Dual-cure adhesives utilize UV light for rapid initial curing and fixturing, followed by a slower, secondary mechanism to complete polymerization, especially in areas the light cannot reach.
| Dual-Cure Type | Secondary Mechanism | Why the Secondary Cure is Essential |
| UV/Thermal Cure (UV + Heat) | Exposure to a specific temperature for a defined time (e.g., 10 minutes at 120∘C). | Ensures 100% cure in shadowed areas (under opaque components) and achieves maximum structural strength and temperature/chemical resistance. |
| UV/Moisture Cure (UV + Humidity) | Exposure to ambient air humidity for a set time (e.g., 24 hours). | Cures material in shadowed areas. Often used for large gaps or when thermal curing is not feasible. The cured material reacts with moisture to complete polymerization. |
| UV/Anaerobic Cure (UV + No Oxygen) | Cures in the presence of metal ions and the absence of oxygen. | Used for potting or bonding deep within metal assemblies (e.g., threadlocking). The UV cure provides quick fixturing, and the anaerobic cure finishes the bond where light and air are excluded. |
2. Importance of Post-Cure Timing and Environment
Even single-cure adhesives often benefit from a controlled post-cure environment, and dual-cure systems absolutely require it.
- Stress Relief and Full Property Attainment: Even after a full UV dose, the adhesive continues to cross-link and consolidate. This final, slower process achieves the adhesive’s ultimate chemical resistance, tensile strength, and dimensional stability.
- Preventing Delayed Failure: Skipping the secondary cure means the adhesive in shadowed areas remains liquid. This liquid material can leach out, swell, or absorb moisture, eventually leading to catastrophic bond failure or material corrosion.
- Achieving Tg and Hardness: The post-cure often determines the final Glass Transition Temperature (Tg) and Shore Hardness of the polymer. An incomplete cure will result in a lower Tg and a softer material, making it unsuitable for high-temperature or load-bearing applications.
Correct Process Steps:
- UV Exposure: Apply the full specified UV dose (J/cm2) for rapid initial cure and fixturing.
- Immediate Handling: Parts are now fixtured and can be handled.
- Secondary Cure (If Required): Subject the parts to the specified heat profile (e.g., in an oven) or humidity profile (e.g., ambient room exposure) for the full duration specified by the manufacturer.
- Cool Down/Final Property Check: After post-cure, the parts are ready for final use.