Complete manual of adhesives for the hottest compo

2022-08-12
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Complete manual of adhesives for composite membranes

adhesives play a very important role in the process of composite materials. Many factors should be considered in the selection of adhesives for food flexible packaging. First, it can store the same amount of electricity as traditional batteries. Generally speaking, as a composite adhesive, the following conditions should be met:

it has high adhesion to a variety of materials, especially foods containing salt, acid, spices, alcohol, water, sugar, fatty acids and other substances. These substances have different characteristics, and they may penetrate into the adhesive layer through the inner layer of the composite film, It corrodes and destroys the adhesive, resulting in delamination and cracking of the composite packaging bag. Therefore, the adhesive must be able to resist the turbidity of the above substances and always maintain sufficient adhesive peel strength

it has high transparency, does not invade the printing pattern ink, and has a high affinity for the ink. No odor, non-toxic; There is no harmful extract after packaging the food

temperature requirements for food storage; It is resistant to heating, pressurization, high-temperature cooking and sterilization (100 ℃ ~135 ℃) in the process of food processing, as well as the instantaneous high temperature (220 ℃) during bag making and sealing. It has fast curing speed and is harmless. Good weather resistance, no yellowing bubbles, no pulverization and delamination

soft, easy to operate and long-lasting adhesion

polyurethane adhesive

the chemical structure of polyurethane adhesive contains many polar bonds such as carbamate bond, ester bond, urea bond, etc., which gives it excellent adhesion and film-forming performance, making it the preferred variety of adhesives for composite films

solvent based polyurethane adhesive

at present, the vast majority of domestic composite film production lines use solvent based polyurethane adhesive, which refers to ester soluble adhesive. This adhesive has a long history, mature technology and low price. In the past, almost 100% of the world used polyurethane adhesives as dry composite adhesives to manufacture composite plastic packaging bags. It can be seen that polyurethane adhesive plays an important role in plastic packaging

for general packaging, composite film polyurethane adhesives are generally used for general packaging, adhesives for general food packaging, and adhesives for cooking food packaging with boiling sterilization resistance of 70 ℃ ~100 ℃. Such adhesives are two-component ester soluble polyurethane adhesives. The main agent is polyether or polyester polyol, which is modified by aromatic isocyanate and contains hydroxyl polyurethane polyol, and the curing agent is the synthesis of aromatic isocyanate and trimethylolpropane. Ordinary polyurethane can not completely adapt to foods with corrosive media and reinforced plastics, such as pickled mustard, pickled and spicy vegetables containing salty, sour and spicy ingredients. Therefore, a food grade polyurethane adhesive resistant to acid and spicy has been developed. The acid and spicy adhesive has the advantages of high solid content, low viscosity, good initial bonding strength, acid resistance, spicy resistance, salt resistance and high peel strength, which can make up for the shortcomings of ordinary polyurethane that is not resistant to corrosive media

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 125 ℃ high temperature cooking and sterilization. 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 using the mixing of polymers to form interpenetrating polymer complex. For example, Japan has developed a series of cooking resistant polyurethane adhesives. Polyester polyols with high molecular weight synthesized from mixed binary acids and mixed binary alcohols are used to react with the curing agent, and epoxy resin, phosphoric acid, anhydride, amine and other compounds are added at the same time, so as to greatly improve the high temperature resistance, water resistance, acid resistance and oil resistance of the adhesives

although solvent based polyurethane adhesive has excellent performance, mature process and many varieties, it still has some shortcomings: the use of organic solvent based adhesive has the problem of volatilization of organic solvents. The solid content of solvent based polyurethane adhesive products is about 20~35%, and then 75% of organic solvents are required to volatilize in a short time in the drying channel at about 80 ℃, and the emission is generally two to four times that of dry glue. Therefore, there are a lot of environmental pollution and safety problems caused by solvents, and a lot of energy is consumed. In addition, in the production process of adhesives, there are also problems of pollution caused by solvent volatilization and flammable and explosive safety

in addition, solvent based adhesives have high cost, high toxicity, and poor solvent removal during dry compounding, which is easy to cause odor in food packaging bags, and even pollute food. Therefore, with the enhancement of people's awareness of environmental protection, the use of this type of adhesive is declining. The United States and the European Union have restricted the use of polyurethane adhesive dry composite film, which can not be used in food and drug packaging. At present, the packaging of such adhesives in developed countries in Europe and the United States has decreased sharply from 80% 10 years ago to 30% now. However, due to the excellent performance of polyurethane adhesive dry composite film products, dry compounding is still the main compounding process of plastic packaging film products in countries that have not made clear restrictions on its use

at present, there are many brands of polyurethane adhesives in the domestic market, most of which are aromatic block polymers with rigid segments composed of flexible segments of polyester polyols and diisocyanates. The difference is nothing more than the connection and arrangement of flexible segments and rigid segments. The aromatic amine generated after hydrolysis of aromatic polyurethane adhesives is a carcinogen, For example, the finished adhesive synthesized by toluene diisocyanate (TDI) in common use contains free TDI, which is hydrolyzed to produce toluene diamine (TDA) during high-temperature cooking. This substance is a medium carcinogen. Therefore, the U.S. Food and Drug Administration (FDA) prohibits aromatic polyurethane adhesives from being used in food and drug packaging, while some countries allow it. They believe that TDA content below 1ppm is harmless to human body, Therefore, while aromatic polyurethane is allowed to be used as food packaging to avoid oil leakage of oil return valve in large tonnage, strict regulations are made, that is, after the cooking bag is filled with 4% acetic acid aqueous solution, after cooking at 121 ℃ for 40 minutes, the aromatic amine content in the solution must be less than 0.008mg/l. China's hygienic standard for food packaging bags stipulates that the migration amount of total aromatic amines (calculated by TDA) should be less than 0.0004mg/l. Therefore, composite manufacturers must understand the mechanical properties and damage characteristics of materials when they are compressed through tightening experiments before using polyurethane adhesives. In addition to doing peel strength experiments under various conditions, they must also ask relevant departments to do toxicity tests, which should not be used easily, Prevent harm to human health. Although the cooking bag sample made of the above composite film adhesive has no small molecules after detection; Inspected according to gb/t regulations, it meets food hygiene standards. However, polyurethane adhesives for composite films in China are basically two-component solution systems, and the general varieties of domestic adhesives can basically meet the production needs of domestic general-purpose composite films. In the past, polyurethane adhesives for the production of high-performance varieties such as aluminum/plastic composite films that can withstand cooking at 121 ℃ or 135 ℃ were mainly imported from Japan, Germany, the United States and other countries. In recent years, some domestic enterprises and research units have successfully developed several varieties of cooking resistant polyurethane adhesives. The properties of the polyurethane adhesive developed for cooking resistant composite bags meet the composite process requirements of cooking resistant composite bags, and are equivalent to the properties of corresponding imported products

the author believes that the solvent based polyurethane adhesives used now should be excessive to environmental friendly adhesives. At present, solvent-free and waterborne polyurethane composite film adhesives are being actively developed at home and abroad, especially the former. At present, corresponding products have been sold in domestic and foreign markets

solventless polyurethane adhesives

the enhancement of environmental awareness has prompted people to actively develop solventless polyurethane adhesives. Solventless polyurethane adhesives are generally two-component adhesives composed of polyols and polyisocyanates or polyisocyanate prepolymers with relatively low molecular weight and polyols or polyamines. This kind of composite film adhesive was applied in 1974, which began in Germany. Since the 1980s, the United States, Italy, France, Germany and other countries have begun to use a large number of solvent-free adhesives, and its use in Europe and the United States has exceeded 60% of the total processing volume of this kind of composite film products

using solvent-free adhesives, not only does it have no solvent pollution and reduce a lot of energy consumption, but also there is no pollution to the contents and peculiar smell caused by residual solvents in the composite products, which greatly reduces the risk of fire in the production process

solvent free adhesives are suitable for the compounding of various plastic films and aluminum foil. The content of free diisocyanate is low, the resistance to contents is excellent, the bonding strength of adhesives is high, and the aluminum-plastic composite film products can withstand high temperature cooking

the quality of solvent-free adhesive composite products can completely reach the quality of solvent-based adhesive composite products. Its main features are:

the main agent adopts isocyanate terminated prepolymer, which reduces the content of free isocyanate in the adhesive, further reduces the pollution to the environment, and changes the problem of poor heat sealing strength

reduces the viscosity of the adhesive, improves the wettability of the adhesive to the film, and improves the coating and dispersion performance of the surface of the adhesive hybrid film. Therefore, improving the quality of solvent-free adhesive composite products can also improve the machine speed and increase the production efficiency

dry compounding with solvent-free adhesive has the advantages of using solvent-based adhesive for dry compounding. Compared with solvent-based adhesive, it also has the following characteristics:

no organic solvent is used, the cost is reduced, and there is no environmental pollution caused by the volatilization of organic solvent

the drying section does not need solvent volatilization, which reduces energy consumption

the composite film has no residual solvent damage and eliminates the erosion of solvent on printing ink

it does not contain organic solvents, which eliminates the damage of the composite substrate caused by solvents and high-temperature desolvent, and makes the dimensional stability of the composite membrane structure good; The equipment is simple, the floor area is small, and the investment is saved

but the problems of solvent-free composite adhesives also include the following points:

the price is higher, resulting in higher production costs

in order to improve the wettability of the adhesive to the film, the viscosity of the adhesive is required to be low. The result is that the initial cohesion of the adhesive is low, the adhesion is not high, and the curing speed is not fast enough, which affects the composite production efficiency

the second problem is mainly caused by the low relative molecular weight of the main agent of polyurethane adhesive, which is generally controlled at 3000 ~ 4000. If the relative molecular weight is increased, the viscosity is too high to be evenly coated on the composite film, which will affect the quality of the composite film. However, the viscosity can be reduced by increasing the coating and compounding temperature (40 ℃)

the key technology of solvent-free two-component polyurethane adhesive lies in the synthesis of appropriate main agent with isocyanate terminated groups, which must meet the following requirements:

achieve the required bonding strength

good coating performance

low viscosity to ensure good process performance

in terms of the selection of synthetic process route, first use diols and dicarboxylic acids to synthesize polyester. Through the selection of chemical structure and dosage ratio of diols and dicarboxylic acids, the molecular weight of polyester can be adjusted to meet the requirements of isocyanate terminated prepolymer, and then combine with different molecular weights and functional groups

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