Bio-based flexible epoxy foam synthesized from epoxidized soybean oil and epoxidized mangosteen tannin

[Display omitted] •Flexible epoxy foam synthesized from epoxidized soybean oil and mangosteen tannin.•Low concentration of epoxidized mangosteen tannin was employed for foam preparation to avoid phase separation.•Epoxidized mangosteen tannin increased compressive strength and decreased compression set.•Cellular structure of epoxy foams showed a mixture of closed and open cells. This work presents a flexible epoxidized soybean oil-based epoxy foam and epoxidized mangosteen tannin (EMT). The flexible foam was synthesized without catalyst, from epoxidized soybean oil (ESO) and methyltetrahydrophthalic anhydride (MTHPA) at a weight ratio ESO:MTHPA of 75:25. EMT was used as a reinforcing material. The advantages of EMT were its rigid benzene ring, co-crosslinking with ESO and curing with MTHPA. Mangosteen tannins were extracted from mangosteen pericarps and depolymerized by thiolysis reaction prior to epoxidation with epichlorohydrin. The effect of EMT content was assessed from 0.5 to 2.0 wt% of ESO. Azodicarbonamide and zinc oxide were used as a blowing agent and an activator, respectively. The experimental results indicated synergistic behavior, as EMT reinforced the epoxy foams by increasing compressive strength and decreasing compression set, whereas foam density increased with EMT content. Fourier transform infrared spectrophotometer analysis, swelling and soluble fraction testing indicated complete crosslinking of the epoxy foam. The addition of EMT also increased glass transition temperature (Tg) and degradation temperature of the epoxy foam. All epoxy foam samples showed subzero Tg (-6.5 to -5.4 °C) and semi-closed cells with irregular shapes. The derived epoxy foam was classified as a flexible foam due to its low Tg and mixture of open and closed cells.