In addition to pigments, resins, and additives, the composition of solvent-based gravure printing ink also contains more solvent, and the ink is generally thinner. The drying form of solvent-based gravure printing ink is volatile drying, which is suitable for the printing of paper, film, aluminum foil and other materials. At the same time, due to the fast printing speed of the gravure printing machine and the thick printing ink layer, it is very important to analyze and understand the influence of the thixotropy, fluidity, viscosity, and drying properties of the ink on the printing quality to improve the printing quality.
Thixotropy
The nature of the change in flow properties caused by the ink under external stirring is called thixotropy. In fact, any kind of suspension dispersion, thinning after stirring and thickening after standing, can be explained by thixotropy.
Generally speaking, good quality ink has low thixotropy and can be used directly on the printing machine without special treatment. However, inks without thixotropy are prone to precipitation, so they are not good. In general, white, yellow, and cyan inks have low thixotropy, and magenta and black inks have large thixotropy.
The thixotropy of ink is generally related to the following factors.
(1) The nature of the pigment used. Like the carbon ink of the same type, the thixotropy of the prepared ink is much smaller than that of the untreated ink after the surface is subjected to ozone oxidation treatment.
(2) The particle size of the pigment. Generally, the pigment particles are small, the thixotropy of the ink is small, and vice versa.
(3) The amount of pigment. If the amount of pigment is large, flocculation will occur between the pigment molecules, so the thixotropy of the ink is also large.
(4) Molecular weight of synthetic resin. The molecular weight of the resin is large, but the ink with a small n-heptane value has a large thixotropy, and vice versa.
(5) Wettability of pigments. The wettability of the pigment is good, the thixotropy of the ink is small, and vice versa. Some ink manufacturers often use the base ink method to pre-disperse in the production of black ink, so that the degree of wetting of the pigment is increased, thereby reducing the thixotropy of the ink.
Solvent-based gravure printing inks should have appropriate thixotropy. When the thixotropy is too large, it first shows that the leveling property is not good, and the viscosity is not easy to control, which may cause paste, uneven coating, poor gloss, and printed products are prone to "orange peel" and other phenomena. If the thixotropy is too small, the printed dots tend to increase, and the text and lines will become thicker. [next]
fluidity
The fluidity of the solvent-based gravure printing ink is a comprehensive reflection of the relative density, viscosity, coagulability, thixotropy and other factors of the ink. In order to obtain better fluidity, the continuous material must have appropriate viscosity, the pigment must have good wettability, and the pigment content should not be too high, the relationship between the liquid and solid interface should be handled well, and the thixotropy of the ink should be minimized.
When testing the fluidity of the ink, in addition to measuring the ink outflow time with the Tu 4 # cup or Zahn 3 # cup, the ratio between the amount of ink remaining on the wall of the cup and the amount of outflow ink is also to be seen. At a certain viscosity, the smaller the ratio between the two, the better the fluidity of the ink, and vice versa. If the fluidity of the ink is not good, there will be many printing failures when printing on the machine.
Viscosity
The viscosity of solvent-based gravure printing ink is related to its fluidity. The proper viscosity of the ink is the key to ensure the good transmission of the ink. Although the transfer rate of ink is closely related to many factors such as resin type, ink adhesion, printing speed, printing pressure, printing substrate, ambient temperature and relative humidity, and the depth of the printing plate cell, the viscosity of the ink is still The decisive factor. The relationship between ink viscosity and transfer rate is shown in Figure 1.
The main factors affecting the viscosity of solvent-based gravure printing ink are the following.
(1) Fix the viscosity of the resin solution. The resin viscosity is large, the ink viscosity is high, and vice versa.
(2) The matching of pigment and resin liquid. If a coagulative reaction occurs between the two, the viscosity of the ink system is high, otherwise it is low.
(3) Miscibility and compatibility of solvent and dispersion. If the solvent can dilute the dispersion in any proportion, the viscosity of the ink is low, and vice versa.
(4) Temperature. Low temperature, high ink viscosity, and vice versa.
If the viscosity of the solvent-based gravure printing ink is too low, it may cause pinholes in the printed product, or the large-area color block is not flat and smooth, and it may make the printed product false and appear halo. When printing a substrate with a smooth surface, the adhesion of the ink is low; when printing paper, it will appear powdering and dusting.
If the viscosity of the solvent-based gravure printing ink is too high, it will cause printing defects such as scratches, bite, paste, and "orange peel". [next]
In summary, when designing and manufacturing solvent-based gravure printing inks, it is necessary to select suitable pigments, resins, and solvents to match, so that the ink reaches a suitable initial viscosity. The printing plant should use the appropriate mixed solvent, preferably the thinner provided by the original ink manufacturer, and adjust the original ink to an appropriate "working viscosity" before it can be printed on the machine, so that high-quality prints will be obtained.
In addition, the viscosity of the ink in the printing process is also related to the printing speed. In general, the faster the printing speed, the lower the viscosity of the ink, otherwise the ink transfer effect is not ideal.
Solvent-based gravure printing inks are volatile dry inks, that is, when the solvent overflows from the wet ink film, the ink film will change from liquid to solid. Therefore, the speed of solvent volatilization has a direct impact on the drying properties of the ink.
When designing and producing solvent-based gravure printing inks, the solvent mixture of medium-boiling point, low-boiling point and high-boiling point is generally selected to meet the drying requirements of the ink. If the resin is improperly selected, the resin softening point is low, the resin does not remove the solvent well, the pigment release to the solvent is not good, too much high boiling point solvent will soften the ink film, etc., will affect the solvent volatilization rate, resulting in solidification Too much solvent remains in the ink film afterwards.
When preparing solvent-based gravure printing inks, the resin is generally selected first, followed by the solvent. The dissolution parameter of the mixed solvent is similar to the dissolution parameter of the dissolved substance, so it is easy to dissolve and the faster the volatilization rate. Due to the addition of pigments in the finished ink, the volatilization rate of the mixed solvent will be greatly reduced, and the larger the pigment particles, the slower the volatilization rate of the solvent, the worse the drying property of the ink. At the same time, the greater the amount of pigment, the slower the volatilization rate of the mixed solvent. This is due to the mutual traction between the solvent and the pigment surface, which resists the removal rate of the solvent.
In addition, the higher the room temperature, the lower the relative temperature of the air and the faster the air circulation, the faster the volatilization rate of the mixed solvent, and the slower the converse.
The printing failures caused by improper drying of the ink mainly include:
(1) At a certain printing speed, the initial drying of the ink will cause problems such as paste, scratches, ink whitening, poor gloss, and poor leveling.
(2) In continuous register printing, because the ink is too slow to dry, sticking rollers will occur when printing at the next station, and it will also cause the printed product to rewind and stick. The evaporation rate of the solvent in the ink depends not only on the solvent's boiling point, vapor pressure, latent heat of evaporation and other factors, but also on the temperature, relative humidity, air volume and ink layer thickness of the printing environment, so it should be adjusted at any time according to the changes in conditions The evaporation rate of the solvent in the ink.
Under normal production conditions, if the ink dries too fast, an appropriate amount of slow-drying thinner should be added; if the ink dries too slowly, the fast-drying thinner should be added. If conditions permit, the temperature of the printing environment should be controlled between 18 ~ 25 ℃, and the relative humidity should be controlled below 65%, which is conducive to the stability of ink performance.
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