A new group of adhesives have been developed that cure under visible light (405-nm wavelength). Traditional light-cure adhesives, by contrast, need ultraviolet (UV) light with wavelengths between 250 and 365 nm. The difference of a few dozen nanometers of wavelength may not seem like a big deal, but it opens up a range of bonding, encapsulation, and sealing applications that were previously unsuitable for light-curing adhesives.
On the factory floor, for example, shifting to visible light eliminates the need for expensive UV lighting. Instead, inexpensive light-emitting diode (LED) lamps can handle the job. And eliminating UV lights means workers need not worry about eye injuries and skin burns.
Researchers have been trying to develop light-curing adhesives that use longer- than-UV wavelengths for a some time, and there are plenty of products on the market that cure at or close to 400 nm. However, none of these have been able to eliminate oxygen inhibition, a process in which atmospheric oxygen curbs freeradical polymerization, a key step in curing adhesives with UV light. The telltale sign of oxygen inhibition is a tacky, partially cured surface.
Oxygen inhibition is most pronounced in applications where the adhesive is exposed to the atmosphere. For example, oxygen inhibition would be worse in any application with an open-air cure than in an application with the adhesive between layers of glass. Thicker adhesive cross sections also increase the potential for oxygen inhibition, which in the past ruled out many potting and encapsulation applications.
A common strategy for minimizing oxygen inhibition is to use adhesives that cure under more intense, shorterwavelength UV light. Unfortunately, this strategy is at odds with a shift to longer wavelength cures.
These new adhesives (which Master Bond calls the LED401 Series) eliminate this problem. Even though they cure under less intense light, the final finish is best described as glassy smooth and completely tack-free.
LED adhesives work with a variety of substrates, including those that block UV-light transmission. They also adhere to glass, polycarbonate, and acrylics, as well as many other polymers and metals.
They also cure deeply with the full cure extending 1/8 of an inch below the surface. Thus, the new adhesives can be used as encapsulants.
Fully cured LED adhesives do not lose any physical or mechanical properties, and they retain their electrical insulation and dimensional stability with little shrinkage. In addition, the adhesives resist water, chemicals, fuels and oils, and many acids and bases.
Most grades of LED401 are optically clear. In thicker sections, the adhesive takes on a slight amber tint, though this does not significantly hinder light transmission. Researchers have developed an opaque white grade for applications that don’t require optical clarity.
Less expensive but safer
The new adhesives can be cured using any lamp capable of emitting light at 405-nm wavelengths with an intensity of 1 to 4 W/cm2. Many off-the-shelf LED lights, including inexpensive spot lamps, meet this criteria. Under one of these light sources, cure times are short. In thinner sections, it takes 15 to 30 sec for a full cure. Thicker sections require longer cure times, but usually less than a minute.
Curing with visible light also makes the factory floor safer in that UV lights can injure eyes and burn skin if operators are not careful. Lower-intensity LED lights pose no such threat.
Menu of new adhesives Master Bond’s new line of LED one-part/no-mix adhesives are available in three grades:
LED401 is a solvent-free compound that creates strong bonds and works with a variety of substrates. It also resists moisture.
LED401LV is a low viscosity, optically clear adhesive that cures quickly. It bonds well to substrates and has excellent dimensional stability.
LED403Med is a high viscosity adhesive intended for medical applications. It cures tack-free without any oxygen inhibition. It meets USP Class VI speci cations and passes the ISO 10993-5 cytotoxicity test. This version is biocompatible, resists various sterilization methods, and it adheres well to many of the plastics used in medical devices.