The ink does not dry on the printing press and maintains a certain fluidity, but once transferred to the substrate, it must be dried immediately. For example, after the offset ink is printed, it becomes a micron-level film on the substrate, the surface area of ​​the ink per unit volume becomes larger, chemical or physical changes occur between the ink itself and the ink and the substrate, the viscosity of the ink rises, and finally completely cured and dried.

On the other hand, printing inks such as offset printing inks, flexo printing inks, gravure printing inks, and screen printing inks are classified according to different printing methods, and the drying methods are also different. This is especially true of sheet-fed offset printing inks and web offset printing inks.

1 Classification of drying methods

The five typical drying methods for printing inks are now explained. Because the printing method is different from the ink used, the drying method is different, and the drying speed is also significantly different.

1) Oxidative polymerization drying type

The ink binder contains a large amount of dry oil or dry modified oil synthetic resin. 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 the drying oil are fatty acids and glycerin lipids. The fatty acid represented by R-COO- is mainly composed of linear unsaturated fatty acids composed of 18 C atoms. In recent years, due to environmental problems, the amount of vegetable oil used in offset printing inks has increased. In particular, soybean ink and its printed materials have obtained SOY sell certification. Therefore, the main component of vegetable oil used in offset inks has been changed from original linseed oil to soybean oil.

Unsaturated fatty acids in vegetable oils produce oxidative polymerization with O2 in the air. Figure 1 shows the reaction mechanism in glyceric acid. The conjugated double bonds are formed by oxygen in the air. The formation of the conjugated double bonds causes the double bonds between the surrounding vegetable oil molecules to undergo secondary polymerization due to the Diels-Alder reaction, thereby completing the drying.

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

In fact, the drying of offset printing is not limited to oxidative polymerization. Offset printing ink is printed on the substrate (paper) with the pigment (solid component) dispersed in the binder (liquid component). After that, the binder and low-viscosity ink or solvent penetrate into the paper through the capillary phenomenon. At this stage, the cohesive force of the resin and the pigment is in an enhanced state. When touched by hand, the ink film formed is in a state that can maintain its shape. We call this state the solid 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 between the binder composed of resin and vegetable oil and oxygen reaches complete drying.

2) Infiltration drying type

This is a drying method for newspaper and phone book printing. The substrate is non-coated paper with good absorption. The low-viscosity solvent components (vegetable oil, mineral oil) in the printing ink binder are squeezed into the paper depressions and paper pores during printing, and then the solvent penetrates into the paper through the capillary between the paper fibers, the ink viscosity It quickly increases and dries.

Of course, the oxidative polymerization reaction will be carried out afterwards, and it will take a considerable time to completely dry and form a strong ink film.

3) Volatile dry type

Ink such as thermosetting ink, flexo ink, silk screen ink, etc., after printing on the substrate, the solvent is separated and volatilized, and the solid content of the ink is left on the substrate. This drying method is called volatile drying.

The ink binder is composed of polymer (resin) and solvent. After printing, the solvent is volatilized by a heating process such as hot air, and the remaining resin and pigment form an ink film, so the drying is completed in seconds.

For example, solvent-based gravure inks use low-boiling solvents (benzene, boiling point 110 ° C; ethyl acetate, boiling point 77 ° C; ethanol, boiling point 78 ° C), and water-based flexo inks use water as the solvent. Both are dried due to solvent evaporation after printing.

On the other hand, there are many types of paper suitable for high-speed web rubber roller thermoset ink from coated paper to wood pulp paper, but there are many ink rollers on the printing machine, which requires the ink to have on-machine stability. Compared with the gravure and flexo inks described above, the thermoset ink uses a high-boiling mineral oil solvent (boiling point 250 ° C to 300 ° C). Therefore, a high temperature type hot oven (paper temperature 100 ° C to 150 ° C) is used to dry the ink by volatilization of the solvent, and then a strong ink film is formed by oxidative polymerization reaction.

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

4) Thermal polymerization drying type

For metal ink, etc., linseed oil-modified alkyd resin or castor oil-modified alkyd resin is used, which is heated in a drying process to cause thermal polymerization reaction.

On the other hand, there is also a heat-curing solvent-free ink, which can be combined with a variety of drying methods (including Diels-Alder reaction of oxidative polymerization). The ink is composed of the following three basic reaction systems:

â‘  Polycondensation reaction occurs from unsaturated polyester resin system and amino resin system including OH group and COOH group.

â‘¡ Crosslinking reaction occurs between epoxy resin and amino resin.

â‘¢ Free radical polymerization reaction between unsaturated polyester resin and unsaturated monomer.

Especially for the purpose of achieving quick-drying, it is necessary to add substances based on different reaction systems such as organic peroxides or metal desiccants and other polymerization aids or catalysts. By adding this special catalyst, to maintain the stability of printing ink storage and on-board stability. Once heating exceeds the specified temperature range in the drying process, the catalyst will act instantaneously and quickly produce a curing reaction.

5) UV and EB curing and drying type

After the ink is transferred to the substrate in the form of an ink film, the photon reaction is generated by ultraviolet (UV) or electron beam (EB) to dry the ink. Therefore, the drying process can be completed in an instant, and the subsequent process can be carried out continuously, which can achieve a significant increase in production efficiency.

On the other hand, this type of ink has attracted attention from the perspective of environmental protection because it is solvent-free, especially the quick-drying property makes the ink a good abrasion-resistant ink. Ultraviolet curable inks are widely used in the printing field of packaging materials.

Although the mechanism of UV curing and drying is complicated, in general, ink uses photoreactive acryl monomers and photopolymerization initiators. Under UV irradiation, a chain reaction occurs to cause polymerization and drying. Especially recently, UV cation reactive materials (alicyclic epoxy materials, oxetanes, vinyl ethers, etc.) with low viscosity that are less irritating to the skin than acryl-based materials have been used for UV peel coating Agents, inkjet printing inks and other products.

EB dry. The previous EB drying equipment is large in size, and the irradiation energy is quite strong (there is a suspense that deteriorates the substrate), and inert gas (N2) must be present during the entire drying process), which is not suitable for printing requirements .

However, a low-energy EB curing concept device has recently appeared, and an exploration experiment of replacing N2 with inexpensive CO2 is in progress. This experiment has been attracting attention as a promising new drying method. The EB does not necessarily have to use a photoinitiator. The situation that the odor must be generated in the UV ink due to the presence of the initiator can be basically avoided, which can further enhance the added value.

The representative drying methods are described above. In addition to this, there are some other drying methods, such as high-frequency drying (microwave drying), infrared drying, catalyst curing reaction, precipitation drying and so on. However, ink drying is not done by only one drying mechanism, but by a combination of the above-mentioned various drying methods. [next]

2 Drying fault

Here is an example of offset printing using offset printing ink to analyze the typical faults in printing related to drying.

1) Influencing factors of ink drying

As mentioned above, the main drying method of sheet-fed offset printing inks—oxidative polymerization drying method is a chemical reaction. Generally speaking, depending on the temperature, humidity, paper pH, and ink pigments, the drying of the ink will be affected by chemical factors.

â‘ The influence of temperature

Drying is a chemical reaction. The temperature rises by 10 ° C, and the drying speed is about twice that of the original. However, once the summer temperature rises, the drying speed is accelerated, and the viscosity of the ink increases, so it becomes the cause of the back side sticking or ink sticking. Therefore, after printing, the printed matter must be blown to control the temperature rise. Blowing printed materials not only blows oxygen between printed materials to accelerate the oxidative polymerization process, but also suppresses the occurrence of faults such as ink sticking.

â‘¡The influence of humidity

In the case of high humidity, the drying speed becomes slow. In the rainy season, on many occasions, there will be more dampening water attached to the non-graphical parts of the printed matter, which needs to be paid attention to.

â‘¢The influence of pH

It is affected by the pH of the paper and the pH of the fountain solution. When the pH value is below 4, the drying becomes very slow. This is due to the reaction of the acidic components in the paper or fountain solution with the desiccant in the ink, and the metal desiccant Co, etc., becomes insoluble salts and loses its catalytic effect. .

â‘£The influence of pigment

Carbon black or vanadium-based pigments used in black inks tend to adsorb metal desiccants to make them ineffective. In addition, metal-containing inorganic pigments tend to accelerate drying.

2) Common offset printing problems

Also known as Set off, it is due to the uneven pressure or extreme pressure after printing, which causes the ink to transfer from the printed sheet to the paper surface that is in contact with it causing sticking. In the state of insufficient drying, this phenomenon will occur once the excess pressure is increased.

Ink sticking

In the ink solidification stage after printing, the printed parts are stuck or stuck together.

Poor drying

After the printing was completed, after several days, the finger pressed hard on the printing surface of the printed matter, and the ink was still dry and sticky. This phenomenon is likely to occur when the following conditions occur: ① When the ink is excessively emulsified; ② When the pH value of the paper or fountain solution is low; ③ When the paper has poor ink absorption capacity; ④ The ink is too thick during printing, and the drying performance of the ink itself becomes Poor time; ⑤ When too much metal drier is added.

Chalking

After the printed matter is dried, the ink surface will wipe off the pigment with one hand. This is due to the excessive penetration of the connecting material in the ink into the paper, so that the remaining pigment cannot stand well on the paper, the wear resistance is reduced, and cannot be formed. Caused by ink film. Ink drying too slowly leads to excessive penetration of the binder into the paper, or the use of paper with fast oil absorption and strong oil absorption capacity is prone to this phenomenon.

Crystallization

When the ink of the previous color is completely dried, the ink cannot be evenly attached when printing the ink of the next color, or the ink drops off with a finger. The time interval between printing the next color ink and the previous color ink is too long, or too much desiccant is added, or oil-absorbing paper is used, or too much wax in the ink is easy to crystallize.

Pass through

The phenomenon caused by the osmotic drying rotary ink, that is, the ink solvent (vegetable oil and mineral oil) oozes out on the back of the printing surface. In severe cases, the pigment will also ooze out. This phenomenon occurs when low-viscosity ink is piled on paper with strong permeability.

migrate

Due to the volatile gas generated in the ink film after printing, the back color of the stacked paper after printing changes from white to yellow.

Cause of migration phenomenon-The volatile gas from the ink film is due to the oxidative polymerization of the binder when the ink is dry, so it contains a variety of components. In particular, the use of inks with many dry oil components is prone to this phenomenon when the temperature and humidity are high.

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