Study On Flame Retarded Polypropylene With APP Modified And Coated By Hyperbranched Polyester Charring Agent

Polypropylene(PP)has been fully applied in many fields,from mall containers in our daily life to big tanks used in chemical plants as well as vehicles and ships,owing to its good acidic and corrosion resistance,excellent heat resistance,light weight,and extremely high elongation at break and impact strength.However,PP is extremely flammable because of low limiting oxygen index(LOI)of 18.2 voL%,which limits the application of PP.Therefore,it is extremely urgent to study the flame protection of PP.At present,phosphorus flame retardants are more used in the flame retardant research of PP,among which ammonium polyphosphate(APP)has been widely studied because of its high phosphorus-to-nitrogen ratio,which can play a dual role of acid source and blowing agent simultaneously.However,its strong moisture absorption and poor compatibility with PP cause the declines of mechanical properties and flame retardancy of PP.Therefore,for the first time,a kind of terminal carboxyl hyperbranched polyester(HBPE,commercially used as a rheology modifier or toughening agent)was selected as the raw material to address the above issues in two ways with silane coupling agent(KH-550).One is HBPE-modified APP(MAPP);the other is HBPE-microencapsulated APP(MCAPP).Besides,two different methods were applied for the preparation of flame retarded PP,such as the flame-retardant coating and the melting blending.As for flame-retardant coating,PP specimens were first treated by plasma to increase their surface energy for better adhesion.Then,the coating liquid composed of MAPP,HBPE and PVFA was coated on the surface of PP specimens by dip-coating.The obtained flame-retardant coating was of excellent flame retardancy,good water resistance,strong adhesion and good flexibility.The specimen PP2-2 with a 180 ?m thick coating had an LOI value of 32.2 voL% and passed the UL-94 V-0 rating.In CCT,the peak of heat release rate(PHRR),total heat release(THR),the peak of smoke production rate(PSPR)and fire growth rate index(FIGRA)of PP2-2 decreased by 82%,30%,60% and 95%,respectively,compared with PP.The strong intumescent char layer formed during the combustion imparted excellent thermal insulation(flame retardant effect in the condensed phase),and generated gases including ammonia gas,carbon dioxide as well as water vapor diluting oxygen and combustible gases,further exerting flame retarding effect in the gas phase and giving a high fire safety for PP.As for blending method,MCAPP and PP were melt-blended to prepare flame-retarded PP composites.20 wt% of MCAPP enhanced the flame retardancy of PP,with LOI value of 32.1 voL% and UL-94 V-0 rating.With 25 wt% of MCAPP,the LOI value further increased to 34.2 voL%.CCT shows that MCAPP also imparted high fire safety to PP.Compared with PP,PHRR,THR and FIGRA of specimen PPM3 decreased by 86.8%,56.4% and 92.6%,respectively.Meanwhile,the smoke suppression of PPM3,i.e.total smoke production(TSP)and PSPR,decreased by 79.6% and 51.8%,respectively.The analyses from SEM,TG-IR,XPS and LRS results of char layers show that MCAPP played the flame retardant effect in condensed phase and gas phase.More importantly,tensile and impact tests show that 20 wt% and 25 wt% MCAPP had little negative effect on the tensile strength of PP matrix,while enhancing impact strength.