From the 1970s on, foreign countries began to study two-component solvent-free adhesives. From the 1980s, the United States, Italy, France, and Germany have begun to use a large amount of solvent-free adhesives. According to the literature, they are currently used in major European countries. It has exceeded 60% of the total processing volume of this kind of composite film products, and Europe and the United States have legislation to limit the use of solvent-based adhesives.
Most of the adhesives used in the domestic food packaging composite film industry are solvent-based, and the use of organic solvent-based adhesives has problems with the volatilization of organic solvents. The solid content of solvent-based Polyurethane Adhesive products is generally 50 to 75%. Due to the requirements of the compounding process, it is necessary to add a solvent to dilute to a solid content of about 20 to 35% during use, and then 75% of the organic solvent is required to be about 80°C. The drying tunnel volatilizes in a short time, and the discharge amount is generally two to four times that of the dry glue. Therefore, there is a large amount of solvent pollution to the environment and safety issues, and consumes a lot of energy. It is estimated that only the food packaging industry emits more than 400,000 tons of organic solvents annually into the atmosphere. In addition, in the production process of the adhesive, there is still the problem of contamination caused by solvent evaporation and flammable and explosive safety problems. The use of solvent-free adhesives not only eliminates the contamination of solvents and reduces a large amount of energy consumption, but also does not cause damage to the safety of the use of residual solvents in composite products, and also greatly reduces the risk of fires during the production process.
At present, China has successfully developed alcohol-soluble two-component polyurethane adhesives with excellent environmental performance, which greatly reduces the toxicity of the adhesive itself and residual solvents. If it can successfully develop solvent-free adhesives, it will be together. It will become a new generation product that will meet the environmental protection trend in the future.
Current situation, level and development trend at home and abroad
With solvent-based adhesives, there are volatile organic solvents that pollute the environment and safety issues, coupled with the rise in the price of solvents, the use of petroleum as a raw material, and the need for energy conservation, which has restricted the development of solvent-based adhesives abroad. The US Environmental Protection Agency stipulates that no more than 2.9 pounds of solvent may be emitted per gallon of adhesive during the coating operation. The solvent-based adhesives used in the domestic coating industry generally have a solids content of 25% and solvent volatilities of about 6 pounds per gallon.
In order to solve the above problems, Europe and the United States have carried out studies on the high solids, low viscosity, and solvent-free systems of polyurethane adhesives since the 60th generation. In recent years, foreign solvent-free adhesives have developed rapidly. From the technical point of view, the first generation of products has been developed to the current third-generation products.
The first generation of solvent-free adhesive was a one-component moisture-curable adhesive. This adhesive is characterized by its large molecular weight, high viscosity, high operating temperatures, and the need for humidification equipment. As the product is cured, carbon dioxide, a by-product, is emitted, and the composite film is prone to blistering; if it fails to provide proper moisture, the problem of poor curing occurs.
The second generation of solvent-free adhesives are two-component reactive adhesives. The advantage of this type of adhesive is that it eliminates the effects of moisture and air humidity, the product has a small molecular weight, the viscosity decreases, and the operating temperature also decreases. The disadvantage is that it contains more free diisocyanate (in order to reduce the viscosity), it is easy to migrate into the film and cause food contamination and heat seal strength is reduced, and then it is not suitable for EVA film and NY film.
The third-generation solvent-free adhesive overcomes the shortcomings of the second-generation solvent-free adhesives, is suitable for the compounding of various plastic films and aluminum foils, reduces the content of free diisocyanates, and is excellent in resistance to the content and properties of adhesives. Adhesive strength increases, aluminum plastic composite film products may be resistant to high cooking.
At present, the quality of the third-generation non-solvent adhesive composite products can fully meet the quality of solvent-based adhesive composite products. The main features of the third generation of products are:
1. The use of anti-catalytic system, that is, the use of isocyanate-terminated prepolymer as the main agent, so that the free isocyanate content of the adhesive is reduced, the pollution to the environment is further reduced, and the problem of poor heat seal strength is also changed.
2. It reduces the viscosity of the adhesive, improves the wettability of the adhesive to the film, and improves the coating and dispersing properties of the adhesive on the film surface. Therefore, the quality of the solvent-free adhesive composite product can be improved, the machine speed can also be increased, and the production efficiency can be increased.
The compounding process using solvent-free adhesives in the domestic laminating industry is relatively late. Therefore, the development of the solvent-free adhesives that are matched with them is also lagging behind. At present, there are nearly 30 solvent-free composite equipments in China, which are mostly used for the packaging of some exported foods to meet the stringent residual solvent requirements in foreign countries. The solvent-free adhesives currently used by domestic laminating manufacturers are imported, and the price is generally about one ton. 4 million.
Due to the introduction of solvent-free adhesive laminating equipment in China, many manufacturers have begun to pay attention to the research and development of solvent-free adhesives. However, due to the late start, the market has not seen mature domestic solvent-free adhesives that are equivalent to foreign products on the market.
(to be continued)
