Thermoresponsive phenolic formaldehyde amines with strong intrinsic photoluminescence: Preparation, characterization and application as hardeners in waterborne epoxy resin formulations

Thermoresponsive phenolic formaldehyde amine (PFA) derivatives with intrinsic photoluminescence were prepared through the modification of PFA with hydrophilic polyethylene glycol diglycidyl ether (PEG) and hydrophobic E20 epoxide resin and benzyl glycidyl ether (BGE). Turbidimetry measurements showed that the cloud point temperature (T-cp) of the obtained PFA derivatives in water could be effectively modulated through adjusting the ratio of E20 and PEG. The thermoresponsive PFA derivatives prepared under the first addition of E20 exhibited a sharper phase transition than those prepared under the first addition of PEG. Air-oxidation of thermoresponsive PFA derivatives led to the depression of T-cp, which was ascribed to the occurrence of slight intermolecular crosslinking reaction verified by the gradual increase of hydrodynamic size. Although no traditional fluorophores existed, thermoresponsive PFA derivatives could emit blue fluorescence centered at ca. 488 nm. Air-oxidation resulted in not only the red-shift of maximum emission wavelength, but also marked enhancement of fluorescence intensity. The QY of thermoresponsive PFA derivative in water was ca. 3.4% before oxidation, while 57-h oxidation led to the QY boost to 6.9%. Such thermoresponsive PFA derivatives were used as polyamine hardener in the waterborne epoxy resin formulations to form coating films onto the surface of mild steel. Corrosion evaluation around the scribe and adhesion tests demonstrated that the coating film cured by oxidized thermoresponsive PFA derivative with a lower T-cp value exhibited a better comprehensive anticorrosion performance. Moreover, this coating film could emit bright blue fluorescence under UV-light irradiation. (C) 2018 Elsevier Ltd. All rights reserved.