For optimal bonding, the adhesive’s surface tension must be lower than the substrate’s surface energy. When the adhesive’s surface tension is higher, the liquid adhesive resists spreading and instead beads up (like water on a waxed car), preventing intimate contact with the surface.
- Low Surface Energy (LSE) Substrates: These materials (e.g., Polypropylene (PP), Polyethylene (PE), Teflon/PTFE) inherently resist bonding and are the most common cause of poor wetting.
- Contamination: Even high-surface-energy materials (like clean glass or metal) become LSE when contaminated with oils, dust, or mold release agents.
2. Solutions by Surface Preparation
The most critical step in fixing poor wetting is raising the substrate’s surface energy above the adhesive’s surface tension.
| Technique | Method/How it Works | Target Substrates |
| Cleaning | Wipe the surface with solvents (e.g., Isopropyl Alcohol (IPA) or acetone) to remove contaminants like oil or dust. This is the first and most basic step. | All substrates (Glass, Metal, Plastics) |
| Abrasion | Roughening the surface with sandpaper or grit blasting increases the surface area and removes the weak outer layer, promoting mechanical adhesion and better wetting. | Metals and Rigid Plastics |
| Plasma/Corona Treatment | Specialized equipment bombards the surface with ionized gas, chemically modifying the top layer to dramatically increase the surface energy. This is highly effective for LSE plastics. | LSE Plastics (PP, PE), Rubber |
| Primers/Activators | A thin liquid layer applied before the adhesive that is specifically designed to chemically bridge the gap between the LSE substrate and the adhesive. | Difficult-to-bond plastics and certain metals |
3. Solutions by Adhesive Selection and Process
If surface preparation is not enough, the adhesive or dispensing process must be optimized.
- Adjust Adhesive Viscosity:
- For bonding large, flat surfaces or to aid capillary action in tight gaps, a lower viscosity adhesive (more watery) will flow and wet the surface much more easily.
- For vertical joints or gap-filling, a higher viscosity is necessary to prevent run-out, but expect a longer dwell time for the material to fully settle and wet.
- Select Proper Chemistry: Choose an adhesive specifically formulated for LSE substrates (often specialty UV acrylates or rubber-modified formulas). These contain additives that inherently lower the adhesive’s surface tension.
- Optimize Dispensing Speed: In automated dispensing, moving the nozzle too quickly can drag the adhesive and prevent it from settling. Allow sufficient dwell time for the adhesive to spread fully across the joint before bringing the substrates together.
- Increase Application Pressure: Applying a slight, even pressure when joining the substrates can help force the adhesive to spread and fill the entire bond line, ensuring full wetting.