Progress of waterborne epoxy coatings cured at the

Progress of room temperature curing waterborne epoxy coatings

progress of room temperature curing waterborne epoxy coatings

April 16, 2004

with the development of coating science and technology and people's new ideas and legal awareness, higher and higher requirements for environmental protection are put forward. The development of coatings towards high solid and water-based coatings has become a consensus in the coating industry. Among them, water-based epoxy coatings, especially water-based epoxy coatings cured at room temperature, are one of the important achievements, and their properties have reached the level matching with solvent based epoxy coatings

as we all know, compared with solvent based coatings, waterborne epoxy coatings have many advantages, such as low v-excellent content, small fumes, safe use, water-based cleaning, etc. Although the raw material price of water-based coatings is high, it still has great attraction in industry and commerce, and is being continuously promoted. Waterborne two-component epoxy coatings have been widely used in the market, such as high-performance architectural coatings, equipment primers, industrial plant floor coatings, transportation vehicle primers, environmental friendly primers for automobile maintenance, and industrial maintenance topcoats. Therefore, room temperature curing waterborne epoxy coatings will have great development prospects, and many new varieties have been studied and developed abroad

1 types of waterborne epoxy coatings

waterborne epoxy coatings widely used in the market are mainly composed of two components: one component is hydrophobic epoxy resin; The other component is hydrophilic glue curing agent. Based on the physical morphology of the two components, e

igoci et al. Divided the waterborne epoxy coatings into the following five types, as shown in Table 1. The safety and stability are not as good as the first transmission system

Table 1 types of room temperature curing waterborne epoxy coatings

types of epoxy resin curing agents

1 liquid or liquid lotion water soluble adhesive

2 solid dispersions water soluble adhesive acrylic cold molding foaming technology containing terminal or glue based functional groups uses more and more common enoic acid

3 liquid or solid lotion dispersion

4 liquid or liquid lotion amine dispersion

5 solid fractions.The Bulk amine dispersion

all these waterborne epoxy resin coating systems in the table have a common feature: epoxy resin is hydrophobic and exists in the form of dispersion (lotion can also be considered as a dispersion). The first and second types have been commercialized at present. Careful analysis shows that the resin is liquid or solid epoxy, which is hydrophobic, while the curing agent is water-soluble glue compound, which is hydrophilic. In this way, there are great differences in hydrophilicity and hydrophobicity between resin and curing agent, which will lead to mismatching in solubility parameters, thus adversely affecting the film-forming property of the coating. Because the coating film-forming process involves the diffusion and coalescence of hydrophobic resin particles and the entry of hydrophilic curing agent into the resin phase. Although many factors will affect this process, such as the viscosity of the system and the properties of the solvent, the mixing degree between the resin and the curing agent, that is, the matching of the solubility parameters of the two, is the most important factor

if the solubility parameters match well, it is conducive to the film formation of the paint and the improvement of various properties of the paint film. The third type is to use the water diluble acrylic dispersion neutralized by the glue as the curing agent, which is collocated with epoxy lotion, commonly known as epoxy acrylic paint. This kind of coating also has the problem of mismatching the hydrophilicity between the resin and the curing agent. The fourth and fifth categories are new room temperature curing waterborne epoxy coatings developed in the late 1990s after considering the above reasons

2 a new class of room temperature curable waterborne epoxy coatings

in order to improve the compatibility between epoxy resin and curing agent, match it with solubility parameters, and improve the performance of room temperature curable epoxy coatings, shell chemical company of the United States has developed a novel room temperature curable epoxy coating, that is, category 5 products in Table 1. There are two kinds of resins used in this kind of epoxy paint: one is the commonly used bisphenol A epoxy resin; The other is epoxy resin with medium molecular weight and multi-functional groups. However, all epoxy resins need hydrophilic group modification, such as modification reaction with hydrophilic polyethylene glycol, alcohol tunneling compound or non-ionic surfactant, introducing hydrophilic group into epoxy resin molecule to make it self emulsifying. The curing agent is relatively confidential. It is an adduct of shuangpan A and glue, which is also modified with hydrophilic groups, and there is no free non-ionic surfactant in the system. It is an aqueous dispersion or water-based lotion

the resin and curing agent are designed in the form of dispersion, which is mainly to synthesize a curing agent with certain hydrophobicity (i.e. lipophilicity) and good compatibility with the resin. The solubility parameters of resin and curing agent are close, which changes the water-soluble state of adhesive curing agent in the past and improves the matching with resin, while epoxy resin improves its hydrophilicity and can self emulsify, which is the characteristics of class 5 epoxy coating

the curing and film forming process of this kind of coating is different from that of the past epoxy coatings (such as class 1 or class 2). The first or second type of epoxy coating uses water-soluble curing agent. When the two components are mixed and cured, the curing agent diffuses from the aqueous phase into the resin particles. Therefore, the concentration of curing agent on the surface of the resin particles is relatively high, making the surface easy to cure and become relatively hard. The formation of the hard shell on the surface of the resin particles will prevent the resin particles from coalescing into a film, and eventually make the resin and curing agent in the paint film unevenly distributed, thus affecting the performance of the paint film

class 5 epoxy coating resin and curing agent are dispersions, and the two components are separated from each other after mixing. When drying and forming a film, it can be considered that a de emulsification process occurs, which occurs quickly. The viscosity of the two dispersed phases is relatively low, and the matching ratio is good. Uniform mixing in the end flow state will occur to obtain a very uniform paint film. The results obtained by scanning the surface of the paint film with atomic force microscope (AFM) technology show that the uniformity of the paint film has been greatly improved

the curing agent of the new class of epoxy coatings is aqueous dispersion. Water can be used to reduce the Mongolian degree of the system without phase separation and without adding other organic reagents. For class 2 coatings, acetic acid is often added to stabilize the adhesive curing agent when diluting. Moreover, generally speaking, the greater the solubility of the curing agent in water that is found for innovative utilization of materials originally developed for other industries, the worse the water resistance of the obtained paint film. It can be seen that the resin and curing agent of class 5 coatings have certain hydrophobicity, so great progress has been made in water resistance

another improvement of the novel epoxy coating is to connect the hydrophilic nonionic surfactant to the molecules of the resin and curing agent. This new technology reduces the amount of surfactant, that is, there is no free surfactant in the system, thus reducing the sensitivity of the coating to aqueous organic solvents. The following is a list of the experimental results of the finish paint obtained by different proportions of curing agent and resin, and compared with the results of class 2 epoxy paint, the drying temperature is (25 ± 1) ℃, and the relative humidity is (50 ± 5)%