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This heading covers wood pulp manufactured by a combination of mechanical and chemical pulping processes. Such pulp is variously described as semi-chemical pulp, chemi-mechanical pulp, etc.
Semi-chemical pulp is produced in a two-part process in which the wood, generally in chips, is first chemically softened in digesters and then mechanically refined. It contains a great deal of impurities and ligneous matter and is used mainly for medium-quality papers. It is generally known as neutral sulphite semi-chemical (NSSC), bisulphite semi-chemical or kraft semi-chemical.
Chemi-mechanical pulp is produced in refiners from wood in chips, shavings, sawdust or similar forms. The wood is reduced to a fibrous state by the abrasive action induced by two closely spaced ridged plates or discs, one or both of which are rotating. Small amounts of chemicals are introduced as a pre-treatment or during refining in order to facilitate fibre separation. The wood may be subjected to steaming for different periods of time at different pressures and temperatures. Depending on the combination of processes employed in its manufacture, and the order in which the processes are carried out, chemi-mechanical pulp is also known as chemi-thermomechanical pulp (CTMP), chemi-refiner mechanical pulp (CRMP) or thermo chemi-mechanical pulp (TCMP).
Chemi-mechanical pulps are used, inter alia, in the production of newsprint (see Note 4 to Chapter 48). They are also used for making tissue and graphic paper.
The heading includes pulps known as screenings.
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