Preparation of Waterborne Mastic Primer-topcoat Coatings for Railway Freight Cars and Their Properties

LIU Wei-zhen, YIN Xiao-yu, HAN Kang, WANG Wei-qiang (Shijiazhuang Paint Company, Shijiazhuang 050051, Hebei, China)

Abstract: A kind of waterborne mastic primer-topcoat coatings was prepared with modified waterborne resin as the main film forming material, high-performance environmentally friendly anti-rust pigments, organic corrosion inhibitors and other functional additives. The effects of anti-rust pigments, organic corrosion inhibitors and other functional additives on coatings and the properties of coatings film were discussed. The results indicated that the waterborne mastic primer-topcoat coatings prepared in the experiment had excellent corrosion resistance and good mechanical properties. In addition, the fast-drying and easy-to-apply coatings could meet the requirements of railway freight cars.

0 Foreword
From 2018 to 2020, the Railway Corporation plans to purchase 216,000 railway wagons The coatings for railway wagons ushered in a great opportunity for development. According to literature data, the number of railway wagons in China has reached 890,000 in 2018. In addition, about 30,000 tons of paint are needed each year for the construction and overhaul of trucks. At present, most of the coatings used for railway trucks are traditional low-solid solid solvent-based coatings with high volatile organic compound (VOC) content. During the production and use of coatings, these VOCs are emitted into the air and participate in the atmosphere. The photochemical reaction caused pollution to the atmosphere. With the increasingly strict national environmental regulations and people’s increasing environmental awareness, the waterborne coating of railway wagons will be fully launched in 2019. Therefore, it is urgent to research and develop water-borne thick-slurry coatings that meet market demands.

In 2007, water-based coatings were tried out on some truck models, and the test models gradually expanded. After several years of running tests, the water-based technology for railway truck coatings has matured [3-5]. The water-based thick paste bottom-in-one coating developed in this paper uses modified water-based resin as the main film-forming material, supplemented with high-performance environment-friendly composite anti-rust pigments and functional additives, with fast drying speed, good initial water resistance, mechanical properties and salt resistance Excellent fogging performance, the thickness of dry film can be 100-120 μm in one coat on the surface of railway car body or accessories, which can meet the requirements of truck painting, and can effectively solve the problem of long railway truck painting cycle.

1 Experiment
1.1 Main raw materials
modified water-based resin, Japan; Neutralizer, BASF; Dispersant, wetting agent, rheological additive, BYK; Defoaming agent, Evonik; Anti-settling agent, Hemings-Germany Qian; Organically modified zinc-aluminum-phosphate molybdenum hydrate, Germany; zinc phosphate and organic corrosion inhibitor, France; sericite powder, Luzhou, Anhui.

1.2 Main equipment
GFJ-04 high-speed dispersing machine, Shanghai Modern Environment Engineering Technology Co., Ltd .; QSMⅡ sand mill, Tianjin Jingkelian Material Testing Machine Co., Ltd .; JJ224BC electronic balance, Changshu Shuangjie Testing Instrument Factory; QAG coating film sag meter , Tianjin Yonglida Material Testing Machine Co., Ltd .; 101-A Electric Blast Dryer, Tianjin Taisite Instrument Co., Ltd .; CTH-SG7525-02F Universal High and Low Temperature Humidity Test Chamber, BGD881 Salt Spray Corrosion Test Chamber, Guangzhou Standard Geda Experimental Instrument Supplies Co., Ltd .; UVTEST11W UV aging test chamber, Atlas, USA.

1.3 Basic Formulas

The basic formulas of water-based thick bottom-in-one coating are shown in Table 1.

Table 1 Basic Formula of Waterborne Mastic Primer-topcoat Coatings

1.4 Preparation of water-based thick paste bottom-in-one coating
Weigh in accordance with the formula in Table 1, add the modified water-based resin to the paint mixing tank, and slowly add the formulating amount of neutralizer, dispersant, and defoamer in order at 500 r / min. And an appropriate amount of deionized water, continue to stir for 3 to 5 minutes to obtain a uniform dispersion; add pigments, fillers and anti-settling agents to the above dispersion while stirring, and after stirring, stir at 1 500 r / min for 10 to 20 minutes , Grind to a fineness of ≤45 μm; then slowly add wetting agent, leveling agent, rust preventive, flash rust preventive, rheological additive and remaining deionized water in the formula while stirring at 800 r / min. Continue to stir for 5-10 minutes to obtain a uniform paint slurry, filter with 160 mesh silk cloth, and pack for future use.

1.5 Preparation of sample plate
Before painting, the grease, rust and oxide scale on the surface of the substrate should be removed to meet the requirements of Sa 2.5 level specified in GB / T 8923.1-2011, and the surface roughness should be controlled within the range of 30 to 80 μm. Remove sand, dust and other impurities on the clean surface before painting. Prepare the template according to 6.2 spraying method in GB / T 1727-1992. When testing the resistance to salt spray and artificial accelerated aging, the dry film thickness is (110 ± 10) μm, and the dry film thickness of other items is (45 ± 5) μm. The salt spray resistance and artificial accelerated aging resistance samples were measured after drying for 14 days at the temperature and humidity specified in GB / T 9278-1998, and other properties were measured after drying for 7 days.

1.6 Technical indicators and test results
According to TJ / CL 252.2-2012 “Technical Conditions for Water-soluble Paint for Railway Freight Cars (Unified on the Underside) (Interim)” issued by the former Ministry of Railways, the performance of the products was tested. The results are shown in Table 2.

Table 2 Performance Indicators and Test Results of Waterborne Mastic Primer-topcoat Coatings

2 Experimental results and discussion
2.1 Selection of matrix resin
At present, the coating of railway wagons is mainly one-component self-drying coatings. In this experiment, several water-based resins commonly used in industrial coatings are selected: water-based alkyd resin (1 #, Tongde) , Water-based epoxy ester resin (2 #, Wanxia), water-based acrylic resin emulsion (3 #, Dow), modified water-based resin (4 #, Japan), paint according to the basic formula, and according to the standard TJ / CL 252.2 —2012 requires the coating to be tested, and the effects of different resin systems on the dryness, initial water resistance, and salt spray resistance of the coating are compared and analyzed. The results are shown in Table 3.

Table 3 Effects of Different Resin Systems on the Resistance of Coatings

It can be known from Table 3 that the water-resistant alkyd resin system 1 # and the water-based epoxy ester resin system 2 # have good anticorrosive properties, but because of their slow drying speed and poor initial water resistance, they cannot meet the requirements of the assembly line operation of railway wagon coating; Resin system 3 # has fast drying speed and good initial water resistance, which is suitable for assembly line operation, but its salt spray resistance is poor and cannot meet the salt spray resistance requirements of truck coatings; modified water-based resin system 4 # has fast drying speed and initial water resistance Good, excellent corrosion resistance, can meet the performance requirements and coating construction requirements of railway wagon coatings. In this experiment, a modified water-based resin (4 #) was selected as the main film-forming substance for a water-based thick paste bottom-in-one coating. This resin is a modified high-molecular epoxy resin with good corrosion resistance, fast drying speed, and initial water resistance. Good, can meet the requirements of high anticorrosion and easy construction of truck coatings.

2.2 Selection of anti-rust system

In this experiment, zinc phosphate and organically modified zinc-aluminum-phosphate molybdenum hydrate were used to form anti-rust system 1 #. Zinc phosphate, organically modified zinc-aluminum-phosphate molybdenum hydrate, and organic slow-release agent were used in combination. The antirust system 2 # was used to evaluate the anticorrosive performance and water resistance of the coating film, and compared with the antirust system 3 # composed of traditional chromium pigments and zinc phosphate. The experimental results are shown in Table 4.

Table 3 Effects of Different Resin Systems on the Resistance of Coatings

It can be known from Table 4 that when the antirust system 2 # composed of zinc phosphate, organically modified zinc aluminum molybdenum hydrate and organic corrosion inhibitor is used, the salt spray resistance and water resistance of the coating can be significantly improved. The system does not contain heavy metals, which can effectively avoid the pollution of heavy metals to the environment in the production and application of products. Comparing antirust system 1 # and antirust system 2 #, it can be seen that the addition of organic corrosion inhibitors has significantly improved the salt spray resistance and water resistance of the coating film. This is because the molecules of the organic corrosion inhibitor are composed of two parts, polar and non-polar. The polar groups can be firmly adsorbed on the surface of the metal substrate, and the non-polar groups can penetrate into the coating, thereby forming A dense single-molecule directional adsorption layer is formed between the substrate and the substrate, which effectively blocks the corrosion of corrosive media such as water and oxygen, and plays a major role in the pre-corrosion of the metal substrate. The anticorrosive mechanism of phosphate-based antirust pigments is mainly the hydrolysis of phosphate ions to generate zinc hydroxide and secondary phosphate ions. The hydrolysis products form adhesion and corrosion inhibition complexes, which cause the surface of the metal substrate to be phosphated and formed in the anode range. A particularly effective protective layer inside [6]. The experiments show that there is a synergistic effect among the three rust inhibitors of the rust prevention system 2 #, which can significantly improve the corrosion resistance of the coating.

2.3 Selection of thixotropic system
Thickeners commonly used in water-based coatings can be divided into inorganic minerals, polyacrylates, polyurethanes, and celluloses according to their components. In this experiment, thixotropic system 1 # was composed of organically modified hectorite clay with strong thixotropy and associative polyurethane thixotropic agent with good leveling and splash resistance. Compared with the use of inorganic mineral thixotropy alone System 2 # and synthetic polymer thixotropic system 3 #. The effects of different thixotropic systems on coating properties were investigated. The results are shown in Table 5.

Table 5 Effects of Different Thixotropic Systems on the Leveling, Sagging and Storage Properties of Coatings

As can be seen from Table 5, the thixotropic system has a relatively large impact on the leveling, sagging and thermal storage stability of the coating. Inorganic thixotropic system 2 # has strong thixotropy, and under the low-shear state of static storage, it has a significant thickening effect on the coating. This is because organic modified hectorite clay is a water-containing layered aluminosilicate mineral. The negative charge between the layers can adsorb water and organic polar groups. After it is dispersed uniformly in water, the subsequent addition of modified aqueous resin, the polar groups such as hydroxyl groups and ether bonds in the molecular chain are attracted by the negative charge between the organic modified hectorite clay layers and inserted between the layers. Formation of organic modified hectorite clay-organic polymer network structure, which makes the organic modified hectorite clay swell, which increases the viscosity of the coating and shows good sag resistance and storage stability , But poor leveling, can not be used alone in this experiment. Synthetic polymer thixotropic system 3 # is an associative polyurethane thixotropic agent. The thixotropic agent has the advantages of good leveling, good splash resistance, and small impact on gloss. However, when used alone, the anti-sag resistance and Poor heat storage stability. In this experiment, an organic modified hectorite clay and an associative polyurethane thixotropic agent were used to form a thixotropic system 1 #. This system combines the strong thixotropy of inorganic minerals and the leveling property of associative polyurethanes. Good storage stability, good thick coating, can meet the coating requirements of railway wagons.

2.4 Determination of paint base ratio 

In the paint formula, the pigment base ratio (P / B) refers to the mass ratio of the pigment to the base material, and the paint base ratio of the coating has water resistance, salt spray resistance and aging resistance. Performance and mechanical properties have a greater impact. This experiment investigated the effects of coatings on water resistance, salt spray resistance, and artificial accelerated aging resistance by designing different pigment-to-base ratios. The results are shown in Table 6.

Table 6 Effect of P / B Ratio on the Water and Salt Spray Resistance and Aging Resistance of Coatings

It can be known from Table 6 that the water resistance and salt spray resistance and artificial accelerated aging resistance of the coating show a trend of first increase and then decrease with the increase of P / B. This is because when P / B is low, most of the pigments The fillers are all wrapped by the resin base material. The performance of the coating film is mainly determined by the resin, and the polymer resin materials generally have a certain water absorption and swelling, which results in the coating film being water-resistant and resistant to light loss and discoloration during artificial accelerated aging. In addition, because the anti-rust pigment in the coating film is relatively low, and most of it is covered by the matrix resin, its hydrolysis reaction is limited, and it cannot effectively form a complete and dense corrosion-resistant layer on the surface of the metal substrate, resulting in salt resistance of the coating film. Poor fogging performance; with the increase of P / B, the proportion of pigments and fillers in the coating film increases, the gloss of the coating film itself decreases, and some “naked” rust-proof pigments are hydrolyzed, and the hydrolysis products form adhesion and corrosion barriers The compound promotes the adhesion, water resistance and salt spray resistance of the coating film, which is manifested by a significant improvement in the water resistance and salt spray resistance of the coating film; but when the P / B is too large, the resin base cannot The pigments and fillers are fully wetted and dispersed, and the bonding effect is limited, resulting in a discontinuous and dense coating film, which cannot effectively block the penetration of water molecules, oxygen and other corrosive media, resulting in water resistance and salt spray resistance of the coating film. The performance and resistance to artificial accelerated aging are reduced. In summary, when P / B is set to 2.2, the comprehensive performance of the coating is optimal, which can meet the requirements of truck coatings for thick slurry coatings such as water resistance, salt spray resistance and artificial accelerated aging resistance.

3 Conclusions
(1) Through experimental research, the waterborne properties of railway wagons prepared by using modified water-based resins as the main film-forming material and adding zinc phosphate, organically modified zinc aluminum molybdenum phosphate hydrate, organic corrosion inhibitors, and other functional additives Thick slurry bottom coating, all properties meet the requirements of industry standard TJ / CL 252.1-2012.
(2) The prepared water-based thick paste bottom-in-one coating uses high-performance environmentally-friendly rust-proof pigments and organic corrosion inhibitors as the rust-prevention system, and has excellent anti-corrosion performance. It can replace the traditional chromium-based rust-proof pigments and is environmentally friendly. Low content, meets the requirements of environmentally friendly coatings.
(3) The prepared water-based thick slurry bottom surface one-in-one coating has fast drying speed, good initial water resistance, easy construction, and can meet the requirements of railway wagon assembly line coating operations.

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