Oxygen transport through epoxy-based powder coatings in humid environments

• Published in: Progress in organic coatings Vol. 175; p. 107295
• Main Authors: Zargarnezhad, Hossein, Zarei, Parham, Wong, Dennis, Lam, C.N. Catherine, Asselin, Edouard
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.02.2023
• Subjects: Competitive permeation; Epoxy coatings; Fusion bonded epoxy; Gas transport; Wet-state conditions; Epoxy coatings; Gas transport; Competitive permeation; Wet-state conditions; Fusion bonded epoxy
• ISSN: 0300-9440; 1873-331X
• DOI: 10.1016/j.porgcoat.2022.107295

Epoxies are used in various industrial applications as corrosion barriers but quantitative time-dependent predictions of their ability to mitigate corrosion attack at the metal/coating interface remain elusive. Permeability data for water and oxygen through epoxy-based coatings are of particular interest because the coating's barrier performance in humid environments is directly related to how quickly these reactants get to the metal/coating interface. There is, however, an absence of literature data to explain oxygen transport within coatings in the presence of condensed water and its associated plasticization effects. We show that water is a dominant player in the barrier performance of epoxy coatings because it blocks the transport of other permeants. Through analysis of the sorption isotherms and empirical permeation data, we determined adjustable parameters that explain water vapor plasticization effects in fusion bonded epoxy (FBE) at 65 °C. At this temperature, evolution of cavity formation and break-up of water clusters result in high mobility of water molecules inside the epoxy network. We also modeled oxygen transport through FBE in wet-state conditions based on time lag measurements, and with reported literature data on vapor and gas sorption in epoxy. In a mixed gas/vapor system, condensed water in FBE blocks and/or significantly decreases gas permeation in the coating. If the service temperature is low (<40 °C), water immobilizes the oxygen gas within microvoid regions in the glassy epoxy. Our experimental measurements, combined with Freeman's theoretical model for upper bound limits, showed that this water-induced blocking mechanism is sufficient to suppress corrosion reactions on the underlying substrate material. At 65 °C and above, the synergistic effect of coating plasticization by water molecules and dissolution of oxygen in a mobile water phase results in significant gaseous transport. We applied mathematical models based on proven sorption and transport models to FBE free-standing films and derived adjustable parameters to quantitatively explain this competitive permeation. Our O2 transport data show that, compared to the single-layer FBE, epoxy-based coatings with additional polyolefin layers can improve the barrier performance by deprivation of micropore channels for gas transport. •Condensed water is a dominant player in the barrier performance of epoxy because it blocks the transport of other permeants.•Adjustable parameters that explain water-induced plasticization effects in fusion bonded epoxy (FBE) at 65°C have been determined.•In a mixed gas/vapor system, water vapor in FBE blocks and/or significantly decreases gas permeation in the coating.•The water-induced blocking mechanism is sufficient to suppress corrosion reactions on the underlying substrate material.•At 65°C and above, dissolution of oxygen in a mobile water phase is the primary driver of gaseous transport in epoxy.