Curing and film formation of waterborne epoxy resin system
Waterborne epoxy resin coating is an emulsion coating, but its film forming mechanism is not only very different from that of general polymer emulsion coatings, but also very different from that of solvent-based epoxy systems. The film-forming process is a physical or physicochemical process. The polymer particles in the polymer emulsion have a low glass transition temperature, and form a tightly packed structure after the water evaporates, and the film is aggregated under the action of capillary pressure. The epoxy resin coating is a reactive two-component coating, which forms a film There are both physical and chemical processes in the solvent-based epoxy system. Both the epoxy resin and the curing agent are in the form of molecules in solution, and the system is homogeneous. The curing reaction proceeds between the molecules, so the curing reaction proceeds Very complete, the film formed is a homogeneous aqueous epoxy system is a multi-phase system, the epoxy resin is a dispersed phase, and the amine curing agent is a continuous phase, so the curing reaction first occurs at the interface, and the curing agent gradually diffuses to the ring In the dispersed phase of the oxygen resin to further cure the epoxy, assuming that the particle size of the dispersed phase in the epoxy emulsion is 500 mm, the density is 1.16 g/mL, and the molecular weight of the epoxy is 100, and ignore its swelling in water, you can It is calculated that each epoxy particle contains about 4.6×103 molecules. Therefore, the curing of the waterborne epoxy system is controlled by the diffusion process. Compared with the solvent-based epoxy system, its curing is a complicated process with a long curing time. And it is less complete and incomplete, and the degree of curing depends on the following factors: (1) The smaller the particle size of the dispersed phase, the easier the curing is to complete. (2) The viscosity and glass of the epoxy resin particles in the dispersed phase The temperature of the disperse phase resin in the type I waterborne epoxy resin is in the liquid state, but as the curing reaction progresses, the viscosity of the surface layer of the epoxy particles continues to rise, or even becomes a solid, and its glass transition temperature will gradually rise, making the curing agent Diffusion is becoming more and more difficult. (3) Compatibility of amine curing agent and epoxy The better the compatibility of amine curing agent and epoxy, the easier it is to diffuse into the resin, which is beneficial for curing in the film-forming process. Due to the evaporation of water, the epoxy particles and the amine curing agent form a close accumulation.If the epoxy particles are still liquid or the glass transition temperature is still below room temperature, the compatibility between the epoxy and the curing agent is good, and a homogeneous phase can be formed. Conversely, transparent and glossy films, if left for too long, and the glass transition temperature of the epoxy particles increase, an incomplete homogeneous film may be formed. The transparency and glossiness are low, and even the film cannot be formed. The above is the curing and filming knowledge of epoxy resin system.