Solvent selection of plastic gravure ink

In plastic gravure inks, solvents not only dissolve a variety of film-forming polymers, adjust viscosity and rheology, but also have an important influence on film quality. One of the key technologies in the formulation of plastic gravure inks is the selection of suitable solvents. The following describes in detail the characteristics of commonly used solvents and how they are selected.

Common solvent classification

Plastic gravure printing ink is divided into four categories: alcohol, ester, benzene and ketone.

1. Alcohols Common alcohol solvents include ethanol (alcohol), isopropanol, n-butanol, and the like. Ethanol and n-butanol are mostly used as cosolvents. Ethanol has excellent solubility in some resins, especially for dyes. Ethanol has high moisture absorption and its solubility will decrease with the increase of water content; Fast, dosage should not be too much. Isopropanol is miscible with water, ethanol and toluene.

2. Ethyl acetate (ethyl acetate) and butyl acetate (butyl acetate) are commonly used as ester solvents for esters. Ester solvents have strong dissolving power and have a strong fruit odor, but the price is high. Printing ink.

3. Benzene Commonly used benzene solvents are benzene, toluene, xylene and so on. Benzene is volatile, cheap, but toxic; toluene is ideal for dilution ratio and volatilization rate, and is one of the most widely used varieties of plastic gravure inks. Xylene volatilizes slower than toluene and is one of the most commonly used solvents in plastic gravure inks.

4. Ketones The commonly used ketone solvents include butanone (methyl ethyl ketone, MEK) and cyclohexanone. Ketones have strong solvency and are particularly suitable for high solid content plastic gravure inks, which help to improve the flowability and leveling of high solid content plastic gravure inks, but the odors are relatively unpleasant.

The nature of the solvent

1. Solubility The solvent's actual ability to dissolve the resin is the primary condition for its ease of use. Solubility can be measured by the solubility parameter δ, which is defined as the square root of the liquid's cohesive energy density:
δ=(ΔE/Vm)1/2
Where: â–³E, cohesive energy, refers to the heat of vaporization of a liquid or solid (J), Vm is the molar volume (cm3).
The solubility parameter formula for the mixed solvent is:

δ mixing=X1V1δ1+ X2V2δ2+ X3V3δ3+......XiViδi/X1V1+ X2V2+ X3V3+......XiVi
In the formula, Xi and Vi are the molar fraction and the molar volume of the mixed solvent, respectively. The dissolution parameters provide a solid theoretical basis for understanding the interaction of solvents with resins, pigments, and solvents, and provide a logical system for correcting and improving the formulation of plastic gravure inks.

2. Evaporation rate The volatilization of solvent is similar to the evaporation of moisture in wet clothes. It is mainly affected by temperature, vapor pressure, surface area/volume and surface air flow rate. It is also an important factor that determines the drying speed of ink film and affects the formation of ink film. And the final quality is the second important condition for selecting a solvent formulation. The volatilization rate of the mixed solvent is calculated by the following formula:
R=∑RiΥiVi
In the formula, Ri, Υi, and Vi are the volatilization rates, activity parameters, and volume fractions of the i component, respectively.
The volatilization of solvents in plastic gravure inks can be divided into two stages. In the first stage, the volatilization rate of solvents is mainly affected by the temperature, vapor pressure, surface area/volume and surface air velocity of a single solvent. The same solvent. With the further volatilization of the solvent, the volatilization rate abruptly slows. In the second stage, the volatilization rate is no longer controlled by the volatilization of the surface solvent, but is affected by the ability of the solvent to diffuse from the interior of the ink film to the surface. At this time, the volatilization rate of the solvent is slower, and those solvents that have a strong effect on the resin will remain in the ink film for a long time, that is, the solvent release rate is slow.

The volatilization rate of the solvent is one of the important factors that determine the properties of the final printed ink film. If you use a lot of fast volatile solvents, such as butanone, ethyl acetate, toluene and other solvents, the viscosity of the ink attached to the substrate will greatly increase, the fluidity will decline, and the level of the ink film will be seriously affected. Skin phenomenon; In addition, due to the rapid evaporation of the solvent, the ink can not be fully leveled to form a lot of pinholes; due to the large amount of solvent evaporation and absorption of heat, resulting in a sharp decline in surface temperature of the substrate, so that the surrounding air water condensation on the ink film The phenomenon of “whitening” occurs; as the solvent volatilizes too quickly, the surface of the ink film quickly solidifies, and the bottom layer solvent cannot escape. When the temperature rises, the residual solvent extrudes from the inside out of the surface to form bubbles, ie, holes.

If a large amount of slow-evaporating (high-boiling) solvent is used, the ink will dry slowly, which will cause the ink film to become sticky, soft, flow-bearing, and thick edges.

3. Surface Tension Surface tension is one of the intrinsic properties of plastic gravure inks. In the preparation and printing of plastic gravure inks, pigment wetting, ink film leveling, and wettability and adhesion to the substrate are all related to the surface tension. Related.
The surface wetting principle can be used to adjust the wettability of the ink. For example, the surface tension of a plastic gravure ink is small, and a solvent with a large surface tension can be added for adjustment. Conversely, a solvent with a small surface tension can be added.

4. Viscosity In view of the difference in the viscosity of various solvents is not more than 0.001Pa · s, it is generally believed that the viscosity of the solvent will not affect the viscosity of plastic gravure ink. In actual fact, different solvents have different viscosity. The reason is that the solvent solubility of the solvent, the hydrogen bond of the solvent, and the viscosity of the solvent will affect the viscosity of the plastic gravure ink.

Formulation design of mixed solvents

After knowing and mastering the solvent's solvency, volatilization rate, surface tension and other properties, how to select and quantify the mixed solvent for ink?

(1) Select the solvent or mixed solvent that can dissolve the resin by dissolving the parameters.
According to the mechanism of action, the solvent is divided into a true solvent, a dummy solvent, and a co-solvent. In a solvent for a plastic gravure ink, a solvent that can dissolve the ink alone is called a true solvent; a solvent that does not dissolve the ink is called a false solvent; a solvent that cannot be dissolved alone and that must be mixed with other solvents to dissolve is called a cosolvent. Adjust the balance between true solvent, spurious solvent, and co-solvent to achieve the desired dissolution force, volatilization rate, and viscosity. The dissolving power of the mixed solvent should not be too strong. If it is too strong, the release rate of the solvent will be slow; if it is too weak, the resin cannot be fully dissolved, which will affect the plastic gravure ink and the printing quality.
The volatilization of the mixed solvent generally belongs to the non-ideal liquid volatilization. If the proportion of the true solvent in the residual solvent is reduced, the dissolving power will be weakened, the wet film is in a poorly dissolved state, or even insoluble, and the phenomenon of phase separation and whitening of the ink film occurs. Therefore, in the mixed solvent formulation, the amounts of fast, medium, and slow volatile components should be balanced, and the distillation curve of the mixed solvent should also show a balanced upward trend, and the vapor phase composition of the mixed solvent and the liquid phase composition should be kept consistent at the ambient temperature as much as possible. , so that the mixed solvent has a good solubility.
(2) Establish volatilization profiles of solvents or mixed solvents. If you do not know volatility diagrams of solvents or mixed solvents as a function of time and temperature, usually select several mixed solvents with different volatility to test to establish their volatility profiles. The figure provides a reference for the choice of solvent or mixed solvent.
(3) Select several solvents or mixed solvents through the above steps, and then determine by experiment which solvent or mixed solvent can meet the requirements of the plastic gravure ink formulation, and sometimes it may be necessary to adjust the composition and amount of the mixed solvent in a small range. In order to improve the performance of a certain aspect of the ink, a mixed solvent that meets the various performance requirements of the plastic gravure ink is finally obtained.
(4) Optimize the solvent or mixed solvent ratio to meet the solubility and other properties of the resin. Under the premise of ensuring that the mixed solvent has sufficient dissolving power, suitable volatilization speed, suitable viscosity and surface tension, an effective, inexpensive and low-toxic solvent should be used, which can effectively dissolve the resin, disperse the pigment, and satisfy the ink film. The performance requirements can also reduce solvent costs.
The research and application of solvent volatilization speed, dissolving power, surface tension, viscosity, flash point, flammability, etc., have been very active in the ink manufacturing industry, and there will be considerable progress in the future.



Source: Ke Yin Media - "Printing Technology"