Colloidal Behavior of Amphoteric Dry Strength Agents

Abstract

Dry strength additives are essential to produce paper that is strong enough to meet various usage requirements. Recently, amphoteric copolymers have been developed as more effective dry strength agents for addition during the papermaking process. To understand the mechanism of such polyampholytes, the fundamental colloidal behavior of amphoteric polyacrylamide dry strength additives was investigated. The present thesis includes potentiometric titrations, viscometric and turbidimetric measurements, charge analysis using microelectrophoresis and colloidal titration with a streaming current detector, and adsorption of amphoteric polyacryamides onto bleached kraft hardwood fibers.

The potentiometric titration results for the polyampholytes were in agreement with the theoretical curves except in a range between pH 4 and 6. The charge analysis showed that the total charge of the amphoteric polyacrylamides depended on pH. The total net charge was reversed at the pH value termed the iso-electric point, where the polyampholytes have no net charge. The iso-electric points determined by the two measurements, microelectrophoresis and streaming current, coincided well, but not with the calculated values. Around the iso-electric point, the turbidity of the polyampholyte solution was maximized, while the viscosity was minimized. In this region of pH, addition of salt led to swelling of the polyampholyte chain conformation, which was an opposite effect from what is seen in simple polyelectrolytes (anti-polyelectrolyte effect). Namely, the turbidity was decreased and the viscosity was increased by addition of salt to the polyampholyte solution around the iso-electric point.

A colloidal titration procedure using a streaming current technique was found to be applicable at either pH 3 or 11 in order to determine the concentration of the polyampholytes. The required titration amount for zero charge was found to deviate from a 1:1 stoichiometric relationship, depending on the salt concentration. The adsorption of the amphoteric polyacrylamide dry strength additives onto bleached hardwood fibers was significantly affected by the pH and salt concentration. Also, it was slightly increased with time for adsorption. The stirring rate during adsorption was not a important factor for adsorption. It was found that the polyampholytes with no net charge were also adsorbed on negatively charged fibers. The maximum adsorption of the polyampholytes was achieved around the isoelectric point. This behavior is attributed to a sufficiently low charge density of the polyampholytes around the iso-electric point, which does not allow the polyampholyte chains to lie flat against the charged surfaces because of electrostatic attractions.