Characteristics of metallocene polyethylene film
Metallocene polyethylene refers to a polyethylene material prepared by the reaction of a metallocene catalyst, referred to as a metallocene. Compared with the conventional Ziegler-Natta catalyst, the mPE has a relatively narrow molecular weight distribution and is relatively uniform. The composition of the composition, so the film produced high strength, vertical and horizontal strength uniformity. Due to the narrow molecular weight distribution, low molecular content, so resistant to chemical extraction and anti-pollution performance, film viscosity is low; at the same time does not contain super High molecular weight, so there are fewer crystal points. mPE film has excellent physical and mechanical properties, such as: puncture resistance, impact resistance, high tensile force, good tearing, etc. mPE film has good heat sealing performance, low sealing temperature, heat Wide sealing temperature range, low melting point peak, shortened heat sealing time, good sealing performance, and greatly reduced leakage and breakage. In addition, mPE film also has moisture resistance, gas barrier, oxidation resistance, oil resistance It is a resin material commonly used in high performance packaging films. It is cooked, chemically resistant, non-toxic, tasteless, does not affect the nutritional content of food, maintains food aroma, printability, and easy opening. The LLDPE film is easy to migrate and oxidize in the presence of oil. Under the action of alkali, acid or strong surfactant, the heat seal weld will often rupture and cause pollution. The mPE film has traditional LDPE, LLDPE, PP. , EVA and other materials can not replace many of the advantages, is gradually replacing some of the traditional packaging materials, it will occupy a very important position in the packaging industry with excellent performance.
Application of metallocene polyethylene film in high performance packaging
Material selection and formula design
Since the mPE resin has certain requirements on the process and equipment during the blown film process, the production cost of the mPE film is restricted. In the production process, the mPE/LLDPE blend extrusion blow molding method is used to obtain the packaging material with excellent performance, which is reduced. The overall cost of the film, and the viscosity of the melt is reduced, the torque transmitted to the screw is reduced, thereby reducing the driving load and enabling the mass production of the film. The production ratio of the high-performance packaging film is generally 60%, LLDPE or LDPE. At 40%, its cost, production process and performance are in an ideal state.
Production process control of mPE film for high performance packaging
Due to its special molecular weight distribution and composition distribution, metallocene polyethylene resin determines that the processing technology and even processing equipment of the polymer are different from traditional materials. In the production process of metallocene film, the normal operation and strictness of equipment must be guaranteed. Control the process conditions to achieve the purpose of stabilizing product quality.
1. Production process: The production process of mPE film is basically the same as that of ordinary polyethylene film process, but the process conditions are different. The process flow is as follows: Batching - stirring - extrusion - inflation - cooling - corona - traction - Winding - inspection - packaging.
2. Blow molding process control: Since the current metallocene polyethylene resin is higher in price than ordinary polyethylene resin, in order to reduce the cost, mPE is mixed with general-purpose LLDPE or LDPE resin. When the raw material is metered into the extruder feeding section, the resin In the melting stage, the viscosity suddenly increases, and the torque is transmitted through the screw to drive the motor current to rise. If the control is not good, it will cause shutdown or other phenomena, and the temperature of the material in the machine will also rise, causing the traction to be unstable when the melt exits the mold. Broken film, broken film, unstable bubble, which affects the width and thickness uniformity of the film. If the temperature is lowered, the film is not plasticized, the crystal point is much, the transparency is poor, the mechanical properties of the film are degraded, and the film quality is lower. Hard, and rough. When combined with other films, local shrinkage, delamination, poor heat sealing effect, etc.. Therefore, the mPE blow molding process preferably follows: the low-high-medium-medium temperature zone extrusion process, thus The melt is plasticized evenly, the discharge is stable, the traction is taken, and the winding is normal. In this way, the feeding section should be kept at a low temperature to ensure timely and powerful thrust feeding; in the compression section, the temperature should be rapidly increased to make the resin Melting in advance, reducing the super-torque reaction of the melt due to viscosity increase; the melt entering the homogenization section should adopt the method of cooling, which is convenient for transferring more heat accumulation, so that the material is in a state of smooth viscous flow, ensuring melt balance Through the filter screen, a stable tube film is formed to eliminate the melt fracture phenomenon, which lays a foundation for the rear traction and cooling.
Cooling is also an important part of mPE film processing. Since the melt extrusion temperature of mPE resin is higher than that of traditional LLDPE and the crystallization temperature is lower than that of LLDPE, it is especially important to transfer the heat of the melt in time. Process requirements. Appropriate blow-up ratio helps to cool the bubble, improve the balanced orientation of the film, and ensure the uniformity of the film thickness. It is better to keep the blow ratio between 1.8-3.5 during the production of mPE film. The height of the wire should be controlled in 2D (D is the diameter of the die die), and the overall quality of the film is guaranteed.
3. Equipment requirements: After the mPE resin enters the extruder, the material is squeezed to melted. When the material enters the middle of the melt, the melt viscosity increases sharply, and the friction of the material increases, causing the screw torque to increase and pressure transmission. The driving part of the extruder is subjected to a large load, which causes the main current to rise. Therefore, when selecting the equipment, it is necessary to select a device whose driving power is sufficient to withstand the torque and load occurring in the production. In the production process, a reasonable process is controlled. Conditions can also improve the load on the mainframe within a certain range to ensure normal production.
The choice of the length-to-diameter ratio of the screw of the extruder is also a key to the production equipment of the mPE film. The length-to-diameter ratio of the screw can be selected to produce a high-quality mPE film, and the production capacity can be guaranteed, thereby reducing the waste and obtaining certain benefits. The screw technical parameters to be used in mPE film processing are not consistent, but the extruder screw should have sufficient extrusion pressure to make the screw have sufficient shearing action in the compression section; after entering the homogenization section, the shearing must be reduced. The melt is slack, and the heat is quickly transferred, and the mold is quickly released to stabilize the production capacity. Generally, the length to diameter ratio of the screw is selected between 25:1 and 32:1 to ensure the yield and product quality of the mPE film.
Conclusion
Metallocene polyethylene materials have been widely used in food packaging and industrial packaging due to their excellent properties. Suppliers of major metallocenes are also actively upgrading technology to develop a new generation of metallocene materials, striving for performance and stiffness. Processing, heat sealing and other aspects of the balance. Different metallocene catalysts have different metallocene properties, users can choose the appropriate metallocene materials according to the needs of their products.