Enhancement of cardanol-loaded halloysite for the thermo-oxidative stability and crystallization property of polylactic acid

Abstract

A natural antioxidant, cardanol (CA), was loaded on halloysite nanotube (HNT) to fabricate a dual-functional green additive cardanol-loaded halloysite (HNT-CA) for the preparation of fully green polylactic acid (PLA) composites with improved thermo-oxidative stability and crystallization property. Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA) and transmission electron microscopy (TEM) measurements confirmed that cardanol was loaded in the lumen and attached on the outer surface of halloysite with approximately 22.9 wt% loading efficiency. The anti-aging behavior of HNT-CA was evaluated by the change of oxidation onset temperature (OOT) and weight-average molecular weight (Mw) of PLA composites stabilized by this additive after a 12-day aging test at 120 °C. The results showed that the OOT values of PLA composites with CA dropped from 298.2 °C to 279.0 °C, whereas the PLA composites with HNT-CA decreased from 292.2 °C to 281.5 °C, showing a higher retention. Similarly, the PLA stabilized by HNT-CA presented a higher retention of Mw than that stabilized by CA after thermal oxidation aging. For the PLA composites with HNT-CA, the migration of cardanol in PLA was inhibited by the restriction of halloysite to cardanol, thus showing long-term thermo-oxidative stability. Moreover, the analyses of polarized optical microscope (POM) and differential scanning calorimetry analysis (DSC) showed that HNT-CA increased the crystallization rate of PLA compared to HNT further due to the improved dispersion in PLA. These results indicated that HNT-CA exhibited excellent performance in improving the thermo-oxidative stability and crystallization property of PLA.