Technology

Mechanism

Cationic UV curing is based on the crosslinking of monomers, oligomers or polymers through the generation of cationic species. UV light activates a photoinitiator, which generates a cation. This cation initiates a chain reaction, linking reactive groups into polymers. Typical monomers for this reaction include epoxides and vinyl ethers.

Advantages

• Dark cure: The adhesive continues to cure even after the UV light source is removed, leading to a complete and thorough cure over time. This characteristic allows for better curing in shadowed areas and improved depth of cure.
• Low Shrinkage: Cationic curing exhibits lower volumetric shrinkage, leading to better dimensional stability.
• Good Adhesion: This method provides excellent adhesion to various substrates.

Limitations

• Slow Cure Speed: Compared to free radical curing, the reaction speed is often slower.
• Moisture Sensitivity: Cationic curing can be disrupted by the presence of water or high humidity levels.
• Sensitivity to daylight: It’s necessary to keep the adhesive and coating systems in the dark until the adhesive is applied.

Application Examples

• Electronics Encapsulation: Protection and insulation of electronic components.
• Laminating Adhesives: Used in composite materials.
• Optical Lenses: Employed in optical applications where low shrinkage is important.

Mechanism

In free radical UV curing, photoinitiators are activated by UV light, generating free radicals. These free radicals initiate the polymerization of acrylates and methacrylates, causing the material to cure.

Advantages

• Fast Curing: This method allows for very rapid curing, often within seconds.
• Tailored Properties: Degree of crosslinking can be precisely controlled by adjusting the UV dose.
• Wide Range of Monomers: A variety of acrylates and methacrylates can be used, offering high flexibility in formulation.
• Low Cost: Compared to cationic curing, materials and processes are often more cost-effective.

Limitations

• Oxygen Inhibition: The process can be inhibited by the presence of oxygen, leading to incomplete curing at the surface.
• Higher Shrinkage: A higher volumetric shrinkage occurs, which can introduce stress within the material.

Application Examples

• Coatings: Fast-drying coatings for furniture or vehicles.
• Packaging Adhesives: Used in fast-curing packaging solutions.
• Adhesive Tapes: Utilized in pressure-sensitive tapes for industrial applications.

Cationic and free radical UV curing are two distinct technologies used for curing adhesives. While cationic curing offers deeper curing and lower shrinkage, free radical curing provides rapid curing and a broader range of materials. Both technologies have specific application areas that depend on their respective properties and requirements.

Courtesy of artimelt AG: