Built-in polycarbonate diffusion technology

Lighting manufacturers are seeking the best options in light-diffusion technologies to offset harsh LED illumination, eliminate hot spots and maximize lumen output in optics and lenses. There are many ways to achieve diffusion, such as adding a diffusion film or texturing the surface of the plastic. Yet in some cases, these methods can either add cost or do not achieve the same aesthetics as other light sources.

To this end, Covestro incorporates precise engineered diffusion additives into Makrolon® polycarbonate resin. The pellets that are subsequently created can be injection molded into components or processed by extrusion into profiles, sheets and films. This “built-in” diffusion offers the advantage of eliminating a secondary operation that adds to system cost. Diffusion additives incorporated directly in the resin also allow for greater design flexibility in the molded shape, otherwise a secondary step may be necessary or may not be possible if the molded shape is complex.

Another aspect to consider with LED technology is meeting UL requirements. Since many LED lamp replacements and luminaires are considered electrical enclosures, meeting UL 94 flame retardancy requirements, such as stringent V-0 or 5VA ratings, is also necessary. Covestro has existing product solutions to meet these ratings and is developing next-generation products, which will meet these ratings at lower part thicknesses.

Molding or extruding parts with thinner wall sections is one way to reduce costs. To achieve this, the main thickness of a diffuser tube for LED lighting continues to decrease. A non-halogen flame retardant material is also requested by most lighting manufacturers. In terms of functionality, the balance of transmission (Ty), half-power-angle (HPA) and color are the key market requirements. Later in the second quarter of 2017, Covestro will launch a new non-halogen flame retardant (FR) Makrolon® polycarbonate extrusion grade with a UL 94 V-0 listing at 1.0 mm for a diffuser LED tube application.

Potential application for new Makrolon® flame retardant extrusion grade with diffusion technology.

To achieve a high lumen output while maintaining an acceptable level of diffusion, Covestro utilizes a light diffusion technology focusing on light refraction versus reflection. An optimized particle mixture is added to the polycarbonate, where the proprietary size and shape allow the light passing through the resin to effectively bend around the particles, dispersing the light while maintaining a high overall level of light transmission. Comparison of this new diffusion technology to older diffusion technology demonstrated an increase in light transmission from 70 percent to 77 percent while maintaining the same level of diffusion.

A combination of surface texture and built-in diffusion can help achieve high performance, while also offering a range of diffusion levels. The figure below illustrates this process. The surface texture of Makrolon® Lumen XT polycarbonate sheet improves the overall performance, resulting in higher light transmission for a given diffusion level. Varying the built-in diffusion yields product options at multiple diffusion levels.

Effect of surface texture applied to built-in diffusion on Markolon® Lumen XT polycarbonate sheet

Diffusion technology not only affects the LED light transmission, but also the color of the LED light exiting the plastic. When an older diffusion package is added, the net effect results in a lower color temperature of the LED light passing through. To mitigate this effect, Covestro created a new diffusion color to minimize absorption at 450 nm and 650 nm, the wavelengths at which standard polycarbonate tends to absorb light. In Table 1 below, the net effect can be seen. The older diffusion package reduces the color temperature by 165K, while the new diffusion package reduces color temperature by 61K.

Scans Color Temperature (K) Color Temperature Difference (K)
LED only 5971 0
LED + old diffusion 5806 165
LED + new diffusion 5910 61
Comparison of correlated color temperature of diffusion packages


But what if the designer desires to change the color of the light? Experts at Covestro’s Color and Design Centers can fine tune the color of the diffusion package to create a broad color palette for customized applications.

Judicious selection of the correct thermoplastic, appropriate diffusion package and color allows LED lighting designers to achieve the desired look for their application. Covestro provides solutions with diffusion technology in Makrolon® resins for injection molding and extrusion applications, and Makrolon® Lumen XT polycarbonate sheet. This allows designers to customize part thickness, the amount of diffusion or the distance between the plastic and the LED source, to create the optimal visual effect and lumen output.

Makrolon® diffusion products were used in lighting components at the Covestro headquarters in India.
For more information, email plastics@covestro.com.

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