The production methods of PVA film include solution casting method, water-added melt extrusion method, anhydrous melt extrusion method, water-added melt extrusion blow molding method, continuous biaxial orientation method and dry extrusion blow molding method. The widest range of applications is now mainly solution salivation method (wet method) and extrusion blown film method (wet method, dry method). These several processes are born out of the traditional plastic film production. Since PVA is a water-soluble substance, the film-making process conditions are different from those of thermoplastics , so the above two processes are directly applied to the production of PVA films. But there are still many technical problems, and there are also some economic problems, such as difficult quality control, low efficiency, high energy consumption, large floor area, and high equipment investment. Therefore, research and improvement of these two types of process technology and equipment to make them more suitable for PVA film production is the research and development focus of the industry at home and abroad.
(1) Strip salivation method
Tapecasting, also known as doctorblading or knife coating, is an important coating forming method . Salivation molding technology is widely used in industries such as papermaking, plastics and coating electronic product production, and it is the pillar technology for the production of water-soluble films. The process was first proposed by GlennN.Howatt in 1947 and patented in 1952. Usually, the specific process of salivation molding is: first, mix the raw materials, additives and solvents in the reactor with ultrasonic or mechanical stirring for a period of time, and after they are uniform, add release agents, plasticizers and other functional additives for mixing to obtain Well-dispersed and stable glue . Then put it into the insulation kettle for heat preservation and defoaming. When forming, the glue flows from the middle and lower part of the insulation kettle through the pipeline to the carrier tape , and the green film layer is formed by the relative movement of the carrier tape and the scraper. The green film layer and the carrier tape enter the drying chamber together, and the solvent evaporates to improve the strength and toughness of the finished film. The hydrosol is mainly composed of five main parts such as PVA, water, release agent, plasticizer and other functional additives. Their functions and functions in the glue are different and affect each other. The salivation process is relatively simple, but the product quality is not easy to control, the film yield is low, the energy consumption is high, and the equipment investment is large.
The PVA film studied in this experiment mainly uses polyvinyl alcohol as the main raw material, adding various additives, and its process is different from the traditional plastic film forming process. Its process is as follows: the raw materials are prepared into a certain concentration of water-soluble glue, and the salivation is applied to the mirror stainless steel belt, and then it is dried in the drying room, then the steel belt is peeled off after entering the drying room to dry to the specified moisture, and finally the edge is cut. Winding to obtain the finished film. At present, the basic technical parameters of the domestic salivation film production equipment are as follows: the film thickness is 0.02~0.2 mm, the width is ≤2000 mm; the production speed is 2~5 m/min.
Its technological process is shown in Figure 2.
(2) Roller film making
Roller film-making refers to the method in which rubber is coated on a rotating drying drum by a roller coater , and the coated stock solution evaporates water on the drum to become a dry film. The most essential difference between is the coating process. The roll drool method realizes the continuous coating of the rubber by the continuous rolling of the roll. This film-making method is to dissolve water-soluble polyvinyl alcohol to make a dilute solution as a film-making stock solution. In order to adjust its processing performance, corresponding plasticizers can be added to the stock solution. The plasticizers used in ordinary polyvinyl alcohol films mainly include ethylene glycol, glycerin, and low molecular weight polyethylene glycol. In addition, coloring agents, release agents, etc. may also be mixed or applied. The film-making stock solution is coated on a rotating drying drum or a belt by a roll coater. The coated stock solution evaporates water on the drum or belt to become a dry film.
As shown in Figure 1-4, the drum drool production line mainly includes a control system, a glue making system, a drool coating system , and a heating system. The salivation coating system mainly refers to the large roller, the heat preservation Water Tank and the glue tank, as well as the motor and reducer that drive the roller . The heating system consists of a heating oil tank, a temperature control device, a sensor and a circulation system. The roller method has high film forming efficiency, but due to its high energy consumption and slow production rate, large-scale industrial production has not been realized in CHINA .
(3) Wet extrusion blow molding method
The production of PVA plastic packaging film by blow molding is mainly based on polyvinyl alcohol, adding water and other additives and blown into film by extrusion blow molding machine. Its process flow is shown in Figure 1-5.
The melting point of polyvinyl alcohol is 220C~240C. Since the dehydration and etherification starts at 160°C, the decomposition ; Vinyl alcohol resins are used for extrusion blown film production. Compared with the salivating method, the extrusion blown film method has obvious improvement in product quality and production efficiency , but PVA must be modified. After the early granulation, the process and the amount of auxiliary materials are increased. After repeated heating, it is necessary to maintain a moisture content of 15-35% in the film. The process is complicated, the technology is difficult, and the energy consumption is high. Moreover, film forming, stretching, and setting are carried out at the same time, and the strength of the obtained film is not high.
(4) Dry extrusion blow molding method
After drying PVA in vacuum for 24 hours, mix it with plasticizing modifier and film forming agent in a high-speed mixer according to a certain ratio , and then use a diameter-reversed Brabender225 single-screw extruder to extrude and granulate, and then go through degassing Film blowing and finalizing treatment to obtain product 8. The process flow chart is shown in Figure 4.
The dry extrusion blow molding method not only retains the excellent properties , but also has the advantages of simple process, low energy consumption, high efficiency and low investment compared with wet and salivating processing technologies. Advantages of saving and so on5. At the same time, PVA dry-process pellets can be used to produce multi-layer co-extruded composite films and injection-molded hollow-molded containers . This technology is innovative, fills the gap in domestic PVA melting processing technology, broadens the application field of PVA resin, and has a good market prospect. In recent years, extrusion blow molding has achieved industrial .
In CHINA, the dry extrusion processing technology of PVA has been one of the unresolved technical problems . my country's market has a large demand for PVA film, most of which rely on high-priced imports from abroad. Due to the large difference in technological process, these methods have different characteristics. The process flow of the solution salivation method is relatively simple, but the product quality is not easy to control, the film yield is low, the energy consumption is high, and the equipment investment is large; while the extrusion blown film method has obvious improvements in product quality and production efficiency compared with the salivation method. However, in the extrusion blown film method , PVA must be modified. After pre-granulation, the process and the amount of auxiliary materials added are increased. The resin and film must be heated repeatedly during the entire production process, and 15% to 35% of the film must be kept. The water content is high, the process is complicated, the technology is difficult, and the energy consumption is high; and the film forming, stretching, and setting are carried out at the same time, the molecular chain is difficult to move, the degree of orientation is poor, the strength of the obtained film is not high, and the inherent properties of PVA resin cannot be fully utilized. characteristic. Because the extrusion blown film method still has many unsolved problems and cannot realize industrialized production, the solution salivation method is mainly used in CHINA at present.
This article is excerpted from "Preparation of PVA Sustained Release Film and Research on Its Sustained Release Performance". The copyright belongs to the original author. If there is any infringement, please contact us immediately, and we will deal with it in time!
