This article describes the development of solvent-free polyurethane adhesives and their coating technology and application technology in composite film packaging materials.
Since the early 1970s, Herberts of West Germany first made composite film packaging materials with solvent-free polyurethane adhesives. Because of its obvious economical, safety, and environmental protection advantages, it has been used in Germany, the United States, Japan, and the world. There is great progress. In Europe and the United States, the amount of solvent-free composite processing in the past was less than 10% of the total processing volume of composite film packaging materials, and more than 30% of the total processing volume was now processed with a solvent-free composite process. Since the introduction of the first solventless dry laminating machine in Japan in 1987 (Showa 52), it has now grown to more than 60 solvent-free composite machines. Solvent-free adhesives for the manufacture of composite film packaging materials have become mature applications. At the same time, the application and development of this 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, the Beijing Chemical Industry Research Institute introduced the solventless polyurethane adhesive production technology and equipment as early as 1985 by HENKEL (Germany) and became the first unit to produce such products in China. However, there was only one solventless laminator in China before 1995, and the demand for solvent-free polyurethane adhesives was small. In recent years, the increase in raw material prices, especially the increase in solvent prices, has reduced the economic benefits of the composite film processing industry. Due to the government’s restrictions on the environment and people’s awareness of environmental protection, combined with the new film processing industry, the new Technology awareness and understanding. It has not been developed rapidly until the last five years. At present, there are more than twenty statistically-based non-solvent dry laminating machines in China. The demand for solvent-free polyurethane adhesives has also increased substantially. This article mainly introduces the development of non-solvent polyurethane adhesives and its application technology in the processing of food packaging composite films.
2. Comparison of solvent-free dry compounding and solvent dry compounding
The main processing method for producing food packaging composite film materials is dry-type compounding. The adhesive used for a long time is a polyurethane solvent-based adhesive with an organic solvent as its medium. This kind of adhesive has many advantages and is widely used. It plays an important role in the processing of composite films. However, in use, a large amount of solvents will be discharged into the atmosphere each year, causing serious environmental pollution and waste of resources. In addition to the advantages of solvent-free adhesive dry compounding in the use of solvent-based adhesive dry compounding, it also has the following features:
(1) Without organic solvents, the cost is reduced. (2) There is no environmental pollution caused by the volatilization of organic solvents. (3) Solvent evaporation drying section is not needed, reducing energy consumption. (4) There is no risk of fire or explosion, no need for explosion-proof measures for solvents, and no need for equipment and warehouses for storing solvents. (5) The composite product has no problem of residual solvent damage, and eliminates the attack of the solvent on the printing ink. (6) It does not contain organic solvents, eliminating the effect of composite substrates being susceptible to solvent and high-temperature drying being damaged, and the dimensional stability of the composite membrane structure is good. (7) In the composite application, the same quality requirement, the coating amount of the solvent-free adhesive is less than that of the solvent-based adhesive, and the cost savings are very economical. (8) The equipment is relatively simple, the floor space is small, and 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 the solvent-based polyurethane adhesive and does not require any solvent (ie, 100% solids content of the adhesive) to perform the coating compounding well. Solvent-free polyurethane adhesives can also be made into single and double components. The appearance of the solvent-free polyurethane adhesive is a very viscous liquid at room temperature, and it is a low-viscosity flowable liquid in the case of heating, and can be applied to the coating of a solventless dryer. It is different from commonly seen hot melt adhesives. Hot melts are solid at room temperature, and most of them are thermoplastic polymer compositions or natural products. Solvent-free polyurethane adhesives are mostly reactive components.
In recent years, many countries in the world have conducted in-depth research on this type of adhesive, developed a variety of practical non-solvent dry composite polyurethane adhesive, and fully achieved the various application properties of solvent-based adhesives, can meet the food packaging composite Membrane materials require a variety of performance first-generation solvent-free adhesives.
One-component solvent-free polyurethane adhesives were first applied in solvent-free dry compounding (such as UR7505 and UR7506 produced by North Chemical Research Institute, see Table 1). One-component polyurethane adhesives contain an NCO group that is cured by reacting with water in the air and the water attached to the coated film. Although the single component has the advantages of convenience and the like, there are also disadvantages. When the coating amount exceeds 2 g/m2, poor curing occurs. This is due to the water (moisture) curable type, and the coating thickness cannot reach a complete reaction with water, so the use is limited and the coating amount can only be <2 g/m2. As a result, the requirement for cooking resistance cannot be achieved, and the range of processed composite products is affected.
(2) Second-generation solvent-free adhesive (two-component solvent-free adhesive)
In order to overcome the shortcomings of one-component adhesives, people developed a solvent-free two-component polyurethane adhesive. This product is also known as the second generation product. This type of adhesive is composed of two polyurethane prepolymers. When used, the two components are uniformly mixed together and cross-linked and cured by mutual reaction to form macromolecules. (For example, UR7330/UR8150 manufactured by North Chemical Industry Park is shown in Table 1.) This type of adhesive has lower viscosity than single-component adhesives. Some varieties can be used at room temperature. And in order to reach the initial high bond strength, the reaction speed is very fast. When two-component solvent-free polyurethane adhesives are used, in order to mix the two components, an apparatus for automatically feeding the coating system is required, and metering pumps are usually used for this purpose. In terms of use, films other than EVA, nylon, and aluminum foil can be used. The transparent film is resistant to low temperature sterilization and steaming. The main reason why it cannot be used on EVA and NY is to use a low molecular weight polyisocyanate to reduce the viscosity of the adhesive. It will react with amides of EVA and NY to form insoluble compounds. Bad heat sealing of composite products.
(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 require adhesives that can withstand high-temperature cooking (120°C for 30 minutes), for this purpose. People developed and researched the third-generation non-solvent polyurethane adhesives (as shown in Table 2 by Henkel). The third-generation non-solvent adhesive has the following characteristics: strong initial adhesion, low viscosity, operating temperature <80 °C, no restrictions on the composite substrate, excellent resistance to content properties. The aforementioned second-generation adhesives solve the problem of poor heat sealing of EVA and NY films, and the problem of easily wrinkling or peeling of films with strong barrier properties.
4. Demand and Development of Domestic Solvent-Free Polyurethane Adhesives
Since 1996, domestic composite membrane production plants have gradually realized the superiority of solvent-free composite processes. They have introduced more than 20 solvent-free composite membrane production lines. Each year, nearly 1,000 tons of solvent-free adhesives and solvent-free composite film adhesives are needed in the domestic market. Demand is growing rapidly. Due to the fact that only our hospital has a supply of products and there are few varieties, we mainly rely on imports. The price of imported adhesive is higher, but also due to the domestic composite film product structure. For raw material differences and other reasons, the same type of adhesive used in Europe and the United States is no problem, but it is often used in China. For example, solvent-free adhesives are used for films with high slip levels, EVA films, CPA films, and the like. Isocyanate monomer migration is prone to occur, and some composite films are poorly sealed during maturation.
In order to change the status of polyurethane adhesives for solvent-free composite membranes mainly relying on imports, our institute has invested a lot of manpower and material resources on the basis of the introduction of technology and developed new products for polyurethane adhesives for solvent-free composite membranes suitable for China's situation.
A: One-component solvent-free polyurethane adhesive UF7508 for low temperature (70°C) coating
Although single-component solvent-free polyurethane adhesives have some disadvantages, they are still widely used due to their ease of use, good initial tack, lack of worry about mismatch, and low price. However, the early-developed one-component solvent-free polyurethane adhesives have a higher temperature (90-100°C) when used for coating of composite films. Although heating can reduce the viscosity and facilitate coating, the temperature is too high and the adhesive reacts quickly. The increase in viscosity is also rapid and is not conducive to coating. At the same time, the coating amount 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 obtain a stable coating amount, it is necessary to minimize changes in the viscosity of the adhesive. And the solvent-free laminating machine's coating roller is immersed in the adhesive at 90-100°C for a long time, which can easily cause deformation and damage of the coating roller. Therefore, in the development of one-component solvent-free polyurethane adhesives, determining its optimum coating temperature is an important factor. The one-component solvent-free polyurethane adhesive UF7508 developed by our institute in 2000 is a new adhesive product that can be applied at a lower temperature (70°C).
One-component polyurethane adhesives are polyisocyanate prepolymers containing NCO groups. They are cured by reacting with water in the air and the water attached to the coated film. The one-component non-solvent polyurethane adhesives are mostly of the following types. 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)
2Polyester 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 blend of these two types. It has the characteristics of high bonding strength of polyester urethanes and improved hydrolysis resistance from polyether urethanes.
B: New two-component solvent-free polyurethane adhesive with low free isocyanate monomer (UR7980/UR8160)
One of the quality problems of current solvent-free composite bicomponent polyurethane adhesives is that the adhesive contains more free isocyanate monomers, and the resulting small molecule compounds easily migrate into the film to cause contamination of the packaged product, and is also small. Molecular products reduce the heat seal strength of the package. In 2000, the new type of 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 solvent-free polyurethane adhesive consists of two polyurethane prepolymers. The chemical structure is briefly shown below.
The main component is generally a polyester polyurethane prepolymer (usually containing an active 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: R, R\, R†in the above formula is 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-link curing. When solvent-free two-component agents are used, the product is required to have a lower viscosity to ensure that the operating temperature is between 40-50°C. The viscosity of the solvent-free adhesive is inversely proportional to the content of isocyanate groups in the prepolymer, and reducing the content of free diisocyanate groups in the prepolymer inevitably lowers the content of isocyanate groups 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 developed by our institute with low free isocyanate monomer not only reduces the content of free diisocyanate groups in the product, but also enables the product to have an appropriate viscosity, and the operating temperature is moderate to meet the requirements of customers.
5. Solvent-free adhesive coating and use
The coating of solvent-free polyurethane adhesives is different from the coating of solvent-based adhesives. When a solvent type adhesive is applied, in order to make the coating uniform, the viscosity for easy application is adjusted by adding an organic solvent. Since solvent-free adhesives do not contain any solvents at all, the viscosity at room temperature is quite large (especially when solvent-free one-component adhesives exceed tens or hundreds of thousands of mpas at room temperature). According to the particularity of the structure of the solvent-free polyurethane agent, as long as the temperature is slightly changed, the viscosity has a great change. In order to make the viscosity drop suitable for coating, the adhesive is usually heated during use. Therefore, a solvent-free adhesive coating composite device should be a dedicated device with a heated coating and gluing system. Solvent-free laminating machine is composed of mixing metering glue system; coating glue system; compound cooling system; unwinding, winding system and other major components.
1). Solventless coating machine coating system
Now our country's introduction of solvent-free compound coating system is mostly a dedicated solventless compound coating system, also known as four-roll coating system (see Figure 1). It consists of A. B. CD composed of four rollers, A roller is a coating roller; B roller is a floating roller; C roller is a metering roller; D roller is a plastic fixed roller, fixed by the fixed roller D and metering roller C between the storage glue.
2). Use and coating of solvent-free adhesives
1 coating viscosity and temperature:
Solvent-free adhesives have large differences in molecular weight compared to solvent-containing adhesives. Solvent polyurethane adhesives generally have a molecular weight of tens to hundreds of thousands. When there is no solvent, the viscosity is also large, and it reaches millions of pascals per second (mpas) at room temperature. It is even a gel and cannot be coated directly. So it is dissolved by solvent to dilute to a coatable viscosity. When the structure of solvent-free adhesive and solvent-based adhesive is very similar, in order to achieve the viscosity required for solvent-free coating operation and reduce the molecular weight of adhesive, the viscosity is much lower than that of solvent adhesive adhesive, but it cannot be directly coated. The structural characteristics of solvent-free polyurethane adhesives are: the attraction between molecules is large (hydrogen bonds can be formed), the viscosity is high at room temperature, and when the temperature rises, hydrogen bonds break and the viscosity drops. Is a one-component solvent-free adhesive viscosity and temperature curve. ) Therefore, the temperature is lowered to a coatable viscosity. Temperature increase Viscosity Decrease Coating is easy; but temperature is too high (one-component glue is greater than 100°C, two-component glue is greater than 80°C), the internal reaction rate of the adhesive accelerates gelation, easy to form polymer resin, resulting in coating Difficulties and uneven coating. Therefore, it is necessary to determine the optimum temperature for coating and maintain the minimum temperature difference. General one-component adhesive coating temperature is controlled at 70-100 °C, the viscosity is controlled at 600-3000mpas; two-component adhesive coating temperature is controlled at 45-80 °C, viscosity 500-1500mpas.
2 One-component adhesive coating and curing.
In addition to the temperature and viscosity required for the control of one-component adhesives, since one-component polyurethane adhesives contain NCO groups (in the range of 3-6%), they cure by reacting with water. When the coating amount is lg/m2, the amount of water required to completely cure is 6.5-13 mg. Water adhering to the surface of the dried film alone is not sufficient (OPP measured at 38% RH at 22° C., and water adhering to both sides of the CPP film is approximately 2-4 mg/m 2 ). The supply of water must be replenished from the outside, so the environment must be humidified.
3 coating amount
In solventless compounding, the amount of adhesive applied is critical to the quality of the composite film. The amount of coating is insufficient, it is easy to cause the adhesive force of the composite film is not large, easy to peel or even delamination. The selection of the optimal coating amount of the adhesive should be based on the comprehensive consideration of the different quality requirements of the composite film products, the properties of the film, the printing effect, and the ink and adhesive properties. General use of one-component coating amount of 0.5-1.8g/m2. When the amount of coating is more than 2g/m2, the coating is thick, which 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, and affecting the bonding strength of the composite product. The use of a two-component solvent-free polyurethane adhesive is No such limit. However, too large a coating amount is not conducive to the composite quality. Generally, the coating amount is controlled at 1 to 3.5 g/m2. When a special composite film is used (for example, for high-temperature cooking), it can be increased to no more than 5.0 g/m2.
4 initial adhesion and bonding strength.
Since the molecular weight of the solvent-free adhesive used is lower than the molecular weight of the solvent-based adhesive, the cohesion force at room temperature is small, and the initial adhesion before the uncured adhesive is low. Therefore, the tension and the traction force should be strictly controlled during the composite production operation. Membrane product quality issues. Now adhesives manufacturers are developing products with strong adhesive strength in the early stage, and this problem is expected to be better resolved.
6. Conclusion
Non-solvent adhesive dry composite technology to manufacture composite film packaging materials has become a mature industrial technology after more than 20 years of development. In particular, in the past five years, China's composite film processing industry has realized and understood this new technology, and successfully introduced more than 20 sets of solvent-free composite equipment. The technology of solvent-free dry composite manufacturing of composite film packaging materials will be achieved in China. Greater development. With the localization of solvent-free composite equipment and the diversification of solvent-free adhesives, this new processing technology will gradually be recognized and accepted in the country with its outstanding outstanding advantages. It is expected that China will develop at a faster rate in the next few years.
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