The formation of positively charged aluminum rosinate precipitates and their effect on paper sizing efficiency

A study was conducted to determine a more efficient means of preparing paper resistant to liquid penetration through addition of rosin and papermakers' alum to cellulose. Paper sizing is accomplished by depositing a hydrophobic aluminum-rosin precipitate on the cellulose fibers. In order to optimize the interaction with cellulose, it is necessary that the precipitate be positively charged. It was hoped that proper control of the particle size and the electrokinetic potential of the precipitate prior to addition to the pulp would result in a more efficient sizing of the cellulose fibers. The appropriate conditions were determined under which a highly dispersed stable positively charged colloidal suspension of rosin acid in the presence of aluminum salts could be prepared. Considerable differences were observed with Al 2 (SO 4 ) 3 and Al(NO 3 ) 3 as a result of the presence of the divalent sulfate ion. A series of sized paper handsheets were prepared using preformed positively charged aluminum rosinate suspensions and their resistance to liquid penetration was determined. Inferior results were obtained with the preformed suspensions when compared to the normal means of in situ sizing where rosin size is added to the cellulose suspension followed by alum. Further paper handsheet trials were carried out in an attempt to determine the reasons for decreased sizing efficiency obtained with the preformed precipitate. The results are discussed in terms of the most important mechanisms involved in the sizing of paper.