By Mark Busa
Sales & Marketing
Epoxy Technology, Inc.
UV curing adhesives offer a convenient way to quickly cure an adhesive in specific applications. In order to receive the full benefits of UV curing products, it is very important to cure these materials properly. Below are some important questions to take into consideration before curing UV adhesives:
- Is the product Cationic (epoxy-based) or Free Radical (acrylate-based)?
- Cationic UV systems are initiated by photo generated ions and cure in minutes, although shorter cure times may be achieved with proper lamp selection. They generally have less shrinkage, higher strength and are not subject to oxygen inhibition. The stronger the photo initiator package, the faster the UV cure response. Many cationic systems can be thermally post cured for enhanced properties.
- Free radical UV systems are known for their very rapid cure in seconds, rather than minutes. They are not sensitive to moisture or other bases, and thermal post curing is not an option; unlike with cationic systems.
- What type of UV lamp, format, filters & intensity is being used?
- Type - UV lamps tend to be metal halide/doped mercury or LED.
- Metal halide lamps have several types and provide a broad wavelength emission spectrum, which is generally the most efficient way to cure cationic UV systems. The most common metal halide lamps are mercury lamps. Standard “H” type mercury lamps are best for thin coatings, while iron doped (“D” or “F”) type mercury lamps are good for a deeper cure of thick coats (pigmented or clear).
- LED lamps have very narrow emission spectra and are specified by a single wavelength (365nm, 385nm, etc).
- Format – UV lamps come in flood or spot formats. Flood lamps tend to be more efficient than spot cures as flood lamps have a larger curing area and generate more heat which benefits a cationic cure. Spot cures are recommended for parts < 0.5” diameter.
- Filters – Always check if the lamp has a filter. Filters can block certain wavelengths and may adversely affect your cure.
- Intensity – Higher intensities may work for small dots or thin layers, but they add stress and are less effective for thick layers. To cure thick layers – reduce the intensity, increase the cure time and use a lamp with a longer wavelength emission spectrum to cure more deeply.
- Any thermal post cure available (cationic systems)?
- Thermal post cures can increase the degree of conversion of areas exposed to an initial UV cure in certain epoxy-based systems. Post cures can also provide enhanced mechanical and physical properties to these products.
- Any shadowed areas of the bond line?
- For shadowed areas, longer wavelength lamp types give the best edge cure. A deeper edge cure is helpful when choosing a low temperature post cure. Shadow curing is only possible with certain cationic systems, not free radical materials.
Keeping in mind these essential points will prepare you to properly select and cure a UV adhesive. A technical data sheet for your UV material should be referred to, as it provides specific cure recommendations to achieve the best results.
For more information on UV curing adhesives and their curing process, please visit our website at www.epotek.com.