The theory of "polarity" of printing solvents

Solvents are volatile organic liquids that are one of the essential components in inks. According to the classification of the boiling point of the solvent, the solvent can be divided into low Buddha spot solvent - boiling point below 100 °C, middle boiling point solvent - boiling point between 100 ~ 150 °C, high boiling point solvent boiling point between 150 ~ 250 °C.

Classified by chemical composition can be divided into: petroleum hydrocarbon solvent, coal cosolvent, helium hydrocarbon solvent, alcohol solvent, ester solvent, ketone solvent, ether ester solvent.

Solvent is not only an important part of the ink, but also can be used to adjust the ink viscosity and drying speed. The speed of solvent evaporation can directly affect the drying speed of the ink and the quality of ink layer formation. If the solvent can be reasonably selected, the purpose of reducing the cost can also be achieved.

In the printing operation, we often need to allocate ink according to the printing task. However, due to the poor understanding of the properties of the ink and the solvent, the ink mixing process may be due to the improper addition of the solvent, or the wrong ink may be selected during the color adjustment process. The phenomenon of ink precipitation or clumping, thus causing unnecessary loss of ink scrap.

The appearance of the above phenomenon is due to the fact that the solvent and the resin are not well miscible with each other, and this factor is related to the theory of "polarity."

First, the polarity of the solvent

Both resins and solvents can be classified into non-polar, polar, and weak polarities according to their molecular structure. This property is affected by, for example, the symmetry of the molecular structure, the type and number of polar groups, and the length of the molecular chain. Etc.

In the molecular structure of polymer resins and organic solvents often contain some polar groups, such as: -COOH, -OH, -CO, -NO and so on. If the molecular structure of the resin and the organic solvent is symmetrical and does not contain polar groups, they are non-polar; if they have asymmetric molecular structure, they contain various polar groups such as carboxyl, hydroxyl, and carbonyl groups. Nitro, etc. Then they all have different polarities.
Therefore, some resins can only be dissolved in a certain solvent, such as: Peroxyethylene resin can only be dissolved in cyclohexanone solvent, nitrocellulose can only be dissolved in polar solvent esters, ketone solvents. Some weakly polar drying oils are insoluble in polar solvents and soluble in non-polar hydrocarbon solvents.

The above theory is applied to the actual printing operation. It is only necessary to grasp the ink of the same type when mixing colors. For dispensing green plastic ink, only blue plastic ink and yellow and plastic can be used. Ink deployment, but not with plastic ink and metal ink deployment.

If the ink is too thick to be incorporated into the solvent, then the plastic ink, which can be used in cyclohexanone, can reduce the cost by erasing the screen plate when xylene and cyclohexanone are used together. Adjusted from dry metal, glass ink, available xylene, pine perfume transfer. Adjustable high temperature curing ink available xylene, toluene, pine perfume. The two-component curable epoxy ink can be adjusted with a mixture of butanol and xylene.

Second, the real solvent, cosolvent, thinner

For volatile dry inks, the solvents it uses can be divided into three categories:
1, true solvent:

The solvent having the polymer resin dissolved in the ink is the true solvent of the resin.

2, auxiliary solvent (potential solvent):

It does not dissolve the polymer resin used by itself, but it can provide a certain degree of solvency by mixing with a true solvent within a limited amount.

3, thinner:

This kind of solvent can't dissolve the polymer resin used, and it doesn't help the solution, but it can be mixed with the true solvent within a certain limited amount. It plays a dilution effect, and the price is lower than the true solvent and auxiliary solvent used, which can reduce the cost. .
The classification of this solvent is relative, it is a true solvent for a certain kind of solvent in a type of ink, and it can only be used as a diluent in another kind of ink.

Third, the boiling point, flash point and auto-ignition point of the solvent

Organic solvents are all flammable products (excluding chlorinated hydrocarbons), and some flash points can reach minus 16.5°C. Therefore, indoors must be air circulated during printing. For the sake of safety, open flames are not allowed in the printing shop.

1, boiling point:

The boiling point of the solvent is the temperature at which the vapor pressure of the solvent reaches one atmosphere. The relative high and low vapor pressures of various solvents at high temperatures are not the same as those at room temperature.

2, flash point:

When the solvent is heated, the vapor is emitted into the air. As the temperature rises, the concentration of the solvent vapor gradually increases. When the flame is met, there is a flame, and the temperature immediately goes out.

3, the evaporation rate:

At a certain temperature and relative humidity, observe the volatilization rate of a drop of the tested solvent on the filter paper in a box of the specified size and shape.

The smaller the value of the evaporation rate, the slower the volatilization rate of the solvent, the greater the value of the volatilization rate, the higher the volatilization rate of the solvent. With the help of the following table, in the case of screen printing, especially when the "solvent-evolving ink" is selected, it is possible to select the solvent for the climate season. (See the table below)