From the 1970s on, foreign countries began to study the two-component non-solvent J-type Polyurethane Adhesive. Since the 1980s, the United States, Italy, France, Germany and other countries have begun to use a large number of solvent-free adhesives, currently in major European countries. The use of the compound film adhesive has accounted for 70% to 90% of the total amount, and Europe and the United States have legislation 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. On the one hand, it causes the waste of solvents, and on the other hand it also brings about environmental pollution and damage to toxic substances. Solid content of solvent-based polyurethane adhesive products is generally 50% to 80%. Due to the requirements of the composite process, it is necessary to add a solvent to dilute to a solid content of about 20% to 35% during use. Then 75% of the organic solvent is required to volatilize in a short time of 80~(2). The discharge amount is generally two to four times that of the dry glue, so there is a large amount of solvent pollution to the environment and safety issues, and the consumption A large amount of energy.It is estimated that only the food packaging industry emits more than 400,000 tons of organic solvent into the atmosphere every year. In addition, there are still problems of contamination caused by volatilization of solvents, and flammable and explosive safety problems in the production process of adhesives. The use of solvent-free adhesives not only eliminates the contamination of solvents and reduces a large amount of energy consumption, but also does not cause any damage to the safety of the use of residual solvents in composite products, and also greatly reduces the risk of fires during the production process.
Second, domestic and international status, level and development trend
With the use of solvent-based adhesives, there are volatile organic solvents that pollute the environment and safety issues, coupled with the rise in the price of solvents (mainly oil as raw materials) and the need for energy conservation, making the development of solvent-based adhesives limited in foreign countries. The US Environmental Protection Agency stipulates that no more than 2.9 pounds of solvent may be emitted per gallon of adhesive during the coating operation. The solvent-based adhesives used in the domestic coating industry generally have a solids content of 25% and solvent volatilities of about 6 pounds per gallon.
In order to solve the above problems, Europe and the United States have been conducting studies on the high solids, low viscosity, and solvent-free systems of polyurethane adhesives since the 1960s. In recent years, foreign solvent-free adhesives have developed rapidly. From the technical point of view, the first generation of products has been developed to the current third-generation products.
The first generation of solvent-free adhesives was a one-component, moisture-curing adhesive that was characterized by its high molecular weight, high viscosity, high operating temperatures, and the need for humidification equipment. As the product is cured, carbon dioxide, a by-product, is released, and the composite film is prone to blistering. If proper moisture is not provided, the problem of poor curing occurs.
The second-generation solvent-free adhesives are two-component reactive adhesives. The advantage of these adhesives is that they eliminate the effects of moisture and air humidity. The product has a low molecular weight, a low viscosity, and a reduced operating temperature. The disadvantage is that it contains more free diisocyanate (in order to reduce the viscosity), it is easy to migrate into the film and cause food contamination and reduce the heat seal strength, and then 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 adhesives, is suitable for the compounding of various plastic films and foils, reduces the content of free diisocyanates, and is excellent in resistance to content and properties. Adhesive strength increases, aluminum plastic composite film products may be resistant to high cooking.
At present, the quality of the third-generation non-solvent adhesive composite products can completely meet the quality of solvent-based adhesive compounding agents. The main features of the third generation of products are:
1. The use of anti-catalytic system, that is, the main agent using isocyanate-terminated prepolymer. In this way, the free isocyanate content of the adhesive is reduced, the pollution to the environment is further reduced, and the problem of poor heat seal strength is also changed.
2 It reduces the viscosity of the adhesive, improves the wettability of the adhesive to the film, and improves the coating dispersion performance of the adhesive on the film surface. Therefore, the quality of the solvent-free adhesive composite product can be improved, the machine speed can also be increased, and the production efficiency can be increased.
The compounding process using solvent-free adhesives in the domestic laminating industry is relatively late. Therefore, the development of the solvent-free adhesives that are matched with them is also lagging behind. At present, there are more than 30 solvent-free composite equipments in China, which are mostly used for the packaging of some exported foods in order to meet the stringent residual solvent regulations in foreign countries. The solvent-free adhesives currently used by domestic laminating manufacturers are all imported, and the price is generally around 40,000 yuan per ton.
Due to the introduction of solvent-free adhesive laminating equipment in China, many manufacturers have begun to pay attention to the research and development of solvent-free adhesives. However, due to the late start, the market has not yet seen mature domestic non-solvent adhesive equivalents with foreign product levels.
Third, the main technology of solvent-free two-component polyurethane adhesive
The key to the technology of solvent-free two-component polyurethane adhesives is to synthesize the appropriate isocyanato-terminated main agent and must meet the following requirements:
1. To achieve the required bond strength;
2. Has good coating performance;
3. Low viscosity to ensure good process performance.
In terms of synthetic route selection, polyesters are first synthesized using dihydric and dibasic acids, and the molecular weight of the polyester is adjusted by the choice of the chemical structure of the dihydric alcohol and the dibasic acid and the ratio of the amounts to meet The requirements of the isocyanate-terminated prepolymer are then reacted with polyethers and isocyanates of different molecular weights, functional groups, to synthesize isocyanate-terminated prepolymers.
The isocyanate-terminated prepolymer is an oligomer of a polyester diol and MDI and a mixture of a polyether diol and an oligomer of MDI; the hydroxyl-terminated prepolymer is a polyester diol and a polyether divers Mixtures of alcohols should have a ratio of hydroxyl groups to isocyanate groups of about 1:1.6 (molar ratio).
In the choice of polyester, the molecular structure of the polyester has a great influence on the properties of the polyurethane. In order to obtain a film with high strength and a certain flexibility, various components in the polyester should be properly matched. Among them, aliphatic dibasic acids such as adipic acid and sebacic acid, whose linear, hydrocarbon chain in the molecule increases the soft segment of the polyester, thus increasing the flexibility of the final polyurethane product and lowering the freezing point. The introduction of an aromatic ring in the chain increases the rigidity and the ability to resist oxidation, and makes it difficult to access the ester bond due to steric hindrance, thereby improving the hydrolysis resistance. Suitable aromatic diacids are isophthalic acid, terephthalic acid, phthalic acid or anhydride.
Simple diols, such as ethylene glycol, 1,3-propylene glycol, or 1,4-butanediol, can form linear polyester chains with excellent flexibility and have a strong tendency to crystallize. Polyesters formed from short-chain diols With a high COO bond density, it has a strong intermolecular attraction. This can explain its strong tendency to crystallize, as well as their poor resistance to polar solvent and poor miscibility with hydrocarbons. Branched diols, such as 1,2-propanediol, interfere with the intermolecular association due to the presence of side chains, and also provide a space protection effect on the ester bond, thus improving heat, oxidation, and hydrolysis resistance. Performance, and have a weak tendency to crystallize and better solvent resistance.
In two-component solvent-free adhesives, the use of polyester alone is far from meeting all of the required properties and requires the use of polyethers for adjustment. First, polyether polyurethanes are flexible and can reduce the viscosity of products. Second, due to the weaker interaction between polyether chains and polyesters, the cohesive strength of the products can be adjusted to improve the adhesive properties of the products. Third, the hydrolysis resistance of polyether elastomers is much better than that of polyester elastomers, which reflects the superior hydrolytic stability of ether groups over the ester groups. Fourth, the use of polyethers can reduce costs.
In the selection of polyethers, suitable polyethers are propylene oxide polyethers, propylene oxide-ethylene oxide copolymerized polyethers, and the viscosity and adhesive strength of the product can be adjusted by changing the molecular weight of the polyether used. Under the inert condition of keeping the molecular weight of the polyester unchanged, the molecular weight of the polyether is changed, and the viscosity of the product decreases as the molecular weight of the polyether increases. This is because the smaller the molecular weight of the polyether, the shorter the molecular chain, the greater the density of urethane bonds in the polymer, the greater the intermolecular attraction and the higher the viscosity of the product.
Fourth, application prospect analysis
At present, due to cost reasons in China, solvent-free two-component polyurethane adhesives are far from universal, and solvent-based adhesives are mainly used. However, from a long-term point of view, solvent-free products have irreplaceable advantages and will continue to increase in the future.
Since more than 20 years ago, German Herberts has used solvent-free polyurethane adhesive composite packaging materials, due to its obvious economic, safety and environmental protection advantages, as well as the continuous development of laminating technology and equipment, it has made it in Europe and other developed countries. The country has rapidly increased from 10% of its usage 2O years ago to about 80% of the current level.
In recent years, with the vigorous development of the food industry, the industry for making two-component adhesives for polyurethane composite films has been continuously expanding. According to incomplete statistics, the current market capacity has reached about 40,000 tons/year. Now there are more than 100 manufacturers of polyurethane laminating adhesives in China, all of which are solvent-based adhesives. In the future, we should gradually focus on the development of solvent-free adhesives, and we should adopt a combination of technology introduction and independent research and development to develop products that meet the requirements of use as soon as possible.
According to the current domestic industry status, the adoption of solvent-free adhesives in China still needs at least five to ten years. However, solvent-based products are currently developing toward high solids and low viscosity, and water-based, alcohol-soluble and other low-toxicity products have emerged. Low residual solvent adhesive products. We believe that with the continuous improvement of the industry level and environmental protection considerations, the prospect of solvent-free adhesives will be even better.
Reprinted from: China Polymer Network
