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Sunday, September 8, 2013

Smoothness: Increasing & Decreasing

The biggest factors affecting smoothness of paper tend to be mechanical, rather than chemical. In particular, the type and extent of calendering usually causes the largest change in paper smoothness and density as the sheet passes through the system. Smoother paper can be achieved by increasing the nip pressures in calendering and by wetting the sheet with a mist spray or wet-box application before the web passes through a nip. Increased refining also tends to yield a smoother sheet.

Since calendering makes paper denser at the same time that it makes the paper smoother, papermakers are continually wrestling with issues of trying to separate these two variables. A common approach is to try to bulk up the paper, decreasing its apparent density, so that it then can be calendered back to the desired final smoothness and caliper. Wet-end additives that have been found to be most effective for lowering the density of paper include thermomechanical pulp (TMP) fibers and structured fillers such as scalenohedral (rosette shaped) precipitated calcium carbonate (PCC) filler.

Very high levels of smoothness can be achieved by conventional water-based mineral coatings, usually applied with a blade. Uncoated products can approach similar levels of smoothness and gloss only when using specialized equipment such as supercalenders, soft-nip calendered, or highly polished Yankee cylinder dryers.

To achieve rougher paper it is first necessary to back off on calendering. One might also consider increasing the amount of coarse fibers, such as softwood TMP or kraft fibers having a low degree of refining.

Smook, G. A., Handbook for Pulp and Paper Technologists, Angus Wilde Pub., Vancouver, 1992, ISBN 0-9694628-1-6. [Calendering practices]

Vreeland, J. H., Ellis, E. R., and Jewett, K. B., "Substrata Thermal Molding. Part 1. A Breakthrough in the Understanding and Practice of the Hot Calendering of Paper," Tappi J. 72 (11): 139 (1989); and "Part . Putting Theory into Practice," Tappi J. 72 (12): 201 (1989).