Synthesis Of Novel Halogen-free Phosphorus-nitrogen-silicon Flame Retardants And Their Application In Polylactic Acid

Polylactic acid(PLA)is a bio-based and biodegradable polymer with great potential to replace traditional petroleum-based plastics.PLA has promising applications in household appliances,sportswear,automotive and aerospace,and healthcare due to its excellent biodegradability,transparency,biocompatibility and thermoplasticity.While,like petroleum-based polymers,PLA is highly flammable and generates large amounts of molten droplets during combustion,which largely limits its useing in many applications.Therefore,research into the flame retardant properties of PLA is essential.A large number of studies have shown that the melt blending modification of PLA with flame retardants is one of the effective means to overcome the flammability and easy dripping of the matrix;and the halogen flame retardants widely used in China,which produce carcinogenic substances such as dioxins during combustion,are poorly environmentally friendly and easy to migrate,that their application has been restricted.Phosphoryl,phosphorus heterophilic and siloxanebased flame retardants because of the advantages of halogen-free and good flame retardancy,so the development of such halogen-free flame retardants is being paid attention to.In this work,two halogen-free flame retardants containing phosphoryl,phosphorus heterophilic groups and amino siloxane chains were synthesized,characterized,and the structures and properties of two new flame retardants,DPPB-AD and BPOB-A-D,were determined;then PLA/DPPB-A-D and PLA/BPOB-A-D flame retardant composites were prepared by melt blending technology,and the structures and properties of the flame retardant composites were characterized with the aid of various The structure and properties of the flame retardant composites were characterized by modern analytical methods,and the effects of the types and addition ratios of flame retardants on the performance of PLA flame retardant materials were investigated by compounding different flame retardant systems.The main research work is as follows.(1)Preparation of flame retardants and study of their structure and properties.DPPC and BPOD were used as raw materials to react with PHBA,APTES and DOPO,respectively.The successful synthesis of the flame retardants DPPB-A-D and BPOB-A-D was demonstrated by modern analytical methods such as FTIR,NMR and EA;the thermal stability of the flame retardants was analysed using TGA tests.(2)Effect of flame retardants on the properties and structure of PLA materials.Two types of PLA/DPPB-A-D and PLA/BPOB-A-D composites with different ratios were prepared by melt blending technology,and the flame retardant properties,thermal stability and mechanical properties of the materials were investigated.The results shown that the LOI test and UL-94 test performance of the composites increased as the amount of flame retardant added increased;their tensile strength and elongation at break decreased;and the thermal stability and carbon residue rate increased significantly.When DPPB-A-D was added at 20 wt%,the LOI value increased from 19.1% to 29.1% for the pure PLA matrix,the UL-94 test improved from NR to V-0 rating,the tensile strength and elongation at break decreased from 68.17 MPa and 8.21% for pure PLA to 42.15 MPa and 6.95% respectively,and the carbon residue increased from 1.27% to 12.08% for pure PLA.When BPOB-A-D was added at 20 wt%,the LOI value increased to 28.7%,UL-94 was upgraded to V-0 rating,tensile strength and elongation at break decreased to 51.44 MPa and 7.56%,and carbon residue increased to 11.78%.In CONE tests,HRR and THR of PLA/DPPB-A-D and PLA/BPOB-A-D flame retardants were effectively reduced,and the residual char layer after combustion increased,reducing the degree of combustion.The results of DSC tests shown that the addition of flame retardants could increase the glass transition temperature of PLA materials and promote PLA crystallization.The EHC and SEM analysis proved that the flame retardant mechanism of the flame retardant material is the synergistic effect of the condensed phase flame retardant mechanism and the gas phase flame retardant mechanism,which can effectively promote the formation of carbon layer and achieve good flame retardant effect.(3)Effect of different compounding systems on the structure and properties of PLA materials.The flame retardants DPPB-A-D(or BPOB-A-D)were compounded with APP,MEL and POSS respectively.The systems formed were melt blended with PLA to prepare PLA compounded flame retardant materials,and their properties were investigated.The results showed that the PLA had the best flame retardant properties with an LOI value of 28.1% for the APP and DPPB-A-D compounded systems with a proportion of 5 wt% and 10 wt% respectively,namely PLA/5 APP/10 D-AD,and the flame retardant achieved a V-0 grade in the UL-94 test;in addition,the compounded APP and DPPB-A-D systems had the effect of promoting the carbon formation of the substrate,with a residual carbon rate of 10.42% at 600°C.The MEL,POSS and DPPB-A-D systems,PLA/7.5 MEL/7.5 D-AD and PLA/7.5POSS/7.5 D-AD,have the best overall flame retardancy,mechanical properties and thermal stability.Similarly,the results for MEL,APP and POSS,respectively,in combination with BPOB-A-D,showed that the best flame retardancy,mechanical properties and thermal stability were obtained for MEL: B-AD mass ratio of 1:1,APP:B-AD mass ratio of 1:2 and POSS: B-AD mass ratio of 1:1.The microstructure analysis also shows that the compound system with the same total addition has better dispersion,better synergistic flame retardancy and a denser carbon layer.