Preparation of a novel flame retardant based on phosphorus/nitrogen modified lignin with metal–organic framework and its application in epoxy resin

To reduce the fire risk of epoxy (EP) resin, a renewable lignin-based flame retardant functionalized with a metal–organic framework (MOF) HKUST-1 (Cu 3 (BTC) 2 , BTC = benzene-1,3,5-tricarboxylate) was prepared by a facile and green method. The chemical structure of flame retardant was characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results showed that HKUST-1 was in-situ grown on the surface of phosphorus/nitrogen modified lignin, which was treated as an efficient flame retardant (termed PN-lignin@HKUST-1) for subsequent EP resin. The flame-retardant properties of EP composites were evaluated by the limiting oxygen index (LOI), vertical burning (UL-94), and cone calorimetry tests. As compared with neat EP thermoset, the presence of 15 mass% PN-lignin@HKUST-1 increased the LOI value from 28.5 to 33.2% and upgraded the UL-94 rating from V-2 to V-0. Besides, the peak heat release rate (PHRR) and total heat release (THR) were reduced by 41.2 and 33.8%, respectively. Observations of char residues could indicate that addition of PN-lignin@HKUST-1 contributes to a compact and dense char layer, which blocks the release and transfer of heat and harmful gases during combustion. Graphic abstract