Sheldahl Red Book
Product Announcement from Sheldahl - A Multek Brand
FIRST SURFACE MIRRORS
A first surface mirror consists of a metallic coating (typically aluminum or gold) on a substrate. For multilayer insulation (MLI) blankets the substrate is usually PET or polyimide film, though FEP is used in some applications. Metallic coatings have very low emittance, so films coated on both sides are typically used for the inner layers of insulation blankets to minimize heat transfer.
Aluminum is the most commonly used coating; it combines low absorptance and emittance with low cost. The surface emittance, and hence energy transfer, can be reduced further through the use of gold coatings. Because gold is nearly inert, it has also been used in applications where the MLI blankets will be subjected to moist (salty) atmosphere for extended periods (e.g. space shuttle). As an alternative in this application, the aluminum can be protected with a corrosion resistant AOC coating.
When the internal layers of the blanket will not rise much above room temperature, PET films are used. They form efficient, cost-effective radiative transfer barrier layers. When high temperature operation or burn resistance is required, polyimide is the substrate of choice.
SECOND SURFACE MIRRORS
When the sun is shining on a surface, it will be heated and reach an equilibrium temperature based on the amount of sunlight absorbed (solar absorptance, a) and the amount of heat energy emitted (emittance, e). The lower the absorptance to emittance ratio, the lower the equilibrium temperature will be. First surface mirrors have an absorptance ratio of between 3 and 15 as seen in Table 1. To reach lower equilibrium temperatures, another device is necessary with a lower absorptance to emittance ratio.
A second surface mirror uses the bulk of the substrate to provide relatively high emittance and a metallic coating to (usually) provide low absorptance. The lowest ratios of absorptance to emittance are obtained by using a clear FEP (Teflon) film and a highly reflective silver coating. Polyimide films are used when high temperature operation is necessary or when the stretchy FEP film does not supply adequate structural stability.
The absorptance and emittance characteristics of the second surface mirror can be tuned through the choice of substrate material, substrate thickness, and coating.
See The Red Book for more information on all of Sheldahl's thermal control products.