Wetting of the hottest printing process 0

2022-08-19
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Wetting in the printing process

the inherent wettability of blanket and ink roller. The change of wettability in the printing process is very important to the normal operation of printing, especially the normal operation of offset printing production

I. wettability and changes of blanket

blanket is the intermediate of ink transfer in offset press. During printing, the blanket is in contact with the ink of the graphic part of the printing plate, as well as the water of the blank part of the printing plate. Therefore, the blanket is mainly composed of non-polar material rubber, so as to ensure that the blanket can absorb and transfer the ink to the maximum extent, and stabilize the sample in a constant wet state for 4 hours to absorb water to the minimum extent; The rubber blanket is also required to have the best performance of oil resistance, acid resistance, oxidation resistance and aging resistance. In order to make the blanket have good printing performance, in addition to selecting appropriate rubber materials to make the blanket, the wettability of the blanket must be maintained as much as possible in the process operation during the printing process

1. wettability of rubber blanket. The blanket is composed of an adhesive layer and a base cloth. The adhesive layer includes an inner adhesive layer for bonding the primer and a surface adhesive layer for transferring ink. The main component of the rubber layer is natural rubber. The viscosity of hydraulic oil will affect the performance of oil pump and oil pressure system of material testing machine and synthetic rubber

natural rubber can be regarded as a polymer of CIS isoprene monomer, that is,

according to experimental determination, the molecular weight of natural rubber is about 200000 ~ 500000. Therefore, the degree of aggregation n is a thousand digits. Under normal conditions, natural rubber is a long linear molecule polymerized by 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 wear resistance and impact resistance, poor humidity effect, easy to become sticky and brittle when heated, and prone to plastic deformation and aging. In order to overcome the shortcomings of natural rubber in physical properties, vulcanizing agent (usually sulfur) is added to natural rubber to vulcanize the rubber, so that some double bonds of linear rubber molecules are opened, sulfur is used as a "bridge" to form a sulfur bridge, cross-linking reaction occurs, and become a three-dimensional structure

the rubber structure model after vulcanization is shown in Figure 1-21. It can be seen from the figure that sulfur atoms connect rubber molecules with linear structure to form many lattice like structures, thus improving the mechanical and physical properties of rubber

figure structural model of vulcanized rubber

although the mechanical and physical properties of vulcanized rubber are improved, the general elasticity decreases and the hardness increases. This is because the more vulcanizing agent is added, the more cross-linking, and the more shaped lattices connected by sulfur atoms. Soft rubber with less sulfur content, and hard rubber with more than 10% sulfur content. The rubber used for rubber blanket has less sulfur content, good elasticity and low hardness. The hardness of rubber ink rollers for printing is different by the amount of sulfur added

the oil resistance of vulcanized natural rubber is still poor, especially when it is in long-term contact with organic solvents such as kerosene, it will gradually swell and deform. Therefore, natural rubber is not suitable for the surface adhesive layer of rubber blanket. Because natural rubber has a large adhesive force, it can firmly bond the bottom cloth of rubber blanket together, and it is generally used as the inner adhesive layer of rubber blanket. However, during the printing process or when cleaning the blanket, it is necessary to strictly prevent the infiltration of organic solvents such as kerosene, so as to avoid the swelling of natural rubber and the "foaming" or "shelling" of the blanket

at present, resin ink is widely used in lithography, and its resin binder contains a certain amount of high boiling point kerosene, which requires that the surface adhesive layer of rubber blanket has good oil resistance, so the surface adhesive layer of rubber blanket generally uses synthetic rubber with good oil resistance as raw material

the synthetic rubber used to make the surface layer of rubber blanket includes neoprene, nitrile rubber, etc. The fasteners bear the axial tension that increases at a constant rate.

neoprene is polymerized from chloroprene, that is,

due to the existence of chlorine atoms, neoprene has better wear resistance, oil resistance, heat resistance, organic solvent resistance, ozone resistance and air tightness than rubber, except that its elasticity and cold resistance are slightly worse than natural rubber, especially its aging resistance is particularly good, and its chemical structure is not easy to be destroyed due to the action of light and oxygen in the air for a long time, It can still maintain good physical and mechanical properties

nitrile rubber is copolymerized by butadiene and acrylonitrile in the presence of initiator, that is,

nitrile rubber has excellent oil and solvent resistance due to the presence of cyano (- C ≡ n or - CN), which is unmatched by natural

rubber

in the molecules of neoprene and nitrile rubber, due to the existence of polar groups such as cyano and chlorine atoms (- CI), they tend to be polar, but they are still non-polar and can be well wetted by ink. In order to further enhance the lipophilic and hydrophobic properties of the rubber blanket surface adhesive layer, some rubber blanket surface adhesive layer raw materials are added with a certain amount of vinyl acetate vinyl chloride copolymer. Nevertheless, the wettability of blanket will change during use

2. Change of surface wettability of rubber blanket. The rubber cloth is wrapped on the rubber drum of the offset press. In the process of mutual rolling, each small unit of the rubber cloth should be in periodic contact with the printing plate and substrate. The ink, wetting liquid, substrate (mainly the paper powder falling off the paper) and blanket on the printing plate 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, which is mainly formed by the accumulation of paper powder coating particles, plant fibers, pigment particles in the ink and other substances. Cover layer A. most of the substances in the screw cap at the lower side of the ball screw are polar, so the non polarity of the blanket is weakened, 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 points, cannot be normally transferred to the substrate through the blanket during high-speed imprinting, resulting in false imprinting and dot loss of the printed matter. On the other hand, because the hydrophilicity of the blanket increases, it will transfer more wetting liquid adsorbed from the printing plate surface to the paper, so that the water content of the paper increases, the size lengthens, and the overprint is not accurate. At the same time, the surface strength of the paper decreases, and the accumulation of paper powder and paper wool intensifies the process of the blanket, making production impossible. This requires cleaning the blanket to restore its original wettability

there are many double bonds on the molecular chain of natural rubber. These unsaturated double bonds are easy to react with oxygen, changing the molecular structure of rubber and causing "aging" phenomenon

after vulcanization of natural rubber, most of the double bonds were destroyed due to the formation of "sulfur bridges", but a small number of double bonds remained. Neoprene, nitrile rubber and other synthetic rubbers also have multiple double bonds on the main chain of their molecular structure, so the inner and surface rubber layers of rubber cloth made of vulcanized rubber and synthetic rubber are also easy to oxidize and aging

the rubber blanket works under the irradiation of light. Over time, the rubber ages and a glass like light aging film is formed on the surface. Because this film is very smooth, it not only masks the original surface properties of the blanket, but also completely eliminates the capillary effect of the surface and reduces the lipophilic ink transfer performance

after the rubber blanket is heated, it will produce thermal aging, making the surface adhesive layer hard, sticky, and even cracked, so it can no longer be used. In addition, in the process of transferring ink, the blanket is subjected to the periodic stress of the plate cylinder and the embossing cylinder. The frequency of this alternating stress is very high (especially in high-speed offset press), so the internal friction of the rubber is large, and the heat released makes the temperature of the blanket rise, which intensifies the thermal aging of the blanket. In order to slow down the thermal aging of blanket, the printing workshop should strictly control the change of temperature and humidity to achieve a constant temperature and humidity working environment. Suitable printing pressure transfer ink is advocated in printing. On ultra-high speed offset press, water and ink roller cooling and heat dissipation methods are adopted to maintain the wettability of blanket surface

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