UV Wavelength, Photoinitiators, & UV Curing

What is Wavelength?

In the 1800s, James Clerk Maxwell showed that light can be described as traveling in the shape of a wave. The wavelength of light is the distance from one peak in this wave to the next adjacent peak (or from trough to trough). Different wavelengths have different energy and penetration qualities.

This makes the wavelength of light you select to cure a material very important. Not all wavelengths cause curing, and some work better than others. With so many different vendors and types of UV curable materials (resin, urethanes, paints, inks, and more), it’s not surprising that the best wavelength varies for each material.

What Are Photopolymerization Initiators?

The photopolymerization initiator (aka photoinitiator) is one of the most important parts of UV curing.  As you might guess from the name, it is a material that initiates polymerization.  But what does that actually mean?

Polymerization is a chemical reaction where small molecules (monomers and oligmers) combine to form a large chainlike molecule (polymer), and in the case of curing, transforms a liquid-like substance into a solid in the process. So, the curing or hardening of resin is an act of polymerization.

How do we control when polymerization starts, then? How can we even start it?

The answer to that question can be found in the name as well. The “photo” in “photopolymerization” means light. So, our initiator is light-activated. In other words, polymerization won’t start until the initiator is exposed to light, specifically UV light in our case. There are two types of initiators - the radical type, and the cation type. The radical type is used more often because it cures quickly and is sold in large quantities, so it is usually more cost efficient. The table below describes their characteristics.

UV LEDs, UV LAMPS, And Finding the Right Wavelength For You

In traditional curing with UV lamps, the polymerization initiator is exposed to a wide range of wavelengths. For example, metal halide UV lamps have peaks, or highest intensity, at 250 nm to 450 nm, and high pressure mercury lamps have peaks at 254 nm, 313 nm, 405 nm, and 436 nm, centering on 365 nm.  So, with all of these wavelengths being emitted simultaneously, it’s not clear exactly which wavelength the initiator is most sensitive to and activated the most by. Instead, it is irradiated with a wide range, and one or more of the many wavelengths within that range causes it to solidify.

UV LEDs are different - a single LED emits a single wavelength (or a very narrow range) in the range of UV-B and UV-A.  Some of them are 285nm, 300nm, 310nm, 365nm, 385nm, 395nm, and 405nm. UV-B, which contains wavelengths 285 nm, 300 nm, and 310 nm, is still uncommon due to weak illuminance. (You can build a UV LED array to emit more than one wavelength, but we’ll get to this later!)

The precise wavelength range of UV LEDs means that it’s crucial to target the wavelength that the material’s polymerization initiator is the most sensitive to. This can make curing with LED’s far more efficient than with traditional lamps, but it comes with a small problem - because UV LEDs are still somewhat new, the best wavelength for a material is often still unknown. 

If this is your situation, testing or asking the manufacturer are the best ways to find the correct wavelength. Some products specify the correct wavelength and illuminance directly, and since material manufacturers have recently become more aware of UV LEDs, this is becoming more common. 

Our recommendations when testing for the best wavelength:

Typically, the best wavelengths to try are: 365nm, 385nm, 395nm, and 405nm.

365nm is generally the most common wavelength.  385nm and 395nm are frequently used for inkjet printing.  When curing thickly-coated materials such as UV-curing gaskets and UV-curing putty, for example, 395 nm and 405 nm are common wavelengths.

It’s also possible to use a UV LED irradiator that emits 2 or more wavelengths. For example, some irradiators that we manufacture combine 365 and 385nm, 365 and 420nm, 365 and 405nm, etc.

UV LEDs with wavelengths of 275 nm, 280 nm, 300 nm, 310 nm, etc. in the deep ultraviolet range have recently been competing against each other in the sterilization market. LED packages in those wavelength bands are also available. In the future, it seems likely that there will be more opportunities for UV LED irradiators to be used for sterilizing.  It is effective in killing influenza virus, staphylococcus, Escherichia coli, and more. In the future, area illuminators such as UV LED illuminators may play an active role in sterilizing knives and foodstuffs such as bread. In fact, we already have a record of delivery to the food industry.

Important Issues In Polymerization and Their Solutions

The key problems of these photopolymerization systems are yellowing and oxygen inhibition. 

Yellowing is when the tint or color of the material changes a slightly after curing. For example, wood coatings such as clear coat often change the tint.

Oxygen inhibition refers to the exposure of the surface to oxygen during curing. When this happens, the surface does not cure sufficiently and remains sticky.  To combat this issue, the range of wavelengths that photopolymerization initiators respond to has expanded in recent years. It now spans from 300nm to 430nm. Wavelengths near 300nm show promising results in reducing the problem of oxygen inhibition. 

However, LEDs near that wavelength are expensive and have low illuminance, so oxygen inhibition will continue to be a major issue and opportunity for LED manufacturers in the future. We do offer irradiators with wavelengths of 285nm and 300nm, so reach out if you’re interested in further information.
Get in touch with us at, info@ocirtech.com