A Flame Retardant Poly(lactic acid) Blend with Modified Cellulose and Ammonium Polyphosphate

Poly(lactic acid)(PLA), a sustainable and biodegradable polymer, has the potential to be the next generation of eco-friendly materials to replace petroleum based thermoplastic. To further develop PLA applications in a wide range of areas such as tissue engineering, 3D printing, and food packaging, enhancing its flammability and mechanical strength remains challenging as most reinforcing fillers are non recyclable or environmentally destructive. Therefore, it is urgent to design a PLA composite with enhanced flame retardancy and balanced mechanical properties by only using green additives.<br/>Cellulose is a renewable source that received lots of studies when combined with solid phase phosphate flame retardant, namely ammonium polyphosphate (APP), since cellulose generates char under combustion which acts as a physical barrier to stop the flame. APP is reported to effectively increase PLA flame retardancy but deteriorate ductility. While, as other scientific studies show, cellulose incorporated in polymer blend actually strengthens the overall tensile strength and maintains polymer toughness. In a polymer/solid phase flame retardant system, the interfacial energy between the polymer matrix and flame additives is essential to flame resistance performance. Contact angle measurement revealed that the high surface tension of PLA/cellulose and PLA/APP limits the efficiency of flame additives when burnt. Resorcinol bis(diphenyl phosphate) (RDP) can be easily coated with cellulose fibers under mild condition due to strong H-bonding interactions which is elucidated by Fourier Transform Infrared Spectroscopy (FTIR). Based on Young’s equation and Owen-Wendt method, RDP lowers the surface tension between each component. Therefore, RDP coated cellulose fibers also act as a compatibilizer in PLA/APP blend, which is correlated with lower Tg analyzed by DMA. So that PLA melt blended with 5%RDP-Cellulose and 5% APP easily passes UL-94 V-0 criteria. The flame retardancy was also confirmed with limiting oxygen index and cone calorimetry. Thermogravimetric analysis (TGA) indicates that PLA/RDP-cellulose/APP has a different thermal decomposition history and higher residue~10% after 800 celsius compared with PLA/untreated cellulose/APP blend. Moreover, the addition of RDP coated cellulose fiber minimizes the negative effect of APP on mechanical properties. Izod Impact and tensile testing showed that the PLA/RDP-cellulose/APP composite has impact strength and tensile strength comparable to those of neat PLA.<br/>Work supported by the ERDC (W912HZ-20-2-0054) and the Morin Charitable Trust.