Rheokinetics of urea-formaldehyde resins gelation

Rheokinetics of gelation of aqueous suspensions of urea-formaldehyde oligomers was studied. Experiments were carried out with a rotational viscometry method using two regimes of deformation: constant stress or constant shear rate. It was shown that a maximum on a viscosity vs. time curve is observed and it reflects sedimentation of rather large gelled particles. Then, gelation of a whole system (determined as the point of the unlimited growth of viscosity) takes place. Experiments carried out at different temperatures gave us an apparent activation energy which depends on the oligomer content in a composition. Characteristic points of a rheokinetic curve do not depend on the concentration of a curing agent. The problem of the temperature increase due to heat dissipation is also discussed. It was demonstrated that shear stress is a kinetic factor influencing the rate of a reaction. A master curve for a rheokinetic curve was constructed, time scale being reduced by the gel time.