A process that extracts fibrous material, cellulose, from wood or other raw material as a prelude to papermaking. The purpose of pulping is to liberate cellulose fibers from other chemicals and impurities in the wood (or other fibrous source). Before wood can be pulped, it must undergo debarking. Bark contains little or no fibrous material, and is removed either mechanically or hydraulically. Debarked logs are then sent to a chipper where they are chopped into small pieces ready for pulping. Pulping can be done either by mechanical, chemical, or semimechanical (a combination of mechanical and chemical) means. Mechanical pulping uses revolving disks to grind wood chips into pulp. Water is added to the process to reduce wood damage due to heat and friction. One of the non-fibrous elements that is not removed during mechanical pulping is lignin, an organic material that binds fibers of cellulose together in the wood. It is the presence of lignin that is primarily responsible for low durability and yellowing with age. Mechanical (or groundwood) pulp is inexpensive to produce and generates the highest yield (100 pounds of wood will yield 80:95 pounds of pulp with this method). Groundwood pulp has many qualities that make it ideal for printing, but also has many disadvantages, such as low strength, low brightness, low permanency, and the tendency to yellow with time. Paper made from mechanical pulp also has a high quantity of imperfections called shives, or unfibered bundles of fiber that were torn from the wood during grinding. Groundwood pulp is used for low-quality papers used for newspapers, telephone directories, catalogs, and "pulp" magazines, as well as household items such as paper towels, tissues, and sanitary papers. Recent technological innovations are improving the quality of mechanical pulps while retaining their cost and yield advantages. (See also Mechanical Pulping, Refiner Mechanical Pulp, Thermomechanical Pulp, Chemi-Thermomechanical Pulping, and Pressurized Groundwood.)
Semimechanical pulping, as its name implies, is a two stage process that uses a chemical mixture (most commonly sodium sulfite and alkaline salts) to soften lignin, followed by a disk refiner to fiberize the cooked chips. However, a substantial portion of the lignin still remains, and pulp yield (60:80% of the original wood) is less than that of mechanical pulping. Semimechanical pulping produces stiff fibers, and is generally used for corrugated board, roll cores, and containers. Semimechanical pulp is not used for paper intended for writing or printing. (See also Semichemical Pulping.)
Chemical pulping results in near-total removal of lignin and other non-fiber constituents of wood. Chemical pulps produce the highest-quality printing and writing papers, but fiber yield is generally 50:55%, lower than the other pulping methods. There are several different chemical pulping methods. In the sulfite process, wood chips are cooked with a solution of sulfurous acid and calcium bisulfite to dissolve lignin. Sulfite pulp is moderately strong, and is soft and flexible. It is used to supplement mechanical pulp. Over the years, stronger pollution laws have resulted in the use of different chemicals. The sulfite process was for a time the most important chemical pulping process, but has been largely replaced by alkaline pulping. In the 1850s, the soda process was devised, which used lye (a caustic soda) to remove lignin from wood chips. Two decades later, sodium sulfate was added to the soda process, resulting in a pulp with greater strength. The advantages of this process—called kraft pulping, after the German word for strength—resulted in its becoming the dominant chemical pulping method. Although kraft pulping is often called the sulfate process, the active chemical has since been discovered to be not sodium sulfate, but rather sodium sulfide. The advantages of kraft pulping include its ability to handle nearly every known type of wood, its efficient chemical and heat recovery system which lowers processing costs, and its ability to produce a strong, high-brightness pulp. The replacement of batch pulping systems with continuous pulping systems and the use of monitoring operations via computer has also improved, speed, economy, and quality of pulp. Wood pulp is brown in color (due primarily to the presence of residual lignin), and can often be used as is for brown wrapping paper or paper bags. More often, however, pulp must be bleached in order to produce printing and writing papers. Chemical pulping can only be done for so long before severe fiber degradation occurs, so bleaching is typically done to remove or alter the lignin without harming the cellulose fibers.
Modified chemical processes are used to pulp nonwoody plants. Although extracting cellulose is easier, their fibrous content tends to be less than that of wood. Cotton and other textile remnants generates higher fiber content, and is used for high-quality and permanent writing papers and is also pulped using a modified alkaline chemical process. Recycled paper is also being used increasingly as a source of pulp.