Adhesion Promoter and Primer for Bonding - When Needed?

When is pre-treatment actually necessary?

Whether you need an adhesion promoter depends mainly on the surface energy of the material. An adhesive only wets a surface when its own surface tension is lower than that of the substrate. Non-polar plastics such as PP, PE or PTFE sit around 29‑34 mN/m and below, so almost no adhesive sticks on its own.

A primer or activator either raises wettability or speeds up curing. On polar, easily wetted materials like clean steel or glass, cleaning alone is often enough, whereas silicones, elastomers and plasticiser-rich plastics nearly always require chemical pre-treatment.

• Always needed: PP, PE, POM, PTFE and other non-polar plastics.
• Usually needed: silicones, EPDM, plasticised PVC, glass-fibre reinforced plastics.
• Rarely needed: clean, degreased metal, glass or ceramic.
• Silane adhesion promoters improve ageing resistance on glass and aluminium.

Which products exist and how do they work?

The terms primer, activator and adhesion promoter are often mixed up but act differently. An activator speeds up curing, a primer forms an adhesion-boosting intermediate layer, and an adhesion promoter in the strict sense couples chemically to the substrate.

How do I select by substrate and adhesive?

Selection always follows two steps: first by the substrate, then by the adhesive family. A primer for polyolefins is fundamentally different from a silane adhesion promoter for mineral surfaces.

• Check the adhesive data sheet: it names the released primer.
• Mind pot life and open time of the primer - do not leave it open too long.
• For outdoor use always check ageing and UV resistance of the system.
• Releases per ISO 4587 (lap-shear strength) give reference values for bond strength.

How do I apply primer correctly?

A primer only works as a very thin, even film. Too much material forms a soft release layer and weakens the bond. Before application the surface must be clean, dry and free of grease.

• Degrease the surface with isopropanol or a suitable cleaner and let it flash off.
• Apply primer thin and even with an applicator, felt or brush - no puddles.
• Respect the flash-off time: 1 to 10 minutes depending on product, until solvent evaporates.
• Bond within the open time, otherwise the primer loses its effect.
• Do not touch with fingers - skin grease destroys the activated layer.

Check the result on a sample part before going into production. A lap-shear or peel test shows whether the break is in the material (cohesive, good) or in the bond line (adhesive, poor).

Frequently asked questions