Coating technology of the most popular polyvinyl a

2022-08-06
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Coating technology of polyvinyl alcohol composite membrane polyvinyl alcohol (PVA) is a kind of polyhydroxy polymer. Due to its molecular structure and high compactness, it is one of a few high barrier polymers with excellent performance. Japan has long produced PVA film by extrusion blow molding, and China has also produced a small amount of PVA casting film in recent years. The film shows unique properties, such as good transparency and gloss, anti-static, tear resistance, moisture permeability, good printability and excellent Yin Qi. In dry environment, the oxygen barrier property of PVA film is even better than EVOH and PVDC, but its Yin Qi property and mechanical strength are greatly affected by environmental humidity, coupled with high softening point and poor heat sealing performance, so it is inconvenient to be used as barrier packaging materials alone. Generally, other general materials such as PP film, PE film or pp/pe film are selected to be compounded with PVA, which can overcome their shortcomings and make comprehensive use of their respective advantages to form packaging materials with extremely low oxygen permeability. 1、 Advantages of coating process] PVA is compounded with PP or PE to form a film. There are two process routes to choose from: (1) PVA film is compounded with PP, PE and other films with the help of adhesive; (2) Coating and compounding with PVA aqueous solution on PP or PE base film. The composite membrane is made of PVA membrane and pp|pe membrane. The front arm must first manufacture PVA membrane and develop adhesive for composite, and actively position itself as a medium and low-end agent. Due to the strong water absorption of PVA, the PVA layer of the two-layer composite membrane is easy to be layered when encountering water. Therefore, the three-layer structure of pp/pva/pe needs to be manufactured to meet the use requirements. In this way, the two-layer adhesive needs to be compounded twice, and the process is complex; In addition, the manufacturing difficulty of PVA membrane and the complexity of equipment and technology result in high comprehensive cost. Coating and compounding with PVA aqueous solution on the base film and storing and analyzing the measured data. First of all, it is not necessary to manufacture PVA film alone on the same day. After the base film is subject to specific surface treatment and appropriate coating compound formula is selected, PVA compound can be directly coated on the base film to save the process of coating adhesive. The drying of PVA solution and 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, It only needs to produce pva/pp (PE) two-layer structure to meet the use requirements. This process is not only simple, but also the comprehensive cost is far lower than that of PVA film composite method. 2、 The principle and measures of coating composite process coating method is to dissolve PVA in water to form a glue solution and modify it appropriately. It is coated on the base film with certain surface properties, and then heated and dried to make PVA composite film. In order to obtain better overall performance of PVA composite membrane, two basic problems must be solved: one is the water resistance of PVA coating, so as to reduce the adverse impact of the environment and maintain high Yin gas; The second is to improve the wetting adaptability of PVA adhesive solution and the surface properties of the base film, and enhance the interface adhesion so that it will not peel off in the process of use. The Hu effect bonding of the interface is the key link in the coating process. According to the adhesion theory, the effective adhesion depends on the effective infiltration of the adhesive solution into the base film. The infiltration of the adhesive solution into the base film is related to the properties of the adhesive solution, the surface conditions of the base film and the coating and film making conditions. According to the wetting thermodynamics, the liquid must automatically spread on the solid surface, which must meet certain thermodynamic conditions, that is, the solid surface tension must be greater than or equal to the sum of the liquid surface tension and the liquid/solid interface tension. The surface tension of PP film and PE film is less than that of PVA; In addition, PVA is a polymer with strong polarity, while PP and PE are non-polar polymers with large interfacial tension. It is difficult for PVA glue to spread on the untreated PP and PE base membrane. In order to obtain the thermodynamic conditions that PVA glue is easy to spread on the base membrane, corresponding measures are taken in the process: first, the PVA glue is modified. Before modification, PVA has a high content of hydroxyl groups in the main chain, and its aqueous solution is easy to form hydrogen bonds with high cohesion energy, so its surface tension is large, and it is easy to form sol in low-temperature aqueous solution with poor fluidity,. Through chemical modification, the hydroxyl content of PVA molecule was reduced, thus the cohesion energy was reduced, the surface tension was reduced, and the flow firmness of the glue solution was greatly improved; The modification also weakens the polarity of pva3 molecules when the experiment is not conducted for a long time, which is conducive to the attraction and adhesion between PVA coating and the base membrane interface. Another major effect of modification is that the insoluble curing film is formed after the coating dries out of water, which significantly improves the water resistance of PVA coating. Secondly, the base film should be properly corona treated. In order to make the adhesive liquid soak well on the base membrane, besides improving the properties of PVA adhesive liquid, corona treatment should be carried out on the base membrane to improve the surface properties of the base membrane. High voltage corona makes the air on the surface of the film highly ionized and discharged, producing ozone and nitrogen oxides, causing thermal degradation and oxidation on the surface of the base film, changing the chemical composition of the surface and improving the surface tension of the base film. At the same time, polar groups are introduced into the surface to increase the interface attraction, thus improving the infiltration and adhesion of the interface. The improvement degree of the surface properties of the base film is related to the voltage value used in the corona treatment process and the treatment speed. In 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. During corona treatment, the base film shall be prevented from softening and deformation under the accumulated high temperature, and the base film shall have a certain reasonable thickness. Base 3: adopt some operating process conditions conducive to wetting and bonding. (1) After corona treatment, the film shall be coated in time to avoid the performance degradation of the base film, which will affect the wetting of the glue solution; (2) As the surface tension decreases with the increase of temperature, the coating operation should be carried out at room temperature. When drying at high temperature, the far-infrared heating method should be used to reduce the direct impact of high temperature on the surface properties of the base film; (3) Control the appropriate coating speed, etc. 3、 Main process parameters maximum coating speed: 8m/min drying temperature 100 ℃ ~110 ℃ drying time: 1~1.5 minutes controllable coating thickness: below 15 microns IV. performance and application effect of the composite film 1. Wrinkle performance: the pva/pp composite film was vigorously rubbed in the hand for more than 20 times without deep embrittlement and peeling, and a strong adhesion was formed between the surface coating and the base film. 2. Mechanical strength the cracking strength and elongation at break of jpva/pp composite film were significantly higher than that of PP film. 3. The hygiene test has been carried out by the public health department and conforms to the provisions of the national hygiene standards. 4. Permeability under the same conditions, the oxygen permeability of PP membrane and pva/pp composite membrane with different PVA layer thickness were measured. A 5-micron-thick PVA layer can greatly reduce the oxygen transmission of the composite film, and its oxygen transmission rate is about one thousandth of that of PP film, which meets the requirements of oxygen insulated fresh-keeping packaging. Considering the friction damage and uneven coating in the use process, the thickness of PVA coating can not be less than 5 microns, which is more reasonable. 5. Fragrance retention performance test the flavor contained in beverages and foods can also be prevented from being affected by external tastes in shelf display or other places. 6. Application effect green tea contains a variety of nutrients and flavor components. These components are very sensitive to environmental factors such as temperature, humidity, oxygen and light, and are easy to cause quality changes. Therefore, packaging materials are required to have the effect of oxygen isolation and shading. Packaging application shows that fresh green tea packaged with pva/pp composite film can be stored in indoor natural environment for seven months. Even if transparent packaging is used, the flavor and color of green tea can still be well maintained. The preservation of milk powder should also prevent the oxidation of lipids. Therefore, it is necessary to isolate oxygen and avoid light exposure. At the same time, it should be moisture-proof and bacteria proof. The pheasant milk powder packaged with pva/pp composite film, stored in a cool place indoors for six months, can still maintain the good quality of the milk powder. 5、 Summary 1. The measurement and application show that good air tightness can be obtained by coating several microns of PVA layer on PP and other base films, which can meet the packaging requirements of a variety of products. The thickness of PVA layer is more reasonable in microns. 2. Through test and determination, pva/pp composite film has good tear strength, high elongation at break, good crease resistance, and conforms to the provisions of health standards. 3. The key of this technology is to improve the water resistance of PVA layer and enhance the interfacial bonding strength. The process features that PVA aqueous solution is used for coating and film making. The evaporation of solvent will not cause harm. The required PVA coating is thin, which can not only save valuable materials, but also realize rapid drying and film forming. Therefore, the process is simple, the composite structure is reasonable, the production cost is low, and it has good practical and promotional value. (wangzhenhua) (China Packaging Research and testing center)

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