Drainage and Pressing Taken Together are the Dewatering Steps on a Paper Machine
On a commercial paper machine, the fiber, minerals and other chemicals that will become the dry sheet of paper are delivered in a mixture called the furnish that contains about 99 percent water. The art of making paper is all about how much of, and how fast, this water is removed in various stages. In the forming section, the relative amount of water is reduced from about 99 percent to about 80 percent. This removal of bulk liquid water is called drainage. Precipitated calcium carbonate (PCC) has less affinity for water than cellulose fiber, that is, PCC is less hydrophilic. PCC also can exhibit relatively large particles that are uniform in size. These factors create the conditions for good drainage, and increasing the amount of PCC in the furnish will facilitate that. By carefully selecting the right PCC morphology, the papermaker can often improve drainage by several percentage points.
Printing and writing papers are mainly flat, two-dimensional materials. Because paper is formed on a rapidly moving belt, there is a certain amount of orienting of the fibers in the direction the belt is moving. There is also some distribution and settling of the PCC through the thickness of the paper, which, while small, is still much greater than the diameter of individual filler particles. Drainage, fiber orientation and filler distribution all impact the formation (uniformity) of the paper, and poor formation can cause one side of the sheet to look, feel and behave differently than the other. This is called sidedness, and excessive sidedness is undesirable. In general, the use of PCC as filler improves formation and reduces sheet sidedness.
There is a trickle-down effect of improving drainage on a paper machine. When the paper web (the paper before it is converted into sheets, envelopes etc.) leaves the forming section, it enters a section of the paper machine where it is pressed to remove still more water. If the paper contains less water entering this section, then the roll presses and other devices can operate more efficiently so that less water remains when the paper enters the drying process.
Drying removes the last of the water from the paper, and if less water remains in the paper because of improved drainage and pressing, then less water must be removed through drying. When the paper moves into the dryer section it is no longer supported by wires or belts, but now must maintain its structural integrity and support its own weight. The unsupported sheet has more opportunities to break and cause a stoppage in paper production. Therefore, the strength of the sheet is important to avoid these breaks. Paper strength is related to moisture content. The more water in the sheet, the lower the strength. Because PCC can often lower the water content of the paper going into the dryer section, it also reduces the number of breaks. This is a huge productivity benefit. Drainage and strength can often be maximized by using PCC of a relatively large size, but large particles are less efficient at providing brightness and opacity in the sheet.
Drying is accomplished using steam-heated rollers, and when less drying is needed less energy is used. Less energy means lower cost, but improved drying has another benefit. Paper machines are often limited in the amount of steam pressure that can be applied to heat the rollers. When less energy is required for drying, productivity gains are often realized since the paper machine can run at faster speeds.
Formation, pressing and drying are all part of a broadly defined quality paper makers call runnability. Every paper mill measures runnability in slightly different ways. In all mills, however, runnability comes down to how much trouble is encountered to keep a paper machine producing good quality paper. Paper mills measure by the rate, in tons per hour, that quality paper is produced. If a paper machine encounters many breaks then its output will be lower. If there is a lot of deposit buildup on the machine, then the quality of the paper may be lower or more breaks may occur. A paper machine exhibiting good runnability is one that encounters few breaks, has little deposit buildup and makes good quality paper. By properly selecting PCC size and morphology, papermakers can maximize paper machine runnability. When choosing fillers, papermakers usually balance the properties required in the final product against paper machine productivity requirements. PCC is unique in its ability to provide the best combination for maximum profitability.
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