The composite film flexible packaging bag has the advantages of high strength, good airtightness, waterproof refraction, heat-resistant sealing and the like, and can make the packaging fresh, color-preserving and taste-proof. Therefore, the application in food packaging is extremely common. Its packaging quality is not only related to the properties of the composite film, but also depends on the properties of the adhesive. Two-component Polyurethane adhesive occupies an important position in food packaging due to its excellent overall performance. Through the choice of film type and different properties of the adhesive and coating amount can be made to meet the cold, oil, acid, heat, gas barrier, transparent and wear-resistant, puncture resistance and other performance requirements of flexible packaging materials.
1 Overview of Polyurethane Adhesives for Food Packaging
Food packaging, divided into ordinary packaging and high-performance packaging. The corresponding polyurethane adhesives can be divided into four categories: (1) general use, (2) steaming, (3) transparent cooking, and (4) aluminum foil cooking. Composite film polyurethane adhesives for general packaging generally include dry food packaging adhesives and plastic food packaging adhesives that resist 70 to 100° C. boiling sterilization. Typically, such adhesives are two-component solvent-based polyurethane adhesives. Its main agent is a hydroxyl group-containing polyurethane polyol modified by aromatic isocyanate (polypropylene), and a curing agent is an adduct of aromatic isocyanate and trimethylolpropane (a ) When using, mix the A component and the B component in a certain proportion, and then dilute to a certain concentration with ethyl acetate and then sizing and coating. At present, this kind of glue has been produced in China. It is mainly based on polyadipate Polyester polyols, and most of them use propylene oxide Polyether as raw materials, and the cost is low. Composite film polyurethane adhesives for transparent cooking bags and aluminum foil cooking bags generally include composite film adhesives resistant to retort sterilization at 121°C and composite film adhesives resistant to high temperature retort sterilization at 135°C. The so-called high-temperature cooking refers to a process of sterilizing food at a high temperature of 121 to 145° C. and a steam pressure of 0.18 to O.40 MPa. This requires not only that a variety of substrates withstand the high temperatures, but also that the adhesives used should be able to withstand high temperatures. Polyurethane adhesives for ordinary packaging are easily hydrolyzed at this high temperature, and the adhesive strength is reduced, which also easily causes the film to fall off. In order to improve the retort resistance of polyurethane adhesives, it is usually to change the molecular structure of the soft segment polyester and the hard segment, introduce moisture- and heat-resistant high-temperature segments, change the type of cross-linker, adjust the density of the cross-linking agent, and even use a total of polymer Mixed to form an interpenetrating polymer network (IPN) approach to solve.
2 Development of anti-cooking polyurethane adhesives by domestic and foreign research institutes
2.1 Difunctional polyester Polyurethane Adhesive
Dainippon Ink Chemical Industries and Japan's Toyo Ink Manufacturing Co. have developed a series of cooking-resistant polyurethane adhesives. They use mixed dibasic acids (terephthalic acid, isophthalic acid, sebacic acid, sebacic acid), and blended two. Polyols with high molecular weight synthesized from monoalcohols (ethylene glycol, neopentyl glycol) react with a curing agent and simultaneously add epoxy resins, silane coupling agents, phosphoric acids, acid anhydrides, amines, etc. It greatly improves the high temperature resistance and water resistance of the glue, and also improves the acid resistance and oil resistance of the glue. Its main performance indicators are shown in Table 1 and Table 2.
Dalian Light Chemical Research Institute developed XK-908 high temperature cooking adhesive. The raw materials with more rigid and heat-resistant structures were selected to synthesize polyurethane adhesives by the reaction of hexanediol, sebacic acid, terephthalic acid synthetic polyester polyol and isophorone isocyanate. The glue introduced the six-member ring structure of isophorone and the rigid structure of terephthalic acid, which improved the heat resistance of the adhesive. XK-908 adhesive can withstand boiling at 121 °C for 35 minutes, its technical indicators have reached the level of imported similar products, has completed 300 tons / year of trial production.
