Waterborne epoxy resin is one of the commonly used thermosetting resins. The structure contains polar groups such as epoxy groups, hydroxyl groups and ether bonds. It has excellent performance and is widely used in coatings and adhesives. It is often used as a primer in the field of coatings, with high adhesion and good corrosion resistance. Adhesives are often used as structural adhesives with high bonding strength and good aging resistance. At present, epoxy resin coatings and adhesives on the market are still dominated by solvents. Commonly used solvents are aromatic hydrocarbons, esters, ketones, etc., with high VOC content. With the increasing emphasis on environmental issues and the increasingly strict environmental protection regulations, low-VOC or zero-VOC waterborne epoxy resin emulsions have gradually become the focus of research.
Waterborne epoxy resin
Aqueous epoxy resin emulsion refers to a stable dispersion system obtained by dispersing epoxy resin in the form of particles or droplets in a medium with water as the continuous phase. Because epoxy resins are lipophilic polymers, emulsifiers must be added to the system or hydrophilic groups must be introduced into the molecular chain to make it self-emulsifying.
Comparison of methods
Conversion method operation is more complicated than self-emulsification method, there are many factors to be considered. Choosing the appropriate emulsifier, controlling the amount of emulsifier, emulsification temperature, stirring speed, water addition speed and other factors can easily prepare a water-based epoxy resin emulsion with small particle size and good stability.
The self-emulsification method involves multi-step synthesis of modified epoxy resin, which has a higher cost, but the emulsification process of the prepared modified epoxy resin is simpler. Generally, it is convenient to prepare smaller particles and better stability by adding water and stirring. Waterborne epoxy resin emulsion.
Using a centrifuge to test whether the layering can measure the mechanical stability of the emulsion, and standing to check whether the layering can measure the storage stability of the emulsion. The mechanical stability and storage stability of the waterborne epoxy resin emulsion prepared by the reverse conversion method are related to the amount of emulsifier. Within a certain amount, as the amount of emulsifier increases, the mechanical stability and storage stability are significantly improved. Generally speaking, the mechanical stability and storage stability of the waterborne epoxy resin emulsion prepared by the self-emulsification method are better than the reverse conversion method, but when the appropriate amount of emulsifier is added, the reverse conversion method can also prepare the stability and self-emulsion method Epoxy resins are equivalent to emulsions. This is because more emulsifiers can form a solid protective layer on the surface of the dispersed particles to prevent the latex particles from colliding with each other to cause sedimentation and cause delamination or demulsification to ensure the stability of the emulsion.
An epoxy resin emulsifier was prepared using epoxy resin E51 and polyethylene glycol, and the advantages and disadvantages of the self-emulsification method epoxy resin emulsion of the reverse conversion method and the emulsifier chemically modified epoxy resin were compared. The results show that When the amount of emulsifier is 10%, the mechanical stability and storage stability of the self-emulsifying waterborne epoxy resin emulsion are better than the waterborne epoxy resin emulsion of the reverse conversion method. When the amount of the emulsifier of the reverse conversion method is increased to 16% Only then can an emulsion with comparable stability be prepared.
Waterborne epoxy resin
3. Dispersed phase particle size and particle size distribution
The particle size of the dispersed phase of the waterborne epoxy resin emulsion can be measured with a laser particle size analyzer. For the reverse conversion method, within a certain amount of emulsifier, the larger the amount of emulsifier, the smaller the particle size of the dispersed phase, which is generally about 1 μm. This is because when the amount of emulsifier is large, before the reverse conversion, emulsification The agent molecules can diffuse to the surface of the newly formed water droplets in time to ensure that the water droplets have a constant size and maintain a small size; after the reverse rotation, the emulsifier molecules can coat the latex particles in time so that they cannot aggregate and become larger, so that the size of the dispersed phase particles can be obtained Smaller oil-in-water emulsion. The particle size of the aqueous epoxy resin emulsion prepared by the self-emulsification method is generally at the nanometer level, and the particle size distribution is narrower.
Using Span-80 and sodium dodecyl sulfate (SDS) as the composite emulsifier, through the screening of the HLB value, concentration and reaction temperature of the composite emulsifier, the optimal conditions for preparing the water-based epoxy resin emulsion were obtained, that is, when compounding When the emulsifier HLB value is 16.2, the mass fraction is 9%, and the temperature is 50oC, the obtained aqueous epoxy resin emulsion has good stability, the dispersed phase particle size is about 150 nm, and the particle size distribution is narrow.
Toluene diisocyanate is easy to hydroxyl reaction characteristics, the hydrophilic group polyethylene glycol-1000 grafted into the epoxy resin segment, to prepare a non-ionic self-emulsifying aqueous epoxy resin emulsion. The emulsion has good stability, the average particle size is around 30 nm, and the particle size distribution range is narrow. The adhesion of the prepared coating film is grade 2, and the water resistance and acid and alkali resistance are good.
4. Coating film performance
Compared with the self-emulsification method, the water-based epoxy resin emulsion prepared by the reverse conversion method has more emulsifier residue, and the gloss, hardness, water resistance, and solvent resistance of the coating film after film formation will be affected to a certain extent, but as long as The proportion of curing agent is appropriate, and the curing process is properly controlled. Two methods can obtain coatings with similar properties. Research shows that for water-based epoxy resins, curing process control is a key factor affecting the performance of the coating film. The influence of the dry film thickness, drying temperature, and film layer microstructure on the coating film performance is much greater than that of the emulsifier. .
An epoxy resin E44 was reacted with hydrophilic group polyethylene glycol (PEG) to synthesize a water-based epoxy resin emulsifier and mixed with epoxy resin E44 in a certain proportion. Lotion. The results show that when the molecular weight of polyethylene glycol is 6 000, the ratio of hydroxyl groups to epoxy groups is 1.00:1.25, and the reaction is about 130oC, the synthesized emulsifier has good emulsification effect. When the amount of emulsifier is 12%, the prepared coating film has good mechanical stability, freeze-thaw stability, adhesion is 2 grade, hardness is 3H.
Waterborne epoxy resin
After decades of development, the waterborne epoxy resin has greatly improved the preparation process and coating film performance, and some have approached or even reached the performance of solvent-based epoxy resins. At present, in the field of rail transportation, the waterborne coating system for subways has been fully promoted, and the research on the waterborne coating system for high-speed rail is being promoted. With the continuous deepening of research on waterborne epoxy resins, it is believed that products with better performance will continue to emerge, and will gradually replace traditional solvent-based epoxy resins to achieve green coating.