Tannin-based non-isocyanate polyurethane adhesives

A process for manufacturing a non-isocyanate polyurethane (NIPU) composition comprising:

• transesterification of a condensed tannin with a carbonate or a carbonate precursor followed by a reaction with a hardener selected from the group consisting of diamines, triamines, polyamines and mixtures thereof, wherein the process comprises:
• a. reacting the condensed tannin with a dialkyl carbonate or a precursor thereof at a temperature in a range from 20 to 40° C. for 1 to 12 h in a solvent selected from the group consisting of water, a C1- to C6 alkanol and a combination thereof in presence of a catalyst which is a Lewis base to obtain a reaction product,
• b. removing the solvent and drying the reaction product at a temperature in a range from 30 to 60° C. for 24 to 72 h to obtain a dried reaction product,
• c. mixing the dried reaction product obtained in b) with a hardener selected from the group consisting of diamines, triamines, polyamines er and mixtures thereof in an amount of 20 to 40 parts by weight of diamine, triamine or polyamine per 100 parts of dried reaction product of b) in an aqueous system at a temperature in a range from 35 to 70° C.,
• d. adjusting the pH of the system obtained in c) to at least pH 10 to obtain an alkaline reaction medium,
• e. thereafter increasing the temperature to 80 to 110° C. and keeping the alkaline reaction medium at this temperature for at least 30 to 90 min to obtain a viscous resin, and
• f. cooling the viscous resin obtained in e).