African tree bark exudate extracts as biohardeners of fully biosourced thermoset tannin adhesives for wood panels

•The exudate extracts of the African trees Vachellia nilotica and Senegalia senegal were found to be effective hardeners of tannin adhesives.•The exudates contained no aldehydes and consisted of 2,5-dihydroxymethyl furan linked to carbohydrate chains.•On reacting with flavonoid tannins 2,5-dihydroxymethyl furan detached from the carbohydrate chain to react with the tannin and harden it.•They cross-linked pine tannin resin by reaction with the 2,5-dihydroxymethyl furan either separated or still linked to the carrying carbohydrate chains.•The particleboards dry Internal Bond strength results obtained were interior grade comparable to those obtained with control panels bonded with UF adhesives. A totally “bio” hardener derived from the exudate extract of the African trees Vachellia nilotica and Senegalia senegal was investigated. It hardened a maritime pine tannin resin without any obvious presence of an aldehyde thus obtaining a completely “bio” adhesive for wood panels. MALDI-ToF and CP MAS 13C NMR analysis indicated that the hardeners cross-linking species was 2,5-dihydroxymethyl furan linked onto carbohydrate chains. The hydroxymethyl furan moieties reacted both after detachment from the carbohydrate chains but also with carbohydrate chains still attached. The carbohydrates too were then involved as part of the binder. The gel times were slower than what was obtainable with paraformaldehyde but sufficiently fast to possibly be of industrial significance.