Application of Food Packaging Dry Composite and Development of Adhesive

First, the status of dry composite materials used for food packaging

The modernization of packaging materials in China, originally in 1955, introduced extrusion composite technology developed in the United States and developed a composite film of cellophane and polyethylene. The disadvantages are the odor of polyethylene oxidation, poor adhesion, poor heat resistance, and the like. Since then, with the development of various films, in 1965, the German dry compound technology was introduced, which solved the disadvantages of extrusion compounding. At the initial stage of adhesives, vinyl acetate, vinyl ester, ethylene vinyl acetate, and rubber adhesives were used. Later, polyurethane adhesives have good processability and high adhesive strength (all kinds of plastic films and Aluminum foil can be bonded, oil resistance, drug resistance, food resistance, cold resistance, heat resistance, excellent softness and other characteristics, almost all in the plastic composite polyurethane adhesive.

The composite film itself was originally a composite of cellophane, polyethylene film, and polypropylene, and was mostly used for the packaging of dried foods such as confectioneries. It was processed using a polyurethane-based single-liquid adhesive. Later, with the improvement of cultural tastes and the development of foods (cooking foods, etc.), changes in circulation institutions (supermarkets, mass retail stores, joint sales stores for frozen products, etc.), and the large and inexpensive supply of plastic films, they were packaged. The foods have also begun to diversify and advanced, and sterilization methods have also evolved from ordinary cooking methods to high-temperature steam sterilization. As a result, the performance and processability of the adhesives have been further improved, and the polyurethane-based two-liquid solvent-based adhesives currently used have been developed and mainstreamed.

Second, the characteristics of dry compound

1. Applicable to all kinds of films, as long as the selected adhesive and coating amount can be combined with a composite film with good heat resistance, oil resistance, airtightness, chemical resistance, and transparency.

2. In the case of composite polyethylene film, there is no oxidation odor and the heat sealing performance is better than extrusion compounding, and the film can be formed at a lower temperature than the extrusion compounding.

3. It can be used flexibly when carrying out multi-variety small-scale production. The substrate and adhesive are easy to replace.

4, better than the extrusion composite peel strength, the composite film transparency is good, the film is strong, suitable for automatic filling.

Third, the use of composite substrates

Cellophane (PT) has chemical resistance, airtightness, firmness, and oil resistance. The disadvantage is that it is hygroscopic and moisture permeable. It is used in the distorted packaging of soap, health ball packaging, and candy biscuits.

Polypropylene film (OPP) has good oil resistance and moisture resistance, but is poor in air permeability. Suitable for seaweed, snacks, instant noodles, frozen food packaging.

Polyester film (PET) has good cold resistance and heat resistance but poor puncture resistance. Suitable for high temperature cooking sterilization of food packaging.

Nylon film (NY) is resistant to puncture, cold and heat resistant, impact resistant, airtight, and hygroscopic. Suitable for liquid, angular foods and pasteurized packaging.

Aluminum foil (AL) can resist UV, moisture, and air permeability, but it is easy to wrinkle. Suitable for high temperature cooking sterilization packaging.

Various aluminized films (VMCPP, VMPET, VMOPP) have good moisture resistance, air permeability resistance, and poor water resistance. It is used in the packaging of snacks, tea and other snack foods.

Four, dry composite with a variety of film precautions

1. Polyolefin film (PE, PP) is a non-polar polymer, so it must be treated by corona treatment to make its surface active. When the film is placed for a long time, the surface must be inspected. At the same time, the film contains additive components that should be prevented from depositing and affecting the fastness of the composite.

2, to check whether there is oil, debris, etc. mixed, not only affect the appearance, but also cause poor adhesion.

3, if the film is loose, wrinkles, uneven thickness, do not use, otherwise it is easy to cause curl, air bubbles and other bad appearance.

4. Hygroscopic films (such as PT, NY, etc.) gradually absorb moisture from the air, affecting the degree of surface treatment, and the moisture can adversely react with the curing agent in the polyurethane adhesive.

Fifth, the precautions of dry composite printing film

1. The ink film must be solvent-resistant, otherwise the ink is easy to lose during dry compounding.

2. The adhesion of the ink to the substrate must be good, otherwise no good adhesion can be obtained after compounding.

3, the wettability of ink and adhesive must be good, otherwise there will be problems with adhesiveness and appearance.

4. When there are many solvents remaining in the printing film, odor will occur after compound drying.

5, ink film must be resistant to processing conditions after composite processing (water resistance, heat resistance, drug resistance).

Sixth, the development direction

Adhesives for flexible packaging have gone through two stages in China: the first stage was the end of the 1970s to the early 1980s, and the domestic economic development was still in its early stage. The technical strength was not strong. The common packaging compound was a one-component rubber adhesive. Main, high-end packaging products 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. The following specific analysis of the advantages and disadvantages of various types of adhesives and development prospects.

1, one-component adhesive

Since most of the one-component adhesives are based on vinyl acetate, vinyl ester, 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 peel strength. It is not high, the compound product smells heavy, and the compound product is monotonous. It is mainly composed of paper-plastic composite and plastic-plastic composite. 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.

2, 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:

a. Good bonding effect, high peel strength, wide range of applicable substrates, and can be used for composites of paper, PET, OPP, NY, PE, CPP and aluminum foil. The composite product has good quality and high strength.

b, outstanding resistance to media, acid, alkali, oil, hot and various oxides and chemicals corrosion.

c, cold resistance and heat resistance, many food packaging requires cryogenic preservation or high temperature 121 ~ 135 °C sterilization, and the use of polyurethane adhesive under the above conditions can still maintain 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) is hydrolyzed at high temperature, it will become toluenediamine (TDA). Toluene diamine is a carcinogenic substance. After the compounding reaction of polyurethane adhesive, the residual monomer of isocyanate contained in the polyurethane adhesive is minimal, but when After high-temperature steaming of the packaging, these residual monomers migrate through the plastic film into the food and are hydrolyzed to form aromatic amines (TDA). Therefore, they have the potential for carcinogenicity. Therefore, the US Food and Drug Administration (FDA) expressly requires that The use of aromatic polyurethane adhesives for cooking and packaging will be severely punished if someone packs steamed foods in packaging bags made from this glue and sells them on the market. Although China, Europe, and Japan are not as strict as the United States, there are clear restrictions on the content of TDA. China's domestic requirements for TDA content shall not exceed 0.004mg/L. Therefore, the use of TDI as a raw material for adhesives on cooking packaging has been limited.

The two-component polyurethane adhesive uses ethyl acetate as the 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, the ethyl acetate solvent-based polyurethane adhesive 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.

3, 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 has its advantages:

a. Free isocyanate monomer free, improving the operating environment.

b, is not sensitive to moisture, improve the stability and adaptability of the adhesive.

c. The use of industrial alcohol as a solvent reduces production costs.

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

e. The composite membrane product has low residual solvent.

At present, alcohol-soluble polyurethane adhesives are only used for composite packaging of common dry foods. From the perspective of technical and alcohol-soluble properties, it is impossible to achieve the peel strength and cooking performance requirements of ester-soluble polyurethanes. In addition to the provisions for the TDA content of packaging adhesives for cooking products, there is no restriction on the packaging used for ordinary products, so it is not possible to replace ester-soluble polyurethane adhesives, so the development prospects are not optimistic.

4, water-based polyurethane adhesive

Waterborne polyurethane adhesives disperse polyurethane resins in water. An emulsion is formed, or the urethane resin is spliced ​​with a hydrophilic group dissolved 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. Under the current situation of increasing energy shortages, 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.

5, 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:

a. With a solvent-free adhesive, there is no contamination of the solvent and the cost is relatively reduced.

b. The solvent evaporation drying process is needed to reduce the energy consumption.

c. The composite products are free of residual solvents, and reduce the risk of “floods” in the production process.

d, solvent-free polyurethane adhesive

Applicable to all types of plastic composite packaging, also has a good adhesive effect on aluminum foil, resistance to content properties and high temperature cooking performance can also be the same as ester-soluble polyurethane, high peel strength.

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 polyurethanes, 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.

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