The process by which paper absorbs ink
- 2024-05-23 17:47:41
- admin
Paper ink absorption is not only related to the degree of porosity and capillary state of the paper, but also related to the surface properties of the paper fiber, the content of fillers, pigments, adhesives, ink composition and characteristics, printing methods, printing pressure and other factors. In actual printing, the absorption of ink by paper can be divided into two stages.
The first stage is the printing press stamping moment, relying on the role of printing pressure to transfer part of the ink to the surface of the paper into the larger pores of the paper, that is, the ink as a whole (including the pigment in the ink) into the paper pores, this process is generally called pressure penetration stage. At this stage, the paper's absorption of ink mainly depends on factors such as the size of the printing pressure, the structure of the paper and the viscosity of the ink. If the printing pressure is large or the structure of the paper is loose, the absorption capacity of the paper to the ink will also be strong. In general, the printing structure of loose paper, such as newsprint, letterpress paper and other non-coated paper, the printing pressure should be smaller, the ink viscosity should also be lower. And paper with tight printing structure, such as coating paper, the printing pressure can be appropriately large, and the ink viscosity can be slightly higher.
The second stage is from the paper to leave the imprint area until the ink is completely dry, this stage mainly relies on the capillary action of the paper to absorb the ink, called the free penetration stage. At this stage, the binder is separated from the ink as a whole and enters the inside of the paper at a fairly slow speed through small pores and the rough surface of the paper fibers. Therefore, this process is actually the process of migration of the binder from the ink to the pores of the paper, because the binder is separated from the ink as a whole, it will change the conjunctival nature of the ink film retained on the paper, and the fixation and drying of the ink stain will also be completed in this process. At this stage, the absorption rate of the paper to the ink determines the gloss of the print, and whether it will occur through printing, powder and other phenomena. When the ink is transferred to the paper, over time, the low molecular binder (solvent) will begin to penetrate into the paper layer, so that the solvent content in the ink layer on the paper will decrease, the ink viscosity will increase, and the ink layer will condense.
The ink absorption of paper depends on the number of paper capillaries and the size of the capillary diameter. Paper is a porous material, and there are many gaps of varying sizes between fibers and fibers, between fibers and fillers, and between pigment particles inside the paper. These gaps are equivalent to many capillaries. Under the action of these capillaries, the binders in the ink can be absorbed, and the thicker the diameter of the capillary, the faster the absorption rate of the ink, therefore, the pore structure of the paper determines its absorption of the ink.
The looser the paper, the larger the pores and the stronger the ink absorption. For example, newsprint is relatively loose and has strong ink absorption, so the ink transferred to newsprint is fixed quickly, which is conducive to the improvement of printing speed. Coated paper is not as loose as offset paper, and the pores are not as large as offset paper, but the fixation of the ink is not necessarily slower than that of offset paper during printing, mainly for two reasons.
First, the ink supply amount of coated paper printing is small;
Second, there are countless small capillaries between the pigment particles attached to the surface of the coated paper, and the absorption capacity of more small capillaries is much larger than that of a few thick capillaries. In addition, the coated paper has a strong absorption selectivity for the binder, allowing the low molecular components with strong fluidity in the binder to penetrate into the paper, while the polymer components in the binder remain in the ink layer, so the fastness and gloss of the ink layer can be improved.
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