Forest Industry

E

Wood Flake and Strand Products

Like particleboard, wood flake and strand products differ from standard lumber and veneer products in that the log is first broken apart, and the products are then formed from these pieces. As their name implies, wood flake and strand (long thin strip of wood) products are made from large flakes and strands of wood that are arranged in layers and then pressed into panels. These panels can be as strong as plywood but are cheaper to make because flakes can be made from lower-quality logs.

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Waferboard and Oriented Strand Board

For waferboard, the bark is removed from the logs, which are then mechanically broken apart into square to rectangular flakes typically 5 to 8 cm (2 to 3 in) long. These fragments are dried, coated with adhesive, distributed randomly, and pressed into a large panel, called waferboard, that is then cut into smaller panels—often with the same dimensions as plywood sheets.

Today, waferboard has been largely replaced by oriented strand board, which is stiffer and stronger than waferboard. Oriented strand board is made by the same method as waferboard but with longer wood flakes—about 8 to 10 cm (3 to 4 in) long. In addition to being longer than waferboard flakes, the flakes used to make oriented strand board are also narrower, allowing them to be more closely aligned. Three to five layers of flakes are stacked with the direction of the flakes in each layer running perpendicular to that of the flakes in the adjacent layers. These layers are then pressed into a panel.

The close alignment of the flakes and the perpendicular layering give oriented strand board nearly the same strength and stiffness as plywood. However, because small, crooked, and otherwise low-quality logs unsuitable for lumber or veneer can be used to make oriented strand board, it can be made less expensively than plywood. Being less expensive, it is replacing plywood in many applications, such as siding in building construction.

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Laminated Strand Lumber and Oriented Strand Lumber

Laminated strand lumber and oriented strand lumber are made of wood flakes 30 cm (12 in) long. These flakes are much longer than the flakes used to make oriented strand board. They are dried, coated with adhesive, and pressed. Unlike oriented strand board, these types of engineered lumber are manufactured to dimensions that correspond to conventional lumber dimensions. Because of their strength, uniformity, straightness, and lower cost, laminated strand lumber and oriented strand lumber are increasingly replacing structural lumber as wall studs, rafters, and joists (floor- and ceiling-supporting beams).

F

Wood Fiber Products

Wood pulp (a mixture of ground up and moistened wood cells or “fibers”) is the main source for paper and many other fiber products. While papermaking has been practiced for centuries, other wood fiber products, such as fiberboard, hardboard, and insulation board, have been developed in more recent years.

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Paper

According to tradition, the Chinese were the first to make paper when they experimented with mixing fibers from cloth rags and old fishnets with the fibrous inner bark of the mulberry plant around ad 105. Later, fibers from the bamboo plant were used as well. The Chinese mixed these plant, rag, and old fishnet fibers with water and pounded them into a pulpy mass using mortar and pestle. The mass was then placed on a cloth screen held rigid by a bamboo frame. As the water drained through the screen, the damp, thin mat of fibers left behind dried into a sheet of paper. Papermaking was confined to China for about 500 years, until it was introduced to Japan about 610, and then to Central Asia about 750.

Paper was first made in Spain around 1150, and the craft quickly spread to other European countries. The first commercially successful papermaking machine was invented by French inventor Nicholas Louis Robert in 1798 and was later modified by British papermakers Henry and Sealy Fourdrinier (for whom the modern Fourdrinier machine is named). The Fourdrinier machine contains a wire-screen conveyor belt that drains water from the wet pulp, leaving a mat of wet fibers, which are air dried and pressed between rollers. Until the 19th century in Europe, rag fibers were pressed into sheets of paper. But as demand for paper increased, cloth rags became increasingly scarce. Frenchman René de Réamur devised the idea of using wood to make paper when he observed wasps making paperlike nests by softening wood fibers with their saliva. The first mechanical wood pulp-making process was introduced about 1840.

Today more than 95 percent of paper is made from wood, while the remainder consists of fibrous materials, such as rice chaff, flax, and rags. More than one-third of wood pulp is obtained from recycling paper and paperboard. About one-quarter is obtained from wood waste generated from sawmills, veneer mills, and other wood industries. About two-fifths is obtained directly from harvested trees that are small, crooked, or otherwise unsuitable for other uses. Other types of raw material used alone or in combination with wood fibers to make paper include cotton fibers, bamboo, wheat straw, and synthetic fibers (see Fiber).

The modern paper industry uses both mechanical and chemical processes to form wood pulp. In the mechanical processes, logs from which the bark has been removed are ground into pulp against a large revolving grindstone. Pulp is also made from wood chips by forcing the chips between revolving metal disks, which tear the chips into fibers. In the chemical processes, wood chips are treated with chemicals such as sodium sulfate or magnesium sulfate that dissolve lignin and other wood-binding materials, breaking the fibers into pulp. There are many variations of mechanical and chemical pulping methods, and some wood pulping plants combine both approaches.

Commercial pulping processes cannot remove all the lignin from wood pulp. Because the residual lignin gives the pulp a brown color, the wood pulp is bleached with chemicals such as chlorine or sodium hydroxide to remove the residual lignin and whiten the pulp.

From the plant, the pulp is pumped to a Fourdrinier (papermaking) machine. This machine contains a wire-mesh conveyor belt that allows water to drain from the wet pulp, leaving a mat of wet fibers on the moving screen. Additional moisture is removed from the pulp by means of air suction pumps. The fibrous mat of pulp then passes through successive sets of felt and metal rollers, which press the mat into smooth paper. After being pressed, most paper is coated with substances such as clay, carbon, or calcium carbonate to increase its printing quality, as well as its color, smoothness, and strength. The paper is then wound onto reels.

Paper products are classified as paper and paperboard with both bleached and unbleached grades. Paper includes newsprint, stationery, text paper, and computer printing paper. Paper products also include sanitary paper products such as paper towels and tissue papers, as well as coarse paper products used in grocery and shopping bags. Industrial papers are used to make filters, gaskets, and other heavy-duty items.

Paperboard is a thick type of paper often used in industrial and packaging applications. One class of paperboard, known as industrial papers, is used to make filters, gaskets, and other heavy-duty items. Another type of paperboard is cardboard, which is used to make cartons and boxes. Paperboard is made the same way paper is made. The major difference is that paperboard is at least 0.3 mm (0.012 in) thick. One type of cardboard, called containerboard, contains corrugated paper, which is two layers of cardboard encasing a layer of cardboard pressed into small ridges. Containerboard is used for packaging and shipping boxes. Other types of paperboard are used for milk and ice cream cartons, cereal boxes, paper plates, book bindings, and other items. Certain types of paperboard are coated with plastics or metal foils to create a moisture barrier and to provide packaging for food products.

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