Flexible packaging refers to the packaging in which the shape of the container can change after filling or taking out the contents. All kinds of bags, boxes, and packages made of paper, aluminum foil, fiber, plastic film, and their composites are soft packaging.

Monolayer films have different advantages and disadvantages. For example, the polyester film has high strength and excellent oil resistance, gas resistance, fragrance wrapping, and moisture resistance, but its heat-sealing performance is poor; The moisture resistance and heat sealing of polyethylene film is good, but the gas resistance and oil resistance are poor. They have performance defects when they are used as plastic packaging alone, but the combination of polyester film and polyethylene film can not only maintain the high strength and good oil resistance, gas resistance, fragrance retention, and moisture resistance of the polyester film but also have good heat-sealing performance.

The function of the composite film is to make multi-layer films composite together, which not only overcomes the shortcomings of the single-layer film but also integrates the advantages of each layer of film to become an ideal packaging material. In addition, because the ink is sandwiched in the middle of the film layer, the ink layer is protected from direct friction, scratch, and the destruction of various corrosive substances, which not only solves the problems of color seepage and fading in plastic printing but also avoids the safety and health problems caused by the ink’s direct contact with food and drugs.

The “composite” in the industry actually means “lamination”, which means that films of different properties are bonded together in a certain way, and then sealed to protect the contents. The composite processing methods of flexible packaging mainly include dry composite, wet composite, extrusion composite, coextrusion composite, and so on.

1 Dry composite

Among various processing technologies of composite membrane, dry composite is the most traditional and widely used composite technology in China. It is widely used in the packaging of food, drugs, cosmetics, daily necessities, light industrial products, chemicals, electronic products, etc. Dry lamination is to apply a layer of solvent-based adhesive on the plastic film with the coating device (generally coated by gravure screen roller), remove the solvent and dry it by the laminating machine, and then composite it with other substrates in the hot pressing state, such as film and aluminum foil. The process flow is shown in the figure below because it is compounded in the “dry” state of the adhesive (solvent-free state), Therefore, it is called dry composite.

Monolayer films have different advantages and disadvantages. For example, the polyester film has high strength and excellent oil resistance, gas resistance, fragrance wrapping, and moisture resistance, but its heat-sealing performance is poor; The moisture resistance and heat sealing of polyethylene film is good, but the gas resistance and oil resistance are poor. They have performance defects when they are used as plastic packaging alone, but the combination of polyester film and polyethylene film can not only maintain the high strength and good oil resistance, gas resistance, fragrance retention, and moisture resistance of the polyester film but also have good heat-sealing performance.

The function of the composite film is to make multi-layer films composite together, which not only overcomes the shortcomings of the single-layer film but also integrates the advantages of each layer of film to become an ideal packaging material. In addition, because the ink is sandwiched in the middle of the film layer, the ink layer is protected from direct friction, scratch, and the destruction of various corrosive substances, which not only solves the problems of color seepage and fading in plastic printing but also avoids the safety and health problems caused by the ink’s direct contact with food and drugs.

The “composite” in the industry actually means “lamination”, which means that films of different properties are bonded together in a certain way, and then sealed to protect the contents. The composite processing methods of flexible packaging mainly include dry composite, wet composite, extrusion composite, coextrusion composite, and so on.

2 wet composite

Wet compounding is to coat a layer of adhesive on the surface of the composite substrate (plastic film and aluminum foil), compound it with other materials (paper and cellophane) through a pressure roller when the adhesive is not dry, and then dry it into composite film through hot drying channel. The characteristics of wet compounding are simple to process operation, less amount of adhesive, low cost, and fast compounding speed. The adhesives used in the wet composite method mainly include polyvinyl alcohol, sodium silicate, starch, polyvinyl acetate, ethylene-vinyl acetate copolymer, polyacrylate, natural resin, etc. The working principle of a wet compounding machine is basically similar to that of dry compounding. The difference is that dry compounding is to heat the film coated with adhesive through drying channel, and then bond with the composite by hot pressing after the organic solvent in the adhesive volatilizes; The wet composite method is to composite the film coated with adhesive directly with the composite material, and then enter the drying channel for drying. When wet lamination is used, at least one of the two substrates is a porous material (such as paper or paperboard) so that the diluent contained in the adhesive can volatilize during drying. Therefore, wet compounding is widely used in the compounding of paper/fiber, paper/paper, paper/paperboard, paper/aluminum foil, and other materials.

3 extrusion composite

Extrusion compounding is to extrude thermoplastic materials such as polyethylene into the flat die after melting in the extruder to form a sheet film. After flowing out, it is immediately compounded with another or two films through the cooling roll and composite pressing roll. Compared with other compounding methods, extrusion compounding has the advantages of low equipment cost, less investment, clean production environment, no residual solvent in the composite film, high production efficiency, and simple operation. Extrusion compounding occupies a very important position in the compounding process of plastics. Extrusion composite film can be prepared by extrusion coating and extrusion composite.

4 extrusion coating film

Extrusion coating is a method to melt thermoplastic plastics such as polyethylene and flow out of the flathead, press it between two closely contacted rollers to another substrate, and make a composite film after cooling. Unwinding base materials are generally printed pet, BOPP, paper, and other materials. For example, an instant noodle packaging bag is a typical example of extrusion coating production. Its structure is BOPP printing film / LDPE (or PP).

5 extruded composite film

Extrusion compounding is to clamp the extruded resin between two substrates. It plays the role of adhesive to combine the two substrates, and it is also a composite layer. The first base material is often a pet, BOPP, PET, paper, and other materials, the second base material is generally LDPE, CPP, aluminum foil, aluminized film and other materials, and the extrusion resin is generally PE, PP, EVA, EAA and other resins. General ordinary washing powder packaging film can be compounded by this method, and the structure is as follows: BOPP printing film / extruded resin / PE film.

 

6 solvent-free composite

The adhesive used in solvent-free compounding has attracted much attention because it does not contain an organic solvent. Solvent-free compounding has become the main production method of Flexible Packaging Composites in Europe, America, and other developed countries. It is a composite method of coating substrate with solvent-free adhesive and directly bonding it with the second substrate. Although the adhesive is used as a dry composite, the adhesive does not contain organic solvent and does not need a drying device. Because of its superior environmental friendliness, the product performance can also be the same as dry compounding, which is the development direction in the future.

7 coextrusion composite

Coextrusion is a molding method for preparing composite film by melting and plasticizing two or more different plastics through two or more extruders and supplying them to a pair of die, or combining the plastics supplied by various extruders through a distributor and supplying them to the inlet die. The different plastics mentioned here can be different kinds of plastics, plastics of the same kind but different brands, or plastics of the same brand but different formulas.

The cost of coextrusion composite is low, which can be reduced by 20% – 30% compared with dry composite, and adhesive or anchor coating agent (AC agent) is not used in the coextrusion composite process, so it has good hygiene and no environmental pollution problem. However, due to the obvious limitations of materials, the composite film used for the coextrusion composite process is only limited to various thermoplastics. If it is necessary to produce composite materials containing aluminum foil, paper, etc., the coextrusion composite method cannot be used. In addition, it is not allowed to print between composite films. When the film needs to be printed, only patterns, words, etc. can be printed on the surface of the composite film.

To sum up, the above composite processing methods have their own advantages and disadvantages. From the development of China at this stage, dry composite has the widest application range and the largest proportion. At the same time, extrusion composite has developed rapidly, and its proportion is second only to dry composite. However, wet composite and Co-extrusion composite processes are greatly limited by materials and cannot become mainstream, Although solvent-free composite technology has become mature in developed countries, it is still in its infancy in China, which is the development direction of composite processing in the future.

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