Study on Polishing Technology of Water-based Paint

Varnishing is a method of coating (or spraying, printing) a colorless and transparent coating on the surface of the printed matter, and after leveling and drying (calendering), a thin and uniform transparent bright layer is formed on the surface of the printed matter. Printing glazing can improve and improve the appearance, performance and protection of printed matter, and is an important means to improve printing quality and product quality.

Among the several glazing processes commonly used in China, waterborne coating glazing is a recognized environmentally friendly glazing method. It is non-toxic and non-polluting, and is particularly suitable for food, medicine, beverages, tobacco and alcohol, and packaging and printing products with strict hygiene requirements .

1. The basic composition of water-based glazing paint

Water-based glazing paint (water-based glazing oil), also known as dispersing glazing agent, is colorless, odorless, strong in transparency and non-toxic, free of organic volatiles, low cost, wide source of materials, good gloss and resistance Foldability, wear resistance and chemical resistance characteristics. Aqueous varnish is mainly composed of main ingredients, solvents, additives and other ingredients.

1. Main agent. The main agent of water-based glazing oil is water-based film-forming resin, which is the most important component of water-based glazing oil and is the film-forming substance of glazing agent. It affects and dominates the various physical properties of the coating and the glazing quality of the film, including gloss, wear resistance, adhesion, flexibility and drying properties.

There are many types of water-based film-forming resins, including natural resins, synthetic resins and modified resins, but the most commonly used at home and abroad are acrylic resin copolymer resins. The performance of the waterborne acrylic copolymer resin depends on the composition of the monomer and the synthesis process of the monomer. The 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 waterborne glazing coatings.

2. Solvent. The main role of the solvent is to disperse and dissolve the film-forming resin and additives. The main solvent of water-based varnish is water. Compared with ordinary organic solvents, the biggest advantages of water are colorless, odorless, non-combustible, wide source, low price, good leveling, and no irritation to human and skin contact. The heat of vaporization of water is relatively high, the volatility is lower than that of general organic solvents, and the surface tension of water is high, which results in poor wettability to the coated substrate. Therefore, some ethanol and a small amount of auxiliary solvent are often added to the solvent to accelerate drying Speed, reduce surface tension, improve processing suitability of water-based glaze. Water freezes below 0 ° C, so water-based varnishes are best stored and used at around 20 ° C.

3. Additives. Adding additives is mainly to improve the coating process performance of water-based glazing oil and the film-forming performance after glazing. The additives mainly include the following: (1) Surfactants. Reduce the surface tension of the aqueous solvent, improve the leveling, and improve the wetting ability of the aqueous varnish on the surface of the printed matter. (2) Booster. Increase the solubility of water-based resin in water, adjust the viscosity of the resin solution, improve the stability of the varnish, improve the leveling and appearance of the coating. (3) Defoamer. It can control the blistering of water-based glazing oil during the production and coating process, and eliminate quality defects such as fish eyes and pinholes. (4) Leveling agent. It helps to reduce the surface tension, improve the wetting ability with the substrate, improve the fluidity, extend the leveling time, and make the coating smooth and smooth. (5) Desiccant. Increase the drying speed of water-based polishing agent, improve the suitability of paper printing. (6) Other additives. Such as slip agent, adhesion promoter, wetting and dispersing agent, plasticizer, etc. There are many types of water-based additives, which need to be added selectively according to the type of water-based glazing oil and the needs of glazing products. The amount of various additives should not exceed 5% of the total amount, otherwise it will affect the Processing suitability.

Second, the characteristics of water-based glazing

1. The biggest advantage of water-based glazing is that it is non-toxic, non-polluting and has no residual odor, which meets environmental protection requirements. The water-based glazing oil uses water as a medium, which reduces the emission of organic volatiles, greatly reduces the environmental pollution of organic solvents, improves the glazing working environment, and benefits the health of employees. Water-based glazing can completely eliminate the harm of toxic and hazardous substances in solvent-based varnish to human body and pollution to packaged goods, and can also prevent fire hazards caused by static electricity and flammable solvents, and reduce the residual odor of solvents on the surface of printed products.

2. Water-based glazing oil has excellent surface properties. Water-based glazing oil is not only safe and environmentally friendly, but its main performance has reached or exceeded the level of traditional solvent-based glazing coatings. Because water-based varnish is a product with low viscosity and high solid content, it has good gloss and fluidity, and after drying and forming a film, it has good water resistance, adhesion, smoothness and wear resistance. Has good heat sealing performance.

3. The water-based glazing oil has fast drying speed and high production efficiency. The water-based varnish of the new acrylate system can generally achieve a drying speed of about 200m / min, or even higher. The water-based glazing oil can be coated with an infrared drying coating glazing machine, or a high-speed flexo printing machine, gravure printing machine, or offset printing machine can be used for online glazing.

4. The coating process is not easy to cause the ink to change color. Water-based glazing oil has a wide and good affinity with most inks, and can be wet-wet glazed without completely drying the ink. The glazing oil usually does not dissolve the ink, so it is not easy to cause ink Discoloration.

5. The cost of water-based varnish is low and easy to use. Water-based varnish can be diluted and cleaned with water, which is very convenient and has a low overall cost.

6. Water-based glazing can adapt to a variety of substrates. Waterborne glazing can be used for glazing of paper, cardboard, aluminum foil, gold and silver cardboard and vacuum aluminized paper. It can also be applied to polyolefin (such as PE, PP, PVC, etc.) and coated with polyolefin. The paper surface of the material is glazed. The water-based glazed printing products can be stored for a long time. Under long-term strong sunlight, the water-based glazed ink is not easy to yellow and change color.

7. Problems with water-based glazing. Low-gram weight paper is easy to cause dimensional deformation; when applying, it is necessary to grasp the indicators such as paint viscosity, printing speed, coating amount and drying time; offset printing online, glazing, pay attention to the principle of incompatibility of oil and water, Choose the water-based varnish should pay attention to the compatibility with the ink. [next]

3. Some issues that should be paid attention to

The quality of water-based glazing depends on the quality of water-based varnish, the glazing suitability of the printed matter and the control of the coating process. In order to obtain a good glazing effect, attention should be paid to the following issues when water-based glazing.

1. Water-based varnish viscosity control. During the coating process, the viscosity and solid content of the varnish must be reasonably controlled. Therefore, the dilution can only be carried out within a certain solid content range. The thinner of aqueous varnish generally uses a mixture of water and ethanol (1: 1 ratio). The viscosity reduction effect is obvious and should be strictly controlled.

2. The water-based coating adaptability of the printed matter. Paper with rough surface and strong absorption is not suitable for water-based glazing. If necessary, you can apply primer to fill the pores of the paper fiber and then varnish; when using thin paper for glazing, you must choose the type of water-based varnish. And conduct experiments in advance. In addition, although water-based glazing can be wet-wet glazed, it has a wide affinity for most inks. However, in terms of glazing quality, the printing ink is still required to be dried as much as possible before glazing to avoid "gloss" (I.e. gloss loss) and "stress cracking" (i.e. cracks appear on the surface of thick large area ink) and other faults.

3. Control of coating amount. The coating amount of water-based varnish is not easy to control, because the evaporation rate of ethanol in the coating varnish is fast, and the water is colorless and transparent. Apply a layer of transparent water-based liquid on white paper with the naked eye It is difficult to distinguish the amount, so the control of viscosity is very important. The content of ethanol should also be increased in time to ensure the stability of the product.

4. Use of anti-sticking agent. When using water-based varnish, the anti-sticking agent can be completely eliminated or reduced, because the water-based varnish is safe to cure, the surface is smooth, and rarely sticks.

5. Drying time control. In multi-color printing, if the last color is coated with varnish, the drying time will be slower because the paper has absorbed more ink and fountain solution. If infrared water-based glazing oil is used, it can be dried by means of an infrared drying device. During the drying process, the varnish is heated, and the bottom layer quickly penetrates and dries. At the same time, the water in the varnish is heated and evaporated by high temperature and discharged through the exhaust pipe. Another way to increase the drying speed is to increase the ethanol content, but it depends on the actual production conditions and the viscosity of the varnish.

4. Common faults and troubleshooting methods of water-based varnish

In the process of water-based coating, some common faults will occur. We should carefully analyze the causes of the faults and find ways to eliminate the faults, so as to ensure good coating quality.

1. Drying is not good and the surface is sticky.

(1) Causes of failure: ① The water-based varnish is coated too thick; ② The viscosity of the varnish is too high; ③ The drying tunnel temperature and hot air are insufficient; ④ The PH value of the varnish is too high; ⑤ The coating pressure is uneven and the local coating is too thick ; ⑥ Machine speed is too fast, especially when the printing carrier is a non-absorbing surface.

(2) Exclusion methods: ①Adjust to reduce the viscosity of varnish, diluent uses a 1: 1 mixed solution of ethanol and water; ②Appropriately reduce the coating amount; ③Adjust the pressure to make the coating uniform; ④Enhance the temperature and hot air of the drying tunnel ; ⑤ PH value of varnish should generally be controlled at 8-9; ⑥ Adjust machine speed according to product conditions; ⑦ Can add quick-drying emulsion in proper amount; ⑧ Change to use quick-drying water-based varnish.

2. The gloss is not good, the brightness is not enough.

(1) The cause of the failure: ①The paper is too coarse, the penetration and absorption is too strong; ②The coating amount is insufficient, the coating is too thin; ③The gloss of the varnish is low, the solid content is insufficient; ④The ink on the surface of the printed matter is not dry; ⑤Coating Low ambient temperature and high humidity; ⑥ The inherent quality of the varnish is poor.

(2) Exclusion methods: ① Appropriately increase the viscosity of the varnish; ② Increase the amount of coating; ③ The ink should be fully dried before glazing; ④ The paper quality is too coarse, and a layer of primer should be applied first; ⑤ Increase the ambient temperature and Drying temperature; ⑥ Replace good gloss water-based varnish; ⑦ A small amount of leveling additives can be added.

3. Uneven coating on the surface, with streaks and orange peel.

(1) Causes of failure: ① The viscosity of the varnish is excessive; ② The coating amount of the varnish is too large; ③ The coating pressure is not adjusted properly; ④ The surface of the coating roller is too rough and not smooth; ⑤ The varnish dries too fast; ⑥ The varnish Poor leveling; ⑦ The ink does not dry and repels varnish.

(2) Exclusion methods: ①Ink should be fully dried and dust removed before glazing; ②Adjust the coating pressure to make the coating uniform; ③The coating roller should be regrinded or remade if it is rough, aging or deformed; ④Appropriate Reduce the viscosity of the varnish; ⑤Appropriately reduce the coating amount; ⑥Add a small amount of slow-drying additives or PH stabilizers.

4. There are many bubbles during the glazing process.

(1) Causes of failure: â‘  The viscosity of varnish is too high; â‘¡ The PH value of varnish is too low; â‘¢ Excessive circulation stirring; â‘£ Insufficient amount of varnish in the rubber disc and rubber barrel;

(2) Exclusion methods: ①Reduce the viscosity of varnish; ②Appropriately add PH stabilizer to increase the PH value; ③Appropriately add water-based defoamer, mix well, and the maximum amount should not exceed 1%; ④Increase the varnish Supply volume; ⑤ Appropriately reduce the coating speed.

5. Difficult to clean varnish and easy to skin.

(1) Causes of failure: ① The water-based varnish dries too fast; ② The PH value of the varnish is low; ③ The varnish has poor water solubility; ④ The shutdown time is long and it is not cleaned in time; ⑤ The ambient temperature is too high.

(2) Exclusion methods: ①Add slow drying agent to reduce drying speed; ②Add PH stabilizer to increase PH value and improve water solubility of water-based varnish; ③Clean in time when shutting down; ④Temporarily shut down, varnish circulation system should Keep running; ⑤When cleaning is difficult, you can use water detergent or ethanol for cleaning.

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