This paper describes the development of solvent-free polyurethane adhesives and their coating processes and application techniques in composite film packaging materials. In the early 1970s, the former West German Herberts company first used solvent-free polyurethane adhesive to make composite film packaging materials. Because of its obvious economy, safety and environmental protection advantages, it has been in Germany, the United States, Japan and the world. There has been great progress. In Europe and the United States, the solvent-free composite processing volume in the past was less than 10% of the total processing volume of the composite film packaging material, and it has been developed to more than 30% of the processing in the solventless composite process. Since the introduction of the first solvent-free dry compound machine in Japan in 1982 (1977), it has grown to more than 60 solvent-free composite equipment. The manufacture of composite film packaging materials with solvent-free adhesives has become a mature application technology. At the same time, the application and development of the process has also promoted the development of research and production of solvent-free polyurethane adhesives. In order to promote the development of China's food packaging composite film processing industry, Beijing Chemical Industry Research Institute introduced the solvent-free polyurethane adhesive production technology and equipment from HENKEL in 1985, becoming the earliest unit in China to produce such products. However, before 1995, there was only one solventless laminating machine in China, and the demand for solventless polyurethane adhesives was very small. In recent years, the rise in raw material prices, especially the increase in solvent prices, has reduced the economic benefits of the composite film processing industry, and the government’s restrictions on environmental impact and people’s awareness of environmental protection, coupled with the new composite film processing industry. Knowledge and understanding of technology. It has not developed rapidly until the past five years. Up to now, there are more than 20 non-solvent dry laminating machines in China. The demand for solvent-free polyurethane adhesives has also increased significantly. This paper mainly introduces the development of solvent-free polyurethane adhesives and its application technology in food packaging composite film processing. 2. Comparison of solvent-free dry compounding and solvent dry compounding The main processing method for manufacturing food packaging composite film materials--dry compounding, the adhesive used for a long time is a polyurethane-based solvent-based adhesive using organic solvent as medium. This type of adhesive has many advantages and is widely used, and it plays an important role in the processing of composite membranes. However, a large amount of solvent is discharged into the atmosphere every year during use, causing serious environmental pollution and waste of resources. Solvent-free adhesive dry compounding has the following advantages in addition to the advantages of using solvent-based adhesive dry compounding: (1) Without an organic solvent, the cost is lowered. (2) There is no environmental pollution caused by the evaporation of organic solvents. (3) No solvent evaporation drying section is required, which reduces energy consumption. (4) There is no danger of fire and explosion, no explosion-proof measures for solvents are required, and equipment and warehouses for storing solvents are no longer needed. (5) The composite article has no residual solvent damage problem and eliminates the attack of the solvent on the printing ink. (6) It does not contain an organic solvent, and the composite substrate is easily affected by the solvent and high temperature drying damage, and the composite film has good dimensional stability. (7) In composite applications, the same quality requirements, the amount of solvent-free adhesive coating is less than the coating amount of solvent-based adhesive, saving cost and has good economics. (8) The equipment is simple, the floor space is small, and the investment is saved. 3. Development of solvent-free polyurethane adhesives Solvent-free dry composite adhesives and solvent-based dry composite adhesives are also polyurethane series. It maintains the characteristics of a solvent-based polyurethane adhesive and can be well coated without any solvent (ie, 100% solids adhesive). Solvent-free polyurethane adhesives can also be made into one-component and two-component. The appearance of the solvent-free polyurethane adhesive is a very viscous liquid at normal temperature, and a low-viscosity flowable liquid under heating, which is suitable for coating without a solvent dry machine. It is different from the hot melt glue that is usually seen. The hot melt is solid at room temperature, most of which is composed of a thermoplastic polymer composition or a natural material, and the solventless polyurethane adhesive is mostly composed of a reactive component. In recent years, many countries in the world have conducted in-depth research on this type of adhesive, developed a variety of practical solvent-free dry composite polyurethane adhesives, and fully achieved the various application properties of solvent-based adhesives, can meet the food packaging compound Membrane materials for various performance requirements First generation solventless adhesives. One-component solvent-free polyurethane adhesives are first used in solvent-free dry compounding (such as UR7505 and UR7506 produced by Beihua Institute, see Table 1). The one-component polyurethane adhesives all contain NCO groups which are cured by the reaction of moisture in the air and water adhering to the coated film. Although the single component has advantages such as ease of use, it also has disadvantages. When the coating amount exceeds 2 g/m2, curing failure occurs, which is not possible to completely react with water due to the water (moisture) curing type, so the use is limited, and the coating amount can only be <2 g/m2. Thus, the requirement of resistance to cooking cannot be achieved, and the range of processed composite products is affected. (2) Second-generation solvent-free adhesive (two-component solvent-free adhesive) To overcome the shortcomings of one-component adhesives, solvent-free two-component polyurethane adhesives have been developed. This product is also known as the second generation. This type of adhesive consists of two sets of polyurethane prepolymers. When used, the two components are uniformly mixed together to form a macromolecule by mutual reaction to achieve cross-linking and curing. (For example, UR7330/UR8150 produced by Beihua Institute is shown in Table 1.) This adhesive has lower viscosity than single-component adhesive, and some varieties can be used at normal temperature. And in order to achieve high initial bonding strength, the reaction speed is very fast. Two-component solvent-free polyurethane adhesives, when used, require a device that automatically supplies the coating system for mixing the two components, typically using a metering pump. In terms of use, films other than EVA and nylon and aluminum foil can be used. The transparent film is resistant to low temperature sterilization and cooking. The main reason why it can not be used in EVA and NY is to reduce the viscosity of the adhesive. Low molecular weight polyisocyanate is used, which will react with the amide compound of EVA and NY to form insoluble compounds. The composite article is poorly sealed. (3) The third generation of solvent-free adhesives. The purpose of research and development of solvent-free adhesives is to maintain the same adhesive properties as solvent-based adhesives. In composite films containing aluminum foil structures, some adhesives that are resistant to high temperature cooking (120 ° C for 30 minutes) are required. The third generation of solvent-free polyurethane adhesives (such as those produced by Henkel in Germany) were developed. The third-generation solvent-free adhesive has the following characteristics: strong initial adhesion, low viscosity, operating temperature <80 ° C, no restrictions on composite substrates, and excellent resistance to contents. The second-generation adhesive mentioned above solves the problem of poor heat sealing of EVA and NY films, and the problem that the barrier film is easy to wrinkle or peel off. 4. Demand and development of domestic solvent-free polyurethane adhesives Since 1996, domestic composite film production plants have gradually recognized the superiority of solvent-free composite processes. They have introduced more than 20 solvent-free composite film production lines, requiring nearly 1,000 tons of solvent-free adhesives per year, and solvent-free composite film adhesives in the domestic market. Demand is growing rapidly. Since there are only products available in our hospital and there are fewer varieties, we mainly rely on imports. The price of imported adhesives is higher, due to the structure of domestic composite film products. Due to differences in raw materials, etc., the same type of adhesive is used in Europe and America, but there are often problems in China. For example, solvent-free adhesives are used for films with high slip agent content, EVA films, CPA films, and the like. The migration of the isocyanate monomer is prone to occur, and the composite film is poorly heat-sealed when the bag is matured. In order to change the status of polyurethane adhesives for solvent-free composite film mainly relying on imports, our institute has invested a lot of manpower and material resources on the basis of imported technology and developed new products for polyurethane adhesives for solvent-free composite film suitable for China. A: Low-temperature (70 ° C) coated one-component solvent-free polyurethane adhesive UF7508 Although the one-component solvent-free polyurethane adhesive has some shortcomings, it is still widely used due to its convenient use, good initial adhesion, no mismatch, and low price. However, the early development of single-component solvent-free polyurethane adhesives used in composite film coating at higher temperatures (90-100 ° C), although heating can reduce viscosity, easy to apply, but the temperature is too high, the adhesive reacts quickly, The viscosity is also increased quickly, which is not conducive to coating. At the same time, the amount of coating has a great relationship with the viscosity of the adhesive. When the viscosity is high, the coating amount is large, and when the viscosity is low, the coating amount is small. To achieve a stable coating amount, it is necessary to minimize the change in viscosity of the adhesive. Moreover, the coating rubber roller of the solventless laminating machine is immersed in the adhesive of 90-100 ° C for a long time, which easily causes deformation and damage of the coating rubber roller. Therefore, in the development of one-component solvent-free polyurethane adhesives, determining the optimum coating temperature is an important factor. The single-component solvent-free polyurethane adhesive UF7508, which was successfully developed in our hospital in 2000, is a new adhesive product that can be coated at a lower temperature (70 ° C). The one-component polyurethane adhesive is mostly a polyisocyanate prepolymer containing NCO groups, which is cured by reacting water in the air with the water adhered by the coated film. The one-component solventless polyurethane adhesive type is mostly the following Two kinds: 1 Polyether polyurethane polyisocyanate: OCN-R-NCO+H-(-0R'-). -OH--→OCN-R-NH-CO-(-OR'-)nO-CO-NH-NCO (diisocyanate) (polyether polyol) (polyether polyurethane polyisocyanate) 2 polyester polyurethane polyisocyanate OCN-R-NCO + HO～OH--→OCN-R-NH-CO-O～O-CO-NH～NCO (diisocyanate) (polyester polyol) (polyester polyurethane polyisocyanate) The UF7508 developed by our institute is a mixture of these two types. It has the characteristics of high bonding strength of polyester polyurethane and improved hydrolysis resistance by polyether polyurethane. B: New two-component solventless polyurethane adhesive with low free isocyanate monomer (UR7980/UR8160) A quality problem in the current two-component polyurethane adhesive for solvent-free composites is that the adhesive contains more free isocyanate monomer, and the small molecule compound generated easily migrates into the film to cause contamination of the packaged product, and is small. The molecular product reduces the heat seal strength of the package. In 2000, a new two-component solvent-free polyurethane adhesive with low free isocyanate monomer developed by our institute met the needs of domestic customers and promoted the development of this technology in China. The two-component solventless polyurethane adhesive consists of two sets of polyurethane prepolymers, the chemical structure of which is as follows. The main component is typically a polyester polyurethane prepolymer (generally containing a reactive group - OH). OCN-R-NCO + HO～OH--→HO～O-CO-NR-R-NH-CO-O～OH (diisocyanate) (polyester polyol) (polyester polyurethane prepolymer) The other component (also known as the curing agent) is a polyisocyanate prepolymer (generally containing NCO groups): OCN-R-NCO+OH~R"-OH--→OCN-R-NH-CO-O-R"-O-CO-NH-R-NCO (diisocyanate) (polyol) (polyisocyanate prepolymer) Note: In the above formula, R, R\, and R" are a hydrocarbon group or a phenyl group. When used, the two components are uniformly mixed together, and the macromolecules are formed by mutual reaction to achieve cross-linking curing. When a solvent-free two-component agent is used, the product is required to have a lower viscosity to ensure an operating temperature of between 40 and 50 °C. The viscosity of the solventless adhesive is inversely proportional to the content of the isocyanate groups in the prepolymer, and the content of the free diisocyanate group in the prepolymer is lowered, which inevitably lowers the content of the isocyanate in the product, thereby increasing the viscosity of the product. The operating temperature is increased and the operating conditions are not easily controlled. The new two-component solvent-free polyurethane adhesive of low free isocyanate monomer developed by our institute not only reduces the content of free diisocyanate in the product, but also makes the product have appropriate viscosity, and the operating temperature is moderate to meet the requirements of customers. 5. Coating and use of solvent-free adhesives The coating of solventless polyurethane adhesives is different from the coating of solvent based adhesives. When the solvent-based adhesive is applied, in order to make the coating uniform, the viscosity which is easy to apply is adjusted and controlled by increasing the organic solvent. Since solvent-free adhesives do not contain any solvent at all, the viscosity at room temperature is quite large (especially when solvent-free one-component adhesives exceed tens of thousands or hundreds of thousands of mpas at room temperature). According to the particularity of the structure of the solventless polyurethane agent, as long as the temperature is slightly changed, the viscosity thereof is greatly changed. In order to make the viscosity drop suitable for coating, the adhesive is usually heated during use. Therefore, the solvent-free adhesive coating compounding equipment should be a dedicated device with a heated coating and gluing system. The solventless compound machine consists of a mixed metering and dispensing system; a coating and gluing system; a composite cooling system; a unwinding and winding system. 1). Solventless compound coating system At present, the coating system of the solventless laminating machine introduced in China is mostly a coating system of a special solventless laminating machine, which is also called a four-roll coating system (see Fig. 1). It consists of A. B. CD four-roller composition, A-roller is a coating roller; B-roller is a floating roller; C-roller is a metering roller; D-roller is a glue-fixing roller, and the glue between the fixed roller D and the metering roller C is stored. 2). Use and coating of solvent-free adhesives 1 coating viscosity and temperature: Solvent-free adhesives vary greatly in molecular weight compared to solvent-containing adhesives. The molecular weight of the solvent polyurethane adhesive is generally from tens of thousands to hundreds of thousands. When there is no solvent, the viscosity is also large, and the normal temperature reaches millions of milliamperes per second (mpas), even a gel, which cannot be directly coated. Therefore, it is diluted to a coatable viscosity by means of solvent dissolution. When the solvent-free adhesive is similar in structure to the solvent-based adhesive, in order to achieve the viscosity required for the solvent-free coating operation, the molecular weight of the adhesive is lowered, and the viscosity is much smaller than that of the solvent-based adhesive, but it cannot be directly coated. The structural characteristics of the solvent-free polyurethane adhesive are: large intermolecular attraction (hydrogen bond formation), high viscosity at normal temperature, hydrogen bond cleavage and viscosity decrease when temperature rises. It is a one-component solvent-free adhesive viscosity versus temperature curve. Therefore, heating increases the viscosity to a coatable viscosity. Temperature increase viscosity drop coating is easy; but the temperature is too high (one-component adhesive is greater than 100 ° C, two-component adhesive is greater than 80 ° C), the internal reaction speed of the adhesive is accelerated to gel, easy to form polymer resin, resulting in coating Difficulty and uneven coating. It is therefore necessary to determine the optimum temperature for the coating and to maintain a minimum temperature difference. Generally, the coating temperature of the one-component adhesive is controlled at 70-100 ° C, the viscosity is controlled at 600-3000 mpas; the coating temperature of the two-component adhesive is controlled at 45-80 ° C, and the viscosity is 500-1500 mpas. 2 coating and curing of one-component adhesive. The coating of the one-component adhesive is in addition to the temperature and viscosity controlled above, since the one-component polyurethane adhesive contains NCO groups (contents in the range of 3-6%), which is cured by reaction with water. When the coating amount is lg/m2, the amount of water that needs to be completely cured is 6.5 to 13 mg. The water adhering to the surface of the dried film alone is insufficient (OPP measured at 32 ° C at 38 ° RH, and the surface of the CPP film adhered to water of about 2-4 mg/m 2 ). Water must be replenished from the outside, so the environment must be humidified. 3 coating amount In solventless composite processing, the amount of adhesive applied is critical to the quality of the composite film. Insufficient coating amount, easy to cause adhesion of the composite film is not large, easy to peel off or even delamination. The choice of the optimum coating amount of the adhesive should be based on the different quality requirements of the composite film product, the film properties, the printing effect, and the performance of the ink and the adhesive. Generally, the single component coating amount is 0.5-1.8g/m2. When the coating amount is >2g/m2, because the coating is thick, it is not conducive to the curing reaction of the adhesive and water, causing the center of the film layer to not cure for a long time, affecting the bonding strength of the composite product, and using the two-component solventless polyurethane adhesive. There is no such restriction. However, if the coating amount is too large, it is not conducive to the composite quality. Generally, the coating amount is controlled at 1 to 3.5 g/m2, and when the composite film of special needs is required (for example, for high-temperature cooking), it can be increased to not more than 5.0 g/m2. 4 initial adhesion and bond strength. Since the molecular weight of the solvent-free adhesive used is lower than the molecular weight of the solvent-based adhesive, the cohesive force is small at normal temperature, and the initial adhesive strength before uncured is low. Therefore, the tension and the traction force should be strictly controlled in the composite production operation, otherwise the composite will be produced. Quality problems of film products. Now adhesive manufacturers are developing products with strong initial adhesion, and this problem is expected to be better solved. 6. Conclusion The technology of solvent-free adhesive dry composite manufacturing of composite film packaging materials has become a mature industrial technology after more than 20 years of development. Especially in the past five years, China's composite film processing industry's understanding and understanding of this new technology, and the successful introduction of more than 20 solvent-free composite equipment experience, solvent-free dry composite manufacturing composite film packaging materials technology will be obtained in China Great development. And with the localization of solvent-free composite equipment and the variety of solvent-free adhesives, this new processing technology will gradually be recognized and accepted domestically with its obvious outstanding superiority. It is expected that in the next few years, China will develop at a faster rate. Mens Work Overalls,Blue Work Overalls,Insulated Work Overalls,Basic Overall Tomax Garments Factory , https://www.tomaxgarments.com