Moldable optical silicones enable LED lamps and luminaire designs (MAGAZINE)

This article was published in the October 2012 issue of LEDs Magazine.

View the Table of Contents and download the PDF file of the complete October 2012 issue, or view the E-zine version in your browser.

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Moldable silicones are emerging as a viable option in LED-based product designs for use as secondary optics, light pipes, light guides, and other optical components. Indeed new formulations designed specifically for solid-state lighting offer the ability to withstand high temperatures associated with the LED semiconductor junction with no optical degradation. The material can also be molded into complex shapes offering great flexibility to the product developer.

Fig. 1. The global lighting market is on the verge of a transformation as LEDs increasingly replace conventional light sources. According to analysts at research firm McKinsey & Company, the market for LED lighting will explode at 30% per annum to exceed $81 billion by 2020, when it will represent close to 60% of the overall lighting market.

Such projections are supported by the accelerating adoption of LED technology in more and more general lighting applications, ranging from low-power, low-lumen fixtures like downlight replacements – where LEDs are increasingly replacing low power compact fluorescent lamps and halogens – to more challenging applications like street lights, industrial lighting, office lighting, high power halogens or illumination of sport venues. As LED-based light sources further penetrate applications that demand higher lumen density and power, physics will demand they operate at higher temperatures – even as lamp and luminaire designers seek to reduce the number of LEDs, or pack the LEDs closer in order to develop sources that are comparable or smaller in size than previous generation devices.

At the same time, LED designers are innovating modules, lamps and luminaires that integrate multiple functions into fewer parts (Fig. 1), or that incorporate smaller or more complicated features. LED manufacturers are seeking new materials that accelerate productivity, deliver higher yields – particularly for larger parts – or reduce waste.

New materials

All of these challenges effectively represent growing pains for an emerging lighting segment that is quickly evolving past its early-generation designs and materials. In response, the industry is exploring new materials, such as silicones that, while less familiar in LED applications, bring a rich history of proven performance in dozens of other industries, including advanced electronics, automotive and communications. Silicones address several of the challenges posed by next-generation LED designs, including the ability to withstand higher temperatures, support higher lumen density, enhance manufacturability and enable more complex designs. Like LEDs, silicones are evolving too. Recently, several leading optical and LED manufacturers have been putting a new class of optical-quality moldable silicones to the test in their designs, and seeing positive results.

While some grades of silicone are transparent, moldable silicones such as those recently introduced by Dow Corning represent a more advanced material engineered expressly for LED applications, which is to say they compare well in performance against today’s best-in-class optical materials. Plus, like conventional silicone materials, moldable silicones exhibit low viscosity before cure, enabling them to be molded more easily into complex shapes than either organic polymers or glass, offering new design options for secondary lenses, light pipes, light guides and other optical components. This quality can also help reduce manufacturing costs and cycle times in injection molding and other processes, and potentially reduce system costs for LED-illuminated lamps and luminaires. Lastly, compared to many organic materials, the chemical backbone of silicones makes them particularly well suited to manage the increasingly high temperatures of today’s and tomorrow’s LED lighting systems.

Hotter LED designs

High-heat applications are where moldable silicones shine. As a class of high-performance materials, they easily withstand temperatures of 150°C and higher without significant loss of optical or mechanical performance. These qualities will become more attractive as LED sources increasingly deliver more intense white light from comparatively smaller package sizes, and as customers seek smaller lamps and luminaires with higher luminous flux, which will also drive up temperatures at the device level.

Fig. 2. As lumen densities increase, the package temperatures within today’s high-brightness LEDs are already reaching as high as 150°C. This not only poses challenges for epoxy encapsulants conventionally used to seal LED packages, it is also raising heat exposure for traditional secondary optics materials such as polycarbonate (PC) and acrylic. In general, the optical quality of these plastic materials declines over time at temperatures above 125°C and 95°C, respectively. The same applies to epoxies at temperatures above 150°C.

Such high temperatures cause traditional optical materials used in LED lighting systems to yellow with age, which diminishes the total system light output. This can have a profound impact on lumen maintenance and efficiency – dropping the expected 80% lumen output below acceptable levels earlier than the expected 50,000 hours of an LED light source’s useful lifetime. Further, yellowing can adversely change an LED’s color temperature over time. Such shifts in a light source’s color are unacceptable to lighting designers and end-users alike.

In comparison to conventional materials, moldable silicones retain their excellent optical stability and transparency even after prolonged exposure to temperatures upwards of 150°C, exhibiting comparatively little or no yellowing and greater reliability across the visible spectrum. Indeed, this emerging class of silicones enables LED optical components to maintain their lumen output and efficiency better over the course of an LED’s useful lifetime.

Thermal testing

Thermal aging tests performed by Dow Corning in an air-circulating oven at 150°C for up to 10,000 hours demonstrated that silicone’s high optical transmission remained steady – ranging from 90-95% in the visible spectrum – under such conditions. Moldable silicones also retained their high performance for other optical qualities during aging under high heat, including reflection, low haze and stable refractive index.

Moldable silicones also stood up well in related tests that compared their thermal and optical stability against conventional materials, such as PC, acrylic and epoxy. Thermally aged for 200 hours at 150°C, moldable silicones retained their high optical quality (Fig. 2). In contrast, incumbent organic materials exposed to identical conditions began to show significant yellowing as temperatures exceeded 125°C.

The outstanding thermal and optical stability of moldable silicones may be enough to inspire new LED designs. These materials can help resolve issues such as glare control while maintaining efficiency, color temperature stability and performance over time. But in addition to this, silicone’s low viscosity before cure further enables designers to consider LED components with more complex shapes, thinner wall configurations, dual functions or very fine features.

Silicone structures

Structures like undercuts, for instance, are difficult to easily achieve with plastics because they cannot be easily released from the mold. Fabricating parts that adjoin thin- and thick-wall sections is also more challenging with plastic, which is more brittle and therefore more prone to cracking or breaking. Lastly, designs that use plastic materials typically keep them away from the heat of the LED light source, which precludes configurations that shift plastic optics closer to or even touching the LED.

Like conventional silicones, optical-grade silicones are well-suited to precision molding applications. Before cure, the viscosity of silicones decreases as heat increases. This allows silicone resins to be injected into a mold at lower pressures than what is typical for other materials, while still achieving good flow and reproduction. For example, their low viscosity enables replication of micrometer-sized features on a lens surface that, in turn, offers advantages in enhancing, focusing or directing light output.

In short, the physical properties of moldable silicones enable new designs that would be otherwise very challenging to achieve with incumbent materials, allowing for the exploration of new shapes, styles and sizes of LED lighting, as well as new applications.

Fabrication techniques

Silicone-based components can be fabricated using a variety of techniques, including injection molding, casting/cavity molding and others. While naturally very flexible, their hardness may be tuned to either absorb vibration or deliver excellent impact resistance. With their low moisture uptake and ability to withstand harsh environmental effects, conventional silicones are already frequently used by the electronics industry to protect fragile components against damage. Moldable optical silicones deliver many of the same advantages.

Fig. 3. Shrinkage is another familiar challenge for plastics that is not as much of a problem for moldable silicones, which need not be cooled in the mold as long as plastic in order to prevent warping. This helps reduce cycle time – particularly for large parts – which is important since the length of time that a part must remain in the machine can represent an important portion of its total cost, depending on the mold, optical part design and process factors mentioned above. In addition, the comparatively low shrinkage of moldable silicones helps minimize or prevent warping in components that integrate straight sections, such as the back of semispherical optics.

The design and manufacturing advantages of moldable silicones cannot be overstated because they allow issues to be solved early in the LED design chain. A finished LED lamp might incorporate over ten different silicone-based components, including adhesives, pottants, secondary optics and encapsulants (Fig. 3). Silicones are well known for addressing challenges at the package level, and as LEDs penetrate into general lighting those challenges will become more common.

Optics example

Referring back to Fig. 1, LEDiL, a leading optics supplier to the world’s lighting manufacturers, recently demonstrated this with the development of its innovative Strada-FT-TPHS lens module. The product of a collaboration between LEDiL, Dow Corning and other suppliers, the module features secondary optics comprising an asymmetric lens fashioned from Dow Corning’s moldable silicones.

Notably, LEDiL’s application of moldable silicone technology enabled the secondary optics to perform dual functions. In addition to creating an asymmetric forward-throw light distribution pattern, the secondary optics also provide ingress protection (IP) for the LED package. By using the lens material as a seal against outside dust and moisture, LEDiL was able to reduce the overall number of parts and address a challenge that would normally have fallen to its luminaire customers further down the design chain. Namely, its customers would have had to seal the entire luminaire.

Moldable silicones enabled further design features for the module, such as an undercut, and thermal and optical stability from -45⁰ C to 150⁰C to prevent yellowing over the course of the module’s lifetime. Neither these features nor the secondary optic’s dual functionality would have been easy or even possible using plastics, underscoring the versatile design possibilities that moldable silicones offer.

The performance demands may vary wildly between one LED design and another in today’s evolving lighting industry. But moldable silicones are finding application at every level of the LED design chain at other top optical suppliers and lighting designers. Further, Dow Corning is continuing to seek new industry partnerships to develop innovative new applications for moldable silicones.

In terms of performance, moldable optical silicones combine and often exceed the best qualities of both organic polymers and glass. As demand for LED lighting accelerates over the next decade, moldable optical silicone materials will play a major role in the development of new high-performing LED light sources, and help expand design and processing options for LED lamps and luminaires. Their good thermal stability, moldability and mechanical properties offer benefits at virtually every stage of the LED value chain – solving challenges to sealing, protecting, adhering, and shaping light. With the addition of their attractive optical qualities, moldable silicones can address design issues such as diffusion and glare control, color temperature variation and performance over time. Moldable silicones offer the potential to advance the adoption of LED lighting, drive down cost and help expand the technology into new markets, such as general and accent lighting for home, office and retail spaces, traffic lights and other outdoor lighting, mobile devices and automotive interior lighting. Silicone-based LED lighting could especially benefit applications that require a cool touch and environmental toughness.