Curing and drying of printing ink (gloss)

The ink is not dry on the press and maintains a certain degree of fluidity, but once it is transferred to the substrate it must be dried immediately. For example, after the offset printing ink is printed on the substrate, it becomes a micron-scale film, the surface area per unit volume of ink becomes large, the ink itself and the ink and the substrate produce a chemical or physical change, the viscosity of the ink rises, and finally it completely cures and dries.

On the other hand, printing inks such as offset printing inks, flexographic printing inks, gravure printing inks, screen printing inks, etc. are classified according to different printing methods, and drying methods are also different. This is also true of offset inks and web offset inks in offset printing inks.

1 Classification of drying methods

Here are five typical drying methods for printing inks. Due to the difference in the printing method and the ink used, the drying method is different, and the drying speed is also significantly different.

1) Oxidation polymerization drying type

The ink binder contains a large amount of synthetic resin containing dry oil or dry denatured oil. Once the ink is printed on the substrate, these dry oil components are absorbed by the oxygen in the air and form soft macromolecules through oxidative polymerization. The ink film is cured and dried.

The main components of dry oils are lipids of fatty acids and glycerol, and the fatty acids represented by R-COO- are mainly composed of straight-chain unsaturated fatty acids composed of 18 C atoms. In recent years, due to environmental issues, the amount of vegetable oil used in offset printing inks has increased. In particular, SOY sell certification has been obtained for soy inks and printed products. Therefore, the main component of the vegetable oil used in the offset printing ink was changed from the original linseed oil to soybean oil.

Unsaturated fatty acids in vegetable oils react with O2 in the air to produce oxidative polymerization. Figure 1 shows the reaction mechanism in glyceric acid. The conjugated double bonds are formed by the oxygen in the air, and the formation of the conjugated double bonds allows the secondary polymerization of the surrounding vegetable oil molecule double bonds due to the Diels-Alder reaction to complete the drying.

However, only the oxidative polymerization reaction of vegetable oils and the drying process are slow, so drying aids such as Co and Mn are used in sheet-fed offset printing inks.

In fact, offset printing does not only involve oxidative polymerization. Offset printing ink is printed on the substrate (paper) with the pigment (solid content) dispersed in the binder (liquid composition). Afterwards, the binder and the low-viscosity varnish or solvent penetrate through the capillary phenomenon into the paper. At this stage, the cohesion of the resin and the pigment is in an enhanced state, and when the ink is touched by hand, the formed ink film is in a state capable of maintaining its shape. We refer to this state as the solidified state of the ink, which is in the same state as the osmotic drying type to be discussed below. After this, the oxidative polymerization reaction of the connecting material consisting of resin and vegetable oil with oxygen reaches complete drying.

2) Infiltration drying type

This is a drying method for newspapers and telephone book printing. Substrate is a good absorption of non-coated paper. The low-viscosity solvent components (vegetable oil, mineral oil) in the printing ink vehicle are squeezed into the depressions of the paper and the pores of the paper during printing, after which the solvent penetrates through the capillary between the paper fibers and penetrates into the paper, the ink viscosity. It will increase rapidly and dry.

Of course, the subsequent oxidative polymerization reaction takes a considerable time to completely dry and form a strong ink film.

3) Volatile drying

Heat-fixing inks such as inks, flexo inks, and screen printing inks are printed on a substrate and the solvent therein is separated and volatilized. Solid components in the ink remain on the substrate. Such a drying method is called a volatile drying type.

The ink vehicle consists of a polymer (resin) and a solvent. Since the solvent is volatilized by a heating process such as hot air after printing, the remaining resin and pigment form an ink film, and drying is completed in units of seconds.

For example, a solvent-based gravure ink uses a low-boiling solvent (benzene, boiling point 110° C.; ethyl acetate, boiling point 77° C.; ethanol, boiling point 78° C.), and aqueous flexographic inks use water as a solvent. Both dry after printing due to solvent evaporation.

On the other hand, high-speed web rubber heat-set inks can be applied to many types of paper, from coated paper to wood pulp. However, there are many ink rollers on the press, which require the ink to have on-board stability. In contrast to the previously described gravure and flexo inks, the heat-fixing ink uses a high-boiling mineral oil solvent (boiling point 250° C. to 300° C.). Therefore, using a high-temperature type hot oven (paper temperature 100° C. to 150° C.), the ink is dried by volatilization of the solvent, and then a strong ink film is formed by oxidative polymerization.

In the past, heat-fixed ink prints were prone to problems such as scalding, hot wrinkles, and broken paper due to high paper temperature. Recently, after the improvement of the ink, the paper temperature dropped to about 100°C, and the above failures have been very rare.

4) Thermal Polymerization Drying

The linseed oil-modified alkyd resin or castor oil-modified alkyd resin is used as the metallic ink or the like, and it is heated in a drying step to cause a thermal polymerization reaction.

On the other hand, there is also a heat-curable, solvent-free ink that can be selected from a variety of drying methods (including the Diels-Alder reaction for oxidative polymerization). The ink consists of the following three basic reaction systems:

1 A polycondensation reaction occurs between an unsaturated polyester resin system containing an OH group and a COOH group and an amino resin system.

2 Cross-linking reaction occurs between epoxy resin and amino resin.

3 The radical polymerization reaction between unsaturated polyester resin and unsaturated monomer.

Especially for applications where fast drying is desired, substances based on different reaction systems such as organic peroxides or metal drying agents or other polymerization aids or catalysts are added. By adding this special catalyst, the storage stability and on-board stability of the printing ink are maintained. If the heating exceeds a predetermined temperature range in the drying step, the catalyst will act instantaneously and a rapid curing reaction will occur.

5) UV and EB curing drying type

After the ink is transferred onto the substrate as an ink film, the ink is dried by ultraviolet (UV) or electron beam (EB) generation of the actinic reaction. Therefore, the drying process can be completed in an instant, and the subsequent processes can be performed continuously, which can greatly increase the production efficiency.

On the other hand, this type of ink has attracted attention from the perspective of environmental protection because it has no solvent, and in particular, the quick-drying property makes the ink an ink with good abrasion resistance. UV curable inks are being used in large quantities in the printing field of packaging materials.

Although the mechanism of the UV curing and drying is complicated, in general, the ink uses a photoreactive acryl monomer and a photopolymerization initiator, and under the irradiation of UV, a chain reaction occurs and the polymerization is dried. In particular, UV cation-reactive materials (cycloaliphatic epoxy materials, oxetanes, vinyl ethers, etc.) that are less viscous than acryl-based materials have recently been used for UV release coating. Agent, inkjet printing ink and other products.

EB dry. In the past, EB drying equipment was bulky, and the irradiation energy was quite strong (there was a suspense that caused the substrate to deteriorate). Inactive gas (N2) must be present throughout the drying process. These are not suitable for printing requirements. .

However, the low-energy EB curing concept device has recently emerged, and exploration experiments are being conducted to replace N2 with inexpensive CO2, which has been attracting attention as a promising new drying method. The EB does not necessarily use a photoinitiator. The presence of an odor due to the presence of an initiator in the UV ink can be basically avoided, and the added value can be further increased.

The above describes a typical drying method. In addition, there are other drying methods such as high-frequency drying (microwave drying), infrared drying, catalyst curing reaction, precipitation drying, and the like. However, the drying of the ink is not accomplished by only one drying mechanism, but drying is achieved by a combination of the above-described various drying methods.