Lightweight tannin foam/composites sandwich panels and the coldset tannin adhesive to assemble them

► Lightweight sandwich panels of tannin foams surface laminated with natural fibres/tannin resin matrix composites were prepared. ► These environment friendly panels have better mechanical properties of the brittle rigid tannin foams core. ► The tannin–resorcinol–formaldehyde cold-set adhesives for bonding the surface composites to tannin foam and foam to foam performed well. ► The composition of the tannin–resorcinol–formaldehyde cold-set adhesives was determined by MALDI-TOF mass spectrometry. Sandwich panels of tannin/furanic foams laminated on the two faces with thin high density natural fibres/tannin resin matrix composites, or other thin panels such as plywood, yield light panels that have better bending strength of the surface composite and better mechanical properties of the brittle rigid tannin/furanic foams. The tannin–resorcinol–formaldehyde cold-set adhesives for bonding the surface composites on to the tannin/furanic foam and bonding foam to foam were examined by Matrix Assisted Laser Desorption/Ionisation Time of Flight (MALDI-TOF) mass spectrometry. The analysis revealed that the adhesive was composed of resorcinol–formaldehyde oligomers, unreacted flavonoids, and resorcinol–formaldehyde oligomers coreacted with flavonoid units of the tannin.