Dissolution pulp, made from softwoods, hardwoods, and non-woody plant fibers, is used in products like paper, rayon, and cellophane. The modified kraft and sulfite processes turn wood chips into cellulose fibers, with the former using an alkaline liquor and sodium sulfide. The pulp is bleached to decrease lignin content, sieved, dried, and stored. Dissolved paste is mixed with other pastes in papermaking to balance printability and durability. Demand declined in the 1970s-1980s but has since stabilized and remains important in various sectors.
Dissolution pulp is a cellulose product that is used in the manufacture of many different products including paper, rayon and cellophane. Most dissolving pastes are made from softwoods which are softwoods. Softwood pulps are known for their long fibers that are particularly suited to making products such as rayon, a synthetic fiber that can be made into fabric for clothing and linens. However, a portion of the dissolved pulp is produced from hardwoods and an increasing amount is produced from non-woody plant fibers.
The two most common practices for converting vegetable fibers into dissolved pulp are the modified kraft process and the sulfite process. Both processes cook the wood in strong chemicals to turn the 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 stripping the bark from logs. The logs are then chipped and added to a vat with the 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 the debris from the fibers.
The pulp in dissolution is bleached to decrease the lignin content. Lignin is a polymer that binds wood fibers together. Lowering the lignin content allows for the eventual loosening of the dark fibers from the lighter ones. After the bleaching process, the pulp goes through a final sieving 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 fuel the pulping process. The combustion process produces more energy than needed, so excess electricity can be sold.
The dissolved paste is used in addition to other pastes to create some products. In papermaking, for example, manufacturers mix mechanical pulps with chemically derived dissolving pulps to balance the printability of long fibers with the durability of short fibers. They can also choose not to bleach their pulp to create brown paper. Although the demand for dissolution cellulose experienced a slow decline from the 1970s to the late 1980s, as competition from other fibers such as polyester increased, it has since stabilized and continues to play an important role in a series of sectors.
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