The key to the development of flexo technology - ink

Since the emergence of flexographic printing in the 20th century, the printing inks that were used in the early days were made from aniline dyes extracted from coal tar and shellac or natural resins such as water and ethanol as solvents. Their printing properties, color, hiding power, The attachment fastness is very defective; more importantly, the aniline ink is toxic. With the development of flexo printing technology, aniline inks have long been eliminated and replaced by water-based, alcohol-based, polyamide-based, acrylic-based, and UV-type inks.

First, the characteristics of flexographic printing ink

Flexo printing inks have two distinct properties, namely low viscosity and fast drying. The ink has a low viscosity and good flow properties, which is in stark contrast to paste inks used in lithographic and letterpress printing. Due to the low viscosity of flexographic printing inks, flexographic printing presses can successfully use a very simple anilox ink infusion system with good ink transfer performance. At the same time, due to the quick-drying properties of flexographic printing inks, dry-overprinting can be achieved even in adjacent colors. This not only avoids overprinting failures due to poor drying but also means higher printing speeds. Compared with overprinting of lithographic or letterpress inks, flexographic printing inks have great advantages. And the quick-drying nature of the ink makes the flexographic printing process ideal for printing plastic non-absorbent materials. Flexographic printing presses generally only need to be equipped with a simple and energy-saving drying system to ensure normal printing. This can reduce flexographic printing costs in both machine manufacturing and operation. High speed and low cost are the main factors that make the flexographic printing process competitive. Over the past few years, the flexo ink market has developed rapidly abroad.

Second, the type of flexographic printing ink

There are three main types of flexographic printing inks: solvents, resin binders, and colorants. According to different solvents, flexographic printing inks can be divided into solvent-based inks mainly used for plastic printing, water-based inks mainly used for paper printing, and general-purpose UV-curable inks.

Solvent ink

The main components of solvent-based inks are pigments, ink binders and solvents. The binder is a mixture of resin and solvent. The resin is the main body of the binder and plays a role in spreading the ink, carrying the pigment or dye, and providing the adhesive force with the substrate. The role of the solvent is to dissolve the resin, so that the ink has a fluidity and a certain viscosity, in order to achieve the transfer of ink during printing, and control the ink drying speed. Alcohol solvents are the least toxic solvents. The colorant provides the ink with a color. According to the colorant, the solvent type ink can be further divided into a dye type ink and a pigment type ink.

There are two main types of dye inks: basic dye inks and lightfast dye inks. The main components of basic dye inks are: basic dyes, mordants and resin binders, solvents, and waxes. Its characteristics are: high color intensity, bright, cheap, but easy to fade, that is poor water resistance. Basic dye inks are generally used only for the printing of paper bags, wrapping papers, waxed bread papers, and confectionery papers, and are not suitable for thin film printing because the dyes easily penetrate the film. Light-fast dye ink main components are: light-resistant dyes, adhesives, solvents, plasticizers, waxes. Its characteristics are: water resistance, light resistance is better than basic dye inks, its strength and brightness slightly lower than the basic dye inks, with excellent transparency, so lightfast dye inks are often used in addition to paper printing, Various metal foil printing. Dye-type inks have few side effects on most rubber and photosensitive resin plates.

Since the dye is completely transparent, the brightness of the obtained color is lower when printing a colored paper such as kraft paper. Adding some titanium white pigment to the dye ink can reduce its transparency, but the diffusion titanium dioxide medium must be compatible with the dye. The addition of pigments to dye-based inks can also produce special color effects, increase the color intensity of the ink, and maintain low viscosity. Of course, the possibility of their interaction should be considered when mixing.

The fundamental difference between a pigment ink and a dye ink is that the dye can be dissolved in the ink vehicle used and the pigment is insoluble. The pigment must be dispersed in the ink vehicle by means of a grinding device. Pigment inks consist of the following ingredients: pigments, binders, plasticizers, solvents, and waxes. Since the light fastness, water resistance, wax resistance or heat resistance of the dye ink cannot meet the requirements of the printed matter, corresponding pigment inks can be used at this time. Pigment inks in the country have high viscosity and poor permeability. In addition to the pigment ink can be formulated directly with pigments, pigment flakes can also be used to prepare. Pigment flakes are pigment flakes in which pigments diffuse in the resin under high shear conditions, where the pigment particles are very small. The ink formulated with the pigment flakes has high gloss, transparency, and coloring power. However, when the pigment particles are small, they also generate heat when they are broken, causing the polymerization of the resin, which may affect the ink flowability. There are many kinds of pigment inks, which can be adapted to the printing of different materials, and can be roughly divided into the following printing materials: paper ink, polyolefin film ink, polyvinyl chloride (PVC) film ink, polyester film ink, nylon film Ink and aluminum foil printing inks.

2, water-based ink

As early as the 1940s, water-based flexographic printing inks based on casein and α-proteins began to be used in the printing of corrugated boxes. In the 1950s, as the printability of ink was improved, the application of water-based inks was expanded to the printing of kraft paper bags. Water-based inks are used in the printing of wax-coated milk cartons due to their ease of printing, good printability, and intrinsic insolubility in hot waxes. With the development of the milk carton packaging industry, the consistency of printability and printing products has been continuously improved, and water-based inks have been continuously developed. As the performance of water-based inks continues to improve, its application range has expanded to various wax paper and board printing sectors, and has entered the newspaper printing market, with an annual growth rate of about 10%.

With the improvement of public awareness of environmental protection, the printing industry has also put forward many requirements for environmental protection, and water-based ink is just in line with the requirements of environmental protection. It can significantly reduce the emission of VOCs (organic volatiles) into the atmosphere, thereby reducing air pollution and improving the printing operation environment. It completely eliminates the harmful effects of certain toxic and harmful substances in sealed inks on the human body and the pollution of packaged goods. It can be called "green" ink and is particularly suitable for the packaging and printing of foods, beverages, medicines and other products. Printing convenience, economy and safety are the reasons for the continuous expansion of the scope of application of water-based inks. At present, water-based inks account for about 35% of flexographic printing inks, but with increasing environmental protection requirements and low solvent costs, etc. Affected, water-based ink will have a broader space for development.

The main components of water-based inks are colorists, water-based binders, auxiliary solvents and dissipating agents, and anti-foaming agents. It differs from solvent inks in that the solvent used in water-based inks is water rather than organic solvents, that is, the binders of water-based inks are mainly composed of resin and water. According to the different link materials, water-based inks can be divided into three categories: water-soluble, alkali-soluble and diffusive.

(1) Water-soluble inks: The binders for water-soluble inks include polyvinyl alcohol, hydroxy cellulose, and polyvinyl pyridone. These binders have a permanent solubility, so the range of use of inks formulated with such binders is limited. They can only be used where the ink film is allowed to have permanent water solubility or no contact with water.

(2) Alkali-soluble ink: The binder of the alkali-soluble ink is an acidic resin containing -COOH and -OH groups.

(3) Diffusion thick ink: The diffusing ink consists of a chain of fine resin particles suspended in water—latex. The significant advantages of this ink are: 1 Latex can have high density and low viscosity, can produce high-quality thin coating; 2 latex contains considerable polymer molecules, large molecules are generally harder than small molecules, wear-resistant Loss, heat resistance, and adhesion. The biggest disadvantage of latex ink is that it is difficult to print and it is difficult to clean, because once the latex condenses, it becomes insoluble. Latex inks are more suitable as coating inks.

In recent years, with the application of new materials and the improvement of the level of science and technology, the three major drawbacks of traditional water-based inks, such as slow drying, poor gloss, and easy to cause paper stretching, have been significantly improved. The application of domestically produced flexographic high-precision ink shows that the drying speed can reach 200m/min and the resolution has exceeded 150l/min. With appropriate application of the printing process method, the print gloss is as good as solvent ink.

3. UV curing ink

UV-curable inks are inks that can change from a liquid state to a solid state when irradiated with ultraviolet light of a certain wavelength. The development of UV inks began in 1946, but in recent years there have been significant developments. Germany began researching solvent-free flexo printing inks and systems in 1990 to replace water-based inks, thereby increasing the drying speed and reducing waste disposal. Ultraviolet curing inks are mainly composed of pigments, photosensitive resins, reactive diluents, photoinitiators and additives. Among them, the photosensitive resin is mainly cyclochloroacrylic resin, polyester acrylic resin, urethane acrylic resin, etc.; most of the active monomers are unsaturated double bond cross-linking monomers, such as acrylic ester monomers; pigments are mostly universal pigments, but Must have lightfastness.

The drying mechanism of the UV ink is that, under the effect of ultraviolet rays, the photo initiator decomposes to generate free radicals or ions to improve the monomer double bond, and then crosslink with the hydroxyl polymer of the system.

The non-drying part of the UV ink on the printing press is not flat and the time is almost unlimited. This non-drying property allows the ink viscosity to remain stable during the long-term operation of the printing press. The rheological properties will remain constant throughout the printing process and no special monitoring is required.

UV inks are "cured" rather than "dry", unlike ordinary inks. If the generally inked ink layer is a 10 μm thick wet ink, the dry ink film is only 7 μm thick due to the inclusion of 30% volatile compounds. The 10 μm thick wet ink film of UV ink is still 10 μm thick ink film after curing. That is to say, with UV ink, not only the same ink film produced by the same anilox roller has better hiding power, but also a small amount of UV ink can achieve the effect of the ink film produced with a large amount of ordinary oil. When the UV ink cures, its energy is about 10 times higher than that of the UV light, which can meet the requirements of today's high-speed printing.

Ultraviolet ink has the best resolution and resolution in flexographic printing, small dot gain, no solvent discharge, no environmental pollution, good printability, no change in physical properties during printing, fast drying, and high production efficiency , Water resistance, wear resistance, good stability.

The greatest impact on flexographic printing in the 1990s was the UV flexo printing technology. Because the UV ink does not have the problem of volatilization of water or solvent, the transfer and reduction of dots are better than traditional inks, and the colors are more vivid, which has greatly promoted the quality of the flexographic printing products.

Due to the late start of our country's flexo printing technology, all supporting equipment and consumables are still not closed. At present, only a few companies in China can mass produce high-precision water-based ink products and flexo-printed UV inks and auxiliary materials, and the quality of inks needs to be further improved. .