Discussion on the application and printing technology of liquid packaging film
in China's current flexible packaging market, the market share of liquid packaging is constantly expanding. The range of liquid packaging is very wide. The liquid packaging mentioned in this paper is only limited to the flexible plastic film packaging of milk, soymilk, wine vinegar and other contents. The plastic film used is mainly PE coextrusion film, which is printed on the surface of gravure printing machine for food manufacturers to use on automatic filling machines.
in the market of plastic gravure surface printing inks, polyamide or modified ones are mostly used as connecting materials. This kind of ink is the earliest developed gravure ink in China, and the production process is relatively mature and perfect. This type of ink is suitable for polyethylene and polypropylene films. The ink layer has good adhesion fastness and good gloss; However, polyamide inks generally have performance defects such as poor water resistance, grease resistance and freezing resistance. Printing this type of ink in high temperature summer is prone to sticky and softening, and freezing will occur when printing in low temperature winter. In the post-processing process of filling, circulation and storage of liquid bags, high-temperature sterilization or freezing processes are required. Therefore, the inherent defects of polyamide surface printing inks cannot meet the requirements of liquid packaging. Color printing manufacturers producing liquid bags must select non polyamide surface printing inks for different types of coextrusion PE films. At present, polyester and modified products are the most suitable inks for liquid bags in the market, There are single liquid and double liquid reactive inks.
in the market, coextrusion PE films for liquid packaging can be divided into three categories in terms of their apparent properties: soft milky PE films, black-and-white coextrusion PE films, and hard high-temperature cooking PE films. Soft milky PE film is mainly used for simple liquid packaging such as milk, soy sauce and vinegar. The coextrusion blowing masterbatch is composed of LDPE, LLDPE and milky white material. The barrier performance of the membrane is relatively poor, the sterilization temperature is not more than 90 ℃, and the shelf life is relatively short; It is generally used in manual or low-speed filling equipment. For example, low-grade milk packaging, generally with a shelf life of up to three days, must be sold as soon as possible.
black and white films are mostly three layers, with a maximum of five layers. During film blowing, the increased intermediate layer is formed by CO extrusion of barrier masterbatch. The black masterbatch in the heat sealing inner layer plays the role of blocking light and isolating ultraviolet rays. Due to the existence of the intermediate barrier layer, which is composed of EVA, Eval and other masterbatches, the shelf life of the contents is long and can be maintained for more than 30 days. However, there are some three-layer coextruded black-and-white films on the market. The intermediate layer is not composed of barrier masterbatches, so such black-and-white films also belong to the first category. Among the three types of films, black-and-white films are the most unfavorable for printing. The black-and-white film contains the largest amount of smoothing agent and brightener when it is formed. In order to meet the requirements of low friction coefficient of black-and-white film on high-speed automatic filling machine, and the added oily substances are extremely unfavorable to the color printing process, which greatly reduces the adhesion of the ink layer. In most high-speed automatic filling machines, the sterilization system of hot hydrogen peroxide is directly installed for the sterilization of packaging film. In this case, we should pay attention to the impact of sterilization conditions on the ink layer. The antioxidant performance of some pigments in the ink is not very ideal. The test shows that the oxidizability of hydrogen peroxide increases significantly with the increase of temperature. Therefore, when providing packaging materials, color printing manufacturers must be clear about the customers and ensure that the comprehensive discharge of wastewater, waste gas and pollutants meets the standard rate of 100%.
hard high-temperature cooking PE film is mainly used for yogurt, soymilk and other dairy drinks. When the contents are sterilized after filling, ultra-high temperature sterilization process is adopted. The stem coextrusion PE film takes HDPE masterbatch as the main body, and some LDPE and color masterbatch are added. Therefore, the hard PE film has good high temperature resistance and can meet the requirements of ultra-high temperature sterilization. Due to the pressure difference during sterilization, it is easy to produce the phenomenon of "bulging bag", so it is required that the hard PE film not only has good heat sealing strength, but also has good tensile strength performance. Color printing manufacturers generally use "three meters high" or "one foot on" to test the effect.
as mentioned above, polyamide ink cannot be used as the ink for liquid packaging, and polyester ink is preferred, which will be explained in detail below. Generally speaking, all coextruded PE films produced by color printing plants and in the middle are mostly produced by themselves. Due to the different proportion of masterbatches, the performance of the film has certain differences. Before printing, we must consider various printing adaptability of the film, such as the composition of masterbatches, the surface tension of the film, the plasticizer content of the base material, etc. Another important consideration is the post-processing conditions of liquid content filling. Zhejiang New Oriental ink company has two kinds of special inks for liquid bags to meet the requirements of customers at different levels in the market. Super Lifu 313A ink is specially used for soft coextrusion PE films, such as the first two film types mentioned above. The ink is alcohol soluble single liquid ink, which has good sanitation, storage and printing functions, and the content of heavy metals meets the requirements of food packaging regulations in Europe and the United States. There are two kinds of sterilization of soft coextrusion PE film: atmospheric boiling and peroxide sterilization. Boiling is carried out after the contents are packaged, while hydrogen peroxide sterilization is carried out after the packaging film is printed and passed through the automatic filling machine. Both require printing inks to have excellent water resistance and high temperature resistance, while peroxide has higher requirements for the antioxidant capacity of inks. 313A ink has good water resistance, and can withstand the sterilization of hydrogen peroxide with a concentration of 30% below 40 ℃. At the same time, 313A ink can withstand boiling. Super Lifu 314 surface printing ink is a kind of polyurethane modified ink with double liquid reaction, which is specially used for surface printing of various coextrusion PE liquid films. When used, 8% of the corresponding hardener is added, which has very excellent high temperature performance and water resistance. It can withstand 121 ℃/40min pressurized cooking. This kind of ink uses ketones and esters as the main solvents, and toluene and other solvents can be added to adjust the volatilization rate of the solvent. After printing, the ink is placed in the curing room for 24 ~ 48h, and the performance is better. With the continuous renewal and development of liquid packaging machinery and equipment, the sterilization conditions of hydrogen peroxide continue to improve, mainly manifested in the increase of sterilization temperature, ranging from 65 ℃ to 110 ℃; The concentration of hydrogen peroxide also increased. The 314 ink of New Oriental ink company can withstand the sterilization conditions of 80 ℃ and 35% hydrogen peroxide. When the sterilization temperature is higher, it can be matched with the 313 high temperature varnish of New Oriental ink company to meet the use requirements.
next, we will discuss the points for attention in the process of liquid packaging printing for the reference of color printing manufacturers.
first, the performance of the selected raw materials.
polyamide surface printing ink is generally not suitable for liquid package ink due to its poor water resistance and grease resistance; Polyurethane inks also have different types, which should be targeted when selecting. The type 313A inks produced by our company are generally only used for soft coextrusion PE films, while the type 314 is suitable for all kinds of coextrusion PE films. The coextrusion PE film used by color printing manufacturers is generally blown by the manufacturers themselves, and the technical level is different. Therefore, we must grasp the selection of masterbatch, and it is advisable to use masterbatch without smooth agent model; After the selected PE film is treated by EDM, the surface energy should be at least 40dpn, preferably more than 42dpn
Second, the customer's post-processing conditions or test standards.
only when color printing manufacturers fully understand the conditions of food manufacturers at the time of filling, as well as the subsequent processing technology, and even the detailed requirements of the circulation of liquid packages in the market, can they be targeted and have a clear idea. The main points for attention include: filling speed, sterilization conditions, shelf life, storage requirements, etc.
Third, printing technology.
on the basis of grasping the first two points, it will not be a problem for the color printing manufacturers to increase the central government budget of 3.99 billion yuan for liquid packaging. The comprehensive quality of printing workers is the key in production, and the technical information provided by ink manufacturers is the basis of production. In addition, the quality of thinner and the change of workshop environment are also factors that cannot be ignored. Fourthly, formulate a set of appropriate factory inspection standards for finished products.
in the production process of liquid bag, the film is changed more frequently, which is easy to cause different performance of the film. The amount of ink used in liquid bag is not much, and the long-term use of old ink printing may also lead to the decline of ink performance. Therefore, there are many variable factors in the production of liquid bag. Color printing enterprises can formulate a set of appropriate finished product testing standards by simulating the use conditions of customers, so as to ensure that unqualified products do not leave the factory and avoid unnecessary losses.
at present, the competition in China's flexible packaging market is very fierce. The direction of our color printing enterprises is to avoid popular orders, continue to innovate, and take a high-tech path. Liquid bag production is currently in a period of rapid growth. How to occupy the commanding height first is very important. Zhejiang New Oriental ink company hopes to create a better tomorrow with color printing enterprises interested in liquid bag production
(Zhejiang New Oriental ink group company/Yu Yongyue)