Soft packaging, also known as soft cans, has exceeded hard packaging such as plastic, glass bottles and cans because of its unique comprehensive properties, such as lightweight and convenient, long shelf life, hygiene, easy storage and transportation, easy disassembly, less garbage and good shelf effect.
Due to its excellent properties, polyurethane adhesives can bond different properties of film materials together to obtain composite films for flexible packaging with various properties such as cold resistance, oil resistance, drug resistance, transparency, wear resistance and so on. At present, polyurethane adhesives have become the main adhesives for the processing of composite films for flexible packaging in domestic and foreign markets. In the domestic adhesives market, polyurethane adhesives are second only to shoemaking in the manufacturing of composite film for packaging. There are many kinds of polyurethane adhesives for packaging, such as water-based polyurethane adhesives, hot melt polyurethane adhesives, solvent-based polyurethane adhesives and solvent-free polyurethane adhesives. The commonly used polyurethane hot melt adhesives can be divided into two categories: 1) thermoplastic polyurethane elastomer hot melt adhesives; 2) reactive hot melt adhesives. The main disadvantage of thermoplastic hot melt adhesive is its high viscosity, so it has a great influence on the apparent quality of coating. Reactive polyurethane hot melt adhesives are a new type of adhesives developed on the basis of traditional hot melt adhesives. They not only have the characteristics of good initial adhesion and excellent post-curing properties of traditional hot melt adhesives, but also have the advantages of polyurethane's changeable composition and wide range of performance adjustment. They have excellent bonding properties for various substrates. In addition, the use of emulsifiers or the introduction of hydrophilic ionic groups in molecules has reduced the water resistance of waterborne polyurethane adhesives for packaging. In recent years, the research on improving the water resistance of waterborne polyurethane adhesives has become a hotspot. At the same time, because of the large heat capacity of water, how to improve its solid content and drying speed is also one of the urgent problems to be solved.