The inherent wettability of blankets and ink rollers, the change of wettability during the printing process, is very important for the normal progress of printing, especially the normal progress of offset production.
Blanket wettability and its changes
Blanket is the intermediate of ink transfer in offset press. When printing, the blanket is in contact with the ink in the graphic and text part of the printing plate, and is also in contact with the water in the blank part of the printing plate. Therefore, the blanket is mainly composed of non-polar material rubber to ensure that the blanket absorbs the ink to the maximum extent and transfers the ink to the minimum to absorb moisture; it also requires the blanket to have the best resistance to oil, acid, oxidation and aging performance. In order to make the blanket have good printing performance, in addition to selecting a suitable rubber material to make the blanket, in the printing process, the wettability of the blanket must be maintained as much as possible in the process operation.
1. The wettability of the blanket. The rubber cloth is composed of an adhesive layer and a base cloth. The adhesive layer includes an inner adhesive layer for bonding the base fabric and a surface adhesive layer for transferring ink. The main components of the adhesive layer are natural rubber and synthetic rubber.
Natural rubber can be regarded as a polymer of cis isoprene monomer, namely
It has been experimentally determined that the molecular weight of natural rubber is about 200,000 to 500,000. Therefore, the degree of polymerization n is a thousand digits. Under normal conditions, natural rubber is a long linear molecule formed by the polymerization of thousands of cis-isoprene, and the flexible molecular chains are curled together, making it highly elastic. However, natural rubber has low mechanical properties such as abrasion resistance and impact resistance, poor humidity effects, and is prone to stickiness and brittleness when heated, and is prone to plastic deformation and aging. In order to overcome the deficiencies of natural rubber in terms of physical properties, a vulcanizing agent (generally sulfur) is added to the natural rubber to vulcanize the rubber, so that some double bonds of the linear structure rubber molecules are opened, and sulfur is used as a "bridge" to form sulfur Bridge, cross-linking reaction occurs, becoming a three-dimensional network structure.
The rubber structure model after vulcanization is shown in Figure 1-21. It can be seen from the figure that the sulfur atoms link the rubber molecules with a linear structure to form many mesh lattices, thereby improving the mechanical and physical properties of the rubber.
Although the vulcanized rubber has improved mechanical and physical properties, its general elasticity decreases and its hardness increases. This is because the more vulcanizing agent is added, the more cross-linking, and the more network lattices connected by sulfur atoms are also caused. The soft sulfur content is soft rubber, and the sulfur content of more than 10% becomes hard rubber. The rubber used in the blanket has less sulfur content, good elasticity and low hardness. The difference in hardness of the rubber ink roller for printing is achieved by the amount of sulfur added.
Vulcanized natural rubber still has poor oil resistance, especially when it comes into contact with organic solvents such as kerosene for a long time, it will gradually swell and deform. Therefore, natural rubber is not suitable for the surface adhesive layer of the blanket. Because natural rubber has a large adhesive force, it can firmly bond the bottom cloth of the rubber cloth together, and is generally used as the inner rubber layer of the rubber cloth. However, in the printing process or when cleaning the blanket, it is necessary to strictly prevent the penetration of organic solvents such as kerosene to avoid the swelling of the natural rubber and the "blister" or "unshelling" of the blanket.
At present, lithographic printing generally uses resin-based ink, and its resin-based binder contains a certain amount of high-boiling kerosene, which requires the surface layer of the blanket to have good oil resistance, so the surface layer of the blanket generally uses good oil resistance. Synthetic rubber as raw material.
Synthetic rubbers used to make the surface layer of blankets include neoprene rubber and nitrile rubber.
Neoprene is polymerized from chloroprene, ie
Due to the presence of chlorine atoms, neoprene rubber has better elasticity and cold resistance than natural rubber, wear resistance, oil resistance, heat resistance, organic solvent resistance, ozone resistance, and air tightness are better than rubber, especially resistant The aging is particularly good. The chemical structure is not easy to be damaged by light and oxygen in the air for a long time, and it can still maintain good physical and mechanical properties.
Nitrile rubber is made by copolymerization of butadiene and acrylonitrile in the presence of an initiator, namely
Because of the presence of cyano group (-C≡N or -CN) in nitrile rubber, the performance of oil resistance and solvent resistance is particularly good, which is unmatched by natural rubber.
In the molecules of neoprene and nitrile rubber, due to the existence of polar groups such as cyano group and chlorine atom (-CI), there is a tendency to be polar, but it is still mainly non-polar and can be well ink Moistened. In order to further enhance the lipophilic and hydrophobic properties of the rubber layer on the surface of the blanket, some raw materials for the surface layer of the blanket are added with a certain amount of vinyl acetate-vinyl chloride copolymer. Despite this, the wettability of the blankets must change during use.
2. Changes in the wettability of the blanket surface. The blanket is wrapped on the blanket cylinder of the offset printing machine. During the rolling process of the cylinders, each logo unit of the blanket is periodically in contact with the printing plate and the substrate. The ink, wetting fluid on the printing plate, the substrate (mainly the paper powder falling off the paper) and the blanket will have physical and chemical effects, and the result will change the wettability of the original blanket.
During the printing process, due to physical adsorption, a cover layer will be formed on the surface adhesive layer of the blanket. This cover layer is mainly formed by the accumulation of substances such as paper powder coating particles, plant fibers, and pigment particles in the ink. Most of the substances in the masking layer are polar, so the non-polarity of the blanket is reduced, the polarity is increased, the lipophilicity is decreased, and the hydrophilicity is increased. In this way, the ink on the graphic part of the printing plate, especially the ink on the tiny dots, cannot be transferred to the substrate through the blanket normally during high-speed imprinting, which causes the printing marks to be dull and dots lost. On the other hand, because the hydrophilicity of the blanket increases, it transfers more damp liquid from the surface of the printing plate to the paper, increasing the moisture content of the paper, extending the size, and overprinting is inaccurate. The surface strength of the paper decreases, and the degree of accumulation of rubber dust on the paper powder and paper wool increases, making production impossible. This requires cleaning the rubber cloth to restore its original wettability.
There are many double bonds in the molecular chain of natural rubber. These unsaturated double bonds easily react with oxygen, which changes the molecular structure of the rubber and thus causes "aging".
After natural rubber vulcanization, most of the double bonds are destroyed due to the formation of "sulfur bridges", but a small amount of double bonds remain. Synthetic rubbers such as neoprene and nitrile rubber also have multiple double bonds on the main chain of their molecular structure. Therefore, the inner rubber layer and the surface rubber layer of the blanket made of vulcanized rubber and synthetic rubber are also easily oxidized and aging occurs phenomenon.
The blanket works under the condition of light irradiation. As the rubber ages over time, a glass-like photo-aging film is formed on the surface. Because this layer of film is very smooth, it not only masks the original surface properties of the blanket, but also completely eliminates the capillary action on the surface, reducing the performance of the lipophilic ink transfer.
After the blanket is heated, it causes thermal aging, which makes the surface adhesive layer hard, tacky, or even cracked, and can no longer be used. In addition, the blanket is subjected to the periodic stress of the plate cylinder and the impression cylinder during the transfer of ink. The frequency of this alternating stress is very high (especially for high-speed offset presses), so the internal friction of the rubber is large. The heat released increases the temperature of the rubber blanket layer, which in turn exacerbates the thermal aging of the rubber blanket layer. In order to slow down the degree of thermal aging of the blanket, the printing workshop should strictly control the temperature and humidity changes to achieve a constant temperature and humidity operating environment. In printing, it is recommended to use appropriate printing pressure to transfer ink. On ultra-high-speed offset printing machines, water and ink roller cooling and heat dissipation methods are used to maintain the wettability of the blanket surface.
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