Wood provides the primary raw material for the European Pulp, Paper & Board Industry. Wood consists of cellulose fibres that are bound together by a material called lignin. Wood is one of the very few raw materials used by a major industry, which is infinitely self-renewing. Paper for Recycling is a secondary raw material.
The pulp & paper industry primarily uses either sawmill residues (wood chips) or trees that are too thin to be processed economically in sawmills.
The industry used to be based almost entirely on softwoods such as spruce, pine, larch and fir. More recently, eucalyptus, birch, aspen and other hardwoods growing in temperate climates, have been introduced as a raw material as they provide improvements in some specific properties of printing and writing papers, fluting for corrugated cases and other grades of papers. Nevertheless, softwood provides longer fibres (average 3mm compared with 1mm for hardwoods) and continue to be used for papers required to have the highest strength characteristics.
Paper & board may also be produced with de-inked pulps (DIP), i.e. pulp made from Paper for Recycling from which inks and other contaminants have been removed.
Fibres for paper and board are produced in a pulp mill. The fibres are separated from one another into a mass of individual fibres. This is called woodpulp. The separation can be undertaken by a mechanical process, where the fibres are teased apart, or by chemical means, where the lignin binding the fibers together is dissolved away by cooking the woodchips in suitable chemicals. After separation, the fibres are washed and screened to remove any remaining fibre bundles.
Generally, the chemical pulp produced in the world today is based on one of two processes: sulphate or sulphite. Most chemical pulp is made by the alkaline sulphate (kraft process), which uses caustic soda and sodium sulphate to "cook" the woodchips. In the unbleached stage, a dark brown, but very strong pulp results, which can be bleached to a high brightness if required. The acid or neutral sulphite mills produce pulps which are easily bleached, usually with hydrogen peroxide. These pulps fulfil today's demand for "chlorine free" raw materials.
Thermo-Mechanical Pulp (TMP) and Chemi-Thermo-Mechanical-Pulp (CTMP) are produced by a combination of the mechanical and chemical processes.
Quality papers require a pulp that does not discolour during storage or go yellow when exposed to sunlight and which retains its strength. One of the most effective ways of achieving all three requirements is by bleaching. This has the additional advantages of improving absorption capacity, removing any small pieces of bark or wood left behind as well as giving a high level of purity.
Stone groundwood: pulp produced by grinding wood into relatively short fibres. This pulp is used mainly in newsprint and wood-containing papers, like LWC and SC papers. Thermo-mechanical (TMP): pulp produced in a thermomechanical process where wood chips are softened by steam before entering a pressurised refiner. TMP has mainly the same end-uses as stone groundwood. Variants of the above two processes produce pressurised stone groundwood pulp and refiner mechanical pulp.
Semi-chemical: pulp produced in a two-stage process, which involves partial digestion with chemicals, followed by mechanical treatment in a disc refiner. The pulp is mainly used in production of fluting medium for corrugated board. Chemi-thermomechanical (CTMP): pulp produced in a similar way to TMP, but the wood chips are chemically treated before entering the refiner. This pulp has properties suited to tissue manufacture. Some CTMP pulp is used in printing & writing paper grades. CTMP pulp is classified under semi-chemical pulps in the Harmonised System of the Customs co-operation council. In FAO, as well as other industry statistics, such chemi-thermomechanical pulps are grouped with mechanical pulp.
Sulphite: pulp produced by cooking wood chips in a pressure vessel in the presence of bisulphite liquor. End-uses range from newsprint, printing & writing papers, tissue and sanitary papers. Sulphite can be either bleached or unbleached. Sulphate (or kraft): pulp produced by cooking wood chips in pressure vessels in the presence of a sodium hydroxide (soda) liquor. The pulp may be unbleached or bleached. End-uses are widespread, with bleached pulp particularly used for graphic papers, tissue and carton boards. Unbleached pulp is commonly used in liner for corrugated board, wrappings, sack and bag papers, envelopes and other unbleached speciality papers.
Pulp made from recovered paper from which inks and contaminants have been removed.
Woodpulp produced in a pulp mill may be fed directly to a paper machine in an "integrated paper mill" or dried and pressed into bales to be used as a raw material by papermills worldwide. Integrated pulp is pulp that is produced for use as raw material in production of paper & board at the same mill, or for shipment by a producing mill to other mills, which it owns, controls or with which is affiliated within the same country. Market pulp is pulp that is sold in open competition with that of other producers. All pulp exported from the producing country is considered to be market pulp.
The paper making process is also highly automated, involving special knowledge and expertise. Production processes are optimised for each grade of paper & board (newsprint, printing & writing papers, packaging, hygiene papers...). Variables are many: raw material composition (mixture of chemical softwood and hardwood pulp, mechanical pulp, recovered paper, fillers, pigments, additives, etc.), machine size (width, speed), type of production equipment, and automation level.