LED tubes rise to lifetime challenges with polycarbonate materials (MAGAZINE)

Solid-state lighting replacements for traditional linear tube lighting require outer covering and optical materials that are as robust as the LED source inside. BILL MARSHALL explains the advantages afforded by polycarbonate for LED tubes.

Fluorescent tube lighting has been prevalent for years in commercial environments such as in office spaces and on factory floors. While this lighting has been effective, the trend now is to replace the tube lights because of the evolution to the more technologically advanced LED lighting. LED lighting is highly desirable because of its efficiency and long life. Replacing common fluorescent linear lamps with LED tubes to take advantage of the lumen-to-power ratio offered by solid-state lighting (SSL) technology presents challenges from both the design and optical perspectives. To meet this challenge, lighting manufacturers typically partner with materials suppliers and use advanced materials to achieve desired results in lens aesthetics, assembly methods, and optical performance. This allows them to compete globally in both performance and cost.

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In this article, we will address the use of polycarbonate as a material for these popular linear lamp replacements, describing its physical and optical properties, as well as other enhancements it delivers for LED lighting designers.

A competitive marketplace

Globally, it's estimated that billions of incandescent lights and fluorescent tubes are to be replaced over the next five years, according to a presentation given by Strategies Unlimited's Philip Smallwood, director of market research, LEDs and lighting, at Strategies in Light 2015. LEDs are gaining rapid penetration in both applications. From an environmental perspective, LED lights have an advantage over traditional fluorescent lights because they do not contain mercury, which is a toxic pollutant on the Environmental Protection Authority's list of industrial pollutants. Additionally, LEDs offer greater energy efficiency and have a longer lifespan than linear fluorescent lamps.

The LED market today is therefore extremely competitive, and is also very fragmented. To stand out in this environment requires a balance between competitive pricing, high product quality, and excellent optical performance. This represents a continuous design and engineering challenge for manufacturers.

A number of criteria need to be taken into consideration to produce materials for LED tube lights. Plastic has become the material of choice, especially polycarbonate, because of its one-of-a-kind properties and the versatility it provides. Considerations include optical performance, mechanical properties, safety, and design flexibility - all of which will be covered here.

Optical performance

An LED can be a very bright, unidirectional source, and manufacturers need materials that either make it possible for the light to shine directly through a surface for maximum brightness, or provide uniform light distribution with no evidence of the light source for a more diffused effect. LED manufacturers and designers know how difficult it is to find a material that hides the LED source while permitting light to be transmitted at optimum levels. This is important not only for aesthetics but ensures optimal energy efficiency - a key goal of LED lighting technology.

FIG 1. Diffusion of the LED tube material is evaluated by measuring relative transmittance (as a percentage).

The covering of an LED source regulates the amount of light that is transmitted or diffused. Customers often look for a material that offers high clarity and purity to ensure the optimum light transmission possible and maximum efficiency. However, depending on the application and the aesthetics needed in the end product, manufacturers are also concerned with the uniformity of light distribution. Polycarbonate can be tailored to specific needs for an application through the compounding process. Light transmission greater than 90% can be achieved for transparent polycarbonate resins. In some cases, the polycarbonate resins contain light diffusion additives, which are generally polymeric materials with a specific geometry, particle size, and refractive index that help obtain the desired balance of light transmission and light diffusion. For these polycarbonate resins, excellent light uniformity can be achieved over the entire surface of a part while hiding the bright LED light source, eliminating "hot spots."

To achieve the optimum light distribution to best meet suppliers' needs and end users' expectations, tests are conducted to accurately quantify the light transmission and light diffusivity of these polycarbonate resins before LED optic or covering production begins. In determining what is referred to as the D50 angle, or the angle at which the amount of light transmitted is 50% of the amount of the transmitted light at angle 0° (as seen in Fig. 1), a variable angle photometer and a direct light source are used to measure the effects of varying light-diffusing agents at differing concentration levels in specimens. Data from these measurements allow developers to select the appropriate diffusing technologies for the desired combination of property characteristics in the end product.

FIG. 2. Key properties of polycarbonate in LED tube lighting, in comparison with acrylic and glass.

It is often a careful balance to adjust properties, because material additives for light diffusion can impact light transmission and the efficiency of the lighting, and vice versa. Time spent doing this, however, optimizes the benefits of the LED technology.

Mechanical properties

LEDs are solid-state devices with no fragile parts or filaments and are therefore very robust. In addition, LEDs have very long lifetimes relative to traditional light sources. Polycarbonate has proven outstanding toughness - far superior to acrylic and glass - and is an ideal material to use as a lens, cover, or housing that will ensure the LED lamp or luminaire remains undamaged over its long lifetime.

While glass and acrylics individually meet some of the requirements for LED lighting, as shown in Fig. 2, they can both fall short when it comes to impact and heat resistance as well as design flexibility. Conversely, polycarbonate and polycarbonate blends are increasingly considered an ideal starting point for LED applications as they have the requisite basic properties and can be customized with other monomers, polymers, or additives to meet specific performance requirements.

For LED tubes, the most desired lens design is a form called "profile extruded," which typically provides excellent hiding of the LED light source while providing the maximum amount of light transmission. Polycarbonate light diffusion grades are ideal for this application. In conjunction with their excellent toughness, polycarbonates also offer the transparency and uniform light distribution needed to help eliminates these troublesome LED hot spots. Transparent polycarbonate resins can achieve light transmission greater than 90%, while polycarbonate resins containing a light-diffusion additive can achieve excellent light uniformity, hiding the bright LED light source. The stiffness of the polycarbonate resin allows for good dimensional stability during the extrusion, and the outstanding toughness of polycarbonate allows the designer to make very thin lenses for optimizing the light transmission through the lens, which results in greater lumen output for the light source.

UL (safety) requirements

The LED tube acts not only as an optical component diffusing light but also as an enclosure. Since with LED technology, the electronic components are placed inside, the tube needs to be UL 94 compliant. This is not the case for standard light bulbs.

Requirements for ignition resistance, or flame retardancy, depend on the UL codes governing the specific application and the materials used. High-powered LED light sources operate at temperatures as high as 80-110°C. Polycarbonate resins offer superior ignition resistance for these types of operating conditions. For lower-voltage applications using Class 2 power sources requiring UL 94 HB and V-2 flammability requirements, polycarbonate, acrylics, and styrenic-based resins such as styrene acrylonitrile (SAN) can be considered as materials for lenses, covers, and optics. For more demanding LED lighting applications where Class 1 power sources are used, the materials requirement for optics and lenses is UL V-0 and, in some cases, UL 5VA. To ensure the required flame retardancy and compliance with their associated UL 94 restrictions, components must be rigorously tested.

FIG. 3. UL94 test setup for V ratings on LED tube materials.

Testing involves using a blue flame, which is applied to the specimen at the appropriate exposure times for the desired flammability rating. Total burn time, dripping volume, flaming and glowing combustion levels as well as cotton-ignition levels are all measured according to the rating requirements. For UL 94 V-O certification, a 1-mm-thick specimen is exposed to a 20-mm flame for 10 seconds, removed for 30 seconds, and then exposed again for an additional 10 seconds. The setup for these tests is illustrated in Fig. 3. Certifications for 5VA require more severe tests with stricter requirements. In these tests, a 125-mm flame is applied to a conditioned square plaque specimen (2.5-3 mm in thickness) for 5 seconds and removed for 5 seconds, repeating the process five times.

Polycarbonate is among the only transparent plastic resins that offer the light transmission, thermal stability, and ignition resistance required for these demanding applications at a reasonable cost. In fact, polycarbonate resins are available over a very broad range of UL flammability requirements that cannot be achieved by other transparent or light diffusing plastics. Polycarbonates are available with UL 94 flammability ratings for HB, V2, V0, and 5VA.

Design flexibility

Despite their increasing use in general lighting applications, LED tubes do not have a standard shape and instead require design flexibility to obtain the desired figure (e.g., co-extruded tubes). One of the advantages of LED lighting is the freedom it offers manufacturers to be creative in their product designs. Unlike traditional incandescent lighting, the lighting industry is no longer restricted in aesthetic configuration and designers can actually "shape the light." Plastic materials used for housing or covering the LED source can be formed into countless shapes and sizes through injection molding, injection blow molding, profile extrusion, and sheet extrusion/thermoforming processes.

Polycarbonate offers this flexibility in design options with a wide range of products available for specific processing requirements. In addition, because of the relative strength and toughness of polycarbonate, parts can be down-gauged for weight, energy, and cost savings. As mentioned earlier, this ability to be made into very thin parts also offers a great advantage for lighting applications as the thin diffusion lens allows more light to pass through, resulting in improved lumen output and greater efficacy, a highly desired feature for optical engineers.

Summary

The material chosen for LED lighting applications has a real impact on the realization of the core benefits of LEDs themselves. Glass, acrylic, and plastics have traditionally been used here, with preference usually based on function of a specific component, the environment the application will need to withstand, characteristics of the LED chip, and regulatory requirements. Plastics have emerged as a preferred material, allowing OEMs and molders the opportunity to reap the maximum benefits from LED technology.

The material used for LED tube lenses needs to offer a range of properties that include excellent LED hot-spot hiding, uniform light diffusion, maximum light transmission, and easy and versatile fabrication while also satisfying the UL flammability requirement. Polycarbonate light-diffusion resins are uniquely suited to satisfy all of these requirements, allowing optical engineers to develop LED tubes that are superior in energy efficiency compared to traditional lamps and are far more robust, which ensures the tube will survive the long lifetime capability of the LED light source.

BILL MARSHALL is senior account manager at Trinseo Performance Plastics/Consumer Essential Markets and a 25-year veteran chemist (trinseo.com).

Materials challenges and new developments are just some of the topics that will be addressed during the Advanced Technology Track taking place at Strategies in Light 2016. Register for the event, co-located with The LED Show, and participate in the discussion.