Classification, Composition and Application of Waterborne Varnish

Varnish is a colorless, transparent or translucent ink. It can increase the gloss of printing, protect the surface of printed matter, improve the abrasion resistance, chemical resistance, and water resistance of printed products, and prevent the printed products from getting dirty or sticking.

At present, UV glazing and solvent-based glazing are commonly used, but the use of water-based lacquers is rare. Varnish is mainly solvent and waterborne. Solvent type is generally based on polyurethane, SBS (styrene-butadiene-styrene block copolymer), EVA (ethylene-vinyl acetate copolymer), polyacrylate as the main resin, with toluene, ethyl acetate, gasoline Such as solvents, supplemented by tackifying resin, plasticizers, antioxidants and other additives. Due to the volatilization of organic solvents, there are disadvantages of flammability, toxicity, and environmental pollution, which are no longer the focus of current research and development. Although UV coating has the advantage of fast drying speed, it is expensive. The water-insoluble polymer material overcomes the disadvantages of the solvent-based coating oil being flammable, toxic, high in energy consumption, polluting the environment, and the like, thus becoming a current paper product. The focus of field development research.

Water is the cheapest and non-polluting coating solvent. This waterborne coating has developed rapidly in the 1990s and is generally welcomed in developed countries. For the printing industry, it has gradually increased the requirements and restrictions on hygiene, environmental protection, and labor protection. In particular, the requirements for food packaging and printing are more stringent. Therefore, water-based coating oils also meet the requirements of international health standards.

First, the classification of waterborne polish

1. Traditional water-based polish. The main agent of the traditional water-based coating oil is a high molecular polymer dissolved in water or suspended in water. Its water content reaches 50% to 70%. Due to the excessive moisture content, the glazing effect is not ideal, and there is a problem of slow drying and uneven flow.

2. Modern new waterborne varnish. The addition of human auxiliaries (mainly surfactants) to traditional waterborne varnishes has resulted in the formation of modern new waterborne varnishes. Among them, 80% water is replaced by ethylene glycol or propylene glycol, and its appearance is milky white. By adjusting the solid content in the glazing oil, different glazing effects such as high gloss, ordinary gloss, and sheen can be obtained.

The new type of waterborne varnish is in line with the requirements of hygienic and environmental protection, and its application is more and more. It is widely used in the packaging of food, medicine and other products, and the use of waterborne varnish can be recycled through biodegradation.

3. Catalytic waterborne varnish. Catalytic waterborne coatings are typically used in web presses with heating devices. It is thermoset and does not re-plasticize when heated, so it can only be calendered using on-line calendering.

Catalytic water-based polishes contain free formaldehyde, which is a carcinogen and harmful to human health. Therefore, from the perspective of hygiene and environmental protection, catalytic water-based coating oils can only be used for prints that do not require high hygiene and environmental protection.

Second, the composition and application of waterborne varnish

Water-based coating oil is mainly composed of three main components: a main agent, a solvent, and an auxiliary agent.

1. The main agent. The main agent of waterborne coating is a film-forming resin, usually a synthetic resin, which influences and dominates the physical properties of the deep layers and the glazing qualities of the film, such as gloss, adhesion, and drying properties.

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 its copolymer resins depend on the composition of the monomers and the synthesis process. Waterborne acrylic copolymer resin has a series of excellent properties such as good film forming property, good glossiness, high transparency, fast drying speed, abrasion resistance and water resistance, and is widely used in waterborne coating oils.

2. Additives. Additives are used to improve the physicochemical properties and processing characteristics of aqueous coatings. Frequently used additives are: curing agents, surfactants, defoamers, desiccants, adhesion promoters, wetting and dispersing agents, and other additives.

There are many types of additives, depending on the type of topcoat. However, the amount of all types of additives should not exceed 5% of the total amount, otherwise it will affect the processing flexibility of coating oil.

3. Solvents. The main role of the solvent is to disperse or dissolve synthetic resins and various additives.

The solvent of water-based coating oil is mainly water, colorless, odorless, non-toxic, with a wide range of sources, low price, almost zero volatility, and very good leveling property. However, there are also deficiencies, such as slow drying speed, easily causing product size instability, etc., therefore, in the use of appropriate amounts of coconut ethanol, in order to improve the drying performance of aqueous solvents, improve the processing of waterborne coating adaptability.

The application of waterborne varnish must be based on different glazing methods, paper types, product quality requirements and other comprehensive considerations to achieve good results. When using it, pay attention:

1 In the coating process, the viscosity and solid content of the coating oil must be reasonably controlled. Dilution can only be performed within a certain solid content range.

2 When selecting the paper type, consider the thickness of the thick paper, and the stability of the paper is poor.

3 Control of Coating Amount It is important to note that the coating amount of waterborne coating is not easy to control, so the control of the viscosity is very important.

4 In the use of water-based coating can completely eliminate or reduce the use of anti-adhesive dirty agent, because the water-based coating is completely cured, the surface is very smooth, very little sticky.

5 The control of drying time should make full use of the osmotic drying of the water-based coating oil. Another way to increase the drying rate is to increase the ethanol content, but it depends on the actual situation and the viscosity of the coating oil, and it cannot be arbitrarily adjusted.

In short, whether they are water-based varnishes or other water-based materials, they are not ideal for the printing of non-absorbent substrates (printability is better on paper with good absorbency), and they need to be further improved. And perfect. (Text / Yu Ruixiang)