In gravure inks, solvents not only dissolve various film-forming polymers, adjust viscosity and rheology to suit printing, but also have an important impact on film-forming quality. One of the key techniques in the formulation of gravure inks is the selection of suitable solvents. The following is a detailed description of the characteristics of the solvent and how to choose and quantify it:
1. Classification of common solvents
1. Alcohols
Commonly used alcohol solvents are ethanol (alcohol), isopropyl alcohol, n-butanol, etc. Ethanol and n-butanol are generally used as co-solvents. However, ethanol has a high hygroscopicity, good solubility with certain resins, and good solubility with dyes. The solubility of ethanol decreases with increasing water content; Fast, the dosage should not be too much. Isopropyl alcohol is miscible with water, ethanol and toluene.
2. Esters
Commonly used ester solvents are ethyl acetate (ethyl acetate) and butyl acetate (butyl acetate), which have strong dissolving power and strong fruity smell. The disadvantage is that the price is too high. Commonly used in solvent gravure inks.
3. Benzene
Commonly used benzene solvents are: benzene, toluene, xylene, etc. Benzene volatilizes quickly and is cheap, but it is highly toxic; toluene is ideal in dilution ratio and volatilization rate, and it is one of the most widely used varieties in gravure inks. Xylene has a slower volatilization rate than toluene and is also one of the most commonly used solvents in gravure inks.
4. Ketones
Commonly used ketone solvents are: methyl ethyl ketone (methyl ethyl ketone, MEK), cyclohexanone and so on. Ketones are highly soluble solvents, especially suitable for high solids gravure inks. They also help to improve the fluidity and leveling of high solids gravure inks, but their odor is relatively unpleasant.
Second, the nature of commonly used solvents
1. Solvency
The actual solubility of the solvent to the resin is the primary condition for its use. The dissolving power can be measured by the dissolving parameter δ. The dissolution parameter δ defines the square root of the cohesive energy density:
δ = （△ E / Vm） 1/2
In the formula: â–³ E cohesive energy refers to the heat of vaporization (cal) of liquid or solid, Vm is the molar volume (cm3).
Formula of the dissolution parameters of mixed solvents:
X1V1δ1 + X2V2δ2 + X3V3δ3 ＋ …… ＋ XiViδi
delta mixing =
X1V1 + X2V2 + X3V3 + …… + XiVi
In the formula: X1V1 is the mole fraction and mole volume of the mixed solvent, respectively.
Dissolution parameters provide a solid theoretical basis for understanding the interaction of solvents with resins, pigments, and solvents, and provide a logical system for revising and improving gravure ink formulations.
2. Evaporation rate
The volatilization of the solvent is similar to the volatilization of moisture in wet clothes. It is mainly affected by temperature, vapor pressure, surface area / volume ratio, and the speed of surface air flow. It is an important factor that determines the drying speed of the ink film, which affects the form of the ink film and In turn, it affects the final quality of the ink film. It is the second important consideration when choosing a solvent formulation.
The volatilization rate of the mixed solvent is calculated by the following formula:
R = ΣRiTiVi
In the formula: R is the volatilization rate of the mixed solvent, RiTiVi is the volatilization rate rate, activity parameter and volume fraction of the I component.
The volatilization of solvent in gravure ink can be divided into two processes (stages), namely the first stage: the solvent volatilization rate is mainly affected by four factors of a single solvent (ie temperature, vapor pressure, surface area / volume ratio and surface air flow rate) The volatilization rate of the solvent is the same as that of the single solvent. With the further volatilization of the solvent, the volatilization rate of the solvent suddenly slows down. At this time, it enters the second stage: the volatilization rate of the solvent is no longer controlled by the volatilization of the surface solvent. Controlled by the diffusion of the inner surface of the coating film, at this time, the evaporation rate of the solvent is slower, even those solvents that have a strong interaction with the resin will stay in the ink film for a long time, and will not be released for a long time, that is, the solvent dilution is slow.
The size of the solvent evaporation rate is one of the important factors that determine the final ink film properties. If a large amount of solvents that evaporate quickly, such as methyl ethyl ketone, ethyl acetate, toluene and other solvents, the viscosity of the gravure ink particles attached to the substrate will be greatly increased, the fluidity will be reduced, and the leveling will be seriously affected, resulting in "orange peel "Phenomenon; In addition, due to the rapid volatilization of the solvent, the gravure ink failed to level sufficiently to form many pinholes. Due to the large amount of solvent volatilization and heat absorption, the surface temperature of the coated object dropped sharply, which was enough to cause moisture in the surrounding air Condensation into small water droplets results in "whitening" phenomenon; finally, because the solvent volatilizes too fast, the surface layer of the ink film immediately solidifies, and the underlying solvent cannot escape. When the temperature rises, the residual solvent extrudes from the inside of the surface, forming bubbles, That is a hole. If a large amount of slow volatilization (high boiling point) solvent is used, the gravure ink will dry slowly, causing stickiness, softness, sagging, and thickened edges.
The solvents listed above like butanone, ethyl acetate, and ethanol are all quick-drying solvents; like butyl ester, xylene, butanol, and cyclohexanone are slow-drying solvents. [next]
3. Surface tension
Surface tension is one of the inherent properties of gravure ink. In the preparation and printing of gravure inks, pigment wetting, ink film leveling, wetting of substrates, adhesion, etc. are all closely related to surface tension.
The principle of surface tension can be used to adjust the gravure ink. If the surface tension of the gravure ink is small, a solvent with a large surface tension can be added. On the contrary, if the surface tension is large, 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 1 centipoise, so people often do not take the viscosity of the solvent as a factor affecting the viscosity of the gravure ink. In fact, it has been proved that the viscosity of the gravure ink prepared by different solvents differs by hundreds of centipoises. why? This is because the dissolving power of the solvent to the resin, the hydrogen bond of the solvent and its own viscosity will affect the viscosity of the gravure ink.
3. Formula design of mixed solvents
After being familiar with and mastering the solvent's dissolving power, volatilization speed, surface tension and other properties, how to choose and quantify the mixed solvent in gravure ink?
First, select the solvent or mixed solvent that can dissolve the resin through the dissolution parameters.
We divide the solvent into true solvent, false solvent and co-solvent according to the mechanism of action. In the gravure ink solvent, the solvent that can dissolve the gravure ink alone is called the true solvent; the solvent that cannot dissolve the gravure ink is called the false solvent; the solvent that cannot dissolve the gravure ink alone and must be mixed with other solvents to dissolve the gravure ink is called the cosolvent . Adjust the balance between the real solvent, the fake solvent and the co-solvent to achieve the desired dissolving power, volatilization speed, viscosity, etc. The dissolving power of the mixed solvent cannot be too large, nor too small. Because it is too large, the dilution of the solvent is slow; if it is too small, the resin fails to fully dissolve, affecting the quality of the gravure printing ink.
The volatilization of mixed solvents is generally a non-ideal liquid volatilization behavior. If the ratio of the true solvent to the residual solvent is reduced, the dissolving power will be weakened, and the wet film will be in a poorly dissolved state, or even insoluble, and there will be phase separation. Make the ink film appear white. Therefore, the amount of fast, medium, and slow volatile components in the mixed solvent formulation should be balanced. The distillation curve equation of the mixed solvent is in a balanced rising shape, and the vapor phase composition and the liquid phase composition of the mixed solvent should be maintained during the volatilization process at ambient temperature. Consistent, so that the mixed solvent has good solubility.
Second, establish the volatilization profile of the solvent or mixed solvent. If the volatilization relationship of the solvent or mixed solvent with time and temperature is not known, it is usually necessary to select several mixed solvents with different volatile substances for testing to establish the volatilization of these mixed solvents The outline drawing provides a reference for the choice of solvent or mixed solvent.
Third, the experiment confirms the selected solvent or mixed solvent. Once several candidate solvents or mixed solvents are selected through the above steps, the next step is to determine which solvent or mixed solvent can meet the requirements of the gravure ink formulation through experiments. Sometimes It may be necessary to adjust the composition and amount of the mixed solvent in a small range to improve the performance of certain aspects of gravure ink, such as viscosity, leveling, volatilization, etc., and finally obtain a mixed solvent that meets all aspects of gravure ink performance.
Fourth, optimize the solvent or mixed solvent to meet the solubility of resin and other properties of gravure ink. Under the premise of ensuring that the mixed solvent has sufficient dissolving power, appropriate volatilization speed, appropriate viscosity and surface tension, according to the above performance data, select an effective, cost-effective and low-toxic solvent, which can effectively dissolve the resin and disperse the pigment. , To meet the performance requirements of the ink film, and reduce solvent costs as much as possible.
4. Conclusion
It is true that the research and application of the solvent's volatilization rate, dissolving power, surface tension, viscosity, flash point, and flammability have been very active in the past and future, and considerable progress will be made in the future. The above explanation is just the author's superficial knowledge. Here, I urge you to correct me a lot, which is also the purpose of this article.
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