A Study on the Performance of Waterborne Epoxy Primers on Different Substrates

LI Mu-ying, CHENG Lu, WANG Rui-hong (Hebei Water Paint Engineering Technology Research Center, Hebei Chenyang Industrial and Trade Group Co., Ltd., Baoding 072550, Hebei, China)

Abstract: The adhesion tests of waterborne epoxy primers on different substrates of passenger cars were briefly introduced. The effects of different epoxy emulsions, different curing agents, wetting agents and pigment-binder ratios on coatings adhesion and other properties were discussed.

0 Foreword
Passenger cars are the main means of transportation for highway passenger transportation. There are many types of base materials for bodywork, including various aluminum materials, galvanized steel plates, and cold-rolled plates. In view of the diversity of passenger car substrates, ordinary primers cannot meet the requirements of high adhesion and adaptability. Waterborne epoxy primers with good comprehensive properties must be selected. This article mainly develops a water-based epoxy primer with good adhesion on different substrates, good salt spray resistance and fast drying speed.

1 Experimental part
1.1 Main experimental raw materials
Waterborne epoxy emulsion and waterborne epoxy curing agent, imported; dispersant, BYK, Germany; defoaming agent, Efka; leveling agent, Germany Deco; thickener, Germany Mingling ; Flash rust agent, Shanghai Haofeng Fine Chemicals; Deionized water, homemade; Pigments and fillers.


1.2 Experimental equipment
High-speed dispersing machine: Shanghai Pushen; Coating film adhesion tester / QFD type: Tianjin Runtong Precision Test Technology Development Co., Ltd .; 3M tape: 3M company; Precision salt spray tester / SST-120: Shanghai Poly New Electronic Technology Co., Ltd.


1.3 base formulation (see Table 1)

Waterborne epoxy primer formula

1.4 Experimental Procedure
fineness of deionized water, various additives, pigments and fillers formulated proportion in a certain order is added to the stock pot, after high-speed dispersion using grinder to ≤25 μm After that, raw materials such as water-based epoxy emulsion, anti-flash rust auxiliary agent, thickener and the like are added and filtered through a 200-mesh filter cloth to obtain component A. The aqueous epoxy curing agent and deionized water were added to a clean preparation pot and dispersed to a uniform state according to the formula ratio, and then filtered through a 200-mesh filter cloth to obtain component B.


1.5 Test substrate
Cold rolled steel sheet, galvanized sheet, aluminum sheet A, aluminum sheet B.


1.6 Preparation of sample plate
Mix the A and B components according to a 6: 1 mass ratio, stir well, dilute with deionized water to an appropriate viscosity, and spray with a W-71 spray gun. The sprayed sample was placed horizontally in a drying box, and the thickness of the coating film was performed in accordance with the provisions of GB / T 13452.2. Then under the prescribed conditions for curing, and finally use the grid method for adhesion testing.


1.7 Adhesion test method
Refer to the standard GB / T 9286-1998 “Cross-section test of paint and varnish paint films”.


1.8 Product test results (see Table 2)

Test results of water-based epoxy primer

2 Results and discussion
2.1 Influence of film-forming substances on coating adhesion
Two different epoxy emulsions and curing agents were used in different experiments. Performance tests and comparisons were performed to select good adhesion and salt resistance on various substrates. Epoxy emulsion and curing agent with excellent haze and fast drying speed. The specific test results are shown in Table 3.

Effect of different epoxy emulsions and curing agents on performance

The two epoxy emulsions used in this experiment are bisphenol A epoxy emulsions, curing agent A is a modified polyamide epoxy curing agent, and curing agent B is a modified fatty amine epoxy curing agent. The molecular structure of epoxy resin has a strong polar ether bond and hydroxyl group, which causes strong adhesion between the epoxy resin and the surface of the substrate, especially the metal surface. Therefore, the epoxy emulsion is used as a film-forming substance. Modified polyamide curing agent has good salt spray resistance, good adhesion to various substrates, great peel strength, good toughness of the coating film, but slow drying speed; modified fatty amine curing agent is resistant to salt Fog is average, but drying is fast. In order to ensure that the product has good adhesion on various substrates, good salt spray resistance and fast drying speed, the epoxy emulsion A is selected as the emulsion. The use of two curing agents A and B alone cannot meet the requirements of the product, so As the curing agent, A and B are used together.


2.2 The influence of different proportions of curing agent on the adhesion of the coating
Epoxy emulsion and curing agent can change the ratio of the two within a certain range, which can improve some properties of the coating film. If the epoxy emulsion is slightly excessive, it is beneficial to improve the resistance to salt spray, but the adhesion becomes worse and the drying speed becomes slower; if the amine curing agent is excessive, it is easier for the aqueous curing agent to emulsify and fuse the epoxy resin, and the crosslinking speed is accelerated. The drying time is shortened and the adhesion is improved to a certain extent, but the salt spray resistance is deteriorated. Therefore, the ratio of epoxy emulsion and curing agent has a great influence on the coating performance. The specific experimental results are shown in Table 4.

Effect of different proportions on performance

It is concluded through experiments that when the ratio of the main agent and the curing agent is 6: 1, the adhesion of the water-based epoxy primer to various substrates is 0, and the salt spray resistance is the best, and the drying speed is moderate.


2.3 Influence of substrate wetting agent on coating adhesion

The substrate wetting agent is also an important factor affecting the adhesion of the coating. Experiments were performed by comparing different substrate wetting agents to select sample 2 as the basis of the system. Wood wetting agent. The specific experimental results are shown in Table 5.

Effect of substrate wetting agent on coating adhesion

A suitable substrate wetting agent can adjust the surface tension of the coating, avoid cracks or shrinkage of the coating film, and improve the adhesion of the coating film. The types of substrate wetting agents include anionic surfactants, non-ionic surfactants, polyether-modified polysiloxanes, and the like. The type and amount of substrate wetting agent should be determined based on factors such as the surface tension of the aqueous epoxy emulsion and the interfacial tension of the substrate. At the same time, experimental screening confirmed that the adhesion of the polyether-modified polysiloxane-based substrate wetting agent is the best. This substrate wetting agent improves the wettability of the substrate. The polar molecules contain polyether segments and remain in the water. In contrast, the hydrophobic part is a hydrocarbon segment that faces the air. This structure creates orientation that greatly reduces the surface tension of the liquid.


2.4 The effect of Pigment binder ratio on coating adhesion 

Pigment binder ratio is an important factor affecting the adhesion of coatings. If Pigment binder ratio is too high, the coating film will easily crack and the adhesion will decrease. If Pigment binder ratio is too low, salt spray resistance will change. Poor, slow drying and high cost. Therefore, in order to ensure the performance of water-based epoxy primer, determine the best pigment-base ratio. The water-based epoxy primer is made according to no base-to-base ratio. After curing the sample in accordance with the regulations, the coating adhesion is tested. The specific experimental results are shown in Table 6.
It can be obtained through experiments that when the base-to-base ratio is 2.0, the adhesion is the best. When the base-to-base ratio exceeds 2.5, the adhesion decreases. Therefore, the formula has a base-to-base ratio of 2.0 and the best performance of water-based epoxy primer.

3 Adhesion test pictures
The adhesion test results of water-based epoxy primer on different substrates are shown in Figure 1.

Effect of pigment-base ratio on coating adhesion, adhesion test of water-based epoxy primer on different substrates

It can be seen from FIG. 1 that, through screening of raw materials and adjustment of formula, the developed water-based epoxy primer exhibits good adhesion on different substrates.

4 Conclusion
Adopt epoxy emulsion A as the main resin, epoxy curing agent choose curing agent A and B to use together, so that the comprehensive performance of water-based epoxy primer is the best.
The substrate wetting agent and pigment-base ratio have a great influence on the adhesion of water-based epoxy primer. Through screening additives and adjusting the formula, it is finally determined that the substrate wetting agent suitable for the system is polyether-modified polysilicon. Oxyalkanes have a pigment-to-base ratio of 2.0. Guaranteed good adhesion of the product to different substrates. At present, the coating of passenger cars in China is still mainly based on traditional solvents. However, with the improvement of people’s awareness of environmental protection, water-based coatings rely on their advantages such as energy saving, environmental friendliness, and construction safety, which will inevitably gradually increase their use in industrial coatings. Share, and therefore, water-based epoxy primer will have a very wide application space.

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