|This article relies largely or entirely upon a single source. (January 2010)|
Dissolving pulp (also called dissolving cellulose) is a bleached wood pulp or cotton linters that has a high cellulose content (> 90%). This pulp has special properties, such as a high level of brightness and uniform molecular-weight distribution. It is used to manufacture various cellulose-derived products.
Dissolving pulp is mainly produced chemically from the pulpwood, in a process that has a low yield (30 - 35% of the wood). This makes up of about 85 - 88% of the production. Dissolving pulp is made from the sulfite process or the kraft process with an acid prehydrolysis step to remove hemicelluloses.
The sulfite process produces pulp with a cellulose content up to 92 percent. It can use ammonium, calcium, magnesium or sodium as a base. The prehydrolysis sulfate process produces pulp with a cellulose content up to 96%.
Special alkaline purification treatments can yield even higher cellulose levels: up to 96 percent for the sulfite process and up to 98 percent for the sulfate process.
The minor part is produced from the shortest cotton linters, normally second cut. These are washed mechanically and chemically to remove proteins, waxes, pectins and other polysaccharides. This is bleached to get the required brightness. Dissolving pulp from cellulose linters gives the purest cellulose and is used for manufacture acetate plastics and high-viscosity cellulose ethers.
Dissolving pulp is used in production of regenerated cellulose. In the regenerated cellulose process the cellulose is converted to cellulose xanthate which dissolves easily in caustic soda. The resulting viscous liquid can be extruded through spinnerettes and regenerated as man-made fibres. Cellulose can also be dissolved in some organic solvents directly and processed to regenerate the cellulose fibres in different forms. The lyocell process uses an amine oxide to dissolve cellulose and Tencel is the only commercial example of this direct-dissolution process, which unlike the viscose process is pollution-free.
The 90-92% cellulose content sulfite pulps are used mostly to make textiles (like rayon) and cellophane. The 96-% cellulose content sulfate pulps are used to make rayon yarn for industrial products such as tire cord, rayon staple for high-quality fabrics, and various acetate and other specialty products.
Cellulose powder is dissolving pulp that has undergone acid hydrolysis, been mechanically disintegrated and made into fine powder.