Synthesis Of Novel Phosphonamide Flame Retardants And Their Effects On The Combustion,Mechanical And Crystallization Behavior Of Polylactic Acid

With the enhancement of energy and environmental awareness,renewable and degradable biopolymer materials have received widespread attention.Bio-based polymer material polylactic acid(PLA)has been widely used in medicine,agriculture,electronics,and rail transit etc,due to its high transparency,excellent mechanical strength and easy processing.However,PLA is flammability,slow crystallization and brittle defects,which limits its application in some emerging fields.Therefore,the preparation of PLA composites with excellent flame retardancy,crystallization properties and high toughness owns important scientific significance and practical application value.In this paper,based on the characteristics of PLA,three novel phosphonamide flame retardants(4,4'-(phenylphosphoryl)bis(piperazine-4,1-diyl))bis(diphenylphosphine oxide)(PDPO),piperazine-1,4-diylbis(diphenylphosphine oxide)(PPDO),N,N',N"-(nitrilotris(ethane-2,1-diyl))tris(P,P-diphenylphosphinic amide)(NTPA)were synthesized through molecular design of flame retardants and the structure of the three phosphonamide flame retardants were characterized by Fourier transform infrared spectroscopy(FTIR),1H,13C,31P nuclear magnetic resonance spectroscopy(NMR),elemental analysis(EA)and other tests.The degradation behavior of flame retardants was studied by thermogravimetric analysis(TGA),and the results showed that three flame retardants possessed good thermal stability.Three flame retardants PDPO,PPDO and NTPA were added to PLA materials through melt blending to prepare flame retardant PLA materials.The cone calorimetry(CONE),limiting oxygen index(LOI)and vertical combustion(UL-94)tests were used to study the combustion performance of materials;the thermal properties and crystallization behavior of materials were studied using differential scanning calorimetry(DSC),TGA and polarized light microscope(POM)tests;the mechanical properties of materials and the compatibility between flame retardants and resins were investigated through the tensile,bending and impact properties and scanning electron microscope(SEM)tests.Meanwhile,the flame retardant mechanism of flame retardants on PLA materials was also studied through SEM,Raman analyzer(LRS),FTIR,X-ray photoelectron spectroscopy(XPS)and thermogravimetric-infrared-gas-mass spectrometry.The results showed that the PLA composites passed UL-94 V-0 rating during the vertical combustion test when 4,3 and 2.5wt%of PDPO,PPDO and NTPA were added to the PLA material,and their LOI values were increased from 19.0%of pure PLA to 29.4,27.8 and 28.5%,respectively.The CONE tests showed that the average effective heat of combustion(av-EHC)of flame-retardant PLA materials were significantly lower than that of pure PLA with the addition of phosphonamide flame retardants,which indicated that phosphoamide flame retardants possessed a good flame retardant effect in gas phase.Besides,the total heat release(THR)of materials were effectively reduced,and the flame retardant performance index(FRI)were significantly improved.The TGA test showed that the addition of flame retardant catalyzed the degradation and charing of the materials.The initial thermal degradation temperature of the materials was reduced,and the char residues of the materials were increased.The analysis of flame-retardant mechanism showed that the addition of the phosphonamide flame retardants promoted the carbonization of the materials,and then formed a dense,partially graphitized and high-quality char layer on the surface of the materials,which exerted an excellent barrier effect in condensed phase.At the same time,the flame retardants released incombustible gases,phosphorus-containing and benzene-containing free radicals during their pyrolysis process and inhibited the combustion reaction in gas phase,resulting in the reduction of the combustion intensity of materials.Thus,PLA composites obtained excellent flame retardancy.Due to the slow crystallization of PLA,it is necessary to add the nucleating agent to promote the crystallization of PLA during practical application.In this paper,the influence of flame retardants on the crystallization behaviors and mechanical properties of PLA material was studied through DSC and POM tests.The test results showed that the incorporation of phosphoamide flame retardants PDPO,PPDO,and NTPA increased the crystallinity of PLA from 27.34%to 32.26,32.14 and 29.01%,respectively,and the crystallization rate of the materials was significantly increased.Among them,the introduction of NTPA kept the obvious spherulitic morphology of PLA materials.The mechanical performance tests showed that the flame-retardant PLA materials maintained good tensile,flexural and impact properties compared with pure PLA,and the SEM test proved that the phosphonamide flame retardants exhibited excellent compatibility with the PLA resin.Although the addition of flame retardants enabled PLA with excellent flame retardancy,crystallization behaviors and good mechanical properties,the inherent brittleness of PLA materials has not been improved.In this paper,hollow kapok fibers(KF)with light weight and high specific surface area were selected for toughening modification of flame-retardant PLA.First,the waxy on the KF surface was removed to obtain MKF by alkali treatment,and it was added to the PLA material.When 0.5 wt%of MKF was introduced,the impact strength of the PLA improved from 3.73 kJ·m-2 to 5.54 kJ·m-2 with the increase of 48.5%.When the addition of MKF was 0.5 wt%and the incorporation of NTPA was 3 wt%,the PLA/NTPA/MKF composites passed UL-94 V-0 grade and its LOI value was 28.3%,and the impact strength of PLA/NTPA/MKF reached 4.43 kJ·m-2.The analysis of the flame retardant mechanism showed that the addition of NTPA promoted the premature degradation and char formation of PLA/MKF materials.NTPA played an excellent flame retardant effect in gas and condensed phase.The flame retardant properties of PLA/NTPA/MKF materials were effectively improved.Therefore,the prepared PLA/NTPA/MKF bio-based composites with outstanding flame retardant properties,high crystallization rate and excellent toughness endowed PLA composites a good application prospect in emerging fields.