Flame-retardant effect of a functional DOPO-based compound on lignin-based epoxy resins

A simple and environmentally benign synthetic route to lignin-based epoxy resins is highly desirable. Alkali lignin and glyoxal are promising renewable and sustainable alternatives to phenol and formaldehyde, respectively. Their use is demonstrated to produce lignin-phenol-glyoxal novolac epoxy resins (LPG-NERs) through a simple one-pot synthesis. Flame retardancy of LPG-NERs was improved by functionalization with a N, S modified 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide (DOPO) material (termed DBS) obtained via nucleophilic reaction between DOPO and a N, S containing intermediate. FTIR and 1 H NMR analyses confirmed the successful fabrication of LPG-NERs and SBD. The introduction of lignin and SBD reduced crosslinking of the epoxy network, weakening mechanical properties , but conferred excellent flame retardancy (including vapor and condensed phases) as determined by LOI (34.5%), UL-94 (V-0 rating), and cone calorimetry tests (lower heat release and smoke production, and higher char formation). The SBD structure imparted a desirable vapor mechanism (blowing out phenomenon caused by the fire quenching effect of PO•/PO 2 • radicals and dilution effect from non-condensable gases such as NH 3 , N 2 , SO 2 ), while lignin (a natural biochar precursor) in synergy with SBD imparted superior charring performance.