On-line water-based coating full contact

Due to the deteriorating global environment, environmental protection has become a major issue that affects the survival and development of the entire human race. Printed matter, which is one of the products that penetrate the most extensive human lives, is naturally taken care of by everyone.

As a new type of environmentally friendly finishing material, waterborne varnish has become the focus of research and development in developed countries in recent years. On-line coating has been widely adopted because of its high production efficiency. Then, how does the environmental protection feature of waterborne coating be reflected, and what deficiencies exist in it? How to apply the maximum advantage of online water-based coating? In this connection, we interviewed Mr. Du Haoxin, Production Director of Jinan Tianyi Printing Co., Ltd., Guo Jian, Director of Beijing Shuangyan Color Printing Factory, Ms. Feng Baoping of Baoji Jinyan Group, and Zhang Jianhua, Manager of Qingzhou Xinhua Packaging Products Co., Ltd. They asked them to talk about water-based glazing according to their own production practices and discussed the application of on-line water-based coating. Here, we have compiled their opinions and suggestions for your reference. Thank you very much for your great support.

Waterborne gloss environmental protection analysis

1. Waterborne Varnish is safe and reliable The main raw materials of waterborne Varnish are waterborne acrylic resin and waterborne acrylic emulsion; the additives are mainly defoamers and filming aids, all of which are free of organic volatiles; the main solvent is water, harmful The material has zero volatility and good leveling performance. It is a true “green material”. However, the drying speed of waterborne varnish is slow, so many companies currently use a small amount of volatile edible ethanol as an auxiliary solvent, which not only improves the drying performance of the aqueous solvent, but also improves the processing adaptability of the aqueous varnish. Of course, more importantly, this mixed solvent does not contain solvents such as acids, ethers, ketones, esters, and aromatic hydrocarbons, so waterborne coating oils are basically colorless, odorless, and non-toxic, safe and reliable.

2. Adding no harmful substances to the glazing process The aqueous glazing oil is applied to the surface of the printed matter, and the glazing effect is achieved after drying. This completes the glazing process. Waterborne varnish is water when it is coated and dried. It is harmless to the environment and the human body. Even if the operator comes into contact with the water-based varnish, it can be rinsed with clean water and it will not harm the skin. After glazing, you can wash the plate, wash the car, and wash the tank directly with clean water, without using any other solvent, so as to ensure no pollution to the environment during the glazing process. In addition, the drying equipment of the water-based coating oil is an infrared drying oven, which avoids that the UV coating must be dried with ultraviolet rays of a certain wavelength to generate ozone (O3) and the solvent-based coating oil contains a large amount of harmful substances that volatilize or remain in the printed matter. The disadvantages that cause harm to the environment and the human body.

3. Polished products can be recovered after polishing. During the subsequent storage and use, the varnish is further dried and the conjunctiva is dried. Although the solvent is still volatile in the process, it is volatile for water-based polish products. The solvent is still water. In addition, if the printed matter after use is recycled, the acrylic resin and a small amount of auxiliaries left on the print can be easily decomposed without leaving any harmful substances. For consumers, buy prints that contain water-based polish, and don't worry about taking toxic odors home.

On-line water coating application

In the on-line water-based coating, the coating effect is affected by many factors such as the printing process, printing equipment, printing materials, printing speed, and product use. It is not easy to obtain perfect results. The following is a summary of the current problems encountered with on-line waterborne coating and the solutions to the problem after interviewing industry experts.

1. Effect of printing process on on-line water-based coating

Water-based coating oils have a wide range of applications and can be used for online operations such as offset printing, flexo printing, and gravure printing. The effect of different printing methods on the glazing effect is mainly reflected in the speed of the glazing and the thickness of the glazing oil, and the printing method has the same effect on the printing speed and the ink film thickness. There are many kinds of water-based coating oils on the market, and different water varnishes with different viscosities and different drying speeds should be used for different printing methods.

Flexographic on-line glazing uses an anilox roller-squeegee short ink inking system that requires a viscosity of 50 to 60 s (painted 4 # cups) and a drying speed of 8 to 10 m/s. The thickness of the coating film is usually controlled by adjusting the number of screen lines of the anilox roller. Since the anilox roller has uniform ink transfer, a flexo coating is used to make the coating layer more uniform.

In the on-line offset coating, the water-based coating oil is liable to be incompatible with the ink. Therefore, in the selection of the resin in the aqueous coating oil, the varnish should be similar to the offset ink or have similar compositions. Offset printing ink systems have a plurality of uniform ink rollers, requiring the drying speed of waterborne coating oil to be relatively slow, otherwise it will dry on the surface of the rubber roller and cannot print normally. In addition, the printing speed of the offset press is much slower than the flexo press, which makes the problem of the drying speed of the aqueous coating oil more prominent.

The gravure printing machine uses an anilox roller-squeegee short ink inking system, but the anilox roller used in the gravure printing machine has a lower number of lines and is rougher than that used in the flexo printing. In order to avoid the coating oil layer being too thick or dry, an aqueous coating with a low viscosity and a fast drying speed should generally be used.

In short, the viscosity of waterborne varnish is relatively low, and there are some special requirements for the ink supply system of printing equipment. Generally, the ink supply systems of flexographic and gravure printing machines are more suitable for on-line water-based coating, while the ink supply systems of offset printing machines are relatively unsuitable for water-based coating, and water-based coating oils with suitable viscosity must be used.

2. The effect of printing process on on-line water-based coating

In the printing process, aqueous coatings are usually used in two process steps. One is to base oil for the substrate before printing to increase its surface smoothness, gloss, prevent fuzzing, and improve printing quality. The other is water-based coating after the printing of yellow, magenta, cyan, black and spot colors to improve the surface gloss and wear resistance of printed products, and the solvent resistance and scratch resistance of printed products are also improved. As the semi-finished products after water-based coating are printed with other colors, the ink is difficult, and even cannot be left on the prints. Therefore, the water-based coating oil is always arranged for the final coating. In addition, the glazing of the printing surface is more conducive to hot stamping and other post-press processing, generally using the following process: printing → water-based coating → hot stamping → die-cutting.

3. Influence of printing materials

Waterborne varnish has a wide range of applications and can be used on paper, aluminum foil, aluminized paper, gold/silver paperboard, and the like. The type and quality of printing materials have a great influence on the glazing effect.

For paper, on the one hand, the absorption of different papers differs greatly. Even if the same coating oil is used, the coating effect will be different. Therefore, according to the different absorption of paper, choose different viscosity and solid content of Varnish. On the other hand, the smoothness of the paper surface also has a direct effect on the gloss effect, especially on the brightness of the gloss. Paper with a high surface smoothness has a good effect after glazing, and has a higher lightness, whereas it has a lower lightness. For example, when the front surface of ordinary white cardboard is lighted, the effect is good; when it is reversed, light effects are basically not seen.

4. Effect of printing speed on on-line water coating

When the printing speed is too slow, the water-based coating oil will dry on the surface of the printing plate, resulting in a paste plate and a dirty plate. In this case, a small amount of water and glycerol should be added to the water-based coating for humidity conditioning to slow the drying speed. When the printing speed is too fast, the water-based coating on the prints is too late to dry, and the prints are stacked together, causing the back of the print to stick. In this case, an appropriate amount of ethanol and isopropanol should be added to the aqueous coating oil to adjust the drying speed.

5. Effect of product use

Waterborne gloss products have a soft, delicate, bright, bright colors and other characteristics on the sensory and visual aspects. At the same time, water-based coating can also improve the brightness and metallic feeling of gold and silver on the product, but the water-resistance is worse than that of other varnish oils. Varnish should be selected according to different product applications. Hard pack cigarette packs printed on white paper or white cardboard require a long drying track on the cigarette packer, which requires a high abrasion resistance of the waterborne coating. The use of coated paper printed soft box cigarette labels, there is a high requirement for the smoothness of water-based coating oil, which has a lot to do with the type of cigarette packaging equipment used by cigarette factories. Wine labels, especially beer labels, have high requirements for the water resistance, alcohol resistance, and friction resistance of waterborne coating oils due to the need for refrigeration and transportation of beer. In addition to cigarette packs and wine labels, water-based polishes are also widely used in medicine boxes and food packaging. In short, according to the different uses of the product, after the polishing of the product in the increase in gloss, improve wear resistance and safety and environmental protection at the same time, with anti-oil, anti-alkali, high temperature, resistance to baking, anti-solvent and other properties.

In the domestic market, the prospects for the development of water-based varnishes are unanimously optimistic. We believe that with the improvement of technology and the enhancement of environmental awareness among the general public, the environmental protection department, together with printing companies and equipment suppliers, will be able to achieve a more healthy and rapid development of the water-based glazing technology in China and make it more suitable for the printing industry in China. Big contribution.



Source: Ke Yin Media - "Printing Technology"