Synthesis Of Piperazine Pyrophosphate And Its Flame Retardant Effect In Polypropylene

Compared with ammonium polyphosphate-based intumescent flame retardants(IFR),the piperazine pyrophosphate(PPAP)-based IFRs have better thermal stability,flame retardant effects and wider applications.However,it also has some problems such as immature synthesis process and formula,unclear action mechanism and the flame retardant effect needing to be improved.The use of synergists is an important means to improve the flame retardant effect of IFR,but the research on the synergistic effect of PPAP-based IFR is almost blank.Therefore,there is important theoretical and practical significance to carry out research on the synthesis,synergy and mechanism of PPAP.The PPAP was synthesized through the catalytic condensation of piperazine diphosphate,and its structure was characterized by infrared spectrum,solid-state ³¹P NMR and ¹³C NMR.Its optimal synthesis conditions were as follows:w(piperazine diphosphate):w(catalyst A)=1:0.012?0.015,and the reaction temperature and time being 210°C and 5 h respectively,the product being purified by methanol washing.Under the above conditions,the overall yield of the product was about 88%,and the water solubility was 0.15 g/100 m L of water.The process has some advantages such as simple process,high yield,low water solubility,good thermal stability and white color of the product.The synergistic flame retardant effect of three kinds of ZnO and PPAP/melamine polyphosphate(MPP)or PPAP/melamine cyanurate(MCA)in PP was investigated by limiting oxygen index(LOI)determination and vertical burning test.The results show that they all have good synergism.For example,when the loading of PPAP/MPP based on the total weight is 16wt%,the LOI and flame retardant efficiency(EFF)of PPAP/MPP/PP are 29.9%and 0.744,respectively,and the vertical combustion level is FV-2,while the LOI and EFF of PPAP/MPP/ZnO/PP with 15.5wt%of PPAP/MPP and0.5wt%of ordinary ZnO are 32.3%and 0.894,respectively,and the synergistic efficiency is 7.58.The type of ZnO has a slight influence on the synergistic effect,nano-ZnO has slightly poor synergism,while the synergy of amorphous ZnO is between ordinary and nano-ZnO;the synergistic effect of ZnO with PPAP/MPP is slightly better than that with PPAP/MCA.The effect of ordinary ZnO on the combustion behavior of PPAP/MPP(MCA)/PP was investigated by cone calorimetry test(CCT).The results show that the ZnO has an obvious influence on the combustion behavior of the above flame retardant PP.Combining a small amount of ZnO can significantly increase the ability of PPAP/MPP(MCA)to inhibit the cracking of PP,reduce various heat release parameters and fire growth index(FIGRA)of flame retardant PP,and significantly increase the fire performance index(FPI).For example,compared with PPAP/MPP/PP,the peak heat release rate,mean heat release rate,total heat release and FIGRA of PPAP/MPP/ZnO/PP were reduced by 44.7%,25.3%,26.1%and 37.0%,respectively.And the FPI increases by 59.4%.The above-mentioned synergistic effects of ZnO with PPAP/MCA are much better than that with PPAP/MPP;Combining a small amount of ZnO can also markedly inhibit the production of smoke and CO during the combustion of flame-retardant PP.The flame retardant mechanism of PPAP/MPP(MCA)and the synergistic mechanism of ZnO were studied by TGA of flame retardant PP,the analysis of the component and structure of heated flame retardant PP and CCT residues.The flame retardant mechanism of PPAP/MPP(MCA)is as follows:during combustion,PPAP is first cracked into phosphoric acid,metaphosphoric acid and their piperazine salt,MPP is cracked into polyphosphoric acid and ammonium polyphosphate,and then further cracked into polymetaphosphoric acid with the increase of temperature.The strong dehydration of polymetaphosphoric acid makes piperazine,its derivatives and the oxide of PP dehydration form amorphous and graphitized elemental carbon,the compounds of carbon and nitrogen.The polymetaphosphoric acid and the above substances are intertwined to form the residue.The inert gases such as N₂,NH₃,CO₂,CO and water decomposed from PPAP,MPP or MCA and PP foam the molten residues to form an intumescent char layer.The layer performs the flame retardant effect by insulating heat and oxygen,preventing the composite from further decomposition and the decomposition products from volatilization,which also results in the suppression on the flame propagation and fire intensity.The synergistic mechanism of ZnO is as follows:ZnO promotes obviously the charring,and increases the residuals of combustion,the degree of graphitization as well as the strength and stability of the intumescent layer,so that the inhibition of PPAP/MPP(MCA)to the pyrolysis of PP is improved,which makes PP the better flame retardant property and inhibiting ability on propagation and fire intensity.