In normal packaging and printing, operators often have to suffer a lot because of the floating color and blooming of the printed ink film, which also greatly reduces the quality of their printed products. This failure is mainly caused by the printing ink system containing more than one pigment or filler. Although occasionally uneven pigment separation occurs, after the batch packaging and printing process, floating color or blooming, that is, pigment changes, can be clearly observed from the surface of the ink film (the text is abbreviated). This is a pigment change that occurs in the state of a wet printed graphic (ink) ink film. However, exploring the different forms of floating color and flowering, and the operator is mistakenly called shrinkage, which is caused by the excessive amount of defoaming agent in the ink system, which is different from the former. To this end, the author will discuss the following points based on years of practical application, and discuss with colleagues in the packaging and printing industry.
1. The expression form of ink film floating color and flowering
Floating color refers to the separation of the pigment in the wet ink film in a horizontal direction once the ink is printed. This is the concentration of one or several transformations of various pigments in the ink film concentrated on the surface and uniformly distributed. However, it is obviously different from the color just produced from the ink factory or the previous formula.
The flowering refers to the uneven distribution of multiple colors in the ink film after packaging printing, usually in the form of stripes or honeycomb. For this reason, people often understand the separation of pigments in the vertical direction.
In addition to floating colors and flowering, after the anilox roller circulates ink or sprays ink, the striped flowering phenomenon that can be seen on the surface of the ink film on the packaging and printing is called silk.
The reason for the occurrence of floating color hair, we know that: all the components containing amphoteric pigments or fillers in the packaging printing ink system may produce floating color and flower bloom, the production process of the ink manufacturer is improper, or the operation of the packaging and printing operator Improperly can also produce floating colors and hair.
1. The effect of pigment settlement and flocculation on floating color hair
We have known in practice for many years: floating color hair is caused by uneven distribution of pigment on the surface of the ink film after packaging and printing (including uneven printing plate, uneven pressure, etc.). Although the reasons involved are many, the stability of pigment dispersion is very important. For example, in an ink system composed of a dispersion of multiple pigments, the separation of the pigment-dispersed ink system is often caused by excessive flocculation or sedimentation of a certain pigment, which is why we often talk about floating colors. Secondly, we learned from the observation of the entire process of packaging and printing ink sedimentation: the effect of particle size is greater than density, and the floatation is more serious. It is also commonly called the density difference of pigments, strictly speaking, it is caused by the particle size of pigments. of. From the packaging and printing ink dispersion system in China, the resin and solvent adsorbed by the pigment particles of small particle size are relatively larger than the pigment particles of large size, and the density tends to be smaller, and the density of the pigment particles of larger particle size will cause the pigment particles to become larger. The sinking of the pigment; and the pigment of small particle size floating up constitutes the floating color phenomenon of the printing ink.
In this regard, in the design of ink formulas, people have adjusted the light control flocculation to prevent the pigment from floating and blooming. Although it has an effect, if it is excessively severely flocculated, it will cause precipitation and cause floating color and blooming. appear. For the convenience of production, after the 1990s, people introduced dispersants, especially coupling agents into the ink system, the effect is very obvious. The purpose of dispersants is to achieve the best adsorption state on the surface of the pigment. The purpose is to bridge between the binder and the pigment, thereby enhancing the storage stability of the pigment and preventing the floating color from blooming.
2. The effect of solvent on floating color hair
Professor Yang Haijiao, known as China's "ink star", believes that the solubility of solvents in resin polymers and the rheology of printing inks and their suitability for packaging and printing processes, especially the formation of ink films, are very important. This represents a half-century of common views in China's packaging and printing industry. In this regard, in order to make full use of the solvent, a variety of solvents (that is, mixed solvents) are generally used for compatibility, that is, the combination of the true solvent and the diluent.
In the design of packaging printing ink formulations, the hydrogen bond of the solvent and the substrate should be similar (close to the viscosity); the solubility of the solvent is similar to the solubility of the resin in the ink system (similarly soluble); the surface tension of the solvent is similar to that in the ink system The surface tension of the resin is similar (nearly flat); the density of the solvent is similar to the density of the binder (resin) (nearly stable); the mixed solvent should be as close as possible to the gap of the gradient of the volatilization (nearly dry); the packaging of the real solvent and the diluent should be kept The printing ink needs to adapt to the printing proportion. If we meet the above six basic conditions, we will not cause the floating and blooming of the printing ink film due to the solvent.
3. The effect of surface tension gradient on floating hair
For a long time, people have been ignoring the cause of the surface tension gradient on the ink film. After evaporating the surface solvent of the packaging printing ink film, the surface tension difference between the upper and lower sides of the printed graphic ink film often causes the formation of the ink film vortex. flower. The difference in surface tension caused by the heating or uneven blowing of the wet ink film surface will also cause the ink film to move in parallel (skewing)-the formation of hair.
If the solubility of the solvent is too strong, the viscosity of the packaging printing ink will drop sharply, which will accelerate the sedimentation speed of the pigment particles. As a result, the difference in sedimentation due to the different pigment particle sizes-that is, the generation of eddy currents, and then floating color hair Created conditions.
In addition, in a mixed solvent system, if the true solvent volatilizes too fast, it will not only produce a difference in surface tension, but also disrupt the balance of the solvent's volatilization gradient. As a result, the resin in the ink system will also be precipitated with the absorbed pigment due to the reduction of the true solvent-blooming, and at the same time, the pigment particles will agglomerate and affect the improvement of the surface flatness of the ink film, but the volatilization of the solvent is too slow It will cause the viscosity of the printing ink to rise slowly, which will cause the printing ink film to flow for too long. As a result, the appearance of the ink film vortex is inevitable-flowering, floating color and sagging.
4. The effect of resin on floating color hair in ink system
There are many types and different resins, and the wetting ability is also different. As for the composition of multiple pigments, the adsorption capacity of various pigments to the resin (linking material) will not be the same, and the pigments that are not wetted are very prone to agglomeration-this is also a main reason for floating color .
In packaging and printing ink systems, especially when the molecular weight and molecular weight distribution of the resin are different, the size and distribution of the polar active groups will not be the same. The difference in these distributions is caused by the different wetting ability of the pigment. Secondly, the adsorption capacity of various pigments to the binder (resin) will not be the same-pigments with a thin ink film layer will easily flocculate-floating color hair. However, in small molecular weight rosin or alkyd resin inks, due to the large polarity and strong adsorption, the formed adsorption ink layer is thin, the dispersion stability is poor, and it is very easy to produce flocculation and float. From a theoretical point of view, when packaging printing ink flocculates, it is more necessary to control the resin concentration during the printing ink grinding process according to the performance of different pigments and binders, forcing the polymer (linker) to adsorb on the pigment On the surface, its purpose is to prevent the flocculation failure of a single pigment.
Poor solubility of polymers (resin binders) in packaging and printing ink systems can also cause the disadvantage of floating colors. Generally speaking, the miscibility is poor when using several resins. Secondly, the dissolving power of the selected solvent does not reach the critical value, that is, people often say that the resin and the resin are poorly mixed, coupled with insufficient affinity for the pigment, often after dissolution and volatilization, there will always be floating color, hair flower. Even if the turbidity and floating color are eliminated under the action of the solvent, it will already begin during the solvent volatilization process. Because the volatilization of the solvent and diluent destroys the balance relationship of the resin binder, which is often said: when the resin binder with poor compatibility is precipitated, it will bring out the adsorbed pigment and cause the ink film to float and Hair flower etc.
5. The effect of packaging and printing conditions on floating color hair
In China, the water-scarce and dry areas to the north of the Yellow River and the humid waters to the south of the Yangtze River in the Yangtze River have also directly caused the floating color of the printed graphics.
Whether it is a dry environment or a workshop with high humidity (except for a workshop with temperature and humidity control), we know that during the printing process, due to the evaporation of the solvent, the temperature of the ink film surface drops, the condensation of moisture, etc. will cause the eddy current of the ink film of the packaging printing ink The formation of the surface will cause the hydrophilic pigment on the surface to adsorb to a certain level, which will cause the agglomeration and separation of the pigment-because the water is brought into the ink layer and the hydrophilic pigment particles are combined, the aggregation and sedimentation often occur through the bridging effect of the non-mutual solution ——Floating colors. The occurrence of this kind of failure is particularly noticeable in packaging printing inks mixed with a hydrophilic solvent that evaporates quickly and is hydrophilic.
Such failures are often seen in water-based letterpress, water-based gravure, screen printing, plastic letterpress, plastic gravure, and color inkjet printing. The generation of insoluble colloidal particles in printing inks causes shrinkage, which is sometimes confused with floating hair. We know that the concentration of surface-active substances changes during the film formation of printing inks, which exceeds its solubility and generates a small amount of incompatible droplets that can cause shrinkage. For example, excessive addition of silicone oil or excessive viscosity may easily cause shrinkage. The difference between shrinkage holes and floating hair is that irregular, spherical, small depressions are formed on the ink film, and a ring or edge is often formed around a drop or a small piece of impurities. From the perspective of the leveling of packaging printing inks: it is a special "dot" type of unevenness that occurs on the surface of the printing ink film (its shape is divided into planes, craters, dews, bubbles, dots, etc. ).
2. Measures to prevent and remedy floating color hair
The expressions of floating hairs around the ink film have been described, and several expressions have been roughly analyzed and summarized. But how can we prevent and remedy the faintly colored floating flowers, there are indeed very few publications and published documents on the market. To this end, the author describes each case based on accumulated experience, hope colleagues to criticize and give enlightenment.
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