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Discussion on cross-linking modification technology of water-based PU Water-based polyurethane (PU) is a polymer material developed by using water instead of organic solvent as the dispersion medium. It was put on the market in the early 1970s and has become one of the most important water-based polymer materials. It not only retains the high strength, wear resistance and other properties of general PU, but is also safe to use, non-toxic, non-flammable and has no environmental pollution. It is called a green environmentally friendly coating and can be widely used in industries such as textiles, printing and dyeing, leather processing, coatings, adhesives, wood processing, construction and papermaking. Especially with the strengthening of people's safety awareness and environmental awareness and the increasingly stringent environmental regulations, water-based coatings have become a new direction for the development of coatings in the future. Water-based PU can be divided into external emulsification type and internal emulsification type according to the preparation method. The former is the earliest water-based PU product. In 1953, the United States Du Wyandott of Pont Company synthesized PU emulsion. The preparation process is to use a difunctional polyol to react with an excess of diisocyanate in an organic solvent to synthesize a prepolymer with -NCO capping. Then add an appropriate emulsifier and undergo strong shearing. The PU emulsion is dispersed in the water medium and uses diamine for chain extension. However, due to the large amount of emulsifier, long reaction time and coarse emulsion particles, which results in poor storage stability and poor physical and mechanical properties of the glue layer, this method is basically not used in the current production of PU emulsion. The latter was invented by Dieterich in the early 1960s. It first synthesizes a prepolymer by reacting polyol and polyisocyanate, and then introduces hydrophilic groups into the molecular chain of the prepolymer, which spontaneously disperses under certain conditions to form an emulsion. The emulsion particles produced by this method are finer and evenly distributed, have good storage stability, and the physical and mechanical properties of the glue layer are also good. The water-based PU currently used is synthesized using this method. However, the presence of hydrophilic groups in the PU structure makes the glue layer poor in water resistance, heat resistance and gloss, severely limiting its scope of use. In recent years, the modification of water-based PU using the "cutting" characteristics of the PU structure and various modification methods has become a research hotspot in this field. Cross-linking modification is one of the most effective modification methods to improve the water resistance and mechanical strength of PU emulsion. This article introduces the cross-linking modification technology of water-based PU as follows. 1． Change the synthetic raw materials of PU. The raw materials for synthesizing PU are polyols, polyisocyanates and chain extenders. As long as any of the components is fully or partially replaced by raw materials with three or more functionalities, a PU product with a certain degree of branching and cross-linking can be obtained. Due to the particularity of the water-based PU synthesis process, generally only a small amount of trifunctional and polyols can be used to replace difunctional polyols. If the amount of polyether triol is too large, resulting in too high system viscosity, the synthesized prepolymer will be difficult to disperse in water, and may even cause gelation. Therefore, when synthesizing cross-linked PU emulsion, this method is generally not used. Instead, a branched chain structure is introduced into the PU molecular chain through a multifunctional chain extender. Therefore, in recent years, research on the screening and synthesis of new cross-linking agents and multifunctional chain extenders has been quite active, and has become one of the main ways to improve the physical and mechanical properties and water resistance of water-based PU. 2． Add internal cross-linking agent The PU emulsion prepared by adding internal cross-linking agent is still a single-component system. The internal cross-linking agent can coexist stably with other components in the emulsion system. Only during construction and application, due to changes in the acidity, temperature, oxygen in the atmosphere, radiation and other factors of the system, it acts as a cross-linking agent to cross-link the glue layer. High-temperature curing internal cross-linking agent This type of internal cross-linking agent is temperature-sensitive and is quite stable at room temperature. At high temperatures, it can decompose active groups to participate in cross-linking. For example, the XP-7063 blocked isocyanate emulsion developed by Bayer Company in Germany can be mixed with other PU emulsions to form a stable one-component emulsion. After drying, the highly reactive NCO is regenerated through heat treatment and reacts with active groups such as hydroxyl, amino, and urea groups contained in the PU molecules to form a cross-linked coating. Amino resins such as melamine and other amino resins developed by Cytec Company under the brand name Cymel 303 belong to this type. Room temperature curing internal cross-linking agents carbodiimide and methimine, as early internal cross-linking agents, can exist stably in PU emulsion. The cross-linking reaction is catalyzed by acid. During the drying process of the coating film, due to the volatilization of water and neutralizing agent, the pH value in the film decreases, providing conditions for the cross-linking reaction to occur. Aziridine compounds have been used in coatings, textiles, medical and other fields for many years. In recent years, they have been used as internal cross-linking agents in PU emulsions. They can not only react with carboxyl groups, hydroxyl groups, etc., but can also self-polymerize in acidic environments, but are quite stable in alkaline environments. Chen et al. conducted a systematic study on diaaziridyl and triaziridyl compounds as internal vulcanizing agents for carboxyl-containing aqueous PU dispersions, and compared the reaction of monoaziridyl compounds with formic acid as a simulated reaction. The results showed that the vulcanization reaction is controlled by the pH value of the system. The vulcanizing agent undergoes a ring-opening reaction with the carboxyl groups in the polymer molecular chain to cross-link. Polyaziridyl vulcanizing agents can produce a cross-linked PU network. Recently, Olin Company in the United States uses urethane as an internal cross-linking agent and introduces it into the PU skeleton. The cross-linking mechanism is that excess diamine migrates into the latex particles and reacts with urethane to form biuret, which triggers an auxiliary cross-linking reaction. This reaction occurs within the latex particles and only increases the relative molecular weight of the PU chains in the latex particles without affecting the stability of the latex. The content, position and amount of diamine are the main factors that affect the degree of cross-linking. Zhang Yaojun and others added self-made latent cross-linking agents during the PU chain extension process to produce two types of self-crosslinking water-based PU coatings, which increased the water pressure resistance index of the fabric from the original 25~30 cm to 40~50 cm. Auto-oxidative cross-linking internal cross-linking agent This type of internal cross-linking agent adopts the cross-linking mechanism of white dry alkyd resin, introducing vegetable oil or fatty acids containing unsaturated bonds into the PU molecular chain, and using organic metal catalysts (such as cobalt, manganese, zirconium salts) to generate free radicals from oxygen in the atmosphere, triggering double bond cross-linking on the main chain. Radiation cross-linking internal cross-linking agent introduces double bonds at the end of the molecular chain of PU emulsion, initiating radiation and cross-linking modification. This type of research has not yet been reported in China due to the large investment. 3． Water-based PU with external linking agent added is also called water-based two-component PU coating. Usually water-based PU is one component and cross-linking agent is another component. The two components are mixed evenly during use. The coating undergoes a chemical reaction during the film-forming process or during heat treatment after film-forming to form a cross-linked structure. This type of PU emulsion is a type of coating with excellent comprehensive properties developed in the late 1980s and early 1990s. From the description of its performance in the literature, it can be seen that the operation process of water-based two-component PU coatings (such as film formation method, drying time, service life, etc.) U foam plastics, steel plates, galvanized copper plates and chromium-plated aluminum materials all have excellent adhesion) and physical and mechanical properties (such as the appearance, gloss, hardness, wear resistance, impact strength and tensile strength of the paint film), which can be comparable to solvent-based two-component PU coatings. More importantly, it uses water instead of organic solvents, and the mass concentration of volatile organic compounds is very small, and there is almost no pollution to the environment. The overseas development overview of this type of coating has been reported. The composition of the cross-linking agent is usually determined by the structure of the water-based PU. When PU molecules contain hydroxyl groups or amino groups, commonly used cross-linking agents include water-dispersed polyisocyanates, propylene oxide compounds, aziridine compounds, amino resins (such as melamine), etc. ； When PU molecules contain carboxyl groups, commonly used external linking agents include polyamines, aziridine compounds and certain metal compounds such as Al(OH)3, Ca(OH)2, Mg(OOCCH3)2, etc. ； When the PU molecule contains a certain special group, the external linking agent can also be selected according to the reaction characteristics of the group. For example, in the PU water dispersion system with epoxy groups, diamine compounds can be used as cross-linking agents. ； In addition, formaldehyde is also a commonly used cross-linking agent for water-based PU. It can form methyl ether cross-linking bonds between allophanates, urethanes and amino groups of PU-urea molecules. In order to better improve the performance of PU, internal cross-linking agents and external cross-linking agents can be added at the same time to cross-link and modify PU through a dual vulcanization system. There are many reports on the research on two-component water-based PU abroad, but it is still in its infancy in China. 4． Interpenetrating polymer network (IPN) modification technology is used. The application of IPN modification technology in water-based PU modification is usually based on in-situ polymerization method to synthesize PU emulsion with IPN structure. That is, when synthesizing PU prepolymer, acrylate or a mixture of acrylate and styrene is used as the solvent. When prepolymers are emulsified, organic peroxides are used to initiate emulsion polymerization. By selecting appropriate monomers, composite emulsions with different cross-linked structures can be produced. These cross-linked structures can exist in the network of their respective molecular chains or between the networks of two molecular chains. Lucas et al. added acrylic monomers with side groups such as epoxy groups such as glycidyl methacrylate to PU emulsion for polymerization. The resulting emulsion not only has an IPN structure, but also has permanent cross-links between the IPN structure and the emulsion. Experimental results show that the 100% modulus stress of this cross-linked IPN emulsion film is significantly increased. The PU-acrylic emulsion with IPN structure synthesized by Chen Yifang using external emulsification method also shows excellent performance. The existence of IPN structure makes the emulsion film have high water resistance, and no whitening phenomenon was observed after being immersed in water for 24 hours at room temperature. Therefore, the use of IPN modification technology to modify water-based PU will also be an important research direction in the future. 5. Conclusion The purpose of cross-linking modification of water-based PU is to improve its performance and reach the level of solvent-based two-component PU coatings. Foreign research on the modification of water-based PU is very active, and new modification technologies and new products are constantly being developed. Domestic research and development in this area started late and the level is relatively low. Although there are dozens of varieties of water-based PU coatings, there are still very few varieties that can meet the needs of high-end leather. There is an urgent need to develop coatings that adapt to trendy leather.