Apparent Density (High, Low , Variable)

HIGH apparent density of paper (basis weight divided by caliper) is a common problem, especially in cases where paper is heavily calendered. Another way to describe this kind of problem is "low caliper at a given basis weight and smoothness." In a typical paper machine operation there is likely to be a close relationship between apparent density and smoothness. Some common causes of high apparent density (at a given smoothness) include (a) over-REFINING of the fibers, (b) fibers that are naturally flexible and conformable, and (c) high filler levels, especially in the case of clay filler. Some strategies for dealing with high apparent density have little to do with wet-end chemistry. For instance, installation of soft calendering equipment [see references] may help to preserve caliper while meeting a certain smoothness target. Another strategy is to add a percentage of fiber having a bulky nature such as chemithermomechanical pulp (CTMP) [see Moberg 1986]. The scalenohedral or "rosette" form of precipitated calcium carbonate is inherently bulky, so one can expect a lower apparent density when rosette PCC is used in place of either clay or ground calcium carbonate filler [Bown 1996]. However, even in the case or rosette PCC, increasing filler levels above about 10% are likely to increase the apparent density at a given smoothness level.

In cases where papermakers choose to back off on refining or wet-press nip loading to avoid densifying the paper, it is very likely that strength properties such as tensile strength and internal bond strength will suffer. Part of the solution, then, may be to use a more effective program of dry-strength agents [Howard, Jowsay, 1987; Linke, 1968; Robinson 1980].

Some practical things to look for if the apparent density of paper is unexpectedly high include (a) basis weight and moisture of the paper, (b) whether the smoothness is correct, and (c) whether it is feasible to decrease the wet-press loading or calendering nip pressure.

LOW apparent density is less often a problem for papermakers. Low apparent density may be caused by stiff fibers, insufficient refining, or insufficient calendering. Paper can be made denser by reversing many of the suggestions just listed. For instance, one might increase the degree of refining, use kraft fibers in place of mechanical fibers, or use clay filler in place of other fillers. One also can increase the wet-press load, increase the calendering load, or increase the moisture going into the calender stack. Refining will tend to increase the bond strength within the paper, and a high level of clay will tend to weaken the paper, so the papermaker has some flexibility in achieving the desired paper property targets.

VARIABLE apparent density or caliper of paper can have various causes, often associated with machine-directional streaks. Possible root causes can include a non-uniform jet from the slice adjustments or headbox flow problems, wake effects on a Fourdrinier table, partial filling of press felts or forming fabrics with pitch-like materials, uneven loading of wet-press nips, uneven drying profiles, and crown problems or localized heating of calender rolls. Some wet-end chemical factors to watch out for include variations in filler content, freeness variations, and drainage rate variations, possibly caused by changes in cationic demand of the system.


Bown, R., "Physical and Chemical Aspects of the Use of Fillers in Paper," in Roberts, J. C., Ed., Paper Chemistry, 2nd Ed., Blackie Academic and Professional, London, 1996, Ch. 11, p. 194. [Fillers versus apparent density]

Howard, R. C., and Jowsey, C. J., "The Effect of Cationic Starch on the Tensile Strength of Paper," Proc. 1987 Paper Physics Conf., 217 (1987).

Linke, W. F., "Retention and Bonding of Synthetic Dry Strength Resins," Tappi 51 (11): 59A (1968).

Moberg, K., "Wet End Chemicals for Reducing Board Density," Southern Pulp Paper 49 (4): 9 (1986). [CTMP fiber for increased bulk]

Robinson, J. V., "Fiber Bonding," in J. P. Casey, Ed., Pulp and Paper Chemistry and Chemical Technology, 3rd Ed., Vol. II, Wiley-Interscience, New York, 1980, Ch. 7, p. 915.

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 2. Putting Theory into Practice," Tappi J. 72 (12): 201 (1989)

PLEASE NOTE: The information in this Guide is provided as a public service by Dr. Martin A. Hubbe of the Department of Wood and Paper Science at North Carolina State University ( Users of the information contained on these pages assume complete responsibility to make sure that their practices are safe and do not infringe upon an existing patent. There has been no attempt here to give full safety instructions or to make note of all relevant patents governing the use of additives. Please send corrections if you find errors or points that need better clarification. Go to top of this page.

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