The pulp and paper industry can be divided into four main segments: pulp, recovered paper, graphic paper, and packaging. The process of paper production depends on many factors, such as the materials used, the grade of pulp refinement, the type of machine used to make the paper, and the finishing treatment applied after the paper is formed.
Paper manufacturers use all types of wood, and the properties of the different types of wood provide for differences in the paper they make. The long fibers of softwood, for example, give paper strength, while hardwoods produce short fibers that give paper a smooth and shiny surface. Different paper products require different combinations of pulp. Most newspapers are a blend of chemical pulp (one-quarter) and groundwood pulp (three-quarters). Some newsprint mills produce 100 percent recycled newsprint, while most others incorporate at least some recycled fiber content. In high-quality papers, such as stationery, money, and art products, linen and cotton fibers are used to enhance the paper's strength and durability.
The process of paper manufacturing begins in the forests with the logging industry. Loggers fell the trees designated as pulpwood and cut them into prescribed lengths. The lumber is then chipped in the woods or in off-mill collection yards, or sent whole to the wood yards of the pulp mills. Pulp may also be taken from the waste products of woodworking industries.
At the wood yards, the cords of lumber are first showered with water to remove dirt and sand. They are then placed on conveyor belts and fed into a giant, revolving barking drum, which strips away the bark and cleans the wood. After barking, the conveyor belt feeds the logs into the chipper, a revolving disk with heavy, sharp blades set at an angle, which quickly reduces the logs to millions of wood chips about an inch square and an eighth of an inch thick. The chipper spits the cut pieces onto a set of screening trays that sort the chips according to size. Chips that are too large are rechipped, and those that are too small are used for fuel.
There are two methods of pulping. In mechanical pulping, barked logs are pressed against a grinding stone, which separates the fibers of the wood by friction. The heat generated by the process softens the lignin in the wood and allows the fibers to be separated, creating a material with the consistency of oatmeal. Mechanical pulping is used to produce newspaper and other papers that do not need to last a long time. Although these papers are weak and tend to yellow easily, they are inexpensive to make, since 90 percent of the tree is used and the process is a simple one.
The chemical pulp process most often used is known as the kraft process. In this process, wood chips travel on conveyors to the digesters, which are like giant pressure cookers. The chips are cooked in a solution of chemicals, which causes the lignin molecules to break down and dissolve. After the pulp is cooked, a valve in the digester is opened, and the pulp enters a blow tank at high speed. The chips, softened by the cooking process, disintegrate into fiber, which is separated from the chemicals and impurities. This process is costly and only uses about one-half of the wood from the tree.
The cellulose fibers, now called pulp, pass through many cleaners and screens to prepare them for the bleaching process, which will yield the whiteness and brightness needed for the grade of paper being manufactured. Once bleached, the wood pulp is refined through a mechanical process that beats and roughens the fiber surface. Refining improves the tearing strength, tensile strength, and bursting strength of the paper.
Various additives are often added to the pulp at this point in the process. Fillers, such as talc or clay, increase the brightness, opacity, smoothness, and printing properties of paper. These papers are often used in books, magazines, or stationery. Sizing agents, such as resin, wax, starch, and glue, make it more water resistant. Finally, dyes are often added to change the color of the paper.
At this point, the pulp, roughly 99 percent water and 1 percent fiber, is ready to be introduced into the paper machine through a headbox that stretches across the machine. Pumps spray a thin film of fibers onto a fast-moving, endless wire screen. As it travels along the wire, the water drops away and the fibers are matted into paper. While still damp, the formed paper is picked up and travels through a press section that squeezes out water, and then it travels into a dryer section, which is a maze of steam drums that press and dry the paper. From there, it is rolled and cut into various sizes.
For recycled paper, the pulping process is slightly different. A large machine called a repulper adds water and breaks down the paper into usable pulp. Chemicals that remove the dye and whiten the pulp, which is often called secondary fiber, are added to the mash. Newsprint is the most common form of recycled paper.
Once the papermaking process is complete, the paper may be sold in sheets or rolls to converters, or manufacturers, who use it in the creation of end products. Converters may include the makers of envelopes, stationery, boxes, and food packaging, as well as many other items.
The paper may also be sold directly to printing companies or newspaper, magazine, or book publishers. Some larger paper mills also function as converters, although typically not at the same plant.
Major advances in pulp and papermaking technology, computer sciences, and managerial practices have brought scientists, engineers, and other professionals into the paper industry. Papermaking generally requires a large investment; a new world-class mill, for example, can cost more than $1 billion. For this reason, companies with major pulp and paper facilities are included in the list of the 500 largest companies in the United States. These huge companies operate in many different sectors of the industry, both nationally and internationally, and in many cases work in other industries as well. Environmental engineers are also employed by the industry ensure manufacturers meet more stringent environmental standards.