Adhesion, also known as blocking, refers to the unwanted sticking between layers of plastic films, especially in printed materials. This issue can occur in two main forms: first, when the printed surfaces stick together, making it hard to open packaging and leading to customer dissatisfaction; second, when the ink layer adheres to another surface, either during printing or after the product is processed, which can cause damage to the printed design or contamination of the contact area.
The first type of adhesion typically results in films that are difficult to separate, especially at the bag opening, which can lead to rejection by end users. To prevent internal adhesion, slip agents and opening agents are commonly added to the resin formulation, improving the film’s release properties and ensuring smoother handling during production.
The second form of adhesion is more severe, particularly in high-speed printing operations. It can cause the roll material to be unrollable, leading to production delays, pattern damage due to ink peeling, and even complete product failure if the films become stuck together and cannot be separated. This makes controlling ink adhesion in gravure printing a critical factor for maintaining quality and efficiency.
To address these issues, anti-blocking products must meet specific performance criteria. First, the material should unwind smoothly, supporting high-speed production processes. Second, the surface should remain smooth and easy to open, enhancing user experience.
When selecting substrates for anti-blocking applications, several factors must be considered, including the substrate's slip properties, ink adhesion, solvent evaporation rate, environmental conditions, drying settings, and printing speed. Common substrates include BOPP, BOPET, aluminum foil, and PET films. Each requires matching inks, solvents, and process parameters to ensure optimal performance.
In gravure printing, the anti-blocking properties of the final product are largely determined by the ink formulation. The design of the ink must take into account the binder resin type, residual solvent levels, and machine parameters. By optimizing these factors, the risk of blocking can be significantly reduced.
During the printing process, special attention should be given to the selection of resins with higher softening points, which help reduce blocking. Additionally, ensuring that the ink dries thoroughly is crucial, as residual solvents can increase the likelihood of adhesion. After winding, the backside of the film may have strong affinity with the ink layer, so techniques like corona treatment, single-side treatment, and controlled printing speed are recommended. Proper cooling using a cooling roller before rewinding is also essential.
Testing the anti-blocking properties of gravure-printed materials follows the GB/T 13217.8-2009 standard. The test involves folding the ink-coated surface and applying pressure under controlled temperature and time conditions. The degree of adhesion is then evaluated based on the amount of ink transferred or removed.
The test procedure includes preparing the sample by printing ink on the substrate or using a wire bar, followed by a 2-minute rest period. If water-based ink is used, the sample is dried at 59–61°C for 5 minutes, then cooled. The sample is folded and placed in an oven at 49–51°C under a 2000g weight for 2 hours. After removal, the folded section is examined for adhesion.
To calculate the adhesion level, translucent millimeter paper is placed over the pressed area, and the number of grids covered by the ink and those where ink was removed are counted. The formula used is:
B (%) = B1 / (B1 + B2) × 100
Where:
- B is the degree of ink adhesion
- B1 is the number of ink-covered grids
- B2 is the number of grids where ink was removed
This detailed testing ensures that the anti-blocking performance meets industry standards, helping manufacturers produce high-quality, reliable printed materials.
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