Authors:F. Langmaier, J. Šivarová, M. Mládek, and K. Kolomazník
Condensation of dimethylol-urea (DMU) mixed with urea (U) and collagen hydrolysate (H), obtained through enzymatic hydrolysis of chrome-tanned leather waste, without added acid curing agents in the solid phase was studied through DSC and TG techniques in a temperature interval up to 220°C. Among both techniques TG proved be more useful.While the DMU+U mix produced methylene-oxide (-CH2-O-CH2-) and methylene (-CH2-) bridges at a ratio of approx. 1:1, urea substituted for collagen hydrolysate increased the proportion of more stable methylene bridges to methylene-oxide bridges to a ratio of approx. 2:1. Methylene-oxide bridges are considered to be the main potential sources of formaldehyde emissions from cured urea-formaldehyde type adhesives, and thus the use of collagen hydrolysate in preparation of urea-formaldehyde adhesive types is a suitable way how to make such adhesives more environmental friendly.
Authors:Gonzalo Vázquez, Jorge Santos, M. Sonia Freire, Gervasio Antorrena, and Julia González-Álvarez
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 ~ 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.
hexamine is not a formaldehyde-yielding hardener, and thus, it leads to cured products with low formaldehydeemissions.
Extensive research study on the reaction of the tannins and hexamine has been reported [ 13 , 14 ]. Results have shown that
performance in the panel, water solubility, and lower price [ 1 ]. The formaldehydeemission from the panel used interior applications was one of the factors affecting sick building syndrome in an indoor environment. Therefore, the formaldehydeemission from