Efficient curing with UV LED s18 June 2012
Rick Slagle, Heraeus Noblelight’s business development manager optoelectronics for North America, outlines the reasons why UV LED light sources are increasingly being adopted for use in different drying and curing applications
In conventional lighting, visible LEDs (Light Emitting Diodes) are seeing explosive growth in a variety of applications, driven by low power consumption, long life and the environmentally responsible materials used in their construction. According to the US Department of Energy, from 2010-2030 the cumulative energy savings are estimated to total approximately 1,488 terawatt-hours – representing $120 billion at today’s energy prices – and would reduce greenhouse gas emissions by 246 million tonnes of carbon.
UV LEDs are also seeing ever-increasing adoption in curing applications for many of the same reasons – high efficiency, greater than 10,000 hours’ lifetime, green technology with no mercury disposal issues, no ozone production and no volatile solvents. Additionally, with optimised formulations of inks and adhesives, cure quality and durability have been found to be superior. Early adopters of UV LEDs have shown a long-term commitment to environmental and energy concerns.
LEDs are semiconductor devices and the manufacturing process is much like that used for making computer chips – many thin layers of semiconductor materials are deposited which, when energised, emit light. For high power UV LEDs, aluminium gallium nitride (AlGaN) is the material of choice. The UV LED chip is quite small (approximately 1mm) in comparison with the packaging, which includes the electrical connections, heat sink and lens.
The light output of UV LEDs is much closer to that of semiconductor lasers regarding wavelength, in that they are monochromatic, emitting at a single wavelength. Adding more aluminium to the semiconductor mix creates shorter wavelength UV LEDs. As such, the wavelength must be specified to match the material being cured.
Companies are increasingly adopting a social responsibility for the environment and energy consumption. Ink, adhesive and equipment manufacturers are exploring ways to meet these requirements. UV LEDs offer many of the answers.
They also offer many health and safety advantages: no high temperature operation, no glass tubes, no damaging UVC wavelengths to the skin, and no toxic ozone or mercury exposure.
In co-operation with formulators, the chemistry has been able to produce materials using UV LED curing to produce results that are durable, resilient and visually dramatic.
UV LED heads are easy to integrate, being quite compact and of low weight. Water cooled units are plumbed externally and air cooled units are also available.
LEDs are instant on/off and require no shutters or venting, and no external reflectors or optics. Due to their small size, they can be easily adapted to any form factor, such as 360-degree exposure for in-situ pipe curing or fibre coating curing. The mini-modules can also be packaged to any linear length.
Because there is no infra-red radiation from a UV LED, it is considered a cold light source, making it ideal for any heat-sensitive materials, such as polymers and medical devices.
As with any new disruptive technology, a few considerations also need to be taken into account. Although the initial capital costs are higher, the return on investment can be substantial.
The single wavelength of UV LEDs requires photo-initiators with an absorption that matches the light source. Nevertheless, the formulators have acknowledged that UV LEDs are the source of choice going forward and have released products optimised for their activation.
Lastly, commercially reliable UV LEDs are currently limited to a minimum wavelength of 365nm and, as such, are not viable for ultra-short wavelength applications. However, forecasts show that by the middle of the decade, suitable LEDs will be available for applications such as medical and disinfection.
Heraeus Noblelight has been involved in UV applications for more than 100 years, having invented the first UV lamp. Its newly formed optoelectronics business division is building on this knowledge to provide high performance UV LED solutions for curing applications.
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