Formation uniformity (Flocs, Streaks)
Nonuniformity of paper within a length scale of 2-20 mm is most frequently associated with a tendency of fibers to form flocs. It is important to keep in mind that a certain degree of fiber flocculation can be expected, regardless of chemical conditions in a papermaking furnish. The flocculation occurs because a typical papermaking fibers have length-to-thickness ratios between about 50 and 100. That means that the fibers tend to collide with each other and become somewhat entangled. At the same time, hydrodynamic shear also tends to break down the fiber flocs, and the degree of flocculation can be understood as a dynamic equilibrium between these two tendencies.
Papermakers employ the following kinds of strategies to try to minimize the level of fiber flocculation in the paper: (a) adjustments of the papermaking equipment, (b) selection of fibers or manipulation of mechanical aspects of the furnish, and (c) adjustment of the chemical environment.
Paper machine adjustments that affect the small-scale uniformity of the product can include the angle of impingement of the jet onto the forming fabric (velocity forming vs. pressure forming), the relative velocities of the jet and the fabric, the design, angle settings, and spacings between hydrofoils or blades on the side of the fabric(s) opposite to the paper, and the use of such devices as dandy rolls. These approaches generally lie beyond the scope of this website, except that it may be pointless to consider approaches involving chemical additives if no attention is being paid to the paper machine equipment settings.
In principle, the simplest way to reduce the tendency for fibers to flocculate in a stirred suspension is to decrease the consistency (dry mass of filterable solids per unit of volume). Studies have shown that the degree of flocculation tends to be related to the product of the consistency and the square of the length-to-thickness ratio of the fibers. In practice, papermakers are able to reduce the consistency only up to the point where (a) there is adequate fan pump capacity, (b) the headbox flow is not so high as to create undesirable wake effects, or (c) the furnish still drains easily enough so that the paper reaches a solids level suitable for wet-pressing by the time it leaves the forming section of the machine. Excessive flow through a headbox, relative to its rated capacity, can cause alignment of fibers at an angle to the machine direction. Some ways to modify a papermaking furnish to reduce the flocculation tendency include refining (to make the fibers more flexible), or the substitution of a higher level of hardwood fibers in place of softwood fibers.
Chemical strategies to reduce fiber flocculation in a sheet of manufactured paper tend to be somewhat counter-intuitive. One would like to think that the best solution would be to add a chemical to help the fibers slide past each other. Such an effect is employed during the production of wet-lay nonwoven fabrics by the addition of high levels of very-high-mass anionic or nonionic water-soluble polymers. The use of such "formation aids" can work during formation of thin sheets of synthetic fibers, since the resistance to dewatering tends to be low. However, when producing paper or paperboard from typical wood-based fibers, as used in most papermaking grades, a similar chemical approach would tend to inhibit dewatering and require a decrease in the speed of the paper machine.
The most important chemical strategies for avoiding poor formation uniformity of paper can involve avoiding excessive effects of chemical flocculants, i.e. retention aids. This can be done in either of two ways. The first involves adding the lowest practical amount of retention aid, consistent with what is needed to keep the paper machine system clean, to minimize the decomposition of sizing agents, and to avoid a strongly two-sided composition of paper (if made on a Fourdrinier former). The second approach involves adding the flocculant before a unit operation such as a pressure screen, where fibers are subjected to high levels of hydrodynamic shear. It has been shown that such hydrodynamic forces tend to redisperse fibers that have been flocculated by high-mass polymers. However, a high proportion of chemically-induced attachments between very fine particles and fibers tend to survive the high-shear exposure.
Some of the most notable recent progress in achieving more uniform paper has involved the use of chemical programs that promote easier release of water during the formation process. Such chemical systems were described earlier, and it is worth noting that treatment programs involving microparticle additives are often associated with efforts to achieve more uniform formation. However, the drainage-promoting chemical strategies can be effective only in cases where they make it possible to make other changes, such as lowering the headbox consistency, reducing the average fiber length, or increasing the level of refining.
Machine-directional streaks in paper are often due to wake effects, possibly originating at the slice of the paper machine headbox. If so, the streaks have to be corrected by mechanical changes, rather than chemical adjustments. For instance, it may be necessary to reset the slice adjustments so that the opening is more even across the entire width of the machine. This is much harder to do that one might imagine, since it is usual for counter-rotating vortices to form just after the slice, and interactions between such vortices can result in the streaks. More serious effects can result from deposits on the slice. Efforts to correct such streaks by adjusting the slice opening at different points sometimes make matters worse. Sometimes streaks can be related to the speed of rotation of a perforated roll in an air-padded headbox. Other possibilities include partially plugged suction boxes and various kinds of misalignments or ridges in the fabrics.
Chemical strategies can be important, relative to streaks, if the problem is related to deposited materials, either at the slice lip, on the forming fabric, or in the wet-press felt(s). See the comments regarding deposits and scale if the streaks seem to be possibly related to the headbox slice. See comments regarding barrier chemical treatment if the streaks seem to be related to a partly-occluded forming fabric. See comments regarding felt filling if the streaks seem to be related to wet-pressing. In some paper machine systems it is possible to obtain streaky formation due to poorly mixed retention aid, assuming that the polymer is added very late to the system. This is especially a concern if the retention aid is added on one side of a main stock line in the approach to the headbox.
Cross-machine streaks or "barring" is most often due to unsteady flow or unsteady pressure at the headbox. A specialist should look at the system and find out whether the stock valve is opening and closing excessively, of whether there are other conditions leading to pulsating flow.
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Jokinen, O., and Palonen, H., "Interdependence of Retention and Formation in the Manufacture of SC Paper," Paperi Puu 68 (11): 801 (1986).
Kerekes, R. J., and Schell, C. J., "Effects of Fiber Length and Coarseness on Pulp Flocculation," Tappi J. 78 (2): 133 (1995).
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Swerin, A., and Mahler, A., "Formation, Retention and Drainage of a Fine Paper Stock during Twin-wire roll-blade Forming. Implications of Fiber Network Strength," Nordic Pulp Paper Res. J. 11 (1): 36 (1996).
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.