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3.6.4.1 Fundamental Aspects of Paper Sizing
Sizing makes the native fiber network hydrophobic and thus prevents or reduces the penetration of water or other aqueous liquids into the paper. Sizing prevents the spreading and strike through of ink or printing colors. Papermaking fibers have a strong tendency to interact with water. This property is important for the development of strong interfiber hydrogen bonds, especially during drying, and is also the reason why paper loses its strength when rewetted. A high absorbency is important for a few paper grades such as toweling and tissue. Also corrugated medium paper must be “absorbent” to a certain degree to convert properly in the corrugating process. On the other hand such properties are disadvantageous for many paper grades e. g. liquid packaging, top layer of corrugated board, writing and printing papers and most of the specialty papers. The water and liquid absorb¬ency can be reduced by the addition of sizing agents to the paper stock and/or by their application to the paper surface.
Since the invention of paper about 2000 years ago, it has been treated or sat¬urated with mucilage and rice starch for surface glazing properties, so that people could write on it. After 1280 A. D. animal glue was the principal sizing agent. Alum (potassium aluminum sulfate) was used to harden the applied glue. Alum rosin sizing entered the scene in 1806 (Moritz Friedrich Illig). In 1876, paper-maker’s alum, Al2(SO4)3, was introduced. Since the 1950s the various forms of rosin size (paste, dispersed, fortified), alkyl ketene dimmer size (AKD), alkenyl succinic anhydride size (ASA) and polymers mainly based on styrene acrylate and styrene maleinate (PSA) have come to the market. Today, beside starch for strength improvement (Fig. 3.2) and polymer binders for paper coating, sizing agents are the most important quality improving additives in paper manufacturing. Amongst the specialty chemicals, sizing agents represent a share of 10 % in dry mass (Fig. 3.3.) and a total worldwide turnover of approximately 950 V 106 €.
3.6.4.2 Product Classes
The main classes of chemicals and their consumption as a proportion of the total worldwide usage of 350 000 tons dry p. a. is shown in Fig. 3.10. Because of the trend to neutral or slight alkaline papermaking and the increasing use of calcium carbonate as a filler and coating pigment, the proportion and also the absolute volume of rosin will decline further and the synthetic products will steadily grow.
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