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Absorbency

Absorbency is the ability of a surface to absorb the liquid (the ink, in the printing process) that penetrates its surface. The absorbency allows the pigment diluted in the ink to transfer to the paper surface.

Absorbency is an essential feature of the inking process. In addition to the receptivity, stabilization and immobilization of the ink, the absorbency also affects the first phase of the process, the drying.

A good result on coated papers could be obtained according to specific absorbency characteristics, determined by the use of porous structures tailor-made for inks, which we could find in coated inks with quick stabilization.

Quite the opposite, uncoated papers consist of porous paper materials, with cellulose fibres with the most varied dispositions that could create interstices of different depths and shapes, from one millimeter upwards, up to touch very high values, even coinciding with the same amplitude of the fibers of which the paper is made.

Coated papers have an additional layer on the surface consisting of a structure that is generally much finer than the underlying one. The pigment of this layer is made of mineral particles joined together by special binder, with very small spaces to accommodate the inks. The porous structure of coated paper is very different from the uncoated ones, and the former are defined as microporous in relation to the latter, with a significantly lower average pore size.

To better explain the process of stabilization of fast inks on coated papers and the defects that comes out when  he coated paper is not adequate to the formulation of the inks, it is necessary to make a distinction between pores of larger size from those of smaller size.

The structure of coated papers is associated with the terms “microporosity” and “micropores”, also used for all the materials with pores of the size within a certain limit in relation to the ink. For coated papers with pores above the limit, however, we could use the terms “macroporosity” and “macropores”.

Fot coated and uncoated papers, the shape and size of the pores closest to the surface will influence the penetration of the ink during the printing process and immediately thereafter, affecting the way the ink soaks the paper. In the case of macropores, the ink can soak both the surface and the pigment. Quite the opposite, in micropores the ink penetration is filtered: one part is able to penetrate the paper, while the other remains stationary at the surface.

Soaking and filtration always happen, even if not always in the same way depending on the types of inks and different papers. For example, a macroporous newsprint paper, with a very high diameter fibers, the soaking of the ink is quite clear. In coated and therefore microporous papers, it is possible to observe the filtration process. Both soaking and filtration directly affect the stabilization of the ink, the first stage of the drying process. Despite it is not yet completely fixed to the support, the ink film could cause problems in the finishing phases, in which supports are stacked one on the other. For this reason it is important, especially for coated papers, to evaluate the overall absorbency and porous structure of the paper. The porosity of a printing paper and the overall absorbency are the elements that influence the use of fast stabilizing inks.

In fact, it should be used when the overall absorption values are high enough (although they must be within certain limits) and there are micropores with dimensions within a certain threshold. In these cases it is possible to obtain a rapid stabilization of the inks with the achievement of a sufficiently high gloss.
 

This page is part of Gerp's glossary of terms. Gerp is a dedicated MIS to packaging industry. You are welcome to browse this website or contact us for more information.

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