Method of converting logs or wood chips into paper pulp for use in papermaking accomplished by mechanical grinding, as opposed to chemical pulping. The purpose of pulping is to reduce wood (or other fibrous raw material) to individual cellulose fibers. A non-fibrous constituent of wood, lignin, binds cellulose fibers together, and is primarily responsible for reducing paper quality and its permanence.
Traditional mechanical pulping involves forcing logs against a revolving stone, which grinds the logs into pulp by abrasive action. The stone is sprayed with water to remove fibers from the pulp stone, and to prevent fiber damage due to friction-generated heat.
The production of mechanical pulp (also called groundwood) results in little removal of lignin content, and consequently produces paper that is not of as high a quality as other pulping methods that remove significant amounts of lignin. The advantages of mechanical pulping are its high pulp yield (100 pounds of wood can generate as much as 95 pounds of pulp), its low cost, and the paper it produces has several desirable printing qualities, such as high ink absorbency, compressibility, opacity, and bulk. Disadvantages, however, include low strength, low permanence, and a tendency to yellow with time (primarily caused by high levels of lignin). Paper made with mechanical pulps also contain shives, or incompletely ground fiber bundles.
Mechanical pulps are primarily used in newsprint, as well as papers used in telephone directories, catalogs, "pulp" magazines, and paper towels and tissues.
Many pulps are bleached following pulping to increase brightness and whiteness, and to dissolve additional amounts of lignin. Mechanical pulps are bleached, but not to any great degree. Although some lignin is removed from groundwood pulps by bleaching (and some lignin is softened and lost by the heat generated by the grinding process), extensive bleaching can result in a decrease of pulp yield, defeating the primary advantage of the process. About 23% of the pulp used in the world is mechanical pulp. In most usages, however, groundwood pulp is combined with pulps produced chemically, to counteract the disadvantages of paper made with mechanical pulps.
The above method of producing groundwood pulp is the oldest pulping process. New mechanical pulping methods are reducing the disadvantages without compromising the advantages of mechanical pulping. Refiner mechanical pulping sandwiches wood chips between two revolving disks. Heat due to friction softens the lignin and allows greater separation of the cellulose fibers, while contributing less fiber damage. RMP has greater strength than traditional groundwood, which reduces the need to supplement it with chemical pulps. RMP pulpmills, however, have been supplanted by even newer pulping techniques. Thermomechanical pulping operates similar to RMP, but under higher temperature and pressure. (In many cases, the wood chips are preheated.) The higher temperatures and pressures soften the lignin even more than can be accomplished using frictional heat, and fiber separation is easier. Additional passes through the system increases defiberization. Thermomechanical pulp is stronger than refiner mechanical pulp, and still retains the high-yield and cost-effectiveness of mechanical pulps. Chemi-thermomechanical pulps use mild chemicals, which increases pulp brightness and reduces shive content. Pressurized groundwood is similar to traditional groundwood pulping, but operates under high pressure, which helps overcome some of the disadvantages of the process.