OptiSizer Curtain

New application method

A new size application method where size is applied as a curtain flow on hard-covered rolls makes it possible to produce high quality coated cartonboards with enhanced properties. The new curtain sizing process has already been demonstrated repeatedly at Valmet’s Paper Technology Center pilot trials. High increase in board stiffness values have been reached with curtain starch application.

The application beam forms a curtain of starch on roll with high solids and viscosity, which gives totally new operating window for sizing compared to pond, film or spray application as seen in Figure 1.

Figure 1 (a and b). Operation window of sizing with curtain application (curtain sizing), size can be only starch or combination of starch and other components, like hydrophobic size, pigments and dyes

As high starch solid contents (20-30%) can be used with curtain application giving the opportunities to improve runnability of size press (less wetting of sheet) and to increase savings in drying energy. Reduction of wetting and increased pressure in the nip also reduce or eliminate the unwanted roughening effect of sizing. Consequently, very good surface smoothness and topography are achieved, which might lead into a possibility to reduce the amount of coating color.

New materials in size

Curtain sizing will give opportunities to mix new materials in size with no risk of plugging rods. Such opportunities include, for example, adding highly refined fibers, micro or even nano-scale fibrillated cellulose to sizing process. Of course, chemicals and pigments, which are already being used in sizing can be also applied with curtain sizing process, like, GCC, clay, hydrophobic sizing agents, color dyes or barrier chemic, etc.   

Combining sizing with long nip metal belt calendering

When the new sizing process is combined with metal belt calendering (OptiCalender Metal Belt) with a long nip, the stiffening effect is further improved. At the same time, the topography and smoothness of the sheet are improved significantly. The effect of metal belt calendering is based on high amount of heat transferred from thermo roll and metal belt to the board web. This combined process of curtain sizing and metal belt calendering has a potential for a completely new performance level of coated boards, see Figure 3.