Encapsulation of Ammonium Polyphosphate in Lignin Nanocontainers Enhances Dispersion and Flame Retardancy in Polylactic Acid Foams
S. Peil, Rakine Mouhoubi, Ruben Streekstra, Hugo Ridard, Lothar Veith, Wael Ali, Thomas Mayer‐Gall, Joost Duvigneau, Frederik R. Wurm
Abstract
High Resolution Image Download MS PowerPoint Slide Ammonium polyphosphate (APP) holds significant potential as a nonhalogenated flame retardant. However, achieving compatibility between polar APP and nonpolar polymer matrices remains challenging. Here, we present a synergistic submicrometer-sized flame retardant system composed of hydrophilic APP encapsulated in lignosulfonate capsules. These lignin nanocontainers (LNCs) are prepared via interfacial cross-linking of lignosulfonate using toluene-2,4-diisocyanate (TDI) in inverse miniemulsion, which is stabilized by polyglycerin polyricinoleate (PGPR), a food additive. Excess isocyanate groups on the surface of the LNCs enable the surface-grafting with hydroxy-functionalized polymers, e.g., poly(lactic acid) (PLA), and enhance the dispersion of APP-loaded LNCs in the PLA matrix. Furthermore, APP-loaded LNCs act as heterogeneous nucleation agents in CO 2 batch foaming. The incorporation of 5–20 wt % LNCs resulted in fine-celled foams, as evidenced by the reduced cell diameters (29 ± 17 μm compared to virgin PLA foams (63 ± 31 μm)) and increased cell density of LNC-loaded PLA foams compared to pristine PLA foams from 2 × 10 7 to 4 × 10 8 cells cm –3 . Notably, the flame retardancy of the LNC-enriched PLA foams was significantly enhanced. The combined carbonization effect of lignin, PGPR, and APP led to an elevated foam char yield, increasing from 0.9 wt % (pristine PLA) to 7.3 wt % (20 wt % LNC-loaded PLA). The experimental limiting oxygen index (LOI) increased from 21% (pristine PLA foams) to 24.5% (20 wt % LNC-loaded PLA foams). UL-94 test proved a V0 rating with only 9 wt % of loaded LNCs. Microcone Calorimeter data further underlines the systematic trend of APP added to the foams with a decreased heat release rate, depending on the amount of FR. Together, our results underline the effectiveness of biobased and biodegradable polymers in lightweight packaging and construction applications with enhanced flame-retardancy.