Taking the Heat: Silicone’s Unique Properties Help Create the Next Generation of LED Lighting

By Kaz Maruyama and Hugo da Silva, Dow Corning Corporation, Midland, MI

As global energy demand rises, the significant energy-saving potential of light-emitting diode (LED) lighting remains a top consideration for lighting manufacturers as well as designers, architects, and consultants looking to offer clients more sustainable options. LED lighting uses one-tenth the power of standard incandescent lighting and lasts 25 times longer — it even has the long-term potential to produce more light, with less cost, than compact fluorescent lamps, or CFLs. In fact, LED lighting may soon replace most lighting in homes, offices, public spaces, and in many other applications.

Silicone optical encapsulants provide superior resistance to light degradation.

The challenge? Like incandescent light, today’s brighter, white LED technology also creates heat. But the heat is in the component area rather than in the light itself, and this can compromise performance and reliability. Heat causes LED lighting products made with traditional materials like epoxy to yellow with age, producing less light and even changing color temperature — a designer’s nightmare. With the chip and components exposed to heat, longevity of the light declines, and many LED lighting products have not always been able to deliver on their claim of long-lasting, efficient light.

But innovative, forward-thinking producers of LED lighting systems know something that many designers and other end users are discovering: silicone can take the heat. Using silicone in LED lighting offers solutions to these challenges and the freedom to explore new styles and applications.

The Silicone Difference

Silicone materials, sometimes called inorganics, offer many advantages over organic materials. One very important advantage is their stability over a wide temperature range. The silicone-oxygen bond, which forms the backbone of silicone products, is strong and relatively inert to heat, water, and UV light exposure. As a result, silicones are used in applications where temperature extremes and environmental exposure are challenging application requirements. The silicon-oxygen bond is also exceptionally flexible. The property translates into flexible materials even at very low temperatures and high flexibility (low modulus) over a wide range of temperatures, so silicone products can expand and contract with temperature easily and relieve stress between bonded/sealed substrates with differential coefficients of thermal expansion. Organic materials, such as epoxies and poly urethanes, tend to be more rigid, maintain their base properties over a smaller temperature range, and are less inert to light exposure.

Silicone physical properties can be modified by incorporating different functionality on the silicon-oxygen polymer backbone, varying the molecular weight of the silicone polymer, mixing different silicone polymers, adding different additives or fillers, varying polymer crosslink density, and by utilizing different curing mechanisms. Thermal and/or electrical conductivity, in particular, are properties that can be created or altered by adding thermally or electrically conductive fillers to silicone formulations. Similarly, viscosity is adjustable by varying polymer molecular weight and the addition of viscosity-modifying fillers. Higher cross-link densities, other things being equal, tend to yield cured products that are harder and have less elongation under stress. Many silicones with low cross-link densities are exceptionally soft and can even self-repair when penetrated.

Another important property of silicones is their low surface energy. Because silicones tend to have very low surface energies — lower than organics and approaching fluorinated olefins — silicones can wet most substrates, maximizing interfacial contact. This is an important property where conductivity is required. Thermal conductivity, for example, is an important requirement for thermally conductive materials, but thermal impedance is also critical. Air is a very poor conductor. Interfacial contact between conducting materials must be maximized to achieve optimal thermal or electrical conductivity. Silicones excel in these applications through low thermal impedance.

Silicone thermal interface materials optimize conductivity and impedance.

Thermal and optical stability are the primary reasons that LED lighting made with silicone based technology — such as wafers, molds, sealants, driver protection, optics, and thermal interface materials — provides light transmittance relatively well. Aging tests set at extreme temperatures (150°C or 300°F) for 200-hour periods show that silicone performs better than organic materials. These materials become significantly yellowed and cannot hold up against temperatures above 250°F (125°C) — a stark contrast to silicone’s high performance. Silicone can also be shaped to protect electronic components and to adhere to a variety of materials or substrates used within the lighting application, and it can better cushion fragile electrical components from outside stress. Silicone absorbs vibration, and with its low moisture uptake, it can also withstand harsh environmental effects. These properties, along with its thermal stability, offer lighting companies greater latitude in the shape, style, or size of LED lighting as well as the types of applications they pursue.

Silicones are also comparatively simple to dispense and cure. Many formulations are one-component products that cure with heat or ambient humidity. Two-component systems are less sensitive to mix ratios than other technologies and can be dispensed using off-the-shelf dispensing equipment. LED lighting designed with silicone components conforms to industry standards and labeling requirements — another reason for designers and end users to feel confident.

Solutions Span the Lighting Value Chain

Silicone’s properties benefit nearly every aspect of the solid-state lighting industry value chain: sealing, protecting, adhering, cooling, and controlling light. Silicone technology like pottants, encapsulants, sealants, and adhesives can resolve challenges in LED fabrication and packaging, as well as enable new designs and offer new solutions for electric components, housing, and assembly in the LED lamp and luminaire industry. And finally, in the manufacture of optical systems, silicone can help resolve issues such as glare control, color temperature variation, performance over time, design limitations, and more.

Across the production and supply chain, silicone can offer more thermal stability, transparency, light output, and reliability, as well as easy processing ability and lower costs than traditional materials. It is the material that can further the adoption of LED lighting, drive down costs, and enable successful expansion into exciting, promising new markets: general and accent lighting for home, office, and retail spaces, traffic lights and other outdoor lighting, mobile devices, LCD displays, and automotive interior lighting. Applications requiring a cool touch and environmental toughness will especially benefit from silicone-based LED lighting.

Case in Point: Silicone Encapsulants

Take just one finished product: an LED lamp. Over ten different silicone-based components — such as adhesives, pottants, secondary optics, and electronics protection — can integrate to create a brighter, longer lasting, less expensive lighting product.

A closer look at just one of these components, silicone encapsulants, illustrates how silicone’s properties can create a better LED lamp. For example, LED semiconductor chips must be encapsulated — or covered — to enhance direct light and to protect them from dust, moisture, and mechanical damage. The brighter LEDs favored by consumers produce too much heat, discoloring older materials like epoxy and stressing the chip even more. Silicone encapsulants manage the heat and also increase light output by more closely matching the refractive index of the chip surface. Encapsulants and other silicone materials also enhance the already strong environmental credentials of LED lighting because they can be used in newer, lead-free solder processing and contribute to a significantly longer service life, reducing solid waste.

Creating the Next Generation of LED Lighting

The marketing, research, and business development experts at Dow Corning, one of the largest silicone producers, recommend that those working in the lighting industry look beyond a catalog of silicone parts or plug-in components. The company, which draws on its strong research and development departments to further silicone-based LED innovation, can collaborate within their business models and strategies and help bring new designs and applications to market. Dow Corning is one of the few silicone LED materials suppliers that actively seeks opportunities to develop the next generation of lighting alongside designers, architects, and manufacturers.