Sizes have been applied to paper surfaces for many years. The principal function of a size is to reduce absorbency of the paper surface, which in turn makes the surface more resistant to spreading of applied inks. The inks then do not run or smear and printed patterns applied to the surfaces remain sharp and clearly defined. In addition sizes may provide stiffness, smoothness, weight and luster to the surface.
In the past most sizes have been based on starch, while others have been based on polyvinyl alcohol (PVA) or carboxymethylcellulose (CMC). Neither, however, is a particularly good film former, so that such sizes have tended to produce porous surface coatings on the paper. Thus printed patterns applied to the paper have been properly defined in some areas, but in those areas where the coating has been more porous, the inks have become mottled (i.e., unevenly distributed on the paper surface), a condition commonly referred to as "poor ink hold out." In order to overcome this problem, the paper manufacturers have generally applied an excess of size, to insure that all areas of the paper surface would be adequately coated and porosity would be minimized. This of course results in unnecessary coating thickness in many areas of the paper and an uneconomical use of size compositions.
The use of excess size also produces problems with unwanted paper penetration. Where the size coating is unduly thick due to higher concentrations, drying is more difficult, leading to cracking or crazing of the surface film.
It would therefore be of advantage to have a size which had good film forming properties, such that the size could be applied in an even, non-porous coating across the paper surface. Having such a coating would permit proper sizing of the paper with the optimum quantity of size and would also allow control of paper penetration by the size.