Methodology for Developing Low-Temperature-Cure Accelerators
Two-component reactive epoxy-amine coating systems are widely used as maintenance and protective coatings (M&PC) due to their excellent chemical resistance, corrosion protection, adhesion and moisture barrier properties. However, sluggish rates of the epoxy-amine reaction at sub-ambient temperatures (less than 10 °C) result in coatings with excessively long dry times and improper cure. Consequently, such coatings do not provide the same level of protection as coatings applied and cured at higher temperatures. In this article, we highlight recent advances in improving cure speed at sub-ambient temperatures using new accelerators. Characterization of cure speed was performed by a novel methodology based on monitoring the coefficient of friction (COF) of the coating surface during cure. Results show that these accelerators significantly improve the cure speed under sub-ambient temperatures (less than 10 °C). Incorporation of these accelerators into liquid epoxy resin-based coating compositions may allow applicators to extend the painting season, especially in colder geographies.
Traditionally, a mechanical dry time recorder is used for measuring cure speed.1 In this technique, the coating is applied on a glass substrate using a film applicator. A needle is dragged through this coating from one end at a constant speed for 24 h until it reaches the other end. As the needle traverses through the coating it creates a scratch, the morphology of which is dependent on the stage of cure. Various stages of cure (set-to-touch, tack-free, dry hard, and dry through times) can be measured by locating the points of transition in the scratch morphology. The dry time recorder is easy to use and is widely used in the coatings industry; however, there are several practical challenges associated with utilizing this instrument to guide research. Firstly, the identification of transitions in the scratch morphology may not be trivial, as many of these transitions are ambiguous. As a result, the various stages of cure cannot be unequivocally discerned. In a few cases, certain transitions may not occur in the duration of the test, which makes interpretation of the cure stage difficult. Secondly, the variability of this technique can be very high. This poses a challenge, especially when conducting research such as developing alternative accelerators. Finally, the instrument is limited to monitoring cure only up to 24 h.