Glass and transparent plastics, especially acrylic resins, have long been used in the lighting industry for various aesthetic and functional purposes. As the industry has evolved, light-emitting diode (LED) lighting has moved from niche applications to more mainstream commercial and residential use, and there has been a growing interest in plastics — especially polycarbonate — because of the properties of the material and the versatility it provides. Polycarbonate is now seen in a variety of areas in LED lighting including lenses, optics, covers, channel letters, sign facing, globes, and light diffusers.

The lighted sign and flashlight depict two properties that are needed by LED lighting manufacturers and molders when choosing materials — clarity in order to have great light transmission, and material that allows the light source to be evenly diffused so there’s no evidence of “hot spots” on the surface.

The mechanical integrity and durability of the product are the qualities that have caused the industry to focus on polycarbonate. Certain features of polycarbonate, such as heat resistance and transparency, are important in the LED lighting industry. But because the LED light source is relatively expensive to produce and can last up to ten years, manufacturers and designers are primarily concerned that the material used for the light lens, or cover, holds up for the same amount of time that the LED light source lasts — in order to protect the costly light source.

Based on Styron testing using ISO 180/A (2mm thick test specimens). Complete protocols and results available upon request.

This article discusses important considerations when selecting a material for use with LED lighting solutions. It focuses on the benefits of polycarbonate and why manufacturers and molders alike consider polycarbonate to be an ideal choice to use with LED lighting.

When selecting polycarbonate, first and foremost, one needs to consider the application. How will the polycarbonate be used? How will the source be housed? Today, the uses of LED light sources are almost endless. LED lights can be seen in storefront signs and channel letters, traffic signals, recessed down lights, task lights, retail and refrigerated displays, street and area lights, LCD TV displays and monitors, mobile appliances, and many other applications. When designing covers, lenses, or optics for these varied applications, one must consider the capabilities of the materials — the durability, optical properties, thermal stability, ignition resistance, design flexibility, and UV stability. What is the environment of the LED source? What elements does it need to withstand? These are some other considerations.

Durability

Durability is the starting point for light source protection, especially outdoors. What is needed is a material that is tough. Since the LED light source itself, a solid-state semiconductor, is a much more robust product than traditional incandescent light sources, the materials used to cover the source should be at least as resilient.

Polycarbonate is much more impact-resistant and is less prone to breakage than other materials available. It has outstanding toughness, even over a wide temperature range. Polycarbonate, which is used for items such as bulletproof and riot shields, aircraft canopies, and hurricane panels, can withstand tremendous impact. In general, polycarbonate resins are ten times more impact resistant than acrylics and up to 30 times more impact-resistant than glass.

Due to this outstanding toughness, polycarbonate is easier to work with and less prone to breakage or chipping when cutting the material into shapes. In many applications, it is possible to down-gauge or decrease the thickness of a fabricated part when using polycarbonate versus another material. This results in part-weight and material-cost reductions, which offer a more environmentally friendly solution due to the fact that less product is being used and a reduced amount of energy is needed.

Optical Properties

Based on Stryon testing using ASTM D 1003. Complete protocols and results available upon request.

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. It often is a careful balance to adjust properties, because material additives for light diffusion can impact light transmission, and vice versa.

Based on Stryon testing using the following methods: Vicat Softening Point, ISO 306B (B/50); Heat Deflecton Temperature (HDT), ISO 75 (1.82 MPa, unannealed); Melt-Mass Flow Rate (MFR), ISO 1133 (300°C/1.2 kg). Complete protocols and results available upon request.

The covering of an LED source regulates the amount of light that is transmitted or diffused. Customers look for a material that offers high clarity and very high purity to ensure the optimum light transmission possible. Depending on the application, 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 percent can be achieved for transparent polycarbonate resins. For polycarbonate resins containing a light diffusion additive, excellent light uniformity can be achieved over the entire surface of the part while hiding the bright LED light source, eliminating “hot spots.”

Thermal Stability

Lighting applications generate heat, and the proximity of the material to the light source determines the thermal properties needed. While LED lights are very energy efficient, they still generate heat, particularly for the higher powered LED light sources where operating temperatures can reach 80 – 110 °C. For optics and lens applications requiring close contact to the LED source, a material with excellent thermal stability is required.

Polycarbonate resins offer superior thermal stability compared to acrylic resins and can be used for continuous use temperatures up to 120 °C.

Ignition Resistance

The requirement for ignition resistance, or flame retardancy, depends on the operating temperature of the appliance and the distance of the diffusing medium, or cover, from the light source. Polycarbonate resins offer superior ignition resistance for high-powered LED light sources. For lower voltage applications such as Class 2 Luminaires requiring UL 94 HB and V-2 flammability requirements, polycarbonate, acrylics, and styrenic-based resins such as styrene-acrylonitrile resin (SAN) can be considered as materials for lenses, covers, and optics. For more demanding LED lighting applications such as in Class 1 Luminaires, the materials requirement for optics and lenses is UL V-0@1.5 – 2.0mm thickness.

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.

Design Flexibility

One of the advantages of LED lighting is the freedom it offers manufacturers to be creative in their product design. Unlike traditional incandescent lighting, the lighting industry is no longer restricted in aesthetic configuration. Plastics materials used for housing or covering the LED source can be formed into countless shapes and sizes by shaping resins/materials through injection molding and sheet extrusion/ thermoforming processes.

Polycarbonate offers this processability option with a wide range of products available for specific processing requirements. In addition, because of the relative strength of polycarbonate, parts can be down-gauged for weight, energy, and cost savings.

UV Stability

Exposure to a light source has the potential to compromise the properties of a material. In an LED environment this exposure can come from two directions — the LED source itself, and also from the natural light of the sun. This constant exposure can result in degradation of properties over time. This makes it especially important to select the right material for LED applications, and when formulating materials, to properly UV-stabilize the product to minimize the impact of this phenomenon.

Polycarbonate offers a number of methods to do this including additives and layered films in extruded product.

The selection of a material is complex and a number of factors must be considered to ensure the right solution for LED lighting applications. Polycarbonate is a leading material choice because it has effectively addressed some of the most difficult challenges of the LED lighting industry. The versatility of the material and the ability to custom-tailor properties results in a great match for manufacturer and molder needs.

About the Company

Styron is a global materials company with a product portfolio that brings together plastics, rubber, and latex businesses that share feedstocks, operations, customers, and end users. The company provides sustainable solutions in industries such as home appliances, automotive, building and construction, carpet, commercial transportation, consumer electronics, consumer goods, electrical and lighting, medical, packaging, paper and paperboard, rubber goods, and tires. The company has announced plans to change its company name to Trinseo, effective in late 2011.


Lighting Technology Magazine

This article first appeared in the May, 2011 issue of Lighting Technology Magazine.

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