Surface Sizing with Starch Solutions at High Solids Contents, 2002 Metered Size Press Forum

In this work, we have studied surface sizing of uncoated fine paper with a metered size press (MSP) at considerably higher starch solids content than conventionally used. The specific aim was to increase the solids content of the starch solution from 8 % to 18 % and keep the applied dry starch amount at approximately 1 g/m²/side at a machine speed of 1200 m/min. Two different modified starch types (e.g. anionic and cationized) were selected with different viscosity levels of 20 mPas and 100 mPas (60 °C, 10 %) as a result of different molecular weight.

The results from the study show a reduced starch penetration with an increased solids content and especially with a starch of a higher viscosity. With a reduced starch penetration, surface strength and stiffness of the paper is improved. Additionally, the air permeability of the paper is considerable reduced since the starch is more on the surface. Starch remaining on the surface also explains the reduced internal strength. If the reduced internal strength is not critical regarding specific paper grade properties (e.g. ink jet paper) or it can be compensated through other sheet structure solutions (e.g. optimization of furnish properties or internal sizing), it is possible to use higher solids contents in the sizing operation. An increased solids content of the starch solution reduces the required evaporation energy consumption and drying capacity investments, but then the dry starch amount is to be kept constant. The starch viscosity needs to be unchanged at an increased solids content or compensated through a rod change (e.g. a smaller rod diameter). Less wetting of the paper web during sizing eventually reduces the number of web breaks at the MSP unit, which then improves the total net efficiency of the paper machine. Additionally, surface sizing with high solids starch solutions makes it possible to use the same surface treatment concept (e.g. drying concepts with the same evaporation capacity and roll covers) for both sizing and coating operations.