A brief analysis of the characteristics of water-based ink and printability (on)

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, pH, and other performance indicators.

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.