Abstract
Lignin can be a candidate as a charring agent applied in halogen-free flame retardant polymers, and incorporation of silicon and nitrogen elements in lignin can benefit to enhancing its thermal stability and charring ability. In the present work, wheat straw alkali lignin (Lig) was modified to incorporate silicon and nitrogen elements by γ-divinyl-3-aminopropyltriethoxysilane, and the modified lignin (CLig) was combined with ammonium polyphosphate (APP) as intumescent flame retardant to be applied in poly(Lactic acid) (PLA). The flame retardancy, combustion behavior and thermal stability of PLA composites were studied by the limited oxygen index (LOI), vertical burning testing (UL-94), cone calorimetry testing (CCT) and thermogravimetric analysis (TGA), respectively. The results showed a significant synergistic effect between CLig and APP in flame retarded PLA (PLA/APP/CLig) occured, and the PLA/APP/CLig had better flame retardancy. CCT data analysis revealed that CLig and APP largely reduced the peak heat release rate (PHRR) and total heat release rate (THR) of PLA, indicating their effectiveness in decreasing the combustion of PLA. TGA results exhibited that APP and CLig improved the thermal stability of PLA at high temperature. The analysis of morphology and structure of residual char indicated that a continuous, compact and intumescent char layer on the material surface formed during firing, and had higher graphitization degree. Mechanical properties data showed that PLA/APP/CLig had higher tensile strength as well as elongation at break.
Highlights
In recent years, biodegradable polymeric materials have been paid more and more attention because of shortage of petroleum source and environmental pollution caused by non-degradable petroleum-based plastics [1,2]
Wheat straw alkali lignin was modified to incorporate silicon and nitrogen elements in by γ-divinyl-3-aminopropyltriethoxysilane, and the modified lignin was combined with ammonium polyphosphate (APP) as intumescent flame retardant to be applied in poly(Lactic acid)(PLA)
Thermal stability testing of the samples were performed on TAQ50 thermogravimetric analysis (TGA) (WATERS Company, Milford, MA, USA) instrument at a heating rate of 20 ◦ C/min from 50 ◦ C to 800 ◦ C, in which containing silicon and nitrogen elements (CLig) was tested under air, while the poly(lactic acid) (PLA) and flame retardant PLA composites under N2 atmosphere, and gas flow rate was 60 mL min−1
Summary
Biodegradable polymeric materials have been paid more and more attention because of shortage of petroleum source and environmental pollution caused by non-degradable petroleum-based plastics [1,2] Among these biopolymers, poly(lactic acid) (PLA) plays a significant role due to its raw material, lactic acid, derived from renewable resources such as corn or sugar beet, as well as its good thermal stability and great biocompatibility [3,4,5]. Wheat straw alkali lignin was modified to incorporate silicon and nitrogen elements in by γ-divinyl-3-aminopropyltriethoxysilane, and the modified lignin was combined with ammonium polyphosphate (APP) as intumescent flame retardant to be applied in poly(Lactic acid)(PLA).
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