Relaxation nuclear magnetic resonance imaging (R-NMRI) of desiccation in M9787 silicone pads.

Abstract

The production and aging of silicone materials remains an important issue in the weapons stockpile due to their utilization in a wide variety of components and systems within the stockpile. Changes in the physical characteristics of silicone materials due to long term desiccation has been identified as one of the major aging effects observed in silicone pad components. Here we report relaxation nuclear magnetic resonance imaging (R-NMRI) spectroscopy characterization of the silica-filled and unfilled polydimethylsiloxane (PDMS) and polydiphenylsiloxane (PDPS) copolymer (M9787) silicone pads within desiccating environments. These studies were directed at providing additional details about the heterogeneity of the desiccation process. Uniform NMR spin-spin relaxation time (T 2) images were observed across the pad thickness indicating that the drying process is approximately uniform, and that the desiccation of the M9787 silicone pad is not a H 2 O diffusion limited process. In a P 2 O 5 desiccation environment, significant reduction of T 2 was observed for the silica-filled and unfilled M9787 silicone pad for desiccation up to 225 days. A very small reduction in T 2 was observed for the unfilled copolymer between 225 and 487 days. The increase in relative stiffness with desiccation was found to be higher for the unfilled copolymer. These R-NMRI results are correlated to local changes in the modulus of the material.