Synergistic effects of polyoxometalate-based ionic liquid-doped sepiolite in intumescent flame-retardant high-density polyethylene

A polyoxometalate-based ionic liquid-doped sepiolite (SEP-PIL) was prepared by electrostatically immobilizing phosphomolybdic acid (PMA) on natural sepiolite embedded with imidazolium cations (SEP-IL), and its structural properties were fully characterized by various methods. Furthermore, a new intumescent flame-retardant system (IFR) for high-density polyethylene (HDPE) was constructed by adding SEP-PIL into conventional HDPE/IFR composites. The retardant properties and thermal decomposition behaviors were comprehensively investigated. The results showed that the HDPE composite containing 24 wt% IFR and 1 wt% SEP-PIL passed UL-94V-0 rating, and the limiting oxygen index increased from 17.8% (pure HDPE) to 27.6%. In addition, the HDPE/IFR/SEP-PIL composite had lower peak heat release rate (PHRR) and total heat release (THR) compared to the HDPE/IFR composite. The thermogravimetric analysis demonstrated that the combination of SEP-PIL with IFR could greatly promote the formation of residual chars. Dynamic rheological measurement further confirmed that the synergistic effect of SEP-PIL and IFR could greatly improve the flame retardancy of HDPE/IFR/SEP-PIL composites.