Adhesives in the packaging line classification and application (below)

2.2.2 Isolated Laminated Materials

Aluminum foil and metal-plated film are good insulating materials, and the low-permeability polymer has become a new type of insulation material. As a replacement product of traditional insulation materials, it occupies a certain position in the market. Table 2 and Table 3 respectively show the oxygen and moisture air permeability of the commonly used polymers for packaging materials.

    From Table 2 and Table 3, it can be seen that the best gas-insulated polymer is polyvinylidene chloride (PVDC), ethylene-vinyl alcohol copolymer (EVOH), which is used as a package because these two materials are expensive. Material cannot be used in a wide range. Polyolefin membranes are very good at isolating humid air. They are inexpensive and have a certain mechanical strength, making them a practical barrier material. The polyolefin film is formed on the surface of the material by a hot melt coating method and used as a separator in the production of a liquid beverage package. According to reports, a low-cost silica-coated PET film has been successfully developed. The ultra-micron coating is formed by plasma decomposition using a mixture of silicone/oxygen/helium gas. The isolation effect is better than or equivalent to that. Laminated structure in EVOH and PVDC.

2.2.2.1 Bonded laminated composites

The manufacture of sealing membranes is an example of the practical application of isolated soft laminate structures. The typical structure is as follows:

-Strength layer - Printed layer ‖ Isolation layer ‖ Strength layer ‖ Heat sealing layer Adhesives Adhesives Adhesives

Generally used adhesives are polyurethane adhesives, which can meet the required pressure and temperature requirements for product sterilization. The strength layer is polyester, and the separator is metal foil, metallized film, PVDC, EVOH, and the like. Heat seal layer materials are required to have heat aging resistance and have peelability after sealing.

2.2.2.2 Laminated laminated composites

Both hard and soft isolated packages can be formed by laminating or blow molding using different films or plates. The plastic bottle containing jam is made by laminating-blow molding, and the isolated laminated structure is as follows:

- Strength layers - Adhesives - Release layers - Adhesives - Strength layers

The strength layer uses polyethylene (PE) or polypropylene (PP) and the barrier layer is PVDC or EVOH. Adhesives are thermoplastic, mainly EVA or polyethylene modified with carbonyl compounds (acids, acrylates, anhydrides). These adhesives have good adhesion to paper, metal, and polymer materials. PE and PP adhesives have better heat resistance than EVA adhesives and can be used for high temperature processing and food packaging.

The legality of the laminating layer can reduce the production cost, enable the production to proceed continuously, and meet the requirements for high-speed production.

2.2.3 Metal composites

2.2.3.1 Metal Surface Protection

In the early 19th century, the food industry began using thin tinplates to make food packaging containers. In order to increase the inertness of the container to certain contents, it is necessary to use a coating on the inner surface of the container for protection. The inner coating material is usually an epoxy polymer that is scratched or sprayed on the metal surface by a solvent method. Now, water-based coatings have been used to gradually reduce the content of volatile organic compounds (VOCs) in coatings.

At present, tinplate, TFS or aluminum have all become raw materials for metal containers. The coating method is still the main method of isolating the active metal surface from the contents. As a more advanced method of isolation, materials for compounding polymer films on metal surfaces have emerged. This structure is a composite of polyester, polypropylene or nylon-polypropylene composite films on both sides of the steel plate. This film is formed by hot pressing and bonding using a bonding material during the process of forming the steel sheet roll. Due to the use of the bonding material, the bonding force between the film and the substrate is improved. The polypropylene film may be either a propylene homopolymer or an ethylene-propylene copolymer, and the adhesive resin is a polypropylene modified with an acid or acid anhydride. The polyester film is formed by laminating a biaxially stretched PET film with a low-melting polyester, wherein the low-melting polyester can achieve a high adhesive strength during the hot-press lamination process. The structure of the nylon-polypropylene film is four layers. This structure uses an adhesive to bind the nylon and the polypropylene together, and then the polypropylene and the steel plate are stuck together. The outermost nylon surface has a high hardness and a low frictional force and is easy to handle in many molding processes.

Metal-polymer lamination technology has the advantages of no solvent and low energy consumption and has been applied in the manufacture of aerosol cans and food cans.

2.3 Bonding of metal cans

Metal can forming, the first to use direct welding. First, the tin plate is processed into a cylinder and both ends are capped; then, the edges of the plated blanks are welded together to form a cylindrical body. In the last 20 years, this process has been replaced by welding rod welding technology. The use of tin-plated steel electrodes can form a complete weld seam.

Due to the continuous rise in tin prices in the international market, Wuxi steel sheets have appeared in the manufacturing process of metal cans. Due to the immature welding technology of Wuxi steel, the initial can-making of this material was the use of stretching tanks and closures. With the development of adhesive bonding technology, there has been a glued food can similar to the traditional welding process. The surface of the Wuxi steel sheet is coated with an epoxy primer. In the margins of the treated edge, the nylon tape coated with polyamide is glued. By adjusting the mixing ratio of polyamide and epoxy paint, the process performance meets the requirements, and through the hot-pressing compounding process, the requirements for bonding are achieved, and the requirements for high-speed production lines can be met.

3 Development of Packaging Adhesives

The development of adhesives for packaging is constrained by strict environmental regulations, human health awareness and production safety requirements. In order to reduce the environmental pollution of the product, the need for adhesives using organic solvents is reduced firstly; secondly, the content of volatile organic compounds in water-based adhesives is gradually reduced. At present, Europe has not allowed adhesive producers to use chlorine-containing organics in aqueous emulsions. As a result, the use of hot melt adhesives and other solvent-free adhesives continues to increase in the production of paper, packaging, labels, and soft composites. In the production of paper packaging, in order to meet the requirements of high-speed production lines, hot melt adhesives are used for sealing, and part of the lamination process increases the proportion of water emulsions used. At the same time, in order to better save energy, the use of low-melting-point hot melt adhesives can be used when the hot-melt adhesives use a less severe temperature range.

In the laminated technology of metal and polymer, since no solvent is used, compared with other laminated technologies, it has obvious environmental advantages and will have rapid development in the future.

In order to reduce ecological damage, packaging materials requirements can be recycled. Adhesives can be easily removed during recycling. For example, the use of adhesives in the production of paper packages requires that they be easily filtered out during the re-pulping of paper products. Another way is to use a biodegradable adhesive, and this type of adhesive will become an important development direction.

4 Conclusion

For a long time to come, the packaging industry will continue to be the main area of ​​adhesive use. With the continuous improvement of the adhesive bonding performance of adhesives, the technical progress of the bonding process will positively influence the selection of packaging materials and the improvement of packaging processing equipment. Recently, the environmental protection requirements faced by the packaging industry and the high speed requirements for manufacturing and assembly lines are two major directions for the development of adhesives for packaging. (Text: Sun Dongzhou Zhang Zhi Shen Xingyuan)