Paper is a flexible web or mat of pulp fibers of plant (usually wood) origin. It is widely used for printing, packaging, and sanitary applications and also has a wide variety of specialized uses. Paper is formed from a dilute aqueous slurry of pulp fibers, fillers, and additives. Fillers are inert materials such inert incapable of reac-as calcium carbonate, clay, and titanium dioxide that make printing papers whiter and increase opacity (the ability to read print on one side of the paper without print on the other side of the paper showing through). Additives are materials used to improve the papermaking process and modify the final product. Additives include dyes, strength agents, and sizing agents (used to make paper resistant to water penetration).

Pulp is obtained by mechanical or chemical means or by a combination of the two. The two most common pulping methods are thermomechani-cal pulping and kraft chemical pulping. Thermomechanical pulp accounts for about 20 percent of pulp production in North America. The process consists of introducing wood chips between two large metal discs (on the order of 2 meters in diameter) that have raised bars on their surfaces and that rotate in opposite directions. The discs are in a pressurized refiner that operates at a temperature of 130°C. The combination of mechanical action and steam forms a wood pulp. This wood pulp retains the original lignin of the wood so the paper made from it is not very strong and yellows with age. Mechanical pulp is the chief component of newsprint.

Kraft pulp is formed by cooking wood chips in a highly alkaline aqueous solution at 170°C. It accounts for about 70 percent of pulp production in North America. In this process most of the lignin is removed. The brown pulp is used in sack paper and for the production of corrugated boxes. The bleached pulp is used in white printing papers and tissue papers.

Wood fiber accounts for about 98 percent of pulp production in North America, while globally it accounts for about 92 percent of pulp production. About two-thirds of the wood comes from softwoods because their high fiber length (3 to 5 millimeters) produces strong paper. The short fibers of hardwood species (approximately 1 millimeter) are used with softwood fibers in printing papers to achieve high strength and surface smoothness. Major nonwood sources of fiber, in decreasing levels of global production, include straw (especially wheat), sugarcane residue, bamboo, reeds, and cotton linters. Hemp fibers can also be used, but their fibers are so long that they must be cut in order to make paper from them.

The paper machine continuously forms, drains water, presses, and dries the web of paper fibers, using a single continuously moving plastic screen. The pulp slurry that is applied to the wire consists of 3 to 6 kilograms of dry fiber per 1,000 kilograms of water. Water is then removed by gravity, vacuum, pressing rolls, and, finally, heat in the drier section of the machine.

At a paper mill, wet pulp is spread onto a moving belt from where the water is drained away.

Twin wire machines form the web between two plastic screens, and cylinder machines form several layers of paper that are combined to form heavyweight boards. Paper is converted to a wide variety of products in operations that may include trimming, rewinding onto smaller rolls, cutting into sheets, coating, printing, and box making. see also Economic Importance of Plants; Fiber and Fiber Products; Forestry; Trees; Wood Products.

Christopher J. Biermann

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