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  With the synthesis and modification of polymer film raw materials and the development of flat film biaxial stretching technology, many biaxially stretched films with excellent performance have emerged one after another. Currently, the main flat film biaxially stretched films (abbreviated as: biaxially stretched films) are BOPP, BOPET, and BOPA.

1.Raw material properties
  PE is the main raw material for BOPE film. With the development of polyethylene synthesis technology, a variety of polymerization methods, various types of catalysts and polymerization technologies have been developed, and thus a number of polyethylene varieties have been developed: high-density polyethylene (HDPE), low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), metallocene polyethylene, etc. Usually, the molecular structure, molar mass and distribution of polyethylene resin are controlled by changing the type and amount of comonomers, polymerization methods and catalyst types.
2.Extrusion system
  PE has a regular molecular structure, and its molecular chain is very soft and can crystallize quickly. It can usually achieve a high degree of crystallinity. Usually, PE will undergo more or less thermal oxidation cross-linking during processing, which will cause crystal points, gels, and even charred products. Therefore, designing a suitable extrusion system is very important in the development of BOPE film.
  The extensibility of crystalline polymer film is basically dependent on the crystallinity, spherulite size and deformation characteristics, while the crystallinity, spherulite size and ductility are controlled by the cooling and solidification conditions during casting. Therefore, when making unstretched thick sheets with spherulite structure for biaxial stretching, it is necessary to fully consider the influence of equipment and operating conditions on the cooling assimilation of polymer melt.
4.Longitudinal/transverse stretch ratio
  The stretch ratio is a very important process parameter. Whether it is longitudinal or transverse, it has a great influence on the physical and mechanical properties of biaxially stretched films. At a certain temperature, the larger the stretch ratio, the larger the product of the longitudinal/transverse stretch ratio, the greater the degree of orientation of the film, and the more the mechanical strength, modulus, puncture resistance, gas barrier properties, surface gloss and other properties of the film will be improved.
  Biaxial stretching can change the comprehensive physical and mechanical properties of the film. With the increase of the stretch ratio and the increase of the degree of orientation, the number of straight segments of the polymer increases, the connecting segments between the wafers gradually increase, the density and strength of the film increase accordingly, and the elongation at break decreases. The difference in the longitudinal/transverse stretch ratio largely determines the difference in the longitudinal/transverse physical and mechanical properties of the film. When synchronous stretching is used, the longitudinal/transverse stretch ratios are not much different, and there is no obvious difference in the molecular orientation in the two directions. When using step-by-step biaxial stretching, it is necessary to consider the effect of transverse stretching preheating on the longitudinal properties of the oriented segments by de-orientation of the longitudinal stretching. Therefore, when using the step-by-step biaxial stretching method to produce films with higher longitudinal properties than transverse properties, the choice of longitudinal/transverse stretch ratio is very important.

  The temperature and temperature distribution of each stretching zone are key factors affecting the film stretching orientation and crystallization. Temperature affects the orientation of polymer molecular chains and the formation of crystals by affecting the mobility of molecular chains.
  For many occasions, BOPE films have transparency requirements. Haze and light transmittance are one of the indicators that downstream users pay close attention to. In addition to additives, temperature also has a great impact on the transparency of the film. Especially for crystalline plastics, the preheating and stretching temperatures should not be near the temperature of the maximum crystallization rate, otherwise a large amount of crystallization will affect the stretching of the film and the thickness uniformity and transparency of the film after stretching. The heat setting temperature needs to be selected near the temperature of the maximum crystallization rate, so that the stretched film can crystallize as soon as possible, so that the film has high strength, good dimensional stability and good appearance at the same time.

  POTOP's biaxial tensile test line has advanced technology and stable data. Taking all factors into consideration, it can provide accurate and detailed tensile data for various materials for large-scale production proofing.



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