1 Technical progress of acrylic emulsion

1.1 Research on the morphology and structure design of acrylic latex particles

The performance of acrylic emulsion has a great relationship with the structure. Different structural designs provide different performances. Figure 1 shows the common acrylic structural design in current literature reports [3]. Among them, monodisperse is the most common and simplest type. Most acrylic emulsions are of monodisperse type, with simple process, easy-to-control structure, and stable product quality; most of the researches on core-shell structure[4], there are also structures with special functions, such as bimodal structure, blended structure, multi-microdomain structure, multipolar structure, hollow sphere structure [5], polymer-pigment composite structure [6], etc. Under the premise of ensuring good film-forming performance, the structure designed by synthesis can improve certain performance, meet customers' requirements for different performance [7], and expand the performance range and application field of acrylic emulsion.

Ma et al. [8] reviewed various cutting-edge design ideas and methods of "core-shell structure" in current emulsion polymerization, including gradient structure, sandwich structure, strawberry structure, snowman structure, etc. Different structures can bring different properties ; It also introduces the core-shell structure materials of inorganic and organic composites, such as the research on SiO2, TiO2, ZnO, CaCO3, etc. as the inorganic core. Wood et al. [9] introduced the modification methods of acrylic acid and PVDF, including cold mixing, core-shell and interpenetrating network methods, the film-forming mechanism after modification, and the performance improvement brought about.

1.2 Modification of acrylic latex particles

Acrylic emulsion has many advantages, but it also has many disadvantages when used alone, such as low hardness, slightly poor mechanical properties, slightly poor compactness of paint film, high film forming temperature, etc., which limit its application range. In order to expand its application field, it is an important way to modify acrylic emulsion to make its properties meet the application requirements. Modification of acrylic emulsion

Properties studies include physical blending and chemical modification [10]. At present, the main directions of chemical modification of acrylic emulsion are: silicone modified acrylic emulsion[11], organic fluorine modified acrylic emulsion[12], epoxy modified acrylic emulsion[13], polyurethane modified acrylic emulsion[14], Inorganic nanoparticles modified acrylic emulsion [15] and so on.

Calovi et al. [16] added functionalized graphene oxide (FGO) flakes to acrylic electrophoretic coatings, and studied the effect of graphene oxide addition on the performance of carbon steel-based coatings. Ma Guanhao et al.[17] synthesized fluorine-silicon-modified styrene-acrylic emulsion through emulsion polymerization, and then prepared silica-modified fluorine-containing silicon-styrene-acrylic composite emulsion with tetraethyl orthosilicate and nano-SiO2 by sol-gel method. The AFM analysis shows that the peak-like structure is formed, the hydrophobic performance is greatly improved, and it has a good application prospect in the field of self-cleaning. Wang Xingwang [18] used trifluoroethyl methacrylate, butyl acrylate, methyl methacrylate and other monomers to prepare fluorine-containing emulsions by emulsion polymerization according to a certain ratio, followed by AgNO3, CuSO4, KI according to a certain ratio Equipped with reversible discoloration material, melamine-formaldehyde-urea is used as wall material, reversible discoloration material is used as core material, microcapsules are prepared as discoloration material, and fluorosilicon emulsion is used as film-forming substance to prepare a medium temperature The reversible discoloration paint can be widely used in temperature display paint, warning paint and other fields. Wu Jintian et al. [19] prepared a water-based ultra-thin fireproof coating with silicone-acrylic emulsion and epoxy-modified acrylate emulsion. After the coating is heated to expand, it forms a dense "honeycomb" structure, which has good fireproof performance. Dewey et al[20] compounded nano-ATO and styrene-acrylic emulsion by semi-continuous dropping method. The research showed that with the increase of nano-ATO content, the light transmittance of the coating gradually decreased. When the mass fraction of nano-ATO was 7%, the coating The heat insulation effect on the glass is better, and it has application prospects in the field of transparent heat insulation. Shu Shujun [21] hybridized epoxy to acrylic acid to obtain acrylic hybrid emulsion, which has the characteristics of fast drying, weather resistance and long service life of acrylic emulsion, and has the properties of high adhesion and salt spray resistance of epoxy , a good balance between the performance characteristics of the two, successfully applied to anti-corrosion, flooring, diesel engines, parts and other fields.

2 Application of acrylic emulsion in industrial coatings

Acrylic emulsion is widely used in various industrial coatings due to its excellent performance, convenient construction, and environmental protection.

2.1 Replacing solvent-based alkyd systems for steel structure anticorrosion coatings[22]

Solvent-based alkyd paint is one of the earliest and most mature products in China. With its excellent construction performance and low price, it has occupied a wide range of industrial anti-corrosion coatings market. In recent years, under the call of the "Battle for the Blue Sky", environmental protection policies in various places have been strictly implemented, and the replacement of solvent-based alkyds is imminent. The market demands to provide a surface gloss effect and salt spray resistance similar to solvent-based alkyd, while providing a solution that dries faster than alkyd and has better weather resistance. Some one-component acrylic emulsions have the characteristics of gloss performance comparable to solvent-based alkyds [23], faster drying speed, excellent early water resistance and salt spray resistance, and excellent weather resistance [24], which are many Recognized by customers, it has become a star product for light anti-corrosion applications in the coatings industry. Despite the rapid development of water-based steel structure coatings, there are still some problems in the water-based steel structure anti-corrosion industry, such as submerged arc welding flash rust, low hardness of one component, slow drying speed under low temperature and high humidity conditions, and low surface treatment tolerance. On the low side, etc., performances like this still need to be further improved and improved.

2. 2 Used in water-based container anti-corrosion exterior paint

With the promotion of the China Container Association and the implementation of the industry self-discipline convention [25], water-based container coatings ushered in the spring. At the end of 2017, standard containers were fully switched to water-based container coatings, and water-based coatings for cold containers will be implemented in July 2021. The outer coating of container standard boxes generally adopts three systems: epoxy zinc-rich primer + epoxy intermediate paint + acrylic emulsion topcoat, almost all standard container topcoats are acrylic emulsion systems, with primer and intermediate paint, Can provide excellent water resistance, salt spray resistance, water whitening resistance, weather resistance and hardness and other properties. Despite the rapid development of water-based coatings for containers and a high proportion of transformation, there are still some problems in water-based coatings for containers, such as coating pinholes, paint film sagging, thick film cracking, and fierce price competition for topcoats.

2. 3 Water-based anti-corrosion coatings for auto parts

China is the world's largest producer of auto parts. From 2010 to 2019, the sales revenue of my country's auto parts manufacturing industry continued to grow. Not only did the domestic market grow significantly, but the export of auto parts also maintained a steady growth. In the classification of auto parts, the number of metal parts accounts for a large proportion, more than half of the share, followed by plastics and accessories [26]. Most of the auto parts need to be coated and protected by coatings to protect them from corrosion, or to increase aesthetics and comfort. At present, the single-component epoxy ester system has problems such as strong odor and high VOC emission in the application of auto parts; the development of high-performance acrylic emulsion to replace epoxy ester will have a large market space. Compared with the epoxy ester system, the one-component acrylic emulsion needs to provide excellent salt spray resistance, fast drying speed and excellent adhesion in addition to lower odor and VOC release to meet market demand.

2. 4 For water-based amino baking varnish

High-temperature baking varnish uses metal as the base material, and quickly forms a film through high-temperature baking. The formed paint film has excellent properties such as gloss, hardness, flexibility, adhesion, and impact resistance. Among them, amino baking varnish is the most common high-temperature baking varnish. It is used in the coating field of various metal products. Solvent-based amino baking varnish contains a large amount of toxic and harmful substances such as butanol and xylene, and the release of VOC is seriously exceeded, polluting the environment and endangering health. With the advancement of water-based technology, water-based amino baking varnishes have gradually replaced solvent-based amino baking varnishes, and have partially replaced solvent-based and successfully applied in areas such as anti-theft doors, elevators, packaging barrels, bicycles, tricycles, wine bottle paints, and hardware [27- 28]. As of the end of 2020, water-soluble acrylic acid occupies a dominant position in water-based amino baking varnishes. However, due to its high VOC and tightening environmental protection policies, environmentally friendly acrylic emulsions are increasingly used in amino baking varnishes. Acrylic emulsion is applied to water-based amino baking varnish. In addition to the real low-VOC environmental protection performance, it also needs to show high gloss, flexibility, impact resistance, excellent weather resistance, boiling water resistance, cost-effective, suitable for electrostatic spraying And other characteristics, in order to usher in qualitative development in water-based amino baking varnish.

2.5 For water-based strippable coatings

Peelable coating is a temporary protective coating, which is applied on the surface of the substrate to isolate the erosive medium and prevent it from being scratched, scratched, aged, polluted, dust, oil, fingerprints, etc. during production, transportation or storage Impact, also known as temporary protective coatings, can provide temporary protection for the surface of various substrates. The general protection period is a few days to several months, and it can be easily peeled off from the surface of the substrate after the protection is over. any negative impact on the surface of the material. Acrylic emulsion, as a strippable temporary protective coating [29], has been successfully applied in shipbuilding, construction, glass processing, automobile manufacturing, auto parts, rail transit and other fields. Although water-based strippable coatings have made great progress and development, there are still many challenges. For example, acrylic systems are difficult to strip or become brittle after high-temperature baking, and the solvent resistance and corrosion resistance of acrylic strippable systems are difficult to meet customer needs. Some substrates, such as fluorocarbon protection, are difficult to peel off.

2. 6 For water-based dip coating protective coatings

Water-based dip coating is widely used in the coating of axles, casting workpieces, sofa brackets, parts, etc., with high efficiency, less waste and high degree of automation. The currently used solvent-based or water-soluble dipping paint consumes a lot of solvent, and the stability of the bath solution is also poor. The paint film is prone to sagging, edge shrinkage, bottom exposure, and blooming. The acrylic emulsion type dip coating bath liquid is stable, non-flammable, low odor, convenient construction, wide application, suitable for large-scale industrial continuous production, showing good adhesion, gloss and color retention, and water resistance. One-component acrylic emulsion is widely used in dip coating applications, such as dip coating protection of engineering machinery crawlers, steel structures, parts, sofa supports and other workpieces. Common coating colors are black, blue, yellow, etc.

2. 7 Water-based coatings for roads, bridges and tunnels

Acrylic emulsion can also be used in roads, bridges, tunnels and other applications, such as water-based road marking paint, tunnel reflective paint, tunnel fireproof paint, etc. GB/T 38597-2020 "Technical Requirements for Coating Products with Low Volatile Organic Content" stipulates that the volatile organic compound content limit of road marking paint is 150 g/L. Acrylic emulsion has the characteristics of excellent fast drying, low VOC release and good adhesion to glass beads. It is used in many highways and urban roads to provide durable wear resistance and excellent reflective properties.

2. 8 For chromium-free metal surface treatment

Surface treatment is a commonly used method in industrial production to prevent metal corrosion. At present, the passivation process containing hexavalent chromium is the most mature and widely used. However, hexavalent chromium is a heavy metal with high toxicity, and seeking high-performance, non-toxic chromium-free passivation technology has become the main research direction at present. At present, chromium-free passivation technology is mainly divided into three categories: inorganic salt passivation; organic compound passivation; inorganic-organic composite passivation. Zhang Zhenxing [30] synthesized self-crosslinking silicone-modified acrylic emulsion, combined with commercially available acrylic emulsion, added silane coupling agent KH560 and different corrosion inhibitors, and passed electrochemical and neutral salt spray tests to obtain composite passivation The optimal formula and process of chemical solution.

2. 9 For wood coatings

At present, the raw materials for preparing waterborne wood coating emulsions are mainly waterborne polyurethane dispersions (PUD), waterborne acrylic emulsions and acrylic modified waterborne polyurethane emulsions (PUA). At present, wood coatings on the market are still mainly solvent-based, and there are still many problems to be solved in water-based systems, such as improving transparency, sealing tannin, anti-swelling, anti-stacking, and solvent resistance. Among them, acrylic emulsion has the characteristics of gloss retention, color retention, good adhesion, high hardness, and good water resistance. It has been widely used in water-based wood coatings. However, acrylic emulsion has the disadvantages of poor solvent resistance, hot stickiness and brittleness. Sealing tannin and anti-stretch properties, while providing good primer sanding properties and anti-stacking properties will be the focus of future research and development.

2. 10 For plastic surface coating

Due to the low surface energy of plastic surfaces, acrylic emulsions are used like ABS (acrylonitrile\-butadiene-styrene), PC (polycarbonate), PET (polyethylene terephthalate), PVC ( Substrates such as polyvinyl chloride) are more effective, while substrates such as PE (polyethylene) and PP (polypropylene) have adhesion problems. In addition to requirements for adhesion, coatings for plastics also require coatings with high hardness, fast drying speed, good scratch resistance, and solvent wiping resistance. Silver powder paint is more widely used in plastic paints. Silver powder paint requires better silver powder orientation effect and better metal texture. The main application fields are mobile phones, computers, household appliances, children's toys, clothing molding, motorcycles accessories etc. At present, the development of water-based coatings for plastics is slow. The main reason is that the surface energy gap between water-based systems and plastic substrates is large, and it is difficult to have good wetting and adhesion, which requires technological breakthroughs.

In addition to the above application fields, there are many industrial fields using water-based acrylic emulsions, such as inks, inorganic coatings, paper coatings, leather coatings, one-component floor coatings, reflective insulation coatings, damping coatings, waterproof coatings, and fireproof coatings , Anti-graffiti coatings, adhesives, seed coatings, interface agents, modified asphalt coatings, one-component floor coatings, etc. Different industrial coatings have different application formulas and application technologies.

3 Development trend of acrylic emulsion

Acrylic emulsions have achieved great success in the field of architectural coatings. Under the call of "Battle for the Blue Sky", acrylic emulsions have entered the fast lane in the field of industrial coatings due to their excellent performance, low odor, and low VOC release. At present, it has achieved success in the application fields of water-based steel structure coatings, water-based container coatings, and water-based wood coatings. In the future, it will definitely develop in the direction of functionalization, high performance, and environmental protection.

In addition, acrylic emulsion will penetrate into more industrial segments. However, there are still many problems to be improved and solved in the acrylic emulsion, for example, the coating hardness of the one-component acrylic emulsion coating system is not enough, the anti-flash rust performance is insufficient, especially the flash rust at the submerged arc welding is serious, and the salt spray resistance encounters a bottleneck; The water-based acrylic emulsion system cannot be constructed in winter in the north, and dries slowly in the low-temperature and humid environment in the south. Through special functionalization, hybridization, blending, modification, etc., it will be the focus of future research to maximize the application space of acrylic emulsions and avoid weaknesses.