Measurements of the Polarization Properties of Foam Materials Useful for mm-wave Polarimeters Windows

We have measured in the W-band, using a custom setup, the absorption and polarization properties in transmission of foam materials (elyfoama"double dagger, styrodura"double dagger, plastazotea"double dagger, and propozotea"double dagger) useful for windows of mm-wave photometers and polarimeters. The levels of the induced polarization degree and of the absorption are very small, and difficult to measure accurately. We find induced polarization degrees lower than 0.6 %, and transmissions higher than 97 % for few centimeter thicknesses of our samples. We describe the instrumental setup, the measurements, and the impact of our findings in the design of precision polarimeters for Cosmic Microwave Background measurements. All these materials, with the exception of black plastazotea"double dagger, feature transmissions higher than 99 %, and induced polarizations lower than similar to 1 % for sample thicknesses around 2-3 cm