Identification of Chemical and Physical Phenomena Affecting the Durability of Silvered Polymer (PMMA) Mirrors

摘要

Mirrors are the devices of choice in concentrating solar radiation for solar thermal applications. Efficiency requirements and economic factors dictate that the mirrors maintain high reflectance (over 90%) for many years (at least 5). Depending on the concentration ratio called for, the mirrors must have such surface perfection (specularity) that they can deliver all their reflected energy into an angle of 1 to 5 milliradians (mrad). Other design factors such as weight, flexibility (shatter resistance), and cost make silvered polymer films highly desirable for solar thermal use. The work reported here reveals some of the factors that affect the durability and performance of silvered polymer films in outdoor exposure. The results presented are based on laboratory analyses of various polymers silvered by various processes. The crucial role in degradation played by ultraviolet (uv) light is shown by several experimental results. Uv screens in the polymer improve weatherability. Inconel uv blockers improve adhesive performance. Weather-Ohmeter studies comparing exposures with and without uv radiation show more rapid degradation due to uv. Fourier transform infrared (FT-IR) studies show synergistic effects with uv, chlorine, and silver; but again control of uv is a major factor in enhancing mirror durability. (ERA citation 10:030551)