Overseas, food soft packaging generally develops towards multi-layer co-extrusion packaging materials. Pay attention to the packaging effect of food, that is, good barrier, longer shelf life, easy to use, this tendency deserves our attention in the packaging industry. The development direction of flexible packaging film in our country.
1. Develop flexible packaging film of environmentally friendly materials. At present, the world recognized PET as an environmentally friendly material, it can be recycled and reused.
2. The development of soft packaging materials to thin type. Reducing cost and saving resources is a new subject of soft packaging industry in the new century.
3. The development of flexible plastic packaging film towards special functional direction. High barrier composite materials will continue to increase market capacity. High barrier film, which has the advantages of simple processing, strong resistance to oxygen and water vapor, and longer shelf life, will be the mainstream of supermarket food soft packaging in the future.
1. Printing Suitability of Common Food Soft Packaging Materials
PE film ink has poor adhesion, which needs to be improved by electric presenting treatment. The wetting tension is often used to evaluate the quality of corona treatment. LDPE and LLDPE films are 38-420mN/m and HDPE films are 40-44mN/m. Polyamide ink is commonly used for printing. LDPE thin films have low expansion strength, high elongation and difficulty in color alignment. Therefore, the tension control in printing should be small and the tension should be constant everywhere. If multi-color overprinting, in order to ensure the accuracy of overprinting, it is best to use large roller printing machine. LDPE film softens when heated a little. It is easy to stretch and deform. Therefore, when printing and drying, the film surface temperature should not exceed 55 C. In order to prevent ink adhesion, the film temperature should be cooled below 35 C during winding.
Like PE, PP film has poor adhesion to ink, and corona treatment should be done before printing. Its wetting tension is better at 10114 mN/m. Commonly used polyamide ink, chlorinated polypropylene ink (composite) printing. When PP film is printed with polyamide ink, the adhesion of ink is low. After about one day of aging, it shows good adhesion. If polyurethane adhesive composite film is used at the same time, the peeling force will be small.
PET film is not easy to stretch, and tension control requires PE film to be low. It is usually printed by lithographic process and compounded with CPP as cooking bag. PET film has good insulation. It is easy to generate static electricity and not easy to disappear in the printing process. In severe cases, the film will be sucked on the roller and can not be printed. Therefore, electrostatic eliminating devices are needed in high-speed printing. The drying temperature of PET film can be controlled to a higher level, but its surface temperature should not exceed 85C. When winding, the film should be cooled to room temperature or slightly higher than room temperature.
PT film has good operability and ink adhesion, no static electricity, and is not easy to absorb dust. PT has good hygroscopicity. After hygroscopicity, its elongation increases, wrinkles or relaxation occur, which affects the quality of printing pictures and texts. In addition, the adhesion of ink will decrease. Therefore, great attention should be paid to the environmental temperature of storage and printing. Its tear resistance is poor. If there is a crack in the edge, it is very easy to tear along the crack. It is necessary to prevent the edge from splitting and printing. It shrinks when it dries, and its strength decreases or becomes brittle. Therefore, the drying temperature should not be too high. In addition, attention should be paid to tension control to prevent tension deformation and wrinkles.
2.Printing and Compound of Food Packaging Materials
Gravure and flexographic printing are the main methods of food soft packaging printing, but there are many differences compared with gravure and flexographic printing used in publishing and commodity printing. For example, soft package printing is carried out on the surface of the roll-like impression. If it is a transparent film, the pattern can be seen from the back. Sometimes it is necessary to add a layer of white paint or use lithography technology.
Lithography refers to a special printing method that uses the plate of the image-reversed text to transfer the ink to the inside of the transparent printing material so as to show the positive image of the text on the front of the printed material. Lithographic printing is brighter and more beautiful than surface printing, with bright colors, non-fading, moisture-proof and wear-resistant. Moreover, after lithographic printing, the ink layer is sandwiched between two layers of film, so it will not pollute the packaging。
The printing color sequence of lithography process is opposite to that of ordinary surface printing. For example, the lithography is the final lithographic background for ordinary printing. Lithographic printing is a unique process of plastic composite packaging printing. It has tended to be diversified, multi-functional and serialized in printing equipment. For example, the imported intaglio printing production lines for plastic packaging have linked blowing, printing, compounding, cutting, bag making and other processes.
Secondly, flexographic printing press is used to print plastic film, and most flexographic printing press and dry composite machine form production line.
Soft packaging materials generally do not use a single material, but mainly composite film. The main compounding methods are as follows:
Wet composite method. A layer of water-soluble binder was coated on the surface of the base material (plastic film and aluminium foil). It was compounded with other materials (paper and cellophane) by roller pressing and then dried by hot drying path to form a composite film. This method is suitable for packaging dry food.
Dry composite method. Solvent-based adhesives were evenly coated on the substrate, then fed into the hot drying channel, so that the solvent was fully volatilized, and immediately compounded with another film. For example, stretched polypropylene film (OPP), after lithographic printing, dry composite process is usually used to compound with other materials. Typical structures are: biaxially stretched polypropylene film (BOPP, 12 micron), aluminum foil (AIU, 9 micron) and unidirectionally stretched polypropylene film (CPP, 70 micron). The process is that solvent-based "dry adhesive powder" is evenly coated on the base material by roll coating device, then fed into the hot drying channel, so that the solvent is fully volatilized and then compounded with another layer of film by composite roll.
Extrusion compound method. The curtain melt polyethylene extruded from the slit of T casting die is pressed by clamping roll, salivated onto paper or film, coated with polyethylene, or supplied with other film from the second feeding paper part, and bonded with polyethylene as bonding layer.
Hot-melt composite method. Polyethylene-acrylate copolymer, vinyl acid-ethylene copolymer and paraffin were heated and melted together, then coated on the base material, immediately compounded with other composite materials and cooled.
Multilayer extrusion compound method. Plastic resins with different properties were extruded into the dies through multiple extruders to form films. This process does not need binder and organic solvent between layers. The film has no peculiar smell and harmful solvent penetration. It is suitable for food packaging with longer shelf life. For example, LLDPE/PP/LLDPE with general structure has good transparency, and its thickness is generally 50-60 um. If the shelf life is longer, more than five layers of high barrier co-extruded films are needed, and the middle layers are high barrier materials PA, PET and EVOH.