There are tens of thousands of small gaps or pores formed between the mesh fibers and natural filler particles inside the paper, forming a structure composed of countless narrow channels, which can absorb the liquid due to the capillary action of the liquid. The thicker the capillary diameter, the faster the absorption rate. The absorptivity of paper or the penetration rate of liquid depends on the number and diameter of pores in the paper, as well as on the nature of the liquid.
It needs to be clear here that oil absorption and water absorption are two completely different properties of paper. Among these two properties, printing workers are more concerned about oil absorption, because most of the printing inks are oil-based materials. The printing process requires that the paper has a considerable absorption capacity so that the paper can absorb the ink as soon as possible. When printing, the paper absorbs the binder in the ink, and the surface pigment particles are left on the surface of the paper to dry quickly. Generally, the pores between the paper fibers are larger than the diameter of the pigment particles. During printing, the ink is pressed into the pores of the paper, and those small fiber gaps smaller than the diameter of the pigment particles absorb the linking material under the action of the fiber capillary. To properly understand the action of liquids on paper, one needs to understand the action of capillaries.
The porous structure of paper can be expressed by porosity, which is expressed as a percentage of the volume of air in the paper:
Porosity=1-actual volume of paper/outline volume of paper=1-outline density of paper/actual density of paper
In the formula, the appearance density (ie tightness) of the paper is the actual value calculated from the quantitative and thickness of the paper. The actual density of paper refers to the density of the fibers that make up the paper itself.
The amount of ink absorption can be discussed by the size of the voids. The paper fibers are interwoven into an infinite number of capillaries. After the ink is filled into the voids, the capillaries in the paper absorb the binder in the ink. If the gaps between the fibers are small, the capillary action of the fibers will be destroyed, and the ink absorption performance will be poor; if the pores are too large, the capillary will absorb not only the binder but also the pigment, resulting in strike-through.
Quantitatively, the law of Hagen-Poiseuille can be used: within a certain time t, the liquid volume V passing through the thin tube is the fourth power of the inner diameter r of the tube and the pressure difference at both ends (P1-P2) is directly proportional to, and inversely proportional to the length L of the thin tube and the viscosity n of the liquid, expressed by the formula:
dV/dt=πr 4 (P1-P2)÷8ηL
This formula shows that the thicker the capillary diameter, the faster the absorption rate of liquid. This formula also explains various factors when the ink is pressed into the paper at the moment of embossing: the relationship between printing pressure, length of imprinting time, ink intrinsic density, paper capillary structure, etc., and oil absorption.
The oil absorption of paper has a great relationship with the quality of printed matter. The paper has low oil absorption, and the ink will not be absorbed by the paper soon after it is transferred to the paper, so that the remaining ink will be transferred to the back of another paper covered above, causing stickiness. If the oil absorption is too high, it will penetrate excessively, which may cause print through, or the surface will lose its luster seriously. If the printing is coated paper, rapid penetration may cause ink pulverization, because all the linking materials penetrate into the paper. In the paper fiber, the pigment left on the surface is not protected.
