Introduction to common solvents for screen printing inks
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Solvents are volatile organic liquids and are an essential component of inks. Solvents can be classified into low-foam point solvents according to the boiling point classification - the boiling point is below 100 ° C, the medium boiling point solvent - the boiling point is between 100 and 150 ° C, and the high boiling point solvent - the boiling point is between 150 and 250 ° C.
According to the chemical composition classification, it can be divided into: petroleum hydrocarbon solvent, coal coke solvent, terpene hydrocarbon solvent, alcohol solvent, ester solvent, ketone solvent, ether ester solvent, and the like.
Solvents are not only an important part of the ink, but can also be used to adjust the viscosity and drying speed of the ink. The speed of solvent evaporation can directly affect the drying speed of the ink and the quality of the formation of the ink layer. If the solvent can be reasonably selected, the cost can be reduced.
In the printing operation, we often need to adjust the ink according to the printing task, but because the performance of the ink and solvent is not very well understood, the ink mixing process may occur due to improper solvent addition or incorrect ink selection during the coloring process. The phenomenon of ink precipitation or agglomeration, resulting in unnecessary loss of ink scrap.
The above phenomenon occurs because the solvent and the resin are not well miscible, and related to this factor is the theory of "polarity".
First, the polarity of the solvent
Both the resin and the solvent 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 the polymer resin and the organic solvent, some polar groups are often contained, such as: -COOH, -OH, -CO, -NO, and the like. If the molecular structure of the resin and the organic solvent are symmetrical and do not contain polar groups, they are non-polar; if they have asymmetrical molecular structure and contain various polar groups, such as carboxyl groups, hydroxyl groups, carbonyl groups. , nitro, etc., then they all have different polarities.
Therefore, some resins can only be dissolved in a certain solvent. For example, peroxyethylene resin can only be dissolved in cyclohexanone solvent, and nitrocellulose can only be dissolved in polar solvent esters and ketone solvents. Some weakly polar dry 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 master the color of the same type of ink. If you need to dispense green plastic ink, you can only use blue plastic ink and yellow and plastic. Ink is formulated, but not with plastic inks and metallic inks.
If the ink is too thick and needs to be transferred to the solvent, then the plastic ink can be used with cyclohexanone. When the screen is wiped, xylene and cyclohexanone can be used in a wipe to reduce the cost. Adjusted from dry metal and glass ink, it can be adjusted with xylene and pine perfume. The high temperature curing ink can be used with xylene, toluene and pine perfume. The two-component curing epoxy ink can be adjusted with a mixture of butanol and xylene.
Second, true solvent, cosolvent, thinner
For volatile dry inks, the solvents used can be divided into three categories:
1, true solvent:
A solvent having a polymer resin in which such an ink is dissolved is a true solvent of the resin.
2. Cosolvent (potential solvent):
It does not dissolve the polymer resin used by itself, but in a limited amount, it can be mixed with a true solvent to provide a certain degree of solubility.
3. Thinner:
This solvent does not dissolve the polymer resin used, and it does not have a dissolution effect. However, it can be mixed with a true solvent within a certain limit. It plays a dilution role, and the price is lower than the true solvent and cosolvent used, which can reduce the cost. .
The classification of such solvents is relative. For a solvent, the role of a solvent in a type of ink is a true solvent, and in another ink, it can only be used as a diluent.
Third, the boiling point, flash point and spontaneous ignition point of the solvent
Organic solvents are flammable (except chlorinated hydrocarbons), and some flash points can reach minus -16.5 °C. Therefore, air circulation must be ensured indoors during printing. For safety reasons, no open flames are allowed in the printing workshop.
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 different from those at normal temperature.
2, flash point:
The solvent is heated, and the vapor is emitted into the air. When the temperature rises, the solvent vapor concentration gradually increases. When there is an open flame, there is a flame, and then the temperature is extinguished.
3. Volatilization rate:
Observe the volatilization rate of a drop of the test solvent on the filter paper in a box of a predetermined size at a certain temperature and relative humidity.
Volatilization rate value = butyl acetate volatilization time / volatilization time of the test solvent × 100%
If the value of the volatilization rate is smaller, the slower the volatilization rate of the solvent, the larger the volatilization rate value, and the higher the volatilization rate of the solvent. With the help of the following table, when screen printing is performed, especially when printing with "solvent-evaporable ink", the solvent can be selected reasonably in the climate season. (see table below)
Fourth, introduce several common solvents
1. Pine perfume:
It is 200# solvent gasoline, which is a petroleum product. It can dissolve rosin, glycerin stearin and alkyd resin. It can adjust metal and glass inks. Its solubility is medium range and it can be miscible with many organic solvents.
2. Turpentine oil:
It is the volatile part of the oily turpentine extracted from pine trees. There are three isomers of turpentine: α-pine, β-pine, and diterpene. It evaporates evenly, and due to the degree of unsaturation in the molecular structure, oxidative polymerization can be produced, and a component in which the agaricol is dissolved in the oil and the resin to form a film is formed. The turpentine oil evaporates evenly, and the oxidative polymerization type ink has an effect of preventing skinning and improving leveling property.
3, xylene:
Industrial xylene is a mixture of three different objects, ortho, meta and para. The dissolving power of xylene is larger than that of turpentine and pine perfume, and the volatilization rate is faster than the two. It is widely used in high temperature curing inks and oxidative polymerization inks.
4, toluene:
It is derived from petrochemical or coal coke industry. Toluene can dissolve ethyl cellulose, maleic anhydride resin, rubber, perchloroethylene, etc. It can adjust glass ink and metal ink.
5. Ethanol:
Depending on the application, its purity is different, the purity of absolute ethanol is 99.5%, industrial ethanol is 95.6%, and pharmaceutical ethanol is 70-75%. Ethanol can dissolve many natural resins, such as: shellac, polyvinyl acetate, cyclohexanone resin, polyvinyl acetal resin (made with the adhesive for screen printing), but the sensitivity of ethanol to solvent The glue has a destructive effect.
6, diacetone alcohol:
The main product which is easy to decompose and decompose is acetone, which can dissolve rosin, polyvinyl acetate, nitrocellulose, shellac, chloroacetate copolymer resin and the like.
7, acetone:
It is a highly volatile organic liquid that dissolves perchloroethylene resin, chloroacetate copolymer resin, chlorinated rubber, and the like. It has a destructive effect on the solvent-resistant photosensitive film. But it is a good solvent for tackifying mesh glue.
8, cyclohexanone:
It has very good solubility, in addition to soluble part of natural resin, it can dissolve on many synthetic resins, and it is miscible with most organic solvents. It is a good solvent for solvent-evaporable inks.
9, methyl ethyl ketone:
It is basically similar to acetone, but has a higher flash point and a slower evaporation rate. It is soluble in water, ethanol, ether and hydrocarbons, and oils.
10. Isopropanol:
It is soluble in water, ethanol, ether and chloroform.
11. Butanol:
It can dissolve many natural resins and synthetic resins, such as polyvinyl acetate resin, melamine resin, etc. Butanol and xylene can be mixed in a ratio of 3:7 for high-temperature curing inks and epoxy-based inks.
12. Ethyl acetate:
It can dissolve ethyl cellulose, nitrocellulose, chlorinated cyanoacrylate, chloroacetic copolymer resin and the like.
13, butyl acetate:
It can dissolve many synthetic resins, such as polyvinyl chloride resin, chloroacetic copolymer resin, chlorinated rubber, polyvinyl chloride resin, etc. It is a solvent for solvent-evaporable inks, but it is not suitable for printing in summer due to faster volatilization.
14, kerosene:
Also known as kerosene, colorless or light yellow transparent oily liquid, it is a petroleum distribution product that can be used to produce printing paste.
Five, solvent considerations for ink adjustment
1. The solvent selected for ink adjustment should have good mutual solubility with the polymer resin in the ink.
2. The volatilization rate of the solvent used should be uniformly reduced as the ink layer is dried to prevent the surface of the ink film from being ill.
3. Organic solvents are flammable and have certain toxicity to the human body. Therefore, it is necessary to keep the air circulation in the printing workshop and avoid the existence of fire sources.