This heading covers chemical wood pulp of dissolving grades only, as defined in Note 1 to this Chapter. This pulp is specially refined or purified to meet the requirements of its intended use. It is used for making regenerated cellulose, cellulose ethers and esters and products of these materials, such as plates, sheets, film, foil and strip, textile fibres and certain papers (e.g., paper of a kind used as a base for photosensitive paper, filter paper and vegetable parchment). According to the final use or to the end product, it is also called viscose pulp, acetate pulp, etc.
Chemical wood pulp is obtained by first reducing the wood to chips or particles which are then treated with chemicals. As a result of the treatment the greater part of the lignin and other non-cellulosic materials is removed.
The chemicals usually employed are sodium hydroxide ( soda process), a mixture of sodium hydroxide and sodium sulphate, which is converted partly into sodium sulphide ( sulphate process), calcium bisulphite or magnesium bisulphite, also known as calcium hydrogen sulphite or magnesium hydrogen sulphite respectively ( sulphite process).
The product obtained is superior in fibre length and richer in cellulose than mechanical pulp made from the same raw material.
The manufacture of chemical wood pulp, dissolving grades, is achieved through extensive chemical and physiochemical reactions. In addition to whitening, its manufacture may require chemical purification, deresination, depolymerisation, ash reduction or adjustment of reactivity, most of which are combined in a complex bleaching and purification process.
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