Dissolving pulps are chemical pulps that are suitable for subsequent chemical conversion into such products as rayon, cellophane, cellulose acetate, cellulose nitrate and carboxymethyl cellulose. Production of “chemical cellulose” pulp grades represents a substantial industry in it own right.
Dissolving pulps can be manufactured by either a modified kraft or sulfite process. In each case, the objective is a relatively pure and uniform cellulose product with a controlled weight-averaged degree of polymerization. Both lignin and hemicelluloses are considered contaminants and are removed. Softwoods are the major raw material, but some hardwood is used. The highest-quality pulps (99% alpha cellulose) are manufactured from cotton linters.
The viscose-rayon process is the largest user of dissolving pulps. In the viscose process, the chemical cellulose is “steeped” in caustic solution to form alkali-cellulose, which is carefully aged and then dissolved in carbon disulfide to form a bright orange solution, called xanthate. Extruding the xanthate into a sulfuric acid bath regenerates cellulose. Depending on whether the xanthate is extruded through small holes or wide slots, either long filament fibers (rayon) or films (cellophane) can be produced.