Rheological behaviour of urea-formaldehyde adhesive resins

Abstract

An attempt is made to describe the complicated rheokinetic behaviour 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 a power law function using extrapolation to zero shear and infinitely high shear rates. During most of the period of viscosity increase, the rheological behaviour of studied resins is similar. The deviation from the 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 the formation of stable methoxymethylene groups.

Rotational viscometry for the study of urea-formaldehyde resins

“…It is clearly seen that the difference in gelation time among studied resins depends mainly on the duration of the period of storage when the viscosity growth is slow. The same conclusion was drawn in [ 8,9 ].…”

“…In [ 8 ], the dependence of the viscosity on the storage time was described by the power law, using the classical rate equation for the description of the polymer network formation. The viscosity , η the kinetic factor k and the average order of non-linear polycondensation n′ were the parameters considered in this approach: A conclusion was drawn that the behaviour of a resin can be described by power law, but not starting from viscosity values typical for fresh resin (300-450 mPa • s).…”

“…During measurements, requirements of ISO 3219:1993 [ 10 ] were followed. Viscosity change during resin storage was measured and described by a power law function (1) between shear stress and shear deformation rate, which enables direct evaluation of the consistency index K and power law index n [ 1,8 ]. The viscosity values extrapolated to zero and to infinite shear rates, 0 max ( ), η η and min ( ), η η ∞ were also obtained on the basis of the power law 1 ,…”

“…Viscosity value, marked 50 η and named midpoint viscosity, is the interpolated value of viscosity to torque 50% [ 8 ]. All calculations are performed using Microsoft Excel common applications.…”

“…where K is the consistency index and n is the flow index. The resins were observed during their storage until alkali-promoted polycondensation led to gelation [ 8,9 ].…”