Atmospheric environmental pollution has always been a widely concerned problem, in addition to sand and other conditions, air pollution mainly comes from the emission of toxic gases, such as VOCS. With the continuous strengthening of people's environmental awareness and the implementation of various national policies, the reform of the printing industry as a key emission of VOCS has become inevitable. Environmentally friendly printing ink has therefore become the focus of printing industry research in various countries. At present, the environmentally friendly inks on the market are mainly water-based ink, energy curing ink, plant-based ink and other types, of which the most widely used is water-based ink, water-based ink has a relatively low proportion of organic solvents can inhibit VOCS emissions, is in line with the concept of environmental protection materials. However, water-based ink also has shortcomings such as slow drying and curing speed, water resistance and poor alkaline resistance, which greatly inhibit its application in traditional industrial inks, therefore, the use of resin composite modification ink shortcomings has also become a key project. In this paper, the development and application of water-based ink, the research on resin modification, and the research progress of printing ink on water-based polyurethane are reviewed.
 
1.experiment
 
（1）The development of water-based ink
Ink is produced with the emergence of printing, has a long history, in 1900 Risor red pigment factory ink began to prevail, countries began to pay attention to the study of ink, and water-based ink is a derivative product of people's higher requirements for ink utility research. In fact, as early as the 1960s, foreign countries have begun to study water-based ink, at that time mainly in order to accelerate the rate of printing, reduce the use of raw materials such as petroleum, the use of some benzene ring organic compounds and shellac, sodium lignate and other substances as the main material of ink, to meet the printing needs at that time. In the 1970s, in order not to change the ink gloss, water resistance and other excellent characteristics, while meeting environmental protection conditions, acrylic monomer and styrene polymerization, developed a core shell and network structure of polymer emulsion resin, to ensure the new generation of ink environmental protection and excellent gloss and other advantages. However, with the continuous strengthening of people's environmental awareness, governments of various countries have increased environmental protection laws, and the proportion of benzene organic compounds in ink has declined. In the 1980s, Western European countries began to formally propose the concept and technology of "green ink printing" and "new printing of water-based ink". The earliest ink industry in China began from the printing of paper money in the former Qing paper mill, which basically relied on imported ink at that time. Until 1975, Tianjin Ink Factory and Gangu Ink Factory jointly developed and produced the first domestic water-based gravure paper ink. By the 1990s, more than 100 sets of flexible letterpress joint printing production lines were introduced from abroad, and water-based ink developed rapidly in China.
 
By 2003, China Industrial Technology Research Institute has successfully developed water-based ink related products. At the beginning of 2004, Shanghai Meide Company carried out fine chemical development and produced a water-based, low-temperature thermosolid ink that could meet the requirements of Japan and Germany. Although China's water-based ink research has developed rapidly in the early 20th century, based on national policies and economic development, Western countries such as Europe and the United States have taken the lead in achieving relatively good research and application results: in the United States, about 95% of flexo printing products and 80% of gravure products use water-based ink; British food and pharmaceutical packaging has been replaced with water-based ink; Specific inks for polypropylene film were developed in Japan. Relatively speaking, China's water-based ink development is relatively slow.
 
In order to better promote the water-based ink market, China launched the first water-based ink standard in May 2007; At the same time, the concept of "green innovation and development" was proposed in 2011, aiming to replace solvent-based inks with water-based inks that meet the concept of environmental protection. In 2016, China proposed that the printing industry "13th Five-Year" development plan will be "water-based environmental protection materials research and development" and "green printing" as the key research direction of the industry; In 2020, the national issue of "Promoting Green printing and digital printing". China's water-based ink market continues to expand.
 
（2）Water-based ink application
In the early 20th century, the United States was the first to use water-based ink in flexographic printing. By the 1970s in China, high-quality water-based gravure inks were gradually widely used in various wrapping paper, thick colored bookshelves and cardboard. In the 1980s, bright and matte water-based screen printing inks were developed abroad to cope with the widespread use of water-based inks on fabrics, paper, polyvinyl chloride, polystyrene, aluminum foil and metal substrates. At present, water-based ink because of its environmental protection, safety and other advantages, mainly used in food outsourcing printing, such as tobacco shell printing, beverage bottle printing and so on. With the continuous improvement of environmental protection laws in various countries, the use of water-based ink is also more diversified and intensive. China is also continuously promoting the use of water-based ink in the printing industry.
 
 
2. Results and discussion
 
（1）Research on resin modification
The properties of inks are affected by resin differences. In general, water-based ink resins mostly choose polyurethane resin, modified acrylic emulsion or polyacrylic resin three categories, in gloss and other tests, water-based polyurethane advantage is higher, in packaging and printing is widely used. Therefore, by changing the performance of water-based polyurethane to improve the environmental protection of water-based ink, gloss and other issues have become the development of the printing industry in various countries.
 
（2）Through the selection of water-based polyurethane modification
Waterborne polyurethane is composed of oligo-polyols, can be divided into polyester type, polyether type and mixed type three types, according to the different properties of polyester type and polyether type polymer, its strength and stability are different. Generally, the strength and stability of polyether polyurethanes are weaker than that of polyester polyurethanes, but they are stronger in terms of high temperature tolerance, and are not easily hydrolyzed, and have higher relative hardness. It is through the selection of polyethylene glycol monomethyl ether compound to increase the "consistency" of the ink, improve the characteristic tolerance of the ink. However, this is only a reference for curvature, and various research institutes generally use the improvement of polyurethane to improve a certain aspect of the purpose. For example, when dealing with the problem of ink viscosity and viscous strength in 2010, epoxy resin, a resin with high toughness impact strength, was used through screening, so as to achieve the purpose of improving the strength of ink; In contrast, in 2006, the "Polyurethane Modification Research" published in Beijing Chemical Industry selected ethylene glycol terminal amino polyurethane for polymerization, forming a special resin with a long flexible segment, improving the flexibility of the ink, toughening the water-based ink from the side, and also achieving the purpose of improving the strength of the ink. Some teams are modified by adding some chemicals: polyurethane is improved by adding silica or silicone, and after polymerization, the water-based resin improves the tensile strength of the ink; The end carboxyl nitrile polyurethane is used to polymerize into a resin, thereby improving the bending performance and viscosity of the ink to adapt to more complex environments.
 
Therefore, usually people choose specific polymeric esters according to the properties of the developed ink, and at the same time meet the requirements of synthesizing temperatureproof polyester polyols by selecting appropriate polyacids and polyols, introducing polar groups with strong adhesion to the material, and selecting appropriate raw materials. There are four general characteristics of improving the crystallization of polyurethane, and improving the moisture and heat resistance of waterborne polyurethane ink adhesive by using coupling agent and other additives.
 
① Selective modification of waterborne polyurethane
Waterborne polyurethane in ink mainly reflects its stability, high temperature resistance, hydrolysis, strength and so on. Improving the performance of the ink is mainly to optimize the water-based polyurethane, which can be modified through the combination of special raw materials, so as to improve the inherent characteristics of the stability of the water-based polyurethane and achieve the purpose of optimizing the water-based ink.
 
② Water resistance modification
The ink is mainly used for outer packaging printing, and the contact time with water is longer, if the water resistance of the ink is poor, it is easy to cause the hardness and gloss of the packaging ink to be reduced, and even fall off and damage, which greatly affects the storage performance of the ink. This mainly relies on the resin in the synthetic ink material, that is, the water resistance of water-based polyurethane, to improve the endurance of water-based polyurethane materials in water, that is, to optimize the storage performance of ink. Since waterborne polyurethane is composed of oligo-polyols, it can be used as a material with good water resistance to improve the water resistance of waterborne polyurethane. For example, the use of acrylic monomers to modify WPU improves the water resistance of ink; The water absorption of WPU emulsion is improved by adjusting the content of epoxy resin to meet the high demand of specific water-based inks.
 
In general, in addition to directly replacing ordinary polyurethane with a polymer with high water resistance to improve the water resistance or water absorption of the ink, most researchers use the addition of an organic or inorganic substance to achieve the adjustment effect. For example, by adding nano-grade silica in the resin to improve the water resistance of the resin, which not only improves the water resistance of the ink, but also improves the strength of the ink, the application is more extensive, most ink industrial production uses this method; The composite PUA was formed by emulsion copolymerization to improve the water resistance of the ink. Modified by polyethylene glycol monomethyl ether; Silicone modified WPU emulsion was synthesized by acetone method. Using silane as modifier, organic silicon modified sulfonic polyurethane was synthesized to improve the water resistance of ordinary polyurethane.
 
③ High temperature resistance modification
Generally, the high temperature resistance of waterborne polyurethane is relatively weak, which also leads to the poor heat resistance of waterborne ink. Due to the number of double bonds, the high temperature resistance of polyether polyurethane is generally better than that of polyester polyurethane. By adding long carbon chain polymers with strong heat resistance or phenyl ester and ether substances as polymeric monomers, the high temperature resistance of the polymer is optimized. It can also improve the heat resistance of water-based ink. In addition to the use of long carbon chain polyether polyurethane to improve the high temperature resistance of ink, some experimental teams also use multi-material composite to increase material complexity, curvature to improve the high temperature resistance of ink, such as: WPU was synthesized from DMPA, polyether 220 and IPDI by acetone method, and a certain amount of nano-tin antimony oxide was added to make the ink layer absorb certain heat energy and achieve high temperature resistance. By adding silica aerogel to improve polyurethane, reduce thermal conductivity and improve heat resistance of ink.
 
④ Stability modification
The stability of waterborne polyurethane greatly affects the storage performance of waterborne ink. In addition to water resistance and high temperature resistance, more important is the molecular weight and structure of polyurethane, polyester resin is more stable than polyether because of the hydrogen bond in the molecular structure. Therefore, a certain amount of esters can be added to participate in the polymerization to form a mixed polyurethane, which has higher stability. For example, a two-component water-based polyurethane is formed by polymerization of isocyanate and silico-propyl aqueous dispersion, which has been greatly improved in stability. The test proves that the material has good wear resistance to ink at the same time. At the same time, heat treatment and cooling can also form more hydrogen bonds to make the molecules more closely arranged, in order to improve the stability of water-based polyurethane and improve the storage performance of water-based ink.
 
⑤ Improved adhesion
The water-based polyurethane optimized by the synthesis of water-based ink materials can be improved in terms of water resistance, high temperature resistance, stability, etc., but due to the molecular weight and polarity of water-based polyurethane, polyurethane ink has poor adhesion performance for polyethylene (PE) plastic products. In general, the industry often adds polymers or monomers similar to the polarity and molecular weight of the material to improve the performance of water-based polyurethane, in order to improve the viscosity of water-based ink for some non-polar materials. For example, through the direct polymerization of two polyurethanes, a copolymer containing two hydroxyl groups of polyvinyl chloride - hydroxyethyl acrylate resin can be effectively improved between the waterproof coating and other ink waterproof bond stability; By adding an acrylic polyester resin as a polyurethane adhesive resin with optimized waterproof performance, a special molecular link structure is formed, which greatly improves the bonding property of waterborne polyurethane. However, these methods usually have a certain impact on the original properties of water-based inks such as gloss. Therefore, the industry also through some special methods to treat the material, without changing the characteristics of the material and ink, improve the adhesion of the ink, such as: the use of electrodes to make the material surface active, generate polar groups, thereby increasing the adsorption force of polyurethane molecules; Using the high temperature characteristics of flame combustion, the material is processed by the flame for a very short time, so that it bombards the invisible "flesh thorn" to achieve the effect of improving the adsorption performance.
 
 
2.conclusion
 
At present, water-based ink has been widely used in food packaging printing, pharmaceutical packaging printing, workshops, books and other coating or printing, the application is more and more widely, but due to the lack of water-based ink's own performance limits a wider range of applications. With the continuous popularization and improvement of environmental awareness and safety awareness brought about by the improvement of people's quality of life, water-based environmentally friendly inks that can inhibit the emission of VOCS are constantly occupying the solvent-based ink market, and the traditional solvent-based ink market is being greatly impacted. Therefore, optimizing its water resistance, heat resistance, stability, etc. is an inevitable choice, and improving the light curing speed and utilization rate of water-based ink is also the key point to strengthen research. Future research on the modification of water-based inks should focus on the light curing rate, special resistance (such as corrosion resistance, mold resistance, etc.), and further develop water-based inks with excellent comprehensive properties.

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