Development and application of two component polyu

2022-07-25
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Development and application prospect of solvent-based two-component polyurethane composite film adhesives

foreign countries began to study two-component solvent-free adhesives in the 1970s. Since the 1980s, the United States, Italy, France, Germany and other countries have begun to use a large number of solvent-free adhesives. According to the literature, at present, its use in major European countries has exceeded 60% of the total processing volume of this kind of composite film products, and Europe The United States has legislated to limit the use of solvent based adhesives. Most of the adhesives used in the domestic food packaging composite film industry are solvent-based. The use of organic solvent-based adhesives has the problem of volatilization of organic solvents. The solid content of solvent based polyurethane adhesive products is generally 50 ~ 75%. Due to the requirements of the composite process, solvents should be added to dilute the solid content to about 20 ~ 35%. Then 75% of the organic solvents are required to volatilize in a short time in the oven at about 80 ℃, and the emission is generally two to four times that of the dry adhesive. Therefore, there are a lot of environmental pollution and safety problems caused by solvents, and a lot of energy is consumed. It is estimated that the food packaging industry alone emits more than 400000 tons of organic solvents into the atmosphere every year. In addition, in the production process of adhesives, there are also problems of pollution caused by solvent volatilization, flammability and explosion safety. The solvent-free adhesive not only has no solvent pollution and reduces a lot of energy consumption, but also has no damage to the use safety caused by residual solvent in the composite products, and greatly reduces the risk of fire in the production process

at present, China has successfully developed alcohol soluble two-component polyurethane adhesive, which has excellent environmental performance and greatly reduces the toxicity of the adhesive itself and residual solvents. If the solvent-free adhesive can be successfully developed, it will become a replacement product in line with the development trend of environmental protection in the future

current situation, level and development trend at home and abroad

the use of solvent-based adhesives has the problems of volatile organic solvents polluting the environment and safety. In addition, the rise in solvent prices (mainly using petroleum as raw materials) and the need for energy conservation have restricted the development of solvent-based adhesives abroad. According to the regulations of the US Environmental Protection Agency, the adhesive per gallon shall not emit 2 More than 9 pounds of solvent. The solid content of the solvent based adhesive used in the domestic cloth coating industry is generally 25%, and the solvent volatilization is about 6 pounds per gallon

in order to solve the above problems due to unstable friction, Europe and the United States have studied the high solid content, low viscosity and solvent-free system of polyurethane adhesive since generation 60. In recent years, foreign solvent-free adhesives have developed rapidly. From the technical level, the first generation products have developed to the current third generation products

the first generation solvent-free adhesive is one component moisture curing adhesive. This kind of adhesive is characterized by high molecular weight, high viscosity, high operating temperature and humidification equipment. Because the by-product carbon dioxide is released when the product is cured, the composite film is easy to blister; If proper moisture is not provided, poor curing will occur

the second generation solvent-free adhesive is a two-component reactive adhesive. The advantage of this kind of adhesive is that it eliminates the influence of moisture and air humidity. The product has small molecular weight, lower viscosity and lower operating temperature. Its disadvantage is that it contains more free diisocyanate (in order to reduce the viscosity), which is easy to migrate into the film, resulting in food pollution and reduced heat sealing strength. In addition, it is not suitable for EVA film and NY film

the third generation solvent-free adhesive overcomes the shortcomings of the second generation solvent-free adhesive. It is applicable to the composite of various plastic films and aluminum foil, reduces the content of free diisocyanate, has excellent content resistance performance, and improves the adhesive strength. Aluminum plastic composite film products may be resistant to high cooking

at present, the quality of the third generation solvent-free adhesive composite products can completely reach the quality of solvent-based adhesive composite products. The main features of the third generation products are:

1 The anti catalytic system is adopted, that is, 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

2. The viscosity of the adhesive is reduced, the wettability of the adhesive to the film is improved, and the coating dispersibility of the adhesive on the film surface is improved. Therefore, it can not only improve the quality of solvent-free adhesive composite products, but also improve the machine speed and increase the production efficiency

the domestic laminating industry uses the solvent-free adhesive late, so the development of the matched solvent-free adhesive is also relatively backward. At present, there are nearly 30 solventless laminating equipment in China, which are mostly used for the packaging of some exported foods to meet the strict requirements of foreign regulations on residual solvents. However, the solventless adhesives currently used by domestic laminating manufacturers are imported, and the price is generally about 40000 yuan per ton

due to the introduction of domestic solventless adhesive laminating equipment, many manufacturers have begun to pay attention to the research and development of solventless adhesive. However, due to the late start, there is no mature domestic solventless adhesive equivalent to the level of foreign products on the market

technology of solvent-free two-component polyurethane adhesive

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

1 Meet the required bonding strength

2. Good coating performance

3. 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 mechanism of diols and dicarboxylic acids and the proportion of dosage, the molecular weight of polyester can be adjusted to meet the requirements of isocyanate terminated prepolymer, and then react with polyether and isocyanate with different molecular weight and functional groups to synthesize isocyanate terminated prepolymer

isocyanate terminated prepolymer is a mixture of oligomer of polyester diol and MDI and oligomer of polyether diol and MDI; Hydroxyl terminated prepolymer is a mixture of polyester diol and polyether polyol. The ratio of hydroxyl to isocyanide group should be about 1:1.6 (molar ratio)

in the selection of polyester, the molecular structure of polyester has a great impact on the performance of polyurethane. In order to obtain a film with high strength and certain flexibility, various components in polyester should be properly matched. Among them, aliphatic dibasic acids such as adipic acid and sebacic acid, the linear and hydrocarbon chains in their components increase the flexible chain segment of polyester, thus increasing the flexibility of the final product of polyurethane and reducing the freezing point. The introduction of aromatic ring into the chain can increase the rigidity and oxidation resistance, and the hydrolysis reagent is difficult to approach the ester bond due to space hindrance, which improves the hydrolysis resistance. Suitable aromatic diacids include isophthalic acid, terephthalic acid, phthalic acid or anhydride

simple diols, such as ethylene glycol, 1,3-propanediol or 1,4-butanediol, can form a linear polyester chain with excellent flexibility and strong crystallization tendency. Polyester

formed from short chain diols has high -coo bond density, so it has strong intermolecular attraction, which can explain its strong crystallization tendency, and is also the reason for their poor polar solvent resistance and poor compatibility with hydrocarbons. The branched diols, such as 1,2-propanediol, hinder the intermolecular association due to the existence of side chains, and at the same time play a role in spatial protection of ester bonds, thus improving the heat resistance, oxidation resistance and hydrolysis resistance, and having a weak tendency to shorten the life of crystallization and good solvent resistance

in the two-component solvent-free adhesive, polyester alone is far from meeting all the required properties. The possibility of only replacing the nucleus pulposus of the intervertebral disc and preserving the remaining structure should be adjusted with polyether. First of all, polyether polyurethane has good flexibility and can reduce the viscosity of products. Second, because polyether has weaker interaction between molecular chains than polyester, it can adjust the cohesive strength of the product and improve the bonding performance of the product. Thirdly, the hydrolysis resistance of polyether elastomer is much better than that of polyester elastomer, which reflects that the ether group has superior hydrolysis stability than the ester group. Fourth, the use of polyether can reduce costs

in terms of polyether selection, applicable polyether includes propylene oxide polyether and propylene oxide ethylene oxide copolymer polyether. The viscosity and bonding strength of the product can be adjusted by changing the molecular weight of the polyether used

under the condition of keeping the molecular weight of polyester unchanged, the viscosity of the product decreases with the increase of the molecular weight of polyether. This is because the smaller the molecular weight of polyether and the shorter the molecular chain, the greater the urethane bond density in the polymer and the greater the intermolecular attraction, the higher the viscosity of the product

application prospect analysis

at present, due to cost reasons, solvent-free two-component polyurethane adhesives are far from being popularized in China, and solvent-based adhesives are still widely used. The main reasons are that on the one hand, solvent-free composite equipment is expensive, and there are high requirements for operators; On the other hand, solvent-free products are prone to composite quality problems, the applicable structure is limited, and the main agent itself still has certain toxicity. These factors restrict its application in China. However, from a long-term perspective, with the increasingly stringent environmental protection regulations and the continuous economic development, the proportion of solvent-free and alcohol soluble products will continue to rise

since Herberts company of Germany used solventless polyurethane adhesive composite packaging materials more than 20 years ago, due to its obvious economy, safety and advantages in environmental protection, as well as the continuous development of laminating technology and equipment, the following will introduce to you that the service environment of 1 concrete pressure testing machine in Europe and other developed countries has rapidly increased from 10% of its use 20 years ago to 60% now

in recent years, with the vigorous development of the food industry, the industry of adhesive for two-component polyurethane composite film has been expanding. At present, according to incomplete statistics, the market capacity has reached about 30000 ~ 50000 tons/year. At present, there are more than 100 manufacturers of polyurethane laminating adhesives in China, all of which are solvent adhesives. In the future, we should gradually pay attention to the development of solvent-free adhesives, and should adopt the route of combining technology introduction with independent research and development to develop products that meet the use requirements as soon as possible

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