Polyvinyl alcohol composite film coating technology

Polyvinyl alcohol (PVA) is a polyhydroxy polymer. Due to its molecular structure and high compactness, it is one of the few high-negative septa polymers with excellent properties. In Japan, PVA film has long been produced by extrusion blow molding. In recent years, China has also produced a small amount of PVA cast film. The film exhibits unique properties such as good transparency, anti-static property, tear resistance, moisture permeability, good printability, and excellent hernia. Under dry conditions, the oxygen barrier of PVA membrane is even better than that of EVOH and PVDC, but its chimney and mechanical strength are greatly influenced by the ambient humidity. In addition, the softening point is high and the heat sealing is poor, so it is inconvenient to use as a barrier alone. Packaging materials, generally selected other common materials such as PP film, PE film or PP / PE film and PVA composite, can overcome the shortcomings, comprehensive use of their respective advantages, combined into a very low oxygen permeability packaging materials. First, the advantages of coating process] PVA and PP or PE composite film, there are two process routes to choose from: (1) PVA film with adhesives and PP, PE and other films composite; (2), with PVA water solution Coatings are applied to base films such as PP or PE. The composite film is made of PVA film and PP|PE film. The front film must be made of PVA film and developed adhesive for compounding. Due to the strong water absorption of PVA, the PVA layer of two-layer composite film is easy to be stratified by water, so it needs to be The production of PP/PVA/PE three-layer structure can meet the requirements of use, which requires the two-layer adhesive to be made by two composites and the process is more complicated; in addition to the difficulty of manufacturing the PVA film and the complexity of the equipment process technology, the overall cost is high. When the PVA aqueous solution is applied to the base film, the PVA film does not need to be separately manufactured. After the base film is treated with a specific surface and a suitable coating compound formulation is selected, the PVA compound can be directly coated on the base film. The process of coating adhesive and the like, the original drying of the PVA solution and the compounding with the base film are completed at the same time, and the PVA compound is modified in advance to improve the water resistance of the coating, and only the PVA/PP (PE) two-layer structure needs to be produced. Can meet the use requirements. This process route not only has a simple process, but also has a much lower overall cost than a method using a PVA film. Second, the principle of composite coating process and process measures The coating method is to dissolve PVA in water and mix it with a suitable glue, apply it to a base film with certain surface properties, and heat and dry to make PVA compound. membrane. In order to obtain better overall performance of the PVA composite membrane, it is necessary to solve the two basic ones is always the water resistance of the PVA coating, in order to reduce the adverse effects of the environment, to maintain high Anoiki; Second, to improve the infiltration of PVA glue The surface properties of the base film and the base film enhance the interfacial adhesion so that it does not peel during use. The bonding of the interface effect is a key link in the coating process. According to the adhesion theory, the effective adhesion depends on the effective infiltration of the base film by the glue, and the infiltration of the base film by the glue liquid is related to the nature of the glue liquid, the surface condition of the base film, and the operating conditions of the coating film. According to the infiltration thermodynamic liquid to be automatically spread on the solid surface, must meet certain thermodynamic conditions, that is, the surface tension of the solid should be greater than or equal to the sum of the liquid surface tension and the liquid/solid interfacial tension. The surface tension of PP film and PE film is less than the surface tension of PVA; in addition, PVA is a polymer with strong polarity, and PP, PE are non-polar polymers, and the interfacial tension is large. PVA glue is untreated. The PP and PE base films are difficult to spread. In order to obtain the thermodynamic conditions for easy spreading of the PVA glue on the base film, corresponding measures have been taken in the process: First, the PVA glue is modified. Before the modification, the content of hydroxyl groups in the main chain of PVA molecules is relatively high. The aqueous solution of PVA molecules is prone to form hydrogen bonds and has a high cohesive energy, so that the surface tension is large, and the solution tends to form a sol and has poor fluidity at a low temperature. Through chemical modification, the hydroxyl content of PVA molecules is reduced, thereby reducing the cohesive energy to reduce the surface tension, the flow stability of the glue liquid is greatly improved, and the modification also weakens the polarity of the PVA molecules, which is favorable for PVA coating. The attraction and adhesion of the layer to the base film interface. Another major effect of the modification is that the coating dehydrates and dries to form an insoluble cured film, significantly improving the water resistance of the PVA coating. Second, moderate corona treatment of the base film. To make the glue well infiltrate the base film, in addition to improving the properties of the PVA glue, the base film should be corona treated to improve the surface properties of the base film. Due to the high pressure corona, the air on the surface of the film is highly ionized and discharged to generate ozone and nitrogen oxides, causing thermal degradation and oxidation of the surface of the base film, changing the chemical composition of the surface, increasing the surface tension of the base film, and simultaneously The introduction of polar groups increases the interfacial attraction, thereby improving the interface's wetting and adhesion. The degree of improvement in the surface properties of the base film is related to the voltage value and processing speed used in the corona treatment process. Within a certain range, the higher the voltage, the lower the processing speed (or the longer the processing time), the better the surface performance of the base film. When the corona treatment is to avoid the softening and deformation of the base film at the accumulated high temperature, the base film should have a certain reasonable thickness. The third is to use some process conditions that are conducive to infiltration and adhesion. (1) Apply the film immediately after the corona treatment to avoid degradation of the base film and affect the Chinese character of the glue; (2) Since the surface tension decreases with increasing temperature, the coating operation should be carried out at room temperature and at high temperature and dry time. Far-infrared heating is used to reduce the direct effect of high temperature on the surface properties of the base film; (3) to control the appropriate coating speed and so on. Third, the main process parameters The maximum coating speed: 8 meters / minute drying temperature 100 °C ~ 110 °C Drying time: 1 to 1.5 minutes can control the coating thickness: 15 microns below four, composite membrane performance and application effects 1, wrinkle performance The PVA/PP composite film was vigorously rubbed in hands for more than 20 times without deep embrittlement and peeling, and a strong bond was formed between the surface coating and the base film. 2, mechanical strength determined by JPVA / PP composite film crack strength and elongation at break than the PP film has improved significantly. 3. Sanitary determination was determined by the health department and complied with the national health standards. 4. Breathability Under the same conditions, the oxygen permeability of PP film and PVA/PP composite film with different PVA layer thickness was measured. The 5 μm thick PVA layer can greatly reduce the oxygen transmission of the composite membrane. The oxygen transmission rate of the composite membrane is approximately one thousandth of that of the PP membrane. It meets the requirements of oxygen-removal and fresh-keeping packaging, taking into account the abrasion damage and coating during use. The inhomogeneity of cloth and other factors, PVA coating thickness can not be less than 5 microns, with 6-10 microns more reasonable. 5, Bao Xiang performance test drinks and food contain fragrance, but also can prevent the display of the shelves or other places affected by the outside world. 6. Application effects Green tea contains a variety of nutrients and flavor components. These ingredients are sensitive to environmental factors such as temperature and humidity, oxygen and light, and can easily cause changes in quality. Therefore, packaging materials are required to have oxygen barrier and shading effects. It showed that fresh green tea was packaged in PVA/PP composite film and stored in the indoor environment for seven months. Even though transparent packaging was used, the green tea's aroma and color could be well maintained. The preservation of milk powder also needs to prevent the oxidation of lipids, so it is necessary to separate oxygen and avoid light irradiation, and to prevent moisture and bacteria. Pack milk powder with PVA/PP composite film and put it in a cool place indoors. It can be kept for six months and still maintain good quality milk powder. V. Summary 1. It was determined and applied that a few micron thick PVA layer was coated on a base film such as PP to obtain good airtightness and meet the packaging requirements of various products. The PVA layer thickness is 6-10 microns more reasonable. 2. After testing and measurement, PVA/PP composite film has good tear strength, high elongation at break, good resistance to crease, and meets the requirements of hygienic standards. 3. The key to this technology is to improve the water resistance of the PVA layer and enhance the interfacial bond strength. The process is characterized by the use of PVA aqueous solution coating film, evaporation of the solvent will not cause harm, the PVA coating required is thinner, not only saves valuable material, but also can achieve rapid drying and filming, so the process is simple, the composite structure is reasonable, Low production costs, with good practical and promotional value. (Wang Zhenhua) (China Packaging Scientific Research and Testing Center)