Shear-induced polymer interactions and gelation in urea-formaldehyde polycondensates

Abstract

Urea and formaldehyde polycondensation was performed in shear and non shear condition under different pH values (3.5, 4, 4.5) and amounts of acetate buffer (0.14%, 0.41%, 0.69%). The reaction was ceased by changing pH from acidic to basic one when gel point was reached. This polycondensate represents a medium concentrated dispersion of polymer clusters in aqueos solution of short polymer chains and the buffer. Structural changes occurring within the reacting system were followed by Theological measurements and chemical analysis. The flow type was a plastic one. The dependence of gelation time (gel point) upon rate of shear was determined. The concentration of the dispersion was further increased in the evaporation process under vacuum and small rate of shear. High concentration products obtained (concentration varied from 60-92%) were subjected to the shear force of varying intensity (shear rate up to 20000s -1 ) which induced polymer chain interaction and gelation in shear. Due to various factors, very complex rheological behaviour comprising shear thinning, rheopexy (antithixotropy) and thixotropy was obtained. The type and mechanism of structural linking in the process of building up of the gel network could be deduced.