The types of drying agents include cobalt, manganese, and aluminum salts of fatty acids, naphthenic acid, and octanoic acid. Offset printing cannot do without offset inks, and thus ink cleaning agents are also essential. This shows the importance that printing machine manufacturers place on ink cleaning agents, which are specifically detailed in the engineering manuals of the printing machines. Additionally, some inks contain fillers such as gelatinous calcium carbonate. Currently, the black pigments used in printing inks are mostly inorganic, while colored inks are mostly organic pigments. The fountain solution consists of water, inorganic compounds, and gum arabic, which has very good hydrophilicity.
The common binders used in offset inks are plant oil-based and solvent oil-based. These salts can be oxidized by oxygen in the air, and the oxidation potential of the metal ions increases, enhancing their oxidizing properties, which can promote the oxidative polymerization reaction of drying oils. Therefore, these additives have a catalytic effect on the film-forming reaction of the oxidative polymerization crosslinking of drying oils. Ink cleaning agents are mainly divided into two categories: one is gasoline and kerosene, and the other is specialized cleaning agents for printing (commonly referred to as 'ink cleaning agents').
Offset inks are organic substances based on their molecular structure and can be dissolved by organic solvents, while the pigments in the inks are neither soluble in water nor in organic solvents, only having a certain affinity for water.
Based on the main mechanisms of ink drying, inks can be divided into four types: oxidative polymerization drying type, permeation and exudation drying type, volatile drying type, and photopolymerization type.
Regardless of the type of printing ink, their common requirements are: during the printing process, they must maintain a stable active state before transferring to the paper surface; after transferring to the paper surface, they must quickly lose their activity and cure; the ink must have good adhesion to the substrate; the ink should have good transferability; the images and texts formed by the printing ink on paper must maintain lasting and accurate colors. According to statistics, offset printing ranks first in the total output value of printing worldwide, hence it is known as the 'queen of analog printing.'
Composition of offset ink
Generally, printing machine manufacturers strictly test the chemical products used on their machines, and there are also strict regulations for ink cleaning agents.
Offset printing is one of the multi-color, high-speed, and best quality printing methods.
The additives used in offset inks mainly include linseed oil to adjust ink viscosity, brighteners to increase ink gloss, and drying agents to promote ink drying. For example, carbon black, titanium white, and chromium green are inorganic pigments characterized by stable chemical properties and strong light and heat resistance, but they have poor coloring power; organic pigments are artificially synthesized organic compounds that have bright colors and a wide variety, with strong adaptability to printing, but they have poor light and heat resistance. Oxidative polymerization drying type inks are made from mineral oils, drying plant oils, and synthetic resins as binders, and are commonly used as offset inks. Printing ink is a complex colloid formed by evenly dispersing solid particles, such as pigments with a particle size of about 0.1 microns, in the binder.
Printing ink consists of several components, including binders, fillers, colorants, and additives. Therefore, it is known that the components of ink cleaning agents need to contain both organic solvents and components that have good affinity with inorganic compounds and pigments, in order to utilize the chemical principle of 'similar structure compatibility' during the cleaning process of the ink, forming an emulsion of 'water-in-oil' and 'oil-in-water' to achieve the purpose of cleaning ink rollers, rubber blankets, and printing plates. During formulation, they are mixed and stirred in a certain proportion according to printing requirements and made through the inking process. For example, Heidelberg printing machines have strict tests and designate three ink cleaning agents that can be used on Heidelberg printing machines: Bottcherin 60, Superdot, and Varn. When such inks are transferred to the printing surface, due to the capillary attraction of the paper, the low-viscosity mineral oil in the ink permeates into the interior of the paper, causing the viscosity of the ink to gradually increase. In this process, the contact surface area of the colloid formed by high-viscosity drying plant oils and synthetic resins gradually increases, and the double bonds of unsaturated fatty acids in the drying plant oils undergo oxidative crosslinking polymerization to form a network structure, causing the viscosity of the ink to rise rapidly, ultimately fixing it on the paper surface and drying. Plant oil types such as linseed oil, tung oil, and dehydrated castor oil contain unsaturated double bond structures of drying plant oils; solvent oil types include non-drying mineral oils such as machine oil and kerosene, as well as volatile organic solvents such as benzene, xylene, methanol, ethanol, and isopropanol. Pigments include both inorganic and organic pigments, with inorganic pigments possibly being inorganic elements, metal oxides, or inorganic salts. Additionally, natural resins such as rosin and asphalt, as well as synthetic resins like phenolic resins and polyamide resins, are also used as binders.