On the paper machine, the size press is used to apply surface size to dried paper.182,183 Starch is the most frequently used binder in surface sizing. Besides raising surface strength, starch also imparts stiffness, lowers water sensitivity, reduces dimensional changes and raises air leak density of the sheet. In conventional practice, the sheet passes through a pond of starch dispersion held above the nip between two large rotating cylinders. In the nip a high, transient, hydrostatic pressure is developed. Excess starch dispersion is drained from the ends of the nip. The surface size is transferred to paper by capillary penetration, pressure penetration and by hydrodynamic force during nip passage.
The quantity of starch transferred to paper by a size press depends on several factors: concentration of dispersed starch in the surface size; viscosity of the starch dispersion; diameter of the size press rolls; size press pond height; cover hardness of the size press rolls; size press nip loading pressure; fluting corrugated paper machine speeds; wet-end sizing of the sheet; and water content of the sheet. The concentration of starch in the surface size liquid can range from 2% to ∼15%, depending on product requirements. Frequently, pigments and other materials are added, which further increases total dispersed and suspended solids content. The viscosity ranges from water thin to several hundred cP (mPa·s).
Viscosity of the starch dispersion is the primary rate-determining parameter for dynamic sorption of starch into paper during surface sizing. Surface size penetration into the capillaries of paper proceeds in lateral and normal directions. Lateral flow takes the shape of an ellipse, according to the bias of fiber orientation in machine direction.184 Contributions by wetting and capillary penetration decrease with increasing paper machine speed, while the contribution by hydrodynamic force increases with speed. As a consequence, starch pick-up will pass through a minimum at a specific speed. The hydrodynamic force depends on the angle of convergence (which is determined by the diameter of the rolls), by the nip length (which is influenced by the hardness of the roll covers), by the paper machine speed and by the opposing loading force between the two rolls. High liquid viscosity, large roll diameter, soft roll covers and high newspaper machine speed increase starch transfer, while high nip pressure counteracts these drivers. Starch cationization has no affect on pressure-driven penetration, provided the hydrostatic pressure is high and the viscosity of the dispersion is low.
The transferred liquid penetrates into the sheet according to the void space between fibers and pigment particles. During drying, starch attaches to the fibers and pigment, and reinforces the sheet by ‘spot welding’ and bridging between paper constituents. The ultimate location of the starch in the sheet can be affected by chromatographic partitioning behind a front of water that advances into the sheet. This effect will primarily occur in heavyweight paper and board and may lead to a gradient in starch concentration in the sheet from the surface to the interior and a weakening of internal bond at the ultimate location of free water. Starch application to the sheet induces some desizing due to coverage of hydrophobic patches by hydrophilic starch.
Application of surface size to paper carries with it the transfer of a substantial quantity of water. As an example, surface sizing of a 75 g/m2 (50 lb/3300 ft2) sheet (with 1% residual water content) by a 5% starch solution for a coat weight of 1.5 g/m2 (1 lb/3300 ft2/side) will raise the water content of the sheet to 43%. This large quantity of water will weaken the paper. Web breaks at the size press can occur, particularly when the sheet is also weakened as a result of edge cracks or holes.
Surface sizing can induce structural changes in the paper sheet185 due to the interaction of water sorption (which causes a relaxation of internal stresses) and machine direction tension (which increases anisotropy and creates additional stresses). Anisotropy can be lowered by reducing tension on the web during sheet passage through the size press and subsequent dryers, and by raising the moisture content prior to the size press.
When surface-sized paper leaves the size press, it will cling to a roll and has to be pulled off. The separation force due to film splitting depends on the free film thickness, its cohesiveness, and the rheological properties of the surface size, especially its viscoelasticity. Transfer defects, such as ribbing, orange peel, spatter or misting may result. It is important to control the starch viscosity, to use the correct take-off angle and to apply appropriate web tension. Surface-size splashing can occur due to the converging motion of paper sheet and roll surfaces in the pond and fluid rejection at the nip. Best pond stability is obtained at high or low viscosity, while intermediate viscosity is most prone to induce pond instability.
The same basic test liner paper machine used to produce writing and printing paper are also used to form paperboard. However, modern paper machines are limited in their ability to produce a single-layer paper sheet with a grammage above 150 g m−2. There are a number of reasons for this limitation. Primarily, thicker single-layer sheets are more difficult to dewater requiring excessive reductions in machine speed. Furthermore, the increased drainage forces applied to thicker sheets in the forming section would cause greater fines removal from the bottom of the sheet resulting in a rougher surface. The topside of a very thick sheet would also be adversely affected since paper is formed on fourdrinier machines layer by layer from the wire side up, which would allow extra time for the fibers in the top layer to flock and produce a ‘hill and valley’ appearance. The combination of these two effects would produce an unacceptably two-sided product.
Manufacturing multilayered paperboard from separately formed sheets provides a solution to the above-mentioned problems. The forming section of paperboard machines are composed of two, three, or even four forming sections that bring individual sheets together at the wet press. Paperboard machines are for this reason large and complex having heights that are two to three times greater than single-former machines. Any one of the former sections in a multilayer machine can be either a traditional fourdrinier or a modified fourdrinier equipped with a top-wire unit for additional dewatering capacity. The use of different furnishes in each former produces a final sheet that is engineered for specific stiffness and smoothness requirements.
Although initially forming two to three separately formed sheets of paper, a multilayer machine forms a single sheet of paperboard when the individual sheets of paper are combined together in the wet press. The individual single-layered sheets prior to the wet press are ‘vacuum dewatered’ with a typical consistency of 20% (80% moisture) and are simply assemblages of fibers held together by capillary forces exerted by the continuous matrix of water surrounding the fibers. When the sheet continues it progress through the wet press and the dryers, this continuous matrix of water is decreased and the fibers are progressively drawn together through surface tension. Eventually, at the end of the drying process with a final moisture content of 4–8%, the surface tension forces between individual fibers will produce pressures sufficiently high enough to form fiber-to-fiber hydrogen bonds resulting in a mechanically strong sheet. During multilayer forming, a single sheet of paperboard is formed from the individual sheets of paper by merging the water matrices of each sheet into a single, hydraulically connected matrix in the wet press. The net result is that the multilayer sheet continues through the wet press and dryer section forming fiber-to-fiber bonds inside layers and between layers as if they were initially formed together. Theoretically, the fiber-to-fiber bonding between separately formed layers will be identical to fiber-to-fiber bonding within a single layer. Differences in interlayer bonding strength (measured by z-direction strength tests) will be found when the individual sheets are wet pressed at moisture contents lower than what is necessary to form a hydraulically connected matrix. (z-direction strength is the maximum tensile force per unit area which a paper or paperboard can withstand when applied perpendicularly to the plane of the test sample.)
The advantage of manufacturing a multilayer sheet is that key paper properties can be engineered into the paperboard that would not be obtainable by single-layer forming. Special top layers can be incorporated that are white and smooth, therefore, having excellent printing properties. Middle layers can be used that are bulky and thus inherently thicker producing the stiffest possible board. These middle layers can also contain recycle fibers or pulp fibers of lower quality that can be covered or masked by higher quality top and/or bottom layers.
Although starch is usually added at the wet end of the coated board duplex paper machine as a liquid feed directly to the furnish, other systems which place the starch directly on the formed sheet while it is still on the wire of the Fourdrinier machine or on the felt of the cylinder machine may be used. Advantages claimed are improved retention and better distribution of starch throughout the sheet, while permitting the use of low-cost unmodified starch.
In one system, a solution of cooked starch or a dispersion of starch granules is sprayed from nozzles directly onto the wet-web of fibers. By varying concentration, spray pressure, and spray location, a variety of effects can be achieved (24, 25). Three types of spray systems are in use: high-pressure air atomization, high-pressure airless atomization, and low-pressure airless atomization. With high-pressure systems, an electrostatic assist is used to prevent loss owing to misting (26).
In another system, low-density starch foam is applied directly on the wet-web immediately before it enters the wet press. The foam is mechanically broken at the press nip, and the starch is dispersed through the sheet. By controlling foam density, bubble size, and starch concentration, a wide variety of results can be achieved (27). As in the spraying system, very high retentions are possible, and low-cost unmodified starch may be used.
In another system, a thin curtain of liquid is applied to the wet-web (28) for high retention of chemicals, including starches. This system is claimed to be suitable for addition of starch to multi-ply paperboard where it increases ply bond strength.
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