One of the final stages in the paper pulping process, especially those employing chemical means to separate fibers of cellulose from non-fibrous materials present in wood. The presence of impurities (such as lignin) in pulped wood chips gives unbleached pulp a dark brown color. (For example, paper grocery bags use this unbleached pulp.) To increase the whiteness of pulp and to dissolve residual lignin and other impurities, pulps undergo treatment with bleaching chemicals in several stages, typically five (called "multistage bleaching"). Pulps produced in the sulfite process and early soda process used calcium hypochlorite, a chemical similar to household bleach, as a bleaching agent in a one-stage bleaching process. This method, however, is unable to bleach pulps produced using the now-prevalent kraft, or sulfate process. Multistage bleaching of kraft pulps has two primary phases: lignin-removal and enhancing whiteness. In the first stage of the bleaching process, the pulp is exposed to chlorine gas, which allows the lignin and its related impurities to dissolve easily in a caustic water solution. Several passes through the system ensure that as much lignin as possible is removed. The second stage involves alkaline extraction to remove the byproducts of the previous chlorination phase. Following the lignin-removal phase is the bleaching phase. In the third stage of the process the pulp is treated with calcium (or sodium) hypochlorite to remove coloration from additional impurities in the pulp, and bring out the white color of the cellulose fibers themselves. The fourth stage uses a second alkaline extraction process to remove detritus from the third stage. The fifth and final stage exposes the pulp to chlorine dioxide, which removes the last of the lignin.
Mechanical pulps are bleached somewhat, but not to any great extent. A side-effect of the bleaching process (related to lignin and hemicellulose removal) is a decrease in pulp yield. The main advantage of mechanical pulping is high pulp yield, so excessive bleaching would counteract this advantage.
The whiteness of pulp is measured using brightness test methods provided by the Technical Association of the Pulp and Paper Industry and the International Standards Organization.