Roles of Aluminum Sulfate in Rosin Sizing

Abstract

Roles of aluminum components originating from aluminum sulfate (alum) in rosin sizing were investigated by X-ray photoelectron spectroscopy, X-ray fluorescence analysis and pyrolysis gas chromatography. Handsheets were prepared with 1% alum and 1% anionic rosin emulsion, where the stirring time of pulp suspensions at pH 6.7 or pH 4.5 between the alum and size additions varied from 3 to 3000 min. Sizing effects of the handsheets prepared at pH 6.7 decreased rapidly as the stirring time proceeded, although the sizing appearance of the handsheets prepared at pH 4.5 was observed even at an addition interval of 50 hrs. Under the neutral papermaking conditions, the Al components once adsorbed on pulp fibers were immediately counteracted by OH^- ions, followed by the Al desorption and the drop in rosin retention. On the other hand, the acidic circumstances preserved the Al retention and performance, resulting in higher rosin content and better sizing efficiency. The Al components present on pulp surfaces were then aggregated gradually, and the ability as a retention aid eventually became absent, although the Al content remained almost unchanged. However, the Al components that had lost the above function could make a great contribution to sizing appearance by tub-sizing the Al-containing handsheets with the same rosin emulsion. Therefore, alum in rosin sizing must play two individual roles in sizing response: a retention enhancer as which Al cations act at the wet end, and a sizing promoter as which non-ionic Al components perform in the dried paper sheet.