Solvent-free polyurethane adhesive and film coating technology

This paper describes the development of solvent-free Polyurethane Adhesive 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 Composite 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.