Offset printing is indirect printing. After printing in Xuzhong, the blank part of the printing plate was first wetted by water, then passed the ink roller, and the ink was on the graphic part, and then the ink on the graphic was transferred to the substrate through the blanket.
At present, most of China's books and periodicals (over 2/3) use offset printing; the central and provincial prefecture-level newspapers and periodicals have all eliminated lead printing and realized offset printing; in the packaging industry, more than 500 offset printing machines have been imported, and their number is far more than Although there are more than 200 gravure printing lines. In the entire printing industry, offset printing has become a very important printing method.
In the offset printing process (after color change or shutdown), it is often necessary to clean the ink on the ink roller and blanket. To remove ink well, first of all, we should dissolve the components of offset printing ink.
Generally speaking, the main components of offset printing ink are: solid resin 48%, liquid resin 6%, colloidal oil 20%, wax 5%, aluminum carbonate 2%, and pigment 15%.
The removal process is actually to use a suitable solvent to dissolve the resin component, so as to achieve the purpose of removal.
The resin in printing ink is generally phenolic resin. Commonly used resins include rosin modified phenolic resin, alkyd resin and polyurethane alkyd resin. To remove them, you must find a solvent that can dissolve these resins. According to the principle of similar miscibility (that is, the solvent of the polar gene dissolves the material with the polar gene, and the non-polar solvent dissolves the non-polar material) and the solubility parameter theory (for non-polar molecular systems, usually the dissolution of the polymer In general, the dissolution parameters of the solvent and solute should be as close as possible, which can be beneficial for dissolution). A solvent can be found. This solvent should have the following properties:
a. Has a high dissolving power.
b. Faster evaporation rate.
C. Strong penetration.
d. Good adsorption wettability.
e. Harmless to human body, use only.
f. Basically no corrosion to blanket and ink roller.
g. low cost.
The solubility parameter of the phenolic resin is between 8 and 11.5, and the solubility parameter of the main solvent in the cleaning agent should also be in this range. The solubility of ketones, xylene, diethylene glycol, valerolactone, etc. is within this range, so one or two of them should be selected as solvents. After testing, the combination of valerolactone and dimethylbenzine as a universal solvent for ink and photosensitive adhesive has good solubility, so choose the mixed component of the two as the solvent; you can also choose to take a hydrocarbon solvent, such as n-heptane Alkanes, high boiling point kerosene and Other solvents adjust the volatilization rate of the entire solution; due to the strong permeability of binary lower halogenated hydrocarbons, such as 1,1,1-trichloroethane, can be used as a penetrating agent for cleaning agents; adding anions Surfactant can improve the absorption, wetting and dispersing properties of the solution. OP-10 is a non-ionic surfactant with easy dispersing and good hydrophilic and oleophobic ability. It can be shared with anionic surfactants as oil / water Type emulsifier penetrant. Because of its greater solubility in cold water than hot water, it is ideal as an emulsifier. The ideal formulation of cleaning agent can be like this: solvent: 45% solubilizer: 30% penetrant 5% diluent: 15% emulsifier: 5%
However, the above-mentioned cleaning agent has an obvious shortcoming, that is, the amount of the solvent is high, the volatilization in the printing workshop is very serious, it has a serious impact on the printing environment and the health of workers, and the cost is high. Therefore, many printing plants simply use gasoline as a cleaning agent. Is this appropriate? Obviously, it is definitely wrong to sacrifice the environment and the health of workers in exchange for the reduction in printing costs. European and American countries passed the Environmental Protection Law (VOC Law) around the 1990s. According to the law, all geographies using organic solvents must meet the following requirements:
1. There should be no smell of organic solvents outside the workshop.
2. The concentration of volatile substances in the workshop must be lower than:
1) Volatile organic solvent 50 m / m3
2) Carbon monoxide 100 mg / m3
3) Nitric oxide 100 mg / m3.
Obviously, this law is closely related to the printing industry.
In the face of VOC laws, printing companies in Western countries must strictly abide by it, even at the expense of printing quality and increasing costs.
While considering the development of the printing industry, China must also look at the problem from the perspective of environmental protection: in the printing industry, the discharge of harmful gases and the use of harmful liquids and solids and the discharge of waste should be strictly controlled.
In view of the fact that our government has long regarded environmental protection as a basic national policy, in order to achieve the coordinated development of economic construction and environmental protection and the sustainable development of the national economy, we must ask the question of environmental protection when formulating the development plan of the printing industry, Formulate practical and effective measures.
Some of the cleaning agents originally used in the printing industry in China are toxic in themselves, and the emission of hydrocarbons in all solvents causes pollution to the atmosphere. At the same time, the cleaning agent used in the offset printing process must be thoroughly reformed.
In order to limit pollution and meet VOC laws, printing plants generally use water-in-oil emulsion cleaning agents for cleaning rubber rollers and blankets. The water-in-oil emulsion cleaning agent can not only completely eliminate the dangers of certain harmful substances in the solvent-based cleaning agent to the printing workers, prevent the pollution of the atmosphere by organic solvents, but also greatly improve the flash point of the cleaning agent and improve the fire safety of the printing workshop. To improve the quality of the overall environment. The so-called water-in-oil emulsion cleaning agent means that the cleaning agent is made into an emulsion, and the external phase is oil (oil or solvent); the internal phase is water; and the surface uses a surfactant as a stabilizer. Due to the presence of water, the volatility of the external phase oil or solvent is greatly reduced, and the content of organic volatiles in the air is greatly reduced. And the presence of water greatly increases the flash point of the solution; it increases the fire safety of the printing workshop.
The key to studying water-in-oil emulsion cleaning agents is to find suitable solvents and surfactants to solve the problems of cleaning speed, protection of blankets and rubber rollers, and emulsion stability. This requires a lot of repeated experiments.
The reference formula is as follows: solvent and solubilizer: 45% surfactant: 5% water: 50%
Although there are some unsatisfactory aspects of the water-in-oil emulsion cleaning agent, such as the best quality cleaning agent, its drying speed is not as fast as gasoline, but analyzing its advantages and disadvantages, it should have become the first choice cleaning agent in the printing shop . At present, there are different grades of cleaning agents on the market. High-end cleaning agents should have faster cleaning and drying speeds, which have no effect on blankets and rubber rollers. The solvents used meet the requirements of green environmental protection. The low-grade cleaning agent is lacking in the influence on the factory rubber and rubber ravioli, or the cleaning speed is slower.
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