According to the different preparation methods, there are four methods for hydration of epoxy resin: mechanical method, chemical modification method, reverse conversion method and curing agent emulsification method.
1) Mechanical method.
The mechanical method is the direct emulsification method. The solid epoxy resin can be pre-ground into micron epoxy resin powder by ball mill, colloid mill, homogenizer, etc., and then the particles can be dispersed in water by mechanical stirring. Or mix the epoxy resin with the emulsifier, heat to the appropriate temperature, and gradually add water to form the emulsion under intense stirring. The advantages of preparing waterborne epoxy resin emulsion by mechanical method are that the process is simple and the amount of emulsifier is less, but the particle size of epoxy resin dispersed phase in the emulsion is larger, the shape of the particles is irregular and the size distribution is wide, and the stability of the emulsion is poor. The particles are easy to collide with each other and the condensation phenomenon occurs, and the film-forming property of the emulsion is not good. Of course, increasing the temperature of stirring and dispersion can promote the more effective adsorption of emulsifier molecules on the surface of epoxy resin particles, so that epoxy resin particles can be stably dispersed in the aqueous phase.
2) Chemical modification method.
The chemical modification method is also known as the self-emulsification method, that is, some hydrophilic groups are introduced into the molecular chain of epoxy resin, or block or graft, so that the epoxy resin can obtain the property of self-emulsification. When the modified polymer is emulsified with water, The molecular chains of hydrophobic polymers will aggregate into particles, and ion groups or polar groups are distributed on the surface of these particles and repel each other because of the same kind of charge, as long as certain kinetic conditions are satisfied. A stable waterborne epoxy resin emulsion can be formed, which is the basic principle of preparing waterborne epoxy resin by chemical modification. According to the different properties of the surface active hydrophilic groups, the waterborne epoxy resin emulsions prepared by chemical modification can be divided into three types: anionic, cationic and non-ionic.
A. Anionic type.
By introducing functional groups such as carboxylic acid and sulfonic acid into the molecular chain of epoxy resin by appropriate methods, the epoxy resin after neutralization has the property of water dispersion. The commonly used modification methods are functional monomer chain extension method and free radical grafting modification method. The functional monomer chain extension method uses the amino group reaction of epoxy group with some low molecular weight chain extenders such as amino acid, aminobenzoic acid, aminobenzenesulfonic acid, etc., and introduces carboxylic acid and sulfonic group into the molecular chain of epoxy resin. After neutralization and salt formation, it can be dispersed in the aqueous phase. The free radical grafting modification method makes use of the high activity of methylene in the molecular chain of bisphenol A epoxy resin, which is easy to form free radical under the action of peroxide and can be copolymerized with vinyl monomer. Acrylic acid, maleic anhydride and other monomers can be grafted into the molecular chain of epoxy resin, and then neutralized to form salt to prepare self-emulsifying epoxy resin.
B, cationic type.
The compounds containing amino groups react with epoxy resin to form epoxy resin containing tertiary amine or quaternary amine base, and then neutralized by volatile organic monobasic weak acid such as acetic acid to obtain cationic waterborne epoxy resin. This kind of modified epoxy resin is rarely used in practice, because the waterborne epoxy curing agent is usually an alkaline compound containing amino group. After the two components are mixed, the system is prone to demulsification and delamination, which affects the performance of the system.
C, Nonionic type.
In general, hydrophilic polyoxyethylene groups are introduced into the epoxy resin chain, while ensuring that there are two or more epoxy groups in each modified epoxy resin molecule. The modified epoxy resin can be self-dispersed in water to form emulsion without adding emulsifier. If a bisphenol A epoxy resin with an epoxy equivalent of 190 and bisphenol A is mixed with a double epoxy end group emulsifier with a molecular weight of 4000 and 20000, the reaction is carried out with triphenylphosphine as catalyst. The epoxy resin containing hydrophilic polyoxyethylene and polyoxypropylene chain end can be prepared. The epoxy resin can be soluble in water without emulsifier and its water resistance is enhanced. In addition, the particles prepared by this method are fine, usually nanoscale, while the particles prepared by the first two methods are larger, usually micron. In this sense, although the chemical method has many preparation steps and high cost, it has practical significance in some aspects.
Hydrophilic polyoxyethylene groups are introduced into the epoxy resin chain, and two or more epoxy groups are guaranteed on each modified epoxy resin molecule. The modified epoxy resin can be self-dispersed in water to form an emulsion without adding emulsifier. For example, polyoxyethylene glycol, polyoxypropylene glycol and epoxy resin are first reacted to form an addition with an end group of epoxy group, which is mixed with bisphenol A epoxy resin with an epoxy equivalent of 190. Epoxy resin containing hydrophilic polyoxyethylene and polyoxypropylene segments can be obtained by using triphenylphosphorus as catalyst. The epoxy resin can be dissolved in water without additional emulsifier, and the water resistance of the film is enhanced because the hydrophilic segment is contained in the epoxy resin molecule. After the introduction of poly (ethylene oxide) and propylene oxide, the molecular weight of the crosslinked curing network chain increased, the crosslinking density decreased, and the formed film had a certain toughening effect.
3) reverse transformation method.
Reverse conversion is an effective method to prepare high molecular weight epoxy resin emulsion. The emulsion used in type II waterborne epoxy resin coating system is usually prepared by reverse conversion method. Reverse transformation refers to the process in which continuous phases in a multicomponent system (such as oil / water / emulsifier) are transformed into each other under certain conditions, as in an oil / water / emulsifier system, the continuous phase changes from water phase to oil phase (or from oil phase to water phase). In the continuous phase transition region, the interfacial tension of the system is the lowest, so the size of the dispersed phase is the smallest. The usual preparation method is to uniformly mix the emulsifier with epoxy resin under the condition of high shear force, and then slowly add water to the system under certain shear conditions. The whole system gradually changed from oil-in-water type to water-in-oil type, forming a uniform and stable water dilutable system. The emulsification process is usually carried out at room temperature. For solid epoxy resin, it is often necessary to reduce the viscosity of epoxy resin by means of a small amount of solvent and heating before emulsifying.
4) emulsification of curing agent.
Waterborne epoxy resin emulsion is usually prepared by curing agent emulsification method. The epoxy resin in this kind of system is generally not emulsified in advance, but is emulsified by waterborne epoxy curing agent before use, so this kind of curing agent must be both cross-linking agent and emulsifier. The waterborne epoxy curing agent is based on polyamine, which adds, grafts, extends and caps the polyamine curing agent, and introduces the Nonionic surface active segment with surface activity into its molecule. It has good emulsification effect on liquid epoxy resin with low molecular weight. The advantage of preparing waterborne epoxy resin system by curing agent emulsification method is that epoxy resin is emulsified directly by curing agent before use, and the storage stability and freeze-thaw stability of epoxy resin emulsion need not be considered. The disadvantage is that the suitable period of the emulsion is short.