DSC and DMA study of chestnut shell tannins for their application as wood adhesives without formaldehyde emission

The use of chestnut (Castanea sativa) shell tannin extracts for the formulation of wood adhesives has been studied. The interest was centred not only on the formulation of adhesives from a renewable resource but also on the possibility of completely removing formaldehyde from the adhesive formulations. Tris(hydroxymethyl)nitromethane (TRIS), glyoxal (GLY) and hexamine (HEX) were used as hardeners and the results were compared with those obtained with the traditional hardener, paraformaldehyde (PAR). Differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) were used to study the chemical and mechanical cure of the formulated adhesives and the influence of the type of hardener on the curing properties. DSC curves were obtained at three heating rates and using an isoconversional method, chemical conversion vs. time at a given temperature was obtained. The curing enthalpy for the adhesive with hexamine as hardener was the highest and decreased in the order HEX similar to TRIS >> GLY > PAR. The highest rate of chemical cure was achieved using PAR as hardener followed by GLY > HEX > TRIS, that required higher curing temperatures to achieve complete chemical cure. Mechanical cure was analyzed from isothermal DMA experiments. The rate of mechanical curing decreased in the order TRIS > HEX > GLY, however, the rigidity after completion of curing increased in the same order. Alternative hardeners increased significantly adhesive pot-life compared to paraformaldehyde, especially tris(hydroxymethyl)nitromethane.