The new, innovative surface sizing method applies the starch as a curtain of size. When the application method is combined with hard-covered nip rolls, which ensure a good penetration of starch, the results in coated cartonboards production are significant: stiffness, quality and production values can be increased, and energy and raw material consumption reduced.
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.
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.
The new opportunities in size composition with curtain application make it possible to produce coated cartonboards with enhanced properties. The optimized balance between size coverage and size penetration can be adjusted by nip load in curtain sizing. For example, low pressure in sizing can be used to maximize board stiffness, and high pressure can be used, if the target is to increase ply bond or internal bond, see Figure 2.
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.
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.