The chemical, kinetic and mechanical characterization of tannin-based adhesives with different crosslinking systems
Résumé
Network formation, cure characteristics and bonding performance of tannin-based resins were investigated in order to establish structure–property relationships between the stage B and stage C. Tannin–aldehyde and base-catalyzed autocondensed tannin resins were synthesized and characterized for molecular weight distribution, cure kinetics and cure chemistry by means of GPC, DMA and 13C CP/MAS NMR spectroscopy and solvent stability tests. The resins performance as wood adhesives was further established from lap-shear tests and microscopic observation of the bondline. Resins prepared with highly reactive aldehydes, such as formaldehyde or glyoxal, exhibited a significant extent of hetero-condensation reactions, fast cure kinetics, a high storage modulus and good solvent stability of the stage C-resin. In contrast, resins prepared with bulky aldehydes of low reactivity, such as citral, were dominated by autocondensation reactions, and exhibited slower cure kinetics, a lower storage modulus and solvent-stability of the stage C-resin, alike those neat autocondensed tannin resins. However, all resin systems fulfilled the standard requirements for wood adhesive bonding for interior applications. Additionally, storage modulus increase during cure was found to be a good predictor of the stiffness of the wood-bonded assembly, useful to discriminate between the autocondensation and heterocondensation cure chemistries.