Walter J. Schrenk

Rapid production of large-area polymeric cold mirror via a simultaneous layer formation process

Recent technology advances in the processing of polymeric multilayers has made possible the production of an all polymeric cold mirror reflector via coextrusion processing. The simultaneous formation of up to 5200 alternating layers of optically dissimilar polymers allows the continuous extrusion of a self supporting cold mirror at high linear speeds and widths of over 0.6 m (2 ft). The material can be post formed into shallow 3-dimensional reflector shapes, given proper attention to the initial film optical properties, the final part geometry, and forming considerations. The self supporting nature of the film allows it to be wound on rolls and stored for later part fabrication. Reflectance of 95-99% over the visible wavelengths has been achieved with commercially available polymers. The polymers used have high transmission in the near infrared region of 700-2000 nm, and sharp cut- on and cut-offs have been achieved. Uniformity of optical properties across the surface of the film will be discussed, as will thickness, angle of incidence, and temperature effects. The large area capability of this material may make possible applications for heat/light separation not previously viable due to deposition chamber size limitations or the high cost per area of conventional deposition technologies. Scale up of the process to much larger widths is feasible and will be discussed. Prototyped parts and potential applications of this new material will also be covered.

All-polymeric infrared reflecting films

Coextruded multilayer films consisting of 657 layers of three different polymers have been made which reflect near infrared wavelengths while transmitting visible light. A repeating multilayer stack of the three polymers in layer sequence A/B/C/B with optical thickness ratios of 2/1/2/1 suppress second through fourth order reflectance when refractive indices of the polymers are chosen so that nB = (nAnC)11/2. Broadband reflectance of solar infrared can be obtained by imposing a layer thickness gradient through the crossection of the film. Compared to conventional vacuum deposited multilayer interference stacks, all-polymeric films can be tough and thermoformable. They do not corrode and there is no metal waste. Potential applications include solar energy management. Multilayer film design, polymer specification and coextrusion of initial films to demonstrate feasibility are described.