Flame-Retardant and Recyclable Soybean Oil-Based Thermosets Enabled by the Dynamic Phosphate Ester and Tannic Acid

The demands of safety and sustainability have driven the development of intrinsic flame-retardant biobased polymers from renewable materials. Herein, a mechanically robust, good flame-retardant, and recyclable thermoset was developed from renewable epoxidized soybean oil (ESO) by using 2-hydroxyethyl methacrylate phosphate (HEMAP) as the reactive flame retardant and tannic acid (TA) as the charring agent. The flame resistance of the obtained ESO-based thermoset achieved the highest UL-94 of V-0 rating and a limited oxygen index value of 26.7% due to the synergistic flame-retardant effect of phosphate and TA. The flame-retardant mechanisms of the gaseous phase and condensed phase were fully investigated by thermogravimetric infrared, scanning electron microscopy–energy-dispersive spectrometry, X-ray photoelectron spectroscopy, and Raman spectra. It is confirmed that the incorporation of phosphate and TA could effectively promote the formation of dense carbon layers and delay the pyrolysis of long aliphatic chains. The ternary crosslinking of ESO, HEMAP, and TA via free-radical polymerization and epoxy-ring opening reaction resulted in a rigid network with a high crosslink density, bestowing the thermoset with superior tensile strength (20.0 MPa), flexural strength (36.3 MPa), and bonding strength (16.7 MPa on steel). Moreover, the ESO-based thermoset exhibited a fast stress relaxation behavior due to the transesterification of dynamic β-hydroxyl phosphate esters, which enables the network with thermal-healing ability and recyclability. This study explores a feasible method to prepare an intrinsic flame-retardant polymer from commercially available and renewable vegetable oils and natural polyphenols.