Light-cure adhesives don't like the dark. It's an obvious problem that has kept these otherwise beneficial adhesives out of applications in which opaque components stand between bond line and light source. Henkel Loctite's newest epoxy adhesive gets around this problem with a patent-pending chemistry that needs light only to start, rather than finish, the curing process.
Susan Levanvoski, a Loctite development scientist, describes this new one-part, liquid adhesive as a "cationic epoxy," meaning that it contains a cationic photoinitiator. UV light in the 245 to 365 nm range kicks off the cure reaction. But once started, that reaction continues even in the dark. The company calls the adhesive a "Pre-Activated Epoxy."
In practice, the adhesive's dual light-and-dark cure mechanism enables opaque parts to be assembled with speeds usually associated with conventional light-cure acrylics. Users simply apply the adhesive to one part, expose the adhesive to UV light for as little as three seconds, and then finish the assembly. According to product manager Steve Ouellette, the adhesive has cure delay in the neighborhood of 30 seconds once its been pre-activated by the light. It cures to fixture strength in about three minutes and reaches its full cure in a few hours at room temperature. "That's very similar to what a light-cure acrylic would offer," he says.
And it leaves conventional two-component epoxies in the dust. These familiar adhesives vary widely in their cure times, but typical industrial products may take 30 minutes to reach their fixture strength and an entire day to reach their full cure.
Henkel Loctite isn't the only supplier to have come up with a light-activated cationic epoxy. For example. Wellomer GmbH (www.wellomer.com) sells similar epoxy adhesives in Europe. In North America, Electronic Materials Inc. (www.electronicmaterialsinc.com) has light-initiated, delayed-reaction epoxy that has seen use in the rarefied world of disc drive manufacturing. What's different here, though, is that Loctite chemists have optimized the pre-activation and delay times of their new adhesive for general industrial applications.
These applications include a variety of assembly jobs that involve one or more opaque or colored parts. Ouellette reports that most likely initial uses for the new adhesive will involve the bonding of housing components for notebook computers and other types of office equipment.
Despite its new cure mechanism, the new adhesive still qualifies as an epoxy—with all the bond strength, chemical resistance, and temperature performance epoxies bring to the table. Typical shear strengths, for example, are 2,800 psi on steel or PVC and 970 psi on ABS. In terms of thermal performance, it has a maximum use temperature of about 300F.
Pre-Activated Epoxy also exhibits a very low shrink, which makes it a good fit for precision bonding applications, including laser optics and other types of lenses. Even though lenses and optical components don't necessarily prevent the use of a light-cure adhesive, they can be sensitive to how much that adhesive shrinks as it cures. "Too much shrink can pull components out of alignment," Ouellette explains. A typical light-cure acrylic, for example, would shrink by roughly two to five percent as it cures. A fast-curing epoxy would have a similar shrink. The Pre-Activated Epoxy, by contrast, shrinks by less than 0.7 percent, Ouellette reports.
Henkel Loctite has been supplying Pre-Activated Epoxy to select customers for several months now. Its full commercial launch takes place this month. Ouellette says that future versions of adhesive will target medical and disc-drive applications. For more information, visit Henkel Loctite at www.loctite.com.