The utilization of phytic acid as a reactive flame retardant in the preparation of a fully waterborne biobased epoxy system

Abstract

A fully biobased waterborne flame-retarded epoxy system was prepared using sorbitol polyglycidyl ether epoxy resin (SPE) and phytic acid (PA) as a reactive flame retardant (FR). The flame-retardant efficiency was evaluated by comparing the reference SPE-PA system with solventborne and waterborne SPE systems. Additional enhancement of intumescence and reduction of flammability was achieved by incorporating ammonium polyphosphate (APP) and melamine (MEL) into the SPE–PA system. PA, serving as a curing agent, contributed approximately 1% phosphorous content, resulting in an increased limiting oxygen index (LOI). UL-94 flammability tests demonstrated improved FR properties with PA, and the addition of 2% phosphorous from APP to SPE-PA achieved a self-extinguishing V0 UL-94 rating. Thermogravimetric analysis (TGA) revealed enhanced thermal stability and higher char yield with PA compared with other curing agents. Mass loss calorimetry (MLC) confirmed the superior charring effect of PA compared with other curing agents. The thermal insulation properties of the residual char were assessed by measuring the temperature on the back surface (Tb) of coated steel plates exposed to a 25 kW/m² heat flux for 1 h. The PA sample containing 3%P of APP exhibited a Tb decrease of 130°C compared with the solventborne reference sample. Scanning electron microscopy analysis of the char morphology supported these findings, indicating the effectiveness of the intumescent FR system. Infrared spectra of the char residues and pyrolysis gaseous products were obtained to gain insights into the flame-retardant mechanism of the different systems.