Rotary offset printing ink is one of the main raw materials used in newspaper printing. The performance of ink itself directly affects the printing quality of newspapers. Usually web offset inks need to meet the following requirements: First, under the high-speed shear of the printing press, it can always maintain a good balance with the fountain solution, and the rheological properties of the ink will not change too much, and the ink can be guaranteed to be The printing machine has good transfer performance, which makes the printed newspaper graphic clear, complete and full; Second, in order to meet the requirements of high-speed rotary machines, the ink permeation and drying speed is faster, and the ink is not in the printing machine and during storage. Will be the crust; Third, with the readers continue to improve the quality requirements of the layout, newspaper version of the ink should have a good luster, higher color density and pure bright colors.
First, the relationship between ink composition and performance
Rotary offset printing ink consists of four parts: binder, pigment, filler and additive.
1. Binder
In the ink industry, the binder is a liquid with a certain viscosity, and its quality directly affects the performance of the ink. Because the pigment is generally a solid powder, it is necessary to rely on the wetting action of the binder material to be finely ground by the grinder, and it is also possible to achieve the transfer in the printing press by virtue of the stickiness of the binder material. In printed matter, the binder also plays a role in protecting the ink film and the substrate and exhibits the necessary gloss. The binder material largely determines the viscosity, viscosity, drying and fluidity of the ink. To obtain high quality inks, high quality binders must be used.
At present, the major resins used in the linking materials produced by several large domestic rotary offset ink production plants are petroleum resins, modified petroleum resins, rosin-modified phenolic petroleum resins, and nitric acid resins; the main vegetable oils are flaxseed oil and soybean oil. The main mineral oils are high-boiling point kerosene and solvent oils for inks.
The proportion of the linking material in web offset printing ink is relatively large, which is 60% to 70%, and will change with the production process, ambient temperature and the change of the printing machine type. If the linking material is not properly prepared or the production process is not appropriate, not only can a newspaper with bright colors and excellent gloss be printed, but it will cause the ink to be printed, the edges of the graphic frame to be dirty, the back of the newspaper to be smeared, the print to penetrate, and the paper to be pulled out. A series of printing failures.
2. Pigment
Pigment is another important component of the ink, the proportion in the ink is 15% to 20%, and the proportion of the ink directly affects the color saturation and ink cost level.
The final performance of the ink in the printed product is the color of the pigment. The role of the pigment in the ink can be summarized as follows: the color of the ink and the color density of the ink are displayed; the ink is given a certain ink property; the gloss of the ink is affected and the water resistance is affected Performance; to a certain extent, affect the drying of the ink. The pigment of the ink must have high coloring power, chemical stability, and light resistance, and must also have good dispersibility in the binder, and the particles should be fine.
Because of its specific transfer process, web offset inks are relatively thin, and therefore require pigments with high color saturation and pure hue. At present, the pigments used in domestic large-scale ink factories have been carefully designed and selected to meet their requirements in terms of density, gloss, and printing performance.
In actual production, carbon black has the advantages of low price, good hiding power, strong coloring power, light resistance, heat resistance, high humidity resistance, and good chemical stability. Most manufacturers choose carbon black as the main pigment of black ink. Because organic pigments are brightly colored, stable in performance, and affordable, ink makers mainly use organic pigments in the production of color inks. For example, azo dyes are used for magenta inks, bis-benzidine yellow pigments are used for yellow inks, and phthalocyanine blue pigments are used for blue inks.
3. Filler
The filler in the offset ink is a white solid powder that can be uniformly dispersed in the binder. The main function is to increase the solid content of the ink. At present, fillers used in web offset inks include ultra-fine calcium carbonate (CaCO3), bentonite and fumed silica (SiO2).
The role of the filler in the ink is to adjust the consistency and ink properties of the ink, such as viscosity, fluidity, density, and concentration; to dilute the effect of the ink; to affect the dryness of the ink; to adjust and reduce the cost of the ink.
4. Additives
As the name implies, it is the material used to improve or highlight a certain characteristic of the ink in ink manufacturing and printing. There are many types of additives used in the ink. The common auxiliary agents for rotary offset inks include thinners and releasants. Driers, surfactants, ink balance agents, etc.
(1) Diluent, its role is two, one is to make the ink thinning, increase the fluidity of the ink; the second is to reduce the viscosity of the ink. At present, most newspaper printers in China use the ink printing method for ink printing. However, in the past two years, some newspaper printing companies have also purchased newspaper printing machines with an ink outlet. This type of printing ink has higher performance requirements for inks. The highlight is the requirement for good fluidity of the ink and sometimes thinner is needed to adjust the flow. In addition, in the printing process due to poor paper quality, resulting in pull paper hair, paste version and other issues, adding a small amount of thinner can reduce the viscosity of the ink.
(2) Release agent, also known as viscosity reducer, is used to reduce the viscosity of the ink and does not affect the body of the ink. Newsprints used in web offset presses have poor paper strength, poor surface strength, poor paper absorbency, insufficient water resistance, or too low a print shop temperature, and the ink itself is too viscous to cause paper picking during printing. The graphics and other parts of the pile of ink and other phenomena, in this case the appropriate use of ungreaser can effectively reduce such failures. The relieving agent is generally added in an amount of 3% to 5% of the amount of ink. When used, it should be added according to actual printing conditions.
(3) Drier, also known as desiccant, is a very important additive in the ink. It is a catalyst for the oxidative polymerization of dried vegetable oils. The combination of oxidized conjunctiva and osmotic drying depends on its catalytic drying. The ink is mainly osmotic drying type ink, so the driers should be used as little or as little as possible during the production of rotary offset inks, otherwise the contact time between the ink and the air will be slightly longer and the oxidized conjunctiva (ink crusting) will appear on the ink surface, affecting the ink. usage of.
(4) Surfactant, also known as dispersant. The surfactant wets the surface of the pigment, shortens the grinding time when the ink is manufactured, favors the dispersion of the pigment, and sometimes significantly decreases the oil absorption of the pigment. In the manufacture of a high-concentration ink, the yield value of the ink can also be reduced, and the aggregation and precipitation of pigment particles in the ink can be prevented. The amount of surfactant added is generally 1% to 3% of the amount of the pigment. The surfactant should be selected so as to reduce the surface tension of the binder, thereby increasing the wetting ability of the pigment. Excessive use may result in aggregation of the pigment into the block, which is not conducive to grinding.
(5) Ink balance agent, also known as anti-emulsifier, is generally used in an amount of less than 1%. The ink film is cut between the ink rollers and is constantly in contact with the fountain solution. Normally, the fountain solution enters the ink. If the amount of water is adequate, there is no obvious damage to the ink's fluidity, viscosity, or other properties. Printing is not affected, that is, printing can achieve the desired ink-water balance. When the ink balance performance is not good, there may be two cases. One is that the ink is too hydrophobic. When the graphic part of the plate accepts dampening solution, the ink is exfoliated due to the adhesion of water droplets or water vapor, resulting in white spots in the newspaper. , Lines become thin, intermittent or fine dots disappear; the other is that the hydrophilicity of the ink becomes stronger, the fountain solution enters the ink too much, the ink becomes shorter, the transitivity becomes worse, and the drying becomes slower; Supplying new ink makes it difficult to achieve ink balance in a short period of time. Therefore, ink performance on a printing press may change due to operations such as ink refilling, shutdown, erasing, and paper change, resulting in instability of newspaper printing quality.
In short, the amount of additives in the ink is not large, but it can significantly improve or change the performance of certain aspects of the ink, while not causing too much impact on the overall performance of the ink. Therefore, ink additives have become an indispensable part of the ink production and use process.
(to be continued)