Use of ionic liquids in SECM experiments to distinguish effects of temperature and water in organic coating swelling

During hygrothermal ageing of organic coatings, water and temperature lead to a coating swelling which can be in situ monitored by scanning electrochemical microscopy (SECM). However, it is difficult to separate the respective influence of water and temperature when ageing is performed in aqueous solution. In order to overcome this problem, room temperature ionic liquids can be a solution.

In this work, a model DGEBA/DAMP polyepoxide resin, with or without pigments (TiO₂) was applied onto steel Q-panels, with a thickness about 100 μm, completely cross-linked, and was aged in a 3 wt.% NaCl aqueous solution. The swelling was measured by SECM for different ageing temperatures in the saline solution. The same epoxy systems were placed in RTIL (1-Ethyl-3-methylimidiazolium ethyl sulfate) and the swelling was measured by SECM at different temperatures.

A typical procedure was applied in order to analyze SECM results obtained in RTIL. The results allowed to evaluate the respective contribution of water uptake and temperature to the global coating swelling. The swelling due to temperature was 0.5%/10 °C for unpigmented coatings and 0.3%/10 °C for pigmented coatings while the total swelling was respectively about 5% and 4%, which is higher than the water uptake at saturation. It was then proposed that the swelling excess (about 2% and 1% for unpigmented coating and pigmented coatings respectively) was related to the internal stress relaxation of the polymer network during immersion.