Development of Highly Weathering-resistant Self-cleaning Waterborne Finishing Varnish for Colorful Coatings

AN Wen-Dong, DONG Zhi-Hui, LIU Xiao-jing, ZHAO Chang-yong, LING Qin (Shijiazhuang Paint Company, Shijiazhuang 050051, Hebei, China)

Abstract: Starting from the design concept of high weather resistance and self-cleaning, a waterborne finishing varnish for colorful coatings with high weathering resistance and self-cleaning function is developed. In this paper, the influence of emulsion of the highly weathering-resistant self-cleaning waterborne finishing varnish for colorful coatings on the weathering resistance of film, the influence of surface additive on tarnish resistance, the influence of thickener on the water resistance of film, etc. are emphasized, and its mechanism is discussed.

0 Introduction
Water-based colorful paint can simultaneously display multiple colors through one spray. The pattern makes various colors alternate with each other, and has a decorative effect similar to natural granite or marble. It is mainly used for the interior and exterior walls of high-end buildings. It combines decoration, safety and economy, and has become a hotspot in the research of architectural coatings in recent years.
In appearance, colorful paints are completely comparable to real natural granite or marble, and the decoration can be more diverse and more humane. In terms of comprehensive properties, such as water resistance, weather resistance, and easy cleaning, they are still similar to natural stone. Great gap. As a finishing varnish that is matched with colorful coatings, its main functions are waterproof, stain resistance, UV aging resistance, and improving the overall life of the coating system. Therefore, high-performance finishing varnishes have a very important role in colorful coatings. Its characteristics are high gloss, excellent clarity, good fullness, excellent weather resistance, self-cleaning ability and chemical resistance. With the development of high-rise buildings and high-grade buildings, the long-term aesthetic performance of colorful overall coatings can be effectively improved, which is conducive to further expanding the colorful coatings market.

The products developed in this article also meet T / CPCIF 0001-2017 “Technical Specifications for Evaluation of Green Design Products for Water-based Architectural Coatings”, DB13 / 3005-2017 “Standards for Limits of Volatile Organic Compound Content in Architectural Coatings and Adhesives”, HG / T 5065- The standard requirements of the 2016 “Gloss Varnish for Architectural Coatings”, the appearance meets people’s pursuit of long-lasting beauty and decorative effect of the wall, and also plays a key role in improving the overall coating performance.

1 Experimental
1.1 The main raw materials are
pure acrylic latex, industrial grade, imported; silicon acrylic latex, industrial grade, domestically produced; film-forming aid, industrial grade, Eastman; alkali swelling thickener, polyurethane thickener, industrial grade, Dow; DI water, homemade.

1.2 Experimental instruments
Contact angle tester (CA-100D): Shanghai Yingnuo Precision Instrument Co., Ltd .; high-speed disperser (WJ-1.1), thermal storage drying box (TIK), gloss meter (CR-10, imported), scrub resistance meter (PS2730 ): Shanghai Pushen Chemical Machinery Co., Ltd .; Xenon lamp aging tester (Q-LAB): Imported; Electric blast drying oven (101-1AD), Shanghai Kuntian Laboratory Instrument Co., Ltd .; General high and low temperature humid heat test chamber ( CTH-SG7525-02F): Zhongke Sailing (Beijing) Technology Co., Ltd .; Transmittance Tester (WGT-S): Shanghai Yixin Keyi; Contamination Resistance Tester, Homemade

1.3 Experimental process
1.3.1 Preparation of overcoat varnish

It is experimentally determined that silicon-acrylic latex is used as a film-forming substance, and other components are matched with it to achieve a product formula with outstanding performance and stability. Refer to Table 1 for the reference formula.

Table 1 Reference Formula of Finishing Varnish

1.3.2 Product performance testing
Product performance testing is performed in accordance with the technical indicators in HG / T 5065—2016 “Surface Varnish for Architectural Coatings”. The specific results are shown in Table 2.

Product Performance Test Results

Through product performance testing, all indicators meet the standard requirements. The product’s resistance to artificial aging and stain resistance is far higher than the standard requirements, which fully reflects the product’s design concept of high weather resistance and self-cleaning.

1.3.3 Environmental performance testing

The environmental performance testing of coatings is based on T / CPCIF 0001-2017 “Technical Specifications for Evaluation of Green Design Products for Water-based Architectural Coatings”, DB 13 / 3005-2017 “Volume Organic Compound Content Limit Standards for Architectural Coatings and Adhesives 》 The standard requires testing, and the test results are shown in Table 3.

Table 3 Limits of Harmful Substances in Coatings

1.4 Experimental method
1.4.1 Test method for stain resistance
The matching white colorful paint was sprayed on the asbestos-free fiber cement slab, and after curing for 2 d at normal temperature (23 ± 3) ℃, it was roller-coated with two coats of varnish, the amount was (1 100 ± 0.1) g / dm2, (900 ± 0.1) g / dm2, and then after 7 days of curing at room temperature (23 ± 3) ℃, the performance test is performed. The stain resistance is according to GB / T 9780—2005 “Test Method for Stain Resistance of Architectural Coatings” Test.

1.4.2 Test method for resistance to artificial aging
Spray the matching white and colorful paint on the asbestos-free fiber cement slab. After curing for 2 d at room temperature (23 ± 3) ℃, apply two coats of clear varnish in a roller amount of (1 100 ± 0.1) g / dm2, (900 ± 0.1) g / dm2, and then after 7 days of curing at room temperature (23 ± 3) ℃, the performance test is performed. The artificial aging test is based on GB / T 1865—1997 “Color paint and varnish. Tests under Artificial Weathering and Artificial Radiation Exposure.

1.4.3 Water resistance white test method
Water resistance white test is made by coating a clear varnish on a glass plate (wet film 120 μm), curing at room temperature (23 ± 3) ℃ for 7 days, and setting in a constant temperature water tank. The water temperature is tested. Water resistance is based on the technical indicators and test methods in GB / T1733-1993 “Determination of Water Resistance of Paint Films”.

2 Results and analysis
2.1 The effect of latex selection on the weatherability of the coating film

The ability to clean, and the main film-forming material of this dense protective film is an emulsion, so the selection of the emulsion should consider the following factors: (1) excellent weather resistance, improve product life, reduce the frequency of refurbishment, and save resources. (2) Super strong stain resistance, which reflects the self-cleaning ability of the product. (3) Better transparency, suitable film forming temperature, and smaller particle size. (4) Good physical and mechanical properties and chemical resistance to meet standard requirements. In this experiment, three kinds of emulsions were used to prepare a high-weather self-cleaning water-based colorful paint overcoat varnish. The prepared products were tested for product performance.

Product Performance Test Results

It can be seen from Table 4 that the artificial aging resistance of X1 is significantly better than that of X2 and X3. This is because the bond energy of Si—O of X1 is much larger than that of C—C and C—O bonds, which makes the resistance of silicone-acrylic latex The heat resistance, weather resistance and anti-oxidation ability are obviously enhanced, which can effectively extend the service life of water-based colorful coatings. When drying and forming a film, the siloxane is hydrolyzed and polycondensed to form a solid cross-linked three-dimensional network (—Si—O—Si—) structure between the polymer molecules and between the polymer and the substrate, so that the coating film is formed. Has strong water resistance and adhesion. In addition, the polysiloxane molecules have a helical structure, the methyl groups are arranged outward and rotate around the Si—O chain, the molecular volume is large, and the cohesive energy density is low, thereby making the emulsion highly hydrophobic. Therefore, the performance of the X1 scheme on water resistance and artificial aging resistance is obviously better than the schemes X2 and X3.

2.2 The influence of the choice of surface additives on stain resistance
Today, with the increase of various pollutants in the air, many pollutants are easily adsorbed on the surface of buildings, and the phenomenon of graffiti on the wall is also common. of. Water-based colorful architectural coatings have high technical content, relatively high market prices, and require long service life. Therefore, the requirements for stain resistance and artificial cleanliness are higher than ordinary architectural coatings. Contamination and manual cleaning should have more prominent advantages. The choice of different surface additives directly affects the self-cleaning function of the coating. In this experiment, three surface additives are selected for comparison tests. The test results are shown in Table 5.

Table 5 Test Results of Different Surface Additives

It can be seen from Table 5 that the stain resistance and transparency of the experimental scheme Y1, Y2, and Y3 meet the standard requirements. The water resistance experimental scheme Y1, Y2 is better than Y3, and the alkali resistance Y2, Y3 is better than Y1, and the contact angle of the Y2 scheme. The results are higher than Y1 and Y3. Based on the above product properties, the experimental scheme Y2 meets the requirements, and the stain resistance of the experimental scheme Y2 is significantly better than the experimental scheme Y1 and Y3. This is because the surface additive in the experimental scheme Y2 contains nanometer two. The addition of silicon oxide and nano-silica can greatly improve the stain resistance of the coating film. Therefore, in this experiment, the surface assistant in experiment scheme Y2 was finally selected.

2.3 Effect of thickener selection on water resistance of coating film
Water resistance is also an important performance indicator of high weather resistance self-cleaning water-based colorful finish varnishes. In addition to the factors affecting water resistance, in addition to the effect of latex itself, the effect of thickeners on water resistance is particularly prominent. Therefore, several aspects need to be considered when selecting thickeners: (1) thickening effect, which can make the coating system thicker, weaken fluidity, and prevent sagging during construction; (2) good compatibility with the system, Does not occur after thickening and delamination; (3) Does not affect the comprehensive performance of the coating film, especially water resistance. In this experiment, two types of thickeners, namely alkali-swelling and polyurethane-based thickeners, were selected for comparison tests. The test results are shown in Table 6. The effect of different thickeners on water resistance is shown in Figure 1.

Table 6 Water Resistance Comparison Test
Fig. 1 Water Resistance of Different Thickeners

It can be seen from FIG. 1 that the alkali swelling thickener is obviously whitened after soaking in water, but the polyurethane thickener has no whitening after soaking in water.
From the test results in Table 6, it can be seen that the water resistance and alkali resistance of the Z2 scheme are significantly better than those of the Z1 and Z3 schemes, and there is no significant difference in storage stability. The effect of water resistance on the high weather-resistant self-cleaning water-based colorful varnish is particularly significant. Importantly, therefore, the polyurethane thickener of experiment scheme Z2 was selected for this experiment.

3 Conclusions
(1) Silicone-acrylic latex is used as the film-forming material in the water-based colorful paint overcoat varnish, which can effectively improve the artificial aging resistance and water resistance of the coating.
(2) Adding a nano-silica surface additive can greatly improve the stain resistance of the coating film.
(3) The effect of thickeners on water-based colorful coatings and gloss varnishes is manifested in water resistance. Polyurethane thickeners have obvious advantages in this regard.
At present, in many engineering cases, the colorful paint has replaced the real stone paint market, and has failed to replace the high-end natural stone exterior wall system in large numbers as expected. One of the important reasons is that the comprehensive performance of the colorful system is far from natural stone. The high-performance water-based colorful paint overcoat varnish can give the colorful system better performance, making it possible for colorful paint to replace some natural stone. Helps increase market share in the exterior coatings industry.

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