Opacity (Low, High Light Scattering)
Opacity is related to the ability of light to pass through paper. Most opacity problems experienced by papermakers involve the visibility of images printed onto the reverse sides or subsequent sheets of a magazine, newspaper, or book. There are two kinds of widely used definitions of opacity. The TAPPI test (Method T245) for opacity compares the diffuse reflectance from a sheet of paper that is backed alternately by (a) a black cavity, from which essentially no light returns to the paper, and (b) a standard white tile surface having a reflectance of 89%. By contrast, the printing opacity (TAPPI Method T519) can be defined as the ratio of reflectance from a paper sheet backed by a perfect black (either a black cavity or a black tile), and from a sufficiently thick stack of identical sheets of paper.
Many problems related to opacity can be anticipated from an understanding of how light interacts with the structure of paper. Opacity usually is highly correlated to the efficiency of light scattering by the paper material (except if there are large variations in the color or in the amount of light-absorbing material). High light-scattering efficiency will be achieved if there is a high incidence of spaces within the paper that have dimensions greater than at least a quarter of a wavelength of light. In rough terms, for the highest light scattering, one wants the greatest number of interfaces between solid and air. Light scattering also can be increased if the refractive index of the solid material is increased, but this effect usually is important only when papermakers are using titanium dioxide as an opacifying agent. Opacity values also can be increased by adding black dye, omitting bleaching stages, etc., but such approaches have only very narrow applications; brightness and other measures of reflectivity are even more important to most buyers of paper than is the opacity.
In general, the opacity of paper can be increased by such approaches as (a) adding or increasing the filler content, (b) adding titanium dioxide in particular, (c) minimizing self-agglomeration of fillers and making sure that they are well dispersed before addition to the furnish, and (d) avoiding excessive compaction of the paper.
Refining tends to densify the paper and make it more transparent, so excessive refining of kraft fibers will tend to hurt opacity. Refining tends to make the fibers more flexible, so that they create a higher relative bonded area when the paper is formed. Sometimes it is possible to back off the refining power and increase the level of dry-strength agents such as cationic starch in order to achieve the overall product goals.
Wet-pressing also densifies the paper, though the effects of wet-pressing on opacity differ in subtle ways from those of refining. Usually there is a strong effect of wet-pressing on paper machine speed, drying energy, and paper strength; that means that often it will be undesirable to back off on the nip pressures. There may be exceptions in the case of some specialty grades or when the paper must meet strict requirements for caliper or apparent density.
Calendering can adversely affect opacity, so excess calendering should be avoided after minimum specification limits for smoothness have been achieved. The adverse effects of calendering on opacity are minimized by treatments that favor surface effects, rather than compaction of the paper as a whole. This is why such practices as soft-nip calendering and wetting the sheet just before calendering are often beneficial.
An inadequate retention aid system may result in low opacity, especially if there are significant losses of mineral to the wastewater treatment system. Though it is theoretically possible for retention aid polymers to self-agglomerate filler particles, retention aids generally have a positive effect on opacity. Scanning electron micrographs of different paper products show a range of filler distributions, from highly self-agglomerated to well-dispersed over the fiber surfaces. However, due to convection, any retention aid added to agitated or flowing paper stock is almost immediately taken up by fibers and fiber fines. The ultimate location of the fillers also depends on the degree of fibrillation of fiber surfaces, the type and amount of fiber fine material, and hydrodynamic conditions in the forming process. There are many variables to play with, including the addition points for fillers and retention aid chemicals, but it is usually unrealistic to try to make quantum changes in how the filler is distributed in a sheet of paper.
Though the results might not be worth the effort and increased complexity, it is sometimes worth considering such strategies as (a) decreasing the brightness of inner plies and increasing the brightness of outer plies, or (b) adding some black dye or decreasing the bleaching and compensating by adding fluorescent whitening agents (FWAs). Both of these approaches are theoretically valid. However, one must carefully weight the costs and consider possible adverse effects such as metamerism or an increase in the mottled appearance of the paper.
Low opacity also can result if air spaces within the paper are being filled in by materials added at the surface. Size-press starch tends to reduce paper opacity. Contrary to appearances, the purpose of size-press starch usually is not to produce a continuous, unbroken film or to fill in the surface. But these tendencies can become significant as papermakers adjust their processes to increase surface strength or minimize dusting by adding more surface starch and other polymers.
If the opacity is higher than the specified limit, some of the available options include (a) reducing the filler content and omitting any titanium dioxide, (b) increasing the level of refining of kraft furnish, or (c) adding wax to the paper, etc. Complaints about excessively high opacity are rare in the majority of paper grades.
Bown, R., "Physical and Chemical Aspects of the Use of Fillers in Paper," in J. C. Roberts, Ed., Paper Chemistry, 2nd Ed., Blackie Academic & Prof., London, 1996, Ch. 11, p. 194.
El-Hosseiny, F., and Abson, D., "Light Scattering and Sheet Density," Tappi 6 (10): 17 (1979).
Kwoka, R. A., "Strategies for Cost Effective Optical Performance," TAPPI 1990 Dyes, Fillers & Pigments Short Course Notes, 21 (1990).
Middleton, S. R., Desmeules, J., and Scallan, A. M., "The Kubelka-Munk Coefficients of Fillers," J. Pulp Paper Sci. 20 (8): J231 (1994).
Robinson, J. V., "A Summary of Reflectance Equations for Application of the Kubelka-Munk Theory to Optical Properties of Paper," Tappi 58 (10): 152 (1975).
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 (firstname.lastname@example.org). 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.