Rheokinetics of the gel formation of solutions of binders based on urea-formaldehyde resins

Abstract

We investigated specimens of urea-formaldehyde resin with a urea:formaldehyde ratio of 1:1.5. The content of dry residue was 57.9%, and the content of methylol radicals 19%. Setting was carried out in the presence of various catalysts - oxalic, succinic, and formic acids (0.13% of the amount of resin used). The reaction proceeded under isothermal conditions directly in the working unit of a "Reotest 2" viscosimeter in the 70-90~ temperature range. The maximum viscosity value, after the attainment of which it was observed to drop sharply as a result of precipitation of the hardened material from solution or separation of the reactive system from the working surfaces of the viscosimeter, was adopted as the point of gel formation t*. Concurrently with determination of the viscosity (by investigation of the rheokinetics of the process), we measured the variation in the optical density D of the solution on as "Spekol II" instrument at a wavelength % = 500 nm. A sharp rise in the values of D is observed at a certain time on the curve of D versus time. The moment was treated as the time required to reach the point of phase separation ts, which causes the reactive solution to become turbid. The error of determination of t* and t s did not exceed 6%. An apparatus, which makes it possible to make simultaneous measurements of viscosity and optical density, was used to investigate the character of the optical density D of the solution under a mechanical field. The optical characteristics of the reactive solution were recorded directly in the gap between the inner and outer cylinders of the viscosimeter at a wavelength % = 500 nm on a "Spekol ii" spectrophotometer. The experiments were conducted in the range of shear rates from 1.9"10 -= to 16.8 sec -I. Typical curves of the viscosity and optical density of the solution of the urea-formaldehyde resin versus time are presented in Fig. i. The data indicated that a sharp increase in optical density, which is associated with the precipitation of insoluble particles from the solution, is observed in the structuring process. It is obvious that these particles are woven formations of colloidal particles, which are insoluble in water. Segregation of