In this article, we describe different methods and machines used in paper production to apply surface size to paper, which strengthens the surface, adds stiffness, reduces water sensitivity, and increases the air leak density of the sheet. The Flooded Size Press and the Metered Size Press are two common machines used to apply surface size. Pigmented Surface Sizing is a process of adding pigment to starch-based surface size to improve various properties of paper such as surface smoothness, ink holdout, and printability. The Water Box and Spray Application of Starch are other methods of adding starch to paper during the production process.
The Size Press in the Paper Machine
The Flooded Size Press
The Flooded Size Press is a machine used in paper production to apply surface size to dried paper. Starch is the most commonly used binder in surface sizing. It strengthens the surface of the paper, adds stiffness, reduces water sensitivity, minimizes dimensional changes, and increases the air leak density of the sheet.
In conventional practice, the paper passes through a pond of starch dispersion held above the nip between two large rotating cylinders. Excess starch is drained from the ends of the nip, and the surface size is transferred to the paper by capillary penetration, pressure penetration, and by hydrodynamic force during nip passage. The amount of starch transferred to the paper by the size press depends on several factors, including the concentration of dispersed starch, viscosity of the starch dispersion, size press rolls’ diameter, size press pond height, cover hardness of the size press rolls, size press nip loading pressure, 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, and pigments and other materials are often added, increasing total dispersed and suspended solids content. The viscosity of the starch dispersion is the primary rate-determining parameter for dynamic sorption of starch into paper during surface sizing.
During drying, starch attaches to the fibers and pigment, reinforcing the sheet by “spot welding” and bridging between paper constituents. However, the transfer of a substantial amount of water to the paper during surface sizing can weaken the paper, and web breaks at the size press can occur.
Surface sizing can induce structural changes in the paper sheet due to the interaction of water sorption and machine direction tension. 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 the surface-sized paper leaves the size press, it will cling to a roll and has to be pulled off. Transfer defects, such as ribbing, orange peel, spatter, or misting, may result. It is essential to control the starch viscosity, use the correct take-off angle, and apply appropriate web tension.
The Metered Size Press
The Metered Size Press is a method used in high-speed paper machines to apply surface size dispersion to paper. It has two design options: pre-metering rolls or short-dwell coater heads.
In the gate roll size press method, the size dispersion is fed into the nip between metering and transfer rolls, and excess is overflowed at the edges. The liquid film that passes the nip is split, and the amount left on the transfer roll is split again in a second nip between transfer roll and applicator roll.
The film remaining on the applicator roll is shared with paper in a third nip between the applicator roll and a backing roll for the sheet. The same action proceeds from the other side in a simultaneous gate roll size press or in the second station of a tandem of gate roll size presses.
Roll speeds in the size press train are scaled, and the metering roll is operated at low speed. There is less pond agitation, which permits running higher starch solids and higher viscosity, leading to less web breaks and increased paper machine speed.
In an alternative method, a liquid film is metered onto a size press roll by way of an attached short-dwell coater head. Excess surface size leaves the chamber by flowing back over a weir. The backflow effectively ‘washes’ the roll surface, removing any debris that might have been pulled from the sheet during nip passage.
The metered film is transferred to paper in the size press nip. Volumetric metering of surface size is obtained by using a profiled (grooved), counter-rotating rod. The blade metering system allows metering a thin liquid film for fluids with low viscosity.
Since metered size presses apply coating fluid simultaneously to both sides of paper, contactless air turning rolls and dryers are required to prevent scaling during the initial stages of coating drying.
Pre-metering the surface size allows control of the depth of starch penetration into the sheet and supports the design of paper products with specific properties of surface strength, stiffness, compressibility, and fold strength.
Pigmented Surface Sizing
Pigmented Surface Sizing involves adding pigment to starch-based surface size to improve various properties of paper such as surface smoothness, ink holdout, optical properties, sheet density, stiffness, and printability. It allows for fiber replacement and higher machine speed, and can accommodate coatings with a wide range of compositions. Metered size presses are used for simultaneous two-side application, resulting in higher surface strength and improved printability, and contactless sheet guidance is required to prevent scaling on the drier cans. Starch remains the dominant binder, but synthetic latices and thickeners are added for adjustments. Many paper grades are produced on-machine with a combination of precoating and top coating, followed by finishing with a soft calender.
The Water Box at the Calender
A different way to add starch to paper, especially paperboard, is by using a water box during the finishing process before coating. This helps to improve surface strength and reduce fiber swelling after coating. To prevent issues with the calender rolls, diluted oxidized starch is used along with a lubricant.
Spray Application of Starch
Starch is added to fiber mat on the papermaker’s wire in the production of corrugating medium, linerboard, and other unbleached (Kraft) paperboard grades. This procedure provides better starch retention in the sheet than adding dispersed starch to the stock. Starch is sprayed as a slurry, preferably after heating to a temperature just below onset on pasting, and dispersion occurs in the paper sheet during drying. Strength improvement by starch spraying ranges from 10% to 25% for 2-4% starch application. To successfully operate a spraying system, careful attention is required for starch suspension temperature, spray pressure, flow rate, drop size, nozzle alignment, spraying pattern, and spray overlap. High molecular weight starches are required to impart internal strength, while lower molecular weight starch is applied when high surface strength is the goal, such as in supercalendered (SC) groundwood grades. A significant application of starch spraying is for bonding plies in the production of multiply packaging grades. However, starch spraying has some disadvantages, including inefficiency at high paper machine speed, gradual plugging of wire and felts, and the accumulation of starch in water discharges due to poor retention.
Starch Selection for Surface Sizing
The selection of starch for surface sizing is determined by factors such as dispersion viscosity, film formation, and resistance to retrogradation. Native and modified starches are used, and for low-cost applications, native corn starch is depolymerized in the paper mill by enzymic or thermal-chemical conversion. Acid-depolymerized starches are suitable for use at both the size press and the water box, while oxidized corn starches have improved water-holding properties, better film-forming characteristics, and reduced gelling tendencies, making them particularly useful for high surface strength and ink holdout. Starch ethers and esters are premium products with superior film-forming and water-holding properties, reduced retrogradation tendencies, and are used in high-quality printing and writing papers, specialty grades, such as greaseproof paper, and medical grades. Cationic starches are used in virtually all grades of paper and paperboard to impart strength, reduce BOD, and enhance film clarity. Anionic starches obtained by carboxymethylation of corn starch provide improvements in film strength, pick resistance, opacity, and air-leak density. Starch grafts are a new class of products for use on the size press, and hydrophobic starches provide improvements in water, ink, and solvent holdout, as well as decreased sheet porosity. The addition of starch insolubilizers, such as formaldehyde-free glyoxal-based crosslinker or zirconium salt, improves surface strength, wet pick resistance during offset printing, and resistance to dusting.