Dissolving pulp is a cellulose product that is used in the manufacture of many different products including paper, rayon, and cellophane. Most dissolving pulps are made from softwoods which are woods from conifer trees. Softwood pulps are known for their long fibers which are particularly conducive to making products like rayon, a synthetic fiber which can be turned into fabric for clothing and linens. Some dissolving pulp is made from hardwoods, though, and an increasing amount is made from non-wood plant fibers.
The two most common practices for turning plant fibers into dissolving pulp are the modified kraft process and the sulfite process. Both processes cook the wood in strong chemicals to transform wood chips into cellulose fibers. The modified kraft process differs from the sulfite process in two ways. It uses an alkaline liquor instead of an acidic one, which protects the machinery from corrosion, and the use of sodium sulfide produces a stronger fiber. The term “kraft” comes from the German word for strong. The process was originally developed in Germany in the late 19th century.
While the modified kraft process has become more popular, both processes begin by removing the bark from the logs. The logs are then chipped and added to a vat with liquor. The mixture is heated to soften the chips, and the residual liquid can be reused in the pulping process. After cooking, the pulp is washed to separate debris from the fibers.
Dissolving pulp is bleached to decrease the lignin content. Lignin is a polymer that binds wood fibers together. Lowering the lignin content allows the final loosening of dark fibers from lighter ones. Following the bleaching process, the pulp goes through a final screening to remove any remaining impurities. Then it is dried and stored. In modern chemical pulping processes, some of the pulp is diverted to a recovery boiler where it is turned into fuel to power the pulping process. The combustion process produces more energy than is required, so excess electricity from the process can be sold.
Dissolving pulp is used in addition to other pulps to create some products. In papermaking, for example, producers blend mechanical pulps with the chemically derived dissolving pulp to balance the printability of long fibers with the durability of short fibers. They may also choose not to bleach their pulp in order to create brown paper. While the demand for dissolving pulp experienced a slow decline from the 1970s to the end of the 1980s as competition from other fibers such as polyester increased, it has since stabilized and continues to play an important role in a range of industries.