Toughening And Modification Of Flame Retardant Polylactic Acid Composites With Different DOPO Structures

Polylactic acid(PLA),which has excellent biodegradability,has attracted much attention as a good substitute for traditional petroleum-based general plastics.However,PLA is extremely flammable and has poor toughness,which greatly limit its application range.Therefore,flame-retardant and toughening modification of PLA has become a research hotspot in recent years.In this paper,the composite materials were prepared by melt blending method.Firstly,effects of 10-(2,5-dihydroxyphenyl)-10-hydrogen-9-oxa-10-phosphaphenanthrene-10-oxide(DOPO-HQ),a type of reactive flame retardant,on the flame retardancy and thermal stability of PLA were studied in detail.And the mechanism of DOPO-HQ improving PLA toughness was explored.Further,the flexible degradable resin poly(butylenes adipate-co-terephthalate)(PBAT)was introduced to toughen the flame-retardant PLA composites containing DOPO-HQ and 6H-dibenz[c,e][1,2]oxaphosphorin,6'6-(1,2-phenethyl)bis-,6'6-dioxide(DIDOPO)respectively.The properties including flame retardancy,thermal stability and mechanical properties of PLA/PBAT/DOPO-HQ and PLA/PBAT/DIDOPO were comparatively studied.The main research contents and results of the thesis are as follows:1.PLA/DOPO-HQ composites were prepared with different contents of DOPO-HQ.The flame retardancy of PLA/DOPO-HQ composites were evaluated by vertical combustion(UL-94),limiting oxygen index(LOI)and cone calorimetry test(CCT).The results show that PLA composites can reach V-0 grade with the addition of 7.5 wt%DOPO-HQ,and the LOI is increased from 21.6%to 28.4%.SEM,Raman and XPS were used to investigate the morphology and chemical structure of the char layer after CCT test.TG and TG-IR were used to study the thermal stability and thermal degradation products of the composites.The analysis results show that DOPO-HQ plays a flame retardant role by promoting the carbon formation of PLA and releasing PO?at the same time.The results of mechanical properties show that the elongation at break and impact strength of PLA composites are improved after adding DOPO-HQ.The results of ³¹P nuclear magnetic,infrared spectroscopy and differential scanning calorimeter(DSC)show that DOPO-HQ has a certain toughening effect on PLA,which is mainly attributed to the chemical reaction between-COOH in the PLA molecular chain and-OH in DOPO-HQ,and the reduction of PLA crystallinity.2.PLA/PBAT/DOPO-HQ and PLA/PBAT/DIDOPO were prepared by PBAT blending flame-retardant composites with different phosphaphenanthrene structures.The flame retardant properties of the two composite were studied through UL-94,LOI,and CCT tests.The results show that the flame retardancy of the composites are enhanced by adding DIDOPO or DOPO-HQ,and the flame retardant efficiency of DIDOPO is better than that of DOPO-HQ under the same addition amount.SEM,Raman,XPS and TG-IR were used to analyze the carbon layers and thermal decomposition products of the two composites.The results indicate that DOPO-HQ and DIDOPO play a main role in gas-phase.TG test shows that,compared with PLA/PBAT,the thermal stability of PLA/PBAT/DOPO-HQ and PLA/PBAT/DIDOPO composites are improved,and the residues is increased after incorporating flame retardant,indicating that the addition of flame retardant is helpful to promote the producing of char of the composite material.Further,Kissinger method and FWO method were used to study the thermal degradation kinetics of the two composites,which confirms the improvement of thermal stability of the composites by adding flame retardant.The results of mechanical test show that the elongation at break of PLA/PBAT/DOPO-HQ-7.5 is 26.5%,which is slightly higher than that of PLA/PBAT/DIDOPO(23%).Compared with PLA,the toughness of two composite materials has been significantly improved,and PLA composite materials with both flame retardancy and toughness has been prepared.