Analysis of Milk Packaging Film Printing Process (I)

With the development of the economy and the improvement of people’s living standards, our country’s dairy industry has grown rapidly and has grown. People’s lifestyles and dietary structures have also undergone major changes. Milk has become a must-have item for ordinary people on the table. Plastic film flexible packaging of milk accounts for a considerable proportion, this packaging of milk is usually used automatic filling machine for filling, first to sterilize the milk and packaging film, and then aseptically filling the milk into the plastic bag . This article analyzes and discusses the printing process of this milk packaging film.

plastic film

Film material type

    The film material used for milk packaging film is mainly polyethylene (PE) co-extruded film, which can basically be divided into the following three according to the number of layers:

1 simple milk packaging film, generally a single film, it is a variety of polyethylene (PE) material is added to a certain proportion of white masterbatch, produced by blown film equipment. This milk packaging film has a non-barrier structure and is pasteurized (85°C/30min) for hot filling. The shelf life is short, only about 3 days.

2 Black and white co-extruded packaging film with three-layer structure. The black masterbatch added in the heat-sealing inner layer plays a role in blocking light. This packaging film uses Ultra High Temperature Sterilization (UHT) and hydrogen peroxide (H2O2) sterilization methods, and the shelf life at room temperature can reach about 30 days.

3 five-layer structure of black and white co-extruded packaging film, blown film increases the middle barrier layer (by EVA, eval and other high barrier resin composition), therefore, this packaging film is a high barrier aseptic packaging film, a long The shelf life can be stored at room temperature for about 90 days.

2. Film performance requirements

It should be particularly pointed out that in order to meet the filling requirements for high-speed automatic filling machines, polyethylene film for milk packaging must meet the following performance requirements:

1 good smoothness

The inner and outer surfaces of the film should have good smoothness, so that it can be smoothly filled on the high-speed automatic filling machine. Therefore, the dynamic and static friction coefficient of the film surface should be relatively low, and the general requirement is between 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, therefore, if the lubricant content is too high, it will affect the printability of the film, which is extremely unfavorable for printing.

2 strong 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 LDPE or HDPE particles with a low melt index (MI) in the blown film process is advantageous for increasing the tensile strength of the polyethylene film.

3 surface wetting tension value to meet the requirements

In order to enable 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, otherwise it will affect the adhesion and firmness of the ink on the surface, thereby affecting the quality of the printed matter. It is generally required that the surface tension of the polyethylene film is preferably 38 dynes or more, more preferably 40 dynes or more. Since polyethylene is a typical non-polar high molecular material, its molecular structure does not contain polar groups, and its crystallinity is high, its surface free energy is low, its inertness is strong, its crystallinity is high, and its chemical properties are stable. Therefore, the film material The printability is poor and 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 excellent heat sealing performance

The film should have good heat-sealing and bag-making properties and have good sealing properties. That is, it is necessary to ensure heat-sealing and it can be smoothly cut so that the molten resin will not stick to the knife. Adding a certain proportion of LLDPE in the blown film process can obviously improve the low temperature heat sealing performance of the milk film and the heat sealing performance of the inclusions, but the amount of DPE added can not be too large, otherwise the viscosity of the polyethylene milk film is too high and the heat is too high. Sealing knife knife failure occurs easily.

Printing ink

1. Requirements for ink performance

    Milk packaging film is usually printed by surface printing, except for the general technical requirements (such as hue, fineness, viscosity, drying, etc.) of surface printing ink. Since ultra-high temperature sterilization (UHT) and hydrogen peroxide (H2O2) sterilization are performed during the automatic filling process, special sterilization conditions impose special requirements on the performance of the ink. This is also worth our attention. One of the matters that needs special emphasis is the following aspects of performance:

1 pair of polyethylene film has strong adhesion

Because the surface of the plastic film is dense and smooth, the ink cannot penetrate on the surface, so the adhesive force between the ink and the plastic film must be very strong. This is a key issue for printing inks for plastic films, that is, for printing milk packaging films. The ink should have high affinity and adhesion to the substrate material - polyethylene film, strong adhesion, and good print fastness, this is to ensure high quality prints and good printing results.

2 strong wear resistance and scratch resistance

Since the printing ink is printed on the outer surface of the film, it will be subject to friction during the circulation of the goods. Therefore, the printing ink used must have strong abrasion resistance and scratch resistance, and the surface of the ink film after printing is required. High strength, good firmness, and resistance to rubbing, otherwise the ink layer may fall off, resulting in incomplete print patterns, thus affecting the appearance and quality of the product.

3 excellent oxidation resistance

Most of the high-speed filling machines are equipped with a hot hydrogen peroxide (H2O2) sterilization system to sterilize the packaging film. Hydrogen peroxide (H2O2) has strong oxidative properties, and its oxidative properties are also significant with increasing temperature. Improve, this puts forward the higher requirement to the anti-oxidation ability of the printing ink. If the oxidation resistance of the pigment in the ink is not very good, ink fading occurs and the quality of the product is affected.

4 excellent water resistance

The surface of milk packaging film exposed to ultra-high temperature instantaneous sterilization (140°C/1 to 3 seconds) and hydrogen peroxide (H2O2) sterilization is exposed to water or water vapor. Therefore, the printing ink used must have excellent water resistance. , water will not fade.

5 heat resistance and freezing resistance

The printing ink used in the milk packaging film must also have a certain degree of heat resistance, so as to ensure that after the ultra-high temperature sterilization treatment, ink softening, stickiness, and imprinting, discoloration and the like of the ink will not occur. In addition, the frozen milk is generally required to be refrigerated at low temperatures during storage, circulation, and sales. Therefore, it is required that the printing ink used must also have good freezing resistance, and the ink layer will not occur under low temperature conditions. Freezing and shedding.

6 good health performance

For milk packaging film, it should also meet the hygienic standards for packaging materials and food packaging regulations. Therefore, the ink used for printing should have non-toxic, odorless, low residual solvents, and no toxic heavy metals. So as not to contaminate milk, and harm consumers' health.

At present, the Chaoli 313A surface printing ink and the Chaolifu 314 surface printing ink produced by Zhejiang New Oriental Ink Co., Ltd. are widely used in the field of milk packaging film printing. These two inks are designed for milk packaging film. Special printing inks, they have good affinity with polyethylene film, good gloss, fast adhesion, excellent heat resistance, grease resistance, water resistance, etc., can be said that milk packaging film printing Ideal for inks. Chaolifu 31 3A ink is an alcohol-soluble polyamide ink that can withstand boiling below 100°C and can withstand hydrogen peroxide sterilization below 60°C. Chaolifu 314 surface printing ink is a two-liquid reaction type polyurethane modified ink, which can withstand 121 °C / 40 min retort sterilization, can withstand 80 °C, a concentration of 35% hydrogen peroxide sterilization. For example, our factory uses the Chaolifu 314 two-liquid reactive ink to print milk film. Its oxidation resistance test and results are as follows: The printing film will be cured after 24 hours at a temperature of 45°C-50°C. 35% of the concentration of hydrogen peroxide soaked for about 15 minutes, after removal of the test, the ink adhesion is good, basically no discoloration phenomenon, fully able to meet the requirements of milk packaging.