Wood pulp and paper

One-third of the world’s timber is processed into wood pulp, and most of it is used to make paper and board. Pulping is necessary because woody tissue is not pure fiber, but is a rigid compound of cellulose (49 per cent), lignin (21 per cent), hemicellulose (15 per cent), and small amounts of minerals, proteins, and nitrogen. The cellulose fibers must be extracted from the binding lignin substances before they are soft enough for processing into paper and board.

In addition to forests planted specifically for pulp, one of the main sources of raw material is forest thinnings, which are trees that have been felled to allow others to grow to their full height. Wood residue from sawmills is an important alternative source of wood for pulping and has the advantage of arriving at the mill already chipped.

Waste paper is increasingly used as a raw material for both economic and ecological reasons, to the extent that the United States uses more than 30 million short tons (27 million metric tons) of recycled paper annually.

Before logs are processed into pulp, their bark is removed. In some factories the bark is removed by powerful jets of water. Alternatively, logs are fed into stripping drums where they jostle together until their bark peels off.

Mechanical pulping

The mechanical or groundwood method of pulping is a traditional process that involves breaking down the wood between rotating grindstones under a constant flow of water. This method produces low-grade pulp, most of which is used for newsprint. For higher quality pulp, the water pressure is increased, breaking the fibers down further. The mixture is screened, and any large lumps are removed.

The main advantages of groundwood pulping are a high yield and, therefore, a low price. This process, however, weakens the fibers so some chemical pulp is normally added to lend adequate strength. Newsprint, for example, requires about 15 per cent of chemical pulp to every 85 per cent of mechanical pulp. In addition, the residual lignin in mechanical pulp tends to go yellow with age, and so it is largely used for making “throw-away” products, such as tissues and napkins. Softwoods, such as conifers (which have long fibers and a low density), are favored for this method.

The refiner-groundwood process has developed as a result of the increased availability of wood chips from sawmills. Chips are fed into mills that reduce them to fiber fragments, and the resulting pulp, although less opaque than groundwood pulp, is much stronger; some hardwoods can be included in the mix for bulk. Nearly 50 per cent of Canada’s mechanical pulp is now made by a second-generation refiner process known as “thermomechanical pulping,” where the chips are preheated by steam, allowing the fibers to soften without discoloration.

Stripped logs are cut into wood chips about one-half inch long before being pulped. In mechanical pulping, chips are mashed between grindstones under a constant flow of water, whereas chemical pulping involves “cooking” the chips in chemicals. After cleaning, the pulp is often bleached, and mixers may be added. Pulp is piped into a head box that distributes the liquid over a fast-moving mesh. Water drains out of the pulp leaving a “sheet” of fibers that are then pressed between rollers into paper.

Chemical pulping

Higher quality pulp can be produced by “cooking” wood chips in chemicals to remove impurities, including the lignin “glue” that binds the fibers together and causes paper to yellow. There are three main methods of chemical pulping: the sulfite or acid liquor process, the soda process, and the more popular sulfate or alkaline liquor process. Sulfite liquor cooking is used mainly for spruce and some hardwoods. Inside a steam “digester,” wood is pressure-cooked in a bisulfite solution mixed with sulfur dioxide gas for up to 12 hours. The pulp is then cleaned of lumps, bark remnants, and chemicals before refining. The remaining sulfite liquor, however, causes serious pollution problems if it is not disposed of carefully.

Alkaline liquor processes are used primarily for hardwoods and nonwoody fibers, such as grasses and rags. In addition they are used particularly for conifers because the alkali helps to dissolve their high resinous content. The soda process involves cooking wood chips in a caustic soda (sodium hydroxide) solution, but this method produces relatively low-grade pulp. The “kraft” or sulfate process is a more popular method where chips are cooked in a solution of caustic soda and sodium sulfide at high temperatures and under pressure. Some mills still use batch digesters in which chips are cooked in separate loads. The more efficient continuous digester allows a constant flow, which not only avoids delay but also guarantees greater uniformity in the quality of pulp.

In the digester, chips are mixed with cooking liquor, heated under pressure, and washed to remove the separated lignin and remaining liquor. The liquor-lignin mixture is burned as fuel to generate steam for the process, and the chemicals are recovered for reuse. The kraft method produces a strong, dark-brown pulp suitable for wrapping-paper.

A combination of chemical and mechanical processes is semimechanical pulp. The chips are softened in a solution of sodium sulfite and sodium carbonate or bicarbonate before they are de-fibered in a refiner.

Preparing pulp

After thorough cleansing, pulp is often bleached to transform its dirty brown color into white. Bleaching entails four stages of chemical treatment, all of which must be carefully controlled to avoid damage to the fibers. The pulp is chlorinated and then treated with caustic soda, sodium hypochlorite, and finally chlorine oxide.

To make fine printing paper, pulp must be refined. The fibers, which are stiff and hollow at this stage, are passed through a series of metal disks that cause them to collapse, break, and fray. This process is essential if the fibers are to spread and interlock to form a strong sheet of paper. The length of time spent in the refiner determines many of the properties of the final sheet; the longer the refining process, the higher the quality of paper.

The final stage of preparation before the pulp is ready to pass to the papermaking machine is mixing, which determines the color, texture, strength, water-resistance, and opacity of the paper. China clay and calcium carbonate may be added to fill the gaps between fibers for a smoother paper and to give exceptional whiteness. Sizing agents, such as resin, are added for water-resistance, and dyes and pigments are added for color; fungicides may also be added.

The pulp-making and papermaking processes are sometimes carried out at different factories. If pulp is to be transported long distances, excess water is evaporated to produce “air dry” pulp. When this pulp arrives at the paper factory, water is added before it is fed into the papermaking machine.

Papermaking

When pulp is piped into the papermaking machine, it consists of 99 per cent water and 1 per cent fibers and additives. By the time the pulp has been processed into paper or board, it contains only 5 to 6 per cent water. Today, most papermaking machines are controlled by computers that monitor the amount of pulp flowing in, the moisture content, weight, and coating, and ensure that the finished reel of paper is consistent.

A paper machine can measure up to 300 feet (91 meters) in length and is able to produce a continuous sheet of paper more than 18 miles (29 kilometers) long in a single hour. Current trends, however, favor smaller machines that are more efficient and cost less to run, the traditional Four-drinier machine being the most widely used today.

The United States is the world’s leading manufacturer of paper and paper-board. In 1992 the U.S. produced more than twice as much as Japan and more than four times as much as neighboring Canada. Unlike the North American countries, Japan had to import nearly all of the timber and pulp used to make paper.

Forming sheets

Pulp is pumped from the refiners to a pressurized head box, which ejects a continuous flow of the liquid material onto a fast-moving wire or plastic mesh. Fibers in the pulp form in the direction of the flow, and as the mesh vibrates from side to side, the fibers interlock to form a web. Much of the water in the pulp drains out, assisted by suction boxes beneath the mesh. The thickness of paper is determined by the speed of the mesh in relation to the amount of pulp released from the head box. The eventual width of the roll of the paper is dictated by the width of the mesh and can vary from 6 to 30 feet (1.8 to 9.1 meters).

Toward the end of the mesh, a cylinder covered in fine wires (a dandy roll) presses the sheet flat and an emblem may be engraved on this roll to imprint a watermark in quality bond papers. If the roll is engraved with cross-hatching, the paper will look “woven,” and if parallel lines are produced, the paper is said to be “laid.” The sheet is further pressed and drained of water by a suction “couch roll.”

From the mesh, the sheet is transferred onto a felt blanket and a series of felted presses extract more water from it by compression. At this stage, the web of fibers has a water content of about 65 per cent before it is squeezed between the felt blanket and about 50 heated drying drums.

Some water is absorbed by the felt and some evaporates in a cloud of steam. For every ton of paper produced, about 2 tons of water evaporate. Finally, only about 5 per cent moisture remains in the paper, and this level is necessary to prevent the paper sheet from cracking.

Handmade paper (left) and machine-made paper (right) are produced using the same basic technique. A watery suspension of wood pulp is floated across a wire mesh, which traps the cellulose fibers when excess water is drawn through it. The mesh is then shaken to make the fibers interlock and form a “sheet.” The final size of the sheet of paper or width of the roll depends on the dimensions of the mesh.

Cardboard is made on the same principle as paper but generally by using a different technique. A cylindrical wire roll rotates in a bath of liquid pulp, and as water is drawn through the roll, a layer of fibers is deposited on the surface. The layer is transferred to a felt blanket, where other layers of fiber are added to build up a thick cardboard, and then put between presses.

Coating and calendering

Coatings provide paper with a smoother, more uniform surface on which to print. They consist of such pigments as titanium dioxide or clay, mixed with adhesives and water, and are applied either by equipment in the drying process or on a separate machine. Coating may be applied to a sheet in several layers and on one or both sides of the paper until the required finish is reached.

At the end of the machine, the paper passes through a calender: a series of polished iron rollers or stacks that give the paper a glossy finish. The more rollers it passes through, the smoother the finish and the thinner the paper. A high-gloss finish is achieved by super-calendering, which involves a series of alternately stacked metal and fiber rolls, separate from the papermaking machine.

After calendering, the paper is wound into a roll around a metal core. It may be supplied as a roll or cut into sheets. High-speed sheeting machines can cut as many as eight rolls at the same time.

Papermaking machines are capable of producing rolls of paper more than 5 feet (1.5 meters) in diameter and more than 30 feet (9.1 meters) wide. Sometimes the rolls of paper are cut on the machine, or they may be transported whole.

Other sources of pulp

Because cellulose fiber is the essential component in pulp for paper, a vast number of plants represent potential raw material for papermaking. Annual plants, for example, contain more nonfibrous cells than fibrous and produce a pulp similar to hardwood, although they demand a milder refining process. Straw is pulped in a mixture of lime and water and is still made into paper or board on a small scale in parts of Europe and Asia.

Esparto (Stipa tenacissima), a Mediterranean grass, has good papermaking properties. Its leaves have a higher cellulose content than most nonwood plants, and its fibers are more uniform in size and shape. The thick-walled fibers retain their springy, sinuous form after drying and make a bulky, opaque, stable, and resilient paper.

Bagasse, the pulp obtained from sugar cane, is a useful source of paper material because it contains 65 per cent fiber, 25 per cent pith cells, and 10 per cent water-soluble substances. Provided the pith is removed during pulping, bagasse produces a relatively smooth paper and is used in Latin America and the Middle East. Bamboo has also been found to produce a satisfactory pulp and, in good conditions, yields more fiber per acre than any other plant because, although it is classified botanically as grass, its stems are unusually dense and hard like those of woody plants.

The highest quality alternative source of paper pulp is cotton rag. Rag bonds, which are usually a combination of cotton and wood fiber, are strong, fine, and smooth and suitable for bank notes, legal documents, and business letterheads. Rag papers are watermarked to specify their cotton content and are priced accordingly.

The printing industry uses a large quantity of paper. Newspapers are usually made mostly from mechanical pulp, whereas high-quality magazines and books are generally made from chemical pulp.