From the 1970s on, foreign countries began to study two-component solvent-free adhesives. From the 1980s, the United States, Italy, France, and Germany have begun to use a large amount of solvent-free adhesives. According to the literature, they are currently used in major European countries. It has exceeded 60% of the total processing volume of this kind of composite film products, 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, and the use of organic solvent-based adhesives has problems with the volatilization of organic solvents. The solid content of solvent-based Polyurethane Adhesive products is generally 50 to 75%. Due to the requirements of the compounding process, it is necessary to add a solvent to dilute to a solid content of about 20 to 35% during use, and then 75% of the organic solvent is required to be about 80°C. The drying tunnel volatilizes in a short time, and the discharge amount is generally two to four times that of the dry glue. Therefore, there is a large amount of solvent pollution to the environment and safety issues, and consumes a lot of energy. It is estimated that only the food packaging industry emits more than 400,000 tons of organic solvents annually into the atmosphere. In addition, in the production process of the adhesive, there is still the problem of contamination caused by solvent evaporation and flammable and explosive safety problems. 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 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.
At present, China has successfully developed alcohol-soluble two-component polyurethane adhesives with excellent environmental performance, which greatly reduces the toxicity of the adhesive itself and residual solvents. If it can successfully develop solvent-free adhesives, it will be together. It will become a new generation product that will meet the environmental protection trend in the future.
Current situation, level and development trend at home and abroad
With solvent-based adhesives, there are volatile organic solvents that pollute the environment and safety issues, coupled with the rise in the price of solvents, the use of petroleum as a raw material, and the need for energy conservation, which has restricted the development of solvent-based adhesives abroad. 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 carried out studies on the high solids, low viscosity, and solvent-free systems of polyurethane adhesives since the 60th generation. 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 adhesive was a one-component moisture-curable adhesive. This adhesive is characterized by its large molecular weight, high viscosity, high operating temperatures, and the need for humidification equipment. As the product is cured, carbon dioxide, a by-product, is emitted, and the composite film is prone to blistering; if it fails to provide proper moisture, the problem of poor curing occurs.
The second generation of solvent-free adhesives are two-component reactive adhesives. The advantage of this type of adhesive is that it eliminates the effects of moisture and air humidity, the product has a small molecular weight, the viscosity decreases, and the operating temperature also decreases. 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 heat seal strength is reduced, 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 aluminum foils, reduces the content of free diisocyanates, and is excellent in resistance to the content and properties of adhesives. 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 fully meet the quality of solvent-based adhesive composite products. The main features of the third generation of products are:
1. The use of anti-catalytic system, that is, the use of isocyanate-terminated prepolymer as the main agent, so that 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 and dispersing properties 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 nearly 30 solvent-free composite equipments in China, which are mostly used for the packaging of some exported foods to meet the stringent residual solvent requirements in foreign countries. The solvent-free adhesives currently used by domestic laminating manufacturers are imported, and the price is generally about one ton. 4 million.
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 seen mature domestic solvent-free adhesives that are equivalent to foreign products on the market.
Solvent-free two-component polyurethane adhesive technology
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 polyester has a great influence 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, have linear, hydrocarbon chains in their molecules that increase the soft segment of the polyester, and thus increase the flexibility of the final polyurethane product and lower the freezing point. The introduction of an aromatic ring in the chain increases the rigidity and the ability to resist oxidation, and because of the steric hindrance, it is difficult for the hydrolysis agent to approach the ester bond, 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-propanediol, or 1,4-butanediol, can form linear polyester chains with excellent flexibility and have a strong tendency toward crystallization. Polyesters formed from short chain diols have a high -COO-bond density and therefore have a strong intermolecular attraction, which can explain their strong tendency to crystallize, as well as their poor resistance to polar solvent and carbon Reasons for poor miscibility of hydrogen compounds. Branched diols, such as 1,2-propanediol, interfere with the intermolecular association due to the presence of side chains, and at the same time provide a steric protective 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 product can be adjusted to improve the adhesive properties of the product. 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 include propylene oxide polyethers and propylene oxide-ethylene oxide copolymer polyethers, and the viscosity and adhesive strength of the products can be adjusted by changing the molecular weight of the polyether used.
While keeping the molecular weight of the polyester constant, 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.
Application Prospect Analysis
At present, due to cost reasons in China, solvent-free two-component polyurethane adhesives are still far from universal, and solvent-based adhesives are used in large quantities. This is mainly due to the fact that solvent-free composite equipment is expensive on the one hand and requirements on operators. On the other hand, solvent-free products are more prone to complex quality problems when compounded, and the applicable structures are limited and the main agent itself still has certain toxicity. These factors restrict its promotion and use in our country. However, from a long-term perspective, the proportion of solvent-free and alcohol-soluble products will continue to rise as environmental regulations become more stringent and economic development continues.
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 use 20 years ago to 60% of the present.
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. The current market capacity has reached an average of 3 to 50,000 tons per year according to incomplete statistics. At present, 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.
Source: China Packaging News
