Development of Flame-retardant Thermosetting Poly (Lactic acid) (PLA) and Its Curing Kinetics

Poly (lactic acid) (PLA) is a bio-based polymer that is derived from renewable plant resources, such as corn, cassava, and other crops. It has a wide range of potential applications. However, currently, the PLA produced on an industrial scale is a thermoplastic polymer, which has poor mechanical strength and poor flame retardancy. These mediocre properties limit its potential uses in housings, structural interior of automotives, and packaging. The present study aimed to overcome the limitations of thermoplastic PLA by developing a synthesis route for producing thermosetting PLA with improved flame retardancy. The synthesis involved reacting 9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) with itaconic acid (IA), polymerizing the product with pentaerythritol (PENTA) and lactic acid (LA) oligomer, and end-functionalizing with methacrylic anhydride (MAAH). The success of the synthesis was confirmed by analyzing the results of Fourier transform infrared spectroscopy (FTIR) and proton nuclear magnetic resonance ( 1 H NMR) analysis. The curing kinetics is elaborated by Kissinger’s and Crane’s methods with outstanding fitting. Increasing the LA-to-PENTA ratio results in higher molecular weight and larger arm length, leading to less crosslinking density and lower phosphorus content. A 10:1 LA-to-PENTA mole ratio contributes to UL-94 V1 ranking, 24% limit oxygen index (LOI), 183 kJ/g heat release capacity (HRC), and 21.4 kJ/g total heat release (THR) indicating the admirable flame retardancy. The development of flame-retardant thermosetting PLA could be a significant step forward in expanding its potential applications and improving its properties.