1, the composition and characteristics of water-based ink
Water-based inks are made of water-soluble resins, high-grade pigments, solvents, and additives that are compounded and ground. The water-soluble resin mainly acts as a binder in the ink, so that the pigment particles are uniformly dispersed, so that the ink has a certain degree of fluidity, resulting in adhesion with the substrate material, so that the ink forms a uniform film layer after printing. The hue in the ink is mainly determined by the pigment. The pigment is evenly distributed in the binder in the state of particles. The pigment particles can absorb, reflect, refract, and transmit light, and thus can exhibit a certain color. It is generally required that the pigment has a vivid color, a proper coloring power and hiding power, and a high degree of dispersion. In addition, depending on the purpose of use, it is also possible to have different resistance. The role of the solvent is to dissolve the resin, so that the ink has a certain degree of fluidity, can be successfully transferred during the printing process, and adjust the viscosity and drying properties of the ink. Water-based ink solvents are mainly water and a small amount of ethanol. Commonly used additives are: pH stabilizer, slow drying agent, defoamer, dilute agent and so on.
1) pH stabilizer
The pH stabilizer is mainly used to adjust and control the pH value of the water-based ink to make it stable within the range of 8.0 to 9.5. At the same time, it can also adjust the viscosity of the water-based ink and dilute the ink. In general, an appropriate amount of pH stabilizer should be added at regular intervals during the printing process to keep the water-based ink in a good printing state.
2) Slow drying agent
The slow drying agent can suppress and slow the drying speed of the water-based ink, prevent the ink from drying on the plate or the anilox roller, and reduce the occurrence of printing failures such as blocking and stencil printing. However, the amount of slow-drying agent should be controlled. Generally, the amount of slow-drying agent should be controlled between 1% and 2% of the total amount of ink. If the amount is too much, the ink will not dry completely and the printed matter will produce sticky or odor.
3) Defoamer
Defoamers function to suppress and eliminate the generation of bubbles. In general, when the viscosity of the water-based ink is too high, the pH is too low, or when the printing press runs faster, air bubbles are more likely to occur. If the amount of air bubbles produced is large, white leakage and uneven ink will occur. It is bound to affect the quality of prints. The general amount added is 1% to 2% of the total ink volume.
4) Diluter
For reducing the depth of color, the amount is not limited, adding a large amount of diluting agent may cause the ink viscosity to increase slightly. At this time, a small amount of diluent may be added to adjust.
The biggest feature of water-based ink is to reduce the emission of VOCs (organic volatiles) to the atmosphere, thus preventing air pollution, improving the printing operation environment, and greatly reducing the potential for fire hazards. Water-based inks also have non-toxic, non-corrosive, non-irritating odors, good safety, and easy transportation. It has high concentration, low dosage, low viscosity, good printing adaptability, stable performance, good adhesion fastness, fast drying, good water resistance, alkali resistance and anti-wear performance after drying; printed product full of blots, bright colors, high gloss .
The development and application of ink and wash ink in our country started late, but it has developed rapidly, especially in recent years, it has been developing at a rapid rate. With the increase in the demand for ink and wash, the quality of domestic ink painting has also been improved. The traditional ink has the disadvantages of slow drying, poor gloss, ineffable water, and imprecise printing, which have now been significantly improved. Under the condition that the price of imported ink is generally high, cheap domestic ink and wash ink has gradually occupied the market.
2, water-based ink application and control indicators
The main application fields of water-based inks are flexo printing and gravure printing. According to information, 95% of the narrow flexographic printing in the US uses water-based ink, and 80% of paper gravure uses water-based ink. At home, the development in recent years is relatively fast, but there is still a big gap compared with foreign countries.
The control indicators affecting the printing quality of aqueous inks include fineness, viscosity, drying speed, and pH.
The fineness of water-based ink refers to the size of the pigment particles in the ink and its uniformity of dispersion in the binder. It is also a very important quality indicator. The fineness of the ink will not only have a certain influence on the coloring power and the color equal performance of the ink, but also affect the rheology of the ink. Ink particles are too thick and can easily cause defects such as hairiness on the edges of the dots and stencils. If the ink particles are too fine, the fluidity of the ink will be too large, which will cause the dots to expand, which will also affect the printing quality.
Viscosity is the most important control indicator in water-based ink applications and is a property that prevents fluid flow. Since the viscosity directly affects the transferability and printing quality of the ink, the control of the ink viscosity is very important. In the printing process, the viscosity of the water-based ink is too low to cause defects such as light color, dot enlargement, uneven ink transfer, etc.; the viscosity of the water-based ink is too high, the transfer performance of the ink is relatively poor, and the dirty plate, paste plate, etc. are prone to appear. Malady.
Flexographic water-based inks are generally used directly without dilution, and their viscosity can range from 30s to 45s/25°C. The viscosity of the flexo-type water-based ink used in the carton can be lower, as long as the amount of stabilizer is regularly added, the viscosity change can be basically controlled. The gravure water-based ink is generally diluted with ethanol and water in a 1:1 dilution according to conventional practice, and the viscosity is generally controlled at 15s to 25s/25°C.
Water-based inks are generally alkaline (mainly dependent on amines), and if the pH of the water-based ink is too high, it will affect the drying speed of the ink; if the pH is too low, the drying speed becomes faster, which can easily cause the printing plate and Anilox roller clogging, causing the layout dirty.
Drying of ink and wash is mainly volatile drying and infiltration drying. The drying performance of ink refers to the time required for the ink to dry after being transferred to the substrate. This is also one of the important indicators to measure the quality and performance of the ink. Therefore, controlling the drying performance of ink is an important means to achieve a good printing effect.
The drying speed of a water-based ink depends on the printing speed of the printing press, the drying capability of the drying equipment, the properties of the substrate material, and the ink's own composition. If the ink dries too quickly, it will dry and build up on the plate and anilox roller, and it may clog the anilox roller, resulting in the loss or destruction of the halftone dots, and the leak in the solid part; if the ink has dried faster Slow, in the case of multicolor overprinting will cause the back of the sticky.
In fact, the viscosity and pH of the ink will also have a certain influence on the drying performance of the ink. For example, if the viscosity is too high, the drying speed of the ink will be reduced. If the pH is too low, the ink will be dried too quickly. Therefore, in the printing process, the viscosity and pH of the water-based ink should be adjusted and controlled first, and then an appropriate amount of quick-drying agent or slow-drying agent should be added to the ink according to the actual printing speed to adjust the drying speed of the water-based ink.
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