Study on Cooking Resistant Polyurethane Adhesive

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.

2.2 Polyfunctional Polyester Polyurethane Adhesives

By partially cross-linking macromolecules, the crosslinking density is increased so that the heat resistance of the adhesive is improved. In the synthesis of polyesters, the introduction of trifunctional or more functional groups can cross-link macromolecules and increase the heat resistance of the gel. A high temperature resistant cooking adhesive developed by the Printing Technology Research Institute of Liaoning Province, using lauric acid, phthalic acid, adipic acid, glycerol, trimethylolpropane to prepare a component, and then reacted with an isocyanate compound to make a polyurethane adhesive. This plastic has good resistance to cooking at 125°C, and has high adhesive strength and good flexibility to plastic films. US COUGHLINJAMES [9] synthesizes polyester diols with ethylene glycol, hexanediol and isophthalic acid, terephthalic acid, and then uses p-toluene diisocyanate to modify to obtain isocyanate-based prepolymers. Then, a polyfunctional polyester polyol with trimethylolpropane, ethylene glycol, hexanediol, and isophthalic acid, terephthalic acid synthesized is mixed with an isocyanate-terminated prepolymer to obtain a polyurethane adhesive. Due to the introduction of multi-functional groups and rigid heat-resistant structures in the structure, the adhesive has increased cross-link density, improved heat resistance, and good boiling water resistance.

2.3 Modified Polyurethane Adhesive

In order to improve the heat resistance of polyurethane adhesives, modifiers are often added to their formulations, mainly epoxy resins. Modification with an epoxy resin can make full use of the rigid, heat-resistant structure of the epoxy molecule and the hydroxyl groups on the molecular chain that react with the isocyanate. The reaction of the hydroxyl group with the isocyanate can increase the entanglement and cross-linking between the macromolecules, so that the heat resistance of the adhesive is improved. Zhejiang Huangyan Oriental Ink Chemical Co., Ltd. and the Department of Chemistry of Hangzhou University jointly developed the HE-95 (A), a cooking resistant adhesive for PET/AL/CPP dry composites with similar properties to the AD-502. It is different from general two-component polyurethane adhesives in that the main agent is not a polyisol (polyester or polyether) with a low molecular weight and is derived from diisocyanate chain extension, but is a polycondensation reaction of a mixed dibasic acid and a mixed glycol. be made of. The one-step high molecular weight copolyester (M>6000) is physically or chemically blended with the modified resin without chain extension to achieve high heat resistance and excellent fastness. The peel strength of the adhesive at 180°C and the peel strength after cooking at 121°C/40min all reached the level of AD-502/CAT-10 adhesive produced in Japan.

3 Research prospects

With the rapid increase in the amount of retort food packaging bags, the amount of polyurethane adhesives is also increasing. Cooking bags in Europe, the United States, Japan and other developed countries increased by an average of more than 10% from 1990 to 1998. In China, cooking foods have long been commercialized, with an average annual increase of more than 12%. Domestic adhesives used as cooking foods have so far mostly been imported, and domestically produced products are still unable to meet cooking requirements. The imported brands include AD502/CAT-10, EPS901/KW80, UK3640, UK6800 and A-310/A-3. Wait. Which AD502/CAT-10 accounted for most of the market share. Therefore, the localization of polyurethane adhesive for composite films with low viscosity, high solid content, high strength and high temperature resistance (135 °C, 30 min) is the development direction. At the same time, the market demand for adhesives is also increasing. Hydrolysis of aromatic diisocyanates commonly used produces diaminotoluene, a moderately carcinogenic substance that is detrimental to food packaging. The use of aliphatic isocyanate to prepare polyurethane adhesives does not present this problem, but due to its high price, it cannot be used on general products and domestic demand cannot be met. Aliphatic isocyanates have been used in foreign countries instead of aromatic isocyanates to produce high-temperature cooking polyurethane adhesives. This is a general trend.



Author / Zhang Shengwen, Wang Tingwei