The polyurethane adhesive refers to a binder containing a urethane group (-NHCOO-) or an isocyanate group (-NCO) in the molecular chain. Polyurethane is used as a binder because it has the advantages of excellent elasticity, mechanical strength, adhesiveness, reactivity, and operability. According to the differences in chemical properties and curing mechanism of polyurethane adhesives, they can be divided into: single-component solvent-based, single-component moisture-curing, two-component solvent-based and solvent-free, water-based and polyisocyanate-based and so on.
The alcohol-soluble polyurethane adhesives discussed here are two-component solvent-based adhesives. Two-component polyurethane adhesives are a very important type in polyurethane adhesives, and have a wide range of uses and a large amount. It is usually composed of two components, the main glue and the curing agent, and the two components are individually packaged. Before use, according to a certain proportion of preparation, after mixing evenly, after the two components are mixed, a cross-linking reaction occurs to produce a cured product. Two-component polyurethane adhesives have the advantages of adjustable performance, high bonding strength, and a wide range of applications. They have become the most widely used products in polyurethane adhesives.
Solvent adhesives have many deficiencies compared to solventless adhesives, and the overall development trend for adhesives is non-organic solvation. However, polyurethane solvent-based adhesives have particularly excellent process and adhesive properties. Moreover, non-organic solvent-based adhesives (including solvent-free and water-based polyurethane adhesives, etc.) currently being developed in composite films have not yet been completely established. The level of solvent-based adhesives is reached, so solvent-based polyurethane adhesives will still be present for a longer period of time.
Alcohol-soluble polyurethane adhesives are two-component solvent-based polyurethane adhesives that have all the advantages of two-component and solvent-based polyurethane adhesives and will be the leader in this field in the future.
1. Alcohol-soluble polyurethane adhesives Alcohol-solvent polyurethane adhesives are non-toxic polymer adhesives that can be widely used for bonding plastics, aluminum foils, aluminized films, papers, etc., with good transparency. , Excellent adhesive strength, room temperature curing, suitable for alcohol-soluble ink, low residual solvent and other outstanding advantages. Mainly used in light food light packaging, can be used for polyethylene, polypropylene, aluminum foil, aluminized film, polyester, nylon and other bonding and compounding, capable of bonding PE, PP, PET, CPP, OPP, BOPP, AL Vacuum aluminum coating and other substrates.
Alcohol Solvents Polyurethane adhesives are excellent in machine operability and have good dispersion properties for a wide variety of materials. They can be formulated with a wide range of solvents and even solvents + water, and therefore can be unaffected by high ambient temperatures. According to the need to add high-speed proportions to adapt to the printing of the composite film, it is widely applied to all types of ink and can avoid blooming.
At present, the smoothing agent is used in many PE or PP films for the needs of composite film manufacturers. After many processes such as compounding and printing, lubricating agents are often eroded to reduce slippage and even lose slipperiness. The alcohol-soluble polyurethane adhesive is specially designed to maintain high slippage of the film and minimize the loss of the lubricant. It is worth mentioning that when the temperature of the composite film increases, the compatibility between the lubricant and the film increases, and the lubricant must be in the surface of the film to function. The increase of the compatibility makes the lubricant no longer stay in the film. The surface of the film, which will cause an increase in the coefficient of friction. If it is a high-speed compound, it will affect the adhesion of the film. Therefore, the film should maintain the minimum temperature at which the solvent can be completely removed during the drying of the sizing.
Alcohol-soluble polyurethane adhesives are two-component adhesives that require the addition of a curing agent. The proportion of the two components can be adjusted within a relatively large range. Both have good stability and are not deteriorated by the influence of humidity in the air, and can be stored for a long time without changing the adhesive property of the adhesive. Compared with general polyurethane adhesives, alcohol-soluble polyurethane adhesives can be cured at room temperature without the need of heating or adding catalysts, have better initial adhesion and can continue to increase the bonding strength in the next few days.
Alcohol-soluble polyurethane adhesives can be diluted with other solvents, such as methanol, n-propanol, isopropanol, xylene, or a mixture thereof, in addition to dilution with ethanol. It is worth noting that ketones or ketone-containing solvents are considered as impurities in alcohol sols because ketones such as MEK, MIBK, etc. can cause odors to remain in the composite film. The acetamidine here is preferably produced by a synthetic method because excessive ethanol impurities in the fermentation method will have a certain influence on the adhesive.
Since alcohol-soluble polyurethane adhesives use ethanol as a solvent, high temperature storage should be avoided, especially in direct sunlight, and must be stored in a cold place.
II. Mechanism of Alcohol-soluble Polyurethane Adhesive The main adhesive is isocyanate-based polyurethane resin dissolved in ethanol. It is prepared by the reaction between polyether polyol and isocyanate. The isocyanate-based polyurethane resin and ethanol are very good. compatibility.
The curing agent is a modified epoxy resin. The curing reaction of the main glue and the curing agent is achieved through the reaction of a hydroxyl group and an epoxy group, and the hydroxyl compound reacts with the epoxy group as a pro-agent. Since the hydroxy compound has low electrophilicity and there is no catalyst between the hydroxyl group and the epoxy group, it is usually not reactive at temperatures below 200° C., which requires adding a certain amount of catalyst to promote the reaction to be 70 to 100. At °C, the catalyst we chose was DMP-30 [2,4,6-tris(dimethylaminomethyl)phenol]. Hydroxyl groups on polymer macromolecules can continue to react with the epoxy resin.
Careful readers may want to ask: Ethanol is also a hydroxy compound, why not react with epoxy resin? This is because the adhesive coated on the film needs to evaporate the solvent to be compounded. When the hydroxyl group on the polyurethane reaches the reaction temperature with the epoxy group, the ethanol is basically volatilized. Therefore, the ethanol and the epoxy are not discussed here. Based on the reaction.
Third, the advantages of alcohol-soluble polyurethane adhesive In order to adjust the viscosity of polyurethane adhesive to facilitate the process operation, in the preparation of polyurethane adhesives often use solvents, through the dilution of the solvent, in order to facilitate the adhesive can be more Good coating on the composite film. Most of the current polyurethane adhesives for food packaging use ethyl acetate as a diluent, while alcohol-soluble polyurethane adhesives can use ethanol as a diluent. Compared with ethyl acetate, ethanol has the following advantages:
1. Toxicity As a binder for food packaging, the bonded composite film is in direct contact with the food. In order to be responsible for the health and safety of consumers, not only the substrate film used for packaging is odorless, odorless, non-toxic, but also the adhesive used has the same performance.
The LD50, which measures the toxicity of solvents, is an index that roughly identifies the size of the acute toxicity and refers to the lowest dose required to kill one half of the tested animals in mg/kg (body weight). The higher the value, the lower the toxicity. The LD50 of ethyl acetate (rat, oral) was 5620 mg/kg, and the LD50 of ethanol (rat, oral) was 10810 mg/kg, showing that ethanol has lower toxicity than ethyl acetate.
In addition, ethyl acetate has a strong irritating odor, which has a great invasive effect on the mucous membrane and can cause keratitis; ethyl acetate sticks to the skin and causes dehydration and whitening of the epidermis. These hazards do not occur with the use of alcohol-soluble polyurethane adhesives, so when working with alcohol-soluble polyurethane adhesives, it is possible to optimize the working environment and reduce the operator's vulnerability.
2. Residual Solvents This is a major disadvantage of solvent-based adhesives. The solvent used to dilute the glue is to be removed by a drying tunnel. Ethanol and ethyl acetate have boiling points close to each other, but evaporation of ethyl acetate is more difficult than ethanol, and under normal circumstances, ethyl acetate will cause larger solvent residues.
3. Cooperation with alcohol solvent ink At present, the printing industry has started to use ink as the solvent to be used for the transition from toluene and other benzenes to alcohols such as ethanol and isopropyl alcohol. The alcohol-soluble polyurethane adhesive uses ethanol as a solvent and will have good compatibility and excellent adhesion to the alcohol-soluble ink. Ethyl acetate, on the other hand, has an aggressive effect on certain printing inks, which can cause blooming of the ink. This problem often occurs in surface printing inks.
4. Economics It is well known that ethanol is a raw material for the manufacture of ethyl acetate. The market price of ethanol is around 4,000 yuan/ton, and the market price of ethyl acetate is around 7,000 yuan/ton. There is a big difference between the two. In addition, alcohol-soluble polyurethane adhesives can be cured at room temperature, eliminating the costs of civil engineering and heating energy in the curing chamber. Considering the above factors, the alcohol-soluble polyurethane adhesive is a good product that is more suitable for composite manufacturers.
IV. Development of Alcohol Solvent Polyurethane Adhesive At present, with the rapid development of the economy and the continuous improvement of the quality of life of the citizens, the environmental protection issues of the composite film packaging products have been receiving increasing attention. The market needs more environmentally friendly composite adhesives, and alcohol-soluble polyurethane adhesives are environmentally friendly products that meet this need.
In recent years, polyurethane adhesives at home and abroad, like the entire adhesives and coatings industry, have been characterized by developments in environmental protection, industrial hygiene, resources, and other policies and regulations. Adjustments have been made to the industrial structure and efforts have been made to reduce the use of organic solvent-based products. proportion. Advanced countries have established environmental protection regulations, require the development of non-hazardous adhesives, minimize the amount of solvent used and the toxicity of solvents, but solvent adhesives have their own uniqueness, and they are not used in certain situations, so in a short time It is impossible to cancel all.
The solution is to use a low-toxic solvent, increase the solid content and closed ventilation operation, and use solvent recovery devices. In recent years, Europe has proposed the concept of “green†solvents, namely those that are less toxic, and some even biodegradable, including solvents that have been removed from the list of toxic release materials (TRIs) and organic volatiles (VOCs), such as Ethanol, etc., polyurethane adhesives using these as solvents have less environmental pressure.
Many cities in China are now increasingly focusing on environmental issues and are increasingly concerned about the pollution of the composite film industry. Many complex manufacturers are eager to find small-contaminated adhesives for production, thereby reducing the environmental pressure. Although the development of adhesives is ultimately going to be solventless, because solvent-free adhesives require specialized solventless laminators, investment in equipment is large and they cannot be widely promoted in the country in the near future. Therefore, alcohol-based polyurethanes are currently Mixtures can still become mainstream products in the market. The promotion and listing of our alcohol-soluble polyurethane adhesives has only just begun. It is believed that with the cooperation of a large number of manufacturers, the entire composite film industry will surely enter a new stage of development. (From "China Packaging News")
Research Progress of Alcohol-soluble Polyurethane Adhesives
December 22 08:08:30, 2023
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