The smaller the surface tension of the lubricant, the better its spreading performance. Under normal circumstances, the surface tension of pure water is high, up to 72.75×10-5N/cm, so it is necessary to reduce its surface tension in order to be suitable for wetting printing plates. At present, the best way is to add surfactants, such as ethanol, to pure water. Isopropyl alcohol and some other low alcohols, low carboxylic acids, etc. The surface tension of the solution decreases rapidly with the increase of the substance concentration, and if the substance concentration continues to increase, the surface tension is almost unchanged, so the surfactant should be added to the solution to the right amount, not too much.
Surfactants are "amphiphilic" molecules, consisting of a non-polar oil-philic part of a hydrocarbon chain and a polar hydrophilic group. Its nature depends on the structure of the hydrophilic group, therefore, the surfactant can be divided into cationic surfactants, anionic surfactants, amphoteric surfactants, and non-ionic surfactants according to whether the hydrophilic group ionizes in water, and the type of ionized ions.
Cationic surfactants, readily adsorbed on solid surfaces in media, often used as flotation agents.
Anionic surfactants can be ionized in water, and the surface active base is negatively charged.
The molecular structure of amphoteric surfactants is similar to that of amino acids in proteins, with both acidic and basic groups. Ionized in acidic solution, the surface active base is positively charged; Ionized in alkaline solution, surface-active base is negatively charged; It is not ionized or charged in neutral solution, and has the effect of anti-metal corrosion and anti-surface static electricity, because its surface activity is not affected by the acid and alkaline solution, so it can be considered for application in the moisturizing solution.
Non-ionic surfactants do not ionize in aqueous solution, and the hydrophilic group is mainly composed of a certain number of oxygen-containing groups, which is not affected by strong electrolytes, inorganic salts, acids and bases, and has high stability. When non-ionic surfactants are added to the moisturizing solution, chemical reactions will not occur with other electrolytes in the moisturizing solution. Therefore, it is most appropriate to use non-ionic surfactants to reduce the surface tension of the solution.
The blank part of the offset printing plate is attached to the moistening solution, and the graphic part is attached to the ink. If the surface tension of the ink is less than the surface tension of the moistening solution, under the action of diffusion pressure, the ink is infiltrated in the direction of the moistening solution, forming the printing dot expansion and dirty in the blank part. On the contrary, if the surface tension of the moistening solution is less than the surface tension of the ink, under the action of diffusion pressure, the moistening solution wets in the direction of the ink, so that the printing dot disappears. Only when the surface tension of the moistening solution and the surface tension of the ink are equal, the diffusion pressure on the interface is zero, and the moistening solution and the ink maintain a relative balance on the interface without infiltration, the printing effect is more ideal. Generally speaking, the surface tension of the offset ink is about 30 ~ 36×10-5N/cm, and the balance of the offset ink is achieved under the dynamic, so the surface tension of the plate should be slightly higher, preferably between 35 ~ 42×10-5N/cm. Therefore, the concentration of the surfactant in the moisturizing solution should be the corresponding concentration when the surface tension of the moisturizing solution reaches 35 ~ 42×10-5N/cm, if this concentration is just the critical concentration of the surfactant micelle, it is more ideal, and a stable surface tension can be obtained.