Waterborne polyurethane is a kind of environment-friendly material with water as the dispersing medium (instead of organic solvent). The coatings prepared by it are widely used in the fields of textile, leather, wood, plastic, architecture, paper making, automobile and industrial maintenance. However, due to the introduction of hydrophilic groups in waterborne polyurethane, it has poor performance in water resistance, solvent resistance, weather resistance, drying speed, etc., which limits its application range.
In order to meet people's demand for environment-friendly functional coatings, water-based polyurethane can be prepared by molecular structure design, composite modification, synthesis process and film-forming technology improvement to meet the market demand, such as polyol molecular design on molecular scale, or introduction of specific molecular structure (or element) into polyol to change. The main chain structure of macromolecules, or nano composite modification on the micro scale, or the introduction of specific functional groups in the film-forming process to improve the function of waterborne polyurethane coating. The functional waterborne polyurethane coating prepared by the functional design has the properties that the general waterborne polyurethane coating does not have, such as flame retardant, anti-corrosion, anti mildew and sterilization, anti graffiti, transparent and heat insulation and other special properties.
This paper mainly introduces the functional design, synthesis technology and application progress of waterborne polyurethane coatings.
1 Synthesis technology of functional waterborne polyurethane
By using the tailorability of its molecular structure, combined with the new synthesis and cross-linking technology, the performance of W PU coating is even better than that of traditional solvent polyurethane coating. The synthesis technology and process of functional WPU are relatively complex. Generally speaking, it can be divided into several process methods, as shown in Table 1.
2 Design and application of functional waterborne polyurethane
The functionalization of WPU is to connect functional small molecules with WPU framework, which can be divided into chemical connection and physical connection. Chemical bonding is to introduce active functional groups into WPU framework by grafting reaction, so as to change the physical and chemical properties of WPU and give it new functions; physical bonding is realized by blending small molecular functional compounds with WPU. Through the functional design of WPU, functional WPU coatings with special properties such as flame retardant, anti-corrosion, anti mildew and sterilization, anti graffiti, transparent and heat insulation can be prepared.
2.1 Design and application of fire retardant waterborne polyurethane coating
2.1.1 Halogenated flame retardant waterborne polyurethane coating
The flame retardancy of WPU can be improved by introducing halogen, N, P and other groups into WPU, and the thermal stability of WPU can be greatly improved by adding organosilicon oligomer.
2.1.2 Halogen free flame retardant waterborne polyurethane coating
Intumescent flame retardant has the characteristics of halogen-free, low smoke and low toxicity. It is a widely used halogen-free flame retardant coating at present. The experimental results show that the intumescent flame retardant system of waterborne polyurethane coating is better than the non intumescent flame retardant system.
2.2 Design and application of anticorrosive waterborne polyurethane coating
Polyurethane coating has excellent cold resistance, wear resistance and corrosion resistance. However, due to the hydrophilic group in WPU molecule, its water resistance, solvent resistance and corrosion resistance are not good. Therefore, epoxy resin, acrylate, organosilicon and other functional organic compounds are introduced into the main chain or side chain of polyurethane to prepare network polymer, or nano particles are added into WPU, so as to improve the corrosion resistance of WPU.
2.2.1 Epoxy modified waterborne polyurethane anticorrosive coating
WPU modified by epoxy resin can give waterborne polyurethane good corrosion resistance.
In addition, the addition of a small amount of aluminum powder can improve the corrosion resistance of waterborne epoxy polyurethane zinc rich coating. The coating has high mechanical strength, convenient construction and wide application prospect in heavy corrosion protection of steel.
2.2.2 Epoxy acrylic composite modified waterborne polyurethane anticorrosive coating
Acrylic resin has good water resistance and weather resistance. By modifying waterborne polyurethane with acrylic resin, the high wear resistance and good mechanical properties of polyurethane can be organically combined with the good weather resistance and water resistance of acrylic acid, so that the performance of WPU coating can be significantly improved. The waterborne polyurethane modified by epoxy and acrylic acid can synthesize the advantages of the three and obtain high-performance WPU anticorrosive coating.
2.2.3 Organosilicon modified waterborne polyurethane anticorrosive coating
Silicone coating has excellent heat resistance, weather resistance and hydrophobicity, while polyurethane coating has outstanding wear resistance, oil resistance and good weldability, but the heat resistance, water resistance and corrosion resistance are not ideal. Therefore, the use of silicone modified polyurethane material can make up for the shortage of polyurethane material. The addition of organosilicon enhances the corrosion resistance, elasticity and mechanical stress of waterborne polyurethane coating, and the thermal stability is greatly improved. This high-performance heat-resistant and anticorrosive coating is suitable for aerospace, ocean, automobile and other fields.
2.2.4 Nano material modified waterborne polyurethane anticorrosive coating
Nano materials have unique surface effect, volume effect, quantum effect and interface effect. Nano particles can be used to modify polyurethane anti-corrosion coating.
2.3 Design and application of mould proof and bactericidal waterborne polyurethane coating
2.3.1 Inorganic antibacterial waterborne polyurethane coating
At present, the domestic research of antibacterial polyurethane is mainly through the addition of inorganic antibacterial agents, mainly silver, and zinc oxide and nano-TiO2 can be added as antibacterial agents. For example, anatase nano-TiO2 can inhibit the attachment of marine bacteria, its photocatalytic oxidation can kill the attached bacteria, and the number of bacteria attachment decreases with the increase of anatase nano-TiO2 content.
2.3.2 Organic antibacterial waterborne polyurethane coating
Organic polymer compounds with antibacterial groups are covalently bound to insoluble carriers. They can not only be reused, but also concentrate on the surface of carriers. Therefore, polymer antibacterial agents are becoming a hot spot of research and development.
2.4 Design and application of anti graffiti waterborne polyurethane coating
Graffiti ads, known as "urban psoriasis", can have a bad impact on the city's appearance and make it difficult to remove. The more effective method is to brush anti graffiti coating. The anti graffiti coating shall have multi-functional characteristics, including good hydrophobicity and oil repellency, stain resistance, scratch resistance, chemical resistance and easy cleaning. The main performance of anti graffiti coating is determined by the resin used. At present, most of them are solvent polyurethane, silicone resin, fluorine resin, etc. With the limitation of VOC emission all over the world, it is imperative to develop modified WPU anti graffiti coating. To improve the anti graffiti performance of the film is to improve the surface performance of the film so that it is difficult to absorb and remove pollutants, and to improve the density of the film so that pollutants are not easy to penetrate these two basic ways. At present, organosilicon and fluororesin are mainly used to improve the surface properties of WPU coatings, and reduce the surface Gibbs free energy.
2.5 Design and application of transparent and heat insulation waterborne polyurethane coating
In recent years, with the rapid development of nanotechnology, water-based nano transparent thermal insulation coating emerged. The heat-insulating laminated glass made of the coating has high visible light transmittance and infrared barrier, which not only meets the needs of indoor lighting, but also has certain heat insulation function. This kind of coating has a good application prospect in building, automobile glass and other fields which need transparent heat insulation function.
At present, the production and application of domestic functional WPU coating is still in its infancy compared with foreign countries, and the product performance needs to be further improved. At the same time, the research of functional WPU coating is developing towards the direction of high performance and multi-function, especially the molecular design and synthesis of WPU. In the synthesis of WPU, various methods are adopted to introduce the molecular chain with special functions to the main or side chain of the molecule, so that WPU has special functions; new and efficient hydrophilic chain extender is developed or special cross-linking agent is introduced by using various functional group reactions, so as to improve the water resistance, solvent resistance and storage stability of WPU film; various nano materials and renewable materials are used to achieve chemical modification. These methods are the future development direction of functional WPU coatings.