Ink solvent residue is a common problem in the production of flexible packaging plastics, which has a great impact on the quality of finished products. Therefore, it is necessary to analyze from raw materials to the production process, and take corresponding measures to control, in order to achieve the purpose of quality control.
1. Raw Materials
Compared with the PET structure, BOPP is less easy to control solvent residues in production, whether in the printing process or in the composite process. Firstly, from the molecular structure analysis of the material, the density of BOPP is 0.96g/cm3, and the density of PET is 1.38g/cm3. BOPP is compared with PET: BOPP material density is small, the gap between molecules is large, it is easy to adsorb the solvent in the printing compound process of ink and glue, and it is not easy to release after adsorption, resulting in different degrees of solvent residue in the printing and composite process.
Different film substrates use the same ink and adhesive in gravure and dry composite, and the production process is the same, but the residual amount of solvent in the product will not be the same, because the adsorption and release rate of solvent on different film substrates is not the same.
The release rate of ethyl acetate from the commonly used film substrate is: PET>PA>BOPP (the adsorption rate is opposite). Usually, the temperature of printed and composite drying boxes is close to the dissolution point of BOPP films, and the irregular activity of its molecules is more intense, and the adsorption rate of solvents such as ethyl acetate and toluene is accelerated. Therefore, for BOPP films, it is more difficult to further reduce the residual amount of solvents.
Second, the choice of printing dilution solvent
One of the reasons for the largest solvent residue is affected by the performance of the solvent, the quality of the solvent itself, water content, boiling point, evaporation rate will cause solvent residue, if the solvent water content in the ink is high, or contains a high boiling point solvent will cause the ink drying speed decline, the formation of high solvent residue. Print quality and solvent quality have a lot to do with it, but many printing manufacturers do not pay much attention to the detection of solvent quality, which causes quality problems.
Therefore, we should pay attention to the balance of the solvent, according to the speed of the printing press to prepare the dilution solvent, especially pay attention to the amount of slow dry solvent in the dilution solvent. For example, the amount of n-propyl ester should be controlled in chlorinated polypropylene ink, and the amount of slow-drying solvents such as n-propyl ester and propylene glycol methyl ether should be controlled in polyurethane ink.
If the slow drying solvent is more, it will directly cause the solvent to volatilize incomplete, and the solvent residue is high. If the slow drying solvent is less, it will cause the ink crust, and it is easy to block the plate transfer. Therefore, in the case of the dilution solvent ratio has been determined, do not easily change the proportion of dilution solvent.
Third, printing drying temperature and ventilation
The drying ventilation device is the key to the solvent release in the printing process. First of all, it is necessary to develop the habit of maintaining and cleaning the dry ventilation device, maintaining a certain air volume and making its inlet and exhaust air balance. Secondly, when the last color is dried, the temperature should be increased by 5~10 ° C, if the equipment and product color number allow, after printing, you can let the roll film air through the two-color unit, further ventilation and drying, so that the solvent is completely volatilized, and finally reduce the residual solvent of the product.
4. Printing environment
The temperature and humidity of the printing workshop will also have a certain impact on the release of the solvent, mainly because the excessive humidity in the workshop will reduce the volatilization rate and balance of the solvent, resulting in the slowing down of the volatilization rate of the solvent in the printing process leading to residue, but the humidity is too low and it will easily cause static electricity in the high-speed printing process.
Therefore, it is necessary to often pay attention to the temperature and humidity in the workshop, maintain a certain humidity state to effectively eliminate static electricity in ink printing, and not to increase the solvent residue due to high humidity.
