Water-based ink characteristics and printability

Water-based ink is a mixture of organic pigments, binders, solvents and related additives. It has two significant features: First, low viscosity, good fluidity; Second, it is conducive to environmental protection. With the enhancement of human environmental awareness, water-based inks have been widely used in packaging printing and commercial printing at home and abroad. This article analyzes the characteristics and printability of water-based inks.

Drying and adhesion mechanism

Drying mechanism

The water-based ink changes from a liquid state to a solid state and adheres to the substrate. This drying process is completed in two stages: First, the initial drying stage, expressed as initial dryness, means that the water-based ink changes from a liquid to a semi-solid state and cannot be transferred anymore. Second, it is a thorough drying stage, and it is indicated by thoroughness that the binder in the semi-solid aqueous ink reacts completely to form a film. The primary drying and dryness are collectively referred to as fixing drying of aqueous inks. According to the different printing substrates, the water-based ink drying methods can be divided into several types, such as volatilization and infiltration.

(1) Volatilization and drying. Water-based ink contains volatile solvents. After being printed on the surface of the substrate, the solvent evaporates. The remaining resin in the aqueous ink forms a solid ink film on the surface of the substrate together with the pigment. If the substrate is a non-absorbent material, its drying method is mainly based on volatile drying, and the drying speed first depends on the solvent evaporation speed in the aqueous ink. The evaporation of solvents in aqueous inks is affected by the following factors.

1 link material and pigment molecules. Since solvent molecules in aqueous ink are attracted by binders and pigment molecules, the volatilization rate is slowed down.
2 resin. The degree of slowing down of solvent volatilization is different for different resins. The greater the solubility of the resin, the lower the solvent volatilization rate.
3 pigment particles. After adding the pigment, the volatilization rate of the solvent is greatly reduced, the finer the pigment particles, the larger the surface area, and the slower the volatilization rate of the solvent.
4 ambient temperature and humidity. The higher the room temperature in the printing shop, the faster the solvent volatilization rate and vice versa. Indoor air circulation, the solvent evaporates quickly, otherwise the solvent volatilizes slowly, so the flexo workshop is equipped with a heating and exhaust system to promote ink drying.

(2) Penetration drying. The osmotic drying mainly depends on the penetration and absorption of water-based ink by the paper and is fixed and dried. When the aqueous ink is in contact with the paper, some of the binder penetrates into the paper, and the pigment remains on the surface of the substrate to complete the drying process. Infiltration-drying water-based inks can only print absorptive materials, such as paper, and the degree of penetration of the linking material into the interior of the paper is directly proportional to the absorption force of the capillary in the paper, that is, to the tightness and organization of the paper. In the actual printing, the influence of printing pressure, imprinting time and ink viscosity should also be considered.

Ink fixation and drying process are related to the permeability of the ink to the absorbent material, the penetration is too little or too light, the ink is not firmly fixed, and it is not easy to dry; if the penetration is too large or too deep, it will result in penetration. Affects the gloss of printed products. In the printing of absorbent substrates, both volatile drying and osmotic drying generally exist.

2. The effect of drying on adhesion

There is mutual attraction between the aqueous ink and the substrate, and the ink film can be firmly adsorbed on the surface of the substrate. For paper substrates, water-based inks can seep into their voids and attach using the "casting anchor effect." For substrates with a smooth, non-absorbent substrate, the adhesion of water-based inks depends on the affinity between the aqueous ink and the substrate molecules. .

With the prolongation of drying time, the adhesiveness of water-based ink gradually increases to its maximum value, but then the adhesiveness will gradually decrease to zero. At this time, the surface of the ink film has smooth and smooth and loses its adhesiveness. Therefore, the ideal multicolor printing should be performed at the maximum viscosity, that is, when the water-based ink will dry and not dry. Since water-based inks are water-resistant, they can be mixed with water before drying. Once dried, they cannot be dissolved in water. Therefore, do not allow the water-based ink to dry on the anilox roller, so as not to block the ink hole of the anilox roller. In addition, the flexographic printing plate is always kept wet by the ink during the printing process to prevent the ink from drying and blocking the graphic on the plate.

Printability The printability of water-based inks is very important, and has an impact on the printing process and print quality, including viscosity, thixotropy, adhesion, and pH values.

1. Viscosity Viscosity is the ability of the ink fluid molecules to attract each other and hinder the movement of molecules to each other. In the printing process, the viscosity of the water-based ink is a necessary condition to maintain the normal transfer and transfer of the ink. The factors that affect the viscosity of aqueous inks mainly include the following aspects.

(1) The viscosity of the binder. For inks using the same pigment, the viscosity of the binder material is large, and the viscosity of the ink is also large.

(2) Pigment particle size, content and dispersion. With the same kind of binder, if the amount of pigment is the same, the larger the particle, the greater the viscosity, and vice versa, the smaller the viscosity. The better the pigment is dispersed, the smaller the viscosity of the ink, and vice versa, the viscosity of the ink is large.

(3) Temperature changes. When the temperature is high, the relative mobility of the molecules in the ink increases, and the viscosity decreases; the temperature decreases, the cohesion of the ink molecules increases, and the viscosity increases.

For offset printing inks, the ink can be evenly coated on the printing plate by the ink fountain roller, and the flexographic printing ink fills the cell site in a short time by its own fluidity and adhesiveness. , and then transferred to the plate, so only the lower viscosity, that is, the thinner ink has such properties. If the viscosity is too large, it will cause the following defects: 1 ink transfer. Difficult to transfer. The flexo press is very fast, generally between 80 and 300 meters per minute. The interval between the first color and the second color is only a few seconds or less. 2 easily cause the paper to pull hair, powder phenomenon, and even cause the paper to peel. 3 The ink is difficult to fill in the cell, and it is very difficult for the scraper to scrape the excess ink. Therefore, the viscosity of the flexographic ink is relatively small, generally 0.15 to 0.3 Pa·s. However, the viscosity of the ink should not be too small, otherwise it will be buoyant. In severe cases, it will cause the emulsification of the ink, making it unable to transfer normally, and gradually accumulate pigment on the anilox roller and printing plate, and when it reaches a certain level, it will cause paste printing.

For relatively smooth substrates, when the ink supply is sufficient, the transfer rate is higher, so the viscosity of the ink is required to be slightly higher; and for the soft structure of the paper, the viscosity of the ink used should be slightly lower.

2. Thixotropy The property of the ink that causes changes in flow properties under the action of an external force is called thixotropy of the ink. In the printing press, due to the action of the ink roller, the fluidity and ductility of the ink are also increased, and after the transfer to the printed sheet, the external force disappears, the fluidity and ductility of the ink are reduced, and the ink is thickened from the thin, thus Guarantee the accuracy and clarity of prints and dots. When the ink has a good thixotropy, it is beneficial to the smooth and uniform transfer of the ink and the transfer rate of the ink. The factors that determine the thixotropy of ink are as follows.

(1) The nature of the pigment. After the ink with strong surface adsorption is made of ink, the thixotropy is large, like a type of carbon black pigment, the surface of which is oxidized with ozone to reduce the surface adsorption, and the ink made is better than untreated carbon black. The resulting ink is less thixotropic.
(2) The amount of pigment. With a large amount of thixotropy, the pigment molecules attract and flocculate each other.
(3) Shape of pigment particles. The needle-like and flaky inks of the pigments have a greater thixotropy than the granular ones.
(4) Wettability of pigments. The wetting ability of the pigment and the linking material is poor, and the thixotropy of the ink is large. On the contrary, it is small.
If the thixotropy of the water-based ink is too large, since the ink path of the flexographic printing is short, the ink supply may not be smooth, and even the ink supply may be interrupted, and the uniform and accurate ink supply amount cannot be guaranteed. If the thixotropy of the water-based ink is too small, the text and lines will become thicker due to the ink's infiltration on the paper and the excessive dots in the exhibition. Different types of flexographic printing have different thixotropy requirements for inks. In general, the screen version, text version, and line version require the thixotropy of the ink to be slightly larger; the large-area field version is slightly smaller.

3. Adhesion

The ability of the ink to hinder the peeling of the ink film is called the adhesiveness of the ink. It is extremely important for the smooth progress of printing. The requirements for ink adhesion of flexographic printing mainly include the following points.

(1) Adhesion of aqueous ink is less than the binding force of paper. When the adhesion of water-based ink is large, ink separation is difficult, and the ink spread on the printing press is not uniform. When the ink layer separates between the paper and the printing plate, if the resistance exceeds the binding force of the paper, a phenomenon such as pulling and even peeling of the paper occurs.
(2) The adhesiveness of the first color ink is larger, and the adhesiveness of the back color ink is gradually reduced. Otherwise, there will be cases where the post-printing ink sticks the ink first.

4. pH value

Water-based inks are generally weakly alkaline, and the normal pH range is 8.5-9.5. At this time, the printing performance of water-based inks is better, and the quality of printed products is stable. The effect of pH on the printability of aqueous inks is mainly reflected in the viscosity and drying of the ink.

When the pH value is higher than 9.5, the alkalinity becomes stronger, the viscosity of the water-based ink decreases, the drying speed becomes slower, and the water resistance deteriorates. When the pH is lower than 8.5, the viscosity of the water-based ink increases when the alkaline is weak. The ink is easily dried and fixed on the printing plate or anilox roller, which can cause the plate surface to be dirty and generate bubbles.

The pH value of water-based inks is mainly maintained by ammonia compounds. However, due to volatilization of ammonia during the printing process, the PH value will drop, which will increase the viscosity of the ink and cause poor transferability. At the same time, the drying speed of the ink will increase, and the watermark will be blocked. Roller ink hole, paste version. To maintain the stability of the ink performance, on the one hand, it is necessary to avoid leakage of ammonia as much as possible, and to cover the upper cover of the ink tank; on the other hand, it is necessary to add the ink stabilizer to the ink tank regularly and quantitatively.

Precautions

1. Before printing, the temperature of each color ink used should be stabilized at the temperature of the printing shop, and the printing speed should be adjusted according to the specific conditions such as temperature. When the temperature is high, the water evaporates quickly and the water-based ink dries quickly. Therefore, the speed of the machine must be increased. When the temperature is low, the water evaporates slowly and the water-based ink can dry slowly. The drying device can be added or the machine speed can be reduced.

2. The quality and performance of paper have a great influence on the drying speed of water-based inks and the quality of printed matter. Therefore, it is necessary to fully understand the properties of the paper used, especially the ink absorption, before printing. If the paper absorbs ink poorly, the ink cannot be quickly absorbed by the paper after it has been transferred to the paper. Then, the ink that has not dried will be transferred to the back of the previous paper, causing it to become dirty. However, if the paper absorbs the ink too much, it will infiltrate excessively and cause print-through, or cause the surface of the print to lose its luster. If a coated paper is printed, all the binder material penetrates into the paper fibers, and rapid penetration may cause the powder to chalk, leaving the pigments on the surface unprotected.

3. According to the condition of the printing plate, the printing material and the printing speed determine the viscosity of the water-based ink.

4. Must be fully mixed before use, the organic pigment particles in water-based ink is generally 2 ~ 5μm in diameter, but the pigment particles can make the diameter of its adhesion to 10 ~ 25μm, so if the uneven pigment dispersion, will affect the ink transfer performance, so before use Must be well-mixed.

5. When the viscosity is high, dilute with water or a special solvent.