Development of Ultra-high Solid Epoxy Prime

TIAN Xue-hui, SUN Teng-yuan, HE Zhi-tong, CHEN Zheng, LI Tian-hang (Shijiazhuang Paint Company, High Solids Coatings Technology Innovation Center of Hebei Province, Shijiazhuang 050051, Hebei, China)

Abstract: This paper introduces the method of preparing ultra-high solid epoxy primer, focusing on the process of screening epoxy resins, fillers, dispersants and curing agents. The prepared coatings’ performance meets national standard HG/T 4566— 2013 Epoxy Resin Primer, and the solid content and VOC content meet group standard T/CNCIA 01005—2018 Definition of Low VOCs Content High Solids, Ultra-high Solids and Solvent-free Epoxy Coatings. Such coatings meet the national environmental protection requirements, and have broad market prospects, economic benefits and social benefits.

0 Preface
China’s existing coating products are mostly low to medium Solid-based solvent-based products use a large variety of organic solvents during production and use. The content of volatile organic compounds (VOC) is high. When used, it can not only cause changes in atmospheric oxidation capacity and acidity, but also cause chemical smog. It seriously damages the human living environment, endangers public health, and causes a large amount of solvent resources to be wasted.

In recent years, China has gone from an increasingly severe “fog battle” to the “most stringent environmental protection law” in history, to the restrictions imposed on the sales and use of traditional low-solid solid solvent coatings in some regions. The introduction of the policy is showing a strong signal that environmentally friendly coatings that meet performance requirements have become the main theme of today. At present, mainstream environmentally friendly products can be roughly divided into 4 categories: powder coatings, radiation-curable coatings, water-based coatings, and high-solids coatings (where solvent-free liquid coatings are the ultimate development of high-solids coatings). However, powder coatings require high-temperature baking and curing, radiation curing requires special curing equipment, and relatively high requirements for construction conditions, which restrict their development in the field of surface anti-corrosion coatings; water-based coatings are extremely likely to cause flash rust during the construction process. In addition, compared with solvent-based coatings, there is still a certain gap between the anti-corrosion performance of the coating film and the application field of high anti-corrosion requirements is limited. High solid coatings have the advantages of low VOC emissions, convenient construction, and improved work efficiency, etc., and are widely used in areas with high requirements for corrosion resistance.

Ultra-high solids coatings have stricter VOC requirements and are the first choice for coatings with fugitive emissions. In this experiment, an ultra-high solid epoxy primer was prepared by screening epoxy resin, filler, dispersing aid and curing agent. It has the advantages of low VOC emission, excellent salt spray resistance and good adhesion. It is used in steel structures, tank inner walls, ballast tanks and other fields with high anticorrosive performance requirements, and has broad market prospects, economic benefits and social benefits.

1 Experimental part
1.1 Experimental ideas
T / CNCIA 01005—2018 “Definition of Low VOCs, High Solids, Ultra High Solids, and Solvent-free Epoxy Coatings” states that the mass fraction of non-volatile matter under standard construction conditions measured by prescribed methods is greater than 88%, and Solvent-based epoxy coatings with a volatile organic compound content of less than 150 g / L are ultra-high solid epoxy coatings.
The calculation formula for the theoretical liquid paint VOC (g / L) is shown in formula (1).
VOC = (1-ω) × ρ × 1 000 (1)

In the formula: ω——mass solid content,%;
ρ——density of liquid coating, g / mL.
From the calculation formula of liquid coating VOC, it can be seen that for the preparation of ultra-high solid coatings, the reduction of VOC needs to consider both the solid content of liquid coatings and the density of liquid coatings.
Therefore, when designing the formulation, the experimenter needs to consider how to find a balance between improving the solid content of the coating and reducing the density of the liquid coating. Through the accumulation and analysis of a large number of previous experimental data, it is concluded that the following three aspects need to be considered: (1) the use of high-solid low-viscosity resins and curing agents; (2) in addition to the necessary pigments, as little as possible or low-density pigments Filler; (3) Adjuvant or reactive diluent with viscosity reducing effect.

1.2 Experimental raw materials
128 epoxy resin, Kunshan Nanya; BE188 epoxy resin, Changchun Chemical; SM618 epoxy resin, Jiangsu Miki; iron oxide red, Xinxiang, Henan; zinc phosphate, Shijiazhuang; feldspar powder, Luzhou; wollastonite powder, Beijing; Precipitated barium sulfate, Shenzhou; dispersant, Afcona; dispersant, BYK; dispersant, radiance; leveling agent, BYK; defoamer, Afcona; anti-settling agent, Sanmei; coupling agent, Capatue; curing agent , Junjiang; curing agent, Cardolite.

1.3 Experimental equipment
Sander, QSM-Ⅱ, Tianjin Jingkelian Material Experimental Machine Co., Ltd .; Multi-functional mixer, U450 / 80-220, Shanghai Weida Industry and Trade Co., Ltd .; Electric constant temperature air drying oven, TST202A-1B, Chengdu Tester Instrument Co., Ltd .; Tu-4 # Viscometer, NDJ-5, Shanghai Pingxuan Scientific Instrument Co., Ltd .; Intelligent Automatic Salt Spray Tester, F-120S, Dongguan Jingzhuo Instrument Equipment Co., Ltd .; Electronic Balance , TC3K, Changshu Shuangjie Testing Instrument Factory; Digital Display Density Meter, YMS (0.1-5.0), Qingdao Chuangmeng Instrument Co., Ltd.

1.4 Composition of the experimental formula
The reference formula of the preliminary paint experiment is shown in Table 1.

Table 1 Reference Formula of Pigmented Coatings

1.5 Preparation process
Add the formulated amount of epoxy resin to the dispersion container, add additives, pigments, fillers, and solvents in sequence at 800 to 1 000 r / min, stir for 10 minutes, and then pour the mixed color slurry into the sand mill. Grind to a fineness of less than 25 μm, and the coating preparation is completed. According to a certain mass ratio, the component A color paint and the component B curing agent are mixed, and after 20 to 30 minutes of curing, it can be used.

1.6 Performance test The
mass solid content, volume solid content, and density of the prepared coating are measured, and the theoretical VOC content of the coating under application viscosity is determined according to formula (1). At the same time, according to HG / T 4566-2003 “Epoxy Primer”, the sample was prepared for the coating and the performance of the coating was tested. Specific performance indicators, test results, and test standards are shown in Table 2.

Table 2 Performance Indicators of Coatings and Testing Methods

2 Experimental results and discussion
2.1 Screening of
epoxy resins 2.1.1 Screening of epoxy resins
Bisphenol A epoxy resin is most widely used in the field of heavy anticorrosion, because the molecules of bisphenol A epoxy resin contain hydroxyl and ether bonds, which makes it have higher wettability and adhesion to the substrate, and is chemically resistant. The properties are also high, and because its structure is an alternating arrangement of rigid benzene rings and flexible hydrocarbon bonds, it gives the coating film good physical and mechanical properties and good workability. The ultra-high solid epoxy primer has a small amount of solvent, which increases the viscosity of the system, which will cause inconvenience to the construction. Resin viscosity is one of the main influencing factors. At room temperature, E-44 resin and E-51 resin are both liquid, but compared to E-51 resin, E-44 resin has a higher softening point (12-20 ° C) and a large molecular weight. Therefore, we choose a lower viscosity, E-51 resin with smaller molecular weight.

Currently, E-51 resins commonly used are BE188 from Changchun, NPEL128 from South Asia, and SM618 resin from Miki. The paint is prepared according to the reference formula in Table 1, and the coating film properties are tested according to standards. The specific test results are shown in Table 3.

Table 3 Effects of Different Resins on Coatings' Properties

Analyzing the test results in Table 3, it can be seen that the coatings prepared by Changchun BE188 resin are slightly inferior in mechanical properties. The coatings prepared by NPEL128 from South Asia and SM618 resin from Miki are qualified. Considering the overall cost, this experiment uses SM618 resin from Miki The main film-forming substance in this experiment.

2.1.2 Determination of Resin Content and Filler Content
Using the determined Miki SM618 resin, according to the reference formula in Table 1, test the performance of the coating film when adding different resin amounts.
Analysis of the formulas in Table 4 and the performance test results in Table 5 shows that when the resin content is higher than 20%, the mechanical properties and salt spray performance can meet the requirements, but when it is less than 20%, the mechanical properties will be reduced. Considering the overall cost, this experiment finally determines that the amount of resin is 20% of the total formula and the amount of filler is 45% of the total formula.

Table 4 Formula Schemes with Different Resin Amounts
Table 5 Performance Test Results of Different Formula Schemes

2.2 Screening of
fillers The selection of fillers has a great impact on the final performance of the coating. Adding an appropriate amount can not only improve the mechanical strength, abrasion resistance and hiding power of the coating, but also reduce the volume shrinkage and reduce the epoxy curing. Cost and give coatings good storage stability. Ultra-high solid epoxy coatings have strict restrictions on the quality of solids. Therefore, in addition to the necessary fillers, try to add or use low-density, low-oil-absorbing fillers.
Based on the previous experimental data, this experiment determined that the amount of resin in the formula was 20% and the amount of filler was 45%. The fillers such as precipitated barium sulfate, wollastonite powder, feldspar powder, and silicon fine powder were compared to determine the final main filler. , Table 6 is the basic information of 4 kinds of fillers.

Table 6 Basic Information of Four Fillers

The comprehensive table 6 is for screening suitable fillers, and the cured coatings need to be further tested. The paints are prepared according to the reference formula in Table 1 to test the viscosity, density and non-volatile volume fraction of the liquid coatings. The specific test results are shown in Table 7 and Figure 1 is shown.

Table 7 Effects of Different Fillers on the Viscosity and Density of Coatings and the Volume Fraction of Non-volatiles Fig. 1 Trends of the Effects of Different Fillers on the Viscosity and Density of Coatings and the Volume Fraction of Non-volatiles

It can be known from Table 6 that wollastonite powder has low oil absorption and moderate density. According to Table 7, it can be seen that the coating made by wollastonite powder has low viscosity, moderate volume solid content, and low density. Therefore, we choose wollastonite powder as The main filler.

2.3 Screening of
dispersants Wetting dispersants are surfactants, which not only promote the wetting and dispersion of pigments and fillers, maintain the dispersed state of the dispersed pigments or fillers, improve the dispersion efficiency, and prevent the coating from flocculation, coarsening, and precipitation. Can play a role in reducing the viscosity of the entire coating system. In this experiment, five kinds of dispersants were selected to make paint according to the reference formula, and the addition amounts were all 0.3%. The coating dispersion efficiency, coating viscosity, and state in the container after 50 ℃ heat storage for 15 days were tested. The specific test results are shown in Table 8.

Table 8 Effects of Different Dispersants on Coatings

Through the analysis of the test results in Table 8, it is found that when the dispersant a is selected, the viscosity of the coating is small, the dispersion efficiency is moderate, and there is no abnormality in heat storage, so the dispersing aid a is selected. Dispersion aid a is a kind of high molecular active polymer. Its molecular weight segment is the dispersing chain of the solvent. The middle segment is the anchoring group of the active-friendly filler, which makes it form a firm adsorption layer on the surface of the pigment particles. , Reduce the contact angle of pigment particles, effectively improve the wetting performance of pigments and fillers, improve the fluidity of the coating, improve the grinding efficiency, and reduce the viscosity of the coating.

2.4 Screening of
curing agent Curing agent is one of the main factors affecting the surface condition and anticorrosive performance of high solid epoxy primer, so the selection of curing agent is very important. Amine curing agents are the main curing agents for epoxy coatings. Common curing agents are fatty amines, alicyclic amines, aromatic amines, polyamine adducts, ketimines, and cardanol-modified amines. Based on the previous experimental data accumulation and data investigation, this experiment selects three high-solid and low-viscosity curing agents for comparison. Table 9 shows the basic information of the three curing agents.

Table 9 Basic Information of Three Curing Agents

It can be seen from Table 9 that the three curing agents themselves have a small gap. In order to screen for suitable curing agents, it is necessary to further test the properties of the cured coatings. According to the reference formula in Table 1, make the paint and calculate the ratio with the three types. The curing agent is mixed and matured to test the performance of the liquid coating film. The specific test results are shown in Table 10.

Table 10 Effects of Different Curing Agents on Coatings' Properties

Based on the analysis of the test results in Table 10, curing agent a is an aromatic amine curing agent. The cured coating film has poor physical properties and salt spray resistance, and is suspected of being carcinogenic. It cannot be used. Curing agent b is cashew phenol. Amine curing agent, the polyamine group introduced by the reaction and the adjacent weakly acidic phenolic hydroxyl group are the catalysts for the curing reaction of the epoxy resin, so that this system can be quickly cured at low temperature, and the polar hydroxyl group increases The polarity of the curing agent enhances the wetting and adhesion to the substrate and has excellent antiseptic properties, but the cardanol-modified amine curing agents have poor physical properties, so they cannot be used; curing agent c is a mixed modified Alicyclic amine curing agent, which has a lower viscosity, can effectively increase the solid content of the product, and is significantly better in flexibility and adhesion than the other two curing agents, and has a greater resistance to salt spray.定 curing agent c.

3 Conclusion
This experiment has successfully developed an ultra-high solid epoxy primer that meets the requirements by screening resins, pigments, fillers, additives and curing agents.
(1) VOC emission under construction viscosity is 148 g / L, which greatly improves the construction environment and conforms to the concept of environmental protection, and has significant economic and social benefits;
(2) Mass solid content is greater than 88%, volume solid content is greater than 85%, viscosity Moderate, convenient construction, improve work efficiency;
(3) It has the advantages of excellent salt spray resistance and good adhesion, etc. The application status and prospects are very broad.

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