Abstract
Cellulose nanocrystals (CNCs) have been used as bio-based carbon source in intumescent system. However, CNCs have the disadvantages of low onset decomposition temperature and decompose and carbonize during processing. 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 was increased from 202.4 ℃ to 272.2 ℃ (increased by 34.5%), and the residual char at 700 ℃ was increased from 24.9 wt% to 55.8 wt% compared with 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 PLA composites were investigated. The results showed that the flame retardant system constructed by 7 wt% APP and 3 wt% P/N-CNCs had the best effect in PLA. PLA/7APP/3P/N-CNCs had the highest limit 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 increased by 7.3%, 18.6% and 29.4%, respectively, compared with these properties of PLA/7APP/ 3CNCs. This work provides a new idea for the design of CNCs with great thermal stability and outstanding charring ability, and offers a new method for the preparation of high-performance flame-retardant PLA composites.