Study on Various Flexible Polyurethane Adhesives

Adhesives for flexible packaging have experienced two stages of development in China: the first stage was the late 1970s to the early 1980s. During this period, the domestic economic development was still in its early stage, and the technical strength was not strong. One-component vinyl acetate, vinyl ester, ethylene-vinyl acetate, and rubber adhesives are the main products, and high-end packaging products are supplemented by imported two-component polyurethane adhesives. The second stage is from the 1980s to the present time, with two-component polyurethane adhesive as the leading and extrusion composite adhesive as auxiliary. Then analyze the advantages and disadvantages of various adhesives and their development prospects.

One, one-component adhesive

Since most of the one-component adhesives are vinyl acetate, vinyl esters, ethylene-vinyl acetate, and rubber-based systems, toluene is the main solvent (only a small part is of polyurethane type but not technically mature enough) and is stripped. The strength is not high, the smell of compound products is heavy, and the composite product is monotonous. It is mainly composed of paper-plastic composites and plastic-plastic composites. The packaging can only be used on non-food packaging, and it has gradually been eliminated, and it cannot be formed into the mainstream. The attractive price is relatively cheap, so some manufacturers are still using them in non-food packaging products with low requirements. With the increasing awareness of environmental protection in the country, such adhesives will be completely eliminated.

Second, two-component polyurethane adhesive

China's two-component polyurethane adhesive was developed in the middle and late 1980s. At that time, several domestic adhesives plants and research institutes such as Zhejiang New Oriental, Beijing Beihua Hospital, Shanghai Lianyin and Jiangsu Province were engaged in research and development. With the advancement of technology, the quality of domestically produced two-component polyurethane adhesives has steadily increased and the price is low, gradually winning the majority of the domestic market. Two-component polyurethane adhesives have the following advantages:

1. It has good bonding effect, high peel strength and wide range of substrates. It can be used in the composite of paper, PET, OPP, NY, PE, CPP and aluminum foil. The composite product has good quality and high strength.

2. Excellent resistance to media, resistance to acid, alkali, oil, hot, and various oxides and chemicals.

3. Good cold resistance and heat resistance, many food packaging requires cryopreservation or high temperature 121-135 °C sterilization, and the use of polyurethane adhesive can still maintain good peel strength after the above conditions.

Two-component polyurethane adhesives are classified into polyester polyurethane adhesives and polyurethane polyurethane adhesives. At present, most of the main polyester- and polyurethane-based adhesives are made of adipic acid and ethylene glycol as the main raw materials, and they are solidified by condensation. The agent is polymerized with TDI and trimethylolpropane as the main raw material. Since toluene diisocyanate (TDI) will be converted to toluenediamine (TDA) after high temperature hydrolysis, toluenediamine is a carcinogen. After the compounding reaction of the polyurethane adhesive, the isocyanate residual monomers contained in the polyurethane adhesive are minimal, but after the package is subjected to high-temperature cooking, these residual monomers still migrate through the plastic film into the food, hydrolyze to aromatic amines (TDA), and cause cancer. As a potential factor, the U.S. Food and Drug Administration (FDA) has categorically requested that no aromatic polyurethane adhesive be used for cooking packaging. If someone uses this kind of plastic packaging bag for cooking foods, it will be sold on the market. Severe sanctions. Although the United States, Europe, and Japan are not as stringent as the United States, there are clear restrictions on the content of TDA. China's domestic requirements for TDA content shall not exceed O. 004mg/L. Therefore, the adhesive made with TDI as raw material is subject to certain restrictions on cooking and packaging.

The two-component polyurethane adhesive uses ethyl acetate as a solvent, which is determined by the characteristics of the product and the process requirements. Due to the flammability of ethyl acetate and its environmental and ecological impact, ethyl acetate solvent-based polyurethane adhesives should be In the limit. From this point of view, the development direction of ester-soluble polyurethane adhesive is high solid content and low viscosity. Only in this way can the ester-soluble polyurethane adhesive occupy about 70% of the share in the next ten years.

Third, alcohol soluble polyurethane adhesive

This product is a polyurethane adhesive developed in recent years with ethanol as a solvent. In a strict sense, this product is not an environmentally friendly adhesive, because ethanol is also an organic solvent. However, alcohol-soluble polyurethane also has its advantages:

1. Free free isocyanate monomer improves the operating environment.

2. Not sensitive to moisture, improve the stability and adaptability of the adhesive.

3. With industrial alcohol as the solvent, the production cost is reduced.

4. Excellent compatibility with alcohol-soluble polyurethane printing inks.

5. Composite membrane products have low residual solvents.

However, at present, alcohol-soluble polyurethane adhesives are only used for composite packaging of common dry foods. With regard to the current technology and the nature of alcohol dissolution, it is not possible to achieve the peel strength and the cooking performance requirements of ester-soluble polyurethanes, and only at home and abroad. The TDA content of packaging adhesives used in cooking products is stipulated, and there is no restriction on the packaging of ordinary products, so it is impossible to replace ester-soluble polyurethane adhesives and the development prospects are not optimistic.

Fourth, waterborne polyurethane adhesive

Waterborne polyurethane adhesives are prepared by dispersing a polyurethane resin in water to form an emulsion or by splicing a hydrophilic group on a polyurethane resin in an aqueous solution. Waterborne polyurethane adhesives have the advantages of no pollution to the environment, non-flammable, non-toxic, and wide adaptability. However, the composite strength of water-based adhesives is still inferior to ester-soluble polyurethane adhesives, and the technical development in cooking is still a problem. At the same time, the drying speed is slow, the energy consumption and the cost are high, and it cannot be further developed under the current situation of increasing energy shortage. Therefore, ester-soluble polyurethane adhesives cannot be replaced within the next ten years.

Fifth, solvent-free polyurethane adhesive

In a real sense, solvent-free polyurethane adhesives are environmentally friendly adhesives, and solvent-free adhesives have been widely used abroad. Its advantages:

1. With solvent-free adhesives, there is no solvent contamination and the cost is relatively low.

2. No solvent evaporation drying process is required, reducing energy consumption.

3. Composite products do not suffer from residual solvents, and they also reduce the risk of flooding during production.

4. It is suitable for all kinds of plastic composite packaging and also has good adhesive effect to aluminum foil and high peel strength.

5. Resistance to content and high temperature cooking performance can be comparable with ester-soluble polyurethane.

Considering the above situation and the current development of polyurethane composite adhesives, it will not be possible to develop alcohol-soluble polyurethane adhesives within the next ten years. Water-based polyurethane adhesives are still unable to compete with ester-soluble polyurethane adhesives, and they are not the mainstream of development. In the next decade, ester-soluble polyurethane adhesives will be gradually reduced, and solvent-free polyurethane adhesives will be gradually promoted in the country. At the same time, water-based adhesives will have room for development.