The manufacturing world is constantly looking for processes that can accelerate production while lowering unit costs and improving product reliability. Each innovation must mesh with the overall production process to achieve high output levels. Bonding processes should be capable of smoothly running in automated processes requiring short cycle times. UV and other light-curing adhesives, like those developed by DELO Industrial Adhesives, meet those criteria better than older adhesives and other joining solutions in a wide range of industrial applications. Without these fast-curing UV and other light-curing adhesives, it would not be possible to produce mobile phones, smart cards, embedded cameras, even shower enclosures, as we know them.
For large-scale joining of components on production lines, UV and light-curing adhesives offer significant advantages over traditional methods:
- They are generally solvent-free and one-component, which allows for easy, safe integration into continuous manufacturing processes.
- They polymerize in seconds when exposed to UV or visible light, for shorter cycle times, while allowing for a high degree of automated processing.
- Light curing or “cold joining” is much less energy intensive than curing by thermal sources such as ovens or welding systems.
- Curing occurs with deliberate exposure to light of a certain wavelength, not just any light, so components can be positioned precisely before the adhesive hardens.
- They support design innovation as well as miniaturization, bonding components too small to be screwed together.
- They have outstanding optical qualities, and when used to bond layers in a touch screen, can reduce reflections and improve readability, even in direct sunlight.
| Figure 1: Match between LED lamp spectra and photoinitiator spectra of acrylate- and epoxy-based adhesives |
UV- and light-curing adhesives can be categorized into two main types: Radically initiated adhesives are usually based on an acrylate matrix, while cationically initiated adhesives are based on epoxy resins. These one-component adhesives are simple to process, with no mixing required. In addition, there are dual-curing derivatives of both types which allow for curing in areas that cannot be directly illuminated by light. These materials usually cure by a combination of light with heat or humidity.
Both types offer their own advantages in different application scenarios.
For both chemistries, it is essential to adapt the wavelength of the photoinitiator with the wavelength of the UV light source to ensure an effective curing reaction. Light sources based on LED technology are favored, because with their distinct “peak” emission spectrum, the photoinitiator of the adhesive is designed to activate near the peak of the LED light source. LED lamps are available with different wavelengths (see figure 1) to suit the adhesive/photoinitiator being used.
Figure 2: Dispensing and curing process of an automated production line for snap-action switch sealing |
Seal bonding in seconds
Light-curing adhesives are particularly well-suited for and gaining market share in high volume applications where rapid curing and high reliability are essential, like the production of microswitches used in the automotive industry and other sectors. The following examples illustrate the range of applications for UV-/light-curing acrylate- and epoxy-based adhesives and their benefits.
A. Electromechanical device
In the automotive sector, process reliability is especially important for assembling microswitches used in safety-relevant items such as airbag deactivation switches or seatbelt lock-monitoring devices. These applications demand fast, in-line-compatible bonding with short cycle times and secure process control, including easy dispensability. Increasingly, light-curing adhesives are being used in place of two-component or heatcuring adhesives and casting compounds because they are more cost-effective and reliable.
In assembling snap-action switches for automotive use, hermetical sealing of the switch housings and connectors is essential for withstanding variable environmental conditions in vehicle operations. It is often necessary to bond the housing and seal the connector pins in a single step (seal bonding). Production volumes are in the many millions so the individual pieces must be joined in a continuous, high-speed, completely automated process, a job for which two-component or heat-curing adhesives and encapsulants are neither particularly reliable nor cost-effective. The complex and high-maintenance processing systems for two-component adhesives work only where bigger adhesive amounts (> 100 mg) are dispensed.
In contrast, UV- and light-curing adhesives like DELO-KATIOBOND are optimized for bonding automotive parts to seal them against fluctuating temperatures, humidity, contaminants, pressure, and shocks. The capital and operating costs of lightcuring lamps is much lower than thermal curing in ovens. Light-curing adhesives are easy to dispense in automated, high volume operations and cure within seconds – much faster than thermal curing. Their constant viscosity and unchanging flow properties allow complete wetting and reliable sealing and the bond holds up exceptionally well through years of use.
In addition, a direct 100 percent in-line control of seal tightness is possible since acrylate-based, UV-/light-curing adhesives like DELO-PHOTOBOND achieve final strength immediately after irradiation. Fully automated camera inspection of the bond can be done in-line by using colorants and/or fluorescent agents in the adhesive, ensuring detection of insufficient adhesive or possible contaminations at the connector pins.
The light-curing adhesives for such an application boast high flexibility over a wide temperature range (-40° to +176°F), essential for maintaining a reliable seal.
B. Mobile device displays
Light-curing adhesives also have played an important role in the rapid improvement of smartphone technology, helping meet the demand for fast, easily automated joining methods in assembling displays that can be joined in seconds. These adhesives enable tension-free, direct bonding to the LCD module (display bonding) and bonding of the cover glass to the touch panel (touch-to-lens bonding), producing the desired high optical transparency.
In order to minimize tensions, which cause display failures, these adhesives are very flexible and “soft” – with a shear modulus around 30 times lower than a “standard” flexible acrylate.
Dual-curing adhesives are also available for displays with shadowed areas (for example black print edges). They cure by exposure to light and have a secondary moisture curing mechanism for reliable curing in the shadowed areas.
C. Bonding of mini-loudspeakers
What applies to the display in your smart phone also applies to its speakers. They get smaller and must be produced in high volume with high quality. Today’s mobile phones include high performance mini-speakers, comprised of membranes, coils or covers – up to eight components – bonded together with a light-curing adhesive like DELOPHOTOBOND products that can cure in just seconds.
With light-curing adhesives, speakers can be mass-produced – up to 6,000 per hour. Testing can be done immediately after curing, so an OEM does not need a buffer stock to assure just in time delivery.
D. Chip and SIM card production
Microchips. There are billions of them embedded in credit cards, bank cards and mobile phone SIM cards. Card production and encapsulation of the microchips on them require extremely fast and reliable encapsulation processes. Epoxy-based, light-curing adhesives like DELO-KATIOBOND Dam&Fill offer the ideal solution: A high-viscous “Dam” compound is quickly dispensed around the chip. Then the chip is encapsulated by a lowerviscous “Fill” compound. Curing takes only seconds. The adapted flow roperties of both encapsulants are particularly important as the chip is placed in a cavity and a maximum cover height must not be exceeded. Light curing adhesives maintain this tolerance exceptionally well.
Wide range of DELO UV/light-curing options
DELO is a world leader in developing UV-/light-curing adhesives and curing lamps and partnering with manufacturers of advanced dispensing systems. The company is more than a product innovator, but also a solutions provider that can deliver optimized and even customer-specific solutions for an ever expanding range of applications. For example, DELO’s new DELOLUX LED lamp range ensures fast curing of products like the DELOPHOTOBOND or DELO-KATIOBOND adhesives for electronic applications. Optionally DELO-DUALBOND adhesives are available which offer a fast light fixation and secure curing in shadow areas due to a second curing mechanism like humidity or heat.
Conclusion
DELO UV-/light-curing adhesive families and DELO-brand curing lamps provide customers with major advantages in terms of quality, safety, and reliability. Short cycle times are achieved through the short curing times of both acrylate and epoxy products, enhancing productivity. Dual-curing derivatives of the two adhesive types offer the added possibility to bond components with shadowed areas not accessible by light. DELO’s ability to align the emission spectra of DELO curing lamps to the adhesive for optimal results can further enhance the customer benefit. With the expanding range of UV/light curing adhesives with application-specific benefits, customers have more incentive than ever to work closely with DELO and its sales and technical representatives to develop total bonding solutions that maximize these benefits and optimize the manufacturing process.