Influence of eco-friendly calcium gluconate on the intumescent flame-retardant epoxy resin: Flame retardancy, smoke suppression and mechanical properties

In this work, calcium gluconate (CaG) as an eco-friendly additive was subtly incorporated into intumescent flame-retardant epoxy resin (EP) aiming to increase the flame-retardant efficiency. As expected, CaG exhibited synergistic effect in combination with APP, which could further decrease the heat release of epoxy thermosets including peak of heat release rate (PHRR), as well as the smoke production covering smoke production rate (SPR) and total smoke production (TSP). Typically, EP81 with 10 wt% flame retardants (8:1, the mass ratio of APP and CaG) easily passed a UL-94 V-0 rating, and got the highest LOI value of 31.8%; besides, during combustion it displayed PHRR value at 336 kW/m(2) with a reduction of 39.7% compared to EP/10 wt% APP. In addition, its TSP value was further reduced to 9.2 m(2) from 18.1 m(2) of EP/10 wt% APP. According to the analyses of char residues, it indicated that the presence of APP-CaG was beneficial for EP to form more continuous and compact char which was more effective to insulate the transfer of heat and the diffusion of fuels. Besides, despite the introduction of CaG the tensile strength of EP/APP material was well maintained.