Abstract: This paper mainly reviews the general situation of adhesives for packaging. Lists three main aspects of the use of adhesives in the packaging industry, describes the classification and composition of the adhesives, introduces the use of adhesives in the packaging industry, and predicts the development trend of adhesives for packaging.
Keywords: Adhesives; Packaging
Foreword
The use of adhesives in the packaging industry has been around for a hundred years. With the development of the packaging industry, the consumption of adhesives has remained at a high level, and the global packaging industry accounts for about 35% of the total amount of adhesives. Based on this, it is speculated that the packaging industry may be the largest consumer market for adhesives.
The use of adhesives in the packaging industry mainly focuses on the following three aspects:
1) Manufacturing of packaging materials
In order to meet the practical needs of easy use and low cost, a large number of packaging materials are made of synthetic materials. In the synthesis of these materials, adhesives are widely used. These materials mainly include six major categories:
(1) Tin plate, Wuxi steel and aluminum;
(2) Metal foil, mainly aluminum foil;
(3) Metallized films, mainly aluminum-plated polypropylene and polyester;
(4) Various polymer materials, such as polyethylene, polypropylene, ethylene-acrylate ion exchange polymers, polystyrene, polyvinyl chloride, polyester, polyamide;
(5) paper, cardboard;
(6) Glass
2) Molding of packaging
The process of forming the packaging mainly includes: forming the packaging board, splicing the packaging board, forming and sealing the packaging container. The modern product line enables the packaging container to be molded, loaded into the product, and sealed in a container. The work efficiency is significantly higher than that of the individual processes. When the package is a soft composite (such as paper bags) and hard plastic, continuous operation can be achieved. The adhesive used for the carton packaging must have special properties in this operation.
3) Bonding, printing of labels, manufacturing of sealing tapes, etc. The application of adhesives in the packaging industry is mainly the first two aspects, but the last aspect is also an area that should not be overlooked.
1 Classification and composition of adhesives
1.1 Classification of Adhesives
Both natural and synthetic adhesives can be used in the packaging industry. Table 1 lists the types of packaging adhesives and their market share.
Compared with synthetic adhesives with superior performance, natural adhesives still play an important role due to their advantages of low cost, ease of removal and compliance with environmental requirements. Natural adhesives are mainly used in the paper and cardboard industry. The adhesive is in the form of an aqueous solution, a paste, or an emulsion and is sizing using a squeegee, a squeegee, a nozzle, or the like. The porous structure on the surface of these tools can rapidly increase the initial viscosity of the adhesive. The dispersion made of poly(vinyl acetate) and its copolymer as the main component has obviously improved its performance and become a new adhesive for paper and board. This type of adhesive can be used for materials that cannot be bonded with natural adhesives such as polyethylene, polypropylene and light panels.
In the packaging industry, the manufacturing speed of products is an important factor that restricts costs. In order to reduce the production cost, the production speed of the laminated paper can reach several hundred meters per minute, and the packaging can form several hundred pieces per minute. In this production line, solvent-based adhesives are being replaced by ethylene-vinyl acetate copolymer (EVA) hot melt adhesives due to the influence of the solvent volatilization rate. The hot melt adhesive is sizing by a spraying method, and after the glued adhesive structure is cooled, a satisfactory bonding strength can be achieved. The advantage of hot melt adhesives is that they can bond paper and cardboard to difficult-to-bond materials such as aluminum foils, polymer films, and light panels.
Adhesives exist in different forms in practical applications:
(1) In the form of an organic solution
This form accounts for about 19% of the total. It is mainly applied to the laminating of soft composite materials and the manufacture of some labels. Usually these labels are hard to stick materials on the surface.
(2) In the form of aqueous solutions, emulsions, or aqueous dispersions
This form is 46%, mainly used in the paper and cardboard industry.
(3) Solvent-free adhesive
The main types are hot-melt adhesives, powders, tapes, films and reactive adhesives. Hot-melt adhesive tapes are mainly used for the rapid prototyping of packaging and the manufacture of pressure-sensitive adhesive tapes, while reactive adhesives are mainly used for the manufacture of composite materials.
The growing awareness of environmental protection and the concept of safe use have prompted the packaging industry to use solvent-free adhesives. Initially made of soft composite materials, the main use of organic solvent adhesives. Currently, the use of solvent-free adhesives and two- or three-component polyurethane adhesives has rapidly increased. Polyurethane adhesives can be used nowadays with the rubber roller. When curing, they rely on the moisture in the air to accelerate the curing speed.
1.2 The composition of the adhesive
Adhesives often consist of multiple components. First of all, the core is the adhesive material, and then the components that make up the adhesive must take into consideration various requirements such as reasonable performance of the adhesive, convenient operation, solvent evaporation time, viscosity, and stable adhesive performance. In general, an adhesive formulation contains at least two of the following.
(1) Adhesives - polymers;
(2) Solvents - Water, Organics;
(3) Plasticizer - lower glass transition temperature;
(4) Tackifiers - usually natural resins, can increase the cohesive strength of the adhesive after film formation;
(5) thickeners and fillers - control the viscosity, increase the solid content;
(6) Surfactant - increase the degree of adhesive infiltration on the adhesive surface;
(7) Insecticides and fungicides - used in natural adhesives, paper and paperboard adhesives;
(8) Emulsifiers;
(9) Waxes - improve the fluidity of the adhesive after melting;
(10) Antioxidants.
2 The practical application of adhesives in the packaging system
2.1 The basis for the selection of packaging materials
According to actual needs, there are several factors that must be considered when selecting or designing a package:
(1) Costs; (2) Consumer needs; (3) Ability to identify items to be packaged, mainly referring to the design of the label; (4) Ability to match the loading tool; (5) Performance, such as mechanical strength, isolation capability ( Can be isolated from organic solvents, microorganisms, moisture, oxygen and carbon dioxide), temperature resistance (can be subjected to boiling, high temperature sterilization, freezing, etc.), no chemical reaction with the packaged product.
When choosing a packaging system, costs are usually an important factor. To reduce costs, a composite structure can be formed by lamination techniques. When designing composite structures, performance optimization needs are met through optimized combinations of different materials. One of the most demanding areas for composite materials is the manufacture of food and beverage packaging containers with a barrier layer. Metal cans or glass bottles are good isolated containers; on the other hand, composite materials composed of paper, isolated polymer, and metal foil also have considerable market demand. In the lamination of these materials, the use of adhesives improves the cost-effectiveness of the composites.
2.2 Application of Adhesives in Composite Material Synthesis
2.2.1 Soft composite materials
2.2.1.1 Food Packaging Film
Food packaging films are typically printable oriented stretched polypropylene (BOPP). To facilitate sealing, the inner side of this POPP film is a layer of hot melt material bonded with a polyurethane adhesive.
2.2.1.2 Isolated Carton
The carton is made of cardboard and aluminum foil. The structure is as shown in the figure below:
- Hot Melt Materials - Printable Paperboard, Aluminum Foil, Hot Melt Materials - Adhesives Adhesives
In this structure, the cardboard provides the necessary mechanical strength, and the aluminum foil can sufficiently block oxygen. The inner and outer surfaces of the carton are polyolefin-based hot-melt materials coated on the surface of paper and aluminum foil by hot-melt coating respectively, and starch-PVA (polyvinyl acetate glue is glued into a composite structure on both sides of the cardboard). to be continued)