Synthesis And Flame Retardancy Of Novel Organic Boron-nitrogen And Phosphorus-nitrogen Compounds And Their Synergistic Flame Retardancy With Inorganic Flame Retardants

Polymer materials are applied to all aspects of daily life due to their excellent properties,but polymer materials have poor flame retardancy and are flammable,and have potential safety hazards.Therefore,it is very important to modify polymer materials to improve their flame retardant performance.Flame retardants are a class of additives that improve the flame retardancy of polymer materials.Halogen-based flame retardants are a commonly used type of flame retardant.However,halogen-based flame retardants are easy to migrate and bioaccumulation,and produce toxic substances during combustion.Therefore,the development of halogen-free,non-toxic or low-toxic and high-efficiency flame retardants is the development direction of today's flame retardant field.At present,organic phosphorus-nitrogen flame retardants are the most studied organic halogen-free flame retardants,while the research on organic boron-nitrogen flame retardants is relatively weak.Boron-based flame retardants have the characteristics of low toxicity and high efficiency.In order to develop novel organic boron-nitrogen and organic phosphorus-nitrogen flame retardants,in this paper,the author synthesized organic compounds containing boron-nitrogen and phosphorus-nitrogen:hexakis(4-boronic acid-phenoxy)-cyclophosphazene(CP-6B),9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide derivative(DOPO-N),and melamine phenyl hypophosphite(MPHP),using nuclear magnetic resonance(NMR),Fourier infrared spectroscopy(FTIR)and High Resolution Mass Spectrometry(HRMS)to characterize and confirm its structure.The flame retardant properties of epoxy resin(EP),polyethylene(PE),and polypropylene(PP)were studied.Their molecular structure is shown in the figure below.Inorganic flame retardants have the advantages of environmental protection,low cost,and abundant sources.However,inorganic flame retardants require a high addition amount to achieve better flame retardant performance.In order to reduce the amount of inorganic and organic flame retardants,in this paper,the author compounded CP-6B,DOPO-N and MPHP with different types of inorganic flame retardants,and studied the synergistic flame retardant properties of organic flame retardants and inorganic flame retardants.The specific research results of this paper are as follows:1)The intumescent flame retardant CP-6B containing acid source,carbon source,and gas source was synthesized.Its flame retardant properties in EP were investigated using thermogravimetric analysis,limiting oxygen index(LOI),vertical combustion(UL 94)and cone calorimeter.When 7%CP-6B was added,the LOI value of EP/7%CP-6B reached 32.3%,passing the UL 94 V-0 rating,and its peak heat release rate(pk-HRR),total heat release(THR),average effective heat of combustion(av-EHC)and fire growth Index(FGI)decreased.This result showed that CP-6B had good flame-retardant properties and simultaneously played a role in both condensed phase and gas phase flame retardant.Through X-ray diffraction(XRD)and FTIR analysis,it was found that CP-6B finally decomposed to produce boron phosphate(BPO₄)with high thermal stability.BPO₄ improved the heat resistance of the char layer,effectively inhibited the escape of combustible volatiles,and played a role of isolation and protection.In addition,the mechanical properties of EP and EP/CP-6B were analyzed.The impact energy of EP was 0.31 J,and the impact strength was 7.7 k J·m^-2;the impact energy of EP/3%CP-6B was 0.55 J,and the impact strength was 13.8 k J·m^-2;the impact energy of EP/7%CP-6B was 0.31 J,and the impact strength was 7.8 k J·m^-2,this result showed that adding a small amount of CP-6B would not cause damage to the mechanical properties of EP.2)The organic CP-6B flame retardant was compounded with the inorganic flame retardant magnesium hydroxide(MH),and its flame retardant performance to EP was studied.When 3%CP-6B and 0.5%MH were added together,the LOI value of EP/3%CP-6B/0.5%MH reached 31.9%,and achieved the UL 94 V-0 rating.Compared with EP,the pk-HRR,THR,av-EHC and FGI values of EP/3%CP-6B/0.5%MH were reduced.CP-6B/MH had a synergistic flame retardant effect.It formed a continuous char layer with high heat resistance and released incombustible gases,reduced the concentration of combustible volatiles,and played a flame retardant effect in both the condensed phase and the gas phase.In addition,the impact energy of EP/3%CP-6B/0.5%MH was 0.39 J,and the impact strength was 9.7 k J·m^-2.The test results of mechanical properties showed that the addition of CP-6B and a small amount of MH could effectively improve the flame-retardant properties of EP without reducing the mechanical properties of EP.3)The flame retardant performance of PE was weak.When aluminum hydroxide(ATH)was added separately,the addition amount of ATH generally needed to exceed 50%to achieve higher flame retardancy.CP-6B was added to the PE/ATH to reduce the total amount of flame retardant.When 40%ATH was added,the LOI value of PE/40%ATH reached 25.1%,and achieved the UL 94 rating V-1.When 20%ATH and 20%CP-6B were added together,the LOI value of PE/20%ATH/20%CP-6B reached 27.0%,and achieved the UL 94 V-0 rating.The pk-HRR,THR,av-EHC and FGI of PE/20%ATH/20%CP-6B were lower than those of PE/40%ATH.The addition of CP-6B improved the flame retardant efficiency of PE/ATH.Through XRD and FTIR analysis,it was found that ATH/CP-6B decomposed to form a char layer containing Al₂O₃,BPO₄ and Al PO₄.ATH/CP-6B exerted condensed phase and gas phase flame retardant mechanism.Through the test of tensile properties,it was found that the elongation at break of PE/20%ATH/20%CP-6B was 224.1%of that of PE/40%ATH.This result showed that the addition of CP-6B reduced the effect of ATH on the elongation at break of PE.4)An organic phosphorus-nitrogen flame retardant DOPO-N was synthesized.The organic DOPO-N and inorganic zinc borate(ZB)were compounded,and the flame retardant properties of PE were studied.When 10%DOPO-N and 20%ZB were added,the LOI value of PE increased from 17.7%to 24.6%.The pk-HRR,THR,av-EHC and FGI values of PE/10%DOPO-N/20%ZB were lower than those of PE.The decomposition of ZB produced zinc oxide and boron oxide,which covered the surface of the substrate and and exerted a condensed phase flame retardant effect.The addition of DOPO-N increased the gas phase flame retardant effect.DOPO-N/ZB had a condensed phase and gas phase flame retardant effect.5)An organic phosphorus-nitrogen flame retardant MPHP was synthesized.The organic MPHP was compounded with inorganic silica(Si O₂),and its flame retardant performance to PP was studied.When 30%MPHP was added,the LOI value of PP/30%MPHP reached31.3%,reaching UL 94 V-0 rating,and the pk-HRR of PP/30%MPHP reduced to 737 k W/m².Adding MPHP alone improved the LOI value of PP,but the improvement of heat release rate was limited.When 27%MPHP and 3%Si O₂ were added together,the LOI value of PP/27%MPHP/3%Si O₂ reached 28.7%,reaching the UL 94 V-0 rating,and the pk-HRR of PP/27%MPHP/3%Si O₂ reduced to 578 k W/m².The addition of a small amount of Si O₂ not only achieved a higher LOI value,but also reduced the heat release rate of the sample.Through the analysis of the residual char,it was found that the decomposition of MPHP/Si O₂formed a heat-resistant char layer containing silicon phosphate(Si P₂O₇)in the condensed phase.MPHP/Si O₂ had a condensed phase and gas phase flame retardant effects.