With the continuous improvement of people's requirements for packaging of goods, plastic film and plastic composite film flexible packaging materials have been widely used in the packaging of goods for their excellent display functions, protection functions, and ease of storage and transportation. In order to obtain a qualified plastic film packaging product, the technician must understand the various materials and various process steps in the plastic film printing and bag making process. Among them, ink is one of the important materials, which has an important influence on the printing quality of plastic films and the post-printing processing of plastic films.
Printing of plastic films is generally based on gravure printing. In this paper, by analyzing the similarities and differences between the printing process and the printing process of plastic film, the characteristics and composition of the ink for plastic film gravure printing will be discussed.
First, the characteristics of plastic film printing
After the plastic film is gravure-printed, it has unique advantages unmatched by other printing materials, such as good transparency, but it also has its own shortcomings, such as some plastic films that require surface treatment. There are many kinds of plastic films used for printing, and there are great differences in their surface characteristics, molecular structures, crystallinity, and chemical stability, which affects the binding fastness of printing inks. Therefore, it is necessary to determine whether or not it needs surface treatment before printing according to the characteristics of different plastics. Films such as polyvinyl chloride (PVC), nylon (PA), and polystyrene (PS) are polar polymers that do not require surface treatment before printing. The most commonly used polyolefin film materials (PE, PP, modified polyolefin, etc.) are basically non-polar polymers, they have high chemical stability and small surface tension (29 to 31 mN/m). Not easily infiltrated by most inks and solvents. Theoretically speaking, if the surface tension of an object is less than 33 mN/m, it is almost impossible to adhere to any of the currently known adhesives. Therefore, if the ink is to obtain a certain printing fastness on the surface of the polyolefin, the surface tension must be increased. According to the process requirements, it should reach 38 mN/m or more. So far, the most commonly used surface treatment method for plastic films at home and abroad is corona discharge treatment. The surface tension of commonly used films for printing is shown in Table 1.
Second, the plastic film gravure printing table printing and printing process
Table printing refers to the process of printing the graphic on the surface of the finished product when the process of cutting, bag making, etc., has become the final packaged product after printing on a plastic film. The surface printing process is generally used for non-transparent films or films with poor transparency, such as various colored films, paper, aluminum foil, and the like. After the table printing, the final ink layer is exposed on the surface, so to achieve a better printing effect, it is required to use the printing ink, in addition to meet the colorful, strong coloring, fineness, viscosity and proper dryness In addition to requirements, it must also have good adhesion and gloss, good scratch resistance, high and low temperature resistance, and stable chemical properties. According to the current situation at home and abroad, a single-layer polyethylene film (including IDPE, MDPE, HDPE, UD), EVA film, and non-stretched polypropylene film may be processed by a surface printing method, and the film is generally Most of the reels are printed.
In India, it refers to the use of reverse-image printing plates, the ink is transferred to the inside of the transparent printing material, so that the front of the substrate is a graphic printing method. After the plastic film is printed on the back, most of the plastic film must be processed in a composite manner. Therefore, the printing of the plastic film can be called a composite printing of a plastic film. Prints printed in India are brighter, brighter, more beautiful, colorful, non-fading, and fade-resistant than watch prints. They are moisture-proof, wear-resistant, firm, practical, long-term, non-adhesive, and non-broken. Because the ink is printed on the inner side of the film (it is compounded and the ink layer is sandwiched between two layers of film), it is not easy to contaminate the packaged articles and meets the requirements of the food hygiene law. Therefore, most of the plastic film packaging and printing at home and abroad use this process. In terms of color, tinting strength, fineness, viscosity, and initial hygiene, the requirements for printing inks are basically the same as the requirements for printing inks. However, since the ink after printing is sandwiched between two layers of film, it will not be directly rubbed or scratched. The resistance to ink is not too high, and the glossiness is also the same. However, in film composite The composite bond strength at the back of the ink must reach a certain degree, which is generally not less than 1.0 N/15mm, otherwise the packaging bag made of this composite material is slightly rubbed or flexed during packaging or transportation, and it easily occurs. The detachment between the materials, so India requires ink to adhesive and AC agent has a good viscosity. At the same time, since the inner layer of the composite film is often made of a film with poor airtightness such as a polyethylene film, the amount of residual solvent in the ink must be kept as low as possible. Otherwise, the residual solvent in the ink will slowly release and contaminate the contents through the inner layer material. .
Third, the characteristics of plastic ink film gravure printing and composition
1. Plastic film gravure printing ink features
Due to the high printing speed of gravure printing and the thick printing ink layer (generally in the range of 9 to 20 m), plastic films do not absorb ink, so gravure printing inks must be mainly volatile and use low-boiling, volatile solvents. Make the ink dry quickly. In addition, in the intaglio printing process, the ink is filled and coated in the gravure texture by its own fluidity and cohesiveness, so only the lower viscosity, ie, the thinner system, can impart such properties. In a short period of time, the ink should be filled. If the viscosity is too high, it will be difficult to fill up. It is difficult for the scraper to scrape off the ink of the non-graphic part; on the contrary, if the viscosity is too small, the ink dot will be deformed due to the imprinting. Make the pattern less reproducible. At the same time, since gravure printing uses a scraper to remove the non-image part of the ink and the remaining part of the ink is returned to the original ink tank, even the ink in the graphic and the ink must be repeatedly in contact with the original ink, only the fluid with the lower viscosity can be easily Removed inks with good reconstitution, low thixotropy, and yield values ​​are suitable for gravure printing, so plastic inks for gravure printing must be of lower viscosity.
2. Composition of plastic film gravure printing ink
The ink for plastic film gravure printing is generally composed of pigments, fillers, binders, and auxiliary materials, as shown in Table 2.
(1) Color
Colorants play a coloring role in inks, including dyes and pigments. Most of the plastic gravure inks use pigments. Dyes are soluble in water and their aqueous solution is a true solution; pigments, however, are insoluble in water, insoluble in oils and binders, and can only be dispersed in the form of fine particles in a suspension state in the binder. To a large extent, pigments determine the quality of the ink, such as ink color, tinting strength, relative density, transparency, and resistance to ink. For this reason, the pigments used in the ink require bright colors, strong coloring power, and dispersion. Good sex.
(2) packing
Fillers can be used without, for example, in the ink with the appropriate amount of pigment has reached the viscosity and thickness requirements, you can not use. If the viscosity of the ink is too small and the thickness is not enough after using a proper amount of pigment, a small amount of filler must be added. Typical fillers are light calcium carbonate, precipitated barium sulfate, and the like.
(3) Binder
The binder is a sticky fluid with a certain viscosity and viscosity. It is the main component in the ink and plays a linking role in the ink. The binder is generally prepared by dissolving a synthetic resin in an organic solvent. As a carrier of the pigment, fine particles such as a powdery pigment are well dispersed therein, and the uniform and fine colloid is formed, and the ink is printed on the plastic film. When the film is dried to become ink film, the coated pigment can be firmly attached to the surface of the plastic film. Among them, the resin is the main material for the plastic firmly attached, the solvent should have the property of dissolving the resin, helping to bond, and quickly evaporating to make the ink dry. Visible, the quality of the link material directly affects the performance of the ink. From the perspective of the ink industry, binders are also more important than pigments because, in the manufacture of inks, the same pigment is changed to another type of binder, and it becomes another type of ink; With the same binder, different pigments, the ink does not change the fundamental properties. For this reason, people concluded from practice that the linking material is the "heart" of the ink.
The difference between the intaglio printed ink and the printed ink is also mainly because they use different binders. Table printing inks mainly include polyamide inks using polyamide and nitrocellulose as binder resins, acrylic inks using acrylic and acrylate copolymers as binder resins, and copolymers of vinyl chloride and vinyl acetate as Vinyl chloride-based inks for binder resins, but the most widely used are polyamides. Ink printing inks mainly include polyamide based inks such as chlorinated synthetic resins and polyamides as binder resins, acrylic inks using acrylic copolymers as binder resins, and polyisocyanates as binder resins. Isocyanate inks.
(4) Auxiliary agent
Auxiliary agents are materials that are added in order to impart certain properties to the ink, or to control its properties. In other words, an ink formulated according to its basic composition cannot meet certain requirements in terms of certain characteristics, or it may be necessary to add a small amount of auxiliary materials when the requirements of printing cannot be satisfied due to certain conditions. This additional material is called It is an adjuvant or additive. Commonly used adjuvants include diluting agents, defoamers, detackifiers, tackifiers, desiccants, and the like.
Fourth, the development of plastic film gravure printing ink
In recent years, the packaging and printing industry has been moving toward green, and most of the plastic film gravure printing inks are solvent-based inks. The organic solvents such as toluene and xylene used not only pollute the environment, but also the operator's body. Health causes harm. With the development of society, benzene-free, alcohol-soluble and water-based inks will become the mainstream of plastic film gravure printing inks. The manufacturer of ink for film gravure printing is only able to maintain its invincible position in the fierce market competition only by continuously improving the formula and improving the printing quality and environmental performance of its products. Users of plastic film gravure printing inks only select those inks with good printability, high printing quality, and good environmental protection as far as possible in many types of inks in order to print good products to meet the needs of the society and the market.