Study on Wet Adhesion and Other Properties of Water borne Anticorrosive Coatings

Lei Yuanzhi1 , Yao Fang2 , Li Huining2 , Han Lei1 , Cao Derong1 (1. College of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, China; 2.Zhaoqing Rivers New Material Technology Co. Ltd., Zhaoqing, Guangzhou 526238, China)

Abstract : It is a typical development trend that waterborne coatings are used for the environmental friendly coating process. The wet adhesion is an important parameter to determine the anticorrosive performance of coatings. This paper describes the principle of wet adhesion mechanism. Meanwhile, the effects of film-forming materials, waterborne epoxy resin, pigment/binder ratio and micro -nano hydrotalcite modification on wet adhesion, water absorption and salt fog resistance of coating are investigated.

0 Introduction
With the rapid development of industry, metal materials have been widely used for their excellent process and mechanical properties, but the amount of metal materials scrapped due to corrosion each year is huge, and the economic loss caused is about It accounts for 1.5% to 4.2% of the gross national product, so the protection of metal materials is of great significance. At present, metal protection can adopt corrosion-resistant alloys, cathodic protection and electroplating, but its use is limited due to the influence of factors such as engineering cost, construction technology and protection effect. Many years of anti-corrosion practice have proved that: coating coating is the most economical, effective and widely used method for corrosion protection. The anticorrosive coating can not only protect the metal from erosion, but also can give the material beautiful performance, easy to apply and wide adaptability, and not restricted by the shape and size of the equipment. China has a long history of using coating anticorrosion methods, and many new technologies have also been continuously applied to anticorrosive coatings, effectively promoting anticorrosive coatings to five aspects of economy, high performance, easy construction, environmental protection and energy saving. The water resistance of anticorrosive coatings has become the main research hotspot, but the corrosion resistance of waterborne anticorrosive coatings is still far from that of solvent-based coatings. Therefore, further research is needed on waterborne anticorrosive coatings.

1 Mechanism of Metal Corrosion and Wet Adhesion
1.1 Overview of Metal Corrosion
The phenomenon of physical dissolution, chemical or electrochemical interaction between the surface of a metal material and the environmental medium, which causes damage to the structure of the material or functional degradation, is metal corrosion. The corrosion process develops rapidly. Once it occurs on the metal surface, it will rapidly develop in depth, exacerbating the damage to the performance of metal materials, and then adversely affecting the national economy and production safety.
According to the principle of metal corrosion, metal corrosion methods are generally divided into three types: chemical corrosion, electrochemical corrosion and physical corrosion. The electrochemical corrosion is the corrosion caused by the electrochemical reaction between the metal surface atoms and the conductive ions in the electrolyte solution, including one or more anodic oxidation reactions and cathodic reduction reactions, and the electron flow formed by the electrons lost by the metal oxidation and the electrolyte solution The ion current in the circuit forms a corrosion cell. Electrochemical corrosion is the most common and most harmful corrosion among the three types of corrosion. The corrosion of metals in humid atmospheres, soils, and oceans and lakes is electrochemical corrosion. Anticorrosive coating is a simple and efficient anticorrosive method. The anticorrosive mechanism is to form an isolation layer on the metal surface to prevent it from contacting with oxygen, water and ions or to extend the invasion path of the corrosive medium, thereby protecting it.

1.2 Mechanism of coating wet adhesion
Wet adhesion refers to the measured adhesion of a coating after it has absorbed water. The wet adhesion of a coating is one of the important factors that determine the failure of a coating. In humid environments, water molecules can penetrate the coating / substrate interface to form a continuous or discontinuous water phase, which can cause electrochemical corrosion. If the lateral pressure generated by the water at the interface is stronger than the wet adhesion, causing the bond between some metals and the coating to break, free water replaces the original coating and brings the local metal interface into contact with the penetrating water. Conditions are formed. Once corrosion occurs, metal ions are generated, thereby increasing the osmotic pressure of the electrolyte solution of the local micro system, and then enhancing the penetration of water to gradually cause the coating to fall off. On the other hand, with the occurrence of corrosion, the system environment causes the hydrolyzable groups in the resin to hydrolyze, which causes the wet adhesion of the coating to be further damaged, causing a large area of ​​the coating film to fall off and the protective performance to be lost. Conversely, when the wet adhesion is very good, the bonding strength of the coating and the metal substrate is good, and the lateral movement of the water phase is blocked, which can only accumulate in the original position, which effectively prevents the water from damaging the metal substrate and delays The loss of coating function makes excellent corrosion protection and anti-corrosion effect.

2 Experimental
2.1 Main raw materials
Water-based epoxy resin and curing agent: American MOMENTIVE company; Foamex 810, Wet 245, Dispers 750w: German Digao; phosphate antirust pigment: Hangzhou Haibo Pigment Co., Ltd .; barium sulfate: Dongguan Changping Zhenxin plastic additives business Department; Titanium dioxide, talc: Shanghai Liangjiang Titanium White Chemical Products Co., Ltd .; Mica powder: Shenzhen Jinlonghui Industrial Co., Ltd .; Modified micro-nano hydrotalcite: Zhaoqing College; Water-based polyurethane resin, polyurethane curing agent: Germany Branch Si Chuang; water-based two-component acrylic resin: self-produced.

2.2 Coating formula and coating process
See Table 1 for the preparation formula of the two-component waterborne epoxy, waterborne polyurethane, and waterborne acrylic primer.

Coating formula

(1) Add the dispersing agent, defoaming agent and wetting agent in order according to the amount in the formula in Table 1, and then add deionized water and stir at low speed for 10 minutes. Add pigment and filler slowly at low speed, and then stir and disperse at high speed for 40-50 minutes until the dispersion fineness is less than 50 μm to obtain pigment and filler slurry. Decrease the rotation speed, slowly add resin and an appropriate amount of deionized water to stir and disperse uniformly, canned and store it in a cool and dry place;
(2) Add the curing agent and then disperse evenly, adjust the construction viscosity with deionized water, and filter by screen. The substrate and treatment are carried out in accordance with GB / T 9271-2008, using air spraying [controlling the thickness of the dry film to (45 ± 3) μm], drying after drying, and then placing at 25 ° C and 50% relative humidity After curing in a constant temperature and humidity room for 7 days, it is used for related tests.

2.3 Wet adhesion test method
The test sample is immersed in a constant temperature water bath with a temperature of (40 ± 0.5) ℃, taken out at a specific time, rinsed with deionized water, and quickly dried with filter paper and weighed. The 1998 standard was crossed and then flattened over the grid with a transparent pressure-sensitive adhesive tape to ensure that the adhesive tape adhered closely to the coating. Then the tape was quickly peeled off and the coating was peeled off to determine the wet Adhesion level.

3 Results and discussion
3.1 Influence of different film-forming resin types on wet adhesion properties of coatings
In order to investigate the wet adhesion performance of different types of film-forming resin primers, two-component waterborne epoxy (1 #), two-component waterborne polyurethane (2 #), and two-component waterborne acrylic (3 #) were used. The test results of the wet adhesion of the three water-based primers formulated are shown in Figure 1.

Corresponding wet adhesion of different resin primers

It can be seen from Fig. 1 that the epoxy primer has the best wet adhesion performance, and the primer wet adhesion performance is: two-component waterborne epoxy> two-component waterborne polyurethane> two-component waterborne acrylic. The wet adhesion grades of epoxy and polyurethane primers after immersion for 8 d were all ≤2, which were within the qualified range. The wet adhesion level of acrylic primer after immersion for 5 d is ≤2, and the wet adhesion level after immersion for 8 d is 3, and the coating is severely peeled off. Epoxy and polyurethane primers have better wet adhesion performance than acrylic primers. This is due to the strong adhesion of epoxy and polyurethane resins to metal surfaces, and the denseness of the coating film after curing. The epoxy wet adhesion performance is better than polyurethane, because the epoxy resin system contains more hydroxyl and ether bonds, which has more excellent adhesion to metals; and the epoxy resin contains a benzene ring, so it is hard, and at the same time It also contains ether bonds to facilitate the rotation between molecules and has a certain degree of toughness. The cross-linking distance of epoxy resin is long, the volume shrinkage rate is low during curing, and the internal stress does not damage the adhesion force, so the adhesion force is good. In addition, the epoxy system has excellent chemical resistance. Compared with polyurethane resins, epoxy resins have only hydrocarbon-based ether bonds and no easily hydrolyzed ester bonds, which further structurally enhances the wet adhesion performance of epoxy resins. The wet adhesion performance of the resin is the best of the three.

3.2 Influence of different water-based epoxy resins on wet adhesion performance
In order to further optimize the resin with excellent wet adhesion performance, the three types of epoxy resins to be compared are domestic epoxy resin (1 #), imported epoxy resin (2 #), and domestic epoxy resin. Epoxy (3 #), the results are shown in Figures 2 and 3.

Water absorption and wet adhesion performance of different epoxy primers, rust after 1 #, 2 # primer soaking

With reference to Figures 2 and 3, it can be seen that the primer made of 2 # imported epoxy resin has low water absorption, strong wet adhesion performance, and the best anti-corrosion effect. Water-based epoxy resins contain a large number of hydroxyl groups and ether bonds that are formed by ring opening or have these groups. These groups have strong hydrophilicity. Therefore, water-based anticorrosive primers prepared using epoxy resins also have a certain amount of water absorption. 1 # The domestic epoxy primer has the largest water absorption rate, and it has a tendency to increase with the immersion time. This is because the water absorption rate and equilibrium water absorption rate of waterborne epoxy primer are mainly composed of free volume and polar groups in the system. Determined by the concentration, the larger the free volume, the more polar groups, the faster the water absorption rate, and the larger the equilibrium water absorption. The water absorption rate and wet adhesion performance of 2 # primer are slightly better than 3 #. It can be seen that the water absorption rate of the coating reflects the strength of the wet adhesion performance of the coating to a certain extent, but the water absorption rate and wet adhesion It is not a simple linear relationship, it depends on the transport behavior of water in the coating. The cured epoxy resin has a high cross-link density region and a low cross-link density region. The high cross-link density region is a spherical “strand ball”. The chain ball includes a basic chain ball (10 nm) and an aggregated chain ball composed of the basic chain ball. The low cross-link density region For the substrate. The chain ball is embedded in the matrix to form two structures. According to the water absorption mechanism of the epoxy resin proposed by Adamson, water first enters the low cross-link density region. This step is faster, and equilibrium swelling can be achieved quickly. Then the free water or water clusters existing in the low cross-link density area penetrates into the high cross-link density area. This step is slower. Therefore, the water absorption of epoxy coatings is related to the degree of resin cross-linking and the three-dimensional network structure. If the layers are dense enough, the penetration of free water is slowed, so the water absorption is low, and the wet adhesion damage to the coating is small.

3.3 Influence of micro-nano hydrotalcite on the wet adhesion performance of coatings 

Micro-nano hydrotalcite is a layered columnar bimetal hydroxide. It is an octahedral structure composed of metal ions and 6 hydroxyl groups. Part of the structure M2 + is M3 + isomorphic. Instead, the laminate is positively charged, and the anions filled in between the layers play a balanced charge function. The classic structure is shown in Figure 4.

Hydrotalcite structure diagram

Micro-nano hydrotalcite has a small particle size and has more hydrophilic groups on the surface, which has poor dispersion stability. It is extremely easy to adsorb each other and cause soft agglomeration. It cannot be uniformly dispersed in coatings. It may also form stress concentration points in the coating. , Which seriously affects the amount of addition in the coating and the comprehensive performance of the product. Graft modification of micro-nano hydrotalcite with γ-APS to improve its ability to stably disperse in coatings, and the use of hydrotalcite itself with more hydroxyl groups to enhance the wet adhesion performance of the coating. The results of the wet adhesion test are shown in Fig. 5 when comparing the epoxy primer coatings with and without modified micro-nano hydrotalcite.

Effect of modified micro-nano hydrotalcite on wet adhesion properties of coatings

It can be seen from Figure 5 that the wet adhesion performance of the coating after adding the modified hydrotalcite is significantly better than that of the unmodified one. This is because after the modified micro-nano hydrotalcite is added to the system, the polymerization reaction branches the polymer to the surface of the particles, so that the interface between the filler and the resin is chemically bonded. The layered columnar double metal hydroxide structure of the hydrotalcite enhances the bonding ability and significantly increases the crosslinking density, so the wet adhesion is improved. 

3.4 Effect of different pigment-base ratios on the wet adhesion performance and salt spray resistance of epoxy coatings
To investigate the appropriate pigment-base ratios of primers, the pigment-base ratios were prepared as 5.5: 4.5, 6.0: 4.0, 6.5: 3.5, 7.0: 3.0 epoxy anticorrosive primer, its wet adhesion and SEM test results are shown in Figures 6 and 7.

Wet adhesion of different pigment-base ratio coatings
Scanning electron microscope images of different base-to-base ratio coatings

It can be seen from Fig. 6 that with the increase of the pigment-base ratio (PVC), the wet adhesion performance of the coating first increases and then decreases. It is optimal when the pigment-base ratio is 6.0: 4.0. The wet adhesion level of the coating is ≤2. In Figure 7, with the increase of the pigment-base ratio, the microcosmic surface of the coating changes from dense to loose. The coating film with the pigment-base ratio of 5.5: 4.5 is better than 6.0: 4.0, but its wet adhesion performance On the contrary, it shows that the proper increase of pigments and fillers in the system increases the wet adhesion performance of the coating to a certain extent.

Salt spray resistance time for different pigment-base coatings

Combining Figure 6 and Table 2, it can be seen that the wet adhesion performance of the coating is positively correlated with the resistance to salt spray. The wet adhesion performance of the coating is: 6.0: 4.0> 6.5: 3.5> 7.0: 3.0> 5.5: 4.5, and salt resistance The fog performance is also: 6.0: 4.0 > 6.5: 3.5 > 7.0: 3.0 > 5.5: 4.5. The wet adhesion performance is good. The adhesion of the coating film to the substrate is stronger than the side pressure generated by water at the interface. Then the water phase The lateral movement is blocked and can only accumulate in the original position, which effectively prevents the damage of the coating by water, thus delaying the loss of the coating function and making the protective effect excellent. Therefore, from the corresponding relationship between wet adhesion and salt spray resistance time, we can use the data of wet adhesion to make a preliminary judgment on the salt spray resistance of the coating, adjust the formula and process in time, and improve the efficiency of research and development.

4 Conclusions
(1) Water-based coatings are the main direction of green environmental protection coatings. This article outlines the mechanism of anti-corrosion and wet adhesion, and tests different types of film-forming water-based resins, different water-based epoxy resins, pigment-base ratio, and micro-nano water. Effect of talc modification on wet adhesion, water absorption and salt spray resistance of coatings. It is pointed out that wet adhesion is an important factor affecting the anti-corrosion performance of coatings. The comparison of this test shows that the epoxy coating has better wet adhesion performance than polyurethane and acrylic systems; the waterborne epoxy and curing agent produced by MOMENTIVE are superior to two domestic products of the same type.
(2) By adding modified micro-nano hydrotalcite, the wet adhesion performance of the epoxy primer is significantly enhanced, and a method for improving the salt spray corrosion resistance of the epoxy coating is obtained.
(3) When the pigment-base ratio is 6.0: 4.0, the wet adhesion performance and salt spray corrosion resistance of the coating are superior. From the correspondence between epoxy primer wet adhesion and salt spray resistance test time, we can use the primer wet adhesion change to make a preliminary judgment of salt spray resistance, which is helpful for timely adjustment of formula and process.

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