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Application of metallocene polyethylene film in high performance packaging
- Sep 25, 2018 -

Characteristics of metallocene polyethylene film

Metallocene polyethylene refers to the polyethylene material prepared by the reaction of metallocene catalyst, referred to as metallocene (mPE). Compared with LLDPE prepared by Ziegler-Natta catalyst, mPE has a relatively narrow molecular weight distribution and a more uniform composition distribution, so the film has high strength and good longitudinal and transverse strength uniformity. Because of the narrow molecular weight distribution, the content of low molecular weight is very small, so the anti-chemical extraction and anti-pollution performance is good, film viscosity is low; and does not contain ultra-high molecular weight, so the crystal point is less. MPE film has excellent physical and mechanical properties, such as: anti-puncture, impact resistance, high tensile force, tear and so on. The heat sealing performance of mPE film is very good, the temperature range of heat sealing is wide, the peak value of melting point is low, the heat sealing time is shortened, the sealing performance is good, and the leakage and fracture phenomenon is greatly reduced. In addition, mPE film has the characteristics of moisture-proof, air-proof, anti-oxidation, oil-proof, freeze-proof, cooking-proof, chemical corrosion-proof, non-toxic, tasteless, and does not affect the nutrient composition of food, maintaining the flavor of food, good printing performance, easy opening and so on. Therefore, it has become a common resin material for high-performance packaging film. Generally, LLDPE film is easy to migrate and oxidize in the presence of oil. Under the action of alkali, acid or strong surfactant, the heat sealing weld often breaks down and causes pollution. And the mPE film has many advantages that the traditional LDPE, LLDPE, PP, EVA and other materials film can not. It is now gradually replacing some traditional packaging materials. It will occupy a very important position in the packaging industry with excellent performance.

Material selection and formulation design

The production cost of mPE film is restricted because of the requirement of process and equipment in the process of blowing film. In the production process, the packaging material with excellent performance was obtained by extrusion blow moulding of mPE/LLDPE blend, which reduced the comprehensive cost of the film, decreased the viscosity of the melt and reduced the torque transmitted to the screw, thus reducing the driving load and enabling large-scale production of the film. The production ratio of high performance packaging film is 60% for general mPE, 40% for LLDPE or LDPE, and its cost, production process and performance are ideal.

Production process control of mPE film for high performance packaging

Because of its special molecular weight distribution and composition distribution, metallocene polyethylene resin has different processing technology and equipment from traditional materials. In the process of metallocene film production, it is necessary to ensure the normal operation of equipment and strictly control the technological conditions in order to achieve the goal of stable product quality.

1. Production process: The production process of mPE film is basically the same as that of ordinary polyethylene film, but the control of process conditions is different. The process flow is as follows: batching - mixing - extrusion - blow - up - cooling - Corona - traction - winding - Inspection - packaging.

2. Blow molding process control: because metallocene polyethylene resin price is higher than ordinary polyethylene resin, in order to reduce costs, using mPE and general-grade LLDPE or LDPE resin mixed use. When the raw material is metered into the feeding section of the extruder, the resin is in the melting stage, the viscosity increases abruptly, 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, the temperature of the material in the machine will also rise, the melt outlet mold will cause traction instability, film breakage, bubble instability, and then affect the film width and thickness uniformity. If the temperature decreases, the film will have poor plasticity, more crystalline points, poor transparency, mechanical properties of the film will decline, the film is hard and rough. When combined with other thin films, there are a series of problems such as local shrinkage, delamination, poor heat sealing effect and so on. Therefore, the mPE blowing process is best to follow: low-high-medium-temperature zone extrusion process, so that the melt plasticization uniform, stable discharge, traction, winding normal. In this way, low temperature should be maintained in the feeding section to ensure timely feeding and strong thrust; rapid heating should be carried out in the compression section to make the resin melt ahead of time and reduce the over-torque reaction caused by the increase of viscosity of the melt; cooling method should be adopted in the melt entering the homogenizing section to facilitate the transfer of more heat accumulation and keep the material in a stable state. The viscous flow state ensures that the melt passes through the filter equally, forming a stable pipe film, preventing the melt from breaking, laying a good foundation for the traction and cooling behind.

Coling is also an important part in the processing of mPE film. Because the extrusion temperature of mPE resin melt is higher than that of traditional LLDPE and the crystallization temperature is lower than that of LLDPE, it is especially important to transfer the melt heat in time. Double layer air ring should be used to dissipate heat in time to meet the technological requirements. The appropriate expansion ratio is helpful for bubble cooling, improving the uniform orientation and ensuring the uniformity of film thickness. In the production process of mPE film, it is better to keep the blowing ratio between 1.8 and 3.5, and the frost line height should be controlled around 2 D (D is the die diameter of the die), so the comprehensive quality of the film can be guaranteed.

3. Requirements of equipment: After entering the extruder, the material is extruded to melt. When the material enters the middle stage of melting, the viscosity of the melt increases sharply, the friction of the material increases, the screw torque increases, and the pressure transfers. The driving part of the extruder bears a great load, which makes the current of the main machine increase. Therefore, when selecting the equipment, the screw torque of the extruder increases. It is necessary to select devices that drive power enough to withstand torque and load in production. In the production process, controlling the reasonable technological conditions can also improve the load of the main engine in a certain range to ensure the normal production.

Selection of the length-diameter ratio of extruder screw is also a key to the production equipment of mPE film. Selection of the length-diameter ratio of screw can produce high-quality mPE film, at the same time can ensure the realization of production capacity, thereby reducing waste and obtaining certain benefits. At present, there are different opinions on the technical parameters of the screw used in the processing of mPE film, but the extruder screw should have enough extrusion pressure to make the screw have enough shearing effect in the compression section; after entering the homogenization section, the shearing should be reduced, the melt should be relaxed, the heat should be transferred quickly, and the die should be made quickly to stabilize the production capacity. In order to ensure the yield and quality of mPE film, the ratio of 25:1 to 32:1 is chosen.

epilogue

Metallocene polyethylene has been widely used in food packaging and industrial packaging because of its excellent properties. Major metallocene suppliers are also actively upgrading technology, the development of a new generation of metallocene materials, strive to achieve performance, stiffness, processing, heat sealing and other aspects of the balance. Different metallocene catalysts have different metallocene properties. Users can choose suitable metallocene materials according to their own product requirements.