Research on High-Edge Corrosion-resistant Cathodic Electrodeposition Coating

introduction

In recent years, the domestic electrophoretic coating has developed rapidly, and the competition in the auto and motorcycle parts market is extremely fierce

In recent years, high-edge coverage cathodic electrophoretic coatings have developed rapidly.

In this study, ketimine and aminosilane reacted with epoxy-terminated and polyepoxy hydroxy resin to prepare an amino-blocking resin containing silico-hydroxyl group, and then subjected to acid neutralization, emulsification, hydrolysis and condensation.

Preparation of microgel additives

Synthesis of microgel

Add epoxy resin 828, bisphenol A and xylene into a four-necked flask, heat to 120℃ under nitrogen protection, then add dimethylbenzylamine, keep it at 160℃ for about 3h, add ethylene glycol butyl ether and amine modified Sex agent, keep at 90℃~100℃ for 2h, then add γ-aminopropyltriethoxysilane A-1100, keep at 85~90℃ for 1h, add lactic acid to neutralize after incubation, then add deionized water for emulsification ，Incubate at 50~60℃ for 5~6h to obtain a light yellow translucent liquid with 28% solid content as a microgel additive emulsion

Preparation of high-edge corrosion-resistant cathodic electrophoretic paint

The self-made HED-epoxy cathodic electrophoretic paint emulsion, color paste, and deionized water are prepared according to the conventional formula, and the synthetic microgel additives are added to obtain high-corner corrosion-resistant cathodic electrophoretic paint

Results and discussion

(1) Infrared characterization of microgel additives

The infrared spectrum of the microgel additive is shown in Figure 1

Figure 1 Infrared spectra of microgel additives

It can be seen from Figure 1 that the characteristic absorption peak of the epoxy group at 920 cm-1 of the epoxy resin disappears, indicating that the epoxy group has been opened; a characteristic peak of -OH stretching vibration appears at 3342 cm-1; at 2126 cm Both the stretching vibration peak and the bending vibration peak of the silicon-hydrogen bond at -1 and 925cm-1 disappear; the absorption peak at 2960cm-1 is the methyl CH stretching vibration peak on Si-CH3; the absorption peak at 1257cm-1 is The methyl CH bending vibration peaks on Si-CH3; the absorption peaks at 1010cm-1 and 1060cm-1 are the characteristic peaks of Si-O-Si, indicating that the target product has been produced
.

(2) Parameter test of different addition amounts of microgel additives

The working fluid for experimental electrophoretic coatings is prepared according to Table 1
.
The effects of different addition ratios of microgel additives on the pH and conductivity of the working fluid and the appearance of the paint film and the salt spray performance of the paint film after the electrophoretic paint bathing were studied
.

Table 1 Working fluid formula and tank fluid parameters and paint film performance pictures

It can be seen from the experimental results that different additions of microgel additives have a certain effect on the various parameters of the electrophoresis working fluid and the appearance of the coating film, which can further reduce the contraction force of the corners of the workpiece when the coating is cured.
Inhibit the fluidity during curing and increase the thickness of the paint film of the coating at this part, thereby improving the anti-corrosion performance of the corners of the workpiece
.
When the addition amount is too small, the microgel effect is not obvious and has no effect on the appearance and performance of the coating film; when the addition amount is too much, the appearance of the paint film will increase in orange peel and the gloss will be lower
.
After experimental comparison, this study chose 5%~10% of the total amount of electrophoresis working solution as the appropriate amount of microgel additives
.

(3) Analysis of the cause of rust at the corners

Electrophoresis is performed on the workpiece.
Because of the large surface curvature at the tip of the corner, discharge occurs at the tip, and electrodeposition first occurs in these parts
.
With the progress of the electrophoresis process, the thickness of the paint film gradually increases, the resistance of the wet film increases, and the insulation increases.
The current density of these parts gradually decreases, and the electrodeposition enters the adjacent area
.
During the electrophoresis process, the paint components are mainly precipitated at the tips of the corners, with good corner coverage and thick paint film
.
After the coating film is baked at high temperature, the viscosity of the coating resin decreases and the fluidity increases.
Due to the effect of surface tension, the coating at the corners continuously shrinks to the middle, moves to both sides and becomes thinner, and the thickness of the coating at the tip is significantly reduced.
Therefore, the paint film coverage of the corners becomes poor, and the metal in these exposed parts is not protected by the coating, and severe rust occurs, and the corners and corners have poor corrosion resistance
.

(4) Corrosion resistance test of electrophoretic paint edges and corners

At present, the corrosion test methods of spare parts mainly include the neutral salt spray test (NSS), the cyclic accelerated corrosion test (CCT), and the neutral salt spray test (NSS) is the main method
.
In the experiment, a sharp-edged blade (Deli art blade, specification 18mm, SK5 high-quality high-carbon steel) was selected to simulate the corners of the workpiece, and the blade was phosphated and electrophoresed, and then the corner salt spray test was carried out.
After the phosphating blade electrophoresis, a neutral salt spray test is carried out, and the results of the neutral salt spray test of the workpiece are compared
.

(1) The blades are degreasing and phosphating, and the blades with good phosphating state and consistent crystals are selected for electrophoresis;

(2) During electrophoresis, the blade faces the electrode side, the electrode ratio is 1:2, and the blade after electrophoresis is dried under standard conditions;

(3) In the salt spray box, the blade edge is upward, and the angle is 15°～30° with the vertical plane.
After 168h neutral salt spray test according to GB/T10125-1997, the corner corrosion is judged
.

(4) Make 3 blades in parallel, and take the average of the test results
.

After the blade undergoes a neutral salt spray test, rust spots appear, and the rust spots are distributed in dots, indicating that the non-defective parts have better corrosion resistance
.
Through the adjustment of the coating formula, the surface tension of the coating resin during the heating and curing of the coating is reduced, and the occurrence of shrinkage and other problems during the heating and curing is avoided, and the corner coverage of the electrophoretic coating is improved
.

Figure 2 The edge of conventional electrophoretic paint and high-corner corrosion-resistant electrophoretic paint after electrophoresis

In the experiment, the blade electrophoresis with consistent phosphating crystals was selected, and the edge coverage of conventional electrophoretic coatings and high-edge corrosion-resistant electrophoretic coatings after electrophoresis were compared, and the salt spray sharp edge corrosion of conventional electrophoretic coatings and high-edge corrosion-resistant electrophoretic coatings was 120h and 168h
.
Before and after adding microgel, the comparison result of electrophoretic paint film salt spray is shown in Figure 2
.
After 120 hours of neutral salt spray, the corners of conventional electrophoretic coatings began to appear rust spots, 3~5; the blades with microgel additives did not appear rust spots; after 168h of neutral salt spray, the corners of conventional electrophoretic coatings appeared There are many rust spots, 20-25, and the corrosion level is 2; the blades with microgel additives also have rust spots, 3~5, and the corrosion level is 1
.

Adding the microgel additive emulsion to the existing conventional epoxy cathodic electrophoretic paint can significantly improve the edge anticorrosive performance of the paint, increase the thickness of the edge coating film, and improve the edge coverage
.
The microgel is convenient to use and can be directly added to the working solution of epoxy system cathodic electrophoretic coatings, which can improve the above-mentioned performance
.

(5) Paint film performance test

The high-edge corrosion-resistant cathodic electrophoretic paint prepared under the above conditions was prepared with an electrophoresis working solution, and electrophoresed after 24 hours of curing, 180V/2min, and cured at 175°C for 30min.
The test performance results are shown in Table 2
.

Table 2 Test performance of coating film

Concluding remarks

(1) Introducing amino silane into epoxy resin, neutralizing, emulsifying and hydrolyzing and condensing siloxane groups to prepare microgel additive emulsion;

(2) The coating resin at the corners is baked at a high temperature, and the corner coverage becomes worse due to the effect of surface tension, and the corrosion resistance becomes worse
.
The use of high-corner corrosion-resistant cathodic electrophoretic coatings prepared by adding microgel additives can effectively improve the edge coverage and edge corrosion resistance of the workpiece, and increase the service life of the workpiece;

(3) Study the effects of different addition ratios of microgel additives on the pH value, conductivity and appearance of the paint film of the working fluid, and the salt spray performance of the paint film
.
The results show that 5%~10% of the total amount of electrophoresis working fluid is the appropriate amount of microgel additives.
Under this condition, the working fluid is in a stable state and the paint film is smooth and smooth.
The salt spray test of the phosphating blade is 168h, and the blade is rusty.
The number of points is 3~10, and the corrosion level is level 1
.

References (omitted)