Characterization of BCTMP fines and their effect on sizing.

Mechanical pulps, such as bleached chemithermo-mechanical pulp (BCTMP), are in increasing demand it?. many grades of paper products. To optimize their use in fine papers, it is important to understand fine sizing behavior. To address this question we carried out a series of studies to gain a better understanding of BCTMP sizing at neutral to alkaline conditions. In this paper we characterized the fines fraction in BCTMP and investigated its effect on sizing, particularly in comparison with fines from bleached kraft pulp (BKP).

The experimental results showed that BCTMP fines have much larger specific surface area and carry more negative charges than BKP fines. Despite these differences, they have a similar effect on rosin ester sizing. The presence of BCTMP fines increases the size retention; however, it drastically decreases the sizing degree of handsheets in both rosin ester and AKD sizing. This is because the presence of lines impairs the size coverage on the fiber surface. The results of scanning electron microscope (SEM) confirmed that fines, not fiber, are the preferred sites for sizes. X-ray photo-electron spectroscopy (XPS) was used to characterize the surface chemical composition of sized handsheets, and the results showed that the presence of fines causes less sizes available to fibers. Under the same conditions, a BCTMP fibers-containing system always has a higher sizing degree than a BKP fibers-containing system. In particular, the fines-free BCTMP fibers are much easier to size than the fines-free BKP fibers. This is attributed to the rich carboxylic groups and unique sulfonic groups in the BCTMP fibers.

Li is postdoctoral fellow and Sain is associate professor, Faculty of Forestry, University of Toronto, Ontario, Canada; Ni is professor and director, Limerick Pulp and Paper Centre, Fredericton, New Brunswick, Canada; email Ni at [e-mail].