In recent years, plastic film flexible packaging of fresh milk and many fruit juice drinks and condiments have appeared in the market. As milk and beverages are fresh drinks, the requirements for hygiene and temperature are very strict. Therefore, there are special requirements for the printing of packaging bags. This makes printing of milk and beverage packaging film different from other printing. Technical characteristics.
First, the characteristics of milk and beverage packaging film
The film material used in milk and beverage packaging films is mainly polyethylene (PE) co-extruded film, which must meet the various requirements for packaging printing, processing, storage and transportation and hygiene. From a hygiene point of view, in the process of film blowing and printing, cutting, storage, transportation, etc., it is necessary to prevent the film from being contaminated, and the workshop environment must meet the requirements for food and medicine packaging. In order to prevent the film from being contaminated, a set of ultraviolet light tubes is generally installed at the printing receiving place and the slitting and unwinding place, so that the film can be instantaneously disinfected in the production process to ensure its safety and reliability. In addition, in order to meet the filling requirements of high-speed automatic filling machines, the properties of polyethylene film mainly have the following requirements: (1) Slip properties. The inner and outer surfaces of the film should have good smoothness to ensure smooth filling on the high-speed automatic filling machine. Therefore, the static and dynamic friction coefficient of the film surface is relatively low, and the general requirement is 0.2-0.4. The smoothness of the film is mainly achieved by the addition of a slip agent. After the film is formed, the slip agent migrates from the film to the surface and accumulates into a uniform thin layer, which can significantly reduce the coefficient of friction of the film and make the film have Good smoothness effect. Slip agent is usually oleic acid amide or erucic acid amide, which is just a kind of mechanical mixing with macromolecule polyethylene. It is not well compatible. The molecular heat movement makes it gradually migrate to the interface of low free energy to form a layer. The weak interface layer produces and enhances the smoothness effect. Pay attention to the addition amount of the slip agent, adding too little to achieve the effect; adding too much will affect the printability of the film, reduce the adhesion of the ink, etc., which is extremely unfavorable for printing. (2) Tensile strength. Since the plastic film is subjected to the mechanical pulling force from the automatic filling machine during the filling process, it is required that the film must have sufficient tensile strength to prevent the automatic filling machine from being pulled off under the action of the pulling force. The use of low melt index LDPE or HDPE particles in the blown film process is very advantageous for increasing the tensile strength of the polyethylene film. (3) Surface wetting tension. In order for the printing ink to spread, wet, and adhere smoothly on the surface of the polyethylene plastic film, the surface tension of the film should be up to a certain standard, relying on corona treatment to achieve a higher wetting tension, otherwise it will affect the ink in its Adhesion and firmness of the surface affect the quality of the print. It is generally required that the surface tension of the polyethylene film should be more than 38 dyne, more preferably 40 dyne or more. As polyethylene is a typical non-polar polymer material, its molecular structure does not contain polar groups, and its crystallinity is high, its surface free energy is low, its inertness is relatively strong, and its chemical properties are stable. Therefore, the printability of thin film materials is relatively high. Poor, the adhesion to the ink is not ideal. In addition, the additives (slip agents, plasticizers, etc.) in the film constantly migrate to the surface to form a weak surface layer, which also affects the printing fastness of the printing ink. Therefore, the plastic film is generally required before printing. Surface treatment, change the chemical structure of the film surface, improve the surface tension of the film, so as to achieve the purpose of improving and improving the printability of polyethylene film, improve the adhesion of the ink to ensure the smooth progress of production. (4) Heat sealability. The most worried about the automatic packaging of the film is the leakage sealing and the virtual cover leading to the problem of bag breakage. Therefore, the film should have good heat-sealed bag-making properties, have good sealing properties, and have a wider heat-sealing range in order to speed the package. When changing, the heat sealing effect is not affected too much, and MLLDPE is often used as the heat seal layer, in order to fully ensure the stability and heat sealability of the heat sealing conditions. In other words, it is necessary to ensure that the heat seal, but also can be successfully cut off, so that the molten resin will not stick knife. Adding a certain proportion of LLDPE in the blown film process can obviously improve the low temperature heat sealing performance of the film and the heat sealing performance of inclusions, but the addition amount of LLDPE can not be too large, otherwise the viscosity of the polyethylene film will be too high, and the heat sealing will be Sticking knife failure occurs easily. For the structural design of the film, according to the different contents of the package and its shelf life, the corresponding structure of the packaging film can be selected.
At present, generally used milk and beverage plastic packaging films can be basically divided into three types according to their layer structure. (1) Simple packaging film. It is generally a single-layer film, which is made from a variety of polyethylene (PE) materials by adding a certain proportion of white masterbatch and produced by blown film equipment. This packaging film has a non-barrier structure and is pasteurized (85 degrees Celsius/30 minutes) for hot filling. The shelf life is short (about 3 days). (2) Three-layer black and white co-extruded packaging film. Using LDPE, LLDPE, EVOH, MLLDPE resin, and high-performance composite membranes that are co-extruded with black and white masterbatches. The black masterbatch added in the heat-sealing inner layer serves to block light. This packaging film uses ultra-high temperature instantaneous sterilization and hydrogen peroxide sterilization method, the shelf life can reach about 30 days at room temperature. (3) A five-layer black-and-white co-extruded packaging film. When the film is blown, an intermediate barrier layer (composed of high barrier resin such as EVA and eval) is added. Therefore, this packaging film is a high barrier aseptic packaging film with a long shelf life and can be stored at room temperature for about 90 days. The three-layer and multi-layer black-and-white co-extruded packaging film has excellent heat-sealing properties, light-proof and oxygen-blocking properties, and has the advantages of low price, convenient transportation, small storage space, and strong practicality.
(to be continued)