How paper is made Pulp

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Text 2 Paper

Nearly all printing is done on paper. Its importance in the production cost equation has increased dramatically over the last fifteen years or so as prices have risen sharply and certain qualities of paper have been in short supply. For some printed items such as magazines, more than half the production cost is accounted for by the price of paper, This has resulted in buyers of printed materials considering changes to lighter weight and lower quality papers.

There are several reasons for the big increases in paper costs. The rising price of oil and a shortage of raw materials have increased the actual production costs of paper making, so raising its selling price. This, together with the increased demand for paper, accounts for its importance in publisher's budgets.

Paper basically consists of vegetable (cellulose) fibres with various additives to control the physical characteristics, print-ability and aesthetics of the finished product. The choice of fibre is important. High-quality papers, which may still be made by hand, might utilize cotton, linen or hemp fibres — all of which are high strength yielding and very durable materials. These raw materials give strength and stiffness. Straw and esparto grass are other materials used to produce fibres which, due to their shape and lack of flexibility, yield a weaker paper but one with even texture, softness, elasticity, good opacity and bulk. Book papers were usually made from esparto but this has now been replaced by the bulky hardwood, eucalyptus. Most paper is now made from wood pulp, largely from softwood coniferous trees such as spruce, pine or eucalyptus.

Chemical pulping The object of all chemical pulping is to dissolve away the ligmn and other glutinous materials so that the fibre within the wood can be extracted.

After the initial de-barking, wood is cut into small chips (16 x 3'18mm/⁵/8 x Vein) and these are then cooked at a high temperature and pressure using a variety of pulping liquors depending on the source of the wood and the process available. Caustic soda and sodium suphide have formed the basis of the 'kraft¹ pulping process since the late 1800s whereas sulphite pulping technology relies principally on the use of sodium, calcium or magnesium sulphite and sulphurous acid for pulping, and is often used for woods with a low resin content, The extracted fibres are washed and bleached before being further processed.

Mechanical pulping Mechanical or groundwood pulp is differ­ent from chemical pulps. Chemical pulps are essentially separated, whole cellulose wood fibres from which the lignin and other bonding material has been removed. The fibre length of these pulps is the full fibre length of the source from which the pulp is made. In the mechanical pulping process, bundles of fibres are torn from the de-barked pulping log and therefore contain a mixture of whole fibres, broken fibres, hgmn and various other wood gums. The groundwood pulp, made into paper is soft, bulky, absorbent and opaque, but it is weak and will deteriorate over time, especially if it is exposed to sunlight, as the Hgnin decays.

Papers for different uses need to have different properties and the properties of groundwood are desirable in many grades where permanence is not essential. Among these grades are newsprint, tissue, towelling, wallpaper and some printing paper, both coated and uncoated.

Other processes exist that combine both mechanical and chemical pulping technology. These processes are covered by the general terms, semi-chemical or chemi-wood.

Exercises :

Дата добавления: 2014-12-23