In packaging and printing, with the continuous improvement of printing technology and equipment, and the increasingly fierce competition in the products, people are paying more and more attention to the appearance requirements of packaging prints, and constantly introducing special packaging methods and processing technologies for the surface of packaging prints. Among them, the glazing technology has been widely used to make commodity packaging leap to new heights. Shining not only protects the prints, but also changes the appearance and visual effects of the prints and promotes the sales of goods. In the context of increasing consumer demand for prints or packaging, print glazing has become an important means of value-added printing and modern merchandising.
The glazing characteristics and the main factors affecting the quality of the coating are the coating of a colorless and transparent coating on the surface of printed products that has already been printed, and obtaining a bright film layer. Due to the leveling of the coating on the surface of the printing product, the smoothness of the surface of the printing product is enhanced, not only the surface gloss of the printed product is improved, but also the surface properties of the printed product are improved, the light resistance of the ink is enhanced, and the heat resistance of the ink layer is increased. The ability to prevent moisture has played a role in protecting printed graphics and beautifying products. It has been widely used in the surface processing of book cover processing and packaging decoration, pictures, large-scale decoration, posters and other prints.
There are many ways to polish, according to the coating coating can be divided into oxidation polymerization coating coating, solvent evaporation coating coating, heat curing coating coating, photocurable coating coating. According to the coating method, it can be divided into spray coating glazing, coating glazing and special glazing coating. According to the glazing equipment, it can be divided into glazing machine and printing glazing unit. In addition, the glazing products can be further divided into the entire surface of light, local glazing, extinction, artistic glazing.
No matter what technology method or material is used, it requires excellent glazing quality to achieve the intended purpose. From the material point of view, the main factors affecting the quality of glazing are printing paper, printing inks and glazing coatings with good glazing suitability.
(1) Printing paper. The greatest impact of the printing paper on coating is the smoothness and surface absorption of the paper, which affects the leveling of the coating and affects the formation of a high smoothness film on the surface of the print.
The smoothness of the paper is high, the surface of the paper is in contact with the paint layer, and the paint is easy to level, resulting in a high smoothness film surface. The surface of the paper is highly absorptive. When the coating is leveled, the paper fiber absorbs the coating too quickly, which makes the paint flow poor and difficult to level. Conversely, the surface of the paper is poorly absorbable, and the paper fiber absorbs paint too slowly. In the leveling coating, the paint is not easily congealed, and it is difficult to form a glossy film with high smoothness. Therefore, different paper should be selected to match the coating, adjust the coating coating leveling time, drying temperature and other process conditions.
(2) Printing inks. The particle size of the ink affects the wetting of the ink layer and thus affects the leveling and brightness of the coating. If the ink particles are large, the surface of the print must be rough, not easy to wet, and difficult to level.
(3) Coatings. Coating coating is also the main factor affecting the quality of coating. Coating coating must not only have colorless, odorless, non-toxic, strong gloss, chemical resistance, fast drying and other properties, but also have good transparency, not easy to change color, and have more Good flexibility, heat resistance, leveling, adhesion. The most important effect of coating coating on coating quality is its viscosity, followed by surface tension and solvent volatility. The solvent in the coating will be less and less due to volatilization, infiltration, and diffusion, and the viscosity will gradually increase. The driving force for leveling is the surface tension of the coating. The surface tension flattens the surface of the trace to a smooth surface during the process of shrinking the coating to the smallest surface shape possible. Therefore, the surface tension of the paint has a greater influence on the leveling.
Matters needing attention in the use of the two main coating oils Coating coatings can be mainly classified into oxidative polymerization (mainly relying on the polymerization reaction with oxygen and drying to form a film) and solvent evaporation (mainly relying on the solvent evaporation in the coating to dry out the film. , Its leveling is good, suitable for all grades of large quantities of printing products glazing), heat curing type (relying on the resin and the catalyst to react and dry to form a film when exposed to heat), photocuring type (polymerization of molecules by light irradiation) Reactive dry film, coating gloss, film wear, folding, heat resistance, etc. 4 kinds. The current ideal and development prospects are better UV varnish and water varnish.
1. Use of UV Varnish and Removal of Some Failures UV Varnish consists mainly of photoinitiators, prepolymers (oligomers), monomers (active diluents), auxiliaries (leveling agents, plasticizers) Agents, antioxidants, etc. The photoinitiator is excited at the appropriate wavelength and intensity of the UV light to generate radicals that initiate the polymerization of the unsaturated group (usually acryloyl) on the prepolymer and monomer, eventually changing rapidly from liquid to liquid. Solids usually take 0.1 to 0.3 seconds.
UV glazing of printed matter is mainly coated with UV oil and cured in two processes. In fact, the printing itself is the process of ink coating on the substrate. The UV varnish can also be applied to the substrate by printing. However, the surface of the substrate is required to be dense and non-penetrating oil because of the viscosity of the varnish. It is much lower than ink. Coated paper, whiteboard paper, gold cardboard, plastic film, etc. can be coated with UV oil.
The UV varnish is applied to the substrate, and the varnish flows into the ultraviolet radiation area and cures after a period of time. The curing process is a chemical cross-linking process, ie, the prepolymer, the unsaturated carbon-carbon double bonds (liquids) in the monomer are cross-linked to form a network-structured molecule (solid) by the energy transmitted by the photoinitiator. However, curing varies with UV oil thickness, time, and UV radiation intensity. The relationship between radiative energy transfer and UV oil thickness is an exponential decline. About 90% of the UV energy is absorbed in the uppermost layer of 1 micron, and only 9% is obtained in the lower layer of 2 microns. If the same energy is to be obtained in the lower 2 microns, the initial UV irradiation energy should be increased by a factor of 10. Generally, the thickness of the coating layer is only 3 to 5 microns, and the upper and lower varnish are almost simultaneously cured. However, when the coating is thick (such as using a wire machine, roller coating machine) will appear upper curing, the underlying uncured phenomenon.
Curing varies with time, and a large amount of photoinitiator initiation time generally ranges from 0.1 to 0.3 seconds, but some of the remaining (unreacted) photoinitiators continue to react for 6 to 24 hours, the so-called post-polymerization reaction. If the light is excessive at the beginning, the UV oil will become brittle as a result of the post-polymerization reaction. Therefore, the amount of exposure of the UV coating should not be excessive.
When using UV curing equipment, generally use a lamp, and adjust the distance between the lamp and the object (normally 5 ~ 15cm), if a lamp can not be cured, you need to add a curing lamp to enhance the exposure.
Curing With the change of UV radiation intensity, when the UV intensity per unit area increases, the curing rate will increase, but it is not a straight line but increases several times. UV Varnish is very suitable for polishing high-grade prints.
In the use of UV lamps, due to the lamp to maintain high temperatures above 800 °C is conducive to the vaporization of mercury molecules conduction, but the objects need to cool down. Therefore, design lamps and cooling devices should be fully considered. When using a focusing type lens, do not put the subject in the focus position to avoid overexposure.
At present, there are mainly two ways to use UV glazing.
1 three roller type (ie ink roller, coating roller, pressure roller). The advantage of this method is that it is fast and adjustable; the disadvantage is that it should not be applied to thin paper. Although some glazing machines have air knives, the glazing effect on thin paper is poor and the oil layer is thicker. Therefore, this method is more suitable for board paper.
2 dentition type (ie impression roller with cavities). The advantage of this method is that thin, thick paper is suitable, fuel-efficient, easy to adjust the thickness of the oil layer, because the diameter of the impression roller is large, the pressure is large; the disadvantage is that the speed is not easy to adjust.
When using UV varnish in packaging printing, due to the influence of factors such as technology, materials, equipment, and the environment, there are often some failures that can be summed up. There are mainly the following problems.
(1) The UV varnish is incompatible with the ink, with streaks and orange peel.
The specific performance is that the UV oil is coated on some kind of ink bead, cured like an orange peel, or even if it can be coated evenly without foaming, but if scraping with a sharp tool, the UV oil layer will be separated from the ink layer. The main reason is that the viscosity of the UV varnish oil is too high; the coated anilox roller is too thick (the coating amount is too large), the surface is not smooth, the coating pressure is not uniform, and the leveling property of the UV varnish is poor. The solution is to first apply a layer of base oil, then apply UV oil, or use EDM treatment and then apply UV oil, and mix the appropriate amount of alcoholic infrared varnishes.
(2) UV light oil adhesion is not good, the oil can not paint or flower. The main reason is that the UV light oil viscosity is too small, the coating is too thin; ink in the oil or dry oil content is too high; printed ink surface crystallization; printing ink additives are not suitable; , dusting) too much; coated anilox roller wire is too thin; light curing conditions are not suitable. The solution is to reduce the viscosity of the UV varnish and reduce the amount of coating; the pressure is adjusted evenly; the applicator roller should be ground and polished; add a bright leveling agent. When printing, the corresponding measures shall be taken according to the conditions of coating; UV varnish may be thickly coated; if necessary, it may be topped with base oil or special varnish; and the base oil with adhesion is coated on the printed product.
(3) The gloss and brightness are not ideal after UV polishing. The main reason is insufficient oil supply; paper surface brightness is not enough or the surface roughness is not dense, absorption is too strong; UV Varnish viscosity is too small, the coating is too thin; ethanol and other non-reactive solvents diluted excessive; UV oil coating uneven . The solution is to start with oil. In addition, UV oil expiration or problems with UV oil quality can also make the glazing not bright. According to the difference of the paper, the viscosity and the coating amount of the UV varnishes can be appropriately increased, and the paper with strong permeability and permeability can be coated with a layer of base oil or replaced with a smooth paper.
(4) The UV oil layer is relatively brittle, and the folds at the creases are brittle. Polyurethane oil can be used instead. It should be indented when folded. Thin paper should not be folded with bamboo plate. If the UV oil layer is too thick, this fault can also occur and the amount of oil can be adjusted.
(5) The alcoholic UV oil is easily dissolved by ethanol after curing. If the wine box is covered with UV oil, if the wine bottle breaks during transportation, the wine will dissolve the UV oil layer. The solution is to use alcohol-insoluble UV oil.
(6) Bad drying, incomplete curing, and sticky surface. The main reason is that the UV intensity is not enough; the UV lamp is aging and its strength is weakened; the UV varnish is stored for a long time; the diluent that does not participate in the reaction is added too much; the machine speed is too fast. Solution: In the case of curing speed requirements less than 0.5s, generally should ensure that the power of high-pressure mercury lamp is not less than 120W/cm; lamp to be updated: if necessary, add a certain amount of UV varnish curing accelerator to accelerate drying.
(7) UV oil coating is too thick and neither economical nor brittle. The solution is to heat the UV oil to 50-55°C or add thinner to the UV oil.
(8) UV oil bursts automatically after curing. The reason is that the UV illumination is excessive and the surface temperature of the object is too high. The solution is to increase the distance between the lamp and the surface of the object to be illuminated, so that the object to be taken away from the focus of the shade, quickly cooled after curing, can be cold or cold roller pressure or air-cooled, water-cooled light source.
(9) Polishing produces oil spots. The main reason is that the thickness of the oil is not enough, the temperature of calendering is not enough, the calender is not suitable, and the paper quality is inconsistent. The appropriate material should be adjusted or replaced for the specific situation.
(10) Polishing coating has white spots and pinholes. The main reason is that the coating is too thin; the rubberized anilox roller is too fine; non-reactive diluents (such as ethanol) are added in excess; the printed surface has more dust. The production environment and printed surface should be kept clean; increase the thickness of the coating; add a small amount of smooth auxiliaries: the diluent is preferably the active diluent involved in the reaction.
(11) Large residual odor. The main reason is that the UV varnish is not completely cured; the UV light is insufficient or the lamp tube is aged; the UV varnish oil is poor in the anti-oxidation ability; the non-reactive type diluent in the UV varnish is added too much. The UV varnish should be thoroughly cured, ventilation should be enhanced, and the UV varnish type replaced if necessary.
2. Matters needing attention in the application of waterborne varnish. Waterborne varnish, also known as dispersion varnish, has a colorless, odorless, transparent and non-toxic, no organic volatiles (VOC), low cost, material source Wide, good gloss, folding resistance, abrasion resistance and chemical resistance. As the water-based polish uses water as a diluent, a small amount of volatile ethanol is used as an auxiliary solution, which can eliminate the harm to the human body and the pollution brought to the environment during the operation, which is beneficial to safe production, improving the labor conditions of employees, and complying with environmental protection requirements. Especially suitable for food, cigarettes, pharmaceuticals and other packaging.
Water-based coating oil mainly consists of three main categories: main agent, solvent, and auxiliary agent. The main agent of water-based coating oil is a film-forming resin, which is a film-forming substance of the polishing agent and is usually a synthetic resin. It affects and dictates deep physical properties and film coating qualities such as gloss, adhesion Sex, dryness, etc. There are many types of aqueous film-forming resins, but the most common ones at home and abroad are acrylic resin copolymer resins.
The properties of the waterborne acrylic copolymer resin depend on the composition of the monomer, the synthesis of the monomer, and the synthesis process. Waterborne acrylic copolymer resin has a series of excellent properties such as good film forming, good gloss, high transparency, rapid drying, wear resistance, and water resistance, and is widely used in water-based polishes and water-based gloss coatings.
Auxiliaries are used to improve the physicochemical properties and processing characteristics of aqueous polishes. Adjuvants mainly include the following.
1 Curing agent: Improve the film forming property of the aqueous main agent and increase the cohesive strength of the film. 2 Surfactants: reduce the surface tension of aqueous solvents and improve leveling. 3 Defoamer: Long-term control of blistering of the polish, eliminating fisheye, pinhole and other quality defects. 4 Desiccant: Increase the drying rate of the water-based polish and improve the printability of the paper. 5 Adhesion promoter: Improve the adhesion of the film forming material and the substrate. 6 wetting and dispersing agent: to improve the dispersion of the main agent to prevent stickiness and improve wear resistance. 7 other additives: plasticizers that can improve the folding performance.
There are many types of adjuvants and they vary depending on the type of polish. For example, the infrared waterborne varnish and the matt varnish are all different kinds of varnish, so the components of the assistant agent are also different.
However, it should be noted that the use of various types of additives should not exceed 5% of the total amount, otherwise it will affect the processing of the polishing agent.
The main role of the solvent is to disperse or dissolve synthetic resins and various additives. The solvent of the aqueous coating oil is mainly water. Colorless, odorless, non-toxic, widely sourced, and inexpensive. The volatility of water is almost zero and the leveling performance is very good. However, the solvents of water-based polishes also have shortcomings, such as slow drying speeds and easy to cause product failures and other process defects. Therefore, ethanol is often added during use to improve the drying performance of the aqueous solvent and improve the processing suitability of the aqueous coating oil.
In addition, since the UV coating is ideal in terms of drying and the like, if it is used in an aqueous coating, it can complement each other. Based on this, a new drying method has now been developed for coating. Ultraviolet lacquer coatings can form bright films in a matter of seconds, as well as dry glazing coatings that use a combination of UV and IR radiation.
Compared with solvent-based coating oil, the advantages of water-based coating oil include:
(1) Strong transparency and good preservation. It can be stored for a long time on the prints it is applied to. Under long-term intense sunlight irradiation, water-based glazing oil is not easy to yellow and does not change color.
(2) Good wear resistance. Packaging-type prints require strong abrasion resistance, especially cigarette packs, which are subjected to high mechanical loads at high speeds of up to 300-500 packs per minute. As a result, prints are glazing to improve wear resistance, and aqueous coatings can meet this requirement.
(3) Conjunctival speed. The drying process of waterborne varnish is 30% through evaporation, 70% is absorbed by the material, and drying (the effect of moisture on the drying device) is rapid and can be completed in a few seconds. When the varnish still contains 20% to 30% of moisture, the film can be formed. Therefore, no drying method is needed. If using infrared or hot air drying method, the drying speed is faster.
(4) good heat sealing performance. At present, cigarette packs widely use PP film as the outer package, and the water-based varnish has good heat-sealing performance. Even if celluloid is used, good heat-sealing effect can be obtained.
(5) Finished products have good flatness and strong anti-rolling force. One of the headaches in gravure printing is the continuous drying of the substrate through several color groups. The moisture content of the paper is extremely low and it is particularly prone to curling. The use of water-based varnish to soften the package, the product through the cut sheet, and then bronzing cut, still maintain a good flatness and roll resistance. This will provide a new way for gravure to change the gravure, especially the need for hot stamping to change gravure.
The application of waterborne varnish must take into account factors such as coating method, paper type, and product quality requirements. When using waterborne varnish, pay attention to the following issues:
(1) Viscosity control: In the coating process, the viscosity and solid content of the coating oil must be reasonably controlled. Therefore, the dilution can only be performed within a certain solid content range. Water-based varnishes generally use a 1:1 mixture of ethanol and water to reduce viscosity.
(2) Select the paper type. Thick paper has good dimensional stability and thin paper has poor dimensional stability. Dimensional stability is one of the key indicators of print quality. It is closely related to the amount of water used in printing, the humidity control of the paper, the time and method of drying, and the control of viscosity. If the printed product is less than 90g/m 2 , care should be taken in the use of water varnish.
(3) Control of coating amount. The coating of waterborne coatings is not easily controlled. Since ethanol is easily volatilized during coating, and water is colorless and transparent, it is very difficult to discriminate the amount of coating with the naked eye on a white paper, so the control of the viscosity is very important. Ethanol content must also be increased in time to ensure product stability.
(4) Use of anti-adhesion agent. When using water-based coating, the use of antifouling agent can be completely eliminated or reduced because the waterborne coating is completely cured and there is little stickiness. The paper can be punched and indented with a press after printing (wet-wet printing) for 2 hours.
(5) Control of drying time. Solvent-based coating oils are almost all volatilized and dried, while the infiltration and drying of aqueous coating oils have great utility value. However, in multi-color printing, if the last color is coated with a coating, the drying time will be slower because more ink and wetting fluid has been absorbed inside the paper. Infrared water-based coating oil can be used and dried by means of an infrared drying device. When the drying coating oil is heated, the bottom layer penetrates quickly and dryness. At the same time, the moisture in the polishing agent is heated and evaporated by the high temperature, and is discharged through the exhaust pipe.