Formation of CO₂ bubbles in epoxy resin coatings: A DFT study

The epoxy resin coating is a fundamental species with epoxy resins used as main components to form the final film. Unexpectedly, bulky CO₂ bubbles that occasionally appeared during the curing process of epoxy resin coatings might destroy the final film properties. With an attempt to thoroughly understand the formation mechanism of CO₂ bubbles and further propose countermeasures to control them, Density Function Theory (DFT) in this paper was employed to calculate the absorption process, the curing reaction and the formation mechanism of CO₂ bubbles. The gas phase basicity (GB) values and pK_a values of common amine curing agents were calculated. The total Gibbs free energies difference of the curing reactions between polluted curing agents and epoxy resins were calculated according to a thermodynamic cycle. Whether in gas phase or resin phase, the energetically negative ΔG_solv indicated that the curing reactions might occur spontaneously and CO₂ molecules would be separated and released from amine molecules. The total Gibbs free energy calculations also revealed that the re-absorption of CO₂ by the curing system was energetically unfavorable. Thus, the formation mechanism of CO₂ bubbles of epoxy resin coatings could be summarize in three steps: (1) Carbon dioxide pollutes accidentally the curing agents. (2) CO₂ molecules are gradually released as the curing process occurs. (3) CO₂ molecules are collected to form big bubbles which can lead to seriously surface and/or internal defects. Finally, based on practical experiences three tips were proposed to control CO₂ bubbles. The present results not only evidenced the nature of the unexpected bubbles of epoxy resin coatings, but also additionally paved to the way to full utilization of the formation mechanism to improve the epoxy coatings' properties.