[Abstract] The chemical drying process and its mechanism of oxidation drying, UV drying and electron beam drying are discussed. The influence factors, application scope and their advantages and disadvantages of several drying methods are analyzed. It is hoped that the production practice will benefit.
Keywords: printing ink; chemical drying; drying process; drying mechanism
Drying of printing ink mainly includes physical drying and chemical drying. The physical drying process of the ink is discussed in [1], which focuses on the chemical drying process and mechanism of the ink. Chemical drying includes oxidative drying and polymerization drying formed by ultraviolet (UV) and electron beam excitation.
1 Oxidation drying
Offset printing is performed on an absorbent substrate such as paper. The ink is first subjected to an osmotic drying process, followed by oxidative drying and polymerization drying. The final drying of the printing ink film is completely completed by the oxidation and polymerization of the drying oil and the resin. The printing ink film resists deinking and abrasion by a certain degree of cross-linking and consolidation, but it must also allow the product used to retain sufficient elasticity.
Oxidative drying of offset inks containing dry oily oils does not require the addition of special equipment. It is done directly by molecular bonding with oxygen in the air. For this reason, the ink layer printed on the paper should receive sufficient oxygen on the delivery pile. By dusting, the necessary gaps between the sheets can be increased so that oxygen diffuses between the sheets. Dusting can not only prevent the stain on the back of the printed sheet, but also help dry the printed sheet.
To accelerate the oxidation drying, a catalyst may be added to the ink. It is important to note that there is an optimal amount of desiccant added for each combination of ink and substrate. Adding too much desiccant can cause the ink to dry on the ink roller in the inking unit. Then the transmission of ink to the substrate is disturbed.
Metal soaps, ie mixtures of cobalt or manganese with oil-soluble acids, are often used as catalysts. Among them, the cobalt desiccant is a "surface desiccant", that is, its drying process starts from the surface of the ink and then slowly dries inside the substrate. Manganese is a full penetration desiccant. In general, the desiccant uses a compound and the drying of the compound is a direct drying process.
Factors that affect the chemical drying of printing inks are: 1) colorants, binders, and desiccants in the printing ink; 2) pH values ​​in the substrate, composition of the coating layer, infiltration characteristics, water absorption, and the delivery paper stack Temperature, etc.; In addition, for the offset printing there is the fountain solution PH value, salt content, hardness, alcohol content and so on.
Far-infrared radiation has no direct effect on chemical (oxidative) drying. Only the increase in temperature can bring about a faster reaction. However, in contrast, UV radiation and ionizing radiation (electron beam) can produce chemical ink drying associated with radiation.
2 UV drying
UV drying of printing inks (and coatings) is based on radiation-polymerizable binders. UV inks containing a suitable amount of UV drying agent are suitable for both sheet-fed and web printing presses. The drying device between the printing units can be used to prevent reverse ink transfer from the ink in the rear inking unit due to the ink film splitting. In flexo and gravure, due to the characteristics of the ink (such as ink overprinting, etc.), each inking unit is followed by a drying device (whether single-side printing or double-sided printing). Sometimes the final drying device is added after the last set of inking units, especially for high speed printing.
If a UV drying device is used, as long as there is ultraviolet radiation, the ink layer is polymerized and completely dried. Polymerization occurred less than one second. However, this drying method requires special inks and additional photosensitizers containing completely different binders. Black has an impediment to the penetration of UV radiation in the ink layer, and at the same time, the black ink has a weaker curing effect than other colored or glossy oils. (to be continued)
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