Application of Waterborne Varnish and Problems to Be Noted

First, what is printing

The so-called printing glazing is the surface of the replica that has been completely printed in the graphic and text printing process. The printing plate is printed once or twice with a solid printing plate or a graphic plate, and the surface of the printing object is given a bright film layer. method. Printing glazing can enhance the lightfastness of the ink, increase the heat resistance and moisture resistance of the ink layer, and play a role in protecting the print and beautifying the product.
Printing glazing and other methods are more cost-effective, easy-to-manufacture, easy-to-operate, and have the greatest advantage over the treatment methods that add glossy film to the surface of the printed product. , Selective glazing, both full version glazing (no special plate, no flat plate for letterpress printing, offset printing, no special new plates, the use of the original plate on the machine), or partial glazing Because the principle of transfer between printing and printing is the same as the principle of printing and graphic printing, the printing plate is used to transfer the adsorption layer on the substrate, and the principle of non-adsorption of the blank is used. According to the customer's requirements and the special requirements of the actual printing process, the coating is applied. .

Second, the characteristics of waterborne varnish

Waterborne Varnish (or Dispersion Coating) has a colorless, odorless, VOC-free, low cost, wide source of material, good gloss, folding resistance, abrasion resistance and Chemical resistance, economic health and other characteristics.

Waterborne Varnish can be divided into three types: high gloss, ordinary luster and matt luster. From the processing technology can be divided into: ink fountain coating process, scouring process (using ordinary offset printing machine water, ink fountain) special glazing machine polishing process, on-line coating process. Waterborne varnish can eliminate the harm to the human body and the pollution brought to the environment during operation, and has been paid more and more attention by food and tobacco printing enterprises.

Compared with solvent-based varnish, water-based varnish has advantages in terms of environmental protection. What are the advantages? The following can be summarized:

1. The transparency is strong, and the printed material can be stored for a long time on the prints it is applied to. In the long-term strong sunlight, the water-based gloss oil is not easy to yellow and does not change color.

2. Strong wear resistance

Packaging type prints require strong abrasion resistance, measured with 50 pressures per square centimeter, generally capable of reaching 150 wipes per minute. Brightness generally can reach 50-70 brightness units, high-grade light can exceed 80, non-absorbing The substrate can reach 90, especially cigarette packs, and at high speeds with speeds of up to 300-500 packs/minute, the cigarette labels are subjected to a large mechanical load. For this reason, printed products not only need to be glazed, but also wear-resistant, while water-based coating oils can fully meet this requirement.

3. Conjunctival speed

Dry quickly, water-based paint drying process 30% by evaporation, 70% by the material absorption, drying quickly, about a few seconds to complete. When the varnish still contains 20% -30% of moisture, it can be conjunctiva. Therefore, it does not need any drying method. However, if infrared or hot air drying methods are used, the drying speed can be increased.

4. Easy to use and store, safe, safe to produce and improve labor conditions

Due to the fact that solvent-based coatings are dangerous goods, transportation, storage and use are quite inconvenient. The waterborne varnish uses water as the solvent without any danger. It is safe and reliable to use and can be cleaned with water.

5. Non-toxic and odorless Because water-based varnish uses water as a solvent and a small amount of volatile edible ethanol as an auxiliary solvent, it is particularly suitable for use on food and tobacco packaging. Tobacco is a very easy-to-smell plant, and the application of water-based gloss oil fundamentally eliminates the possibility of odor.

6. High temperature resistance, good heat sealing performance

At present, cigarette packs are widely used as outer packagings of PP film. Waterborne varnish has good heat sealing properties, and even with Celluloid, good heat sealing effect can be obtained.

7. Good finished product flatness, strong anti-rolling force

One of the headaches in gravure printing is the continuous drying of the printed material through several color groups. The moisture content of the paper is extremely low, and it is particularly prone to curling, so that the tobacco factory cannot use it. 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.

8. Reasonable price

At present, the market price of water-based varnish is RMB 26 (including tax) per kilogram, solvent-based varnish is RMB 10 (per tax), but the coating amount of varnish is only 30% of solvent-based varnish. %, so the actual cost is about 80% of solvent-based varnish.

In addition to the above advantages, the use of waterborne varnish on gravure printing machines is more convenient than that of general solvent varnish. It is mainly to control the viscosity of the varnish, the depth of the anilox roller and the temperature of the oven. According to our experience, the viscosity of waterborne coating is generally controlled at about 15 seconds (measured at 4° DIN cup at 20°C). The anilox roll depth is 20 μm, 200 l/in: oven temperature 120°C (100kw x 2) at 150m Under the conditions of /min coating, the coating amount is 5g/m2, and the coating oil is instantaneously dried and can be directly hot stamped and die cut.

Waterborne varnish differs from oily coatings in the way it is used on offset presses. Oily glazing can use the original printing unit, ink fountain, printing directly using the PS version of the glazing. The disadvantage of this glazing method is that the brightness is not enough, the fastness is not strong, easy to fade, and wet wet glazing, especially the full version of the field printing. On the offset printing line, water is coated on the surface by applying the coating method. It is different from the oil offset printing on the original offset printing set. It uses the principle of gravure ink transport, through the metal anilox roller and squeegee, transfer the varnish to the blanket or resin plate, and finally coated on the substrate. Under normal circumstances, the viscosity of the coating oil is about 20 seconds (measured at 3#ZAHN measuring cup 20°C), the electric engraving depth of the metal anilox roll is 23 μm, 150l/m2, the coating amount is 5g/m2, and the temperature is 65 °C, infrared power is 5.4kw x 5 x 2 groups, the total power is about 54Kw, the machine operating speed is 13000 / hour, instant dry, not dusting, can immediately bronzing, die-cutting, but to pay special attention is Even under the wet and wet conditions of the machine, the ink may not be fully dried, and it may take about 4 hours to dry naturally before it is bronzing.

In summary, the waterborne varnish has the advantages of advanced technology, convenient use, economical application, non-toxic and non-polluting, etc. It has a wide range of development prospects and will gradually replace solvent varnish and oil varnish, occupying in the printing industry. leading position.

Third, the composition of waterborne varnish

Waterborne varnish mainly consists of three main categories: main agent, solvent and auxiliary agent.

1. Main 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.

2. Additives

Auxiliaries are used to improve the physical and chemical properties and processing characteristics of aqueous polishes. The types of additives are: 1 Curing agent: Improves the film forming property of the aqueous main agent and increases 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 to the substrate.
6 wetting and dispersing agent: to improve the dispersion of the main agent to prevent dirt and improve wear resistance.
7 Other additives are plasticizers that can improve folding resistance.

3. Solvent

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. Water is colorless, odorless, non-toxic, widely sourced, and inexpensive. The volatility of water is almost zero. Its leveling performance is very good. However, water as a solvent for aqueous coating oils also has disadvantages such as a slow drying rate and a process defect that easily causes product dimensional instability. Therefore, ethanol is added during use to improve the drying performance of the aqueous solvent and improve the processing suitability of the aqueous coating oil.

Fourth, the application of water-based polish

With the rapid and steady development of the market economy and commodity economy, people pay more and more attention to the printing and decoration of books, magazines, covers, calendars, pictures, medicine boxes, and cigarette packs. Not only does the printing require exquisite color images, but also it is magnificent. The surface gloss, which is the special effect produced by applying varnish on the printed material. Applying varnishes on prints not only increases the value of printed products, but also enhances the lightfastness of inks, increases the heat and moisture resistance of inks, protects the imprints, improves the performance of printed products, and brings considerable benefits to printing houses. Economic benefits.

Coating glazing market is a diversified market. All kinds of books and periodicals printing, commercial printing, trademark printing and folding carton printing plants have various degrees of glazing requirements for various packaging products. The application of waterborne varnishes must be based on different glazing methods, paper types, product quality requirements and other factors in order to achieve good results.

Fifth, several issues that affect the quality of glazing and should pay attention to

The main issues affecting the quality of coating are paper properties, paper type, viscosity, coating weight, drying conditions, and the like.

1. Paper performance

The effect of paper on coating quality is mainly reflected in the effect of paper smoothness on coating quality. High-smoothness papers have a significant effect upon polishing, while papers or paperboards with low smoothness have a poor effect after polishing, since the varnish is almost completely absorbed by rough paper or paperboard. In order to solve this problem, you can use the base oil before the last light, or twice the light.
2. 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 moisture content of paper, the time and method of drying, and the control of viscosity. If the printed product is under 90g/m2 paper, please use water-based coating carefully.

3. Varnish viscosity and solids content

In the coating process, the viscosity and solids content of the coating oil must be properly controlled. Therefore, the dilution can only be performed within a certain range of viscosity and solid content. Water-based varnishes generally use ethanol and water as the diluent, and the ratio is 1:1. The dilution amount is 10% to 30%. It should not be too large and should be strictly controlled.

4. Coating amount

The coating of waterborne coating should be controlled at 4-6g/m2. Because the evaporation speed of ethanol is very fast in coating the varnish, the control of the viscosity is very important. Ethanol content must also be increased in time to ensure product stability.

5. Dry conditions

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 glazing oil, the drying time will be slower because the inside of the paper has absorbed more ink and moist water. Must use infrared drying device. During the drying process, the coating oil is heated, the bottom layer penetrates and dries quickly, and at the same time, the moisture in the coating oil is heated and evaporated by the high temperature, and is discharged through the exhaust pipe.

Another way to increase the drying rate is to increase the ethanol content, but depending on the actual situation, depending on the viscosity of the coating oil, it cannot be arbitrarily adjusted.

Sixth, concluding remarks

With the development of society, people’s awareness of the protection of the living environment has also increased. For the printing industry, it has gradually increased many requirements and restrictions on hygiene, environmental protection, and labor protection. In particular, the requirements for food packaging and printing have become more stringent. Only in this way can the requirements of international health standards be met. Therefore, it is imperative to develop and apply pollution-free, pollution-free “green” materials.

Waterborne varnish has the characteristics of non-toxic, odorless, high glossiness, and easy to use. In recent years, it has been widely used in the field of packaging and printing, and has achieved a rising trend.