Disclosed is a method of preparing a bio-based thermosetting foam material with improved properties, in particular improved flame retardant properties, which method includes the steps of a) producing a prepolymer by condensation of at least one phenolic compound and formaldehyde in a ratio of 1:1.0 to 3.0 using 0.15 to 5 wt % of an alkaline catalyst in the temperature range of 50 to 100° C. until the refractive index of the reaction mixture is 1.4990 to 1.5020, b) adding 2 to 40 wt % of at least one natural polyphenol at a temperature of 50 to 100° C., c) adding 2 to 10 wt % of one or more emulsifiers and mixtures thereof, d) adding 2 to 10 wt % of one or more foaming agents and mixtures thereof and e) adding 10 to 20 wt % of a curing agent and f) curing. All wt % being related to the amount of the raw materials used.