Size Press Efficiency (Breaks, Hold-out, Surface Strength)

The purpose of a size press on a paper machine is to apply a solution of starch or other material onto the surface of the dry paper, after which the paper is dried again. Usually the main role of the applied material is to increase the surface strength of the paper. Other benefits can include reduced dusting tendency, increased stiffness, and reduced air-permeability. Papermakers also can add copolymers such as styrenemaleic anhydride (SMA), styrene acrylates (SA), or urethane copolymers to size-press starch if they want to achieve increased resistance to liquids or higher levels of surface strength.

WEB BREAKS AT THE SIZE PRESS

Because the surface of a paper web is re-wetted at a size press, there tends to be an increased possibility that the web will break. The likelihood of web breakage is considerably greater if the size press is of the traditional "puddle" type, in which the paper comes into contact with a bulk phase of starch solution. Many modern paper machines, especially those producing printing papers, are fitted with film-applicator size presses. In such size presses the starch is metered onto a transfer roll by means of a blade, smooth roll, or grooved roll. In all cases there is a danger that absorption of water into the paper will weaken the paper to such an extent that the web breaks - especially if there is a preexisting weak point or hole in the sheet. It is worth noting that excessive rewetting of the paper web at the size press also can result in dimensional stability problems in the finished paper. This may show up as register problems during printing, in a mottled appearance of print images, or in image deletions in the case of xerographic printing.

Important variables, with respect to avoiding size press breaks, include the level of hydrophobic sizing, the moisture content of the paper entering the size press, and the overall quality and dry-strength of the sheet. Since the exposure of paper to size press solution happens very quickly, a low to moderate treatment with an internal sizing agent such as alkenylsuccinic anhydride (ASA), alkylketene dimer (AKD) or a rosin product ought to be sufficient. Papermakers often rely on ink-penetration tests, such as the well known HST test, to predict whether a sheet has a sufficient level of sizing to run well through their size press equipment. In critical cases it may be important to collect samples of paper before the size press - perhaps during threading of the machine - to be able to evaluate the pre-size-press resistance properties of the paper. Baseline data from such tests can be useful when papermakers are working to solve future issues related to size press runnability.

The importance of web moisture before the size press can be appreciated by anyone who has tried to clean up a kitchen spill with a very dry cellulosic sponge. Usually much better results can be achieved if the sponge is first wetted and wrung out before it is used to clean up the spill. In the same way, a paper web with higher moisture is likely to pick up more size press solution, compared to an over-dried web. However, it is important to keep in mind that papermakers often over-dry paper before the size press in order to overcome moisture streak problems, to cure AKD size, and various other reasons. In some cases over-drying of the web before the size press will decrease web breaks. The down-side of such an approach is that the paper will tend to become more brittle, compared to paper that is dried only down to a moisture content of 5 to 9%, i.e. the natural moisture content of paper at a typical level of relative humidity.

If size-press breaks don't appear to be related to excessive rewetting of the paper, some other factors to consider are the presence of holes, deposits, or spots in the web. More information about those factors is given elsewhere in this site.

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 (m_hubbe@ncsu.edu). 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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