Papermaking/Raw materials
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Most people know that fibers from wood are needed to produce paper. But a paper only existing of fibers would not be able to be formed into a sheet. At least the presence of water in the paper sheet is necessary. Without water the individual fibers would not bind to each other and the sheet would disintegrate. As will be discussed later, water is acting as a binding agent between the fibres by forming molecular bridges with hydrogen bonds. A hydrogen bond is a special type of dipole-dipole force that exists between an electronegative atom (the oxygen of the water molecule) and a hydrogen atom from another water molecule. This bond bridges the gap between electronegative atoms on the surface of one fibre to the other fibre. Wood pulp is quite expensive and therefore it would be good to fill the sheet with a less expensive material. Some papers, especially the ones used for offset print, are filled with inexpensive anorganic fillers. The fillers also reduce thickness and increase smoothness of the paper. During the forming process of the paper sheet on the paper machine the fibres need to be formed to the sheet. In order to prevent the fibres from flowing away with the removed water a special chemical, a retention aid, is used to bind the fibres to each other in the formed sheet. This chemical also increase dewatering speed and increases rentention of the filler in the forming sheet. Most paper have a colour, even white papers are coloured. Wood pulp is naturally somewhat yellow, this can be compensated by introducing blue and violet dyes into the wood pulp. In order to increase whiteness further fluorescent whitening agents are mixed in the pulp. The fibres, untreated, would absorb a lot of water or solvents during for instance writing or printing processes on paper. To prevent this the papermakers mix sizing agents into the pulp. Also the surface of the formed paper is treated, mostly with a coating of starch (unmodified or modified) or with a coating with fillers and binders. A lot of papers disintegrate when put in water. For some papers this is not a problem. For hygienic papers that would be a disaster. Certain papers are therefore treated with wet strength agents, for instance so that a paper handkerchief does not break just after you sneeze your nose into it (imagine what could happen in the toilet). Following raw materials used in papermaking will be described
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