Aging Studies of Filled and Unfilled VCE

VCE is an ethylene, vinyl acetate, vinyl alcohol elastomer used to bind high volumes of microparticles. Surveillance data for this material is both scarce and scattered, complicating the assessment of any aging trends in systems. In addition, most of the initial surveillance efforts focused on mechanical properties such as hardness and tensile strength, and chemical information is therefore lacking. The initial work conducted in FY09 focused on the effects of gamma radiation up to 25 MR on filled and unfilled VCE material produced by the Kansas City Plant (KCP) using WR-qualified processes. The work performed in FY10 focused on the effects of temperatures up to 150°C on the VCE material produced by KCP, as well as on a comprehensive data integration to generate a mechanistic understanding of degradation mechanisms in the VCE material. A series of structural and chemical characterization techniques were employed including: Toluene Swelling, Tensile Testing, Solid Phase Micro Extraction – Gas Chromatography – Mass Spectrometry, Infra-Red Spectroscopy, and UV-vis Spectroscopy. Overall results from FY09 and FY10 show a ‘yellowing’ of the material, an increased release of acetic acid and an increased Young’s modulus with increased temperature and increased gamma irradiation dose. When subjected to gamma radiation, VCE samples also showed increased crosslinking, reduced mobility, and hindered IR deformation modes. These observations may be explained by a model in which acetic acid is eliminated from the vinyl acetate group via a molecular rearrangement in which a Hydrogen atom from the ethylene backbone moves to the acetate group. This mechanism releases acetic acid and forms a C=C double bond in the polymer backbone, therefore reducing chain mobility, hindering ethylene deformation modes, affecting the mechanical properties of the VCE material and generating a yellow color through the formation of polyenes. The work showed that acetic acid may constitute an effective chemical signature to probe the aging of VCE material in systems. Future efforts will involve work with actual VCE parts to develop a comprehensive aging model of VCE in systems.