In recent years, fresh milk in plastic film flexible packaging has appeared in the market dazzlingly. This packaging form first sterilizes milk and packaging film, and then uses an automatic filling machine to fill milk into plastic packaging bags. Since milk is a fresh drink, the requirements on hygiene, bacteria and temperature are very strict. Therefore, there are special requirements for the printing of packaging bags, which makes the printing of milk packaging films different from other printing technical characteristics.
The selection of flexible packaging milk plastic film The selection of milk packaging film must meet the requirements of packaging and printing, processing, storage and transportation and hygiene. At present, the commonly used film material is mainly polyethylene (PE) co-extruded film, which is melt extruded polyethylene resin and blow molded.
The performance of polyethylene film mainly depends on the resin density, melt index and molding method. Polyethylene is insoluble in many solvents, but when the temperature exceeds 70 ℃, polyethylene can be dissolved in toluene, amyl acetate and other solvents in a small amount. This property provides conditions for surface finishing. The plastic can be soaked in heated solvent to swell the surface and destroy some crystals to improve the adhesion of the ink film on the plastic surface. In addition, the mixture of fuming sulfuric acid, concentrated nitric acid, chromic acid and sulfuric acid can slowly act on polyethylene at room temperature: sulfuric acid and nitric acid can quickly destroy polyethylene at 90-100 ° C; use potassium dichromate, sulfuric acid, The mixed liquid of distilled water is immersed in polyethylene plastic at 70-75 ° C, which is beneficial to enhance the adhesion of the ink film on the plastic surface.
The films currently used for milk packaging can be basically divided into three types according to their layer structure.
①Simple packaging film. It is generally a single-layer film, which is produced by adding a certain proportion of white masterbatch to a variety of polyethylene materials through film blowing equipment. This packaging film is a non-blocking structure, which is pasteurized (85 ℃ / 30min) for hot filling and has a short shelf life (about 3 days).
â‘¡ Black and white co-extruded packaging film with three-layer structure. Adopt LDPE, LLDPE, EVOH, MLLDPE and other resins, combined with black and white masterbatch co-extrusion blown high-performance composite film. The black masterbatch added in the inner layer of the heat seal plays a role in blocking light. This packaging film adopts ultra-high temperature instant sterilization and hydrogen peroxide sterilization, and the shelf life at normal temperature can reach about 30 days.
â‘¢ Five-layer structure of black and white co-extruded packaging film. The middle barrier layer (composed of high barrier resins such as EVA and EVAL) is added during film blowing. Therefore, this packaging film is a high barrier sterile packaging film with a long shelf life and can be stored for about 90 days at room temperature. Three-layer and multi-layer black-and-white co-extruded packaging films have excellent heat-sealing performance and light and oxygen barrier properties. They also have the advantages of low price, convenient transportation, small storage space, and strong practicality.
In order to meet the requirements of milk filling and printing, the polyethylene film has the following requirements.
â‘ Smoothness. The inner and outer surfaces of the film should have good smoothness to ensure that it can be smoothly filled on the high-speed automatic filling machine. Therefore, the coefficient of dynamic and static friction on the film surface is relatively low, generally 0.2 to 0.4. Slippage is mainly achieved by adding 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 friction coefficient of the film and make the film have a good cooling effect. Slip effect.
The slip agent is usually oleic acid amide or erucic acid amide. It is only a mechanical mixture with high molecular polyethylene and is not well compatible. The thermal motion of the molecule gradually migrates to a low free energy interface to form a layer The weak interface layer produces and improves the slippery effect. Due to the characteristics of the slip agent, pay attention to the amount of slip agent added. If the amount of slip agent is added too little, the slip effect will not be achieved, but if added too much, it will affect the printability of the film and reduce The fastness of ink adhesion, etc., is extremely detrimental to printing.
â‘¡ Tensile strength. Since the plastic film is subjected to mechanical pulling force from the automatic filling machine during the filling process, the film must have sufficient tensile strength to prevent it from being broken under the pulling force of the automatic filling machine. The use of LDPE or HDPE particles with a low melt index during the film blowing process is very beneficial for improving the tensile strength of the polyethylene film.
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