Design And Synthesis Of Phosphaphenanthrene-benzimidazole Derivatives And Study On The Flame Retardant Properties Of Polylactic Aci

Poly（lactic acid）（PLA）,a biodegradable material,is widely used in various fields of people’s production and life,while the demand for it is increasing year by year with the strengthening of environmental awareness.However,the flammability and severe melt-dripping of PLA greatly limits its application scope.9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide（DOPO）and its derivatives are often used as flame retardants for PLA materials by their good flame retardant properties and environmental friendliness,but the flame retardant efficiency of DOPO is still in urgent need of further improvement.Recently,benzimidazole and its derivatives have attracted much attention for their excelent flame retardant and carbon forming properties.The nitrogen-containing structures in benzimidazole moieties can play P-N synergistic flame retardant effect with DOPO structures,thus improving the flame retardant efficiency of DOPO and its derivatives.In addition,the introduction of benzimidazole and its derivatives can have the effect of improving melt-dripping by forming strong hydrogen bonding interactions with the polymer molecular chains to restrict the thermal motion of the molecular chains due to the presence of hydrogen bonds andπ-πstacking of benzimidazole groups.Thus,this thesis designed and synthesized different structures of phosphophenanthrene-benzimidazole derivatives by combining DOPO with benzimidazole groups through P-N bond,to study and analyze the effect law of different structures on the flame retardant properties of PLA films and further explore the possible flame retardant mechanism.The main research contents and results are as follows:（1）Three phosphophenanthrene-benzimidazole derivatives were synthesized by using the Atherton-Todd reaction mechanism,which were phosphophenanthrene-benzimidazole derivatives containing amino group（D-AB）,bis-phosphophenanthrene（2D-AB）and methyl group（D-MB）.The results of fourier transform infrared spectroscopy（FT-IR）,nuclear magnetic resonance spectroscopy(¹H-NMR,³¹P-NMR)and liquid chromatography-gas chromatography-mass spectrometry（ESI）indicated that the target products were successfully synthesized.The results of thermogravimetric analysis（TGA）showed that the three phosphophenanthrene-benzimidazole derivatives have good thermal stability,among which the thermal stability of D-AB and 2D-AB were better than that of D-MB.Moreover,the residual char of D-AB and 2D-AB could reach more than 18%at 600℃under nitrogen（N₂）and air（Air）atmosphere.In addition,the results of thermal cracking-gas chromatography/mass spectrometry（Py-GC/MS）tests showed that the three phosphophosphazene-benzimidazole derivatives were cleaved to DOPO fragments and benzimidazole fragments during the pyrolysis process.These two phosphorus-and nitrogen-containing fragments would further cleave to release PO·radicals that trap reactive H·and HO·radicals and inert gases（NH₃ or N₂）that dilute flammable gases.The result of linear fitting calculations by the Ozawa method showed that all three phosphophenanthrene-benzimidazole derivatives had high activation energies,with D-AB and 2D-AB having higher activation energies than D-MB.（2）A series of PLA/D-ABs and PLA/2D-ABs films with different components were prepared by solution blending of D-AB and 2D-AB with PLA using dichloromethane（CH₂Cl₂）volatilization film formation method.The results of FT-IR,UV-visible near-infrared diffuse reflectance（UV-vis）,X-ray diffractometer（XRD）,scanning electron microscopy（SEM）and rheological tests showed that the reactive functional groups of D-AB could form intermolecular hydrogen bonding interactions with PLA molecular chains and restrict the thermal motion of its molecular chains relative to PLA/2D-AB films,thus improving the complex viscosity of PLA.The results of flame retardancy analysis of PLA/D-ABs and PLA/2D-ABs films by vertical combustion（UL-94）,limiting oxygen index（LOI）and microcalorimetry（MCC）showed that PLA films with thickness of 0.15 mm passed VTM-0 grade at the addition of 5 wt.%and 10 wt.%of D-AB and 2D-AB,respectively,with the inclusion of D-AB improving the melt-dripping of PLA.The analysis of the flame retardant mechanism using TG-IR,EDS,Raman spectroscopy（Raman）and X-ray photoelectron spectroscopy（XPS）showed that both D-AB and 2D-AB could exert flame retardant effects through both gas phase and condensed phase,among which the gas phase flame retardant effect of 2D-AB was better than that of D-AB,and the condensed phase flame retardant effect of D-AB was better than that of 2D-AB.Finally,the mechanical property test results showed that the PLA/D-ABs films had the best mechanical properties due to the better compatibility of D-AB with PLA compared to 2D-AB.（3）A series of PLA/D-MBs films with different components were prepared by solution blending of methyl-containing D-MB with PLA.The PLA/D-MB10%film had better thermal stability at the initial decomposition temperature under N₂ and Air atmosphere compared to the PLA/D-AB10%film,but the carbon formation performance was poor.Meanwhile,the activation energy of PLA/D-MB10%film was higher than that of PLA/D-AB10%film,which indicated that the addition of D-MB was more favorable to inhibit the degradation of PLA films.It was only to a certain extent that PLA/D-MBs films could increase the complex viscosity of PLA throughout the temperature and frequency range,and the complex viscosity of PLA/D-MBs did not change much with the continuous increase of D-MB content.The PLA film with 5 wt.%D-MB addition also passed VTM-0 grade with LOI of 30.4%during combustion,with a longer flame duration and severe melt-dripping than the PLA/D-AB5%film.The flame retardant mechanism showed that PLA/D-MB10%films could only achieve flame retardancy by trapping reactive radicals in the gas phase,and the poor char layer formed after combustion with a large number of cracks implied that the exchange of thermal oxygen could not be effectively inhibited.Moreover,the mechanical properties of PLA/D-MB10%film was slightly worse than that of PLA/D-AB10%film Design and Synthesis of Phosphophenanthrene-Benzimidazole Derivatives and Study on the Performance of Flame Retardant Poly（lactic acid）...