Discussion on Structure and Process Technology of Color Printing Flexible Packaging Materials

The so-called composite packaging means that two or more layers of film material are laminated together in a certain way to become a new packaging material. It not only has the advantages of each layer of film, but also overcomes the shortcomings of a single material, can replace the traditional glass bottles and metal cans, mainly used for packaging beverages, instant noodles, mustard, fish, sausages, spices, candied fruit, biscuits, milk powder, Washing powder, medicines, pesticides, etc.
The author to milk powder bag as an example to analyze and combine with other products, the color printing soft packaging material structure and process technology points are described below for the reader reference.
First, the choice of substrate and structural design As we all know, in the 1980s China's milk powder packaging used BOPP20μ/LDPE50μ, its shelf life is only six months, in order to extend the fragrant, color retention, durability, taste protection period, prevent milk powder agglomeration, seeking high gas barrier High moisture-resistant materials have become an important issue for the development of milk powder packaging. Since vacuum evaporation of plastic film can greatly reduce the amount of water vapor transmission, it is 45g/m2.24h, which is about 100 times lower. The vacuum aluminum film also has the properties of gas barrier, heat insulation, shading, and anti-infrared, etc. The composite flexible packaging is able to maintain the color, fragrance, taste and nutrition of foods, coupled with its dazzling metallic luster, and is well received by consumers.
Since a single material has its specific advantages and disadvantages, it is not possible to have multiple functions at the same time. The high-resistance milk powder bag has been researched and tested using a three-layer film composite structure. The outer layer has a certain strength, transparency, low cost, easy to print BOPP film, inner layer has a good heat sealing performance, can withstand low temperature and the CPE membrane. The key to the milk powder bag lies in the selection of high-intermediate layer barrier materials. At the beginning, we chose the OPP/AL/PE structure. Due to the use of plastic film combined with aluminum foil, the water vapor transmission rate is greatly extended, and the shelf life is extended, together with its gorgeous metallic luster. It is very attractive to consumers. However, aluminum foil is fragile, brittle, easily corroded, and its water vapor transmission rate is inversely proportional to the thickness (see Table 1).
Table 1 Relationship between thickness of aluminum foil and water vapor transmission

In order to improve the barrier properties, the AL foil must have a certain thickness. With the increase in the thickness of the AL foil, the amount of aluminum metal needs to increase. At the same time, the production of aluminum foil requires a lot of power, and its cable accounts for more than 50% of the material cost. The vacuum aluminum-plated PET film has excellent barrier properties (see Table 2). The transmission of ultraviolet rays and visible light is close to zero, which extends the freshness, durability, and taste retention by more than 3 times. It also has a beautiful metallic luster in BOPP. After printing on the back side of the film, it is compounded with it. As the light passes through the surface of the plastic film, the amount of reflection is increased, which can produce a pleasing optical effect. It is particularly suitable for export product packaging and can increase the added value of the product. At the same time, aluminum consumption of aluminum film is only a few thousandths of aluminum foil, so as to achieve energy saving, save metal materials and reduce the weight of packaging bags, and facilitate the purpose of reducing costs.
Table 2 Comparison of Barrier Properties of Several Films

Second, the process According to the special requirements of high resistance milk powder bag (film), determine the following process route. (As shown below)
VM
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BOPP→printing→rewinding→dry compounding→curing→bag making→inspection→packaging storage ↑ ↑
Ink adhesive

High resistance milk powder bag production process flow chart

Third, inks, adhesives and other auxiliary materials selection (1) ink: printing ink with plastic compound, its requirements are: A, good composite fastness; B, easy to dry, less residual components; C, reproducibility; D ,fair price.
(2) Adhesive: Must have strong adhesion, good compatibility with ink, uniform coating, transparent color, and meet the requirements of food hygiene. To this end, we chose to spend Huangyan New Oriental food-grade two-component polyurethane adhesive PU-180.
(3) Toluene, xylene: used as ink solvent. Because coking xylene contains carcinogens, and often mixed with a mixture of benzene, toluene, ethylbenzene, etc., in coking production, there is sometimes an unpleasant coke without odor, which is not suitable for the production of food packaging as a solvent. Toluene. The same reason. Toluene is also preferred for petroleum toluene (produced by oil refineries) without coking toluene.
(4) Ethyl Acetate: As a two-component polyurethane adhesive, ethyl acetate for solvent, the purity must be ≥98%, the content of water and alcohol should be ≤0.2%, otherwise the curing agent isocyanate reacts with OH and destroys the adhesive. The active gene, thereby reducing the adhesive efficiency, affect the composite fastness.
Fourth, process technology points (a) printing 1. Plastic film and its printing performance: plastic film variety specifications, there are thick and thin, narrow and wide, a variety of substrate properties are also different. Therefore, the material tension changes during the printing process is not the same. Changes in the tension of the material have a direct effect on the overlay accuracy of the printed pattern. In order to accurately register the printed pattern, the printing operator must properly control the tension of the film. In addition, in order to improve the printing fastness, the surface of the film must be corona treated to make it more than 38dyn.
2. Ink and solvent: The printing ink must have less impurities, good reproducibility, easy drying, less residual components, and high composite strength. Milk powder bags generally use chlorinated polypropylene printing ink.
Flexible packaging ink solvents are usually toluene, xylene, butanone (methyl ethyl ketone), ethyl acetate, isopropyl alcohol, butanol and so on. Because of the solvent's solvency and volatility vary from resin to resin, there are regulatory requirements for printing on which film to print, what ink to use, and what solvent to use for dilution. The correct use of solvents is closely related to improving the quality of film printing.
The blending ratio of ink and solvent varies depending on temperature, printing, and ink.
3. Drying: In order to completely volatilize the solvent in the film of various colors of printing ink, the film needs to pass through the oven after printing one color. At this time, special attention should be paid to the process parameters such as air flow, wind speed, and drying temperature to prevent printing adhesion, unclear appearance, and odor of products. The general drying temperature is 40-80°C.
4. Affect the printing factor roll accuracy, tension control, stencil technology, film grade, degree of drying.
(B) Composite 1. Selection of polyurethane adhesive for dry composite (1) Dry food packaging: New Oriental PU-180
(2) For boiling: Anji 175 and Huangyan New Oriental PU-8850
(3) For transparent cooking bags: domestically produced plastic (4) aluminum foil cooking: imported cooking glue, such as the United States Morton Company 585K, Japan Toyo Ink Chemical Company AD-502, Takeda Pharmaceutical Company A-310, Germany Henkel Corporation UK3640 /6800
2. Dilution and Coating (1) The diluent solvent is ethyl acetate. Polyurethane adhesive preparation sequence is: gradually add the solvent to the main agent according to the amount and stir it well, then add the curing agent and mix evenly, and then use it after becoming uniform solution. The adhesive concentration is usually 20% to 5% (dry basis includes Solids), 3 # viscosity cup (Zahn Cup) 18 ~ 25s.
(2) Roller rollers and light rollers are generally used for the coating roller. The required coating amount depends on the packaging application. Insufficient amount of coating often causes quality accidents, such as poor appearance of the composite film and poor adhesion, delamination, heat sealing, boiling, cooking, etc., all kinds of resistance decline...... The best coating of the adhesive one, to the substrate structure , The size of the printing surface, the post processing conditions of the composite film, and the end use. The standard coating amount of dry composite adhesives is shown in Table 3.
Table 3 Standard coating weight of dry composite adhesive