Study On The Synthesis Of Free Radical Graft Polymerization Of Lignin And Its Flame Retardancy And Toughening Effect On Polylactic Acid

As one biodegradable polymeric material,Polylactic acid(PLA)features high mechanical strength,excellent biodegradability,and ease of processing.However,PLA is extremely flammable with a limiting oxygen index(LOI)of only 19%.Moreover,it is inherently brittle with very low ductility,typically an elongation at break less than 10%.These two shortcomings has significantly hampered its extensive applications in industry.Despite encouraging progress in addressing these two issues,the use of non-biodegradable flame retardants and toughening agents often compromises the biodegradability of PLA.Meanwhile,the addition of them normally reduces mechanical strength of PLA to different degree.Therefore,it remains a huge challenge to develop high-performance bio-based flame retardants and toughening agents for PLA so far.In response to this challenge,this work aims to develop high-performance lignin-based flame retardants and lignin-based toughening agents by using inexpensive lignin as the starting material through free radical graft polymerization.This work then investigates their flame retardancy and toughening effects on PLA.Firstly,a flame retardant intermediate(P/T)has been successfully synthesized by reacting 3-isopropyl-dimethylbenzyl isocyanate(TMI)with pentaerythritol phosphate(PEPA).Then the intermediate(P/T)was grafted onto the backbone of lignin by free radical graft polymerization by using NaCl-H2O2 as an initiation system.This enables the synthesis of four kinds of lignin-based flame retardants(TP-g-lignin)by simply adjusting the mole ratios of lignin/intermediate(1:1,1:2,1:4,1:8).The flame retardant PLA composites were prepared by melt blending with PLA.The results show that the PLA composites are able to pass a V-0 rating during vertical burning tests,when one adds 10%PT-lignin(1:2)or more than 5%TP-g-lignin(1:4 and 1:8).In particular,when 5%of TP-g-lignin(1:4)is added,the flame retardant PLA composite maintains a comparable tensile strength of 59MPa to the PLA bulk(58-65MPa),in addition to a V-0 flame retardant rating..Secondly,we have synthesized three kinds of lignin-derived toughening agents(Mlignin)by chemically grafting two biobased acrylate monomers,lauryl methacrylate(LMA)and tetrahydrofurfuryl methacrylate(THFMA)in three different ratios via free radical copolymerization with the NaCl-H2O2 as an initiation system.The toughened PLA composites are then manufactured by facilely melt-blending PLA with Mlignin.The results demonstrate that the addition of 20%Mlignin(TFMFA:LMA is 15:5),the elongation at break of PLA increases to 220%and the fracture toughness augments to 54.6 MJ/m3,about 14 times of that of the PLA bulk(4.4 MJ/m3).Such largely enhanced toughness of PL A are attributed to the improved interfacial compatibility between Mlignin and PLA,and the toughening effect of the soft graft copolymer segment.This work has successfully developed high-performance lignin-based flame retardants and toughening agents for PLA by using industrial lignin as the raw material through free radical polymerization.As-developed TP-g-lignin and Mlignin have demonstrated excellent flame retardancy and toughening effects on PLA.This work provides technological support for the development of advanced lignin-derived flame retardants,offers a new strategy for the value-added utilization of industrial lignin,and opens up a new way to the development of high-performance PLA materials.