The mechanism of sizing with soluble rosin soap size differs from the sizing mechanism of dispersed rosin acid size. Cationic fixing agent has to be used with the latter size to assure proper attachment of the size to furnish components. The problem of size attachment is not unlike that for the alkaline AKD size emulsion. The relative extent of attachment is assessed by a high shear stirring test.
Sizing involves a complex array of interactions in which surface reactions play an important role. The large surface area fines and fillers in the furnish reduce sizing efficacy. Size accumulates on fillers/fines to a higher extent than expected from surface area alone. On the basis of experimentally determined adsorptivities, size distribution in a furnish can be estimated. Size on fiber appears to be more efficient than on fillers/fines, compared at equal surface coverage. This requires that we minimize the amount of fine particulates in the furnish. Fillers/fines capture a significant amount of applied size; thus, first pass fillers/fines retention has to be maximized to retain size in the sheet. Size distribution also can be affected by the proper choice of process conditions.