Structure-property relationships in one-component polyurethane adhesives for wood: Sensitivity to low moisture content

The causes of strength loss of wood joints and their consequent delamination from one-component polyurethane adhesives used for bonding structural wood when used at a low moisture content was investigated by testing wood joint strength and elongation at rupture at different wood moisture contents and by C-13-NMR spectroscopy and scanning electron microscopy of the hardened bond line. The combination of the relative proportion of the still-reactive free -NCO groups on the polyurethane, of the wise choice of degree of polymerization of the resin, and of a slower rate of reaction were the three parameters found to be important in overcoming the problem of poor or no bonding of wood at low to very low moisture contents from one-component polyurethane adhesives. The results obtained indicated that one-component polyurethane adhesives that had a combination of a higher proportion of still-unreacted-NCO groups, a lower degree of polymerization, and a slower reaction rate were capable of overcoming the problem of the high sensitivity of polyurethane gluing at low to very low wood moisture contents. (c) 2006 Wiley Periodicals, Inc.