An MD&DI September 1999 Column
Use of cyanoacrylates for high-speed medical device assembly has become a widely accepted practice because of the adhesive's rapid cure time and high bond strength. In this month's MPB special section, Patrick Courtney and Christopher Verosky provide a general overview of cyanoacrylate adhesives in device manufacturing (see Advances in Cyanoacrylate Technology for Device Assembly). Since they wrote the article, however, a new type of cyanoacrylate adhesive has been developed by Loctite Corp. (Rocky Hill, CT) that cures either by exposure to UV light or by the standard cyano-acrylate anionic polymerization mechanism. Photoinitiators in the UV-light-curable cyanoacrylate absorb light energy and dissociate to form radicals that trigger the polymerization process in the adhesive. In essence, the photoinitiator acts as a "chemical solar cell" to convert light energy into chemical energy for the curing process, says Courtney.
By combining the processing benefits of cyanoacrylate and light-curable adhesives, the technology addresses certain limitations of conventional UV-only products and cyanoacrylate products. Courtney explains that, "with cyanoacrylates, you generally could not cure them with exposure to UV light. Now you've got a product that not only cures by the cyanoacrylate method, but that can also be cured with UV light.
"I guess the main advantage would be that you have a UV product with a secondary cure mechanism that doesn't require heat or chemical activators," Courtney explains. "Generally, that's been a limitation with UV-only products. If there's a shadowed area in the device near the bond line that won't see light, finding a means to cure the product that is consistent with a high-speed automated manufacturing process is pretty tricky. But cyanoacrylates cure rather quickly in tight gaps, so you can have a cure mechanism that you can be very assured of working — even in shadowed areas." Verosky adds that, "Actually, you can get better, faster cure response with the lower-intensity light sources than even light-cured acrylics can achieve in terms of tack-free time because [the process] is not inhibited by oxygen."
The light-cured cyanoacrylate is be-ing used with success in certain device assembly applications, say Courtney and Verosky. "I think where it's been very well received is in syringe bonding," Verosky adds. "Some customers are looking at using polypropylene hubs versus the traditional plastics, such as polycarbonate. That's the major medical device application that we've seen the most success with, and that was the one that we initially had in mind when we formulated these adhesives." Describing the use of the new adhesive for syringe assembly, Courtney explains that, "There's always a concern that product can potentially travel down past the section where the cannula meets the hub, and what might happen with the product. With this type of technology, you can be assured that material that wicks down there will still cure by the cyanoacrylate mechanism."
The authors suggest that one additional benefit of UV-curable cyanoacrylates is that applications in which there is the potential for blooming or an excess fillet to be cured can be addressed with a low-power light source. Says Courtney, "For newer things coming down the line, that's really a nice option to have." He emphasizes that this is especially true in medical markets, where aesthetics are important. "The blooming issue with cyanoacrylates always has to be addressed in one fashion or another. And this gives medical device manufacturers a really neat tool for solving that problem."