Dissolving pulp, also known as dissolving cellulose, is made from bleached wood pulp or cotton linters with a high cellulose concentration (more than 90%). It possesses unique qualities such as excellent brightness and a consistent molecular-weight distribution. This pulp is made for applications that demand high chemical purity and, in particular, a low hemicellulose content, because hemicellulose, which is chemically identical to cellulose, can interfere with following processes. Because it is not turned into paper, dissolved pulp is dissolved in a solvent or by derivatization into a homogenous solution, making it totally chemically accessible and removing any remaining fibrous structure. It can be spun into textile fibres (viscose or Lyocell) or chemically reacted to make derivatized celluloses, such as cellulose triacetate, a plastic-like material produced into fibres or films, or cellulose ethers, such as methyl cellulose, used as a thickener.
Dissolving pulp is mostly generated chemically from pulpwood in a low-yielding technique (30 - 35 percent of the wood). This accounts for around 85-88 percent of total production. Dissolving pulp is produced using the sulfite or kraft processes, plus an acid prehydrolysis step to remove hemicelluloses. It should be obtained from fast-growing hardwoods with low non-cellulose content for the best quality.
The sulfite process yields pulp with up to 92 percent cellulose content. As a base, it can be ammonium, calcium, magnesium, or sodium. The prehydrolysis sulphate process yields pulp with up to 96 percent cellulose content.
Special alkaline purification procedures can increase cellulose levels even further: up to 96 percent for the sulfite process and up to 98 percent for the sulphate process
A 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 to manufacture acetate plastics and high-viscosity cellulose ethers.
No comments:
Post a Comment