Fast ultraviolet light and the broader-spectrum light-curing resins now available (Table 1) offer expanded possibilities to reduce cost from many assembly processes. First introduced in the early 1980s for applications beyond inks and thin coatings, light curing has become the method of choice for many more industrial-bonding, sealing, coating, potting and tacking applications. Because faster cures provide more-efficient manufacturing processes and lower total-assembly cost, and because light-curing adhesives are being used in more kinds of applications, both the range of resins and curing equipment now available have expanded dramatically.
Some applications efficiently accomplished with certain UV technologies include bonding glass and most plastics, medical disposable-device assembly, military and consumer electronics, microelectronics and optical encapsulation, lens assembly, conformal and decorative coatings, tacking and strain-relief applications, as well as gasketing in small and large automotive and consumer-product assemblies. Today’s opportunities for light-curing resins are limited only by (1) the ability of the resin to be exposed to and polymerized by light and (2) the imagination of the engineer. (Table 2).
First-generation UV-curable resins did not possess the entire range of properties found in the other adhesive families. However, the second generation of light-curing resins is now broad enough to be considered for many applications that once required slower-curing silicones, epoxies, urethanes, pressure sensitive tape, cyanoacrylates, modified acrylics and other methods, including mechanical fasteners (Table 3).
The challenge now is to choose the best and most cost-effective system for a particular application with today’s large array of light-curing resin and equipment offerings. A simple, four-step evaluation process can indicate the right direction.
1. Match The Adhesive To The Application
To make sure the adhesive, coating or encapsulant matches the “life” requirements of the particular assembly, most adhesive manufacturers provide product data sheets with laboratory test data describing the physical cured and uncured properties of the resin. Curing recommendations may be provided as well. Some resin manufacturers offer adhesive selection as well as process-development assistance. This information provides guidelines for determining feasibility of an adhesive for a particular application.
2. Match The Lamp To The Adhesive
Make sure that the system is optimized — the curing lamp should be matched to the adhesive’s curing chemistry. The spectral output of the lamp needs to be matched to the adhesive’s curing chemistry for cure depths and surface curing to be optimized. For example, most UV inks and coatings require very fast dry cures [typically run rates of 50 - 150 feet per minute (fpm) to be economically practical], and are thus formulated to cure at very high speeds in very thin layers. These coatings exhibit good adhesion to porous surfaces, like paper, and curing requires high-intensity lamps mounted on wide conveyors. It is critical that these lamps emit high doses of short-wavelength UV light to cure these products optimally.
On the other hand, for most high-strength bonding, coating, tacking and potting applications requiring depths of cure from 0.003 to 0.250 inches, moderate- or high-intensity lamps with broad-spectrum curing capability, with concentrations in the long-wave and visible spectrum, are most suitable. Cure speeds with these resins and lamps are far slower than for the typical ink or coating application. However, the 1- to 15-feet-per-minute process rates, or 1- to 30-second exposure times, needed to achieve cures are generally far faster than those offered by other resins and fastening alternatives.
Because resin formulations vary significantly in their cured properties and curing characteristics, the process for selecting the adhesive and equipment is crucial. As shown in Table 4 (page 32), there is quite a range of curing capabilities within the universe of light-curing adhesives. The table represents only typical values for some products. It does not include the rates for slower-curing UV cationic epoxies or UV silicones.
3. Matching The Lamp To The Application
The wide variety of curing equipment that has been devised to cure resins for different applications is perhaps the strongest testament to the success of light-curing technology. Most curing equipment can be categorized by the shape or “footprint” of the light that is produced where cure takes place. On the basis of shape, light can be categorized into spot, beam and flood configurations.
Spot lamps can be divided into those that produce a 1” to 2.5” spot of medium- to low-intensity (10 to 100 mW/cm2
) UV or “black” light, higher-intensity (1,000 to 10,000 mW/cm2
) spots and high-intensity wand-style lamps. Wand-style lamps have become very popular over the last 10 years and are useful wherever small spots of adhesive need to be cured.
A rule of thumb for whether or not to use a wand-style lamp for a particular spot-curing application is that the light “spot” should completely cover the adhesive drop at the closest proximity to the resin. Light-guide tips are typically held close (within 1/8”) to the resin to take optimum advantage of the intensity of the lamp.
Flood-style lamps usually provide low- to moderate-intensity light. These lamps have the advantage of being able to cure a tray of parts, or parts with large bonded or coated areas. Manufacturers commonly use these kinds of lights mounted over indexing tables or conveyorized transport systems. Fairly deep cures can be achieved by these relatively inexpensive units in 10- to 30-second exposure times.
Wide-area flood lamps are used successfully to cure substrates that are somewhat heat-sensitive, such as certain plastics. Even low-intensity, “cool” black lights, available in various lengths, are being used as an inexpensive curing method in some bonding applications.
There are a wide variety of focused-beam lamps available in the market today. Frequently, these are mounted on conveyors to effect very rapid cures for coating or encapsulation applications. Various conveyor lengths from small, compact, 2-ft-long models to larger units with belt widths of 6, 12 and 20 inches, are commonly available. These lamps produce sufficient intensity to overcome the effect of oxygen inhibition at the surface layer of the coating that some UV resins exhibit.
Generally speaking, the higher the intensity, the faster the cure, though a situation of diminishing returns is rapidly reached for intensities above 2,000 mW/cm2. One of the advantages of UV/visible curing is that many resins are so sensitive that they cure at intensities far lower than those emitted by many UV lamps. This allows for degradation of most lights to a half or more of rated intensities before a limit at which adequate cure does not occur is reached.
4. Define Curing Parameters Based On Specific Application
After determining the best adhesive or sealant and most appropriate lamp for an application, the key to light-processing success lies in determining: (1) The speed at which the process needs to proceed and (2) The minimum amount of light intensity required to achieve cure in that time. (The definition of cure is the minimum time required to achieve the adhesive or coating properties needed for the particular application.) Scheduled radiometer readings (a radiometer is a device that measures UV intensity) at the bond line, or at the level of the coating, provide the most complete and cost-effective check of intensity.
A great advantage of light curing is that most second-generation UV/visible adhesives cure quite adequately with relatively low levels of intensity. Some applications may require 10 mW/cm2, while others need 100, 500 or 1,000 or more mW/cm2 for cure. Additionally, they usually cure just as well at 10 or more times the minimum amount of energy needed for cure. This is a valuable feature due to the fact that the intensity of most UV-emitting bulbs decreases over time.
Therefore, in order to maintain a practical bulb life, one should choose a lamp whose intensity at half its initial rating provides more than two times the intensity required for cure. Fortunately, there are many UV-curing lamps commercially available that emit 10 to 100 times the intensity needed to cure most newer UV/visible formulations.
Higher-cost, high-intensity, electrodeless UV lamps best avoid the issue of bulb (intensity) degradation because bulb life is quite long and they shut themselves off when intensity drops to a certain level. These lamps are the best choice where very fast processing speeds or special cure considerations are required.
Some wand-style lamps, in an effort to deal with the bulb-degradation issue and assure a constant amount of light, use an internal variable-aperture mechanism or other mechanical adjustment to control the amount of light intensity escaping into the light guide. Light is monitored internally and triggers the enlargement of the aperture as the light intensity drops off, so that more light escapes and the intensity entering the light guide stays constant. When it drops below a preset level, an indicator light comes on.
While this feature seems a convenience, there is a significant expense associated with such equipment. And, whether the intensity of the light is held constant by mechanical means, or simply allowed to decrease to a predetermined minimum, has no effect on the quality of the cured adhesive. Inevitably, light must always be measured at the end of the light guide with a radiometer whether or not light is being internally monitored and controlled. This is necessary because the wands themselves will deteriorate slowly over time.
100% In-Line Quality Assurance
Academic and/or theoretical models of energy dose required for cure generally are not useful as indicators of complete cure because they do not assure 100% integrity of the bonded part. Application-specific, in-line testing yields immediate and consistent quality assurance. One of the great conveniences of UV/visible processing is the ease of incorporating 100% in-line inspection or bond testing because of the instant-curing characteristic of the adhesives. Simple, mechanical tests are frequently done immediately in-line, as soon as the curing process has taken place. Cumbersome and costly off-line processes can thus be eliminated, and 100% quality bonds are assured.
For example, look at the Typical Bulb Degradation Curve in the figure above. If evaluations determine that a cure schedule of 200 mW/cm2 is required to cure an adhesive in a 10-second exposure time, one can set a bulb-replacement regimen for approximately every 1,000 hours. Monitoring the real intensity at the bond line can always ensure having more than the minimum intensity required for cure. One solution for a light-curing bonding process that has been set on the edge of the curing capability of the adhesive and lamp system may be to select a faster light-curing adhesive.
Photo-Curing May Not Be Best Choice
As with any technology, there are reasons not to choose a photo-curing system. The primary reason would be when some aspect of the assembly or process blocks resin from exposure to light.
For most practical purposes, light-curing adhesives and sealants require direct exposure to light in order to cure. There are exceptions to this rule — the main one being that some UV-curing resins also have other secondary curing mechanisms that work well in some applications. UV-curing catalysts have long been combined with activator and heat-curing systems in resins.
More recently, UV-curing resins have been developed as two-part systems, and as systems that are also sensitive to curing upon exposure to oxygen and moisture. Usually, these systems are only practical where UV curing is the primary curing mechanism, and the majority of the resin is light-cured.
Another exception is that of the UV epoxies that claim to continue to cure following exposure to light. This is a very compelling claim as it inspires thoughts of a brief initiation starting a chain reaction that can continue regardless of circumstances until cure is complete. In fact, the circumstances are limited and difficult to predict consistently.
Because of the processing and performance advantages of UV/visible formulations, a rapidly growing industry of service, equipment and resin providers has arisen. The number and type of applications where the resins have been used successfully have provided the impetus for this growth. As competition in the future intensifies, this technology, as all others that lower a manufacturer’s processing costs, can be expected to continue to grow and diversify.
Additional information on complete light-curing adhesive systems is available from Dymax Corp., 51 Greenwoods Rd., Torrington, CT 06790; call 860-482-1010; fax 860-496-0608.