Wetting of blankets and ink rollers
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Chapter 1 Wetting during printing
Section 3 Wetting of blankets and ink rollers
The inherent wettability of the blanket and the ink roller, the change in wettability during the printing process, is very important for the normal operation of the printing, especially the normal production of the offset printing.
First, the wettability of the blanket and its changes
The blanket is an intermediate for ink transfer from offset presses. At the time of printing, the blanket is in contact with the ink of the printing plate portion and also with the water of the blank portion of the printing plate. Therefore, the blanket is mainly composed of a non-polar material rubber to ensure that the blanket absorbs the ink to the maximum extent and transfers the ink, and minimally adsorbs moisture; the blanket is also required to have the best oil resistance, acid resistance, oxidation resistance and aging resistance, etc. performance. In order to make the blanket have good printing performance, in addition to selecting a suitable rubber material to make a 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 blanket consists of a layer of glue and a base fabric. The glue layer includes an inner glue layer for bonding the base fabric and a surface glue layer for transferring the ink. The main components of the rubber layer are natural rubber and synthetic rubber.
Natural rubber can be thought of as a polymer of cis isoprene monomer.
The molecular weight of natural rubber is determined to be about 200,000 to 500,000. Therefore, the degree of polymerization n is a thousand digits. Under normal conditions, natural rubber is a long-line molecule composed of thousands of cis isoprene polymers, and the compliant molecular chains are curled together to make them highly elastic. However, natural rubber has low mechanical properties such as abrasion resistance and impact resistance, poor humidity effect, easy to be sticky and brittle due to heat, and prone to plastic deformation and aging. In order to overcome the physical properties of natural rubber, a vulcanizing agent (usually sulfur) is added to the natural rubber to vulcanize the rubber, so that some double bonds of the linear structural rubber molecules are opened, and sulfur is used as a "bridge" to form sulfur. The bridge undergoes a cross-linking reaction and becomes a three-dimensional network structure.
The rubber structure model after vulcanization is shown in Figure 1-21. As can be seen from the figure, the sulfur atom bonds the rubber molecules of the linear structure to form a plurality of mesh lattices, thereby improving the mechanical and physical properties of the rubber.
Although the mechanical properties of the vulcanized rubber are improved, the general elasticity is lowered and the hardness is increased. This is because the more the vulcanizing agent is added, the more cross-linking, and the more mesh-like lattices that are connected by sulfur atoms. Soft rubber with a small amount of sulfur, and a hard rubber with a sulfur content of 10% or more. The rubber used for the blanket has less sulfur content, good elasticity and low hardness. The rubber ink roller for printing differs in hardness by the amount of sulfur added.
Vulcanized natural rubber is still poor in oil resistance, especially when it is in 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 rubber layer of the blanket. Since the natural rubber has a large adhesive force, the base fabric of the blanket can be firmly bonded together, and is generally used as an inner layer of the blanket. However, in the printing process or when cleaning the blanket, it is necessary to prevent the penetration of organic solvents such as kerosene to prevent the natural rubber from swelling and causing the blanket to "bubble" or "shell".
At present, lithographic printing generally uses a resin-based ink, and the resin-type binder contains a certain amount of high-boiling kerosene, which requires that the surface rubber layer of the blanket is quite oil-resistant, so the surface rubber layer of the blanket generally has good oil resistance. Synthetic rubber as a raw material.
The synthetic rubber for making the surface layer of the blanket is neoprene or nitrile rubber.
Neoprene is polymerized from chloroprene.
Due to the presence of chlorine atoms, neoprene has better wear resistance, oil resistance, heat resistance, organic solvent resistance, ozone resistance and air tightness than rubber except for its elasticity and cold resistance. The aging property is particularly good, and it is exposed to light and oxygen in the air for a long time, and the chemical structure is not easily broken, and the physical and mechanical properties can still be maintained.
Nitrile rubber is a copolymer of butadiene and acrylonitrile in the presence of an initiator.
Due to the presence of cyano (-C≡N or -CN) in nitrile rubber, oil and solvent resistance are particularly good and natural.
Rubber can't be compared.
In the molecules of neoprene and nitrile rubber, there are polar groups such as cyano and chlorine atoms (-CI), which tend to be polar, but they are still non-polar and can be well inked. Moisten. In order to further enhance the lipophilic and hydrophobic properties of the rubber surface of the blanket, a certain amount of vinyl acetate-vinyl chloride copolymer is added to the rubber surface of the blanket. However, the wettability of the blanket changes during use.
2. The change 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 rollers, each small unit of the blanket is periodically contacted with the printing plate and the substrate. The ink on the plate, the dampening solution, the substrate (mainly the paper powder that has fallen off on 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 masking layer is formed on the surface adhesive layer of the blanket. This masking 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 that the non-polarity of the blanket is weakened, the polarity is increased, the lipophilicity is lowered, and the hydrophilicity is increased. In this way, the ink on the image portion of the printing plate, especially the ink on the micro dot, cannot be normally transferred to the substrate through the blanket during high-speed imprinting, resulting in the embossing of the printed matter and the loss of the dot. On the other hand, because the hydrophilicity of the blanket increases, it will transfer more dampening liquid from the surface of the printing plate to the paper, so that the water content of the paper increases, the size is elongated, the overprinting is not accurate, and at the same time, the paper The surface strength is degraded, and the degree of accumulation of blankets of paper powder and paper wool is increased, which makes the production impossible. This requires cleaning the blanket and restoring the original wettability of the blanket.
There are many double bonds on the molecular chain of natural rubber. These unsaturated double bonds are easy to react with oxygen, which changes the molecular structure of the rubber and causes "aging".
After the natural rubber is vulcanized, most of the double bonds are destroyed by the formation of a "sulfur bridge", but a small amount of double bonds remain. Synthetic rubbers such as neoprene and nitrile rubber have a plurality of double bonds in the main chain of the molecular structure. Therefore, the rubber inner layer and the surface rubber layer which are made of vulcanized rubber and synthetic rubber are also easily oxidized and aged. phenomenon.
The blanket is operated under the condition of light, and the rubber is aged for a long time, and a glass-like photoaging film is formed on the surface. Since the film is very smooth, it not only masks the original surface properties of the blanket, but also completely eliminates the capillary action of the surface, and the lipophilic transfer performance is lowered.
After the blanket is heated, it causes heat aging, which makes the surface layer hard, sticky, and even cracked, and can no longer be used. In addition, during the process of transferring the ink, the blanket is subjected to the periodic stress of the plate cylinder and the impression cylinder, and the frequency of the alternating stress is high (especially a high-speed offset printing machine), so the internal consumption of the rubber is large. The heat released causes the temperature of the blanket layer to rise, which in turn exacerbates the heat aging of the blanket layer. In order to slow down the degree of thermal aging of the blanket, the printing workshop should strictly control the change of temperature and humidity to achieve a constant temperature and humidity environment. In the printing, it is recommended to use suitable printing pressure transfer ink. On the ultra-high speed offset printing machine, the water and ink roller are used to cool and dissipate the heat to maintain the wettability of the surface of the blanket.
Second, the wettability of the ink roller and its changes
The ink roller is mainly used to transport ink on the printing press. In order to spread the ink evenly between the ink rollers, the printing machine is generally arranged in an alternating manner between soft and rigid ink rollers, so that adjacent ink rollers are in good contact.
1. The conditions under which the ink wets the ink roller. Each ink roller having good lipophilicity receives ink from the previous ink roller and then transfers the received ink to the next ink roller. In this process, the ink wets the surface of the ink roller and then adheres to the ink roller. If the specific surface energy of the surface of the ink roller is γSG, the surface tension of the ink is γLG, and the surface tension between the ink roller and the ink is γSL, the adhesion work Wa of the ink roller to the ink is:
Wa=γSG+γLG-γSL
(1-23)
The cohesive work Wc of the ink itself is
Wc=2γLG (1-24)
When the cohesive work Wc of the ink is smaller than the adhesion force Wa of the ink roller to the ink, that is, Wc<Wa, the ink wets the surface of the ink roller and adheres to the ink roller; when the cohesive work of the ink is greater than the adhesion of the ink roller to the ink When the work is Wc>Wa, the ink cannot wet the surface of the ink roller and is detached from the ink roller. Considering the two formulas (1-23) and (1-24), the conditions for the adhesion and shedding of the ink on the surface of the ink roller are respectively
γSG>γLG+γSL (ink adhesion)
γSG<γLG+γSL (ink shedding)
The soft ink roller used on the printing press is generally made of high-polymer materials such as vulcanized natural rubber, synthetic rubber, gelatin and polyurethane. Although the roll surface of the soft ink roller is a low-energy surface, the ink roller material has good lipophilicity, so the ink can wet the roller surface and adhere thereto.
The hard ink roller used on the printing machine is generally made of a metal material such as iron or copper. The roll surface of the metal ink roller is a high-energy surface, γSG>>γLG+γSL, and it is apparent that the ink can wet the roll surface well and adhere thereto.
2. The change in wettability of the ink roller. Soft ink rollers are mostly made of rubber material, and the wettability and other properties are basically the same as those of the blanket. However, the diameter of the ink roller is several times smaller than the diameter of the plate cylinder. Therefore, the angular velocity of the ink roller is about several times larger than the angular velocity of the plate cylinder, so that the heating temperature rises under a high frequency of rolling friction. It is very remarkable that heat aging first occurs on the surface of the ink roller, causing surface hardening, cracking, and even small pieces to fall off. Only the wear of the heat aging layer that has been formed can restore the original oleophilic ink transfer performance of the ink roller. In order to slow down the heat aging rate of the rubber, the inking roller in the printing press inking unit should be equipped with a cooling and cooling device. During the printing process, it is also necessary to prevent the ink from drying out on the surface of the ink roller and drying the ink on the ink roller at an early stage, and timely removing the ink residue remaining on the ink roller. At the end of printing, the ink roller must be cleaned.
Hard ink rolls vary in wettability during printing due to differences in materials.
The offset rollers of some offset presses are made of metal iron, such as J2101 offset press. The iron itself has poor lipophilicity. When the supply of dampening liquid is large and the ink is emulsified seriously, the electrolyte in the dampening fluid Or a hydrophilic colloid, which is dispersed in the ink by the fine wetting liquid, and is adsorbed by iron. The hydrophilic film is formed on the tandem roller, and the lipophilic ability is lowered, and the ink is difficult to adhere to the surface of the ink roller, and often appears to be off. In the case of ink, only the physical or chemical method is used to remove the hydrophilic film layer, and the ink roller can restore the original performance of adsorbing ink and transferring ink.
A layer of metallic copper is plated on the surface of the iron ink roller to form a metal copper roller instead of the iron ink roller. For example, the J4102 offset press uses a copper squeegee roller, which clearly has better ink transfer performance than an iron squeegee roller. However, copper is a metal which is easily oxidized, and particularly in the emulsified ink, the droplets of the wetting liquid contain an oxidizing agent, and the copper roll surface is oxidized to form CuO (copper oxide). On the other hand, the soft rubber in the printing machine's inking system will slowly desulfurize during the ink transfer. The copper ink roller will be in contact with it for a long time, and will produce black streaks in the contact part, which is caused by copper and sulfur. The chemical reaction produced Cu2S (copper sulfide) and CuS (copper sulfide). The copper surface covered by CuO, Cu2S, CuS, the oleophilic ink absorption is reduced, leading to the phenomenon of "deinking", which is usually removed with 10% dilute nitric acid to remove this film on the surface of the copper ink roller.
Polymers such as synthetic resins and plastics are non-polar substances and have good lipophilic properties. If a strong and wear-resistant polymer is poured onto the roll core of the metal ink roller to form a hard rubber roller, not only the ink transfer performance is enhanced, but also a hydrophilic inorganic salt layer or adsorption is not deposited on the roll surface. Hydrocolloid, the ink roller rarely deinks. Some new types of printing presses are now competing for the use of high polymer materials for the squeezing rollers. However, the mechanical strength of this type of ink roller is not as good as that of metal. In use, avoid collision and ink knife scraping.
The polymer rigid ink roller is the same as the rubber soft ink roller and blanket. The solvent resistance is poor. When cleaning the ink roller, it is necessary to select a suitable solvent to prepare the cleaning agent. Otherwise, the surface of the ink roller will swell and dissolve, and the rubber layer will be affected. Lost and cannot be used.