Grafting cellulose nanocrystals with phosphazene-containing compound for simultaneously enhancing the flame retardancy and mechanical properties of polylactic acid

Cellulose nanocrystals (CNCs) can be used as a bio-based carbon source in an intumescent system. However, CNCs have low onset decomposition temperature and decompose and carbonize that may have adverse impact on performance. We, herein, demonstrated the design of phosphazene-containing CNCs (P/N-CNCs) with great thermal stability and outstanding charring ability. The TGA results showed that the initial decomposition temperature of P/N-CNCs increased from 202.4 to 272.2 °C, and the residual char at 700 °C increased from 24.9 to 55.8 wt% comparing to CNCs. Then, flame retardant PLA composites were prepared by blending PLA, P/N-CNCs with ammonium polyphosphate (APP), melamine (MPP), aluminum hypophosphite (AHP) and piperazine pyrophosphate (PPAP), respectively. The thermal stability, flame retardant properties and mechanical properties of the PLA composites were investigated. The results showed that the flame retardant system constructed with 7 wt% APP and 3 wt% P/N-CNCs had the best effect in PLA. PLA/7APP/3P/N-CNCs had the highest limiting oxygen index value (28.1%), the lowest peak heat release rate (266 kW/m2), and reached UL 94 V-0 rating. Moreover, the tensile strength, impact strength and elongation at break of PLA/7APP/3P/N-CNCs were improved by 7.3%, 18.6%, and 29.4%, respectively, comparing to these properties of PLA/7APP/3CNCs. This modified CNCs provides good thermal stability and charring ability. It also offers a new method for the preparation of high-performance flame-retardant PLA composites.