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.