Wang Yaji, Liu Chong, Gao Hao, Jiao Jian
(Hengshui Xinguang New Material Technology Co., Ltd., Hengshui, Hebei 053011, China)
Abstract : Additives can effectively improve the stability and coatability of the waterborne industrial coatings, and have significant effects on the protective performance of the coated products.In this article, the effects of different pH regulator, dispersing agent, wetting agent and antirust agent on the protective performance of the film so as to obtain the optimal types of additives.
As the environment becomes worse, the intensity of environmental governance is increasing. On January 26, 2015, the Ministry of Finance and the State Administration of Taxation issued the “Notice on Levying Consumption Tax on Batteries and Coatings”, with an applicable tax rate of 4% and a volatile organic compound (VOC) content of less than 420g / L (including ) Coatings are exempt from consumption tax. At the same time, the Law of the People’s Republic of China on the Prevention and Control of Air Pollution (2015 Revision) also began to be implemented in 2016. Solvent-based coatings that are widely used in the field of rust prevention will be greatly restricted.Under the increasingly stringent environmental protection measures, coatings companies are also facing new opportunities, namely, water-based coatings. This paper discusses the effects of the main additives on the performance of water-based antirust primers prepared with the environmentally friendly antirust pigment zinc phosphate.
1 Experimental process1.1 Preparation of water-based antirust primer
1.1.1 Composition of the formula.Antirust primer formula is shown in Table 1.
1.1.2 Preparation process
Deionized water, dispersant, wetting agent, defoaming agent, propylene glycol, AMP-95 are mixed evenly, and iron oxide red, modified zinc phosphate, wollastonite powder are added in sequence under stirring, and mixed at high speed for 20 minutes, and ground to fineness ≤30 μm, add the emulsion, slowly add dipropylene glycol butyl ether, wetting agent, anti-flash rust agent, thickener, mix well to make up the remaining water. Filtering and passing the test are finished products.
1.2 Performance testing
According to HG / T 4758—2014 “Waterborne Acrylic Resin Coating” type II primer requirements for routine testing, and GB / T1771—2007 “Test for neutral salt spray resistance of colored paints and varnishes”, The test results are shown in Table 2.
2 Results and discussion
2.1 Anti-rust mechanism
In antirust coatings, once the resin is selected, the antirust pigment will greatly affect the protective effect. Antirust pigments are generally divided into chemical antirust pigments and physical antirust pigments. Chemical antirust pigments can form a passivation film on the coated metal surface, or form a metal soap to prevent corrosion, such as red dan, zinc and chromium Yellow, zinc phosphate, etc. The physical anti-rust pigment fills the paint film structure with its fine particles, which improves the denseness of the paint film, acts as a shield, reduces the permeability of the paint film, and thus acts as an anti-rust. Such as iron oxide red, mica, glass flakes, etc. In view of the fact that lead- and chromium-containing rust preventive pigments cause serious pollution to the environment and endanger human health, and considering the high dielectric constant of water, we have selected modified zinc phosphate and iron oxide red as the rust preventive pigment.
The chemical formula of zinc phosphate is Zn3 (PO4) 2 · 4H2O. It is a colorless orthorhombic crystal or white microcrystalline powder with a density of 3.99 g / m. It is soluble in inorganic acid, ammonia and ammonium salt solutions, and is almost insoluble in water. Its rust prevention mechanism: In the initial stage of rusting, local anodes and cathodes are generated at the damage. The acid from hydrolysis reacts with zinc phosphate particles near the coating of the corrosion zone to generate phosphoric acid and zinc ions. Zn2 + forms insoluble complex with Fe2 + Substances, used as cathodic inhibitors, to form cathodic protection. Phosphoric acid interacts with the metal surface to form an insoluble iron phosphate that acts as a protective agent to achieve rust prevention. At the same time, zinc phosphate reacts with carboxyl and hydroxyl groups in the emulsion to form a complex, which forms a tight protective film with the corrosives, thereby protecting the metal substrate from corrosion.
2.2 Selection of pH regulator
In water-based coatings, a suitable pH value is very important for pigment dispersion and emulsion stability. Generally, the pH value is adjusted to 8-9, and the auxiliary agents used are generally ammonia and organic amines. However, the antirust pigment zinc phosphate selected in this system will be affected by ammonia or ammonium salt, the solubility will change, and the pH value of the system will decrease. The pH values of reagent-grade zinc phosphate and commercially available zinc phosphate in ammonia and AMP-95 were compared, as shown in Table 3.
After 7 d normal temperature test and thermal storage at 50 ℃ for 7 d test, we can see that after adjusting the pH, the pH value of the modified zinc phosphate will decrease greatly with the increase of storage time. Considering AMP- 95 has the advantages of small odor and low volatility, which is more conducive to the stability of the pH value of the system. At the same time, it has the functions of dispersing and anti-flash rust. AMP-95 was selected as the pH regulator of the system.
2.3 Selection of dispersant
The disadvantages of zinc phosphate as a chemical anti-rust pigment are its low solubility and slow hydrolysis rate. Modification treatment is required during application. Particle miniaturization is the most basic means to improve the activity of zinc phosphate pigments. Similarly, the dispersion fineness and dispersion stability of the modified zinc phosphate in the system are also important factors determining the protective performance of the coating film. We compared the ammonium salt dispersant, homopolymeric sodium salt dispersant, hydrophobic copolymerized sodium salt dispersant and non-ionic dispersant. The experimental results are shown in Table 4.
Note: A—ammonium salt dispersant; B—sodium salt homopolymer dispersant; C—hydrophobic sodium salt copolymer dispersant; D—nonionic dispersant.It can be seen from Table 4 that a more hydrophobic dispersant can effectively reduce the grinding fineness, but because it is greatly affected by the cations eluted in the system, the storage stability is poor, and it also affects the salt spray resistance. The hydrophilic dispersant is stable in storage, but has high grinding fineness and is not ideal for water and salt spray resistance. Using a stable nonionic dispersant and ammonium salt dispersant can achieve a good coating film effect, as shown in Figure 1 (400 times magnification).
2.4 Selection of wetting agent
In high surface tension water-based systems, the use of wetting agents is necessary to ensure the stability of the coating and good application performance. Wetting agents can effectively improve the surface energy of liquid / gas and liquid / solid, and affect the adsorption of dispersant on powder. The surface energy and liquid surface tension of common substrates are shown in Table 5.
It can be seen from Table 5 that water cannot completely wet the steel substrate. Only the substrate wetting agent can spread well on the substrate to form a perfect coating film. We selected the non-ionic wetting agent TO-8 in the formula in combination with the macromolecular substrate wetting agent TEGO 245, and exerted their respective maximum effects through the order of addition.
2.5 Selection of flash rust inhibitor
Since the water-based paint itself is a conductor, a charge migration channel is formed between different regions where a potential difference exists, so that corrosion occurs within a short time of the film-forming process of the water-based paint and flash rust is formed. Obviously, the longer the wet paint film is kept, the more severe the flash rust phenomenon can be, which can explain the reason why the flash rust phenomenon is more serious at low temperature and high humidity.Method for selecting anti-flash rust agent: Use a 150 mm × 70 mm sandblasted steel plate, apply a 150 μm wire rod, and dry film thickness (50 ± 5) μm. After horizontally standing for 24 hours, observe the surface of the paint film for rust penetration. Immediately after soaking the test plate in a solution of xylene and butanol 3: 1 to remove the paint film, observe the substrate for rust spots. If there is no rust penetration on the surface of the paint film and no rust spots on the substrate, it means that flash rust suppression is acceptable.This water-based rust-preventive primer has chosen Raybo60 as the anti-flash rust agent, which has obvious anti-flash rust suppression effect and has no adverse effect on other properties of the coating.
This article mainly introduces the effects of pH adjusters, dispersants, wetting agents and anti-flash rust agents on the performance of coatings in water-based antirust primers. conclusion as below:(1) AMP-95 is used as the pH adjuster of the system, with low odor and good stability, and has the functions of dispersing and anti-flash rust;(2) Using a non-ionic dispersant with good stability and ammonium salt dispersant can achieve the best grinding fineness and storage stability;(3) The water-based rust-preventive primer prepared by using TO-8 and TEGO 245 together and prepared in the specified order has good construction properties;(4) Select Raybo60 anti-flash rust agent, the water-based paint prepared has excellent flash rust suppression effect.