Size Press Starch Holdout Problems
The degree to which size press starch is held out at the surface of paper depends on many factors, some of which are related to wet-end additives. Excessive penetration of the starch into the interior of the sheet can hurt the dimensional stability of the paper, and it can greatly reduce the effectiveness of the starch in terms of surface strength, reducing the air-permeability of the sheet, and increasing stiffness. On the other hand, a base sheet that allows more penetration of starch solution can be expected to show greater increases in internal bond strength, and the amount of starch solution taken up at the size press tends to be greater.
TOO MUCH PENTRATION
Many of the measures that papermakers use to increase the hold-out of starch at the size press are the same measures that they use to minimize size press breaks of the paper web. These include increasing the levels of hydrophobic sizing agents added to the furnish.
Denser paper sheets have greater resistance to penetration by hot starch solutions, compared to lower-density sheets, but papermakers seldom have the option of varying the density of their products outside of strict limits. What they can do more often is to employ strategies to form structures with smaller air-spaces, cracks, and channels available for fluid flow. Aqueous polymer solutions resist passage through small capillaries; the pressure required to maintain a given average velocity of flow through a cylindrical capillary is inversely proportional to the square of the pore diameter or radius. This means that it is theoretically feasible to create bulky paper, full of air spaces, and still resist liquid penetration, by the use of high surface area materials. These materials would include such things as groundwood fibers, highly refined kraft fibers, structured fillers such as small scalenohedral calcium carbonate particles, and lots of fiber fines. One thing to keep in mind, however, is that the relatively high surface areas required for such a strategy imply that the dosages of sizing agents needed to cover those surfaces also will tend to be higher.
Some practical measures to increase the hold-out of size-press starch include increasing the solids content and viscosity of the starch solution and switching to a type of size-press starch tending to stay out at the surface. An example of the latter is cationic size-press starch.
LOW PENETRATION OF SIZE-PRESS STARCH
To encourage greater penetration of size-press starch, one needs to do the reverse of various things mentioned above. Internal sizing treatments should be reduced. Refining of kraft fibers might be reduced as well, depending on the constraints of strength specifications. The moisture of the paper going into the size press could be increased.
VARIABLE PENETRATION OF SIZE-PRESS STARCH
One of the key things to suspect, if uptake or penetration of size press starch is highly variable, is that the internal sizing of the sheet is not constant. Variations in sizing efficiency might be due to problems with the additive itself, with its retention in the paper, with large variations in the area of filler and fines in the furnish, or with large variations in the amounts of surface-active materials. Most of these possibilities can be tested. Some relevant tests to be carried out over several days or weeks would include assays of the "percent active" sizing agent in the formulation being added to the machine, tests of the pre-size-press sizing response, tests of white water consistency, headbox or machine chest furnish ash determinations, headbox or machine chest furnish fines determinations, and surface tension measurements of the process water. Once the key factors have been narrowed down, the options would include either (a) minimizing variations of those factors, or (b) compensating by varying the dosage of sizing agent, or other suitable variables.
Brungardt, B., "Improving the Efficiency of Internal and Surface Sizing Agents," Proc. 83rd Annual Meeting, Tech. Section CPPA, B109 (1997).
Cushing, M. L., "Surface Sizing," in J. P. Casey, Ed., Pulp and Paper Chemistry and Chemical Technology, 3rd Ed., Vol. III, Wiley-Interscience, New York, 1980, Ch. 20, p. 1667.
Dill, D. R., "Control and Understanding of Size Press Pickup," Tappi 57 (1): 97 (1974).
Fineman, I., and Hoc, M., "Surface Properties, Especially Linting, of Surface-Sized Fine Papers," Tappi 61 (5): 43 (1978).
Hoyland, R. W., and Howarth, P., "Practical Use of the Size Press," Paper Technol. 13 (2): 38 (1972).
Kane, T. G., "Polyvinyl Alcohol/Starch Sizes Can Reduce Pulp Costs for Fine Paper," Pulp Paper 52 (2): 125 (1978).
Tompkins, T. W., and Shepler, J. A., "Combination Sizing. The use of Synthetic Surface Size with an Alkaline Internal Size," Proc. TAPPI 1991 Papermakers Conf., 191 (1991).
Walter, J. C., "Handling, Circulation, and Filtering in Metering Size Press Installations," TAPPI J. 81 (11): 95 and (12): 68 (1998).
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 (email@example.com). 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.