Porosity (Too permeable, Not permeable enough)
Some confusion regarding paper porosity issues can arise due to different ways of measuring and defining porosity. Many papermakers rely upon Gurley Densometer tests to evaluate the porous nature of paper (TAPPI Method T460). Gurley numbers tend to increase with decreasing air-permeability of the paper, since what one is measuring is the time required for a selected volume of air to leak through a defined area.
AIR-PERMEABILITY TOO HIGH
Suppose that the air-permeability of the paper is too high (i.e. low Gurley Densometer value). Papermakers know intuitively that they can solve this problem by greatly increasing the basis weight, but they also know that it would be very difficult for them to stay in business with such an approach. Rather, they would prefer to use something cheap and effective that will allow them to make the paper less porous without excessively changing other properties such as apparent density, strength, or smoothness. Increased refining usually results in a denser, less porous sheet. By contrast, high permeability can be expected especially if the furnish consists mainly of relatively coarse fibers and a relatively low level of fiber fines. These are some of the same conditions that yield high freeness, and you can expect there to be a fairly strong correlation between water-permeability during formation and air-permeability of the final sheet.
Extremely non-uniform paper is likely to have excessively high air-permeability, since air is expected to move preferentially through the thin areas. This kind of problem ought to be readily apparent if one holds paper up to the light. Some moderate degree of fiber flocculation can be considered "normal," so it is important to compare the product with a standard sample that was prepared under the same general conditions of furnish solids and forming equipment. One of the common causes of excessive flocculation of fibers in paper is excessive use of very-high-mass acrylamide copolymers, that is, retention aids. Solutions can include either reducing the dosage of retention aid treatment or moving the addition point to the upstream side of a pressure screen. Factors affecting formation uniformity are discussed in more detail elsewhere in this guide.
While you are in the process of holding that paper sample up to the light, it is worth also paying attention to whether there are significant numbers of pin-holes.
Practical mechanical measures to decrease air-permeability include (a) increased refining, (b) increased wet-press loading, (c) increased calendering, (d) reduced internal sizing to allow more uptake of size-press starch, and (d) increased size-press starch viscosity to achieve a better film.
The wet-end additive that comes the closest to being a possible remedy for high air-permeability is delaminated clay. Delaminated clay has a highly plate-like structure that has the potential to either block air passages or increase the length of the path that air must take to get through the paper.
Some of the most promising ways to decrease air-permeability through paper involve surface applications. To maximize the effect of size-press starch, with respect to sealing the paper, it makes sense to take measures that tend to hold the starch out at the paper surface. Such measures include internal sizing, increasing the solids content or viscosity of the starch solution, and the use of film-applicator types of size press. In addition, one can add certain copolymers to the formulation. Sodium alginate (from seaweed), polyvinyl alcohol, styrenemaleicanhydride (SMA), and similar copolymers are often found to decrease the air-permeability of paper to a greater degree than starch alone.
Delaminated clay added at the size press can be expected to make the paper less permeable, though the use of minerals at the size press depends on having suitable equipment and procedures.
Those who are familiar with the properties of coated papers will recognize that coated paper is much less air-permeable than typical grades of uncoated papers. So, in a gross sense, the coating process can be seen as a solution to high air-permeability. An even more aggressive approach is to laminate the paper with polyethylene or other films, as in the case of milk cartons.
AIR-PERMEABILITY TOO LOW
The first thing to consider doing if the air-permeability of paper is too low is to back off on refining. Also back off on any of the measures that were mentioned in the paragraphs immediately above. In theory one can make paper more air-permeable by fractionating the furnish to remove the fines; however it is then necessary to find another use for the fines.
The combinations of wet-end additives that are likely to have the greatest effects in making paper more air-permeable, especially if the paper web is evaluated before the size press and calender stack, fall under the category of microparticle systems. The net effect of such treatments can be viewed in terms of an increased fiber-to-fiber friction within the papermaking furnish. The idea is that during the forming process the fibers remain in contact where they first touch, and they do not slide past each other as much as they would in the absence of treatment. Whether or not this mechanism is accurate, the results seem to be consistent with this description.
Baker, C. F., "Good Practice for Refining the Types of Fiber Found in Modern Paper Furnishes," Tappi J. 78 (2): 147 (1995).
Han, S. T., "Compressibility and Permeability of Fiber Mats," Pulp Paper Mag. Can. 70 (5): 65 (1969).
Knauf, G. H., and Doshi, M. R., "Calculation of Aerodynamic Porosity, Specific Surface Area, and Specific Volume from Gurley Seconds Measurements," Proc. TAPPI 1986 Intl. Process and Materials Quality Eval. Conf., 33 (1986).
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.