Intumescent Flame Retardation, Nano-flame Retardation And Synergistic Flame Retardation For Polypropylene

Polypropylene (PP) is widely applied to various fields like chemical,construction,light industry,household appliance,and package due to its overwhelming advantagessuch as high transparency,nontoxicity,ease of processing,low hygroscopicity,highimpact-strength,resistance to chemical erosion,excellent electric insulation,and highperformance-price ratio.Currently,its consumption is only second to that ofpolyethylene among the top-five general plastics.Unfortunately,PP is ratherflammable with a limited oxygen index (LOI) value of around 17.0.In addition,itcombusts too fast and generates a great of heat accompanied by dripping,whichsignificantly limits its wide applications,thus it is of significance to reduce itsflammability.Firstly,based on the molecular design,we synthesized an amino terminatedoligomeric intumescent flame retardant,poly (4,4-diamino diphenylmethane-O-bicycli pentaerythritol phosphate-phosphate),PDBPP for short,on thebasis of phosphrous oxychloride,pentaerythritol,and 4,4'-diaminodiphenyl methane,and the chemical structure of PDBPP was characterized by IR,¹H-NMR,and XPS.The results of thermal analysis showed that PDBPP had excellent thermal stabilityand char-forming ability,yielding 55.0 wt% char residue at 600℃in air atmosphere.Besides,it could match well the processing temperature of PP.Incorporation ofPDBPP not only resulted in a remarkable improvement in the thermal stability andLOI values,but markedly reduced the heat release rate,total heat release and massloss rate of PP.Using maleic anhydride grafted polypropylene (PP-g-MA)as thecompatibilizer,the interfacial tension was improved and the sizes of PDBPP domainsin PP matrix were significantly reduced to one tenth of those of PP/PDBPP samplesbefore compatibilization.Moreover,compatibilization led to a further improvement inthe thermal stability and flame retardancy of PP,while tensile strengths of PP/PDBPPsystems were restored to the levels similar to that of pure PP even if with a 30 wt%PDBPP loading. Secondly,we employed fullerene (C₆₀) to reduce the flammability of PP.TGA andcone calorimeter measurements displayed that at very low loading ((?)2wt%),C₆₀could remarkably improve the thermal stability and dramatically reduce the heatrelease rate and mass loss rate,in which it could match carbon nanotubes (CNTs).Investigations on rheological analysis,heat-treatment and char residue demonstratedthat C₆₀ could trap the free radicals created by the decomposition of PP and otherderivative free radicals like H·and·OH,which will make the melt viscositydramatically increase and consequently make the small molecules need much moreenergy and time to spread into the flame zone to support the combustion as fuels.Inthis way,C₆₀ improved the thermal stability and flame retardancy of PP.Dynamicmechanical thermal analysis (DMA) exhibited that C₆₀ had the reinforcing effect butaccompanied by the plasticization effect for PP.Thirdly,we employed C₆₀ to decorate PDBPP and fabricated a dendrimer-likemacromolecular nano/intumescence synergistic flame retardant,C₆₀-d-PDBPP,whosechemical structure and morphology were characterized by IR,XPS,TEM and SEM.C₆₀-d-PDBPP could readily disperse well in PP matrix through employing PP-g-MAas the compatibilizer since many active amino and hydroxyl groups existed inC₆₀-d-PDBPP molecule.The combination of the high free-radical-trapping ability ofC₆₀ and excellent char-forming ability of PDBPP further heightened the thermalstability and reduced the flammability of PP,evidenced by the observation of muchhigher thermal degradation temperature and much lower peak heat release rate.Subsequently,through the hydroxylation,amino-functionalization,and C₆₀-functionalization of CNTs,we synthesized a novel nano/nano synergistic flameretardant,C₆₀-d-CNTs which was characterized by IR,XPS,TEM and SEM.Compared with C₆₀ and CNTs,C₆₀-d-CNTs was much easier to disperse in PP matrixbecause its structure contained plenty of active amino and hydroxyl groups and thusthe interfacial adhesion could be improved through compatibilization reaction withPP-g-MA as the compatibilizer.Synergistic flame retardancy effects could beobserved from the further reduction in peak heat release rate of PP.Both the highfree-radical-trapping ability of C₆₀ and the barrier effect caused by the CNTs network were responsible for the elevation of thermal stability and the improvement in theflame retardancy of PP.Finally,through hydroxylation and phosphorylation of CNTs,and in-situcondensation polymerization,we fabricated an intumescent-flame-retardant wrappedCNTs,IFR-w-CNTs,which was characterized by IR,XPS,TEM and SEM.Thediameter of IFR-w-CNTs could be controlled through adjusting the feed ratio of IFRto CNTs.Due to the fact that IFR-w-CNTs contains active amino and hydroxyl groups,we employed PP-g-MA as the compatibilizer to reduce the interfacial tension betweenIFR-w-CNTs and PP,thus IFR-w-CNTs could disperse better in PP matrix relative topristine CNTs.The cone calorimeter measurements clearly indicated that,comparedwith CNTs,IFR-w-CNTs could make the peak heat release rate reduce to a greaterextent at the same loading level,which was due to the fact that the char residuecreated by the decomposition of IFR adhered onto the surface of CNTs andconsequently made the CNTs network more compact.Interestingly,when the ratio ofIFR and CNTs was 1:2,not only the optimum synergistic flame retardancy achieved,but the reinforcement effect also reached the peak value,for the tensile strength of thePP/IFR-w-CNTs sample increased up to 40.8 MPa relative to 35.1 MPa of pure PP.