Microporous boron based intumescent macrocycle flame retardant for poly(lactic acid) with excellent UV protection

A low loading microporous boron-based intumescent macrocycle (BMC) flame retardant (FR) was prepared and incorporated into PLA to reduce its flammability, improve the mechanical properties, and enhance its ultraviolet protection. Fourier transform infrared spectrum, scanning electron microscope, Raman spectroscopy, and N₂ adsorption-desorption measurements were used to investigate the properties of BMC. The FR efficiency was evaluated by cone calorimeter, vertical burning test (UL-94), and limiting oxygen index (LOI). A maximum of 3 wt% BMC loading resulted in ~23%, and 37% reductions in the peak heat release rate and the total heat release, respectively. CO, CO₂, total smoke release also reduced by ~71%, 26%, 56%, correspondingly. The fire performance index improved by ~41%, with an LOI value of 30.5% and a V-0 rating in the UL-94 test. The composites generally had excellent ultraviolet protection factor (UPF), whereas the pristine PLA could not pass the UPF rating. Raman spectroscopy analysis of the residual char showed increasingly robust amorphous intumescent glassy char content with the increasing BMC loading. The tensile strength and the Young's Modulus increased by ~34% and 53%, respectively; however, the elongation at break got compromised marginally due to the limited polymer chain mobility resulting from the restricted in-plane and out of plane movements in the polymer matrix. Physical topology interlock was identified as the main mechanism for composite reinforcement. This work provides important insight into the use of sustainable low loading microporous boron-based intumescent flame retardant for improving the properties of PLA.