How Light Curing Adhesives are Affected by your UV Curing Source

By Bruce Campbell

The suitable selection of a UV curing adhesive for any application was largely based on experience in the past. Some adhesives would perform effectively with a given light curing system, while others would result in tacky surfaces or an un-cured bond with what seemed to be a similar setup. Process engineers eventually realized that there were many factors that can affect a UV curing process, such as the joint design, the use of pigmentation, and last but not least the light curing source.

Most commercial UV curing adhesives perform with peak efficiency between the region of 200-400nm, however near visible output in the range of 400-450nm is also important when working with pigmented bonds. Due to compatibility, nearly all of the past developments in UV curing chemistry have focused around the broad spectral output from a medium pressure mercury arm lamp, which has resulted in the majority of industrial applications using this technology. The absorption profile for most UV curing adhesives matches up nicely with the broad spectral output from a medium pressure mercury arc lamp to provide a full cure. However, if you're planning to design a process around a narrower UV band, such as the light output from a doped arc lamp or a UV light emitting diode (LED), several other factors should be considered.

LED UV curing solutions are now commercially available in either single chip or array platforms to deliver focused peak UV light output at 365nm, 385nm, or 405nm bandwidths. When using an LED light source in your process it is therefore important to select a UV curing adhesive that has been formulated to efficiently absorb light in the 365-410nm range of output. Due to the increasing adoption rate of LED light curing systems in industrial applications, it's become more common to see many adhesive formulations that have been long-wave sensitized for improved cure properties and performance.

In addition to the impact of pigments and joint design in a UV curing process, the properties of a resulting bond are also highly dependent on adhesive coating thickness the compatibility between light curing output and chemical absorption. A process may result in a high degree of one cure characteristic over another (i.e. surface cure or depth cure) when an imbalance exists between spectral light output and adhesive absorption. The broad spectral output from a common mercury arc lamp source provides a blend of short and long-wave UV light, resulting in a fully cured bond and a high degree of compatibility. While one of the limitations to the use of LED UV curing systems is a narrow band of light output that makes adhesive selection difficult, depending on its industrial use.

When first examining the economic benefits of an LED light curing solution versus more commonly used arc lamp technology, LED's have several advantages. However, before making any changes to your UV curing process, it's a good idea to test the final cure properties of one solution over the other. In many of today's industrial applications you may find that a UV arc lamp still provides an improved balance of depth and surface cure characteristics, resulting in faster and more reliable UV curing assembly.

About the Author:

Want to learn more about light curing, then visit Bruce Campbell's resource on the most advanced UV curing equipment for precision bonding applications.