Using a corona pen, it's easy to analyze subtle changes in the surface energy, hydrophilicity, and wettability of different solids. The analysis method is simple and effective, requiring only a single scratch on the substrate surface to quickly obtain accurate results. This method is designed for production line testing and is performed by factory-trained operators.
When testing, a mid-range value should be selected as a starting point, such as 38mN/m. If the pen wets the substrate within 2 seconds, the substrate's surface tension is greater than or equal to the selected value. A second test should be performed with a pen of a higher value, and so on, until the result shrinks to a water bead (spherical shape) within 2 seconds. The previous value is then considered the substrate's surface energy and used for comparative analysis.
If the pen shrinks to a water bead (spherical shape) after the first test, a second test with a lower value should be performed until the surface is wet. This method accurately measures the substrate's surface tension and wettability, and helps determine whether the substrate's surface conditions meet the requirements before the test, allowing adjustments to be made to meet the specific requirements.
In industrial practice, the surface energy (surface tension) of plastics is measured by applying a 100mm long ink strip of ink with varying surface energies to the film to be tested. The strip is observed to see if more than 90% of the strip shrinks and forms droplets within two seconds. If so, the strip is applied again with an ink of a lower surface energy and the same observations repeated until no shrinkage or droplets form. The surface energy of this test ink then corresponds to the surface energy of the film. This method accurately measures the surface tension and surface wettability of the substrate and determines whether the substrate surface conditions meet the requirements before application, allowing adjustments to the ink, coating, and viscosity to meet the desired requirements.
Surface tension and surface wettability are very reliable criteria for accurately measuring the adhesion of printing inks and other materials to surfaces. However, other factors, such as static electricity and various additives, also affect viscosity. However, these factors are often not reflected during testing, and even good test results may not meet actual requirements. This requires discussion of these technical issues with the raw material supplier. Generally, these conditions are unlikely to occur, and a surface value of 38-41 mN/m meets the viscosity requirements. Surface tensions below 37 mN/m result in many blank pages (no printed content), while viscosity is poor below 35 mN/m.
Generally speaking, a substrate's ability to form ink droplets, coat, and adhere is related to its surface energy. If the substrate's surface energy is lower than the surface tension of the applied test fluid, beading and line shrinkage will occur. Therefore, for most solvent-based printing and plastics, the required test pen is between 36 and 40 dynes/cm. Some ink-based fluids require a test pen between 40 and 44 dynes/cm. Some laminating and coating applications require a surface energy of 50 dynes/cm or higher. Clearly, surface energy estimation is necessary before printing, coating, or laminating.
Corona pens perform well on most non-adhesive materials. It is important that the test fluid does not alter the substrate's surface properties. For example, if the test fluid penetrates a fibrous substrate (such as paper) and causes it to swell, the result may indicate susceptibility to moisture. Biochemical reactions between the substrate and the test fluid invalidate the results. To ensure reproducible test results, material preparation and testing methods must be standardized. ASTM STD D6182 protocol specifies test conditions and methods. Testing theory emphasizes the importance of standardizing the ambient environment, substrate, and test method. Trainers instruct testers to minimize all variations. Relative humidity should not exceed the standard; excessive humidity can exacerbate data variability. Finally, the test duration of extrusion or coating (printing) must be controlled. This accurately measures whether the film's surface tension reaches the test pen value. This allows users to clearly understand whether the film is suitable for printing, lamination, vacuum aluminum deposition, and other applications. This effectively controls quality and reduces losses caused by substandard materials.