Chemical ageing mechanism

In addition, any chemical aging (Section 23.5.3) could affect these mechanisms for instance, if an elastomer surface is degraded by contact with hostile chemicals, access to the interior may be facilitated and its permeation characteristics would change accordingly. 

This section deals with chemical aging and related physical phenomena, such as diffusion and embrittlement. Apart from the chemical problem there is mechanical deterioration which is also related to long term environmental effects, but this was covered briefly in the preceding cumulative damage discussion. 

Chemical aging resulting from water absorption (i.e., hydrolysis) has not been as widely studied as physical aging. It is relatively well understood at the molecular scale (chemical mechanisms). But macromolecular (kinetics of decrease of the elastically active chain concentration) and mechanical aspects (effect of chain scissions on mechanical properties) are far from being elucidated. 

More recently Braganca, Faulkner, and Quastel (B15) showed that this inhibition of acetylcholine synthesis in brain slices by ammonia is consistent only in the diminution of bound acetylcholine. They further showed that the addition of inhibitors of glutamine synthesis, such as methionine sulfoxide, ethionine sulfoxide, and methionine sulfoximine (the toxic product, causing convulsions, formed in flour chemically aged with nitrogen trichloride) would partially reverse the ammonium inhibition. These observations were confirmed and extended to a wide variety of ATP-requiring reactions by Weil-Malherbe. In support of this suggestion was the observation that ammonia is taken up by the brain in hepatic coma (B8). The observations are valid and have been confirmed, but the interpretation of the data and the hypothesis are questionable. A quantitative basis for the evaluation of this mechanism can... 

Our results show that thin-film samples are suited to monitoring specific substrate influences on the aging behavior in the interphase. Nevertheless, further experiments are needed to correlate the observed evolution of IR bands definitely with chemical reaction mechanisms. 

In summary, the chemical degradation mechanisms of the epoxy systems are determined by the curing agent, the type of aging, and, occasionally, the combination of epoxy system and steel substrate. 

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