Flexo Ink Adhesion: Surface Energy Theory and Practical Measurement for Substrate Compatibility
Ink adhesion to the substrate is a fundamental requirement for print durability and functionality. It is governed by the thermodynamic work of adhesion, which depends on the surface energies of the ink and the substrate. For good adhesion, the ink's surface tension must be lower than the substrate's critical surface tension (γc), and the interface must have high contact. This article explains the theory and practical measurement of surface energy for flexo applications.
The critical surface tension (γc) of a substrate is the surface energy below which a liquid will spread; above which it will bead. For printing, the ink must have a surface tension less than γc to wet the substrate. Typical γc values: untreated PE/PP ≈ 28-30 dynes/cm; corona-treated PE/PP ≈ 38-44; PET ≈ 38-42; paper/board (high) >45. The ink's surface tension is typically 25-35 dynes/cm. The driving force for wetting is the difference (γc - γ_ink). A larger difference promotes spreading and adhesion.
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Measurement: The dyne test (using dyne pens or solutions) is a quick method to estimate γc. The test liquid (with known surface tension) is applied to the substrate; if it wets and spreads within 2 seconds, the substrate's γc is at least that value. For accurate measurement, the contact angle method is used: a drop of liquid is placed on the substrate, and the contact angle (θ) is measured. The surface energy is calculated using the Owens-Wendt or Fowkes model, which separates polar and dispersive components. This provides a complete surface energy profile, enabling better prediction of adhesion.
Adhesion mechanisms: Good wetting is necessary but not sufficient. Chemical bonding (hydrogen bonds, covalent bonds) and mechanical interlocking (due to surface roughness) contribute. For polyolefins, corona treatment introduces polar groups (carbonyl, hydroxyl) that enable hydrogen bonding with ink resins (e.g., polyurethane, acrylic). The degree of treatment is measured by the dyne level; a higher level (e.g., 42 vs. 38) increases adhesion but may reduce over time (decay). Printing should be done within 24-48 hours of treatment.
Adhesion promotion: When the substrate's inherent γc is too low (e.g., untreated PE), primers or adhesion promoters are used. A primer is a thin coating (e.g., chlorinated polyolefin) applied before printing, which provides a compatible interface with both the substrate and the ink. Alternatively, coupling agents (e.g., silanes) can be added to the ink to bond with the substrate's surface.
Testing adhesion: The tape test (ASTM D3359) is standard – a crosshatch cut is made, tape is applied and pulled; adhesion is rated from 0 (poor) to 5 (excellent). For quantitative measurement, a peel test (e.g., 180° peel) is used to measure the peel force. The adhesion strength should be at least 2-5 N/cm for most packaging applications.
Troubleshooting poor adhesion: Common causes: insufficient corona treatment (dyne level <36), contaminated substrate (oil, dust), ink that is too hard (excessive crosslinker), or wrong resin for the substrate. Solutions: increase corona power, clean the web, adjust ink formulation (softer resin), or apply a primer. Also, check drying temperature – over-drying can embrittle the ink, reducing adhesion. By mastering surface energy principles, converters can ensure reliable adhesion across diverse substrates, preventing delamination in final products.
