New Water-based UV Curing Screen Printing Ink and Application (Part 2)

Five, light-cured water-based ink raw materials

1. Photocurable resin

Light-curable aqueous inks generally consist of a base resin, a photoinitiator, additives, and water. The base resin is the main component of the ink system, and the performance of the ink final cured film is mainly determined by the base resin. Commonly used resins include: unsaturated polyesters, urethane acrylates, acrylated polyacrylics, polyester acrylates.

Unsaturated polyesters for light-curing aqueous inks are prepared by esterification of dihydric or polyhydric alcohols with maleic anhydride. In order to make the resin hydrophilic, a dihydric alcohol (such as polyethylene glycol) can be introduced into the hydrophilic structure, and a photoactive can be obtained by capping with a photoactive group-containing acid or alcohol.

Urethane acrylate resin is the most studied system. The only difference from typical urethane acrylates is how to introduce hydrophilic structures in the molecule. Similar to the above unsaturated polyesters, hydrophilic structures can generally be introduced using glycol units. For example, a non-ionic hydrophilic segment can be introduced using polyethylene glycol; a carboxyl group can be introduced using dimethylol propionic acid (DMPA) to obtain an ionic self-emulsifying resin. The photoactive group can be introduced by reacting the hydroxyl group of hydroxyethyl (meth)acrylate (HEA or HEMA) with a polyurethane-terminated isocyanate group (-NCO), and can also be obtained by esterification of acrylic acid with a hydroxy group of a hydroxyl terminated polyurethane.

Acrylated polyacrylic acid has the advantages of low cost, easy preparation, full ink film and good gloss. It is generally possible to introduce hydrophilic carboxyl groups by copolymerizing acrylic acid with various acrylates, and to introduce hydroxyl groups or epoxy groups by copolymerization of hydroxyethyl (meth)acrylate or glycidyl (meth)acrylate to further introduce acryloxy groups, thereby obtaining Photoactivity.

2. Photoinitiator

In addition to the same requirements as the photoinitiator used in the oily system in terms of absorption properties, no yellowing, etc., the photoinitiator used in the aqueous system must also have the requirements of compatibility with the aqueous environment, low volatility, etc. to ensure the ink film Pre-drying (usually at 60-80°C) still maintains the desired concentration. At present, the most used photoinitiator is Darocur 2959. Since it has one more hydroxyethoxy group than Darocur 1173 on the molecule, its solubility in water is relatively large (the solubility of the parent Darocur 1173 is only 0.1%), so it is relatively easy. Volatilize during pre-drying.

Six, water-based UV-curable ink wetting properties of the substrate

In order to ensure the wettability of the water-based UV-curable ink to the substrate material, on the one hand, the surface energy of the plastic surface should be increased by surface modification, and on the other hand, the surface tension of the ink should be minimized when designing the ink formulation. For general UV-curable inks, the surface energy is about 35-38 mN/m, which is obviously too high for low surface energy, surface-untreated polyolefin materials, which can cause printing failures. However, the surface energy of water-based UV-curing inks is much larger than that of general UV-curable inks. At this time, low surface tension monomers should be added to the water-based UV-curable inks, and the plastic surface should be modified to ensure that the ink is on the plastic surface. Wetting properties.

Seven, plastic surface treatment and static control

For some plastic substrates with higher surface energy, better adhesion can be obtained by adjusting the UV curing ink formulation and improving the curing process conditions. However, some plastic products and plastic films (such as PE and PP plastics) have small surface polarities and low surface energy. When printing, they may encounter poor adhesion of ink. Printing effects and adhesion fastness are difficult to meet, and the quality of printing is affected. . Only surface treatment can improve the above problems and improve printing quality.

There are many ways to deal with plastic surfaces:

â–² mechanical method: sandblasting and grinding, etc.;

â–²Physical methods: flame, corona, high-energy radiation, etc.;

â–² Chemical methods: surface oxidation, grafting, displacement and cross-linking.

In the surface treatment, appropriate methods should be selected according to different plastics and process conditions, such as non-polar plastics such as polyolefins such as PE and PP. Flame and corona treatment methods are generally required to increase surface energy, and nylon can be treated with phosphoric acid. . The following describes various surface treatment methods:

1. Flame and Corona (Electro spark) treatment. Flame and corona treatment are two good prepress treatment processes, which can greatly increase the surface energy of plastic sheet and make the surface form a very thin oxide layer.

Flame and corona treatment should pay attention to appropriate, so as not to "burn" the surface, the formation of an excessively thick oxide layer, causing the ink layer together with the oxide layer after printing off. Under normal circumstances, when the flame is treated, the plastic is heated to a temperature slightly lower than the heat distortion temperature and maintained for a certain period of time. These two methods can temporarily increase the surface energy of the plastic. After the treatment, the printing job must be completed within 20 minutes. Otherwise, the treatment effect will drop rapidly.

2. Degreasing treatment. The surface of plastic products stained with oil or the release agent will affect the adhesion of the ink. It can be cleaned by alkaline aqueous solution, surfactant, and solvent to achieve the purpose of surface degreasing and cleaning. If sanded to roughen the surface can also play a role in degreasing.

Plastics, such as polystyrene, which adhere well to screen printing inks, can be mostly degreasing. Degreasing should be done with solvents that do not dissolve the plastic, such as methanol, ethanol, isopropanol, etc. Polyamide plastics can also be wiped with acetone.

3. Chemical surface treatment. Solvent vapors and chemicals are used to treat the plastic surface in order to allow the smooth plastic surface to erode to a controllable uneven surface. It is also possible to oxidize and introduce hydrophilic functional groups or other groups to make the plastic surface amorphous. Dissolves to form a rough surface.

Solvent steam treatment. The polyethylene, polypropylene plastic parts in the hot solvent (such as toluene, trichloroethylene) steam in the treatment of 10 ~ 20s, can significantly improve the ink's fixation fastness.

Chemical oxidation method. 90 g of ammonium persulfate, 0.6 g of silver sulfate, and 1000 g of distilled water are prepared into a solution. Polypropylene and polyethylene plastic products are placed therein, treated at room temperature for 20 minutes or more, or treated at 70° C. for 5 minutes. This method is applicable to hard plastic products. Can also be used for soft plastics. Acid treatment method. Complex PE and PP plastic products can be pretreated by acid etching and treated with mixed acid of chromic acid and sulfuric acid at 50 °C for 10 minutes.

4. Ultraviolet or plasma irradiation. Scrubbing polyethylene and polypropylene plastic surfaces with trichloroethylene first and then irradiating with ultraviolet light or plasma can increase the surface energy remarkably, and the fixation fastness of silk screen printing ink can be increased by more than 5 times. It is suitable for soft plastic products and Hard plastic products. This treatment method is mainly to use high-energy radiation on the plastic surface, resulting in oxidation reaction, the surface of the polar groups, improve the ink adhesion.

5. Static Control in Plastic Printing

It is best to install humidity and temperature control equipment in the printing shop, which not only can effectively control static electricity, but also can better guarantee the printing quality.

Since plastic is a fairly good insulating material, static electricity can have an effect on the printing quality of the plastic during the printing process. The print failure caused by static electricity mainly has the following performance:

(1) Printing materials stick together and cannot be separated;

(2) Printed materials cannot be laminated together or hardly stacked;

(3) The printing material is pasted on the bottom of the screen plate;

(4) The automatic printing machine cannot feed normally;

(5) The operator was shocked.

Since one of the important characteristics of plastics is good insulation performance, it is easy to bring static electricity, and it is not easy to eliminate, leading to printing failures. For this reason, the problem of removing static electricity should be taken into consideration when printing plastics. Currently, the commonly used method is to remove static electricity with an electrostatic remover.

Source: Screen Printing Industry Author: â–  Wen / Zhou Zhen