Analysis of polyurethane adhesives for several com

2022-09-30
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Analysis of several polyurethane adhesives for composite films on the market

polyurethane adhesives are adhesives containing urethane groups (-nhcoo-) or isocyanate groups (-nco), which have been widely used in composite packaging in recent years. It mainly has the following characteristics: when bonding with wood, paper, rubber, plastic and other materials with smooth surface, the bonding force is large; Toughness is adjustable; The bonding process is simple; Excellent low temperature resistance and excellent stability

composite film packaging materials have been widely used in food, medicine, electronics and other industries because of their high strength, good air tightness, waterproof refraction, heat-resistant sealing and other advantages according to different product types. With the prosperity and progress of technology, composite packaging materials are developing in a deeper direction, and the technology of polyurethane adhesive supporting packaging materials is also improving. The following introduces several main polyurethane adhesives for composite membranes in combination with the market, so as to facilitate manufacturers' selection

two component polyurethane adhesive

this is the most widely used polyurethane adhesive. Its main agent is polyurethane polyol compound containing hydroxyl after isocyanate modification of polyether or polyester polyol (called component a), and the curing agent is mostly the adduct of aromatic isocyanate and trimethylolpropane (called component B). When using, dilute and mix component a and component B in a certain proportion with ethyl acetate before sizing and coating. At present, the production technology of this kind of polyurethane adhesive in China is very mature

cooking resistant polyurethane adhesive

the so-called high-temperature cooking refers to the sterilization of food under the heating of high-temperature steam at 121 ~ 135 ℃, which requires that the substrate and polyurethane adhesive can withstand the test of high temperature and high humidity. The polyurethane adhesive for ordinary packaging is easy to hydrolyze at high temperature, and the bonding strength is reduced. It constitutes a normalized industry university research collaborative innovation mechanism, and it is also easy to cause the film to fall off

therefore, to ensure the performance of polyurethane adhesive in cooking environment, special molecular design must be carried out. There are mainly the following methods

(1) in order to improve the heat resistance, it is usually to change the molecular structure of soft segment polyester and hard segment, and introduce high-temperature chain segments resistant to humidity and heat, such as terephthalic acid, isophthalic acid, bisphenol A, etc

(2) there are two reasons for the hydrolysis of polyurethane adhesive: one is that the ester group of polyurethane adhesive chain is not resistant to hydrolysis, and the other is that its high acid value plays a catalytic role in promoting hydrolysis. In order to improve the water resistance of polyurethane adhesives, aliphatic polybasic acids and alcohols with good water resistance, such as azelaic acid, sebacic acid, hexanediol, butanediol, etc., are usually introduced, or the acid value of polyester is reduced

(3) in order to improve the softening point, monomers such as pyromellitic anhydride and trimellitic anhydride are usually introduced to increase the crosslinking density, and polymer blending and interpenetrating polymer complex (IPN) can also be used

adding epoxy resin, silane coupling agent, phosphoric acid, carbodiimide and other additives after the reaction of polyester polyol and curing agent will greatly improve the high temperature resistance, water resistance, chemical resistance, oil resistance and other properties of polyurethane adhesive

alcohol soluble polyurethane adhesive

two component alcohol soluble polyurethane adhesive is a new type of polyurethane adhesive in the domestic composite film market in recent years. Its main characteristics are as follows

(1) good adaptability to alcohol soluble ink

(2) the residual solvent of composite products is low, and alcohol solvent can be used to dilute the solvent, so as to reduce the production cost

(3) insensitive to humidity, stable storage and strong adaptability

(4) alcohol soluble polyurethane adhesive can be used for the composite of plastic, plastic paper and other substrates. The two-component alcohol soluble polyurethane adhesive was successfully developed abroad in the 1980s, while the development and research work in China is currently in its infancy, and only a few domestic enterprises have R & D and production capacity

waterborne polyurethane adhesives

at present, most of the popular polyurethane adhesives in the market are solvent-based. It contains a large amount of organic solvents and a certain amount of free isocyanates, which will affect the environment and human health. With the importance of environmental protection in various countries, waterborne polyurethane adhesives have developed rapidly

although the history of waterborne polyurethane adhesive is not long, its development is very rapid. In 1943, hlack, a West German, successfully made aqueous polyurethane adhesive for the first time. In 1967, Pu lotion was industrialized for the first time and put on the market in the United States. In 1972, Bayer took the lead in using polyurethane aqueous lotion as leather finishing agent, and aqueous polyurethane adhesive began to gradually become an important commodity. In the 1990s, some foreign companies successfully developed one component self crosslinking waterborne polyurethane adhesives by introducing reactive groups into polymers

waterborne polyurethane adhesive can be prepared by the following two methods: first, the original polyurethane adhesive is directly emulsified into a stable water dispersed lotion by adding emulsifier, but the existence of emulsifier will affect its water resistance and bonding strength. Secondly, hydrophilic ionic groups are introduced into the molecular framework of polyurethane to form a self dispersing and self emulsifying system. The advantages of this system are that it does not contain emulsifiers, has good film-forming property, and can be mixed with other anionic or cationic polymerization systems. Researchers can confirm the influence of humidity on the hydrogen bond interaction between go layers and interlayer spacing to reduce costs

in recent years, the research on waterborne polyurethane adhesives in the industry mainly focuses on increasing solid content, drying speed, bonding strength and water resistance. With the advent of water dispersive polyisocyanate curing agent, the performance of two-component polyurethane adhesive has basically reached the performance of solvent based two-component polyurethane adhesive. So far, the application field of waterborne polyurethane adhesives has expanded to the coating of wood floors, plastic, metal, concrete and other surfaces, and a variety of waterborne polyurethane adhesives have been applied in industry

although waterborne polyurethane adhesive has achieved good results in the composite film industry, there are still some problems in its application

(1) it is not as volatile as organic solvents, so when drying, it is necessary to increase the temperature of the drying channel or lengthen the drying channel, which will cause a waste of energy

(2) the bonding strength of waterborne polyurethane adhesive can not meet the requirements of some high-end packaging

(3) due to the limited product variety of waterborne polyurethane adhesive, there are almost no products specially designed for the composite film industry

although the application of waterborne polyurethane adhesive is not perfect, it has broad development space. Therefore, waterborne polyurethane adhesive is both an opportunity and a challenge for domestic adhesive manufacturers

solventless polyurethane adhesive

the application of solventless polyurethane adhesive in composite membranes began in Germany in 1974; Japan introduced this technology in 1977, and by 1997, it had more than 60 production lines; In Europe and the United States, half of the composite membrane products have been prepared with solvent-free polyurethane adhesives

solventless polyurethane adhesives have developed to the third generation. The first generation is polyether or polyester type isocyanate terminated prepolymer, which has the disadvantages of fast viscosity change, slow curing speed and easy to produce carbon dioxide bubbles; The second generation is a two-component type of hydroxyl terminated prepolymer and isocyanate terminated prepolymer. The coating device has a precise automatic glue feeding and metering mixing system, but it has poor adhesion to EVA, aluminum foil and other materials; The third generation is a two-component system improved on the second generation, which can withstand high temperature cooking and can also be used for bonding aluminum foil

because solvent-free polyurethane adhesive does not contain solvent, it has the characteristics of solvent saving and environmental protection. With the improvement of national environmental protection requirements and the gradual limitation of solvent emissions, the application of solvent-free polyurethane adhesives will develop rapidly

there are also some problems in the use of solvent-free polyurethane adhesives

(1) since it is impossible to produce a precise automatic glue supply metering and mixing system in China, expensive equipment must be imported from abroad

(2) the speed of solvent-free compounding machine is very high (generally above 200m/min), which requires high compounding technology

(3) the most important thing is that now solventless polyurethane adhesives are expensive, most of them need to be imported, and only a few domestic companies' products can meet the requirements

therefore, there is still a long way to go for solvent-free polyurethane adhesives to be widely used in China

there are many varieties of polyurethane adhesives, which can play a role in many fields with the change of formula and raw materials. The development direction of adhesives is also changing from general type to special type and then to compatible type (general type), so as to give full play to the optimal performance of adhesives

over the past 20 years of reform and opening up, the polyurethane adhesive industry has achieved rapid development. Especially after China's entry into WTO, a large number of products from foreign manufacturers have poured into China's market, and the prices of domestic products are getting closer and closer, which undoubtedly brings great pressure to domestic manufacturers. Therefore, how to speed up technological progress, seize opportunities, meet challenges and develop in a timely manner has become a problem we must face. We should give full play to our own advantages and seek breakthroughs on the basis of learning, so that our products can reach or even surpass foreign products, and revitalize national industry

source: PRINT world

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