Rheological behavior of urea-formaldehyde adhesive resins

An attempt is made to describe a complex rheokinetic behavior of urea-formaldehyde resins during storage, in close connection with the changes in their chemical structure. The alkali-promoted non-linear polycondensation during resin storage is evaluated by pre-gel viscosity measurements. Viscosity changes are described by power law function using extrapolation to zero shear and infinite high shear rates. During most of the period of viscosity increase, the rheological behavior of studied resins is similar. The deviation from power law dependence occurring in the initial low-rate growth of viscosity takes place because of formaldehyde redistribution which does not lead to covalent network build-up. In the presence of methanol this period is extended due to formation of stable methoxymethylene groups. Our conclusions are that alkali-promoted polycondensation during storage of UF adhesive resins can be described by shear thinning behavior in advancement of covalent network. UF resin is a complicated model but because of its industrail importance, it should not be avoided in study. The covalent network formation is coupled with intricate physical interactions. The quantitative rheokinetic description suffers because of ill-defined aqueous dispersion, sedimentation of which changes in network buildup. The storing of resins in bulk is not comparable with the conditions of thin resin films between wood substrates. © 2005 Matrice Technology Ltd.