The most frequently asked question from the printing industry sales staff, technical service representatives and sheetfed printing staff is the drying of ink on coated and uncoated paper. To find the answers to these questions, you must understand the ink and the drying process of the ink when printing on sheet paper. The drying process of sheet-fed offset printing ink is a very complicated process. These inks use an oxidized drying mechanism, which means that the ink needs to be dried by oxidation. To control the oxidation and drying process, it is important to understand the composition of the ink. The ink contains the following components: Pigments-used to form colors. Binder-used to provide viscosity. They usually include a certain amount of rosin ester or hydrocarbon resin. Oil-used to enhance fluidity. Oils used include linseed oil (dry), soybean oil (non-dry), petroleum (non-dry), and other oils, such as tung oil, cottonseed oil, and the like. Solvent-used to thin the ink and reduce the viscosity of the ink. Low-viscosity petroleum distillates are usually used. Desiccant-used to help the ink harden. They are a catalyst used to accelerate the drying reaction of oxidants. Additives-provide wear resistance, gloss, etc. Let's take a look at the ink drying process, which includes two steps. The first step is oxidation. The oxygen is combined with the oil in the ink and the ink to form a solid. The second step is to absorb. Through this process, the solvent penetrates into the paper and allows oxygen to contact the oil or ink. If the solvent does not penetrate into the paper or coating fast enough, it will reduce the reaction speed of oxygen with oil and ink, eventually extending the drying time. When the ink is initially printed on the substrate, the solvent in the ink will penetrate into the fissures of the substrate's fibers or coating. The pigment remains in the dry oil part, which fixes the pigment on the printing surface. However, this is not completely dry. The ink film becomes very viscous due to the loss of solvent. In this way, the ink loses fluidity and stops there to prepare for conversion. As the solvent is lost, oxygen reacts with the oil, and the resin begins to dry. At this stage, the ink surface is fixed and will not be transferred to another paper in the stack. However, the ink located in the center of the ink film still maintains a certain liquid state. Depending on the composition of the ink, the thickness of the printed ink film, the nature of the paper or other material, and the environmental conditions, it takes two minutes to more than one and a half hours to completely fix the ink. After the printing is completed, oxidation occurs in the alkyd resin or dry oil, and may also occur in the resin. This polymerization results in a three-dimensional, network structure of chemical binders. Oxygen reacts with all the oil, resin, etc. in the ink to convert the liquid material in the ink film to a solid, so that the ink is completely dried. If you leave linseed oil open, the same reaction will occur. Oxygen will react and cross-link with flaxseed oil, eventually hardening. A paper or coating with a very tight surface will cause the solvent penetration rate to be very slow. Therefore, there will be residual solvent in the ink, and will slow down the reaction between oxygen and oil and ink. Of course, this will also increase the ink drying time. In order to speed up the drying process, "drying agents" are used in the ink to accelerate the oxidation process. The two most widely used and longest desiccants in industry are: Cobalt: can accelerate the surface drying process (fixation). Because cobalt is blue, it turns brown during drying, and it is easy to fade white. Moreover, it is easily soluble in organic acids, so acidic fountain solution with a pH value that is too low (below 4.0) affects it. Manganese: It can accelerate the internal drying process and is a more powerful desiccant than cobalt. This desiccant is brown, but has less effect on white than cobalt, and does not easily penetrate into the fountain solution. A mixture of these desiccants is generally used during printing operations to accelerate both surface and internal drying to complete the drying process.
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