Because the printing ink is dried by solvent volatilization, there is a certain amount of solvent residue in the packaging bag. The odor and toxicity produced by organic solvents will contaminate food and endanger human health. At present, the national standard stipulates that no square meter is10Therefore, the residual solvent of the printed matter is an important indicator for inspecting the quality of the ink and the printing quality of the packaging bag. Now we will discuss how to control the residual amount of the solvent.1. FilmAfter the mixed organic solvent is coated on the film by the printing machine, the residual amount is determined as follows:MST>KM>KOP>OPP>CPP>PT>PET>NY.If the residual solvents that are easily generated for various films are selectively listed as follows: ①CPP, toluene②OPP, toluene③PET, ethyl toluene acetate④NY, isopropanol, toluene⑤KM, methyl ethyl ketone⑥MST, isopropanol, toluene⑦PT, isopropanol, toluene⑧KOP, Toluene.For a film with a high permeability to a certain solvent, the residual amount is large due to its affinity with the solvent. The film with low permeability has a small residual amount.Second, the printing press(1) If the wind speed in the dryer is large enough, the residual amount of solvent is inversely proportional to the rise of temperature, and the residual amount is small if the temperature is high: if the wind speed is not enough, the dependence of the residual amount of solvent on the temperature tends to disappear. From this result, it can be understood that the thermal conductivity of the ink coating film affects the residual tendency of the solvent.(2If the amount of ink transferred to the film is in a semi-dry state due to certain conditions before entering the dryer. If the distance from the printing film to the dryer is too long or the printing speed is too slow, or a large amount of air in the dryer leaks out and hits the printing surface, use an abnormal drying method to dry the printed matter, or dry the printed matter in the dryer. The effect of wind speed is small and depends on temperature to dry. On the contrary, the residual volume rate may increase. This abnormal example is generally referred to as”crust”, that is, the dry film on the ink surface layer hinders the diffusion of solvent vapor, and the entire coating film is not uniformly dried. In this case, the diffusion of the solvent vapor must have a large energy, but there is a limit to this energy due to failures such as shrinkage, wrinkles and relaxation of the film itself.3. Solvent useDue to the volatilization of the solvent during the gravure printing process. In order to maintain a certain viscosity of the ink, the solvent must be added frequently. In order to control the solubility and drying speed of the ink. Solvents are generally not used alone, but are mostly mixed with several solvents. The solvent used for dilution is based on different types of ink, considering the re-robustness of the ink, the stability of long-term printing on the printing machine, the stability of the remaining ink, the balance of mixing with the new ink, and the solvent residue of the printed product. Conditions are set. Therefore, it would be dangerous to arbitrarily use other statistics in place of the set solvent.In order to consider the high-speed printing of gravure, the ink is required to have quick-drying and solvent residue after printing compound food packaging bag requirements, so gravure ink can only use solvents with low boiling point and fast evaporation.From the above discussion, it can be seen that the residual solvent content of plastic packaging bags is not only related to the ink, but also related to the printing speed, heat, coating amount, added solvent, etc. Sometimes we must also pay attention to the relationship between the solvent and the film. Consciously use less solvent with good affinity for the film. Only by organizing the above relationships can we ensure the quality of printed matter and compound the requirements of food packaging.
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