Q: In the past, we have seen some light-curable adhesives impacted by a form of inhibition, leaving a tacky residue on the surface. What causes this and how can we avoid it?
Here's how it works...
Light-curable acrylate adhesives cure by exposing photoinitiators to certain wavelengths of light, which break apart the photoinitiators into radical species, which react with oligomers to create long chains and crosslinks. (When you shine a light on the adhesive, it turns into a solid). Usually, by picking a high intensity light of >1 W/cm2 (keep in mind the sun is 0.002-4 W/cm2), and across a broad spectrum range from 300-450 nm, there is so much energy that the adhesives crosslink extremely fast, leaving a firm, tack-free surface. However, once in a while, some monomers and oligomers, which are the building blocks of these adhesives, may be susceptible to "oxygen inhibition" during the cure process. Oxygen inhibition occurs during the curing step, whereby if there is oxygen present at the surface, the oxygen can penetrate into the surface and interfere with the radical polymerization, leaving unreacted monomers and oligomers at the surface. This is the tack that some people feel as they rub a finger across the surface and may get traces of wet residue on their gloved hand.
Many of the newer adhesives are designed to go tack free under medium- and high-intensity conditions and when using the proper light wavelength. Generally, the higher the energy of light (lower wavelength (200-300 nm)) the better surface cure you get, but you limit the depth of cure. The lower the energy of light (higher wavelength (400-500 nm)), you get great depth of cure, but may get more oxygen inhibition. The UV/Visible light spectrum in the 300-450 nm range seems to have the best blend of surface and depth of cure. You also have to consider the intensity of the lamps. Higher intensity lamp systems can put out massive amounts of intensity, measured in mW/cm2. Some spot lamp systems emit up to 15-20 Watts/cm2 (as measured at 365 nm). It will be very easy to cure most adhesives to a tack-free state with such a high intensity light emitting both UV and visible light.
We have observed that most adhesives have a minimum intensity threshold where they will go tack free within a specific time, as well as a minimum total energy threshold to get full cure. Let's look at the equation "Joules/cm2 = Watt/cm2 x seconds". You can vary the intensity in Watts against the time of exposure in seconds to get the same amount of Joules/ cm2. 2 Joules/cm2 = 2 Watts/cm2 x 1 second, OR 2 Joules/cm2 = 0.02 Watts/cm2 x 100 seconds. While it may seem like you get the same amount of total energy in both situations (high intensity for short time, or low intensity for long time), the minimum intensity threshold may show that if you fall below 0.1 Watts/cm2 - you will never get a tack-free state.
Sometimes, some people find it beneficial to tackle the problem a different way and flood the curing area with nitrogen or argon gas during the cure. The nitrogen molecules are larger and are not able to penetrate into the surface as easily, and therefore give a tack-free surface even with low-intensity light sources. This is helpful in situations where a substrate is very heat sensitive or UV sensitive.
Adhesive selection also plays a part. The black magic happening in the chemistry labs allows the chemists to create new adhesives that go tack free at lower intensities. Many of these find roles in potting and coating applications, with all different properties. It used to be that only high-durometer materials went tack free (D60-80). Now chemists have found ways to get soft, yet tack-free adhesives with low durometer ranges down into the A40-60 range.
It all boils down to: With the proper adhesive, process, light wavelength, time, and lamp intensity, you should be able to get a tack-free surface.