Preparation of waterborne dispersions of epoxy resin by ultrasonic-assisted supercritical CO2 nanoemulsification technique

Waterborne nanoemulsion of diglycidyl ether of bisphenol A type epoxy resin (DGEBA) with droplet size of around 124 nm was prepared by using an ultrasonic-assisted supercritical carbon dioxide (scCO2) technique in an autoclave reactor at a low temperature (32 °C). A view cell positioned in-line with the ultrasonic probe allowed observation of the emulsification process. From the image analysis and droplet size measurement, the influence mechanisms of the ultrasonic power, the degree of mixing of scCO2 with DGEBA, the adding amount of emulsifier, and the system pressure on emulsification process and emulsion droplet size were investigated. In the emulsification process, scCO2 penetrated into the mixture and absorbed on the DGEBA molecular. The interactions between CO2 and the functional groups of DGEBA reduced the chain-chain interactions of polymer segments and therefore efficiently reduced the viscosity of DGEBA at a low temperature. Meantime, the cavitation and acoustic streaming of ultrasound provided a shear force for the nanoemulsification and a disturbance force for the homogeneity of the emulsion. Therefore, the combination of scCO2 and ultrasonication made it possible to prepare a long-term stable nanoemulsion under a low temperature. This ultrasonic-assisted scCO2 emulsification method provides an efficient and solvent-free process for the preparation of waterborne nanoemulsions of, for example, some heat-sensitive and water-insoluble active substances at low temperature.