Current situation and development of the hottest a

The present situation and development of adhesives for packaging

preface

adhesives have been used in the packaging industry for nearly a century. With the development of the packaging industry, the consumption of adhesives has maintained a high level. The annual consumption of adhesives in the global packaging industry accounts for about 35% of the total consumption of adhesives. Therefore, it is speculated that the packaging industry may be the largest consumption market of adhesives

adhesives used in the packaging industry are mainly concentrated in the following three aspects:

1) in order to meet the practical needs of convenient use and low price, a large number of packaging materials are made of synthetic materials. Adhesives are widely used in the synthesis of these materials. These materials mainly include six categories: (1) tinned steel plate, Wuxi Steel and aluminum

(2) metal foil, mainly aluminum foil

(3) metallized film, mainly aluminized polypropylene and polyester

(4) various polymer materials, such as polyethylene, polypropylene, ethylene acrylate ion exchange polymer, polystyrene, polyvinyl chloride, polyester, polyamide

(5) paper and paperboard

(6) glass

2) packaging molding

packaging molding process mainly includes: packaging board molding, packaging board splicing, packaging container molding and sealing. The modern product production line can make the packaging container molding, loading products, and container sealing complete continuously, and its work efficiency is significantly higher than the process of completing each process alone. When the packages are soft composite materials (such as paper bags) and hard plastics, continuous operation can be realized. The adhesive used for packaging cartons needs to have special properties in this operation

3) the application of adhesives in the packaging industry is mainly in the first two aspects, such as the bonding and printing of labels and the manufacture of sealing tapes, but the last aspect is also a field that cannot be ignored

1 classification and composition of adhesives

1.1 classification of adhesives

natural adhesives and synthetic adhesives can be used in the packaging industry. Table 1 lists the types of adhesives for packaging and their market share

compared with synthetic adhesives with superior properties, natural adhesives still play an important role because of their advantages of low price, easy removal and compliance with environmental protection requirements. Natural adhesives are mainly used in the paper and paperboard industry. The adhesive exists in the form of aqueous solution, paste or lotion, and is applied by glue spreading tools such as rubber roller, scraper and nozzle. The porous structure on the surface of these tools can rapidly increase the initial viscosity of the adhesive. The dispersion synthesized with polyvinyl acetate and its copolymer as the main body has significantly improved its performance and has become a new adhesive for paper and paperboard. The characteristic of this kind of adhesive is that it can be used for some materials that cannot be bonded with natural adhesives, such as polyethylene, polypropylene and bright board

in the production of packaging industry, the manufacturing speed of products is an important factor restricting costs. In order to reduce the production of vertical marking book with swing rod alignment, the production speed of laminated paper can reach hundreds of meters per minute, and the packaging can be formed hundreds of pieces per minute. In this production line, the solvent based adhesive is being replaced by ethylene vinyl acetate copolymer (EVA) hot melt adhesive due to the influence of solvent volatilization rate. The hot melt adhesive is applied by spraying. The bonding structure of the spraying adhesive is selected according to the maximum bearing capacity of the experimental material. The maximum experimental force is the range of the experimental machine. After cooling, the satisfactory bonding strength can be achieved. The advantage of hot melt adhesive is that it can bond paper and paperboard with some materials whose surface is difficult to stick, such as aluminum foil, polymer film, bright board

adhesives exist in different forms in practical applications:

(1) they exist in the form of organic solution, which accounts for about 19% of the total. It is mainly used in the lamination of soft composite materials and the production of some labels, which are usually difficult to stick on the surface

(2) it exists in the form of aqueous solution, lotion or aqueous dispersion. This form is 46%, which is mainly used in the paper and paperboard industry

(3) solvent free adhesive

the main types are hot melt adhesive, rubber powder, tape, film and reactive adhesive. Hot melt adhesive tape is mainly used in the rapid prototyping of packaging and the manufacture of pressure-sensitive adhesive tape, while reactive adhesive is mainly used in the manufacture of composite materials. The increasing awareness of environmental protection and the concept of safe use have prompted the packaging industry to use a wide range of solvent-free new materials, a wide variety of adhesives. At the beginning of manufacturing soft composite materials, adhesives with organic solvents were mainly used. At present, the use of solvent-free adhesives and two-component or three component polyurethane adhesives is increasing rapidly. Polyurethane adhesive can be used and prepared on the glue roller. When curing, it depends on absorbing moisture in the air to speed up the curing speed

1.2 composition of adhesives

adhesives are often composed of multiple components. First, the core is the adhesive, and then the composition of the adhesive should take into account the reasonable performance, convenient operation, solvent evaporation time, viscosity, stable bonding performance and other requirements. Generally, an adhesive formula contains at least two of the following contents

(1) binder polymer

(2) solvent water, organic matter

(3) plasticizer - reduce the glass transition temperature

(4) tackifier - usually natural resin, which can improve the cohesive strength of adhesive after film formation

(5) thickener and filler - control viscosity and increase solid content

(6) surfactant - increase the degree of infiltration of adhesive on the bonding surface

(7) pesticides and fungicides - used in natural adhesives, adhesives for paper and paperboard

(8) emulsifier

(9) wax - improve the fluidity of adhesive after melting

(10) antioxidant

2 practical application of adhesives in packaging system

2.1 basis for selection of packaging materials

according to actual needs, multiple factors must be considered when selecting or designing a package

(1) cost; (2) Consumer demand; (3) Being able to mark the packed items mainly refers to the design of labels; (4) It can match with the filling tool; (5) Performance, such as mechanical strength, isolation ability (can isolate organic solvents, microorganisms, moisture, oxygen and carbon dioxide), temperature resistance (can withstand boiling, high-temperature disinfection, freezing and other conditions), and no chemical reaction with the packed material

when choosing a packaging system, the cost is usually Engineering Plastics: an important factor for suppliers to develop new technologies for high-performance plastics. In order to reduce the cost, a composite structure can be formed by lamination technology. When designing composite structures, different materials are optimized to meet the needs of performance. The largest demand for composite materials is in the manufacturing of food and beverage packaging containers with isolation layers. Metal cans or glass bottles are good isolated containers; On the other hand, the composite materials made of paper, isolation polymer and metal foil also have considerable market demand. In the lamination process of these materials, the use of adhesives improves the cost performance of composites

2.2 application of adhesives in composite material synthesis

2.2.1 soft composite material

2.2.1.1 food packaging film

the film used for food packaging is usually printable oriented stretched polypropylene (Popp). In order to facilitate sealing, the inner side of this Popp film is a layer of hot-melt material bonded with polyurethane adhesive

2.2.1.2 isolation carton

this kind of carton is composed of cardboard and aluminum foil, and the structure is shown in the following figure:

│ hot melt material │ printable paper ‖ cardboard ‖ aluminum foil │ hot melt material │

adhesive adhesive

in this structure, cardboard provides the necessary mechanical strength, and aluminum foil can fully isolate oxygen. The inner and outer surfaces of the carton are polyolefin hot-melt materials coated on the surface of paper and aluminum foil respectively by hot-melt coating method, Starch PVA is usually used on both sides of the paperboard (polyvinyl acetate paste is bonded to form a composite structure.

2.2.2 isolation laminated material

aluminum foil and metal coated film are insulation materials with good performance. The polymer with low air permeability has become a new type of isolation material, which occupies a certain position in the market as an update of traditional isolation materials. Table 2 and table 3 list the oxygen and wet air permeability of commonly used polymers of packaging materials respectively.

from table 2 2. It can be seen from table 3 that the polymers with the best gas isolation effect are polyvinylidene chloride (PVDC) and ethylene vinyl alcohol copolymer (EVOH). Because these two materials are expensive, they cannot be widely used as packaging materials. Polyolefin membrane can be well isolated from humid air. It is cheap and has certain mechanical strength. It has become a practical isolation material. Polyolefin film is formed on the surface of the material by hot melt coating method, which is used as an isolation layer in the manufacture of liquid beverage packaging boxes. It is reported that a low-cost silica coated PET film has been successfully developed. This submicron coating is formed by plasma decomposition using silicone/oxygen/helium mixture, and the isolation effect is better than or equivalent to EVOH and PVDC laminated structure

2221 the manufacture of adhesive laminated isolation composite

sealing film is an example of the practical application of isolation soft laminated structure. Its typical structure is as follows:

│ strength layer │ printing layer ‖ strength layer ‖ thermal sealing layer

adhesive adhesive adhesive

generally, the adhesive used is polyurethane adhesive, which can meet the pressure and temperature requirements required for product sterilization. The strength layer is polyester, and the isolation layer is metal foil, metal plating film, PVDC, EVOH, etc. The material of thermal sealing layer shall have heat aging resistance and be peelable after sealing

2222 laminated isolation composite

hard and soft isolation packaging can be formed by lamination or blow molding with different films or plates. The plastic bottle containing jam is made by lamination blow molding method, and its isolation lamination structure is as follows:

│ strength layer │ adhesive │ isolation layer │ │

strength layer strength layer

uses polyethylene (PE) or polypropylene (PP), and the isolation layer is PVDC or EVOH. The adhesive is thermoplastic, mainly EVA or polyethylene modified with carbonyl compounds (acid, acrylate, anhydride). This kind of adhesive has good adhesion to paper, metal, and polymer materials. PE and PP adhesives have better heat resistance than EVA adhesives, and can be used in high temperature treatment and food packaging. The pressing layer method can reduce the production cost, make the production continuous, and meet the requirements of high-speed production

223 metal composite

2231 protection of metal surface

at the beginning of the 19th century, the food industry began to use thin tinned steel plates to make food packaging containers. In order to increase the inertness of the container to some contents, it is necessary to protect the inner surface of the container with a coating. The inner coating material is usually epoxy polymer, which is scraped or sprayed on the metal surface by solvent method. Now, water-based coatings have been used to gradually reduce the content of volatile organic compounds (VOC) in the coating. At present, tinned steel plate, Wuxi steel plate (TFs) or aluminum have become the raw materials of metal containers. Coating method is still the main method to isolate the active metal surface from its contents. As a more advanced isolation method, materials that compound polymer films on metal surfaces have emerged. This structure is composed of polyester, polypropylene or nylon polypropylene composite film on both sides of the steel plate. This kind of film is used in the forming process of steel drum