Synthesis Of Phosphine Oxide-Containing Hyperbranched Flame Retardants And Study On Flame Retardancy And Mechanisms Of Their Typical Polymer Composites

Polymers are widely used in every aspect of production and life due to their low weight and cost,good insulativity and corrosion resistance.However,their high contents of carbon and hydrogen elements make them combustible materials.This increases the risk of fires.Therefore,the flame retardancy of polymers must be improved.For environment and human healty concern,halogen-containing flame retardants have been restricted.As a kind of excellent alternative,phosphorus-containing flame retardants are highly-efficient with low toxicity and smoke production.As is well known,the hydrolysis of flame retardants always leads to the decrease,or even loss of flame retardance.In comparision with a P-O-C bond,a P-C bond is very resistant to hydrolysis,acid and base,therefore,phosphine oxides with sole P-C bonds can maintain flame retardance well.The phosphorus content of trialkyl phosphine oxide is always higher than that of the triaryl one.However,only a little application of them in flame retardance was reported until now.Generally,better flame retardance can be obtained with higher phosphorus content of flame retardant.On the other hand,tetrakis(hydroxymethyl)phosphonium salts are environment-friendly and cheap industrial products with low toxicity.They can be used to facilely synthesize trialkyl phosphine oxides with alkali.And the sulfate ions can be simply removed through the reaction of tetrakis(hydroxymethyl)phosphonium sulfate with barium hydroxide.Besides,the products of tetrakis(hydroxymethyl)phosphonium sulfate with alkali contain three hydroxyls and they are easy to be used to synthesize hyperbranched polymers.Hyperbranched polymers have the merits of low melt viscosity,good solubility,high functionality and easy modification.Therefore,according to the features of different polymers,we designed and synthesized six hyperbranched polymers containing phosphine oxide from tetrakis(hydroxymethyl)phosphonium sulfate and introduced them into corresponding plymer to study their flame retardant effects and mechanisms.The main contents are as follows:1.Hyperbranched poly(phosphonate-phosphoramide)containing phosphine oxide was synthesized from the polycondensation of phenylphosphonic dichloride and thrhydroxymethylphosphine oxide before endcapping with anline.It was used as an additive-type flame retardant for epoxy resin.DSC results show that flame retardant is compatible with epoxy resin.UL-94 V-0 rating can be achieved for epoxy resin with 5%flame retardant and the limited oxygen index(LOI)is 35.5%.With 7.5%flame retardant,the peak of heat release rate(PHRR)and total heat release(THR)were decreased by 66.2%and 37.3%respectively,and the time to ignintion was delayed.This epoxy composite formed intumescent char layer with millimeter-scale big pores in it.Thermal decomposition temperature and glass transition temperature(Tg)of flame retardant epoxy resins are decreased,while their char yields(CY)are increased.The simultaneous gas and condensed phase flame retardant mechanisms were verified by cone calorimetry.The phosphine oxides released from thermal decomposition of flame retardant are speculated to generate PO radicals during combustion and inhibit flame.In the condensed phase,flame retardant promoted the formation of abundant,highly-graphited and phosphorus-containing carbonaceous char,with higher thermal and thermal-oxidative stability and strength.Thus,the char layer was compact and intumescent.It hindered flammable volitiles from heat and oxygen,protected the inner matrix and reduced fuel supply.2.Hyperbranched poly(aminomethylphosphine oxide-amine)was synthesized from the polycondensation of trihydroxymethylphosphine and piperazine before oxidation.It was used as flame retardant co-curing agent for epoxy resin.The nonisothermal DSC traces show slightly changed curing reactivity and increased activation energy with the increase of hyperbranched polymer content.The flame retardant epoxy resins have decreased thermal decomposition temperatures and increased CY.The flame retardant is compatible with epoxy resin,which is verified by SEM.The polyfunctional hyperbranched polymer improves the cross-linking density of epoxy resin,and increases their Tg.With 3%flame retardant,epoxy resin passed V-0 rating and the LOI value is 30.7%.The blowing-out effect upgrades UL-94 rating.When the addition amount exceeds 1%,heat release,carbon monoxide and smoke production all decreased.The condensed phase flame retardant mechanism is proved to be increased CY and the barrier and protect effect of char layer.The gas phase flame retardant mechanism is speculated to be the flame inhibition effect of released phosphorus-containing species in the combustion process.The impact strength of epoxy resins increases as flame retardant cotent increases.Therefore,when more than 1%hyperbrached polymer incorporated,fire safety,Tg and toughness of epoxy resin are simultaneously improved.3.Hyperbranched polyurethane containing phosphine oxide was synthesized from trihydroxymethylphosphine oxide and 4,4'-diphenylmethane diisocyanate.It was used as char forming agent and mixed with ammonium polyphosphate in the ratio of 1:2 to form the flame retardnat.With 25%flame retardant,the polypropylene composite passed V-0 rating and the LOI value is 25%.Compared to the neat polypropylene,the flame retardant polypropylene has lower thermal decomposition temperature under nitrogen while higher one under air,and higher CY.It showed significantly decreased PHRR and THR and increased CY in cone test.The condensed phase flame retardant mechanisms are mainly ascribed to increased CY and the barrier and protect effect of the char layer.The mixed flame retardant showed better effects than either of its two components with the same addition amount,thus,synergistic effect between hyperbranched polyurethane containing phosphine oxide and ammonium polyphosphate was concluded in the flame retardance of polypropylene.4.Hyperbranched polyester polyol containing phosphine oxide was synthesized from the polycondensation of trihydroxymethylphosphine oxide with phathalic anhydride before reaction with propylene epoxide.TGA results show that the hyperbranched polyol promotes the degradation and the formation of more char of the free foaming rigid polyurethane foam both under nitrogen and air.When the content of flame retardant polyol increases,the density of foam is improved due to the increased cross-linking density and the LOI values increase.The LOI value of the foam with 50%polyether polyol substituted by hyperbranched polyol is 24.5%.In the cone test,THR decreases,however,only the foam with 50%polyether polyol substituted has lower PHRR and higher CY.The smoke production rate and total smoke production increase as the fraction of flame retardant polyol increases.The flame retardant mechanisms of hyperbranched polyol are increased char yield and speculated flame inhibition effect of PO type radicals.5.Aluminium hyperbranched polyphosphonate containing phosphine oxide was synthesized from trihydroxymethylphosphine oxide and phenylphosphonic dichloride before hydrolysis and reaction with aluminium sulfate.It degrades at more than 600?,which originates from the formation of cross-linking.The little bound water in hyperbranched polymer leaded poly(butylene terephthalate)(PBT)composite lower thermal decomposition temperature.Hyperbranched polymer has little influence on the main degradation stage of PBT,however,its high CY leads to the raised CY of flame retardant PBT.LOI values of PBT composites increase to 25.5%for 30%flame retardant incorporated while still no UL-94 rating was obtained.PHRR and THR of PBT composites decrease and C Y increases in cone tests.Because the ratios of THR and total mass loss decrease slightly,hyperbranched polymer mainly functions in the condensed phase.The limited flame retardant effect is attributed to mismatching of degradation temperature ranges of flame retardant and polymer.6.Melamin hyperbranched polyphosphate containing phosphine oxide was synthesized from trihydroxymethylphosphine oxide and phosphorus oxychloride before hydrolysis and reaction with melamin.It was introduced into polystyrene.The polystyrene composites show decreased thermal decompostition temperatures and increased CY due to the loss of the bound water and the high CY of flame retardant,respectively.Their LOI values increase with more flame retardant added.When the addition amount reaches 30%,the flame retardant polystyrene has an LOI value of 24%,while still achieves no UL-94 rating.The improvement of LOI values is related to the formed intumescent char layer during combustion process.PHRR and THR of polystyrene composites are significantly decreased,and their CY increases.The condensed phase flame retardant mechanisms of hyperbranched polymer in polystyrene are the increased CY and the barrier and protect effect of the intumescent char layer.