Research status of the hottest cooking resistant p

Research status of cooking resistant polyurethane adhesive

composite film flexible packaging bag has the advantages of high strength, good air tightness, waterproof refraction, heat-resistant sealing and so on, which can keep the packaging fresh, color and taste. Therefore, it is common to apply the knowledge of extreme friction and wear testing machine in food packaging. 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 plays an important role in food packaging because of its excellent comprehensive properties. Through the choice of film type, adhesives with different properties and coating amount, flexible packaging materials that meet the requirements of cold resistance, oil resistance, acid resistance, heat resistance, gas resistance, transparency, wear resistance, puncture resistance and other performance requirements can be prepared

1 overview of polyurethane adhesives for food packaging

food packaging is divided into ordinary packaging and high-performance packaging. The corresponding polyurethane adhesives can be divided into four categories: (1) general use (2) For boiling, (3) transparent cooking, (4) aluminum foil cooking. Composite film polyurethane adhesives for general packaging generally include adhesives for dry food packaging and adhesives for cooking food packaging that can withstand boiling and sterilization at 70 ~ 100 ℃. Generally, this kind of adhesive is a two-component solvent polyurethane adhesive. The main agent is polyether or polyester polyol, which is modified by aromatic isocyanate and contains hydroxyl polyurethane polyol (called component a), and the curing agent is the adduct of aromatic isocyanate and trimethylolpropane (called component B). When used, component A and component B are mixed in a certain proportion, and then diluted to a certain concentration with ethyl acetate before sizing and coating. At present, this kind of glue has been produced in China It mainly uses polyester polyols of adipic acid as raw materials, while most foreign countries use propylene oxide polyether as raw materials, with low cost. Composite film polyurethane adhesives for transparent cooking bags and aluminum foil cooking bags generally include composite film adhesives for 121 ℃ cooking and sterilization and composite film adhesives for 135 ℃ high temperature cooking and sterilization. The so-called high temperature cooking refers to the high temperature of 121 ~ 145 ℃ and 0.18 ~ 0 The processing technology of food sterilization under 40MPa steam pressure. This requires not only that a variety of substrates can withstand the test of high temperature, but also that the adhesives used should be able to withstand high temperature. Polyurethane adhesive for ordinary packaging is easy to hydrolyze at this high temperature, which reduces the bonding strength and easily causes the film to fall off. In order to improve the cooking resistance of polyurethane adhesive, it is usually solved by changing the molecular structure of soft segment polyester and hard segment, introducing moisture and heat resistant high temperature chain segment, changing the variety of crosslinking agent, adjusting the density of crosslinking agent, and even adopting the ways of polymer blending and forming interpenetrating polymer complex (IPN)

2 development of cooking resistant polyurethane adhesive by domestic and foreign research institutions

2.1 difunctional polyester polyurethane adhesive

Japan ink chemical industry company and Japan Toyo ink manufacturing company have developed a series of cooking resistant polyurethane adhesives respectively, using mixed dibasic acids (terephthalic acid, isophthalic acid, azelaic acid, sebacic acid), And polyester polyols with high molecular weight synthesized by mixed diols (ethylene glycol, neopentyl glycol) react with curing agent, and add epoxy resin, silane coupling agent, phosphoric acid, anhydride, amine and other compounds at the same time, which greatly improves the high temperature resistance, water resistance, acid resistance and oil resistance of the adhesive. See Table 1 and table 2 for the main performance indicators of the product

xk-908 high temperature cooking adhesive developed by Dalian Institute of light chemical industry. Using raw materials with more rigid heat-resistant structure, polyester polyols synthesized from hexanediol, azelaic acid and terephthalic acid react with isophorone isocyanate to synthesize polyurethane adhesive. With the introduction of Isophorone, a six membered ring structure, and the rigid structure of terephthalic acid, the development strategy of energy and environment has improved the heat resistance of the adhesive. Xk-908 adhesive can withstand cooking at 121 ℃ for 35 minutes, and its technical indicators have reached the level of imported similar products. A 300 ton/year pilot production plant has been built

2.2 multifunctional polyester polyurethane adhesive

the heat resistance of the adhesive is improved by partially crosslinking the macromolecules and increasing the crosslinking density. When polyester is synthesized, the introduction of trifunctional groups or more can make macromolecules cross-linked, so as to improve the heat resistance of the adhesive. A kind of high-temperature cooking adhesive developed by Liaoning printing technology research institute uses lauric acid, benzoic acid, adipic acid, glycerol, trimethylolpropane to prepare component A, and then reacts with isocyanate compound to make polyurethane adhesive. This adhesive has good cooking resistance at 125 ℃, high bonding strength and good flexibility to plastic film. Coughlinjames et al. [9] of the United States synthesized polyester diols from ethylene glycol, hexanediol, isophthalic acid and terephthalic acid, and then modified with p-toluene diisocyanate to obtain isocyanate terminated prepolymers. Then, the polyfunctional polyester polyol synthesized with trimethylolpropane, ethylene glycol, hexylene glycol, isophthalic acid and p-phthalic acid is mixed with isocyanate terminated prepolymer to obtain polyurethane adhesive, which is one of the most important markets of aoshengde. Due to the introduction of multi-functional groups and rigid heat-resistant structure in the structure, the crosslinking density is increased, the heat resistance is improved, and the boiling water cooking resistance is good

2.3 modified polyurethane adhesive

in order to improve the heat resistance of polyurethane adhesive, modifiers are often added to its formula, mainly epoxy resin. Modification with epoxy resin can make full use of the rigid heat-resistant structure of epoxy molecules and the hydroxyl groups on the molecular chain that can react with isocyanates. The reaction between hydroxyl and isocyanate can increase the entanglement and crosslinking between macromolecules, and improve the heat resistance of the adhesive. Zhejiang Huangyan Oriental ink chemical company and the Department of chemistry of Hangzhou University jointly developed a cooking resistant adhesive he-95 (a) for pet/al/cpp dry composite with similar properties to ad-502. It is different from the general two-component polyurethane adhesive. Its main agent is not made of low molecular weight polyols (polyester or polyether) by Diisocyanate Chain Extension, but by the polycondensation reaction of mixed dibasic acid and mixed dibasic alcohol. The high molecular weight copolyester (m6000) obtained by one-step method is physically or chemically blended with the modified resin without chain expansion to achieve high heat resistance and excellent fastness. The peel strength of the adhesive at 180 ℃ and 121 ℃/40min after cooking reached the level of ad-502/cat-10 adhesive made in Japan

3 research prospects

with the rapid growth of the amount of cooking resistant food packaging bags, the amount of polyurethane adhesive is also rising. Cooking bags in Europe, the United States, Japan and other developed countries increased by more than 10% annually from 1990 to 1998. In China, cooking food has long been commercialized, with an average annual increase of more than 12%. However, domestic adhesives used for cooking food are mostly imported so far, and domestic products can not meet the cooking requirements. The imported brands include ad502/cat-10, eps901/kw80, uk3640, uk6800, a-310/a-3, etc. Among them, ad502/cat-10 accounts for most of the market share. Therefore, the localization of polyurethane adhesive for composite film with low viscosity, high solid content, high strength and high temperature resistance (135 ℃, 30min) is its development direction. At the same time, the market requirements for adhesives are also improving. The high-temperature hydrolysis of aromatic diisocyanates usually used will produce diaminotoluene, which is a medium-level carcinogen and is unfavorable to food packaging. The use of aliphatic isocyanate to prepare polyurethane adhesive does not exist this problem, but because its price is too high, it can not be used in general products, and the domestic can not meet this demand. Aliphatic isocyanates have been used abroad to replace Aromatic isocyanates to produce high-temperature cooking polyurethane adhesives, which is the general trend

source: Chinese glue