Rotary offset printing ink is one of the main raw materials for 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 ink consists of four parts: binder, pigment, filler and additive.

1. 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 effect of the binder material to be carefully ground by the grinder, and at the same time, to achieve the transfer in the printing press by virtue of the viscous effect 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 linseed oil and soybean oil. The main mineral oils are high-boiling point kerosene and solvent oils for inks.

The proportion of linking materials in web offset printing inks is relatively large, which is 60% to 70%, and will change with changes in the production process, ambient temperature, and the type of printing press. 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 result in printing without ink, graphic edge smudging, dirty newspapers on the back, print through, and pull paper 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 pigment directly affects the color saturation and ink cost of the ink.

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, so the color saturation of the pigments is required to be high and the hue should be pure. 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. As the name implies, it is the material used to improve or highlight a certain characteristic of ink in ink manufacturing and printing. There are many types of additives used in the ink, and the common auxiliary agents for rotary offset ink are mainly diluents. Release agent, 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 ink on the way. 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, add 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 methods depends on its catalytic drying. The ink is mainly osmotic drying type ink. Therefore, the driers should be used as little or as little as possible when producing rotary offset ink. Otherwise, the contact time between the ink and the air is slightly longer, and the oxidized conjunctiva (ink skinning phenomenon) 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 reduces 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 the pigment particles in the ink can be prevented. The amount of the 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. Under normal circumstances, 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.

Second, the quality control of rotary offset printing ink

The quality control of offset printing inks is mainly achieved through the one-by-one detection of ink performance indicators.

1.Viscosity Viscosity refers to the ability of the ink to resist intermolecular friction between molecules in the ink. Viscosity has a great influence on the print quality, and it controls the transferability of the ink, the reproducibility of the dots, and affects the gloss and drying speed of the ink. Viscosity is too large or too small to be used properly. The viscosity depends on the speed of the press and the tightness and looseness of the paper structure. The viscosity of web offset ink is Laray viscosity 50-80Pa·s/25C°.

At present, there are rotary viscometers, laray viscometers, and balance plate viscometers for measuring ink viscosity.

2. Viscosity Viscosity is the size of the ink's ability to resist splitting forces between the ink films. Viscosity and viscosity are the manifestations of different forces. There is no definite rule to follow. However, in general, the greater the viscosity, the greater the viscosity.

The faster the printing machine rotates, the greater the viscosity exhibited by the ink. Therefore, the viscosity of the design ink must be determined according to the speed of the printing press, so as not to affect the quality of the printed matter. Otherwise, the phenomenon of pulling and flying ink will occur. . Under normal circumstances, high-speed ink stickiness is 3 to 4, medium-speed ink stickiness is 4.5 to 5.5, and low-speed ink stickiness is 6 to 7.

3. Fluidity The fluidity is the ability to measure the outward flow of ink under non-external forces. The larger the expanded diameter, the better the fluidity of the ink.

4. The ink produced by the color ink factory is required to have the same color for each type and batch. At present, many newspaper companies have purchased different types of printing presses, and the style of each newspaper is not the same. This requires the ink factory to carry out “special ink special-purpose special offers” according to the requirements of the newspapers. Although it is also a four-color ink, The hue and other characteristics of the four colors used to supply different newspapers are not the same. However, for a newspaper office, the required four-color ink should be relatively stable in terms of hue and other characteristics.

There are two main methods for detecting the color of inks: 1 Routine test methods, ie, scraping contrast methods. This method is based on the person's eyes and the sample under the standard light source for parallel observation and comparison. 2 Use the printability tester (IGT) to print the proofing method.

5. Drying ink drying is too slow, or even dry, newspapers will appear dirty, pressure dirty, sticky hands and other serious impact on the appearance of newspapers.

The drying method of newspaper printing is mainly osmotic drying, and the main body that affects printing ink drying is the binder.

The reasons that affect the ink penetration drying rate are:

1 The compatibility of resin and mineral oil. When the compatibility of the resin and the mineral oil is poor, the release of both rapidly proceeds and quickly penetrates into the paper; when the compatibility of the resin and the mineral oil is good, the release and penetration of the solvent on the surface of the paper becomes slow; Absorption; 3 ink viscosity; 4 ink emulsification level; 5 newspapers on the graphics section of the ink layer thickness.

At present, the measurement method for the dryness of the cold-set web offset printing ink is the pressure rubbing method, which causes the ink on the newsprint paper and the blank newsprint to rub under a certain pressure for a certain time, and when the blank newsprint is no longer dyed due to friction, the scratching is calculated. Friction time, which indicates the dry value of the ink.

6. Glossiness At present, domestic ink companies use reflectance gloss meter to measure the gloss of ink. The gloss meter is placed on the printed sample of the printability meter and the gloss value can be read directly.

Fineness The fineness of ink used to determine the ink at present is a scraper fineness meter method. The ink fineness is less than 15μm. The fineness of the ink in the ink has a great influence on the ink gloss, and the finer the pigment is, The higher the gloss of the ink, the finer the ink will directly affect the printing resistance of the PS plate. The higher the ink's fineness is, the higher the printing plate's resistance to printing is.

8. Water-resisting offset printing is accomplished by using the principle of ink repulsion. Ink roller and water roller will make the ink and fountain solution contact each other under the printing pressure, causing the emulsification of the ink, and the traditional Chinese medicine water in the printing process is reversed. , it is easy to transfer to the ink fountain, so that the fluidity of the ink changes, even causing the ink to dry