In the mechanical pulping process, the wood fibers are separated from each other by mechanical energy applied to the wood matrix causing the bonds between the fibers to break gradually and fiber bundles, single fibers and fiber fragments to be released. It is the mixture of fibers and fiber fragments that gives mechanical pulp the favorable printing properties. In the mechanical pulping the objective is to maintain the main part of the lignin in order to achieve high yield with acceptable strength properties and brightness. Mechanical pulps have a low resistance to aging which results in a tendency to discolor.
The main processes and techniques are Stone Groundwood Pulping (SGW), Pressure Groundwood Pulping (PGW), Thermo-Mechanical Pulping (TMP) or Chemi-Thermo-Mechanical Pulping (CTMP). The main raw materials, yields on wood and end-uses of pulps are summarised in the table below:
|Pulping process||Raw material||Yield on wood||Typical end-uses|
|Groundwood pulp||Spruce and fir(softwood)||95-97%||Printing & writing papers and newsprint|
|TMP||Spruce and fir(softwood)||93-95%||Printing & writing papers and newsprint|
|CTMP||Dominantly spruce, but also aspen and beech, NaOH, SO2 and H2O2||90-94%||Printing & writing papers, tissue and packaging boards|
Overview over the mains steps in mechanical pulping:
There are two processes used mainly for the manufacturing of mechanical pulping. In the stone ground wood process (SGW) or in the pressurised ground wood process (PGW) logs are pressed against a rotating grinder stone with simultaneous addition of water. Refiner Mechanical Pulps (RMP, Thermo-Mechanical Pulps = TMP) are produced by defiberizing wood chips between disc refiners. The elements causing the mechanical action – grits on a pulp stone in the grinder and bar edges on a steel disc in the refiner – will give the resulting pulps a typical blend of fibers and fiber fragments. Groundwood pulp has a higher proportion of fine material and damaged fibers giving the pulp good optical and paper-surface properties. The more gentle treatment in the refiners produces a higher yield of intact long fibers which gives the pulp higher strength, which is valuable in furnishes for products with a high requirement on runnability.
The characteristics of the pulp can be affected by increasing the processing temperature and, in the case of refining, by the chemical treatment of the chips. Both steps will increase the energy consumption as well as the pollutant level because of a lower pulping yield. The chemi-thermo-mechanical-pulping process (CTMP), in which the wood is pre-softened with chemicals, is generally considered to be a mechanical pulping technique since the chemicals principally soften the lignin prior to the mechanical stage rather than fully dissolve it out as in true chemical pulping processes.
Most mechanical pulping is integrated with paper manufacture. Mechanical pulp is typically included in a paper furnish to increase the opacity of the paper product.