UV metal ink

3.12.1 Metal packaging printing

Metal packaging materials, as an important packaging material in the packaging field, have many advantages over other packaging materials, such as recyclability, good protection of the contents, diverse appearance and shapes, and bright colors. They have great potential for development and are recognized by consumers. Nowadays, the green environmental protection trend has swept into the packaging and printing industry, and “green packaging” has become a hot topic within the printing industry and one of the development trends of printing industry process technology. The metal packaging printing industry’s high energy consumption and large exhaust emissions during the production process have become important factors restricting the development and growth of metal packaging companies, and have also set obstacles for the green development of metal packaging.

In recent years, the UV printing process has become increasingly popular in the metal packaging printing industry due to its obvious advantages in energy conservation and environmental protection. Coupled with its huge cost advantages, it has become an innovative way of energy conservation and environmental protection, and is increasingly sought after by metal packaging companies.

3.12.1.1 Traditional tinplate printing process

Tinplate is coated on the inside and outside and then ready for colour printing. Offset printing is generally used for tinplate printing. Tinplate has a smooth surface and is not absorbent, which is very different from paper. Therefore, heat-setting inks are used for printing on tinplate, which require high-temperature drying. In other words, the printing process requires the use of a special drying device to dry the ink. The drying temperature is usually about 150°C, and the time is controlled at 10 to 12 minutes. At present, the domestic tinplate printing industry mostly uses tunnel ovens (hereinafter referred to as drying rooms) to dry the ink. The drying room is about 30 meters long and 6 meters high, and is connected to the back end of the printing press to dry the printed products. In the traditional tinplate printing process, no matter how many printing passes are required to complete a product, after each printing pass is complete, the printed sheet needs to pass through the drying oven to dry the ink. Each printed product must pass through the drying oven multiple times, which not only consumes a lot of energy, but also emits a lot of VOCs. Therefore, many companies have begun to consider using other methods to replace the traditional heating and curing method, and UV curing has stood out due to its advantages of being highly efficient and energy-saving.

3.12.1.2 UV tinplate printing process

The technology of applying UV in the printing process is to use UV ink to quickly cure under ultraviolet light, which has excellent physical and chemical properties and a high surface brightness. Because the ink in the UV printing process can dry quickly under ultraviolet light, after the adoption of UV technology, each printing unit is equipped with a UV drying device, which is responsible for promptly drying each color of ink. The tunnel oven part of the traditional equipment is no longer needed. Compared with the traditional printing process, the main advantages of the UV printing process are: fast curing speed, short curing time, no need for an oven, which not only improves production efficiency and saves energy, but also reduces VOC emissions and is good for the environment.

3.12.2 Preparation of UV metal inks

UV metal inks are light-curing inks that can be directly printed on the surface of metal materials (including metal substrates with surface treatments and metal materials with surface finishes). Commonly used metal materials in printing include copper, aluminum, iron, stainless steel, and mirror-finished titanium plates, as well as metal materials with a surface treatment such as anodized porous aluminum plates, iron phosphating plates, galvanized iron sheets, nickel-plated iron, and chromium-plated iron, and metal materials with a surface finish such as metal sheets coated with powder paint or baked enamel.

Different metals have different surface properties, and the type of UV ink used should also be different, otherwise there may be problems such as poor adhesion and brittle cracking of the ink layer when the metal bends.

UV metal inks are divided into the following types: general metal UV inks, special metal UV inks, elastic UV metal inks, high-temperature resistant UV metal inks, UV metal inks with special decorative effects, UV anti-corrosion inks for metal etching, and UV metal varnish series.

Each UV metallic ink has an optimal printing color sequence. The light curing speed of UV inks of different colors varies, with some curing slowly and others quickly. It is not possible to print any color first, as you can with self-drying solvent inks. Screen printing UV metallic inks, especially when printing in multiple colors, generally follows the principle of printing dark colors first and light colors last.

UV metallic inks of different colors have an optimal curing order. The curing order of UV metallic inks is:

gold, silver → black → blue → red → yellow → colorless transparent varnish

Dark inks require more UV energy, dry more slowly, and UV light does not easily penetrate the ink layer, making it more difficult for the underlying layer to cure. Therefore, dark inks must be printed first; light inks cure easily and only require one light exposure. If light inks are printed first, the light-colored ink will inevitably be over-cured, the ink layer will become brittle, and the adhesion will be poor, while the dark ink layer will not be cured enough, the surface hardness will be low, and the wear resistance and solvent resistance will be poor. UV metallic inks can be cured immediately after printing, and the ink layer is cured once after each color is printed. When the second color ink is cured, the first color ink has already been exposed to light twice. If it is a four-color pattern, when the fourth color ink is cured, the underlying ink has already been exposed to light and cured four times.

Fresh metal surfaces have a high surface free energy (500 to 5000 mN/m), which is much higher than that of organic polymer materials (<100 mN/m). This high surface free energy is very beneficial for ink adhesion. In fact, many metals are prone to oxidation in the air, forming an oxide film on the surface, which reduces the surface free energy and affects the adhesion of the ink. However, the surface free energy of most metal oxide films is still higher than that of UV inks, so UV inks have a good wetting effect on metal substrates. However, a common problem with UV inks applied to metal substrates is that the adhesion of the ink to the metal is not good. Without the addition of an adhesion-promoting additive, it is difficult for UV inks to achieve ideal adhesion to metal. This may be because the surface of the metal substrate is dense, making it difficult for UV inks to penetrate and absorb. The effective contact interface is small, unlike paper and wood, which have rough surfaces with pores, and plastics, which can be swollen by oil to form a permeable anchoring structure. In addition, because UV inks cure quickly, the internal stress caused by volume shrinkage cannot be released, and the reaction acts on the adhesion of the ink layer to the metal substrate, reducing adhesion. Metal surfaces are often easily contaminated with grease, which is also not conducive to coating adhesion and metal corrosion protection.

In order to achieve good adhesion, corrosion protection and a clean surface on metal surfaces, cleaning, physical treatment and chemical treatment are usually carried out before printing with ink. The easiest way to clean is to wipe the metal surface with a cotton cloth soaked in solvent, or to immerse the metal parts directly in the solvent for washing. A more effective method is vapor degreasing, which involves hanging the metal parts on a conveyor and transporting them over a boiling halogenated solvent in a tank, so that the solvent condenses on the surface of the metal parts and dissolves the grease, thus achieving the purpose of cleaning. Physical treatment, such as sandblasting the metal surface, removes the corroded surface and forms a new rough surface. This is mainly used for some crude industrial parts, such as bridges, tanks, etc. In addition, there are vacuum alumina blasting, steel grit or water-soluble adhesive cleaning, plastic pellet blasting, and sometimes high-pressure water blasting are also used for surface cleaning. Chemical treatment commonly involves the use of phosphoric acid or phosphate to gently etch the metal surface with an acid, forming a layer of iron/ferrous phosphate salts of a certain form to improve the adhesion of the coating, but the corrosion resistance is only slightly improved. The treated metal surface must be thoroughly cleaned to remove soluble salts. Aluminium surfaces are covered with a thin, dense layer of aluminium oxide, so generally only the surface needs to be cleaned.

The core problem with UV metal inks is also to solve the adhesion between the ink layer and the metal. Oligomers and reactive diluents in the ink formulation can form hydrogen bonds or chemical bonds with the metal surface, which can greatly improve the adhesion between the coating and the metal. Generally speaking, oligomers and reactive diluents containing carboxyl groups and hydroxyl groups, especially those containing carboxyl groups, have a more significant effect on metal substrates and and have a significant effect on improving adhesion (Table 3-48). At the same time, the use of oligomers and reactive diluents with low volume shrinkage also helps to improve adhesion. Some reactive diluents have a certain permeability to metals, which also helps to improve adhesion (see Table 3-49).

Table 3-48 Effect of carboxyl-containing monomers on the adhesion of UV inks to metal

Table 3-49: Reactive diluents that are easily permeable on metal substrates

Adding adhesion promoters is an important means of improving the adhesion of UV metal inks. Commonly used are resins with carboxyl groups, acrylates containing carboxyl groups, acrylate phosphates, siloxane coupling agents, titanate coupling agents, etc. Mercaptans cannot be used because they are too smelly, but they have a strong effect on gold surfaces, which are extremely inert. For metal adhesion promoters suitable for UV metal inks, see Table 3-50. Acidic monomers or resins contain acidic groups that can slightly corrode metal surfaces and form complexes with surface metal atoms or ions, strengthening the adhesion between the ink layer and the metal surface. Generally, the amount of phosphate ester adhesion promoters in the formula is low, not exceeding 1%. Silicone coupling agents promote adhesion to metal substrates because after hydrolysis, they can condense with the oxides or hydroxyl groups on the metal surface to form an interfacial chemical bond and improve adhesion. Suitable silicone coupling agents include KH550, KH560, KH570 and some silicone-modified UV resins. Titanate coupling agents are used in UV metal inks to improve adhesion to metal substrates. Suitable titanate coupling agents include tetraisooctyl titanate, tetraisopropyl titanate, and n-butyl titanate.

Table 3-50 Adhesion promoters for UV metal inks

Compared to radical photopolymerization systems, cationic photopolymerization inks are more likely to achieve good adhesion on metal. Cationic curing has low shrinkage and a large number of ether bonds formed after polymerization that can act on the metal surface, all of which can improve adhesion. However, the super-strong protonic acid produced by the photolysis of cationic photoinitiators not only initiates cationic polymerization and cross-linking, but also corrodes the metal substrate, which is obviously harmful to coating adhesion and does not help to improve adhesion. Only by reducing the concentration of the cationic photoinitiator can adhesion be improved. In addition, the commonly used cationic photoinitiators, such as thiourea salts or iodide salts, have ultraviolet absorption <300nm, which is not compatible with UV light sources. Their photoinitiation efficiency is extremely low. A small amount of a free radical photoinitiator such as ITX must be added, which can absorb light energy in the long-wave region of the violet spectrum and transfer energy to the thiourea salt, indirectly exciting the photoinitiator and improving the photoinitiation efficiency.

Since the binder of UV printing inks consists of unsaturated acrylic monomers or prepolymers, it has different solubility properties from the binder of traditional heat-curing inks (mainly alkyds). Unsaturated acrylic monomers are highly aggressive, causing the synthetic rubber in the rollers and blankets to expand and damage the photosensitive layer on the surface of the PS printing plate, causing the image to peel off. Therefore, when printing with UV printing inks, it is necessary to use rollers, blankets and wash water specially designed for UV printing inks. The PS plate must be baked at high temperature to enhance the corrosion resistance of the image layer.

3.12.3 UV metal etching ink

Metal etching is a technical means of using chemical treatment (chemical etching, chemical sanding) or mechanical treatment (mechanical sandblasting, embossing, etc.) to process a shiny metal surface into a concave and convex rough crystal surface. Light scattering produces a special visual effect, giving the product a unique artistic style. As a precise and scientific chemical processing technology, chemical etching is widely used on a variety of metal materials. The key to etching metal materials is twofold: protect the part that needs to be etched; and completely etch away the part that does not need to be etched, so as to obtain the desired image.

It is classified according to the type of chemical reaction during etching:

① Chemical etching. Process: pre-etching → etching → rinsing → acid dipping → rinsing → resist stripping → rinsing → drying.

② Electrolytic etching. Process: loading → power on → etching → rinsing → acid dipping → rinsing → resist stripping → rinsing → drying.

Chemical etching can be classified according to the type of material to be etched as follows:

① Copper etching. The process: clean the surface of the polished or brushed copper plate → screen print UV-resistant resist ink → UV curing → etching → rinsing → remove screen printed resist ink layer → rinsing → post-treatment → drying → finished product.

In this process, UV-resistant ink is used to directly screen print the image, so as to protect the desired part from corrosion. The unprinted part is etched away during etching. Therefore, the UV-resistant ink used requires strong adhesion to the metal, acid (or alkali) resistance, and electroplating resistance.

② Stainless steel etching. Process: cleaning the surface of the plate → screen printing liquid photoresist ink → drying → exposing with a film → developing → washing → drying → inspecting and repairing the plate → curing the film → etching → removing the protective layer → washing → post-treatment → drying → finished product.

This process involves coating the plate with photopolymerisable resist ink, exposing it to light, developing to form a resist pattern, and then etching.

Spraying, brushing, rolling, or dipping can be used to apply a uniform layer of photolithographic resist ink to the metal surface to form a photosensitive film. However, for flat surfaces of a small size, screen printing is the most convenient and reliable method. Photolithographic resist inks also require strong adhesion to the metal, acid (or alkali) resistance, and resistance to electroplating.

For the preparation of UV-resistant and photoimageable resists, see Chapter 4 on PCB inks.