Online formation of epoxy resin multi-hierarchical char layer to improve mass and heat barrier performance via designable “heterogeneous char-forming agent”

Fabrication of the char layers with efficiently suppressing mass and heat spreading is vital to improve fire safety of polymers. However, the designable microstructure of char layer composed by the compact external and internal-surface layers, and the expanded porous mid-char layer had not been fabricated. Herein, we reported the functionalized design and online formation of epoxy resin (EP) multi-hierarchical char layer with improved mass and heat barrier performance under fire environment using oxidized microcrystalline cellulose (OMCC) as heterogeneous char-forming agent (HCA). The morphologies and components evolution of char layers of EP-intumescent flame retardants (IFRs) via roasting at different temperature were characterized. Strikingly, when the content of HCA was 5%, EP with microencapsulated ammonium polyphosphate (MFAPP) (3.75%) could achieve continue and compact outer surface char layer, formed the honeycomb-like char cells in the inner layer and served as the supporting effect for highly expanded char layer. Noteworthy, this multi-hierarchical char layers of EP-IFRs declined the heat spreading, its outer surface temperature stabilized at 42 °C when placed on 240 °C hot plate. Self-designed device combustion test showed this EP-IFRs could obtain the self-extinguished property within 2 min and high residue (96.3%). Compared with those of neat EP, the cone calorimeter test results of EP-IFRs exhibited significant decrease in heat release rate (HRR, 48.9%), total heat release (THR, 28.3%), smoke production rate (SPR, 47.6%), total smoke production (TSP, 52.5%) and obtained a UL-94 V-0 rating with a limiting oxygen index (LOI) of 28.6%. Correspondingly, the mechanism of the fabrication of multi-hierarchical char layer of EP-IFRs was proposed. This contribution provides a facile strategy to develop the sustainable bio-based IFRs.