with an increase in flame retaedant loadings.The thermal stability of modified EP thermosets were scrutinized by TGA.The results showed that BPS-BPP and BPA-BPP react with EP matrix during heating that effectively improves the charring of EP matrix at lower temperatures and enhances the thermal stability of EP at higher temperatures.The flame retardancy of the modified EP thermosets was evaluated by limiting oxygen indexes?LOI?and vertical burning tests?UL-94?.The results showed that with only 9 wt%addition,EP/BPS-BPP passed UL-94 V-1 rating with LOI value of 29.7%and EP/BPA-BPP passed UL-94 V-1 rating with LOI value of 28.7%.In order to study the combustion behavior of the modified EP thermosets,microscale combustion calorimeter and cone calorimeter tests were conducted.The results showed that The peak of heat release rate?PHRR?,total heat release?THR?,total smoke production?TSP?values of the modified EP thermosets were declined conspicuously,indicating good flame-retardant and smoke suppression efficiency of BPS-BPP and BPA-BPP.Dynamic mechanical analysis?DMA?demonstrated that the storage modulus?E'?of EP/9%BPS-BPP and EP/9%BPA-BPP were enhanced to a higher level compared with neat EP.3.BPS-BHP and BPA-BHP were mixed in different proportions with DGEBA/DDM system in order to prepare a series of flame retardant EPs,respectively.The DSC results showed that the T_g of EPs containing BPS-BHP or BPA-BHP has only a slight decrease in comparison to EP,which indicates an moderate plasticizing effect of BPS-BHP and BPA-BHP.The thermal stability of modified EP thermosets were scrutinized by TGA.The results showed that BPS-BHP and BPA-BHP have a high thermal stability,and they effectively improves the charring of EP matrix and enhances the thermal stability of EP.The flame retardancy of the modified EP thermosets was evaluated by LOI and UL-94.The results showed that with only 9 wt%addition,EP/BPS-BHP passed UL-94 V-1 rating with LOI value of 31.4%and EP/BPA-BHP passed UL-94 V-0 rating with LOI value of 33.9%.In order to study the combustion behavior of the modified EP thermosets,microscale combustion calorimeter and cone calorimeter tests were conducted.The results showed that The PHRR,THR,TSP values of the modified EP thermosets were declined conspicuously,indicating better fire safety and smoke suppression were obtained.Furthermore,the results of DMA revealed that the incorporation of BPS-BHP and BPA-BHP decreased E'slightly compared with neat EP.4.To elucidate the flame retardant actions of bridged-cyclotriphosphazene for EP,comprehensive measurement including Fourier transform infrared?FTIR?,Scanning electron microscopy?SEM?,X-ray photoelectron spectroscopy?XPS?,Laser Raman spectroscopy?LRS?,pyrolysis gas chromatography/mass spectrometry?Py-GC/MS?and TG-FTIR techniques were adopted to analyze the char layers and gaseous products.As a result,the flame retardant mechanism was proposed:Bridged aniline-cyclotriphosphazenes or its decomposition products react with EP matrix during heating that effectively improves the charring of EP matrix at lower temperatures.Aniline and NH₃ decomposed from cyclotriphosphazene were released to gas phase.This causes the residue to foam and produces an intumenscent char layer,which acts as insulating barrier to prevent the oxygen and the feedback of heat from reaching the underling material during the process of combustion.Therefore,the enhanced flame retardancy of EPs modified with bridged aniline-cyclotriphosphazenes mainly depends on the protective char formed in the condensed phase.Bridged phenol-cyclotriphosphazenes also enhance the charring of EP matrix in condensed phase.Moreover,phenoxyl radicals decomposed from phenol-cyclotriphosphazenes act as radical scavengers during combustion and inhibit the free radical chain reaction of combustion.Accordingly,bridged phenol-cyclotriphosphazenes showed better flame retardanc on EP than the aniline ones because of the effective synergistic flame retardant action occurred in both condensed and gaseous phases.