Some paper machines contain a unit for the application of certain liquid media to the web surface. When applying a starch solution, a sizing agent, or a mixture thereof, this process is called sizing. The main objectives of sizing are to increase the strength of the paper and to modify the surface properties with respect to liquid uptake during writing, printing, or coating.
Sized papers show a considerably higher strength. This is, for instance, im¬portant for liner and corrugated media, but also for other packaging papers as well as for graphic papers. For tensile strength increase, penetration of the size into the paper is desired. If the main objective is to increase surface strength, for example for printing, the size should remain on the surface.
Sizing also reduces the penetration of liquids into the paper. This limits, for example, the ink spread when writing or printing onto the paper. The same effect is desired for coating base papers, since sizing improves the coating hold-out in the subsequent coating process.
Sizing is usually performed in a size press or a film press. In a size press (Fig. 6.62), the web is passed through a pond of the sizing agent, which is located above a roll nip. As a result of both capillary action in the pond and the hydraulic pressure in the roll nip, the paper web absorbs the sizing liquor.
The amount of size pick-up and the degree of penetration depend upon the pond height, the concentration and viscosity of the size, the absorption behavior (poros¬ity, moisture content, temperature, etc.) of the paper web, and the nip pressure and nip length. Control of the size pick-up is mainly by variation of the size concentra¬tion, but also by variation of the pond height or nip pressure.
Typical size concentrations lie between 5 and 12 %. The pick-up is usually 0.8–3 g m–2 dry substance in total. The size press is limited in speed due to pond turbulences, which become unacceptable above approximately 1000 m min–1. Size concentration and pick-up are also limited.
The modern film press overcomes these limitations (Fig. 6.63). In a film press a film of the application medium is formed on the rolls and is then transferred to the paper web in the roll nip.
To meter the applied film, a profiled (or grooved) rod is usually used. Different profile geometries allow for different film thicknesses. Therefore, variation of the size concentration for pick-up control is no longer needed. With a given rod pro¬file, an additional but limited variation of the size pick-up is possible by adjusting the rod pressure.
Today’s size presses do not limit the speed of the production line, 1700 m min–1 has been achieved and even higher speeds seem possible.
A size concentration of 8–15 % is typical, 25 % or more can also be reached. Typical pick-up amounts are 1–4 g m–2 dry substance, and 7 g m–2 and more are possible.
The water applied in the size press or film press increases the moisture content of the paper web from around 2–4 % to 60–75 %. The water is evaporated in the after-dryer section. The first cylinder in particular must be protected against the build-up of coatings.