Casting polypropylene film based on dielectric property improvement and preparation method and application thereof

Abstract

The invention provides a casting polypropylene film based on dielectric property improvement as well as a preparation method and application thereof. The preparation method comprises the following steps: drying polypropylene granules; adding the granules into a charging barrel for melting; carrying out homogenization treatment through an extrusion die, and converting the granules into molten polypropylene; pushing melt polypropylene to an extrusion die head through a filter screen by utilizing an extrusion screw to form a polypropylene film shape; and quickly cooling and shaping on the surface of a casting roller to obtain the casting polypropylene film based on improvement of the dielectric property. By means of the method, the polypropylene granules are subjected to melt casting, the out-phase point of the casting polypropylene film can be removed, meanwhile, the breakdown performance of the casting polypropylene film can be improved, and the preparation method of the casting polypropylene film is easy and convenient to operate, and the dielectric property of the product is excellent.

Description

Technical Field

[0001] The present invention belongs to the technical field of plastic films, and particularly relates to a cast polypropylene film based on improved dielectric properties, its preparation method and application. Background Art

[0002] Polypropylene (PP) films have high electrical strength and low loss, and are widely used in capacitors. In the 1970s, with the surge in demand for high-transparency and high-uniformity films in food packaging and electronic components (such as capacitors), the traditional blown film method was difficult to meet the requirements due to problems such as rough surface and uneven thickness, which promoted the technological breakthrough of the melt extrusion casting method.

[0003] The melt extrusion casting method is an important key technology for preparing polypropylene films. With the high transparency and uniformity of the polypropylene films prepared by it, it occupies an irreplaceable position in the packaging and electronic fields. This polypropylene film has high dielectric strength and low loss, and is often used as a capacitor film in the electronics industry.

[0004] However, the dielectric strength of the polypropylene films currently prepared by the melt extrusion casting method still needs to be further improved, and there are easily heterogeneous points (such as unmelted grains, impurity agglomerates, etc.) in the cast polypropylene films. As a result, the dielectric film capacitors prepared from the cast polypropylene films often have impurity heterogeneous points and macromolecular entanglement heterogeneous points, and still face problems such as local electric field distortion, breakdown strength decline and life attenuation. In the prior art, in order to remove the heterogeneous points in the polypropylene films, a high-temperature melt filtration process is often required. However, such a filtration method has a contradictory limitation with the production cost, and the complete removal of the heterogeneous points is difficult to achieve. The remaining heterogeneous points may still cause micro-region dielectric failure, further highlighting the importance of removing the heterogeneous points in the cast polypropylene films and improving their dielectric strength. How to use the melt extrusion casting method to mass-produce polypropylene films without heterogeneous points and with excellent dielectric strength is a problem that needs to be focused on. Summary of the Invention

[0005] The purpose of the present invention is to provide a cast polypropylene film based on improved dielectric properties, its preparation method and application, so as to overcome the problems of macromolecular entanglement heterogeneous points and insufficient breakdown field strength in the cast polypropylene films in the prior art.

[0006] The present invention is realized through the following technical solutions: A preparation method of a cast polypropylene film based on improved dielectric properties, comprising the following steps: Take polypropylene pellets and dry them; Add the dried polypropylene pellets into the barrel for melting, and perform homogenization treatment through an extrusion die to convert the pellets into molten polypropylene; Using an extrusion screw, the molten polypropylene is pushed through a filter screen towards an extrusion die head to form a precursor of a polypropylene film. The precursor of the polypropylene film is cooled and shaped on the surface of a casting roll to obtain a cast polypropylene film with improved dielectric properties.

[0007] Furthermore, the drying environment is a vacuum with a vacuum degree of 80 - 120 Pa, a drying time of 6 - 10 h, and a drying temperature of 80 °C.

[0008] Furthermore, the melting temperature of the barrel is set to 220 - 260 °C.

[0009] Furthermore, the temperature of the extrusion die is set to 210 - 240 °C.

[0010] Furthermore, the rotational speed of the extrusion screw is set to 45 - 65 rpm.

[0011] Furthermore, the mesh number of the filter screen is 2000 - 2500 mesh, and the width of the extrusion die head is 140 mm.

[0012] Furthermore, the temperature of the casting roll is 50 - 80 °C, and the rotational speed is 5.5 - 7.5 rpm.

[0013] Furthermore, the width of the cast polypropylene film is 80 mm, and the thickness is 15 - 30 μm.

[0014] A cast polypropylene film with improved dielectric properties is prepared by the above preparation method.

[0015] An application of a cast polypropylene film with improved dielectric properties in optimizing the properties of cast polypropylene films.

[0016] Compared with the prior art, the present invention has the following beneficial technical effects: The implementation method of the present invention is simple, and the implementation conditions are easy to achieve, which can be realized based on an ordinary desktop casting machine. By vacuum drying the polypropylene pellets and controlling conditions such as the barrel temperature, the mesh number of the filter screen, the screw rotational speed, and the casting rod rotational speed within a certain range, the dielectric properties of the prepared cast polypropylene film can be improved. At the same time, according to the above process chain, it is possible to mass-produce cast polypropylene films without phase points and with excellent dielectric properties based on the existing production line.

[0017] The cast polypropylene film prepared by the present invention can effectively remove moisture and low-molecular volatile substances in polypropylene pellets under vacuum drying, and inhibit the generation of heterogeneous points from the source. On this basis, through the coordinated optimization of three core process parameters: using an ultra-high mesh filter to physically intercept micron-sized heterogeneous particles (such as catalyst residues, oxidation aggregates), significantly reducing the internal defect density of the film; controlling the rotation speed of the extrusion screw to precisely adjust the melt shear rate, avoiding molecular chain breakage caused by too high a rotation speed or melt retention or local degradation caused by too low a rotation speed; appropriately regulating the rotation speed of the casting roll, and combining the roll temperature to achieve dynamic slow cooling crystallization, forming a fine and uniform spherulite structure. The synergistic effect of the high-mesh filter, screw rotation speed, and casting roll rotation speed can ensure the integrity of high-mesh filtration: the control of the screw rotation speed can provide a stable melt pressure, ensuring that the melt passes through the ultra-high mesh filter efficiently without clogging; at the same time, the casting roll rotation speed can also match the screw rotation speed, avoiding the concentration of grain boundary stress caused by the mismatch between the melt flow rate and the cooling rate, and synchronously inhibiting crystallization defects. The synergistic effect of the above process chain can ultimately eliminate the heterogeneous points of the cast polypropylene film and improve its dielectric properties. Description of the Drawings

[0018] In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings required for use in the embodiments will be briefly introduced below. The following drawings only show some embodiments of the present invention, and therefore should not be regarded as limiting the scope. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0019] Figure 1 SEM scanning electron microscope surface morphology diagram of Comparative Example 1.

[0020] Figure 2 GPC gel permeation chromatography test spectra of Example 1 and Comparative Example 1.

[0021] Figure 3 Breakdown field strength of the cast polypropylene films prepared in Example 1, Comparative Example 2, and Comparative Example 3.

[0022] Figure 4 Breakdown field strength of the cast polypropylene films prepared in Example 2, Comparative Example 4, and Comparative Example 5. Detailed Embodiments

[0023] The present invention will be described in detail below: A method for preparing a cast polypropylene film based on improving dielectric properties, comprising the following steps: Dry the polypropylene pellets; add the polypropylene pellets into the barrel for melting; conduct homogenization treatment through an extrusion die to convert the pellets into molten polypropylene; use an extrusion screw to push the molten polypropylene through a filter screen towards the extrusion die head to form a polypropylene film shape; quickly cool and shape it on the surface of a casting roll to obtain a cast polypropylene film based on the removal of the heterogeneous point.

[0024] Preferably, in the method for preparing a cast polypropylene film based on the improvement of dielectric properties, the drying environment is a vacuum, the vacuum degree is 80 - 120 Pa, the drying time is 6 - 10 h, and the drying temperature is 80 °C.

[0025] Preferably, in the method for preparing a cast polypropylene film based on the improvement of dielectric properties, the melting temperature of the barrel is set to 220 - 260 °C.

[0026] Preferably, in the method for preparing a cast polypropylene film based on the improvement of dielectric properties, the temperature of the extrusion die is set to 210 - 240 °C.

[0027] Preferably, in the method for preparing a cast polypropylene film based on the improvement of dielectric properties, the rotation speed of the extrusion screw is set to 45 - 65 rpm.

[0028] Preferably, in the method for preparing a cast polypropylene film based on the improvement of dielectric properties, the mesh number of the filter screen is 2000 - 2500 meshes.

[0029] Preferably, in the method for preparing a cast polypropylene film based on the improvement of dielectric properties, the width of the extrusion die head is 140 mm.

[0030] Preferably, in the method for preparing a cast polypropylene film based on the improvement of dielectric properties, the temperature of the casting roll is 50 - 80 °C, and the rotation speed is 5.5 - 7.5 rpm.

[0031] Preferably, in the method for preparing a cast polypropylene film based on the improvement of dielectric properties, the width of the stretched film is 80 mm, and the thickness is 15 - 30 μm.

[0032] A cast polypropylene film based on the improvement of dielectric properties is prepared by the above preparation method.

[0033] An application of a cast polypropylene film based on the improvement of dielectric properties in improving the properties of cast polypropylene films.

[0034] The implementation scheme of the present invention will be described in detail below in conjunction with embodiments. This embodiment is the preferred scheme of the present invention, and the scope of the present invention cannot be limited thereby. Unless otherwise specified, the test methods used in the following examples and / or comparative examples are all conventional methods; the materials, reagents, etc. used, unless otherwise specified, can all be obtained from commercial channels.

[0035] Polypropylene: Borealis, HC-300BF, with a melt mass flow rate of 3.3 g / 10min measured according to ISO1133 at 230 °C and 2.16 Kg.

[0036] Example 1 A method for preparing a cast polypropylene film based on improving dielectric properties, comprising the following steps: Place the polypropylene pellets in a vacuum drying oven and dry them at 80 °C and a vacuum degree of 80 Pa for 6 h to obtain dry polypropylene pellets; add the polypropylene pellets to a barrel at a temperature of 220 °C and homogenize them through an extrusion die at a temperature of 210 °C to convert the pellets into molten polypropylene; set the rotation speed of the extrusion screw to 55 rpm, make the molten polypropylene pass through a filter screen with a mesh number of 2000, pass through an extrusion die head with a width of 140 mm to form a polypropylene film shape, and finally rapidly cool and shape it under the action of a casting roll at a temperature of 50 °C and a rotation speed of 7 rpm to obtain a cast polypropylene film with a width of 80 mm and a thickness of 20 μm.

[0037] Example 2 A method for preparing a cast polypropylene film based on improving dielectric properties, comprising the following steps: Place the polypropylene pellets in a vacuum drying oven and dry them at 80 °C and a vacuum degree of 100 Pa for 8 h to obtain dry polypropylene pellets; add the polypropylene pellets to a barrel at a temperature of 240 °C and homogenize them through an extrusion die at a temperature of 220 °C to convert the pellets into molten polypropylene; set the rotation speed of the extrusion screw to 45 rpm, make the molten polypropylene pass through a filter screen with a mesh number of 2300, pass through an extrusion die head with a width of 140 mm to form a polypropylene film shape, and finally rapidly cool and shape it under the action of a casting roll at a temperature of 65 °C and a rotation speed of 7.5 rpm to obtain a cast polypropylene film with a width of 80 mm and a thickness of 15 μm.

[0038] Example 3 A method for preparing a cast polypropylene film based on improving dielectric properties, comprising the following steps: Place the polypropylene pellets in a vacuum drying oven and dry them at 80 °C under a vacuum of 120 Pa for 10 h to obtain dry polypropylene pellets; add the polypropylene pellets to a barrel at a temperature of 260 °C and perform homogenization treatment through an extrusion die at a temperature of 240 °C to convert the pellets into molten polypropylene; set the rotational speed of the extrusion screw to 65 rpm, allow the molten polypropylene to pass through a filter screen with a mesh number of 2500, and through an extrusion die head with a width of 140 mm to form a polypropylene film shape. Finally, rapidly cool and shape it under the action of a casting roller at a temperature of 80 °C and a rotational speed of 5.5 rpm to obtain a cast polypropylene film with a width of 80 mm and a thickness of 30 μm.

[0039] Comparative Example 1 Place the polypropylene pellets in a vacuum drying oven and dry them at 80 °C under a vacuum of 80 Pa for 6 h to obtain dry polypropylene pellets; add the polypropylene pellets to a barrel at a temperature of 220 °C and perform homogenization treatment through an extrusion die at a temperature of 210 °C to convert the pellets into molten polypropylene; set the rotational speed of the extrusion screw to 55 rpm, allow the molten polypropylene to pass through a filter screen with a mesh number of 300, and through an extrusion die head with a width of 140 mm to form a polypropylene film shape. Finally, rapidly cool and shape it under the action of a casting roller at a temperature of 50 °C and a rotational speed of 7 rpm to obtain a cast polypropylene film with a width of 80 mm and a thickness of 20 μm.

[0040] Comparative Example 2 Place the polypropylene pellets in a vacuum drying oven and dry them at 80 °C under a vacuum of 80 Pa for 6 h to obtain dry polypropylene pellets; add the polypropylene pellets to a barrel at a temperature of 220 °C and perform homogenization treatment through an extrusion die at a temperature of 210 °C to convert the pellets into molten polypropylene; set the rotational speed of the extrusion screw to 100 rpm, allow the molten polypropylene to pass through a filter screen with a mesh number of 2000, and through an extrusion die head with a width of 140 mm to form a polypropylene film shape. Finally, rapidly cool and shape it under the action of a casting roller at a temperature of 50 °C and a rotational speed of 7 rpm to obtain a cast polypropylene film with a width of 80 mm and a thickness of 20 μm.

[0041] Comparative Example 3 Place the polypropylene pellets in a vacuum drying oven and dry them at 80 °C and a vacuum degree of 80 Pa for 6 h to obtain dry polypropylene pellets; add the polypropylene pellets to a barrel at a temperature of 220 °C and carry out homogenization treatment through an extrusion die at a temperature of 210 °C to convert the pellets into molten polypropylene; set the rotation speed of the extrusion screw to 20 rpm, make the molten polypropylene pass through a filter screen with a mesh number of 2000, pass through an extrusion die head with a width of 140 mm to form a polypropylene film shape, and finally rapidly cool and shape it under the action of a casting roller at a temperature of 50 °C and a rotation speed of 7 rpm to obtain a cast polypropylene film with a width of 80 mm and a thickness of 20 μm.

[0042] Comparative Example 4 Place the polypropylene pellets in a vacuum drying oven and dry them at 80 °C and a vacuum degree of 100 Pa for 8 h to obtain dry polypropylene pellets; add the polypropylene pellets to a barrel at a temperature of 240 °C and carry out homogenization treatment through an extrusion die at a temperature of 220 °C to convert the pellets into molten polypropylene; set the rotation speed of the extrusion screw to 45 rpm, make the molten polypropylene pass through a filter screen with a mesh number of 2300, pass through an extrusion die head with a width of 140 mm to form a polypropylene film shape, and finally rapidly cool and shape it under the action of a casting roller at a temperature of 65 °C and a rotation speed of 2.5 rpm to obtain a cast polypropylene film with a width of 80 mm and a thickness of 15 μm.

[0043] Comparative Example 5 Place the polypropylene pellets in a vacuum drying oven and dry them at 80 °C and a vacuum degree of 100 Pa for 8 h to obtain dry polypropylene pellets; add the polypropylene pellets to a barrel at a temperature of 240 °C and carry out homogenization treatment through an extrusion die at a temperature of 220 °C to convert the pellets into molten polypropylene; set the rotation speed of the extrusion screw to 45 rpm, make the molten polypropylene pass through a filter screen with a mesh number of 2300, pass through an extrusion die head with a width of 140 mm to form a polypropylene film shape, and finally rapidly cool and shape it under the action of a casting roller at a temperature of 65 °C and a rotation speed of 9 rpm to obtain a cast polypropylene film with a width of 80 mm and a thickness of 15 μm.

[0044] (1) Cut the cast polypropylene film prepared in Comparative Example 1 smoothly, spray a 10 nm gold layer on the surface to eliminate the charging effect caused by poor conductivity, and then fix it on the conductive tape and clean it. Observe the sample morphology at an acceleration voltage of 15 kV through a scanning electron microscope (SEM), select the analysis area, and obtain the surface morphology map of the cast polypropylene film. The results are as Figure 1 shown.

[0045] (2) Weigh an appropriate amount of the cast polypropylene films prepared in Example 1 and Comparative Example 1 above, and conduct GPC gel permeation chromatography characterization tests. Dissolve them in an organic solvent at 150 °C for 3 h. Weigh 10 PS standard samples and also dissolve them in the same organic solvent to prepare a calibration curve. Then, perform calibration according to the general calibration parameters commonly used in China to obtain the GPC chromatogram of the cast polypropylene film. The results are as Figure 2 shown.

[0046] (3) Place the cast polypropylene films prepared in Example 1, Example 2, Comparative Example 1, Comparative Example 2, Comparative Example 3, Comparative Example 4, and Comparative Example 5 into a withstand voltage tester for DC breakdown tests. The two breakdown electrodes used in the tests are both spherical copper electrodes with a diameter of 25 mm, and the voltage rise rate of the test is 1.5 kV / s. Each sample film is measured 10 times to ensure the test effectiveness. Analyze the breakdown results through the Weibull function. The results are as Figure 3 and Figure 4 shown.

[0047] It can be seen from Figure 1 that there are only slight scratches and cracks on the surface morphology of the cast polypropylene film prepared in Comparative Example 1, showing the normal polypropylene morphology, and there is no morphology exhibited by metals and other impurities. It can be seen that this heterogeneous point is not caused by impurities.

[0048] It can be seen from Figure 2 that Comparative Example 1 has a significant ultra-high molecular weight region, while Example 1 does not show it. The appearance of this region is a significant feature of the heterogeneous point caused by macromolecular entanglement. It can be seen that the prepared cast polypropylene film successfully eliminates the heterogeneous point caused by macromolecular entanglement.

[0049] It can be seen from Figure 3 that the characteristic breakdown field strength of Example 1 is 564.04 kV / mm, the characteristic breakdown field strength of Comparative Example 2 is 477.49 kV / mm, and the characteristic breakdown field strength of Comparative Example 3 is 516.87 kV / mm. Compared with Comparative Example 2, the breakdown field strength of the film in Example 1 is increased by 18.1%. Compared with Comparative Example 3, the breakdown field strength of the film in Example 1 is increased by 9.1%. It can be seen that the cast polypropylene film prepared by the present invention has more excellent dielectric strength while effectively removing the heterogeneous point caused by macromolecular entanglement by controlling the rotation speed of the extrusion screw at the intermediate value, using a high-mesh filter, and adjusting the preparation process of the cast polypropylene film.

[0050] It can be seen from Figure 4It can be seen that the characteristic breakdown field strength of Example 2 is 574.56 kV / mm, the characteristic breakdown field strength of Comparative Example 4 is 461.32 kV / mm, and the characteristic breakdown field strength of Comparative Example 5 is 489.98 kV / mm. Compared with Comparative Example 4, the breakdown field strength of the film in Example 2 is increased by 24.5%. Compared with Comparative Example 5, the breakdown field strength of the film in Example 2 is increased by 17%. Obviously, by changing the rotation speed of the casting roller and optimizing the casting process, the cast polypropylene film prepared by the present invention has stronger dielectric properties.

[0051] Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person familiar with this technology can make various modifications and decorations without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be defined by the claims.

Claims

1. A preparation method of a cast polypropylene film based on improving dielectric properties, characterized in that, It includes the following steps: Take polypropylene pellets and dry them; Add the dried polypropylene pellets into the barrel for melting, conduct homogenization treatment through an extrusion die, and convert the pellets into molten polypropylene; Use an extrusion screw to push the molten polypropylene through a filter screen towards the extrusion die head to form a precursor of a polypropylene film; Cool and shape the precursor of the polypropylene film on the surface of a casting roll to obtain a cast polypropylene film with improved dielectric properties.

2. The preparation method of a cast polypropylene film based on improved dielectric properties according to claim 1, wherein, The drying environment is a vacuum, the vacuum degree is 80 - 120 Pa, the drying time is 6 - 10 h, and the drying temperature is 80 °C.

3. The preparation method of a cast polypropylene film based on the improvement of dielectric properties according to claim 1, characterized in that, The melting temperature of the barrel is set at 220 - 260 °C.

4. A method for preparing a cast polypropylene film based on improved dielectric properties according to claim 1, characterized in that, The temperature of the extrusion die is set at 210 - 240 °C.

5. The preparation method of a cast polypropylene film based on the improvement of dielectric properties according to claim 1, characterized in that, The rotational speed of the extrusion screw is set at 45 - 65 rpm.

6. The preparation method of a cast polypropylene film based on the improvement of dielectric properties according to claim 1, characterized in that, The mesh number of the filter screen is 2000 - 2500 meshes, and the width of the extrusion die head is 140 mm.

7. The preparation method of a cast polypropylene film based on the improvement of dielectric properties according to claim 1, wherein, The temperature of the casting roll is 50 - 80 °C, and the rotational speed is 5.5 - 7.5 rpm.

8. The preparation method of a cast polypropylene film based on the improvement of dielectric properties according to claim 1, characterized in that, The width of the cast polypropylene film is 80 mm, and the thickness is 15 - 30 μm.

9. A cast polypropylene film based on the improvement of dielectric properties, characterized in that, It is prepared by using the preparation method described in any one of claims 1 - 8.

10. Application of a cast polypropylene film with improved dielectric properties described in claim 9 in optimizing the properties of a cast polypropylene film.

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