How to understand the surface tension of gravure ink

The surface tension of the liquid is the ability of the liquid surface to automatically shrink in the air. For printing, it is as important as the surface tension of the substrate. In the printing, the surface tension of the ink changes more. Gravure inks are liquid inks that are prone to some printing quality problems related to the surface tension of the ink.

Surface tension factors

There are four main factors that affect the surface tension of gravure ink:
1) The original ink formulation, such as resin, dispersant, etc.

2) The dilution rate of the original ink and the diluent used. Generally, the surface tension of the ink resin is greater than the surface tension of the printing substrate, and the surface tension of the commonly used diluent is less than the surface tension of the printing substrate. For example, at 20° C., the surface tension of toluene is 2.85×10 −2 N/m, the surface tension of ethanol is 2.28×10 −2 N/m, and the surface tension of acetone is 2.37×10 −2 N/m. Diluents with low surface tension can reduce the surface tension of the ink. Obviously, high dilution results in low ink viscosity and low ink surface tension. With the same dilution rate, a thinner surface tension diluent is more likely to lower the surface tension of the ink than a surface tension higher.
3) Temperature. High liquid temperature, low surface tension; low temperature, high surface tension.
4) Additives. Such as defoamers, leveling agents, etc. can reduce the surface tension of the ink.

Effect of surface tension on printing

Ink dilution is the process of decreasing the surface tension. The higher the dilution rate, the lower the surface tension. After the ink is transferred to the substrate, the surface tension of the ink gradually increases as the solvent volatilizes, reaching a maximum at the time of drying.

There are two reasons for the increase of ink surface tension in the printing process: First, the solvent with low surface tension gradually decreases, and second, the solvent, especially the quick-drying solvent, volatilizes, which causes the temperature of the ink to decrease, thereby increasing the surface tension of the ink. high.

Changes in the surface tension of the ink can have the following effects on the printing process:
1) Affect the ink leveling. The ink with low surface tension has a good leveling.
2) Affect the adhesion of ink. The degree of wetting of the ink by the substrate affects the adhesion to the printed substrate. The lower the surface tension of the ink, the better the degree of wetting of the printed substrate.
3) Lead to printing failures such as shrinkage holes.

Printing failure analysis and solution

After the gravure ink is normally diluted, the surface tension is low, which is beneficial to the leveling and adhesion of the ink; after the ink is dried, the surface tension of the ink film is higher, which is favorable for the overprint of the next color ink or the smooth progress of the next process (such as compounding and coating glue).

1. Ink transfer process

The wetting of the ink can be divided into two stages, the wetting of the printing plate by the ink (ie, the wetting of the ink by the cells) and the wetting of the ink in the web by the printing substrate. As long as the wetting is not good at any stage, the transfer of the ink cannot be performed normally. The defining principle of ink “wetting” is:
When the contact angle θs<90° is wettable, and when θs>90°, it is not wettable. FIG. 1 shows the static wetting state of the ink.

However, there is dynamic wetting during the printing process, as shown in Figure 2. In the picture, the droplet moves to the left (or is moved to the right by the wetting object). At this moment, two contact angles are generated, namely the moving contact angle. A contact angle greater than θs is the advancing angle θa; another contact angle is smaller than θs and is the receding angle θr. At a certain speed, the smaller the surface tension of the droplets, the smaller the advancement angle; The greater the speed of droplet movement, the greater the advancement angle θa; when the velocity is large to a certain extent, the dynamic contact angle is greater than 90°, resulting in the wettable system becoming nonwettable (ie, the lyophile system becomes non-wettable Turbulent system). The maximum allowable interface motion speed under wet conditions is called critical wetting speed.

In the first stage of the printing process, the ink in the ink tank is stationary and the plate cylinder is rotating, which is a dynamic wetting state. In the second stage, the ink and the substrate move at the same speed and are in a state of static wetting. Due to the high surface tension of the ink with high viscosity, dynamic wetting may not be completed, that is, the ink cannot fully wet the cells; it may also result in static wetting that cannot be completed, ie, the ink in the cells cannot fully wet the printing substrate, and finally make the ink Poor transfer.

2. Shrinkage failure analysis

When the surface tension of the printed substrate does not match the surface tension of the ink, such as a low surface tension of the substrate or a high surface tension of the ink, the ink cannot fully spread on the surface of the substrate to form a dewdrop, which may cause shrinkage failure. The causes of shrinkage and related cases are listed below.

1) The surface tension of the substrate is low.
2) The substrate is not clean and oily. The surface tension of most oil stains is lower than the surface tension of the substrate.
3) The substrate may have some foreign material with low surface tension.
In a certain packing factory, the extrusion workshop is closely adjacent to the printing workshop, and shrinkage often occurs in the printing when the sizing machine works. The shrinkage hole disappeared after the crowded workshop was removed. Originally, the air intake of the active air supply device installed in the workshop of this factory was just close to the exhaust outlet of the squeezing machine. When it was produced under a high temperature environment of 300°C or more, the volatile low-molecular polymer was discharged to the outside and immediately It is sucked into the printing shop, and the static electrodes produced by high-speed printing of the printing press make it easy for these low molecular weights to adhere to the substrate and cause shrinkage.
4) The ink formulation is irrational.
5) Inconsistent ink viscosity. The formation of shrinkage cavity needs to meet both the thermodynamic and dynamic requirements. Thermodynamics require the surface tension of the ink to be too high to wet the substrate; the dynamics require that the ink be fluid and have enough time to shrink. The darkness of printing graphics and the actual volume of ink in the field are large and the ink layer is thick. When the solvent volatilizes, a large amount of heat is absorbed. If the external heat does not make up for the heat lost in the ink layer, the surface tension of the ink increases due to the decrease of the surface temperature of the ink. The lower temperature of the ink is higher and the surface tension is relatively lower (or the surface tension of the substrate is lower). According to Benar vorticity theory, ink with low surface tension spreads to the ink layer with high surface tension, and the ink film leveling deteriorates, causing severe shrinkage. If the viscosity of the ink is increased, the content of the solvent in the ink is small, the amount of volatilization is small, and the temperature decrease is small; the fluidity of the ink is deteriorated, and the time required for the ink to transfer from the substrate to the ink loses flowability is shortened, and the ink does not have enough time. Shrinkage, causing shrinkage defects in the print.
6) The ink thinner dries too fast. The reason is basically the same as above. Can be solved by reducing the drying speed.
7) The external cause causes the solvent to volatilize too quickly before the ink layer enters the drying oven, and the surface tension of the ink increases.
In the same workshop of a certain printing plant, there are one nine-color gravure printing machine and one dry-type laminating machine, and the air inlet of the drying box is in the workshop, resulting in a power of about 70kW for the negative pressure of the workshop. When working in the evening, the shrinkage hole often occurs. After 30 minutes of shutdown, the shrink hole disappears again, but after 30 minutes, the shrinkage hole is generated again. There are two main reasons for this phenomenon.

First, because the large negative pressure in the workshop led to the generation of large airflows, especially in the evening when only the door of the workshop was opened, the wind at the printing unit was very large, and the wind played a major role in the drying of the ink. The wind generated by the negative pressure blows onto the ink film. Before the ink layer has entered the drying oven, the solvent has evaporated. The temperature of the ink layer drastically decreases, causing the surface tension of the ink layer to rise. At this time, the ink layer still has fluidity. Shrinkage holes are easily formed.

The second reason is that the lower temperature at night causes the temperature of the ink to decrease and the surface tension of the ink is higher, which can also lead to shrinkage failure.

If both conditions exist at the same time, it is more prone to shrinkage failure.

Why does the shrinkage hole disappear when it is turned on again after shutdown?
After a period of downtime, the airflow in the workshop was reduced or disappeared, and after the engine was turned on again, the workshop gradually formed a large negative pressure, the air flow became larger, and the shrinkage hole regenerated. It can be solved by increasing the viscosity of the ink, reducing the drying speed of the ink, and eliminating the air flow in the workshop, but the best method is to eliminate the negative pressure, such as the air inlet of the gravure printing machine and the dry complex machine, and then install it in the workshop. Active air supply device.

3. Improve ink leveling

The low surface tension of the ink facilitates ink leveling. Reducing the surface tension of the ink can be solved by adding a leveling agent, using a diluent with a low surface tension, and selecting an ink with a suitable viscosity.

Author: Lv Haiping

Reprinted from: Printing Technology