Effect of outdoor exposure on the moisture diffusion and mechanical properties of epoxy polymers

Specimens of epoxy polymers of four compositions based on 2,2 di-(4-hydroxyphenyl) propane cured by a cycloaliphatic amine hardener were exposed during 3, 6, 9, and 12 months on an open bench in a moderately warm marine climate of Gelendzhik. In the initial state and at various stages of exposure the moisture diffusion coefficient and maximum moisture content of flat rectangular specimens-coupons are determined for drying and humidification at a temperature of 60 °C. Properties of polymers in three states have been studied: after a stop of climatic tests without conditioning and after additional stages of humidification and drying using stress-strain tensile tests, dynamic mechanical analysis, optical microscopy, fractography. After the exposure in the climatic conditions, different amounts of moisture accumulate in the specimens depending on relative air humidity and precipitation amounts in different year seasons. At the drying stage, moisture transfer obeys Fick's second law. At the humidification stage, a change of mass of specimens depends on sorption of moisture and desorption of hydrolysis products. Tensile strength decreases after the climatic action and differs by a value of 20-66% for the dried and moistened specimens. A method of dynamic mechanical analysis revealed processes of post-curing and destruction depending on the polymer composition and duration of exposure. The humidification also affected a failure mode of specimens. In the initial state and after different exposure times in open climatic conditions after the humidification on fractographs an area of a mirror zone corresponding to plastic fracture increases. After drying, an area of a rough (relief) zone increases. It is shown that in order to obtain reliable information about mechanisms of climatic aging of epoxy polymers it is necessary to compare the properties of controlled specimens taking into account the moisture content to determine a degree of reversible plasticizing action of moisture against the background of irreversible changes.