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Abstract: A water-borne alkyd lotion was used as the raw material to prepare water-borne alkyd antirust primer, which was compared with the performance of solvent based alkyd resin antirust primer (anti-corrosion, dryness, mechanical performance, workability, and compatibility). And the performance of coatings composed of different formulations was analyzed and discussed.

0 Introduction

Since the beginning of the 21st century, with the rapid development of the global economy and the gradual strengthening of environmental protection efforts by governments around the world, countries around the world have attached great importance to promoting environmentally friendly coatings and restricting the use of solvent based coatings. Due to the serious air pollution and direct harm to human health caused by the organic solvents evaporated from solvent based coatings, especially the smog pollution that has been criticized in China in recent years, it has aroused extreme attention from the government and the public to air pollutants. The paint industry is developing on a large scale towards water-based, low VOC (volatile organic compound) content, and eco-friendly direction.

Today, vigorously developing water-based coatings is not only the trend, but also imperative. Starting from February 1, 2015, if the VOC volatilization of paint products exceeds 420 g/L, the government will impose a consumption tax of 4% on the paint price. After July 1, 2016, Guangdong Province implemented a “ban on oil and water” policy. In 2017, more and more regions have enacted local legislation to restrict the use of solvent based coatings, which has become a wildfire. Especially in the traditional field of solvent based coatings, the pressure of water-based coatings is increasing in the industrial paint category.

In the field of industrial paint, the most widely used in the market is alkyd coatings. It has good physical and mechanical properties, excellent durability and corrosion resistance, good substrate adaptability and coating construction, and its cost is also relatively low. The production of alkyd resin coatings accounts for 60% to 70% of the total production of solvent based coatings, and occupies an extremely important position in the coatings industry. Therefore, achieving the hydration of air drying alkyd coatings is an important development direction for the coatings industry.

Due to the continued demand from customers for the final workability and physicochemical properties of water-based alkyd coatings, as well as traditional solvent based coating products, practitioners need to consider them in a targeted manner. To meet the demands of this market and satisfy customer needs, this study analyzed the formula of water-based alkyd primer and conducted performance comparisons through experiments.

1. Experimental section

1.1 Main raw materials and testing instruments

Main raw materials: water-based alkyd dispersion lotion, domestic; Dispersant, domestically produced; Iron red, domestically produced; Organic silicon defoamer, imported; Environmentally friendly water-based drying agent, imported; PU thickener, imported.

Experimental instrument: AJ-100 electronic balance, Swiss METTLER company; CM-1100 sand mill disperser, Shanghai Saijie Chemical Equipment Co., Ltd; QGS paint film drying time tester, Dongguan Wanguang Weida Experimental Machine Co., Ltd; Electric hot air drying oven, Tianjin Tianyu Experimental Instrument Co., Ltd; Salt fog box, Q-Lab ； Paint flow tester, Taiyou Company, Japan; QTX paint film flexibility tester, Shanghai Pushen Chemical Machinery Co., Ltd.

1.2 Preparation of Coatings

The basic formula is shown in Table 1.

According to the basic formula in Table 1, adjust the proportion of filler and resin lotion, and prepare three samples with PVC (pigment volume concentration) of 40, 45, and 50 respectively.

Painting process:

After mixing raw materials 1-8 evenly, disperse and grind them with a sand mill disperser until the fineness is ≤ 10 μ m. Then, add raw materials 9-14 under medium speed stirring and stir evenly to obtain the finished coating product.

1.3 Preparation of Samples

Add an appropriate amount of deionized water in a certain proportion to the prepared coating, adjust the viscosity of the product to about 30 seconds (coating -4 cups), and use an air spray gun to prepare the plate.

(1) Dry, physical and mechanical performance testing sample: Spray the sample paint onto a polished tinplate surface, and control the dry film thickness to (20 ± 3) μ m/pass.

(2) Water resistance and salt water resistance test sample: Spray the sample paint on the surface of a manually polished 1mm thick steel plate, and control the dry film thickness (40 ± 5) μ m/2 layers.

(3) Neutral salt spray resistance test sample: Spray the sample paint on the surface of a 1 mm thick steel plate that has been sandblasted to Sa 2.5 level, and control the dry film thickness (40 ± 5) μ m/2 layers.

1.4 Paint film performance testing standards

The performance testing standards for paint film are shown in Table 2.

The neutral salt spray resistance test is one of the main methods for detecting the corrosion resistance of coatings. The painted sample needs to be placed under constant temperature and humidity conditions for 7 days before being placed in a neutral salt spray box. Take out the sample and rinse it with water every 24 hours, observe the surface condition of the coating, and then place the sample again in the salt spray box to repeat the test until the coating shows bubbling, rusting, peeling, and other conditions. The salt water resistance test involves immersing the sample in a 3% NaCl aqueous solution, washing the sample with water every 24 hours, and observing the surface condition of the coating.

2 Results and Discussion

2.1 Coating performance testing results

The results of coating performance testing are shown in Table 3.

2.2 Effects of waterborne alkyd resin

The synthetic methods of waterborne resin can be divided into lotion polymerization (primary dispersion) and solvent polymerization (secondary dispersion). Primary dispersion has the characteristics of high relative molecular weight and fast drying, but the paint film has breathability. After drying, fine pores are formed on the surface of the paint film, which is not conducive to corrosion prevention. The secondary dispersion has small relative molecular weight, compact surface state and good mechanical stability of lotion. As the steel structure coating industry places greater emphasis on the anti-corrosion performance of coatings, it is required that there should be no rusting, blistering, or peeling after coating for a long period of time. Therefore, secondary dispersion is more in line with application requirements, and this study also used secondary dispersion.

Different from the film forming mechanism of solvent alkyd resin, the water-based lotion needs to wait for the water to evaporate completely before the polymerization of the resin molecule. After the lotion particles are close to each other, the resin body will be mixed and polymerized (Figure 1). The process is slower than that of solvent based resin. Moreover, due to the role of the emulsion, it may be spread unevenly, which will affect the corrosion resistance. Therefore, this study focuses on investigating the drying performance and water resistance of coatings. According to Table 3, water-based alkyd primer takes a long time and dries slowly during the initial water evaporation process. Especially when PVC is low and lotion content is relatively high, this problem is more obvious. So it is recommended to add fan dehumidification and low-temperature baking equipment during construction.

2.3 Influence of pigment volume concentration

The formula design of this study will change the element setting to the volume concentration of pigment (PVC), and investigate the changes in coating performance under different PVC. Under the same coating and different PVC conditions, the product performance generally deteriorates gradually with the increase of PVC. The experimental results covered by this study basically conform to this pattern. As PVC increases, the salt spray resistance, impact resistance, and other properties of water-based alkyd anti rust primer gradually decrease. Compared with solvent based coatings, water-based alkyd coatings have slightly weaker corrosion resistance at the same PVC value. When PVC=45, water-based alkyd coatings can achieve qualified anti-corrosion performance, while solvent based alkyd coatings can still meet anti-corrosion requirements when PVC=50. This is particularly evident in the resistance to saltwater immersion and salt spray. The larger the PVC, the more fillers there are, the less film-forming substances there are, and the coating cost decreases. Therefore, water-based coatings with the same performance will have slightly higher costs compared to solvent based coatings. But in terms of environmental friendliness, water-based coatings are easier to achieve low VOC content. Moreover, the drying agent for water-based alkyd coatings is controlled to use lead-free and chromium free products at the source, which easily meets the requirements of international environmental standards such as Rohs.

3. Conclusion

In a word, in terms of film appearance, water resistance, salt spray resistance, mechanical properties, paint manufacturing process and coating construction process, water-based alkyd primer products can be very close to solvent based alkyd primer products, and the antirust primer prepared with domestic water-based alkyd lotion can completely meet the anti-corrosion technical requirements of existing solvent based alkyd antirust primer. Based on this, customers are more likely to accept the performance and construction process of such products during the process of coating hydration. If it can be applied in the field of industrial steel structure coating, it can better meet customer needs and has high market promotion value.