Stable waterborne epoxy resins: Impact of toughening agents on coating properties

Abstract

Waterborne epoxy coatings are extensively utilized. However, traditional bisphenol A-based formulations are prone to chalking and cracking in low-temperature, high-humidity conditions, which undermines their protective qualities. To address the limitations, this study first synthesized a series of cyclic long-chain dicarboxylic acid compounds (CAs), which were used to replace bisphenol A (BPA), to develop two groups of waterborne epoxy resins. The CE waterborne epoxy resins (CEs) were prepared by completely replacing BPA with CAs in reactions with liquid epoxy resin. The CEB waterborne epoxy resins (CEBs) were synthesized through blending CA-4 with BPA at controlled ratios, followed by reaction with liquid epoxy resin. The research examined how the chain length in each CA influenced the properties of the resins, films and coatings. The findings revealed that all CAs effectively made the coatings ductile. Among the tested samples, the CE-4 film showed an elongation at break of 193 %, and its epoxy primer demonstrated excellent flexibility, passing a 1 mm cylindrical mandrel test without cracking. Furthermore, the epoxy varnishes formed transparent, continuous films at 5 °C. CA-4 significantly balanced the mechanical properties and durability of the CEB coatings, e.g. tensile strength 23.3 MPa and elongation at break 156 % for the CEB-80 film. At molar ratios of 2:3 and 3:2 of CA-4 to bisphenol A, the coatings not only showcased superior film-forming at 5 °C but also rivaled the salt spray resistance of commercial bisphenol A-based waterborne epoxy coatings.