Current situation and development trend of the hot

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Present situation and development trend of coextrusion technology (1)

multicomponent composite products can be prepared by many methods, and coextrusion process is the most simple and easy method. It has become one of the most advanced plastic molding and processing methods. Polymer coextrusion process is a processing process in which several extruders are used to supply different melt streams respectively, and multi-layer composite products are obtained by merging and coextrusion in a composite head. It can make multi-layer materials with different characteristics compound with each other in the extrusion process, so that the products have the excellent characteristics of several different materials, and complement each other in characteristics, so as to obtain the performance and appearance with special requirements, such as the barrier ability of oxygen and moisture resistance, coloring, heat preservation, thermoforming and thermal bonding ability, as well as mechanical properties such as strength, stiffness and hardness. These multilayer composites with comprehensive properties have extremely wide application value in many fields. In addition, it can greatly reduce the cost of products, simplify the process, reduce equipment investment, and the compounding process does not use solvents and does not produce three wastes. Therefore, coextrusion technology is widely used in the production of composite films, plates, pipes, strengthening basic research and technology accumulation, profiled materials and wires and cables

the following focuses on the coextrusion technology of composite pipes, composite films, flat films and cast films, PVC core foaming composite pipes, plates and profiles, which have been widely used in recent years

composite pipe coextrusion

aluminum plastic composite pipe integrates the advantages of plastic and metal. It has the advantages of non-toxic, smooth, corrosion resistance, light texture, high strength, good heat resistance, low embrittlement temperature, convenient installation, generous appearance, long service life and so on. It can be used for the transportation of corrosive liquids and gases in cold and hot water and drinking water pipelines, ground and underground heating pipelines, gas pipelines, and petrochemical industry, Compressed air transportation and the transportation of beverages, wine, milk and other liquids in the food industry are likely to gradually replace galvanized pipes, copper pipes and plastic pipes in the near future. In industrialized countries, aluminum plastic composite pipes account for about 15% of pipe materials. This technology was patented by the British engineer Itzhak barnoach in 1974, and then kitech company in the Netherlands, unicor company in Germany and klopper company in the Netherlands improved the pipe structure, processing equipment and manufacturing technology, so that its performance has been continuously improved. In the early 1990s, it began to be commercialized in Europe and Australia. In the mid-1990s, China began to introduce the technology of aluminum-plastic composite pipe production line and began to produce and apply aluminum-plastic composite pipes

the aluminum-plastic composite pipe is composed of five layers (polyethylene, hot melt adhesive, aluminum foil, hot melt adhesive, polyethylene), with cross-linked polyethylene (XLPE) as the inner and outer layers, and the middle layer is welded aluminum pipe to increase the strength of the pipe. The inner and outer surfaces of the aluminum pipe are coated with adhesive to bond with the plastic layer, and formed by coextrusion process

the structure of steel plastic composite pipe is basically the same as that of aluminum plastic composite pipe, which is composed of five layers. Its inner and outer raw materials are generally HDPE, and the hot melt adhesive layer is between the steel layer and the inner and outer HDPE. Steel plastic composite pipes with a diameter of MM mostly use steel plate skeleton reinforced composite plastic pipes. It is a new type of composite pressure pipe with cold-rolled steel strip and thermoplastic as raw materials, porous thin-walled steel pipe formed by argon arc butt welding as reinforcement, and double-sided composite thermoplastic on the outer and inner layers. Because the reinforcement of porous thin-walled steel pipe is wrapped in thermoplastic, which is made of 95% recycled materials, this composite pipe overcomes the shortcomings of steel pipe and plastic pipe, and has the common advantages of steel pipe and plastic pipe. At present, polyethylene pipes with steel wire winding as the skeleton are mostly used for large-diameter pipes for gas and water transmission. The winding and welding speed of steel wire skeleton is slow and the production efficiency is low

the key to the production of steel plastic composite pipe is the design of five layer coextrusion composite head, which requires the complete combination of inner and outer plastic layers, hot melt adhesive layer and intermediate steel layer in a coextrusion head. The key of the compounding process is to adopt the method of combining physical compounding and chemical compounding. At the set temperature, a layer of affinity agent is squeezed between the steel wire and the inner and outer HDPE layers to form a five layer structure of the pipe body. Under high temperature and high pressure, the affinity agent in the molten state is uniformly coated on the inner and outer surfaces of the steel pipe through the coextrusion head, and then grafted onto the polar activated functional groups of HDPE tape, And form a bonding interface with the steel pipe surface. After such a co extrusion process with physical and chemical composite effects, the steel plastic composite pipe can have the common characteristics of plastic and steel pipe at the same time, making it have high strength, long life, high air tightness, high corrosion resistance and high weather resistance

composite film coextrusion

multilayer composite film is characterized by good barrier to oxygen and water vapor; The film has high strength and puncture resistance; Good heat sealing; Strong adhesion; It has good anti fog, anti slip and coloring properties. Therefore, it is widely used in the field of packaging

coextrusion film blowing method is mainly used to produce high barrier packaging film, shrink film, hollow fresh-keeping film, geomembrane, etc. it is widely used in food, medicine, daily chemical product packaging, agricultural greenhouse, water conservancy engineering, environmental engineering and other fields

the coextrusion blown film production process is adopted, and the function of the film is quantitatively controlled through the effective adjustment of the thickness. The structure combination of each layer of the film is convenient and flexible, and the selection range of substrate is wider. Thus, the cost of the composite membrane is reduced, the strength is improved, the barrier property is increased, the added value is increased, and the market adaptability is enhanced

the technical difficulty of coextrusion film blowing method lies in the runner design of the composite head. The runner design should ensure that the flow rate of each layer of molten material is uniform, the shear stress of the bonding layer is consistent, and the material temperature of each layer of head should be able to be controlled independently. For this reason, companies such as Brampton engineering in Canada, Battenfeld Gloucester in the United States, Marco engineering in Canada, AMUT in Italy and MA in the United States have successfully studied multi-layer disk annular coextrusion heads, concentric spiral mandrel coextrusion heads and multi-layer conical disk annular coextrusion heads respectively. The structural shapes of each layer of flow channels include annular flow channels, heart-shaped envelope flow channels, spiral branch flow channels and so on

at present, the latest technology of coextrusion film blowing head is the combination of multi-layer conical disk annular coextrusion head and spiral branch flow channel. The temperature difference between adjacent layers can reach 80 ℃, and the product thickness error is within 5%. Here, the hexanediol refrigerating unit is used to replace the cooling water machine. The cooling adopts the combination of internal cooling in the mold and external cooling in the air ring to improve the cooling and crystallization speed, so as to increase the transparency, strength and toughness of the film. The maximum number of layers of composite membrane is 11, and the maximum width of greenhouse membrane is 24 meters

the shortcomings of coextrusion film blowing method are:

1, the number of layers is not allowed to have more changes

2. The ratio of each layer of film shall not fluctuate significantly

3. With the increase of the number of layers and the outer diameter of the die, the residence time of the melt of the outer film in the die increases, which is in danger of decomposition

4. When the melting point and viscosity of adjacent layers of resin differ greatly, if the temperature of each layer is not controlled properly, some resins with poor thermal stability may form decomposition layers

coextrusion of flat film and cast film

the forming principle of cast film is to extrude the plastic melt in the extruder through the T-shaped die, directly enter the aqueous solution or quench roll, and then make the cast film after cooling and traction. This processing method can give full play to the properties of the processed materials while maintaining the best dimensional accuracy. Most thermoplastic films can be produced by tape casting. It is especially suitable for semi crystalline thermoplastic

the forming principle of flat film extrusion is: extrude the plastic melt that has been plasticized evenly in the extruder from the flat film machine head, cool and solidify through the contact of the cooling roller, and finally cut into a certain width of film, and roll into a roll. The structure of each layer of coextrusion film can be symmetrical or asymmetric. When the adhesion performance between the two layers is poor, it is necessary to add a thin adhesive layer between the two layers to improve the heat sealing performance and boundary adhesion performance

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information source: International Plastics business information Beijing University of Chemical Technology Jiang Bo, Zhou Hongzhi and Xu Shuhua

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