UV-curing adhesives are often the first choice when it comes to fast and efficient bonding of components. Curing within seconds, these adhesives allow high throughputs to be achieved in serial production. For optimal curing processes, exposure lamps, in particular, play a crucial role in quite challenging industrial sectors such as automotive or consumer electronics.
by Robert Saller
UV-curing adhesives are particularly well established in consumer electronics. No wonder: within seconds, components of miniature speakers have to be bonded, displays sealed, or microswitches encapsulated. Also in the automotive sector, the use of UV-curing adhesives enables fast bonding processes, such as in electric motors, where the stator is bonded to the housing.
What makes the adhesives so special is their curing mechanism. The adhesives contain a so-called photoinitiator, a chemical compound that breaks down into functional groups through absorption of UV light, thus initiating the polymerization. In the curing reaction, the monomers in the adhesive combine to form a polymer, resulting in a tightly networked structure. A light pulse with a duration of less than one second may be sufficient to fully cure the adhesive and permanently bond components.
The role of high-performance LED lamps
Today, high-performance LED lamps are primarily used for curing UV-curing adhesives. Such lamps are highly efficient and energy-saving and have become well established in various other areas of lighting. The typical lifetime of LED curing lamps often exceeds 20,000 hours. That is 20 times longer than that of conventional lamps.
While discharge lamps had to be preheated for up to 20 minutes, LEDs reach their full luminosity in milliseconds. The narrow emission spectrum of the light-emitting diodes and their high intensities enable optimum adaptation between the adhesive and the curing lamp. However, to truly benefit from the advantages of UV-curing adhesives and high-performance LED lamps, some important factors need to be considered.
The lamp must match the adhesive as well as the component
A key criterion for the light-curing reaction to occur at all is that the wavelength spectrum of the curing lamp overlaps the absorption spectrum of the photoinitiator.
Photoinitiators have a typical absorption spectrum that ends at 370 to 480 nm, depending on the chemical compound. In order to prevent an uncontrolled curing process, the absorption spectra are usually chosen such that daylight, for example, cannot cure the adhesive, or only very slowly. Analogously to the absorption spectra of the adhesives, LED curing lamps emit light at 365, 400, or 460 nm.
For a specific light-curing bonding process to actually take place, it is important that a second condition is met: at least one of the components to be bonded must be translucent within the adhesive’s absorption range. The component’s transmission spectrum also needs to be known in order to select the ideal LED lamp. The spectrum can be determined using suitable measuring devices.
How light intensity influences the curing process
The curing process is also influenced by the intensity of the lamp. Commercially available LED lamps for adhesive curing, such as those made by DELO, have an intensity of up to 12,000 mW/cm².
The major advantage of such high intensities is that they help reduce process times, even for rather difficult bonding tasks: for example, in case of large, design-related distances between the light source and the components to be bonded, or if rather thick adhesive layers are required to cure very quickly. They even allow curing of adhesives layers between partially transparent components, which often let pass only 10 % of the initial intensity value. But this is not the only advantage of high-intensity lamps. The lamp’s lifetime can be increased beyond the manufacturer’s specifications if full LED intensity (e.g., 2,000 mW/cm²) is not required and curing takes place at lower intensities. By presetting the power, the intensity for LED lamps can be continuously adjusted between 0 and 100 %.
For proper selection of the intensity level, it is extremely important to look at the overall process. It is not always beneficial to use particularly high intensities, since they may in some cases lead to improper curing. The overall process includes the working distance, the transmittance of the components and the adhesive layer thickness. For example, the intensity reaching the component to be bonded depends on the distance between light source and component. The smaller the working distance, the narrower the light intensity profile or the higher the intensity. This effect is even more significant for spot lamps than for area lamps, with only two mm already making a clear difference. As a general rule of thumb, if the working distance is doubled, the intensity decreases by a factor of four.
Choosing the lamp according to the application
The industry distinguishes between two lamp types: spot lamps and area lamps. Spot lamps are used for punctiform or linear bonding, as is often the case in the production of microelectronics. Area lamps are chosen for batch exposure of large surfaces such as displays or a large number of components at the same time (e.g. sealing of microswitches). So, depending on the application, different lamps may serve the purpose best.
Homogeneous curing of surfaces
Area lamps enable simultaneous exposure of large surfaces or continuous exposure in long production lines. When designing production lines, it is advisable to provide for area lamps that can be arrayed modularly in different ways without creating shadows, which can occur if the housing is larger than the exposed area. With its two design versions, particularly slim edges, and a light exit area of 100 x 100 mm² and 200 x 50 mm², respectively, the DELOLUX 20 and DELOLUX 202 area lamps perfectly meet this requirement.
The goal should always be to expose the entire surface to be bonded with the same intensity. If large areas are exposed, it is also crucial to guarantee stress-free curing of the adhesive to prevent warpage. Only a homogeneous exposure can ensure that the adhesive cures evenly and completely in all areas and achieves the properties required for the application.
Lower quality lamps often have an inhomogeneous exposure profile due to cheap optics and LEDs, so that the intensity in the center of the bulb is high, but drops off at the edges. The consequences are poor adhesive properties or even component failure.
Spot lamps are usually chosen when tiny surfaces of only a few square mm need to be exposed reliably. Most spot lamps use screw-on focusing optics the user can choose flexibly, as is the case for the DELOLUX 50 curing lamp, for example. This allows achieving spot sizes with a diameter between 1 and 10 mm and intensities of up to 12,000 mW/cm², depending on what is best suited for the specific application. Spot lamps are usually cooled passively; a special feature is provided by the DELOLUX 80 spot lamp (23 mm Ø exposure area), where the LEDs are cooled by a permanently installed, maintenance-free water-cooling system.
UV-curing adhesives, in combination with powerful lamp technology, enable fast curing processes, provided that all parameters have been properly tuned. For optimally designed processes, it is always essential to look at the overall system including adhesive, curing lamp (wavelength, intensity, and type), components, and the production line itself. Furthermore, it is advisable to choose a lamp type allowing production lines to be expanded as desired in order to have great flexibility in the design of such lines.
(Robert Saller is Managing Director at DELO Industrial Adhesives)