Correlation between waterborne epoxy resin and additives

1、 Waterborne epoxy resin

Waterborne epoxy resin is a stable dispersion system prepared by dispersing epoxy resin in the form of particles or droplets in a dispersion medium with water as the continuous phase. As epoxy resin is a thermosetting resin with linear structure, waterborne epoxy curing agent must be added before construction. Chemical crosslinking reaction occurs at room temperature. After curing, the epoxy resin changes its original soluble and fusible properties into an insoluble and fusible spatial network structure, showing excellent performance.

2、 Background

With the increasing awareness of environmental protection in recent years, low VOC environment-friendly waterborne coatings have been highly valued. Paint additives play an important role in coating components. The selection and addition of additives have an important impact on the performance and application of waterborne coatings. At present, waterborne coating additives (such as defoamers, rheometers, thickeners, etc.) have more impact on the performance of waterborne coatings, and what is the relationship between waterborne epoxy resin and additives. It includes the influence of the amount of dispersant, thickener, defoamer, wetting agent and pigment on the film performance of waterborne epoxy resin coatings.

3、 Test

1. Test

Preparation of test materials

2. Preparation of waterborne epoxy resin coating

First, carry out pulping, mix the prepared slurry with water-borne epoxy resin lotion according to a certain proportion, and then add various additives according to the lotion state and required performance to prepare water-borne epoxy resin coating

3. Film performance test

The coating shall be evenly and smoothly applied to the tinplate by brush coating method, and after it is fully dried, a uniform water-based epoxy resin coating film shall be obtained. Refer to GB/T 6739-2006 Paints and Varnishes Determination of Film Hardness by Pencil Method, GB/T 9286-1998 Cross cut Test of Film, GB/T 9754-2007 Determination of Gloss of Paints and Varnishes Film without Metallic Pigments and GB/T 1733-1993 Water Resistance Test of Film to test the hardness, adhesion, gloss and water resistance of the film.

4、 Test results

1. Effect of dispersant on film properties

The addition of dispersant can promote the dispersion effect and stability of the coating, and the water resistance and leveling property of the film will also be enhanced. However, when the dispersant is excessive, the film performance will be significantly reduced.

2. Effect of Dispersant on the Stability of Coatings

The surface free energy of pigments and fillers is high. When the amount of dispersant is insufficient, the pigments and fillers cannot be completely covered. Therefore, the free energy of the paint system is high, which is easy to agglomerate and settle, reducing the stability of the paint; With the increase of the amount of dispersant, the dispersant can completely cover the pigment and filler, thus preventing the agglomeration of the pigment and filler in the system and enhancing the dispersion; But when m (dispersant)>1.5%, the particles in the system are curly structure, which makes the bonding strength between particles in the system insufficient, and the waterborne coating tends to be unstable.

3. Effect of dispersant on water resistance of waterborne coatings

There are hydrophilic groups in the molecular structure of the dispersant, which has strong hydrophilicity. Therefore, when the dispersant is dissolved in the paint, the water absorption of the film will be improved and the water resistance will be reduced. Therefore, in practical application, the amount of dispersant should be as small as possible while ensuring that other properties besides water resistance meet the demand.

4. Effect of Dispersant on Gloss of Coatings

Dispersants help to disperse titanium dioxide, and can effectively enhance the dispersion effect. Good dispersion effect is conducive to reducing the fineness of the slurry and improving the glossiness.

5. Effect of Thickener on Film Properties

Thickener plays a thickening role by changing the structure of colloidal particles in lotion. Before adding thickener, lotion particles intertwine with each other, and the viscosity is low; With the addition of thickener, when the critical micelle concentration of the system is lower than the thickener concentration, the micelles in the water-borne epoxy resin lotion will adsorb on the lipophilic end of the thickener to form a network structure, thus increasing the viscosity; However, when the thickener is excessive, the surface gloss of the film will be significantly reduced.

6. Effect of Thickener on the Stability of Coatings

Due to the existence of thickener, the association between colloidal particles in lotion is a network structure, which can effectively inhibit the aggregation of pigments and fillers, thus improving the stability.

7. Effect of Thickener on Water Resistance of Coatings

The water resistance of the film decreases with the increase of the thickener. The existence of hydrophilic groups in the molecular structure of the thickener makes the external water molecules penetrate into the interior through the surface of the film, which reduces the water resistance of the film.

8. Effect of Thickener on Gloss of Coatings

When the viscosity of the paint system is low, an appropriate amount of thickener can effectively improve its leveling property, resulting in a smooth surface of the formed film, reducing the surface roughness, and thus enhancing the glossiness. However, if the amount of thickener is too large, the viscosity of the coating system will increase rapidly. It is difficult for the high viscosity coating to maintain its smoothness and smoothness during the coating drying process, resulting in a decrease in glossiness.

9. Effect of Defoamer on Film Properties

The antifoaming effect of antifoaming agent in the paint system is obvious, but the addition amount of antifoaming agent needs to be strictly controlled. When excessive amount is added, the coating will show obvious shrinkage cavity, and the amount of defoamer has little effect on other properties of the film.

The reason why defoamer suppresses foam is that the surface tension of defoamer is low. When defoamer with low surface tension is dispersed in water-based epoxy resin lotion, it will immediately adsorb on the surface of bubble liquid film, thus reducing its surface tension. However, a stable lotion system needs to maintain the balance of forces between colloidal particles, so the bubbles covered by defoamer with low surface tension will flow to the liquid with high surface tension. In this process, the wall thickness of the liquid film will gradually become thinner, and the stability will decline until the bubbles break [5]. However, if excessive addition is made, a large number of low surface tension points will gather, which will easily lead to concentrated fracture in the process of maintaining the balance of system forces, thus forming shrinkage cavities.

10. Effect of Wetting Agent on the Properties of Coatings

Bernard vortex phenomenon often occurs in the film forming and drying process of coatings. The volatilization of water and cosolvent reduces the surface tension, resulting in pinhole, shrinkage cavity, shrinkage edge, orange peel and other defects of the coating film. The addition of a proper amount of wetting agent can alleviate these defects, reduce the imbalance of surface tension during film drying, improve the fluidity, and maintain the flatness of the coating film.

The proper amount of wetting agent can effectively improve the leveling property of the coating, and with the increase of the amount of wetting agent, the glossiness and adhesion of the coating film are improved. However, when the wetting agent is excessive, the phenomenon of shrinkage will occur. This can be attributed to the influence of low surface tension of polyether polymer material wetting agent. When the wetting agent is excessive, the low surface tension points in the system increase sharply and flow rapidly to the high surface tension, thus forming shrinkage cavities.

11. Effect of leveling agent on film properties

Leveling is a process in which the coating slowly shrinks to the minimum area and forms a plane on the substrate surface under the effect of surface tension. The coating molecules constantly move in the leveling process, forming the Bernard vortex phenomenon, which affects the leveling property. The film is prone to pinhole, sagging and other defects in the film forming process, most of which are caused by poor leveling. As the viscosity of the coating gradually increases at the flow level, the molecules of the system constantly move to form a Bernard vortex.

An appropriate amount of leveling agent can effectively improve the leveling property of the film. A good leveling agent can moderately improve the hardness of the film and also improve the appearance of the film.

12. Effect of pigments on film properties

The adhesion of the coating can be improved by adding appropriate amount of pigment. This can be attributed to the fact that titanium dioxide pigment has a small size effect and can form a good interface with the substrate. In addition, titanium dioxide has a very strong surface activity and a strong force with the molecules on the surface of the substrate, which improves the adhesion of the coating.

The influence of pigment on the glossiness of the coating can be attributed to: on the one hand, the addition of pigment will enhance the hiding power of the coating, thus improving the light absorption and scattering ability of the coating film, resulting in the reduction of glossiness; On the other hand, titanium dioxide particles exposed on the surface during film formation will increase the surface roughness of the film, and diffuse reflection will occur when light rays enter, resulting in a decrease in glossiness.

With the addition of pigments, the water resistance of the film decreases. The influence of pigments on the water resistance of the coating film can be attributed to the fact that titanium dioxide in the pigment has strong hydrophilicity due to the polarization dissociation of Ti-O bond. Because the molecules in the system keep moving, the hydrophilic exposed titanium dioxide molecules will migrate to the surface of the coating film, thus reducing its water resistance.

Pigments affect the stability of coatings because excessive pigment addition causes a large number of titanium dioxide particles to be exposed without being coated. These particles are agglomerated due to the reduction of surface energy, thus reducing the stability of coatings.

5、 Conclusion

The effects of additives and pigments on the stability, glossiness, water resistance and other properties of waterborne epoxy resin coatings were analyzed and studied through this test. The amount of additives needed to prepare waterborne epoxy resin coating with good comprehensive performance is as follows: wetting agent 0.2%, leveling agent 0.3%, dispersant 1.5%.