Coating paper machines

Paper may be coated either on equipment that is an integral part of the paper machine, ie, on-machine coating, or on separate converting equipment. Many plants include both types of coating equipment and utilize each to its maximum advantage for paper and paperboard. The combination of techniques is of particular value where more than one coating must be appHed to the sheet in order to obtain a product of desired quaUty. 

In 1933, the first roU coater was installed as an integral part of a paper machine. These on-machine coaters produce a low cost coated paper used largely for magazines. Coating of paper off-machine at speeds greater than paper machine manufacturing speeds is possible with the use of the various available blade coaters (17). 

The type of paperboard used by the carton industry is boxboard. Boxboard may be categorized, based on the raw material, as combination or soHd boxboard. Combination boxboard, of which there are many grades, normally is made on a multicylinder paper machine using a substantial percentage of waste paper with virgin pulp. SoHd boxboard usually is made on a Fourdrinier paper machine using only virgin pulp and it is bleached or coated. 

It is also important to study the interactions of papermaking additives (4) in the paper machine water system some additives act synergisticaHy, so that the performance of each is enhanced by the presence of the other. However, some additives have a negative impact on the performance of other additives, or on other desirable paper properties. Thus, optimization of the addition points and usage rates of the entire additive system is necessary in order to maximize performance of the chemical additives and the paper sheet properties, and to minimize cost and negative interactions both on the paper machine and in the white-water system. This is especially tme as unanticipated additives enter the wet end of the paper machine from recycled furnishes, including coated broke (5). 

Packaging (paper and plastic) packaging adhesives release coatings barrier coatings Photochemical machining (89) micromechanical parts optical waveguides... 

Dispersion at temperatures of 90—110°C is a common final step io European mills processiog wax-coated old cormgated containers. Dispersion temperatures less than 90°C are reported to reduce wax particle size to improve pulp drainage properties on paper machines while improving paper strength (45). Dispersion has been used to reduce hot-melt adhesive, plastic coating, and asphalt particle size. These low density particles can then be removed from the pulp by flotation (46). 

Paper may also be colored by surface appHcation of dyestuff solutions after the paper has been formed and dried or partially dried by utilizing size-press addition, calendar staining, or coating operations on the paper machine. In addition, paper may be colored in off-machine processes by dip dyeings or absorption of dyestuff solution and subsequent drying, such as for decorative crepe papers. 

High-quality paper grades for printing are produced, on the paper machine or in a separate process, by coating the sheet surface with a concentrated dispersion of... 

Anionic starches are obtained by reaction with phosphoric acid and alkali metal phosphates or by derivatization with carboxymethyl groups.30,31 This modification is primarily used to introduce amphoteric properties into cationic com starch for application on the wet end of the paper machine. Anionic starches with carboxymethyl substitution are used as thickeners in coating colors or as binders in coatings for specialty paper grades. Oxidized starches are inherently anionic but without thickening action. Potato starch already carries sufficient natural anionic charge to provide amphoteric properties after cationization.32... 

Introduction of new starch products will require extensive technical services, especially for adaptation to closed paper machine wet-end systems, for use with deinked pulp and for the high shear conditions of high-speed paper coating.46... 

Fluid passage through a bent blade nip is driven by hydrodynamic force and thus depends on paper machine speed and viscosity. Non-uniform fluid pressure in the feed chamber and fluctuations of viscosity can lead to variations in coat weight. 

A reduction in the volume of water needed at the paper machine is almost impossible because paper formation will be disturbed if the consistency of the pulp suspension becomes too high. Currently, the stock suspension (i.e., pulp and additives) in the head box of a paper machine contains 97%-99.9% water. The constraint of maintaining pulp suspension consistency means a reduction in water intake has to be realized by reusing the process water. Due to the different purposes and applications in which water is used, the water quality demands vary considerably. Membrane processes can be chosen to produce a specific water quality and are attractive alternatives to facilitate the reuse of water, either alone or combined with biological processes or other chemical and physical processes. In addition, membrane processes are an efficient way to recover raw materials, e.g., coating pigments, from effluents. 

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