EPR and Mössbauer characterization of RTV polysiloxane foams and their constituents

We have used electron paramagnetic resonance (EPR) and 57Fe Mössbauer spectroscopies to investigate potential aging mechanisms in filled RTV polysiloxane foams, diatomaceous earth, and their other constituents. Intense, broad EPR resonances in the RTV foams were recorded at room temperature as a function of microwave power. These signals were shown to come from the diatomaceous earth filler (Celite® 350) in the RTV foams. 57Fe Mössbauer measurements for neat Celite® 350 showed the presence of iron compounds in the form of lepidocrocite. Further EPR measurements showed that the intense, broad signals from Celite® 350 and the RTV foams were due to lepidocrocite that has been annealed to 175 °C and is in an excited state. EPR measurements of annealed samples of Celite® 350 also indicated that structural (chemisorbed) water is not easily released by the lepidocrocite. Therefore, the presence of intermediate forms of iron oxides in Celite® 350 and RTV foams indicates that water is not active in these materials.