Select the most effective wavelength for an LED systemSelect the most effective wavelength for an LED system

Other benefits of LED which may also be a consideration include: they are environmentally friendly as the UV LED systems use about 80% less power versus a comparable lamp system and contain no mercury; the instant on/off nature of LEDs eliminates the need for a shutter; and, the electrical connection to the UV LED heads makes it easier to place the controller remote as compared with using a light guide to a lamp system.



1. UV LED systems offer multiple wavelengths: 365, 385, and 400 nm.

As with any technology, there are limitations to using a UV LED system, such as smaller spot sizes, lower optical power, and limited depth of cure. Also, due to narrowband spectral distribution and narrow beam pattern of LEDs with lenses, it’s more difficult to get an accurate measurement from a radiometer. Testing remains the best method for determining if LED technology is well suited for your application.

There are many variables to consider when developing a UV curing process, such as the adhesive, substrates, curing conditions, and environmental conditions, all of which will affect the cured product’s physical properties. The variables which are controlled by the UV spot curing system are:

In most good quality UV spot curing systems, the first three factors can be directly controlled. Heat is a byproduct of the curing process resulting from the combination of the first three factors along with the substrates being joined. Although it’s not directly controlled, managing heat during the curing process can be an important consideration when dealing with plastics or other components which are sensitive to heat.

With a maximum irradiance reaching up to 9.5 W/cm2, UV LED systems can likely offer a sufficient irradiance level, in small spot sizes, for most curing processes. However, there’s often a significant difference in the peak irradiance available from the LED system, depending on the wavelength being offered. In general, 365-nm LED systems offer the lowest peak irradiance, with available peak irradiance increasing as the wavelength increases. In some cases, a 385- or 400-nm LED system can have significantly higher irradiance as compared to 365 nm. Therefore, the question will often arise as to which is more important in an LED system, wavelength or peak irradiance.

A light-cured adhesive must receive a sufficient exposure of the correct spectrum of light to be fully cured. Because of the narrow spectral bandwidth of LED output (10 nm), it’s critical that the UV LED system’s wavelength matches the absorption spectra of the adhesive’s photoinitiator. If the spectral output of the UV LED system doesn’t match the spectral absorption of the photoinitiator, then regardless of the irradiance level or exposure time, the adhesive won’t cure. However, increased peak irradiance can be of benefit in decreasing curing times or improving depth of cure for some applications.

The adhesive spec sheet specifies a spectrum of light required for curing, based on the photoinitiators contained in the adhesive. Many of the current light-curable adhesives specify 365 nm energy as a requirement for curing. This is designed to match the 365-nm peak output of mercury lamps which have been the standard for UV curing.

However, the limited information provided on some adhesive spec sheets may not always tell the whole story. It’s the range of spectral absorption of the photoinitiator which will determine the wavelengths suitable for curing. This range will vary between adhesives, but is always more than a single wavelength. If there’s only a single wavelength specified, such as 365 nm, then this may unnecessarily limit the wavelengths available for curing the adhesive.

Results of our testing on a select number of adhesives where 365 nm was specified as the required wavelength showed that our 365-nm source, at the irradiance level tested, works well with all the tested materials. Our 400-nm source also worked well on many of the materials. However, it failed to provide adequate curing on several occasions. Although the irradiance of the 400-nm source is higher than the 365-nm source, for a number of materials, the wavelengths were not a good match which resulted in inferior curing ability. While the lower irradiance of the 365-nm LED source may affect the efficiency of the curing, it was able to cure all of the adhesives by extending the exposure time where required.

This initial testing confirms that 365-nm LEDs are suitable for curing many adhesives where 365 nm is the specified wavelength. However, there are other considerations to the curing application which could affect the wavelength selection. This includes cure time; depth of cure verses surface cure; and transmission through parts.

Cure time
A light cured adhesive requires a sufficient exposure before it’s fully cured. An exposure is made up of the irradiance multiplied by the cure time. Therefore the higher irradiance of a 400-nm LED may allow for a decrease in the cure time. The time required for the UV adhesive to cure can be directly controlled by the irradiance level of the light source as given by:



The Rp (polymerization rate or curing speed) is proportional to (I0)0.5 where I0 is the UV irradiance being absorbed by photoinitiators in the adhesive formulation. As long as the UV source’s wavelength matches the absorption spectra of the photoinitiator, then increasing the peak irradiance will have an effect of accelerating the polymerization rate. By accelerating the polymerization rate, it’s possible to reduce the curing time, resulting in increased throughput for the process. However, if the UV source’s wavelength doesn’t match well to the photoinitiator absorption peak, large portions of the UV are wasted and sometimes could be harmful.



2. Penetration into the adhesive depends on the optical density of the material and is also controlled by the peak irradiance and wavelength of the UV source. Longer wavelengths (400 nm) will provide better penetration than shorter wavelengths (365 nm). Higher irradiance will provide better penetration through the material.

Depth of cure vs. surface cure
For transparent UV-curable adhesive formulations, the penetration of the UV from all LED systems (365 nm and up) was found to be sufficient to cure the adhesive to a reasonable thickness. Applications using adhesives, which include some tint or filler or where depth of cure is important, would benefit from the higher irradiance and longer wavelengths of a 400-nm UV LED, if the adhesive is compatible. Conversely, in applications where the adhesive is open to the atmosphere, testing showed that in general, the lower irradiance, shorter wavelength 365-nm LED provided a better surface cure of the material as compared to the higher irradiance 400-nm LED.

Transmission through parts
A light-cured adhesive must receive a sufficient exposure to be fully cured. For many applications where two parts are being joined together, this means that the light must travel through one of the parts being joined to reach the adhesive surface. In these cases, the transmission or absorption spectrum of the parts through which the light must pass is a key consideration when selecting the wavelength of LED for curing.

In one of our test examples, the adhesive datasheet specified a 365-nm light source for curing. Testing showed that both systems were able to cure the adhesive, but the 400-nm example was able to cure much faster and with lower heating of the parts. In this case, following information from the spec sheet alone would not have shown the most effective wavelength for assembly of the parts.



3. Testing of the plastic parts being joined showed that they had significantly higher absorption at 365 nm as compared to 400 nm. As a result, about 60% more of the light energy at 400 nm would make it to the adhesive as compared to the 365nm light.

We’ve focused on the importance of matching the narrow band wavelength of the LED with the absorption spectra of the adhesive’s photoinitiators. This is a minimum requirement for curing the adhesive. However, it should be noted that many of the currently available light-curable adhesives have been designed to work with broadband light sources and as such may include multiple photoinitiators with different spectral absorption ranges. It’s also important to remember that light outside of the spectrum required by the photoinitiators will also have an effect on other components of the adhesive as well as the substrate. It’s the combined effect of the full light spectrum on all components of the adhesive, as well as the substrate, which results in final physical properties of the cured adhesive.

In some cases, it may be found that using a narrow band light source such as an LED on an adhesive that’s designed to work with a broad band light source may not provide the optimal physical properties in the cured materials. Unfortunately, a number of adhesives have not been able to match the physical properties of curing with a broadband curing system such as when a UV LED system is used. It may be found through testing that using multiple wavelengths of LEDs has some benefit to the adhesive’s final cured properties. However, it may be determined that a UV LED light source isn’t appropriate for the adhesive being tested.

Mike Kay, a senior marketing analyst, has been with Lumen Dynamics Group for 10 years. He has extensive experience developing UV curing solutions for medical, electronics, and optoelectronics manufacturing. He holds a Bachelor of Science degree from McMaster University in Hamilton, Ontario.