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In the production of recyclable flexible packaging, solvent residue exceeding the standard is often encountered, especially in lamination production. Improper control of solvent residue exceeding the standard can affect delivery, and in severe cases, it may result in loss of customers and even require payment of a large amount of compensation. Therefore, in daily production, enterprises should pay attention to the following influencing factors and make corresponding process adjustments to ensure that the residual value of lamination solvents can meet the standard.
Compared to PET structure, BOPP is less likely to control solvent residue in production, whether in printing or laminating processes. Firstly, we analyze the molecular structure of the materials. BOPP has a density of 0.96g/cm3, while PET has a density of 1.38g/cm3. Compared to PET, BOPP has a lower density and larger intermolecular gaps, making it easier to adsorb solvents from ink and glue during printing and lamination processes. Moreover, after adsorption, it is not easy to release, resulting in varying degrees of solvent residue exceeding the standard in printing and lamination processes.
Different film substrates use the same ink and adhesive in gravure printing and dry lamination, and the production process is also the same. However, the residual solvent content of the product may vary because different film substrates have different rates of solvent adsorption and release.
The release rate of ethyl acetate from commonly used film substrates is: PET>PA>BOPP (with opposite adsorption rates). Usually, the drying oven temperature for printing and lamination is close to the melting point of BOPP film, and its molecular irregular activity is more intense, accelerating the adsorption rate of solvents such as ethyl acetate and toluene. Therefore, for BOPP film, it is more difficult to further reduce the residual solvent content.
During the printing process, the drying oven temperature of BOPP structure is low, and excessive vehicle speed can easily cause excessive residue in the printing process.
The improvement method is to reduce the speed of printing, extend the residence time of the printing film in the drying oven, and ensure that the printing film is fully dried to reduce solvent residue. Usually, it is necessary to ensure that the solvent residue detected by the printing film under the machine is below 2mg/m2, in order to facilitate the production of composite processes.
The temperature of the first stage of the drying oven should not be too high: for those with two stages of drying oven (such as Bosch high-speed laminating machine), the temperature of the first stage should be controlled at 70-75 ℃, and the temperature of the second stage should be increased as much as possible to ensure that the printing film is not stretched. Currently, there are generally lamination machines with three or four drying chambers, and the temperature of the drying chamber follows a gradient setting. The first drying chamber must control the air volume and temperature well. If the temperature at the beginning of entering the drying oven is too high and the drying is too fast, the solvent on the surface of the adhesive layer will evaporate rapidly, causing a local increase in the concentration of the adhesive on the surface layer, and the internal solvent will be trapped inside. After passing through the oven, the solvent evaporation rate will slow down, and the internal solvent cannot be fully evaporated.
BOPP has a strong adsorption capacity for solvents, and the larger the amount of glue applied, the more severe the solvent residue exceeds the standard after compounding. This can reduce the amount of glue applied to reduce solvent residue. Using a 140 or 150 line glue roller to apply 36% to 38% high concentration coating can reduce BOPP’s adsorption of solvents. At the same time, appropriately reducing the amount of glue applied to 2.5~2.6g/m2 can effectively reduce solvent residue in the composite process.
If the customer does not use high thread count wire roller high concentration coating to reduce residual amount, the pressure of the coating roller can be reduced, the scraper pressure can be increased, the scraper angle can be adjusted, and the composite vehicle speed can be reduced. This method can appropriately reduce the amount of glue applied, prolong the residence time of the glue film in the drying oven, and also reduce solvent residue.
In short, the residual solvent of the printing film should be controlled below 2mg/m2, and the total residual solvent of the laminating machine should be controlled below 5-8mg/m2. After the product is placed in the curing room for curing, the residual solvent can be controlled below 5mg/m2, which not only meets customer needs but also meets quality standards.
Longdapac as the leading recyclable flexible packaging manufacturer in China, we are putting efforts in improving our technology. So we will be able to well control the residual during production.