With the rapid development of China's economy and the continuous improvement of people's living standards, waterborne coatings have faced good development opportunities in recent years and have received unprecedented attention [1]; in terms of metal protection, waterborne protective coatings are increasingly replacing traditional solvent-based coatings in fields such as building steel structures, chemical corrosion protection, industrial equipment, auto parts, and containers [2]

Enhancing Waterborne Metal Protection with Functional Additives

Rust Prevention Properties of Coatings

Zhang Jinlian

(Shanghai Haochang Fine Chemicals Co., Ltd., Shanghai 200030)

     Abstract :  Combining theory with practice, this paper introduces the methods and approaches for improving the rust prevention performance of waterborne protective coatings on metal surfaces using functional additives such as emulsifiers, anti-flash rust agents, organic corrosion inhibitors, wetting agents, and adhesion promoters.

     Keywords :  Waterborne metal protective coatings; Anti-flash rust agent; Organic corrosion inhibitor

 

0  Introduction

With the rapid development of China's economy and the continuous improvement of people's living standards, waterborne coatings have faced good development opportunities in recent years and have received unprecedented attention. ^[1] In terms of metal protection, waterborne protective coatings are increasingly replacing traditional solvent-based coatings in fields such as steel structures in buildings, chemical corrosion protection, industrial equipment, automotive parts, and containers. ^[2] Due to the "flash rust" phenomenon when waterborne coatings come into contact with metal substrates, the difficulty of wetting and dispersion using water as a medium, and higher environmental requirements, there is more research content and space to ensure and improve the rust and corrosion prevention performance of waterborne metal protective coatings.

Therefore, based on practical needs, this paper focuses on the promoting effect of functional additives such as phosphate ester emulsifiers, anti-flash rust agents, organic corrosion inhibitors, wetting agents, and adhesion promoting monomers on the rust prevention performance of waterborne coatings used for metal protection, in order to contribute to the development of waterborne protective coatings.

1  Phosphate Ester Emulsifiers

    Alkyl alcohol ether phosphates are used as basic and efficient anionic emulsifiers in the synthesis of coating emulsions. Phosphate salts can provide typical particle size control, low gel content, and good emulsion stability. By adjusting the ratio of monoesters and diesters, the amount of surfactant and the charge density on the surface of the latex particles can be changed, reducing the amount of emulsifier.

Taking the Rhodofac series of phosphate ester surfactants from Rhodia, France, as an example, they not only improve emulsion stability, improve emulsion water resistance, and increase the adhesion of the paint film to the substrate, but also make the paint film have higher gloss, better anti-blocking properties, stain resistance, scrub resistance, and enhanced pigment dispersion, and can also inhibit corrosion, making them the best surfactants for emulsion synthesis.

Figure 1 below shows a comparison of the corrosion inhibition of pure acrylic clear lacquers with a solids content of 50% prepared using different emulsifiers on metal surfaces. From the figure, it can be seen that the corrosion inhibition of emulsions synthesized using phosphate ester emulsifiers is better than that of sulfate esters. Among phosphate ester emulsifiers, alcohol ether types are superior to phenol ether types, and linear alcohol ether types are superior to branched phenol ether types.

 

    

Figure 1 Comparison of corrosion inhibition of pure acrylic clear lacquers synthesized using different emulsifiers

2  Anti-flash Rust Agents

    When waterborne coatings are applied to metal surfaces, metal oxidation and corrosion occur within minutes during the presence and evaporation of water, a phenomenon known as "flash rust." In appearance, flash rust quickly transforms into rust frost or water frost over large areas. The color of flash rust varies depending on the degree of aging and composition of the metal, and the duration of the "wet state" of the substrate before the paint film dries. The flash rust phenomenon is more serious on sandblasted surfaces and metals with higher carbon content, and anti-flash rust additives must be added to address this.

In the past, sodium nitrite or sodium benzoate were commonly used as anti-flash rust agents. Although they provide short-term anti-flash rust effects, they are gradually being phased out due to their lack of environmental friendliness, poor salt water resistance of the paint film, and other side effects.

The Synthro-cor series of anti-flash rust agents launched by the French company, Innovachem, belongs to this category, see Table 1 below.

Table 1 Synthro-cor Anti-flash Rust Agents for Waterborne Systems

Product	Synthro-cor CE 660 B	Synthro-cor B	Synthro-cor V 609 B
Type	Organic sodium salts	Nitrogen-containing compounds	Organic sodium salts
Performance	Optimal balance between contact flash rust protection, long-term rust protection, and environmental friendliness	Can be used for liquid and gas phase corrosion protection	Balance between contact flash rust protection and in-can corrosion
Suitable Substrates	Iron, polyiron, steel surfaces	Iron and its alloys, copper and its alloys, aluminum, tinplate, etc.	Iron and its alloys, copper and its alloys, aluminum, tin, etc.
Dosage	0.1-0.5%	0.1-0.5%	0.1-0.5%

To compare the anti-flash rust performance of the products in the table above, different anti-flash rust agents were added to a white water-based paint based on Acronal 290 D. The paint was then sprayed onto clean, rust-free, scratched iron plates with a wet film thickness of 80µm. After drying at room temperature for 10 minutes, it was further dried at 50°C for 10 minutes, and the flash rust on the surface of the paint film was observed. The results are shown in Table 2 below.

Table 2 Synthro-cor Anti-Flash Rust Performance Evaluation

Type of Anti-Flash Rust Agent	Active Ingredient / Total Formulation (w/w)
	0.15%	0.30%
Blank Test	Severe Flash Rust	Severe Flash Rust
Synthro-cor CE 660 B	No Rust	No Rust
Comparative Product 1	No Rust	No Rust
Synthro-cor B	Rust	No Rust
Comparative Product 2	\	Rust
Synthro-cor V 609 B	\	Slight Rust

As can be seen from Table 2, Synthro-cor CE 660 B is the best contact-type anti-flash rust agent. Synthro-cor CE 660 B does not contain APEO, VOCs, or nitrites and is non-toxic and harmless. Although Comparative Product 1 has similar performance, it contains nitrites and is not environmentally friendly.

3  Organic Rust Inhibitors

Organic rust inhibitors can replace toxic heavy metal rust-proofing pigments or be used with non-heavy metal rust-proofing pigments to achieve long-term or short-term corrosion protection. Commonly used organic rust inhibitors include alkaline sulfonates, phosphates, sulfonate derivatives, amines, and amine salts; they are generally liquid, easy to use, low in dosage, and do not affect gloss, and can be used in clear coats. Sulfonates or phosphates can form chemical adsorption on the surface of oxidized metals, forming a densely packed molecular layer that effectively prevents the diffusion of metal ions. At the same time, due to their "umbrella" shape, they have a blocking effect, reducing water penetration; they are also highly efficient, and when used with corrosion-inhibiting pigments, they can improve corrosion resistance; when certain corrosion-inhibiting pigments are restricted due to regulations, they can partially or completely replace these pigments without affecting the final corrosion resistance; at the same time, they aid pigment dispersion at low dosages.

Polyethylene glycol ether phosphates have good rust-proofing effects on the surfaces of iron, copper, magnesium, nickel, and other metals. The rust-proofing effect is best when using a fully neutralized product (PH=9.0-9.5). Data shows that using C16-fatty alcohol polyethylene glycol ether phosphate as a corrosion inhibitor is more effective than C16-fatty alcohol phosphate, and C16-fatty alcohol polypropylene glycol ether phosphate is even better. ^[3] Generally speaking, the longer the carbon chain, the larger the non-polar group, the larger the coverage area on the metal surface, the greater the van der Waals force of attraction between the alkyl groups, the greater the cohesive force generated, resulting in more adsorption on the metal surface, a more dense protective film, and higher corrosion inhibition efficiency and better effect. ^[4] 。

Table 3 below shows the Synthro-cor series of organic rust inhibitors for water-based coatings. 　

 

 

Table 3 Synthro-cor Organic Rust Inhibitors for Water-Based Systems

Synthro-cor	CA 50 B	MG 50B	ZN 50B	AM 35B	CU
Type	Calcium sulfonate derivative	Magnesium sulfonate derivative	Magnesium sulfonate derivative	Ammonium sulfonate derivative	Copper sulfonate derivative
Suitable Substrates	Iron and its alloys	Iron and its alloys	Iron and its alloys	Iron and its alloys	Copper and its alloys
Dosage	Used in conjunction with corrosion-inhibiting pigments at 25-50% of the amount of corrosion-inhibiting pigment; if used alone as a corrosion inhibitor, the dosage is 3-8% of the total formulation.

　　Different sulfonate derivatives were used in water-based coatings to replace different proportions of calcium ions in amorphous silica (Shieldex AC 5) and zinc phosphate. The test plates were placed at 35°C and 100% relative humidity for 10 days, and the rusting of the paint film was observed. The results are shown in Figures 2 and 3. The results show that Synthro-cor AM 35 B combined with Shieldex AC 5, or

Synthro-cor Ba 50 MS combined with zinc phosphate, can achieve the best corrosion protection effect.

 

　Figure 2 Effect of Synthro-cor Replacing Zinc Phosphate

Figure 3 Effect of Synthro-cor Replacing Shieldex AC 5

 

4 Wetting Agent                         

Because the surface tension of water is relatively high (72.5 mN/m), more than twice that of general organic solvents, the wetting ability of water-based coatings to the substrate is poor, easily causing surface defects such as pinholes, affecting the adhesion of the coating to the substrate, and thus reducing the corrosion resistance of the anticorrosion coating. ^[5] 。

Phosphate esters are surfactants with low surface tension and low foaming properties, making them uniquely suitable as wetting agents for the substrate in water-based coatings. Generally, the surface tension of phosphate diesters is lower than that of monoesters, and the surface tension of isomeric alcohol phosphate esters is lower than that of normal alcohol phosphate esters. Furthermore, the longer the alkyl chain, the lower the surface tension. Their surface activity often varies with the pH of the solution. Table 4 shows the effect of solution pH on the surface activity of polyoxyethylene dodecyl ammonium phosphate. ^[6] 。

 

 

 

 

Table 4 Effect of Solution pH on Phosphate Ester Surface Activity

 

pH value	6	7	9	11
CMC, mmol/L	5.01	1.51	1.69	5.01
Ycmc, mN/m	34.00	25.50	29.00	30.00

 

The phosphate ester group can form covalent bonds with the metal surface (as shown in Figure 4), significantly improving the adhesion of the coating to the metal surface. Using suitable phosphate ester surfactants as substrate wetting agents alone improves substrate wetting, improves flow, prevents pinholes, avoids haze in clear coats, has a degassing effect, and aids pigment wetting in color coats. Furthermore, phosphate ester surfactants containing a large number of polyoxyethylene groups, polyethers, and other compounds with multiple hydrophilic groups in the molecule provide better improvement in the "upward" wetting of the coating, significantly improving interlayer adhesion.

 

Figure 4 Covalent bond action between phosphate ester group and metal surface

Figure 5 shows the results achieved using Modarez X 043, a phosphate ester substrate wetting agent from Arkema, in a water-based acrylic magnetic paint. It can be seen that adding 0.3% (by weight) of Modarez X 043 allows the water-based acrylic magnetic paint to wet the metal substrate well.

Figure 5 Effect of Modarez X043 on improving metal substrate wetting

 

5 Adhesion-promoting monomer

To improve the adhesion of water-based coatings to metal substrates, adhesion-promoting monomers can be used during resin synthesis to enhance interfacial interaction and reduce surfactant migration. Commonly used adhesion-promoting monomers include allylurea, methacrylamidoethyl ethyleneurea, and phosphate ester functional monomers. Taking Sipomer PAM-100, a methacrylic adhesion-promoting monomer from Rhodia containing a phosphate ester group, as an example, the effects on the dry and wet adhesion of aluminum and phosphatized steel plates are listed in Table 5.

The data in Table 5 shows that when the amount of Sipomer PAM-100 reaches 3% BOTM, the dry and wet adhesion of the coating on the surfaces of aluminum and phosphatized steel plates can achieve very good results.

  Table 5 Improvement of Metal Substrate Adhesion by Sipomer PAM-100

 

Sipomer PAM-100 % BOTM	Coating on aluminum surface		Coating on phosphatized steel surface
	Dry adhesion	Wet adhesion*	Dry adhesion	Wet adhesion*
0	4B	0B	5B	1B
1	4B	3B	5B	3B
2	5B	3B	5B	3B
3	5B	5B	5B	5B
4	5B	4B	5B	4B

 

*Wet adhesion refers to the adhesion tested after the test plate is immersed in water for 24 hours.

6 Conclusion

Phosphate ester emulsifiers, anti-rust agents, organic corrosion inhibitors, wetting agents, and adhesion-promoting monomers, etc., improve the rust and corrosion resistance of water-based metal protective coatings to varying degrees. Water-based additive and coating professionals should consider the rationality, economy, and environmental protection of products to develop, select, and use cost-effective functional additives to ensure and improve the protective performance of water-based coatings, in line with the country's long-term sustainable development strategy.

References

[1] Kong Zhiyuan, The Current Situation and Development of Water-Based Coatings in China, Proceedings of the 6th Water-Based Wood Coatings Technology Seminar and 2008 Water-Based Polyurethane Industry Annual Meeting, May 2008, Shanghai: 19-22

[2] Zeng Fanhui, Chen Hongmei, et al., Study on Salt Spray Resistance of Water-Based Epoxy Anticorrosive Coatings, Proceedings of the 3rd Seminar on Environmental Protection Water-Based Resins and Coatings Technology, December 2004, Wuxi: 115

[3] N-Hunfelt, translated by Su Juhan, et al., Manufacturing, Properties and Analysis of Nonionic Surfactants, Light Industry Press, June 1990, Beijing: 740

[4] Wang Zum, Xu Yup, eds., Amphoteric Surfactants, Light Industry Press, Beijing, October 1990, 132

[5] Zhu Wanzhang, Film Formation of Water-Based Paints, Shanghai Coatings, 2004. (2)