Thermal stability of structural one-component polyurethane adhesives for wood-structure-property relationship

The relationship between the chemical structure of commercial polyurethanes and temperature-dependent creep properties was determined in full scale tests and the results were compared with thermomechanical analysis. Comparison of mechanical performance with ¹³C-NMR spectroscopy studies elucidated important structure-property relationships, which either allow the reduction or elimination of temperature-dependent creep in one-component polyurethanes (1C-PUR) adhesives for wood. The combination of the relative content of still reactive, free -NCO groups on the polyurethane, careful selection of the degree of resin polymerization and a slower rate of reaction are the three most significant parameters that have to be controlled to overcome the problem of temperature-dependent creep found in IC-PUR adhesives. The results obtained indicate that adhesives presenting a combination of a higher content of still unreacted -NCO groups, a lower degree of polymerization and slower reaction rate are capable to counteract problems of high sensitivity of polyurethane to temperature-dependent creep. Two commercial polyurethanes that fulfil the latter requirements and exhibit almost no creep were identified and characterized.