Química da parte úmida em processo de fabricação de papel - interações em interfaces sólido-líquido.

A cationic polyelectrolyte (polyamine), with low molecular weight and high charge density, usually applied as anionic trash coagulant, was used for the retention and drainage studies in the papermaking. The use of sodium carboxymethyl cellulose to simulate the anionic trash content and its effect on the filler retention was an important feature of the work. It could be noted that the increasing of the cationic polymer dosage improves both the filler retention, evaluated by the turbidity of the filtrate, and the system drainage, evaluated by the flow speed. However, high dosages of this polymer compromised the results of these parameters due to the reversal of the system charge. It can also be confirmed that excessive shear forces affect the filler retention. Furthermore, a polyampholyte, with high molecular weight and charge density, containing positive (N-[3-(N′,N′dimethylamino)propyl]acrylamide), negative (methylene butanedioic acid), and neutral (acrylamide) groups in the same chain, was tested as a dry strength agent. All of the studies at molecular level concerning to the polyampholyte behavior in the solution and its adsorption behavior on charged model surfaces at different conditions of pH and ionic strength, were important to explain both the adsorption phenomena, involving cellulosic fibers and polyampholyte, and its impact on the paper strength. It was observed that the polymer solubility increases as the pH moves away from its isoelectric point, pHIEP 7.3, and decreases when the pH approaches close to pHIEP. The sizes of the structures of the polyampholytes depend on the pH of the dispersion medium. Also the size characteristics of polyampholyte, both in individual and aggregated forms, were measured by dynamic light scattering technique. The viscoelastic properties of adsorbed layers, as well as the amount of the adsorbed polymer, were measured by quartz crystal microbalance technique with energy dissipation. These two measurements, associated with the atomic force microscopy images, were important to understand the practical results of polyampholyte usage as a dry strength agent. Best results of paper strength, evaluated by paper strength index, were achieved at pH close to the isoelectric point on which one were found the following features: (1) larger sizes of the polymer structures in solution, (2) higher amount of mass in the adsorbed layers, and (3) the formation of more viscoelastic layers. The phase separation phenomenon, associated with the change in the solubility of the polymer due to the balance of the positive and negative groups throughout the studied pH range, was considered the main aspect for the variation in size of the aggregates. Although this polymer shows antipolyelectrolyte behavior due to the expansion of the its chain and the increasing in charge density with the ionic strength, considering the effect of ionic strength at pH 4.3, the adsorption behavior of polyampholyte was evaluated as a monocharged polyelectrolyte behavior with high charge density. Major and minor amounts of adsorbed masses were found for intermediates and extremes values of ionic strength, respectively. The electrostatic interactions were considered the main cause of the adsorption on charged surfaces. However, the electrostatic screening was considered the explanation for the low values of adsorbed mass at higher values of ionic strength.