Effects of radiation and thermal cycling on Teflon® FEP

Surfaces of the aluminized Teflon(R) FEP (fluorinated ethylene propylene) multilayer thermal insulation on the Hubble Space Telescope (HST) were found to be cracked and curled in some areas at the time of the second servicing mission (SM2) in February 1997, 6.8 years after HST was deployed in low Earth orbit (LEO). In an effort to understand what elements of the space environment might cause such damage, pristine second-surface aluminized Teflon(R) FEP was tested for durability to various types of radiation, to thermal cycling and to radiation followed by thermal cycling. Types of radiation included synchrotron vacuum ultraviolet and soft x-ray radiation, simulated solar flare x-ray radiation, electrons and protons. Thermal cycling was conducted in various temperature ranges to simulate HST orbital conditions for Teflon(R) FEP. Results of tensile testing of the exposed specimens showed that exposure to high fluences of radiation caused degradation in tensile properties of FEP. However, exposure to radiation alone in exposures comparable to those experienced by HST did not produce reduction in ultimate tensile strength and elongation of Teflon(R) similar to that observed for HST-retrieved aluminized Teflon(R). Synergism of radiation exposure and thermal cycling was evident in the results of three experiments: thermal cycling following electron and proton irradiation, thermal cycling following x-ray exposure, and additional thermal cycling of a sample retrieved from HST. However, irradiation and thermal cycling with comparable HST SM2 exposure conditions did not produce the degradation observed in the FEP material retrieved during HST SM2.