I. Principles and Steps of Using a Corona Pen to Detect Film Corona
A corona pen is a tool for quickly determining a film's surface energy (corona value) by measuring ink wettability. Its principle is based on the relationship between dyne values (a unit of surface energy, 1 dyne = 1 mN/m) and ink spread. The specific steps are as follows:
1. Clean the surface: Wipe the film with an alcohol wipe to prevent contaminants from affecting the test results.
2. Line Test: Use a corona pen to draw a line approximately 5 cm long on the film surface and observe the ink's behavior for 2 seconds.
- If the ink shrinks into droplets, it indicates insufficient surface energy (the corona value is lower than the value marked on the pen).
- If the ink spreads evenly, the surface energy is at or above the value marked on the pen.
3. Gradient Verification: Repeat the test using corona pen values of different dyne values (e.g., 38, 40, and 42 dynes) to accurately determine the film's actual corona value.
*Note*: The testing environment must be maintained at 25±5°C and humidity below 60% to prevent temperature and humidity from affecting ink performance (refer to ASTM D2578).
II. Corona Value Specifications and Testing Differences for Laminated and Matte Films
1. Laminated Film Corona Value: The industry standard is 38-42 dynes (data source: ISO 8296-2016). Main reasons:
- Laminated films are typically made of BOPP/PET materials, which require a high surface energy to ensure ink or glue adhesion.
2. Matte Film Corona Value: The general standard is 32-36 dynes (data source: TAPPI T458 standard). Differences include:
- Matte film surfaces are frosted, and increased roughness reduces the effective dyne value;
- When testing, use a corona pen with a thicker tip to avoid misjudgment due to surface unevenness.
III. Special Scenarios and Common Problems
1. Troubleshooting Abnormal Corona Values on Matt Films:
- If test results fluctuate significantly, it is recommended to retest with a plasma treatment system (accuracy ±1 dyne).
- If corona is insufficient in certain areas of the matte film, it can be supplemented with a corona treatment system (power should be adjusted to 50-100W).
2. Professional Explanation of Conflicting Values:
- Different locations on the same film may experience differences of 2-3 dynes, which is normal (ISO standard tolerance range).
- If the film corona value is less than 38 dynes, composite delamination may occur (refer to the peel strength test requirements of GB/T 8808-2023).