Effect of DOPO-containing flame retardants on poly(lactic acid): Non-flammability, mechanical properties and thermal behaviors

Abstract

The flame retardancies of three kinds of 9,10-dihydro-9-oxa-10-phosphaphenan-threne 10-oxide(DOPO)-containing flame retardant(A1, A2, A3)/poly(lactic acid)(PLA) composites[PA-n/(Ax-y), n=1—12; x=1, 2, 3, denoting three kinds of flame retardants; y=10%, 20%, 30%, 40%, denoting the mass fraction of Ax] were greatly enhanced by melt blending of flame retardant Ax with PLA, including twin-screw extrusion and injection-molding processes. With only 10%(mass fraction) of Ax added to PLA, good flame retardancy with limiting oxygen index(LOI) values of more than 33% was achieved. As the Ax mass fraction was further increased to 20%, PA-n/(Ax-20%) composites showed much better flame retardancy(LOI≥35% and UL-94 V-0 rating). Moreover, the thermal degradation behaviors and mechanical properties of PA-n/(Ax-y) composites were investigated via thermogravimetric analysis(TGA), differen-tial thermal analysis(DTA), tensile testing, notched impact-bar testing, and dynamic mechanical analysis(DMA). TGA results show that PA-n/(Ax-y) composites have slower rate of mass loss and much higher char yield, compared to neat PLA. With the addition of Ax to PLA, the DTA and DMA results indicate slight variations in glass transition tempe- ratures(Tg) of PA-n/(Ax-y) composites. Based on TGA results under nonisothermal conditions, the thermal degrada-tion kinetics of PA-n/(Ax-y) composites were studied by Kissinger’s and Ozawa’s methods. These thermal degrada-tion dynamic analyses show lower activation energies(EK or EO) for PA-n/(Ax-y) composites, corresponding to higher mass fractions of Ax(from 10% to 40%). The PA-n/(Ax-y) composites with good flame retardancy and good mecha- nical properties obtained in this study could be potential candidates for fire- and heat-resistant applications in auto-motive engineering and building fields with more safety and excellent performance.