Abstract: 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 China's first unit to produce such products. However, there was only one solventless laminator in China before 1995, and the demand for solvent-free polyurethane adhesives was very low. 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 governmentâ€™s restrictions on the environment and peopleâ€™s awareness of environmental protection, combined with the new film processing industry, the new Technology awareness and understanding. It was not until the last five years that it developed rapidly. At present, there are more than 20 Taiwan-based statistical 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 solvent-based polyurethane adhesives, and does not use any solvents (ie, 100% solids content of adhesives) to perform coating compounding. 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 Film material requirements for various properties. Some people have standardized the development of this series of adhesives into three stages of development.
(1) The first generation of 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 cures by reacting with moisture 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)
(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 (120Â°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 Â°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 waviness or peeling of films with strong barrier properties.
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