Preparation And Properties Of Flame Retardant PLA/g-C₃N₄ Composites

Polylactic acid?PLA?are biodegradable green polymer materials that are widely used in various fields due to their excellent properties.However,PLA is a highly flammable,which seriously affects its further development and application,so it is very important to improve the flame retardancy of PLA.It is demonstrated that graphite carbon nitride?g-C₃N₄?,as a two-dimensional nanomaterial,has excellent thermal stability and chemical stability,and can be applied to the flame retardancy of polymer materials.In this thesis,PLA/graphite carbon nitride composites were prepared by melt blending method and their flame retardancy were studied.Using melamine as raw material,the multi-walled two-dimensional nanomaterial g-C₃N₄was prepared by high temperature polymerization.Then,using hexachlorocyclotriphosphazene,cobalt chloride and melamine as raw materials,g-C₃N₄ dopants g-C₃N₄/P and g-C₃N₄/P/Co were also prepared by same method.The morphology and molecular structure of the product were characterized by scanning electron microscopy?SEM?,fourier transform infrared spectroscopy?FTIR?,X-ray diffraction?XRD?and X-ray photoelectron spectroscopy?XPS?.The preparation and doping of g-C₃N₄ multi-walled two-dimensional nanomaterials were proved to be successful;and its thermal stability was studied by thermogravimetric analysis?TGA?.To prepare flame retardant PLA composites,g-C₃N₄,g-C₃N₄/P,g-C₃N₄/P/Co was added to PLA,respectively.Through TGA,SEM,limiting oxygen index?LOI?,vertical combustion,cone calorimeter test,differential scanning calorimeter?DSC?and other tests,the flame retardancy and mechanism,crystallization and mechanical properties of PLA composites were studied.The study found that the thermal stability and flame retardancy of the three composite materials were improved;The flame retardancy of g-C₃N₄ is slightly better than that of pure PLA,and the flame retardancy of g-C₃N₄/P and g-C₃N₄/P/Co are better than that of g-C₃N₄.Respectively,adding 15wt%g-C₃N₄/P and 15wt%g-C₃N₄/P/Co to PLA composites,the LOI value was increased to 24%,22.5%,and UL-94 rate reached V-1 level;Compared with pure PLA,their peak heat release rate?PHRR?decreased by 24.7%and 24.1%,and their total heat release value?THR?decreased by 15.8%and 12.1%,respectively.The addition of 15wt%g-C₃N₄/P/Co greatly inhibited the production of toxic gases,and peak CO production?PCOP?was reduced by 52.9%.Study indicated that the addition of three flame retardants leads that tensile strength of PLA composite decreases,but tensile modulus is improved;the addition of g-C₃N₄ is beneficial to the improvement of PLA crystallization performance,while the addition of two other flame retardants limits the crystallization ability of PLA.The g-C₃N₄ was respectively compounded with 9,10-dihydro-9-oxa-10-phosphaphe-nanthrene-10-oxide?DOPO?and melamine polyphosphate?MPP?in different proportions to prepare binary flame-retardant PLA composites to further enhance the flame retardancy of PLA/g-C₃N₄ materials.It is indicated that the thermal stability of binary flame retardant PLA composites is decreased,but their flame retardancy is greatly improved;Their flame retardancy are superior to PLA/g-C₃N₄,PLA/DOPO and PLA/MPP composites using single flame retardant;In binary flame retardant system,the flame retardancy of PLA composites with10wt%DOPO and 10wt%MPP is the best.The LOI values of PLA/10DOPO/g-C₃N₄ and PLA/10MPP/g-C₃N₄ are 29.5%and 25%,and UL-94 rate reaches V-0 and V-1,respectively.Compared with pure PLA,their THRR value decreased by 25.2%and 24.1%,and their THR value decreased by 23.6%and 21.1%,respectively.Study demonstrated that the addition of DOPO greatly reduced the tensile strength and tensile modulus of the PLA composite,but the flowability of PLA composite became better and the flow pattern is similar to the liquid viscosity flow.;The addition of MPP had little effect on the mechanical properties,and the viscosity of the material is increased,and the flow pattern is similar to the solid elastic flow;the addition of DOPO and MPP can improve the crystallization property of the PLA.