An MD&DI October 1999 Column
Efficient light penetration through opaque materials and the avoidance of heat-induced substrate damage are among the benefits of pulsed curing systems.
When materials such as certain adhesives or coatings are cured using heat, a problematic by-product can be produced. During the thermal drying process, the solvents in the adhesive or coating evaporate. Unless they are recovered, these vapors from the solvents can create what amounts to toxic air pollution.
In contrast to heat-cured materials, substances that cross-link in the presence of ultraviolet (UV) light cure without emitting toxins into the air. In UV curing, specially formulated liquid mixtures harden instantly into adhesives or coatings when exposed to UV light. These chemical formulations contain photoinitiators, which catalyze the photopolymerization reaction. No solvents are used, so there is no air pollution from the UV curing process.
Pulsed UV systems provide even illumination and little heat buildup in the substrate. Photo courtesy of Advanced Polymers Inc.
With one particular type of UV curingpulsed UV curingadditional environmental and safety benefits can be realized. Whereas most other types of UV curing systems employ lamps containing mercury, pulsed UV uses xenon lamps. This frees the user from concerns about compliance with occupational and environmental regulations regarding mercury use and disposal. Furthermore, pulsed systems use only electrical energyand not microwavesto power the lamps, eliminating the need for shielding to protect personnel from microwave radiation in the manufacturing environment.
Figure 1. Curing can be accomplished via the short, intense bursts of pulsed UV systems while using no more total energy than with continuous-power units.
This article discusses the above benefits as well as other key characteristics of pulsed UV curing for medical device manufacturing, including lower substrate temperatures, high peak power for penetrating thick and opaque materials, instant (in microseconds) on-and-off power application, lamp design flexibility, and safety.
About half of all medical devices are made of plastic, the majority from heat-sensitive polymers. Processing steps that overheat the plastic can cause melting or microscopic stress cracking. These microcracks are virtually undetectable, but can propagate and cause the product to fail while in use. For medical devices, this is clearly unacceptable.
The lower substrate temperatures that can be achieved with pulsed UV curing can help prevent microcrack formation. There are five key reasons for low temperature buildup in the substrate during the curing of devices with pulsed UV light:
- The short-duration pulses, which are too fast for heat buildup.
- The presence of a cooling period between pulses.
- The fact that pulsed UV lamps don't have to operate at a temperature high enough to vaporize mercury (and thus run cooler than mercury lamps).
- The high peak power of the pulses, which eliminates the need for high average power.
- The lack of continuous infrared radiation, as the lamps can be turned completely on or off in microseconds.
The above characteristics can bring distinct advantages to certain complex manufacturing applications. For example, adhesive bonding of products such as thin-walled, high-strength tubes or angioplasty balloons presents one of the most challenging problems in medical device fabrication. Tube assemblies as small as 0.5 mm in diameter, with bonds shorter than 1 mm in length, are often required to hold pressures as high as 290 psi. The subassemblies consist of thermoplastic materials such as polyamide, polycarbonate, PVC, polyethylene, and PET, through which UV light must penetrate to cure the adhesive.
Because many of these materials are delicate and heat-sensitive, it is important that heat transfer be minimized. Continuous UV curing of such devices has resulted in discoloration as well as microcrack propagation, both of which can be reduced with pulsed processing. The high peak energy output of pulsed UV systems results in better surface curing of some adhesives, minimizing the surface residuals that can result from the use of continuous-wave units. High peak output levels also provide greater assurance that the entire adhesive area has received enough radiation to thoroughly curea critical factor in medical devices because of bond-strength and biocompatibility concerns.
With its high peak power, pulsed UV light penetrates opaque materials more effectively than does continuous light. (As an example, the ability of pulsed light to penetrate is the reason that the Federal Aviation Administration mandates that lights on tall towers must be pulsed, since pulsed light can be seen from a greater distance and penetrates mist and fog better than continuous light.) With opaque materials, a greater percentage of light gets absorbed or reflected with each unit of substrate thickness penetrated. By its very nature, an adhesive to be cured tends to be sandwiched between two materials, both of which may be opaque. The manufacturer must somehow get enough light to the adhesive to cure it, while keeping the heat low enough to avoid thermal damage and using as little energy as possible.
Pulsed UV curing brings together the requisite energy efficiency and depth of penetration. The same amount of energy will power a 600-W/in. continuous lamp 1 minute or a 100-W/in. pulsed lamp with 20-kW peak power during a 1-second pulse (Figure 1). Whereas a continuous mercury lamp will deliver most of its energy as heat near the surface of an opaque substrate, light from a pulsed lamp penetrates much farther into the substrate before being totally absorbed or reflected. Pulsed light is thus more effective at delivering light energy for curing where it is needed most.
Most UV curing today is carried out with standard mercury-vapor lamps that must be left on continuously. For many applications, the warm-up time and mechanical shielding necessary for the noncuring phase of the cycle for these lamps can result in undesirable process constraints. Pulsed UV lamps provide full curing energy in microseconds, compared with several minutes of warm-up time necessary for electrode mercury-vapor lamps.
With pulsed systems, processes can be repeatedly started or stopped with the pressing of a button or sending of an electronic signal. This greater degree of control can translate to process improvements and to significant savings in time and lamp maintenance on high-volume manufacturing lines. It is also safer for the lamp to be completely off when not curing.
Figure 2. Lamps for pulsed systems can be configured for specific applications. This circular lamp provides 360° illumination for curing balloon catheters. Photo courtesy of Xenon Corp.
The ability of an operator to turn the lamp off without time penalties can also be important. With some production lines, products could be subjected to enough energy for them to melt or catch on fire if the line stopped and the light source remained on. Pulsed UV lamps can be stopped instantly without incurring a subsequent time penalty when restarting the line.
The instant on/off capabilities of pulsed systems are also beneficial when assembling devices with UV-sensitive components. As each device moves under the light source, the light can be turned on when the area to be cured travels by and then shut off before the UV-sensitive areas move past. In addition, the pulse rate can be varied in relation to line speed to provide full cures at a range of operating speeds. The use of pulsed lamps can also allow for significant simplification of plant ventilation systems, since fans and ducts required to remove the heat and ozone generated by mercury lamps would not be needed.
LAMP DESIGN FLEXIBILITY
Unlike mercury lamps used for continuous curing, pulsed UV lamps can be constructed in lengths and shapes tailored to customer requirements. For example, lamps developed for curing balloon catheters have been designed to encircle the area to be bonded, thereby providing 360° illumination (Figure 2). Another application involved guidewires for arterial catheterization that are coated with a lubricious formulation that must be uniformly cured without exposing the 20-in.-long, thread-like guidewires to much heat. Taking advantage of the design flexibility of pulsed systems, the company placed a specially shaped lamp at the center of the coating machine, allowing the coated wires to travel around the lamp for curing (Figure 3). Yet another custom curing lamp was constructed for a heart valve maker in the shape of the company's arterial filter (Figure 4).
SYSTEM SAFETY AND VERSATILITY
Pulsed UV curing systems offer two additional characteristics that, while not critical for most applications, nevertheless provide added benefits: they can be used with nonvolatile chemicals, and they emit broad-spectrum light.
Figure 3. A pulsed lamp for curing coated guidewires is integrated into an automated production environmentin this case, a coating machine. Photo courtesy of Guidant Corp.
Neither the lamps used with pulsed systems nor the curing formulations contain toxic chemicals. (The mercury contained in most standard UV lamps represents a hazard should the lamp envelope be damaged.) Adhesives and coating formulations used with UV lamps are solventless, and cross-link only upon exposure to light. The liquids convert completely to a solid, and the material cures rapidly without emitting volatile gases.
Figure 4. A custom pulsed curing lamp constructed in the shape of a company's arterial filter. Photo courtesy of Medtronic Avecor.
Current pulsed lamps can be supplied in models that emit light in both the UV and visible regions of the spectrum. This capability is ideal for use with newly developed adhesives that require both visible and UV light to cure. These formulations won't cure when exposed to fluorescent room lightingas do visible lightcurable adhesivesand require less UV light than adhesives that cure with UV light alone. By filtering out unwanted wavelengths, a single pulsed light source can satisfy a wide range of wavelength requirements, from 200 nm (in the UV) through the visible spectrum.
Although application design requirements and cost-effectiveness must always be considered, the appropriate pulsed UV curing system can solve a variety of problems for medical device manufacturers. Pulsed processing offers a curing technology for adhesives and coatings that avoids heat-induced damage to plastics and penetrates farther through thick and opaque materials than do standard systems. Furthermore, pulsed UV lamps lend themselves to complex custom manufacturing applications because they can be made in a variety of shapes to fit specific requirements and can be switched on and off without warm-up or cool-down periods. Finally, pulsed systems are both extremely safe and particularly versatile in meeting various wavelength specifications for curing advanced adhesives.
Louis R. Panico is CEO of Xenon Corp. (Woburn, MA), a supplier of high-peak, narrow-pulse UV curing equipment for a variety of medical applications.