This article was published in the April/May 2012 issue of LEDs Magazine.
View the Table of Contents and download the PDF file of the complete April/May 2012 issue, or view the E-zine version in your browser.
Market demand for energy-efficient lighting solutions has been rising rapidly, and LED lighting products are the focus for the next-generation of fixtures in architectural, commercial, industrial, and residential lighting. New materials are playing a pivotal role in the optimization of these LED replacement lamps and outdoor fixtures (Fig. 1), driven by the specific thermal and mechanical needs of LED lighting.
Several different grades of polycarbonate (PC), a type of thermoplastic, have been optimized for use as LED parts including reflectors, diffusers and lenses. PC reflectors provide a diffuse light from LED-based troffers, while PC-based heat sinks and PC lenses with ultraviolet (UV) protection can offer increased design flexibility to the manufacturers of LED replacement lamps and luminaires.
Polycarbonate vs. PMMA
The increasing adoption of LED lighting creates applications for polycarbonates that were previously the preserve of materials such as glass and metal. For instance, because LEDs emit “cold” light without infrared (IR) radiation, thermal stress on the lamp components is reduced, making it possible to replace glass lenses with those made of transparent thermoplastics, most notably polycarbonate and polymethyl methacrylate (PMMA). PC and PMMA have already been applied to LED lamps and luminaires, especially as parts of housings or transparent covers.
PC and PMMA components can be lighter and thinner than glass, and offer design flexibility. In addition, the components have been cost effectively scaled to production volumes using injection-molding processes. Relative to PMMA, polycarbonate benefits from greater heat resistance, higher impact strength and increased resistance to breakage. PC is also more flame-retardant. Benefits of PMMA over PC include its higher light transmission (> 92%) and better resistance to UV radiation.
Special grades of PC have a light transmission in the visible-wavelength range just under 90%, but they absorb radiation in the UV as well as mid- and far-IR regions. UV exposure will damage standard grades of PC, resulting in an increasing yellowness that impairs the transparency of lenses and covers for lighting fixtures. To counter this phenomenon, a new infusion process has been developed to concentrate UV protection at the surface of PC products (see sidebar).
If PC is to be used in LED lenses, its optical properties must remain unchanged after long-term exposure to LED light. Testing has been performed on 4-mm thick polycarbonate lenses exposed to commercial LED lighting with an intensity of 46 lumens per square centimeter for over 6000 hours at 90°C. Light transmission, clouding and yellowing values (yellowness index, ASTM E 313) changed little under these conditions.
A further advantage to polycarbonate in lens applications is its high refractive index, which allows thinner lenses to be fabricated. Bayer MaterialScience offers several polycarbonate grades that differ in viscosity, color and degree of UV protection, but transmission remains a function of PC thickness (Fig. 2).
PC grades can also be optimized for flame-retardant characteristics. For example, TerraGlo’s Alba Series of luminaires (Fig. 3) are designed with the driver circuitry and LED board underneath the fixture enclosure. As a result, the fixtures needed an enclosure material that was rated to the flammability standard UL94 5VA. The company selected a 3-mm transparent lens cover made from Makrolon FR7087 polycarbonate. The material has been tested by UL including glow-wire flammability and ignition testing.
Diffusers and reflectors
Often, a diffusing effect is needed to evenly distribute the light emitted by LEDs – to reduce glare or provide even backlighting. With PC, there are two ways to achieve a diffusing effect. The material can be made translucent by incorporating appropriate additives. In this case, diffusion strength and transmission depend greatly on the additive type and concentration. Alternatively, irregular, satined surface structures (grain size of 10 µm) can diffuse light due to refraction.
The diffusing effect is measured by using a goniophotometer to determine the half-value angle as defined in DIN 58161. This corresponds to the reflecting angle at which the luminance has fallen to half the light intensity passing through in a straight line from the diffuser component. The larger the half-value angle, the higher the light diffusion.
An additional advantage to PC-based diffuser components is that they can provide a single-piece alternative to multi-component film and lens systems, thereby simplifying lamp assembly.
In order for polycarbonate to be used as a reflector, it can be colored or light-reflecting pigments can be added to the material. This produces a product that reflects light diffusely.
A recent example of a commercial reflector is the Twelve LED troffer reflector that has been injection molded by Fraen Corporation’s Optics Division in Reading, MA. This reflector design uses Makrolon 6265X, a polycarbonate grade that is also flame retardant. The diffuse illumination from the reflector could also be applied to low-bay commercial and architectural fixtures.
If directed light from the LED source is required, a thin layer of metal can be applied to the surface of an injection-molded component. For example, the Makrolon and Bayblend (a blend of polycarbonate and acrylonitrile-butadiene-styrene copolymer) grades can reflect up to 95% of the incident light in such applications.
PC reflectors can be more cost effective than metal reflectors, which often require expensive, time-consuming processes such as vacuum coating or wet-chemical electroplating. As with diffusers, additional cost reduction can result from designing the reflector and adjacent components into a single component.
Polycarbonate heat sinks
PC grades with high thermal conductivity have been developed to act as heat sinks in LED retrofit lamps. For example, at the Strategies in Light show in February, Bayer MaterialScience demonstrated a functional PAR30 LED replacement lamp with a PC heat sink and PC lens. The TC8060 PC features a thermal conductivity of 22 W/mK. The material is flame-retardant, complying with UL 94 V-0 rating at a thickness of 2.5 mm.
When PC is used for heat sinks there is greater design flexibility than can be accomplished using aluminum. Components with non-traditional shapes and more complex geometry can be fabricated. With injection molding, the heat sinks also can be lighter and don't require rework.
Flat or rounded designs
In many cases, it can make sense not to use injection molding to make PC components for LED lamps and luminaires, but rather semi-finished products such as thin films or sheets.
PC films can be printed and are easy to shape and reinforce through back-injection with a thermoplastic. Such extrusion films can provide homogeneous, diffused light based on either the surface structure or light-diffusing particles in the polycarbonate matrix.
Reflector films can also be produced with appropriate additives. Makrofol LM 327 polycarbonate, for example, is just 250-µm thick and easy to thermoform, yet its light reflection is ≥ 97% (based on ASTM E 1331, against a white background). Diffuser and reflector films have great potential for use in the backlight units of LCDs (Fig. 4).
Light-directing films can also be made of polycarbonate. The required effect is achieved using a finely impressed grid of lines that also ensures good homogenization of the light. Applications include transparent covers for strip lighting and LED tubes. A newer application for PC films involves light-extraction films for organic LED (OLED) panels.
Solid polycarbonate diffuser sheets are compatible with flat cover designs. In some cases, modification of the color temperature is also possible. For instance, The Makrolon DX warm diffuser sheets convert cool LED light to a warmer light with a transmission of over 70%. The half-value angle is very high at more than 50°, which means that thin diffuser elements can be made. Possible applications of diffuser plates include large covers for LED street lights (Fig. 5), smaller LED indoor lamps and LED billboards or signposts.
Improved manufacturing of LED lenses
Polycarbonate lenses represent a current area of research and development. At Bayer’s Technical Service Center in Leverkusen, Germany, it is testing collimator lenses (Fig. 6), typically focusing lenses with free-form surfaces and asymmetrical geometries. One target application is automotive LED headlamps using a single-lens design. Today, LED headlamps are comprised of several separate components that are heavier than polycarbonate-based lenses.
One downside to injection molding of PC lenses with free-form surfaces is the very long cycle times, sometimes up to 20 minutes, to achieve high optical quality with low stress and high dimensional accuracy. To improve on this approach, the company has developed a multi-layer injection molding process combined with new techniques for dynamic mold-temperature control. The component is gradually assembled in several layers, resulting in shorter cooling and cycle times.
Multi-layer injection molding can result in improved component quality, lower yellowness index and less transmission loss. There are fewer defects such as shrinkage-related sink marks, for example, because a greater proportion of the shrinkage takes place during the premolding step. In addition, lower injection pressures result in lower internal stresses.
To further optimize the injection-molding process, modeling of the multi-layer process makes it possible to calculate the necessary adjustment of temperature-dependent factors that impact component quality, such as warpage, shrinkage, stresses, and creep behavior. The next step will involve the use of simulation to fully optimize PC component quality.
Advanced polycarbonate materials can bring desirable properties to LED lamp and luminaires, including design flexibility, low weight and UV protection. To date, indoor LED troffers, alternative heat sinks and LED street-light covers have been optimized using these materials.
As PC grades are optimized further for specific applications, increased design flexibility will be brought to the makers of LED replacement lamps and luminaires.