|ITW Devcon's Tru-Bond 18400-series UV-cured pressure-sensitive adhesives cure rapidly to an optically clear, nonyellowing finish|
When it comes to medical device manufacturing adhesives, the choice of curing method can make all the difference in the world. While such products as heat- or time-cured adhesives produce very strong bonds, UV-cured adhesives, although not as strong, have one major advantage: they cure virtually instantaneously. Rapid curing, in turn, can reduce material consumption and decrease production costs.
UV adhesives can also bond a range of difficult-to-assemble materials, including UV-inhibited and tinted plastics. And because they also offer good gap-fill capability, flexibility, strength, and variable viscosities, these adhesives have become a popular option for a host of medical device applications.
UV-cured adhesives have three major advantages, remarks Michael Lucey, president of MLT/Micro-Lite Technology (Mesa, AZ). First, their chemical bonding reaction happens very quickly. Second, UV-adhesive bonds display a range of physical properties that are similar to those of thermal or air-dried adhesives. Third, UV light replaces the use of thermal sources that can color adhesives in undesirable ways.
"Permanent materials that can be applied to glass, stainless steel, or difficult-to-bond substrates, UV adhesives cure when UV light penetrates through the substrate, causing a chemical reaction that transforms them from a liquid into a solid," Lucey explains. "And as soon as they are cured, UV adhesives display various crucial physical properties, including high tensile strength and desirable elasticity between the substrate and the adhesive." For example, UV adhesives can have a high compressive modulus, which determines the amount of stress the material will exhibit under strain, and a low elastic modulus, which ensures the adhesive's stretchability.
"UV-cured adhesives basically mimic the chemistries that you would find in any other polymer," Lucey explains. "For example, they mimic the properties of old-fashioned amine-cured epoxies and isocyanate-cured urethanes. Basically, you're just substituting the functionality that causes the cross-linking and doing it in a very short time."
But UV-cured adhesives are more than just fast. They are also beneficial for medical device applications because the excitation of their photoinitiators occurs in the UV region, preventing some adhesives from yellowing. Curing mechanisms that operate in the visible-light region, in contrast, have yellow photoinitiators--an undesirable characteristic that can affect the adhesive's color.
Boasting this nonyellowing capability, MLT's Lite Fast A-1005 is a high-temperature UV material used for bonding glass to glass. Employed in the manufacture of video-based invasive medical devices, this adhesive is used to bond fiber-optic cables together and polymerizes within 20 to 40 seconds after exposure to a high-intensity UV spot-cure system with a peak wavelength intensity at 365 nm.
"Designed for OEM applications, A-1005 is a permanent adhesive for adhering difficult-to-bond materials," Lucey comments. "One of our clients uses it for bonding together three or four strands of fiber-optic glass, each with a diameter equivalent to that of a human hair. These bonded strands then become part of such instruments as endoscopes and cytoscopes."
Besides being strong and flexible, A-1005 can also withstand such harsh sterilization techniques as autoclaving. "When a medical device is autoclaved, the adhesive must be able to withstand a pressure-cooker-type atmosphere of 123°C for approximately 15 to 30 minutes," according to Lucey. "And you have to be able to do that over and over--200, 250, 300 times--before the adhesive joint breaks." Bond-line failure is another concern, Lucey adds. "Usually, the component that the adhesive is on will fail before the adhesive itself fails."
Like MLT, MasterBond Inc. (Hackensack, NJ) is well aware of the importance of UV adhesives for medical device assembly applications. "UV adhesives have a variety of advantages in medical device applications," remarks Walter Brenner, the company's research and development manager. "But the beauty of them is that they don't require mixing, and they're fast, clear, and easy to use."
|Master Bond's high-viscosity UV15X-6MED2 can encapsulate large open spaces.|
The company offers many UV adhesive formulations that cure in seconds. But besides speed, they can withstand hostile environments up to 200°F when employed in pretreated polyolefin-, polyethylene-, and polypropylene-based catheters and in other applications. In addition, they offer good peel strength because the structure of the plastic enables users to build flexibility into the adhesive. "Moreover, they can be made from urethanes, acrylics, or epoxies, all of which can be rigid or flexible," Brenner says.
Adjustable viscosities are another physical property of UV adhesives. Most catheters, Brenner comments, require the use of low-viscosity adhesives, especially if the area of engagement is very small. In other applications, such as those that involve sealing and bonding, manufacturers prefer a high-viscosity adhesive. Such materials include Master Bond's high-viscosity UV15X-6MED2, which can encapsulate large open spaces. "With UV-cured adhesives, you can adjust many parameters," Brenner adds. But whenever a formulation is altered, it must be retested to comply with USP Class VI specifications.
Despite their many advantages, UV-cured adhesives are still not as physically robust as standard, non-UV medical materials, such as epoxies, in terms of tensile strength, elongation, flexibility, and peel strength. As a result, they are not suitable for applications requiring repeated autoclaving. While MLT's Lite Fast A-1115-M can withstand some autoclaving, UV-cured adhesives cannot be autoclaved over long periods of time.
Master Bond is attempting to overcome this limitation by developing optimized chemistries and improving cross-linking. "At least theoretically, the tighter the adhesive's structure, the better its performance should be in a hostile steam environment at elevated temperatures up to 200°C," Brenner says. "But at present, UV adhesives are primarily used in disposable devices, which require a limited number of sterilization cycles."
Cure Now, Bond Later
While the UV-cured adhesives offered by MLT and Master Bond are used in invasive medical device applications, ITW Devcon (Danvers, MA) offers formulations that are used to bond components in such noninvasive devices as handheld patient monitors. One such technology is the company's Tru-Bond PSA 3000, a 3000-cP pressure-sensitive UV/visible light-cured adhesive that is suitable for form-in-place gasket assembly.
"Normally, UV adhesives are applied between two surfaces and cured in place by passing UV light through one of the substrates," remarks Jim Dunn, manager of technical sales development at ITW Devcon. "Bonding occurs at that point." This operation typically requires that one of the substrates be either clear or translucent so that the light wave can pass through the adhesive bond.
In contrast, when Tru-Bond PSA 3000 is applied, subjected to UV light, and transformed into a pressure-sensitive adhesive on the surface of a substrate, bonding does not occur simultaneously. "Uniquely, you can apply our adhesive to the part and then assemble it five minutes from now, an hour from now, or a month or a year from now," Dunn comments. "And because our material is cured on exposed surfaces and does not require light to pass through a substrate, it works with opaque materials."
Capable of adhering to a range of substrates, the adhesive displays the rapid curing times typical of other UV adhesives. "What happens is that the adhesive cold-flows into the little nooks and crannies of the mating surface, and it increases the surface area contact," Dunn says. "This provides a higher shear strength."
In addition to offering shear strength, the adhesive bonds almost all substrates with a range of peel strengths. For example, its peel strength can be as low as 1 to 3 lb/lineal in. on polypropylene substrates in typical form-in-place gasket installations and as high as 9 lb/lineal in. on PVC. Even higher peel strengths can be attained on metal, glass, and ceramics.
As a pressure-sensitive material, ITW Devcon's adhesive often competes with double-sided tapes, Dunn says. The difference is that the use of tapes results in waste. "If tapes need to have a die cut so that they can go around structures such as gasket areas, a lot of center is lost because material must be cut away," Dunn adds. "With our material, since there is no center waste, we are more cost-effective than die-cut applications."