Characteristics of polyurethane adhesives for flexible packaging

In the 1960s and 1970s, the packaging of most commodities in China was based on straw paper and waste newspapers, glass bottles, and metal cans. By the end of the 1970s, the first dry laminating machine and composite adhesives were introduced in China, marking the country's flexible packaging. Begin the bud and development. After the 1980s, China's flexible packaging began to develop vigorously. Various kinds of plastic flexible packaging gradually replaced grass and paper, waste newspapers, glass bottles, and metal cans and became the mainstream of packaging. After the 1990s, the development of flexible packaging reached its peak. Various new materials and new structural packaging emerged in an endless stream, from plastic and plastic packaging to aluminum-plastic packaging and aluminum-plated packaging; from double-layered compounding to three-layered and four-layered compounding; from two-layer co-extrusion to Four-layer and five-layer coextruded compound.

Throughout the development of flexible packaging, the corresponding adhesives for flexible packaging have experienced two stages of development in China: the first stage was from the late 1970s to the early 1980s. During this period, the domestic economic development was still at an early stage, and the technical strength was not strong. The compounding of ordinary packaging is based on one-component rubber adhesives, and the high-end packaging products are supplemented by imported two-component polyurethane adhesives. The second stage is from the 1980s to the present, 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 single-component adhesives are based on vinyl acetate, vinyl ester, ethylene-vinyl acetate, and rubber-based systems, toluene is the main solvent, (only a small fraction of which are polyurethane types but not technically mature enough). The peel strength is not high. The compound product smells 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 cannot form the mainstream. The only thing that attracts people is that the price is relatively cheap, so some manufacturers are still using it 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 adhesive factories and research institutes such as Zhejiang New Oriental, Beijing North Chemical Research Institute, 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. Good adhesion effect, high peel strength, wide range of applicable substrates, and can be used for compounding paper, PET, OPP, NY, PE, CPP and aluminum foil materials. The composite product is of good quality and high strength.
2, anti-media outstanding performance, acid, alkali, oil, spicy, and a variety of oxides and chemicals corrosion.
3, cold resistance and heat resistance, many food packaging requires cryopreservation or high temperature 121 ~ 135 °C sterilization, and the use of polyurethane adhesive in the above process can still maintain a good peel strength.

Two-component polyurethane adhesives are classified into polyester polyurethane adhesives and polyurethane polyurethane adhesives. At present, most polyester and polyurethane adhesives are made of adipic acid and ethylene glycol as the main raw materials, and are used as curing agents. TDI and trimethylolpropane as the main raw material polymerization. Since tolylene diisocyanate (TDI) will be converted to toluenediamine (TDA) after high-temperature hydrolysis, and toluenediamine is a carcinogen, the polyurethane adhesive contains isocyanate residual monomers after compounding reaction has been minimal, but when packaged After being subjected to high-temperature cooking, these residual monomers migrate through the plastic film into the food and are hydrolyzed to aromatic amines (TDA). Therefore, they have the potential for carcinogenicity. Therefore, the US Food and Drug Administration (FDA) specifically requested that it be prohibited. Aromatic polyurethane adhesives are used for retort packaging. If someone uses this kind of plastic packaging for steamed foods and sells them on the market, it will be severely punished. However, although there are no such strict regulations in the United States, Europe and Japan, there are no clear restrictions on the content of TDA. China's domestic requirements for TDA content shall not exceed 0.004mg/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 the solvent. This is determined by the product's characteristics and process requirements. Due to the flammability of ethyl acetate and its environmental and ecological impact, ethyl acetate solvent-based polyurethane adhesives should be Limits. From this point of view, the development direction of ester-soluble polyurethane adhesives is high solid content and low viscosity. From the perspective of domestic production and economic development, ester-soluble polyurethane adhesives will still account for about 70% of the share in the coming decade.

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 isocyanate free monomer improves the operating environment.
2, is not sensitive to moisture, improve the stability and adaptability of the adhesive.
3, using industrial alcohol as a solvent, the production cost is reduced.
4, with the alcohol-soluble polyurethane printing ink used in conjunction with excellent reproducibility.
5. The residual solvent of composite membrane products is low.

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 at home and abroad. Only the content of TDA for packaging adhesives used for steaming products is regulated, but the packaging used for ordinary products is not limited, so it is impossible to replace the ester-soluble polyurethane adhesive, so the development prospect is not optimistic.

Third, 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 adhesive strength and weather resistance of waterborne adhesives are still inferior to those of ester-soluble polyurethane adhesives, and the technical development in cooking is still a problem at present. At the same time, the drying is slow, the energy consumption is high, and the cost is high. In the current situation where energy is increasingly tight, it cannot be further developed. Therefore, although there will be some development, due to cost and energy consumption, ester-soluble polyurethane adhesives will not be replaced within the next decade.

Fourth, 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, the use of solvent-free adhesive, no pollution of the solvent, the cost is relatively reduced.
2. Solvent evaporation drying process is not required, reducing energy consumption.
3. There is no residual solvent in the composite products, and flood risks are also reduced in the production process.
Solvent-free polyurethane adhesives are suitable for various types of plastic composite packaging. They also have good adhesion to aluminum foil. The resistance to high content and high temperature cooking performance can also be similar to that of ester-soluble polyurethane, and the peel strength is high. Based on the above situation and the current development of polyurethane composite adhesives, it will be impossible to develop alcohol-soluble polyurethane adhesives in the next ten years. Water-based polyurethane adhesives are still unable to compete with ester-soluble polyurethanes, and they are not the mainstream of development. In the next ten years, the ester-soluble polyurethane adhesive will be gradually reduced, and the solvent-free polyurethane adhesive will be gradually promoted in the country. At the same time, the water-based adhesive will have a certain space for development.

Source: "Guangdong Packaging" (6th Issue 2005)