Water based coating is a kind of coating with water as dispersing medium, which has the advantages of less resource consumption, low VOC, non toxicity and no pollution. Water based polyurethane coating combines the advantages of high solid content, corrosion resistance, strong mechanical performance, good film adhesion with the low VOC content of water-based coating, reducing the harm to the environment With the gradual improvement of environmental protection awareness and strict control of organic volatile content in coatings by environmental protection laws and regulations of various countries, the development of water-based polyurethane coatings with low pollution and high performance has become a hot spot. The application of earlier water-based polyurethane coatings has the characteristics of low crosslinking degree, high fracture elongation rate and drying at room temperature. Compared with traditional solvent based polyurethane coatings, water-based polyurethane coatings Due to its low chemical resistance, solvent resistance, poor hand feel and gloss, ester coatings need to be modified to improve their performance. This paper reviews the research progress in the modification of waterborne polyurethane coatings in recent years.

１　Organic resin modification

Epoxy resin has the advantages of excellent physical and chemical properties, strong bond strength, high thermal stability, etc. in the reaction of epoxy resin and polyurethane, branch points are introduced into the main chain, forming part of the network structure, forming cross-linking, improving the relative molecular weight of polymer, so as to improve the mechanical properties, water resistance, and adhesion of waterborne polyurethane. Wang Xiaorong et al. Through water orientation Polyurethane / polyacrylate emulsion (WPUA) was introduced with glycidyl methacrylate (GMA) and polymerizable emulsifier allyl hydroxypropyl sulfonate (AHPS). The waterborne polyurethane / polyacrylate.GMA with a solid content of about 43% was prepared with double bonds and epoxy groups, and epoxy groups were reacted with carboxyl groups in DMPA for cross-linking. AHPS could reduce the interfacial energy of the emulsion. The results show that the water resistance, hardness and adhesion of the modified coating are improved.

Li Bing et al. First opened the epoxy group through the primary amine in KH550, and then modified waterborne polyurethane as the macromolecule chain extender. The waterborne polyurethane emulsion was successfully prepared in the street waterborne polyurethane prepolymer. The structure of the resin was characterized by the instrument. The results showed that KH550 successfully opened the epoxy group and joined the organic silicon functional material to make the synthesis. The resin contact angle is greatly improved.

Acrylate has the advantages of good water resistance and weather resistance, and it can not change yellow when exposed to UV. The composite of acrylic resin and polyurethane resin can play a complementary role. The prepared waterborne polyurethane coating has the comprehensive properties of both. Zhu et al. Showed that under UV irradiation, the resistance of aliphatic polyurethane coating decreased, and the increase of coating capacitance and porosity is higher than that of propylene It takes 28 days for the coating resistance of acrylate polyurethane and aliphatic polyurethane to reduce to 106 Ω· cm - 2, and 35 days for the composite coating of acrylate polyurethane, which is mainly due to the slow fracture of C = O bond during the fracture of acrylate polyurethane. From this, we can see the advantages of acrylate polyurethane compared with the same type of polyurethane. The modification of waterborne polyurethane by acrylate mainly includes Physical modification is to blend acrylate with waterborne polyurethane to improve the mechanical properties of materials. Chemical modification is realized by preparing core-shell structure. The outer surface of waterborne polyurethane micro particles has hydrophilic ion group, and the acrylate particles have hydrophobic groups and swell from the outside to the inside in the opposite direction to polymerize in the polyurethane particles. In recent years, researchers have developed a new process suitable for the preparation of waterborne polyurethane dispersion modified by acrylic acid.

 Qiu et al. Prepared a series of uv-wpua / SiO2 Composite Coatings by in-situ polymerization of isophorone diisocyanate, polyether polyol and dimethylolpropionic acid. On this basis, a series of uv-wpua / SiO2 composite coatings were prepared with tetraethoxysilane and propyltrimethoxysilane as silane coupling agents. The experiments showed that SiO2 particles in the composite coatings were fully dispersed and dispersed in the polymer With the increase of tetraethoxysilane content, the average particle size and viscosity of uv-wpua / SiO2 dispersion are also increased. The hardness, tensile strength and elongation at break are better than those of pure uv-wpua film. When the content of tetraethoxysilane is 0.5%, the mixture has the best water resistance The results show that it is a kind of high-performance water-based UV curing material, which has a good application background.

Su Jiahui and others used isophorone diisocyanate, polyethylene glycol, 2,2-dihydroxymethylpropionic acid as the main raw materials to synthesize waterborne polyurethane dispersion. Three kinds of polyurethane acrylate aqueous UV resins with 2,4,6 functional degrees were prepared by introducing carbon carbon double bond into methacrylic acid-2-hydroxyethyl ester, Dimethylacrylate and pentaerythritol triacrylate, respectively It is found that the performance of three kinds of resins with different functional degrees is not different, while the increase of double bond content has a significant impact on the basic performance of resin film. The higher the functional degree is, the greater the degree of cross-linking is, the higher the glossiness, hardness and heat resistance of the resin are, and the lower the water absorption and flexibility are. The polyurethane acrylate with 4 functional degrees has The best flexibility and comprehensive performance.

Chen Zhonghua, et al. Prepared two-component waterborne polyurethane automobile top coat by using bayhydrol a XP 2695 containing hydroxy acrylic resin and bayhydur XP 2655 curing agent with excellent performance, and measured that the material ratio of - NCO group to - OH group was 1.4h, and the comprehensive performance of the film was the best. In addition, the paper also discussed the influence of the volume concentration of pigment and filler on the performance of the film Now when the PVC value is 15%, the gloss, impact resistance, adhesion and other properties of the paint film reach the optimal value. By measuring the change of the viscosity time of the paint, it is determined that the trial period of the self drying two-component waterborne polyurethane paint is 4H.

Organosilicon compounds have the advantages of weather resistance, fire resistance, water resistance and good stability. The composite modification with polyurethane can significantly improve the water resistance and stability of waterborne polyurethane. At present, the more commonly used modification method is copolymerization or blending. Blending method is to add organosilicon as modifier to polyurethane system, and improve the water-resistant polyurethane by mechanical mixing method However, due to the absence of chemical bond, the hydroxyl silicone oil is easy to transfer, resulting in short silicon sensing time. Therefore, the hydroxyl silicone oil, amino or alkoxy terminated silane coupling agent and isocyanate based prepolymer are polymerized by copolymerization to produce organosilicon modified polyurethane.

Fan et al. Studied the method of blending silica and polyurethane to prepare waterborne polyurethane film. The silica / polyurethane mixture was prepared by adding tetraethoxysilane or methyltriethoxysilane on the surface of polyurethane dispersion through hydrolysis and condensation reaction. These silica particles were distributed on polymer collective, and the agglomerate could completely avoid silica and dispersion In situ modification of Si-CH3 group in polyurethane matrix. The composite film prepared by this method shows excellent hardness and wear resistance, even when the silicon content is 2% - 3%, its performance is better than that of unmodified silicon dioxide. The uniform dispersion of SiO2 in polyurethane matrix is better compatible with SiO2 and organic parts, improves the hardness of the film, thus improving the wear resistance, and the resistance of leather and paper There is a good application prospect in abrasive coatings.

Fu et al. Used the chemical bond between polymer matrix and silica nano materials to prepare castor oil-based waterborne polyurethane silica hybrid materials. Nano silica particles in polyurethane matrix play an important role in improving hydrophobicity and thermal stability. The increase of silicon content enhances the roughness, hydrophobicity and thermal stability of the film, but the transparency of the film This is helpful to adjust the best silicon content to achieve the best stability and light transmittance. Hua Mingyang invented a preparation method of wood polycaprolactone siloxane waterborne polyurethane coating, which uses diisocyanate, alkoxy silicone oil and polycaprolactone to form NCO prepolymer, and then uses hydrophilic monomer, coupling agent and diethylene glycol monohydrate to expand the chain reaction, Finally, adjust the pH value to neutral, add water to stir. The paint has high strength, the film remains yellow, and no harmful solvent volatilization.

２　Modification of inorganic nano materials

Nano materials have the advantages of surface effect, small size effect, macro quantum tunneling effect, high surface activity, good light absorption performance, etc. the composite with polyurethane can significantly improve the aging resistance, wear resistance and hardness of the materials. At present, the main method of nano modification of waterborne polyurethane is mechanical blending of nano materials, such as bin, which is modified by silane coupling agent KH560, and infrared Spectrograph and transmission electron microscope confirmed that nano CaCO3 was successfully modified by KH560, but the thermal conductivity of nano CaCO3 was not affected by KH560. The roughness and gloss of a large number of nano CaCO3 composite coatings were measured by roughometer and luminometer. The results showed that nano CaCO3 modified by KH560 improved the dispersion of nano CaCO3 on polyurethane matrix. The polyurethane coating was tested by TGA The results show that the thermal conductivity of the modified nano CaCO3 polyurethane coating increases with the increase of the mass fraction of nano CaCO3.

Through free radical polymerization, Kima et al. Mixed calcium carbonate, titanium dioxide and loess into traditional PUA to prepare environmental protection PUA floor tile composite film. The research shows that with the decrease of relative molecular weight of polyol, the elasticity of PUA film increases, and the glass transition temperature also increases. The contact angle experiment shows that the addition of non-metallic particles leads to the increase of hydrophobicity of composite coating, which is a characteristic Enough for PUA composite material to be applied to the coating material of environmental friendly floor tiles.

Akbarian et al. Studied the effect of silver nanoparticles on the corrosion resistance of polyurethane coating on the surface of low carbon steel in sodium chloride solution. Adding silver nanoparticles to polyurethane coating with high solid content had no significant change on the corrosion resistance. Adding silver nanoparticles to waterborne polyurethane coating had no significant change on the corrosion resistance The results show that there is no obvious structural change when silver nanoparticles are added to the high solid content polyurethane coating, but degradation in the water-borne polyurethane coating. Infrared spectrum analysis shows that compared with the high solid content coating, the carbonyl component of the water-borne coating is greatly reduced This may be due to the decomposition of isocyanate group in water. The carbonyl component containing silver nanoparticles prevents the reaction of dispersed chloride ions and provides a stable composite. The low content of carbonyl component in water-based coatings may be the cause of degradation of water-based coatings containing silver nanoparticles. The current work shows that adding high solid content polyurethane coatings with silver nanoparticles does not need to worry about corrosion There is a loss in the environment, and the waterborne polyurethane coating is best used far away from chlorine corrosion.

Fang et al. Developed waterborne polyurethane mixed with nano particles and low crystallization based on polypropylene glycol. The experiment shows that when the ratio of hard segment to soft segment is 4 or 5, waterborne polyurethane has the best performance. With the increase of the ratio, the relative solid content, viscosity, acid value, electrolyte and storage stability of waterborne polyurethane increase. The ratio of hard segment to soft segment is 4 The results show that the nano modified waterborne polyurethane coating has a wide range of applications in waterborne ink adhesives.

３　Modification of vegetable oil

Vegetable oil has many characteristics, such as low price, wide sources and various kinds. It is a natural compound composed of fatty acid and glycerin. Its structure contains side chain of unsaturated fatty acid. There are many advantages to synthesize polymer materials with it as raw material. First, vegetable oil contains polyhydroxy and unsaturated double bond structure, which is easy to cross-linking reaction with isocyanate and acrylic acid In the process of preparation, Azadirachtin oil reacts with diethanolamine first, and then bisphenol-A is used to modify fatty acyl The polyurethane coating was prepared by the reaction of methylene diphenyl diisocyanate and polyether amide from amines. The TiO2 nanoparticles were synthesized and modified by silane coupling agent. The properties of the products were characterized by corresponding standard instruments. It was found that the addition of nano-TiO2 significantly improved the gloss, pencil hardness, chemical resistance and thermal stability of the coating materials The polyurethane coating based on the base material has a good prospect, and may even replace the petroleum based coating.

Fu et al. Also used castor oil modified diisocyanate and castor oil-based carboxylic acid hydrophilic chain extender to prepare a new type of completely biological aqueous polyurethane dispersion. First, a new linear terminal saturated diisocyanate was synthesized from castor oil derived undecanonic acid by mercaptan coupling and Curtis rearrangement, and then the hydrophilic chain extender was prepared with castor oil and castor oil-based carboxylic acid Agent reaction. The hydrophilic chain extender is prepared from castor oil and 3-ryukypropionic acid. The water-based polyurethane product has good heat resistance and hydrophobicity, and may be used as a suitable substitute for petroleum based coating. Similarly, Gao also uses castor oil and polyethylene glycol to prepare water-based polyurethane, and the product has good biocompatibility.

The environmental friendly polyurethane dispersions were synthesized by Sariah using Jatropha oil-based polyols and IPDI. The particle size of the dispersions was widely distributed from 53nm to 1.2nm. With the increase of - Oh amount of Jatropha oil, the hard segment component and DMPA component increased. The film of Jatropha oil coating had good thermal stability at the lowest degradation temperature of 286 ℃, in addition, its young's modulus value was from 1 to 28Mpa The tensile strength is from 1.8 to 4.0Mpa, and the elongation at break is from 85% to 325%.

４　Conclusion and Prospect

Although the research on modification of waterborne polyurethane has made great progress, it still needs to strengthen further research. For example, from the direction of basic research, people need to further explain the relationship between micro reaction mechanism, structure and performance in the modification process, which needs to be characterized by modern analysis and testing technology in the future research. In the industrial application, how to reduce the production cost, It is the guarantee to promote the wide application of water-based coatings to improve the cost performance and compete with solvent based coatings.