Causes and Prevention of Floating Color and Flowering of Printing Ink Film

1.
The manifestation of ink film floating color and blooming

Floating color refers to the separation of the pigments in the wet ink film in the horizontal direction once the ink is printed

The blooming refers to the uneven distribution of various colors in the ink film after packaging and printing, usually in the form of streaks or honeycombs

In addition to floating color and blooming, after the anilox roller circulates the ink or prints the ink, the streak-like blooming phenomenon that can be seen on the surface of the ink film for packaging printing is called silk pattern

The causes of floating color and blooming, we know: all compositions containing amphoteric pigments or fillers in the packaging and printing ink system may cause floating color and blooming, due to improper production processes of ink manufacturers or the work of packaging and printing operators.

1.

We have known from many years of practice that floating color 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 various, the stability of the pigment dispersion is very important
.
For example, in the ink system composed of the dispersion of various pigments, the separation of the pigment dispersion ink system is often caused by the excessive flocculation or sedimentation of a certain pigment, which is the reason why we often talk about the floating color
.
Secondly, we know from the observation of the whole process of packaging and printing ink settling: the effect of particle size is greater than that of density, and the floating color is more serious, which is usually caused by the density difference of the pigment.
of
.
In China's packaging and printing ink dispersion system, the resin and solvent adsorbed by small-sized pigment particles are relatively much larger than those of large pigment particles, and the density tends to become smaller.
The sinking of the small particle size of the pigment constitutes the floating phenomenon of the printing ink
.

In this regard, in the design of ink formulations, people have adjusted the slight control of flocculation to prevent pigment floating and blooming.

2.

Professor Yang Haijiao, known as the "star of ink" in China, believes that the solubility of the solvent to the resin polymer, the rheology of the printing ink and the adaptability to the packaging and printing process, especially the formation of the ink film, are very important
.
This represents the common view of China's packaging and printing industry for half a century
.
In this regard, in order to make full use of the solvent, a variety of solvents (ie mixed solvents) are generally used for compatibility, that is, the combination of true solvents and diluents
.
In the design of packaging and printing ink formulations, the hydrogen bonds of the solvent and the substrate should be close (close and sticky); the solubility of the solvent is close to the solubility of the resin in the ink system (nearly compatible); 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 try to pay attention to the closeness of the volatilization gradient (close and dry); the real solvent and diluent should be kept in packaging The amount of printing ink that needs to be adapted to the printing ratio
.
If we meet the above six basic conditions, there will be no floating and blooming of the ink film of the printing ink caused by the solvent
.

3.

For a long time, people have been ignoring the cause of the surface tension gradient to the ink film to bloom.
Everyone knows that after the surface solvent of the packaging printing ink ink film volatilizes, the surface tension between the upper and lower surface of the printing ink film is poor, which often causes the formation of eddy currents in the ink film.
flower
.
The difference in surface tension caused by the uneven surface heating or blowing of the wet ink film will cause the ink film to move in parallel (cutting) - the formation of blooming
.

If the solubility of the solvent is too strong, the viscosity of the packaging and printing ink will drop sharply, which will accelerate the sedimentation speed of the pigment particles.

In addition, in the mixed solvent system, if the real solvent volatilizes too fast, it will not only produce a difference in surface tension, but also destroy the balance of the volatilization gradient of the solvent

4.
Influence of resin in ink system on floating color and blooming

There are many types and different resins, and the ability to wetting is also different
.
For the composition of multiple pigments, the adsorption capacity of various pigments to the resin (linker) will not be the same, and the pigments that are not wetted are prone to agglomeration, which is also one of the main reasons for floating color.

.

In the packaging printing ink system, especially when the molecular weight and molecular weight distribution of the resin are different, the size and distribution of polar active groups will not be the same
.
Secondly, the adsorption capacity of various pigments to the binder (resin) will not be very the same - the pigments with a thin layer of adsorption ink film are prone to flocculation - floating color and blooming
.
In the rosin or alkyd resin ink with small molecular weight, due to the high polarity and strong adsorption force, the formed adsorption ink layer is thin, and the dispersion stability is poor, and it is very easy to cause flocculation and float color
.
From a theoretical point of view, when packaging and printing inks flocculate, it is more necessary to control the concentration of the resin during the grinding process of the printing ink according to the properties of different pigments and binders, so as to force the polymer (linker) to be adsorbed on the pigment.
Ostensibly, the aim is to prevent flocculation failures produced by pigment singly
.

The poor solubility of the polymer (resin binder) in the packaging and printing ink system will also cause the disadvantage of floating color
.
In general, the miscibility is poor when several resins are used
.
Secondly, the solubility of the selected solvent cannot reach the critical value, which is often referred to as poor mixing between resin and resin, coupled with insufficient affinity for pigments, often after dissolving and volatilizing, there will be floating color and hair color.
flower
.
Even if the turbidity and floating color are eliminated under the action of the solvent, it will already start during the solvent volatilization process
.
Because the volatilization of the solvent and the diluent destroys the balance of the resin binder, it is often said that the resin binder with poor compatibility will bring out the adsorbed pigment when it is precipitated, resulting in the floating color of the ink film and the Flowers
etc.

5.
The influence of packaging and printing conditions on floating color and blooming

In China, the water shortage and dry areas in the north of the Yellow River and the wet plum water in the south of the Yangtze River also directly cause the floating color and blooming after the printing of graphics and texts
.
Whether it is a dry environment or a workshop with high humidity (except for temperature and humidity controlled workshops), we all know that during the printing process, due to the evaporation of the solvent, the drop in the surface temperature of the ink film, the condensation of moisture, etc.
, the ink film eddy current of the packaging printing ink will be caused The formation of the surface of the hydrophilic pigment adsorption to a certain level, and then cause the coagulation and separation of the pigment - once the water is brought into the ink layer and combined with the hydrophilic pigment particles, it often occurs through the bridging effect of the immiscible liquid.
- Floating color
.
This failure is especially noticeable in packaging printing inks mixed with fast-evaporating and hydrophilic solvents
.

This kind of failure is often more common in water-based letterpress, water-based gravure, silk screen, plastic letterpress, plastic gravure and color inkjet ink
.
Shrinkage cavities are caused by the generation of insoluble colloidal particles in printing inks, which are sometimes confused with floating colors
.
We know that the concentration of surface active substances changes in the process of printing ink film formation, exceeding its solubility and generating a small amount of incompatible droplets, which will cause shrinkage cavities
.
For example, excessive addition of silicone oil or excessive viscosity may easily cause shrinkage cavities
.
The difference between shrinkage cavity and floating color flower is: irregular, spherical, small depressions are formed on the ink film, often with a drop or a small piece of impurities as the center, and a ring-shaped edge is formed around it
.
From the point of view of the leveling of packaging printing ink: it is a special "dot-type" flow unevenness generated on the surface of the printing ink film (its shape is divided into plane, crater, exposed bottom, bubble, dot, etc.
)
.

2.
Measures to prevent and remedy floating color and blooming

The manifestations of floating color and blooming around the ink film have been described, and several manifestations of them have been roughly analyzed and summarized
.
But there is very little published and published literature on how we can prevent and remedy the fait accompli of fait accompli
.

Packaging printing inks are ultimately a storage stability
.
Ink storage stability is the result of interaction between components
.
For example, the polarity of the solvent is different, and the adsorption on the pigment surface of the active agent is also different, just like the order of the adsorption amount of octadecylamine in each component solvent is: toluene>dichloroethane>mixed solvent
.
Most people in the scientific community believe that various solvents only affect physical adsorption and do not affect chemical adsorption.
At the same time, it has been proved to be correct that the binder and the active agent will produce competitive adsorption on the surface of the pigment
.
This shows that the competitive adsorption depends on the surface affinity of binder, active agent and pigment and filler
.
If the pigment or filler is hydrophilic, the more polar substance is selected for adsorption in a non-polar solvent
.
If it is hydrophobic, a substance with stronger hydrophobicity should be selected for adsorption in polar solvent
.

The physical adsorption of the surfactant will not affect the chemical adsorption of the polymer, because the physical adsorption of the surfactant on the surface of the pigment and filler is not firm, and it is easily extruded from the surface of the pigment by the resin in the solvent
.
The surfactants that form chemical adsorption with the surface of pigments and fillers cannot be extruded from the surface of pigments and fillers by resin polymers
.
The greater the apparent adsorption saturation of the surfactant, the greater the resistance to polymer adsorption
.
On the other hand, if the time for dispersing and grinding during ink production is a little shorter, the ink will not be easily extruded from the surface of the pigment by the resin in the solvent during the manufacturing process or when the ink is stored, but other phenomena may still occur.
, to be practiced
.

In polar solvents, ultra-fine active calcium and alkyd resin can be adsorbed on the surface of Tillandsia and medium chrome yellow.
This is because there are two active centers on the surface of the pigment, so the slightly acidic active calcium and alkyd The resin can still be adsorbed on the surface, but its covering rate is low, so the wetting and dispersibility is also poor.
When it is dispersed to 7.
5MM, the surface will have floating color; when it is dispersed to 1-2MM, it will disappear.

.
_ For example, after adding thinner during the printing process, the floating color will appear again
.
Appropriate dispersants must be selected.
Adding wetting and dispersing agents can not only reduce the grinding time of pigments and fillers, but also save energy without improving work efficiency
.