A method for preparing polypropylene film for film capacitors

Abstract

The invention belongs to the technical field of dielectric film materials, and in particular relates to a method for preparing a polypropylene film for a film capacitor, comprising the steps of: S1, sheet casting preparation; S2, primary synchronous stretching; S3, primary transverse stretching; S4, secondary transverse stretching; S5, secondary synchronous stretching; S6, post-processing. The method for preparing a polypropylene film of the invention can prepare an ultra-wide polypropylene film for a film capacitor with uniform thickness, high longitudinal tensile strength, high transverse elongation, and low thermal shrinkage.

Description

Technical Field

[0001] The invention belongs to the technical field of dielectric film materials, and in particular relates to a method for preparing a polypropylene film for a film capacitor. Background Art

[0002] Polypropylene film has the advantages of high transparency, good gloss, good barrier properties, high impact strength, and low temperature resistance, and is an important material for manufacturing high-performance capacitors. With the rapid development of the electronics, power and other industries, the demand for high-performance capacitors continues to increase. The production of polypropylene film for ultra-wide capacitors can not only meet market demand and promote the development of related industries such as aviation, military industry, and electric vehicles, but also improve production efficiency and product quality, reduce production costs, and enhance the market competitiveness of enterprises.

[0003] At present, the width of biaxially stretched polypropylene film produced by enterprises is mostly below 6 meters, and more often it is 1 to 3 meters. If the width of biaxially stretched polypropylene film can be increased to more than 7 meters, such as 7.2-8.5 meters wide, the application scope and production efficiency of polypropylene film will be greatly improved.

[0004] However, the preparation of ultra-wide polypropylene films faces many challenges, which are far beyond the scope of simple size enlargement. The complexity of this process lies mainly in that as the width of the film increases, higher requirements are placed on the uniformity of raw materials, the accuracy of processing equipment, and the stability of process control. What is more difficult is that as the width of the polypropylene film increases, the uniformity of the film's thickness, stress distribution, mechanical properties, etc. will be greatly reduced, and it is easy to produce defects such as fish eyes, crystal points, bubbles, micropores, tears, wrinkles, etc., which ultimately lead to the polypropylene film produced not meeting the use requirements.

[0005] Among the many factors that affect the preparation of ultra-wide polypropylene films, there is a special factor that directly affects the production of ultra-wide polypropylene films - the unevenness of the thickness of the cast sheet or intermediate film in the width direction. This defect is difficult to overcome by raw material quality control, processing equipment precision improvement, etc. As described in the Chinese patent with the publication (announcement) number CN 105916654B: In the part of the thermoplastic resin in the width direction of the film after the self-forming die is melt-extruded, since the thermoplastic resins exist adjacent to each other, the flow direction of the thermoplastic resin is restricted, so that the film is plane-stretched along the prescribed surface inside the thermoplastic resin, thereby suppressing the shrinkage of the film in the width direction and shrinking mainly in the thickness direction. On the other hand, in the parts of the thermoplastic resin at both ends of the width direction of the film after the self-forming die is melt-extruded, since there is no adjacent thermoplastic resin on the outer side, the thermoplastic resin flows freely, so that the film is uniaxially stretched with the prescribed axis inside the thermoplastic resin as the center, thereby shrinking the thermoplastic resin not only in the thickness direction, but also in the width direction. Therefore, in the formed film, the boundary between the central part in the width direction and the two ends in the width direction is concave in the thickness direction due to the difference in the shrinkage mode of the thermoplastic resin, thereby making the thickness thinner. Furthermore, when such a film is heated and stretched, there is a problem that cracks are easily generated in the thinner boundary part, which makes the entire film easy to break. This phenomenon is more obvious in the preparation process of ultra-wide polypropylene film, and has become an important factor limiting the increase in the width of polypropylene film.

[0006] In addition, for polypropylene film used in film capacitors, the performance requirements for the film are special. In the length direction of the wound film, it is usually hoped that the polypropylene film has a higher tensile strength and a lower elongation, and in the width direction of the wound film, it is expected to obtain a higher elongation.

[0007] In view of this, research on the preparation process of ultra-wide polypropylene film to make the production of high-quality ultra-wide polypropylene film possible is one of the technical problems that technicians in this field have been committed to solving for a long time. Summary of the invention

[0008] The object of the present invention is to provide a method for preparing a polypropylene film for film capacitors in view of the above-mentioned technical problems.

[0009] In view of this, the present invention provides a method for preparing a polypropylene film for a film capacitor, comprising the steps of:

[0010] S1, casting sheet preparation: the polypropylene film raw material is melted, extruded, cooled and formed to obtain a casting sheet;

[0011] S2, one-time synchronous stretching: after the formed casting sheet is preheated, it is subjected to one-time synchronous stretching. During the one-time synchronous stretching, the longitudinal stretching ratio is 1.4-2.5 times the transverse stretching ratio;

[0012] S3, one transverse stretching: the intermediate film after the one synchronous stretching is subjected to one transverse stretching. During the one transverse stretching process, the temperature distribution of the film in the transverse direction is as follows: the film temperature in the middle region is higher than the temperature in the two side regions;

[0013] S4, secondary transverse stretching: the film after the primary transverse stretching is subjected to secondary transverse stretching, and the temperature distribution of the film in the transverse direction during the secondary transverse stretching is as follows: the film temperature in the middle area is lower than the temperature in the areas on both sides;

[0014] S5, secondary synchronous stretching: performing secondary synchronous stretching on the film after secondary transverse stretching;

[0015] S6, post-processing: the film after secondary synchronous stretching is subjected to heat setting, cooling, edge trimming and corona treatment to obtain a finished product of ultra-wide polypropylene film for film capacitors.

[0016] Furthermore, in the step S2, the stretching ratio of the longitudinal stretching is about 6 to 8 times, and the stretching ratio of the transverse stretching is about 2.5 to 4 times.

[0017] Furthermore, in step S2, the temperature of the preheating treatment of the casting is 145-160°C, the preheating treatment time is 80-120s, and the temperature of the first synchronous stretching is 160-178°C.

[0018] Furthermore, the temperature distribution curve T1 of the film in the transverse direction during the first transverse stretching process is a curve that is positively correlated with the thickness of the film before the first transverse stretching.

[0019] Furthermore, the temperature distribution curve T1 of the film in the transverse direction is obtained by fitting according to the thickness distribution of the film in the width direction before the first transverse stretching, and the process is as follows:

[0020] (I) First, the film is divided into several equal parts in the width direction before the first transverse stretching, and the thickness data of the film in each width region is measured in turn to obtain the distribution data of the thickness of the film in the width direction before the first transverse stretching, and a scatter plot is drawn based on the data, the abscissa of the scatter plot is the width of the film, the center point of the width of the film is the 0 point, the left side of the 0 point is the negative direction, and the right side is the positive direction, and the ordinate of the scatter plot is the film thickness data corresponding to each width partition;

[0021] (II) Fitting and connecting the film thickness data corresponding to each width partition, we get Figure 2 The thickness distribution curve L1 of the film in the width direction is shown;

[0022] (III) Dividing the film into a middle area and two side areas according to the thickness distribution of the film in the width direction;

[0023] (IV) Determine the temperature distribution curve T1 of the film in the transverse direction based on the thickness distribution of the film in the width direction.

[0024] Furthermore, in the step (IV), the process of determining the temperature distribution curve T1 of the film in the transverse direction according to the thickness distribution of the film in the width direction is as follows:

[0025] (a) First, select a distance of 40% to 80% of the total width of the film from the center point in the width direction of the film to both sides as the reference distance, and use the film thickness data corresponding to each width partition within the reference distance as the sample for calculating the thickness reference value;

[0026] (b) Perform Anderson-Darling normality calculation on the thickness values ​​of each sample selected for calculation to obtain its overall mean value h0 and standard deviation δ1;

[0027] (c) The standard deviation δ1 is used to measure the discreteness of the array, and N1 times δ1 is used as the discrete standard level. The allowable deviation range of the film thickness before the first transverse stretching is obtained as: h0±N1.δ1;

[0028] (d) The temperature value of each point in the width direction of the film is determined according to the allowable deviation range of the film thickness before the first transverse stretching, and then the temperature distribution curve T1 is obtained by fitting the temperature values ​​of each point in the width direction of the film.

[0029] Furthermore, in the step (d), the temperature value at each point in the width direction of the film is determined as follows:

[0030] First, determine whether the thickness value of the point is within the allowable deviation range of the film thickness h0±N1.δ1:

[0031] If so, the temperature value of this point is the reference temperature value h 基准 ;

[0032] If not, the temperature value of this point is the reference temperature value h 基准 Correction value h 校正 .

[0033] Furthermore, the correction value h of the reference temperature value 校正 The calculation process is as follows:

[0034] Firstly, determine the correction amplitude △h of the film temperature value;

[0035] The reference temperature value h is determined according to the deviation between the actual thickness of the point and the overall average value h0 and the correction amplitude △h of the film temperature value. 基准 Correction value h校正 , where the correction value h 校正 = reference temperature value h 基准 ±α*correction amplitude △h, where the value range of α is 0~1.

[0036] Further, in the step S4, the temperature distribution curve T2 of the film in the transverse direction is determined according to the temperature distribution curve T1 of the film in the transverse direction in the step S3, and the process of determining the temperature distribution curve T2 of the film in the transverse direction is as follows:

[0037] 1) First, the film is divided into several equal parts in the width direction before the secondary transverse stretching, and the thickness data of the film in each width area is measured in turn to obtain the distribution data of the thickness of the film in the width direction before the secondary transverse stretching, and a scatter plot is drawn accordingly. The abscissa of the scatter plot is the width of the film, where the width center point of the film is 0, the left side of 0 is the negative direction, and the right side is the positive direction. The ordinate of the scatter plot is the film thickness data corresponding to each width partition;

[0038] 2) Fitting and connecting the film thickness data corresponding to each width partition to obtain the film thickness distribution curve L2 in the width direction;

[0039] 3) Divide the edge area of ​​the film according to the thickness distribution of the film in the width direction;

[0040] 4) Obtain the total width of the middle area and the areas on both sides of the curve L1 and the curve L2 respectively, record the total width of the middle area and the areas on both sides of the curve L1 as W1, record the total width of the middle area and the areas on both sides of the curve L2 as W2, and calculate the ratio between W1 and W2;

[0041] 5) According to the ratio of W1 and W2, the temperature distribution curve T1 of the film in the transverse direction during the first transverse stretching is proportionally stretched to obtain a temperature distribution curve T1', wherein the ratio of the distance X1 between the two ends of the curve T1 and the distance X2 between the two ends of the curve T1' is equal to the ratio of W1 and W2;

[0042] 5) Establish a temperature baseline B, where the temperature baseline B is a horizontal straight line, and the vertical coordinate temperature value of the temperature baseline B is 0.5~1℃ higher than the average temperature of the film during a transverse stretching process;

[0043] 6) The temperature distribution curve T1' is flipped mirror-symmetrically relative to the temperature baseline B to obtain the temperature distribution curve T2 of the film in the transverse direction during the secondary transverse stretching.

[0044] Furthermore, in step S5, the stretching ratio of the longitudinal stretching is about 2 to 4 times, and the stretching ratio of the transverse stretching is about 1.3 to 2 times.

[0045] The preparation method of the polypropylene film of the present invention can prepare an ultra-wide polypropylene film for film capacitors with uniform thickness, high longitudinal tensile strength, high transverse elongation and low thermal shrinkage. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 It is a schematic diagram of a method for preparing a polypropylene film for a film capacitor according to the present invention;

[0047] Figure 2 is a schematic diagram of a thickness distribution curve L1 of a film in the width direction before the first transverse stretching according to the present invention;

[0048] Figure 3 is a schematic diagram of a temperature distribution curve T1 of a film in a transverse direction during a single transverse stretching according to the present invention;

[0049] Figure 4 is a schematic diagram of a thickness distribution curve L2 of a film in the width direction before secondary transverse stretching according to the present invention;

[0050] Figure 5 It is a comparison diagram of the thickness distribution curve L1 of the film in the width direction before the first transverse stretching and the thickness distribution curve L2 of the film in the width direction before the second transverse stretching according to the present invention;

[0051] Figure 6 It is a schematic diagram of a curve T1' obtained by proportionally stretching the temperature distribution curve T1 of the film in the transverse direction during the first transverse stretching in the present invention;

[0052] Figure 7 is a schematic diagram of a temperature distribution curve T2 of a film in the transverse direction during secondary transverse stretching obtained by mirroring the curve T1' in the present invention;

[0053] Figure 8 The temperature distribution curve T1 is obtained by proportionally stretching the temperature distribution curve T1 in the width direction according to the width change in the present invention. a 、T1 b Schematic diagram of . DETAILED DESCRIPTION

[0054] The following will be combined with the drawings in the embodiments of the present application to clearly describe the technical solutions in the embodiments of the present application. Obviously, the described embodiments are only part of the embodiments of the present application, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments in the present application belong to the scope of protection of this application.

[0055] It should be noted that, in the present application, the terms "include", "comprise" or any other variant thereof are intended to cover non-exclusive inclusion, so that a process, method, article or device including a series of elements includes not only those elements, but also other elements not explicitly listed, or also includes elements inherent to such process, method, article or device. In the absence of further restrictions, an element defined by the sentence "includes one..." does not exclude the presence of other identical elements in the process, method, article or device including the element. In addition, it should be noted that the scope of the method and device in the embodiment of the present application is not limited to performing functions in the order shown or discussed, and may also include performing functions in a substantially simultaneous manner or in reverse order according to the functions involved, for example, the described method may be performed in an order different from that described, and various steps may also be added, omitted, or combined. In addition, the features described with reference to certain examples may be combined in other examples.

[0056] like Figure 1 The method for preparing a polypropylene film for a film capacitor comprises the following steps:

[0057] S1, casting sheet preparation: the polypropylene film raw material is melted, extruded, cooled and formed to obtain a casting sheet;

[0058] S2, one-time synchronous stretching: after the formed casting sheet is preheated, it is subjected to one-time synchronous stretching. During the one-time synchronous stretching, the longitudinal stretching ratio is 1.4-2.5 times the transverse stretching ratio;

[0059] S3, one transverse stretching: the intermediate film after the one synchronous stretching is subjected to one transverse stretching. During the one transverse stretching process, the temperature distribution of the film in the transverse direction is as follows: the film temperature in the middle region is higher than the temperature in the two side regions;

[0060] S4, secondary transverse stretching: the film after the primary transverse stretching is subjected to secondary transverse stretching, and the temperature distribution of the film in the transverse direction during the secondary transverse stretching is as follows: the film temperature in the middle area is lower than the temperature in the areas on both sides;

[0061] S5, secondary synchronous stretching: performing secondary synchronous stretching on the film after secondary transverse stretching;

[0062] S6, post-processing: the film after secondary synchronous stretching is subjected to heat setting, cooling, edge trimming and corona treatment to obtain a finished product of ultra-wide polypropylene film for film capacitors.

[0063] As some embodiments of the present invention, the polypropylene film raw material is high-purity electrical grade polypropylene, with an isotacticity greater than 98%, an ash content less than 20 ppm, and a melt index of 2.0-8.5 g / 10 min.

[0064] As some other embodiments of the present invention, the polypropylene film raw material includes, by weight: 70-100 parts of linear polypropylene, 20-30 parts of long-chain branched polypropylene, 1-3 parts of nano-silicon dioxide, and 10-20 parts of low-density linear polyethylene.

[0065] Preferably, the weight average molecular weight of the linear polypropylene is 2×10 5 ~4×10 5 , molecular weight distribution M w / M n The viscosity is 3 to 5, and the melt index is 3.0 to 10.0 g / min.

[0066] Preferably, the low-density linear polyethylene has a melt index of 1.0 to 6.0 g / min and a density of 0.88 to 0.91 g / cm 3 .

[0067] As some examples of the present invention, step S1 includes: drying the raw materials of the polypropylene film, mixing them evenly, and then adding them into a twin-screw extruder, melting them at 240~260°C to form a uniform melt, and then continuously extruding them through a T-shaped die at 230~250°C, and cooling them through a cooling roller at 80~90°C to obtain a cast sheet.

[0068] Furthermore, in step S2, the temperature of the preheating treatment of the casting is 145-160° C., and the preheating treatment time is 80-120 seconds.

[0069] Preferably, in step S2, the slab preheating zone is divided into three sections, and the temperatures of each section along the feeding direction are: 145-152°C, 150-155°C, and 155-160°C.

[0070] Furthermore, in the step S2, the temperature of the synchronous stretching is 160-178°C.

[0071] Preferably, in step S2, the stretching area of ​​the synchronous stretching is divided into four sections, and the temperatures of each section along the feeding direction are: 160-165°C, 163-170°C, 168-175°C, and 172-178°C.

[0072] Preferably, in step S2, the stretching ratio of the longitudinal stretching is about 6 to 8 times, and the stretching ratio of the transverse stretching is about 2.5 to 4 times.

[0073] Furthermore, in step S3, the difference between the film temperature in the middle area and the film temperature in the two side areas is ≤5-10°C.

[0074] Preferably, Figure 3As shown, during the first transverse stretching process in step S3, the temperature distribution of the film in the transverse direction is shown as curve T1. The temperature distribution curve T1 of the film in the transverse direction is a curve that is positively correlated with the thickness of the film before the first transverse stretching.

[0075] Specifically, the temperature distribution curve T1 of the film in the transverse direction can be obtained by fitting according to the thickness distribution of the film in the width direction before the first transverse stretching, and the specific process is as follows:

[0076] (I) First, the film is divided into several equal parts in the width direction before the first transverse stretching, and the thickness data of the film in each width region is measured in turn to obtain the distribution data of the thickness of the film in the width direction before the first transverse stretching, and a scatter plot is drawn based on the data, the abscissa of the scatter plot is the width of the film, the center point of the width of the film is the 0 point, the left side of the 0 point is the negative direction, and the right side is the positive direction, and the ordinate of the scatter plot is the film thickness data corresponding to each width partition;

[0077] (II) Fitting and connecting the film thickness data corresponding to each width partition, we get Figure 2 The thickness distribution curve L1 of the film in the width direction is shown;

[0078] (III) Dividing the film into a middle area and two side areas according to the thickness distribution of the film in the width direction;

[0079] (IV) Determine the temperature distribution curve T1 of the film in the transverse direction based on the thickness distribution of the film in the width direction.

[0080] As some examples of the present invention, in step (I), the film may be divided into several equal parts along the width direction according to the temperature control accuracy requirements, such as 30 parts, 50 parts or 100 parts, etc. It can be understood that the more equal parts the film is divided into, the closer the measured thickness distribution curve L1 is to the actual situation, and the higher the subsequent temperature control accuracy is.

[0081] It should be noted that in the step (III), due to the gripping and fixing effect of the clamping jaws during stretching, there is a certain width of film on the edge areas on both sides of the film, and the thickness is much higher than the rest of the film. This part of the film will be cut off after the film is prepared. Therefore, no special attention is required during the preparation process, and its temperature can be kept near the average temperature of the film. Preferably, in order to ensure the stability and firmness of the clamping of the edge area, the film temperature of the edge area can be kept at a level slightly lower than the average temperature, such as 2~5° lower than the average temperature. In view of this, in the present invention, the "two side areas" do not include the above-mentioned "edge areas" that will be cut off after the preparation is completed. Generally, the width of the "edge area" on one side is about 5~20cm.

[0082] On this basis, the division method of the "middle area" and "two side areas" described in the present invention is as follows:

[0083] First, according to the thickness distribution curve L1 of the film in the width direction before the first transverse stretching, the dividing lines Z1 and Z2 between the middle area and the two side areas, as well as the dividing lines S1 and S2 between the two side areas and the edge area are set. Among them, the thickness of the film between Z1 and Z2 is relatively uniform, and its value is closer to the average thickness of the film, while the film between S1 and S2 has the phenomenon of necking, etc., so the thickness is thinner. During the first transverse stretching, the film temperature in this area is also relatively low.

[0084] As some examples of the present invention, the average thickness of the film can be firstly Figure 2 A horizontal straight line A is set in the middle, and the ordinate of the straight line A is 1.5 to 3 times the average thickness of the film. There are two intersections between the straight line A and the two ends of the curve L1. Vertical straight lines are drawn through the two intersections to obtain the boundary lines S1 and S2 between the two side areas and the edge areas. Then, by observing the change in film thickness between the boundary lines S1 and S2, the thinner areas on both sides of the film are set as "side areas", and the area in the middle with more uniform thickness is set as "middle area". Preferably, the "middle area" and "side areas" are symmetrically arranged in the width direction of the film.

[0085] Furthermore, in the step (IV), the process of determining the temperature distribution curve T1 of the film in the transverse direction according to the thickness distribution of the film in the width direction is as follows:

[0086] (a) First, select a distance of 40% to 80% of the total width of the film from the center point in the width direction of the film to both sides as the reference distance, and use the film thickness data corresponding to each width partition within the reference distance as the sample for calculating the thickness reference value; Note that the reference distance here is a different concept from the aforementioned "middle area", and the "reference distance" mainly emphasizes the sampling width;

[0087] (b) Perform Anderson-Darling normality calculation on the thickness values ​​of each sample selected for calculation to obtain its overall mean value h0 and standard deviation δ1;

[0088] (c) The standard deviation δ1 is used to measure the discreteness of the array, and N1 times δ1 is used as the discrete standard level. The allowable deviation range of the film thickness before the first transverse stretching is obtained as: h0±N1.δ1, where the value of N1 is 0.8~2;

[0089] (d) The temperature value of each point in the width direction of the film is determined based on the allowable deviation range of the film thickness before the first transverse stretching, the overall average value h0 and the standard deviation δ1, and then the temperature distribution curve T1 is obtained by fitting the temperature values ​​of each point in the width direction of the film.

[0090] Furthermore, in the step (d), the temperature value of each point in the width direction of the film is determined in the following manner:

[0091] First, determine whether the thickness value of the point is within the allowable deviation range of the film thickness h0±N1.δ1:

[0092] If so, the temperature value of this point is the reference temperature value h 基准 , wherein the reference temperature value h 基准 The specific value of can be obtained through experiments, etc.

[0093] If not, the temperature value of this point is the reference temperature value h 基准 Correction value h 校正 .

[0094] Among them, the correction value h of the reference temperature value 校正 The calculation process is as follows:

[0095] Firstly, determine the correction amplitude △h of the film temperature value;

[0096] The reference temperature value h is determined according to the deviation between the actual thickness of the point and the overall average value h0 and the correction amplitude △h of the film temperature value. 基准 Correction value h 校正 , where the correction value h 校正 = reference temperature value h 基准 ±α*correction amplitude △h.

[0097] Generally speaking, the greater the deviation between the actual thickness of a certain width area and the overall average value h0, the greater the correction value h of the width area. 校正 With reference temperature h 基准 The larger the deviation between them, the larger the value of α, where the value range of α is 0~1. In addition, the correction value h of a certain width area 校正 It is positively correlated with the actual thickness of the width area, that is, when the actual thickness of the width area is greater than the average value h0, the temperature correction value h of the width area 校正 = reference temperature value h 基准 +α*correction amplitude △h; when the actual thickness of the width area is less than the average value h0, the temperature correction value h of the width area 校正 = reference temperature value h 基准 -α* correction amplitude △h.

[0098] As some examples of the present invention, the correction amplitude △h of the film temperature value is 5-10°C, which represents the correction value h in step (d) 校正 With reference temperature h 基准 The correction amplitude △h of the film temperature value can be selected according to the material properties of the film, the overall thickness of the film or through experiments.

[0099] In the process of determining the temperature distribution curve T1 described in the present invention, two processing methods are used for the deviation between the thickness measurement value of each width area and the overall average value h0. For the area where the thickness value is within the allowable deviation range h0±N1.δ1 of the thickness h, a uniform temperature, i.e., the reference temperature value h0, is used during a transverse stretching process. 基准 The film temperature in most areas can be controlled to a certain value by processing, which effectively reduces the temperature control accuracy and difficulty, and will not have an adverse effect on the film stretching process and performance differences. For areas where the thickness value is outside the thickness h deviation allowable range h0±N1.δ1, the corresponding temperature reduction treatment is carried out according to the thickness deviation degree. In this way, the degree of deformation during the stretching process can be improved by reducing the temperature, so that the thickness of the film after a transverse stretching, especially the thickness distribution in the width direction, tends to be more uniform.

[0100] Furthermore, in the implementation of the method for preparing the polypropylene film for film capacitors described in the present invention, the stretching zone of the transverse stretching device used should be provided with a matrix temperature control device, such as a matrix temperature heating device, by which the film in the stretching zone can be precisely heated and temperature controlled in different regions.

[0101] It should be noted that, in the present invention, the temperature distribution curve T1 of the film in the transverse direction during the first transverse stretching process is the temperature of the film when it enters the stretching zone from the preheating zone, that is, the film temperature at the junction of the preheating zone and the stretching zone in the first transverse stretching device. Similarly, during the second transverse stretching process, the temperature distribution curve T2 of the film in the transverse direction is the temperature of the film when it enters the stretching zone from the preheating zone, that is, the film temperature at the junction of the preheating zone and the stretching zone in the second transverse stretching device.

[0102] Specifically, during the first transverse stretching and the second transverse stretching, the film may be preheated at 120-150°C. When the film enters the stretching zone from the preheating zone, the temperature of the film may be precisely controlled according to the temperature distribution curve T1 or T2, so that the film temperature distribution at the junction of the preheating zone and the stretching zone in the transverse stretching device satisfies the temperature distribution curve T1 or T2 in the transverse direction. After entering the stretching zone, Figure 8As shown, the temperature distribution curve T1 or T2 can be elongated in equal proportion in the width direction according to the width change, but not scaled in the ordinate direction representing the temperature value. In this way, a series of temperature distribution curves T1 after elongation in different proportions can be obtained. a 、T1 b 、T1 c , ..., and T2 a 、T2 b 、T2 c , ... and according to the elongated temperature distribution curve T1 a 、T1 b 、T1 c , ..., or T2 a 、T2 b 、T2 c , ... During the first stretching or second stretching process, the transverse temperature distribution of the film at the corresponding width is regulated.

[0103] Preferably, in step S3, the temperature range of the film during the first transverse stretching is 155-165°C, and the stretching ratio is 1.3-1.8 times, that is, the temperature distribution curves T1, T1 a 、T1 b 、T1 c The maximum value of... must be less than 165℃, and the minimum value must be greater than 155℃.

[0104] Furthermore, in step S4, the temperature of the secondary transverse stretching is 155-168°C, and the stretching ratio is 2-4 times, that is, the temperature distribution curve T2 a 、T2 b 、T2 c The maximum value of... must be less than 168℃, and the minimum value must be greater than 155℃.

[0105] In actual operation, due to the limitation of temperature control accuracy, according to the temperature distribution curve, such as curve T1, T1 a , T2, T2 a When the temperature of the film is regulated, it is preferred that the deviation between the actual temperature of the film and the temperature value on the set temperature distribution curve is no more than 0.5°C.

[0106] Furthermore, in the step S4, the temperature distribution curve T2 of the film in the transverse direction is determined according to the temperature distribution curve T1 of the film in the transverse direction in the step S3.

[0107] Specifically, in step S4, the process of determining the temperature distribution curve T2 of the film in the transverse direction is as follows:

[0108] 1) First, the film is divided into several equal parts in the width direction before the secondary transverse stretching, and the thickness data of the film in each width area is measured in turn to obtain the distribution data of the thickness of the film in the width direction before the secondary transverse stretching, and a scatter plot is drawn accordingly. The abscissa of the scatter plot is the width of the film, where the width center point of the film is 0, the left side of 0 is the negative direction, and the right side is the positive direction. The ordinate of the scatter plot is the film thickness data corresponding to each width partition;

[0109] 2) Fit and connect the film thickness data corresponding to each width partition to obtain the following Figure 4 The thickness distribution curve L2 of the film in the width direction is shown;

[0110] 3) If Figure 4 As shown, the edge area of ​​the film is divided according to the thickness distribution of the film in the width direction;

[0111] 4) If Figure 5 As shown, the thickness distribution curve L1 of the film in the width direction before the first transverse stretching and the thickness distribution curve L2 of the film in the width direction before the second transverse stretching are plotted in the same plane rectangular coordinate system, and the total width of the middle area and the two side areas on the curve L1 and the curve L2 are respectively obtained, that is, the distance between the straight lines S1 and S2, and the total width of the middle area and the two side areas on the curve L1 is recorded as W1, and the total width of the middle area and the two side areas on the curve L2 is recorded as W2, and the ratio between W1 and W2 is calculated;

[0112] 5) If Figure 6 As shown, according to the ratio of W1 and W2, the temperature distribution curve T1 of the film in the transverse direction during the first transverse stretching is proportionally stretched to obtain the temperature distribution curve T1', wherein the ratio between the distance X1 between the two ends of the curve T1 and the distance X2 between the two ends of the curve T1' is equal to the ratio of W1 and W2, that is, W1 / W2=X1 / X2;

[0113] 5) If Figure 7 As shown, a temperature baseline B is established, wherein the temperature baseline B is a horizontal straight line, and its ordinate is a temperature value above the average temperature of the film during a transverse stretching process; preferably, the ordinate temperature value of the temperature baseline B is 0.5-1°C higher than the average temperature of the film during a transverse stretching process;

[0114] 6) If Figure 7 As shown, the temperature distribution curve T1' is flipped mirror-symmetrically with respect to the temperature base line B to obtain the temperature distribution curve T2 of the film in the transverse direction during the secondary transverse stretching.

[0115] In actual operation, the film width corresponding to the transverse temperature distribution curve T2 of the film obtained in the above step 6) may be slightly larger or smaller than the actual width of the film, which mainly depends on the deformation of the edge area gripped and fixed by the clamps. At this time, it is only necessary to shorten the curves at both ends of the curve T2 by the corresponding width dimension or fill in the missing temperature curve according to the temperature at the end of the curve T2, so that the transverse temperature distribution curve T2 of the film is finally consistent with the actual width of the film before the secondary transverse stretching.

[0116] In the secondary transverse stretching process described in the present invention, the temperature distribution curve T2 is used to perform "delay compensation" on the area with lower temperature during the first transverse stretching. This can not only improve the consistency of the total amount of heat received by each area of ​​the film during the stretching process and improve the consistency of its performance in the width direction, but also alleviate the difference in transverse shrinkage rate between different areas of the film in the width direction.

[0117] Furthermore, in the step S5, the temperature of the secondary synchronous stretching is 165-180°C.

[0118] Preferably, in step S5, the stretching area of ​​the secondary synchronous stretching is divided into four sections, and the temperatures of each section along the feeding direction are: 165-170°C, 167-175°C, 172-178°C, and 175-180°C.

[0119] Preferably, in step S5, the stretching ratio of the longitudinal stretching is about 2 to 4 times, and the stretching ratio of the transverse stretching is about 1.3 to 2 times.

[0120] More preferably, during the secondary simultaneous stretching, the stretching ratio of the longitudinal stretching is 2 to 3 times the stretching ratio of the transverse stretching.

[0121] As some examples of the present invention, the thickness of the ultra-wide polypropylene film for the film capacitor of the present invention is 1-50 um.

[0122] Furthermore, after the secondary synchronous stretching, the film is subjected to a heat setting treatment at 150-160° C., cooling, edge trimming and corona treatment to obtain a finished product.

[0123] The following is an example of the method for preparing the polypropylene film for film capacitors of the present invention:

[0124] Example 1

[0125] A method for preparing an ultra-wide polypropylene film for capacitors comprises the following steps:

[0126] S1, casting sheet preparation: high-purity electrical grade polypropylene is dried and mixed evenly, then added into a twin-screw extruder, melted at 240°C to form a uniform melt, then continuously extruded through a T-die at 230°C, and cooled through a cooling roller at 80°C to obtain a casting sheet;

[0127] S2, one-time synchronous stretching: after the formed casting sheet is preheated at 145-160°C for 80s, it is subjected to one-time synchronous stretching, the temperature of one-time synchronous stretching is 160-178°C, the stretching ratio of the longitudinal stretching is about 8 times, and the stretching ratio of the transverse stretching is about 4 times;

[0128] S3, primary transverse stretching: determining the temperature distribution curve T1 of the film in the transverse direction during the primary transverse stretching according to the thickness distribution of the film in the width direction before the primary transverse stretching according to the method described in the aforementioned steps (I) to (IV), and heating the film according to the temperature distribution curve T1 of the film in the transverse direction during the primary transverse stretching, wherein the temperature of the center point of the middle area during the primary transverse stretching is 162°C, the temperature distribution of the thinnest parts of the film in the two side areas is 158.9°C and 158.5°C, and the stretching ratio is 1.5 times;

[0129] S4, secondary transverse stretching: according to the method described in the above steps 1) to 6), the temperature distribution curve T2 of the film in the transverse direction during the secondary transverse stretching is determined according to the temperature distribution curve T1 of the film in the transverse direction during the primary transverse stretching, and the film is heated according to the temperature distribution curve T2 of the film in the transverse direction during the secondary transverse stretching, the temperature of the center point of the middle area during the secondary transverse stretching is 163.5°C, the temperature distribution of the thinnest parts of the film thickness in the two side areas is 166.7°C and 166.2°C, and the stretching ratio is 2 times;

[0130] S5, secondary synchronous stretching: the film after secondary transverse stretching is subjected to secondary synchronous stretching, the temperature of the secondary synchronous stretching is 165-180°C, the stretching ratio of the longitudinal stretching is about 3 times, and the stretching ratio of the transverse stretching is about 1.5 times;

[0131] S6, post-processing: the film after secondary synchronous stretching is heat-set at 150°C, cooled, trimmed and corona treated to obtain a finished ultra-wide polypropylene film for film capacitors, and the width of the finished product is 8m.

[0132] Example 2

[0133] A method for preparing an ultra-wide polypropylene film for capacitors comprises the following steps:

[0134] S1, preparation of casting sheet: 80 parts of linear polypropylene, 20 parts of long-chain branched polypropylene, 2 parts of nano-silicon dioxide, and 15 parts of low-density linear polyethylene were dried and mixed evenly, and then added into a twin-screw extruder, melt-treated at 250°C to form a uniform melt, and then continuously extruded through a T-die at 240°C, and cooled through a cooling roller at 85°C to obtain a casting sheet;

[0135] S2, one-time synchronous stretching: the formed casting sheet is preheated at 145-160°C for 100s, and then subjected to one-time synchronous stretching. The temperature of one-time synchronous stretching is 160-178°C, the stretching ratio of the longitudinal stretching is about 7 times, and the stretching ratio of the transverse stretching is about 3 times;

[0136] S3, primary transverse stretching: determining the temperature distribution curve T1 of the film in the transverse direction during the primary transverse stretching according to the thickness distribution of the film in the width direction before the primary transverse stretching according to the method described in the aforementioned steps (I) to (IV), and heating the film according to the temperature distribution curve T1 of the film in the transverse direction during the primary transverse stretching, wherein the temperature of the center point of the middle area during the primary transverse stretching is 162.7°C, the temperature distribution of the thinnest parts of the film in the two side areas is 157.5°C and 157.3°C, and the stretching ratio is 1.8 times;

[0137] S4, secondary transverse stretching: according to the method described in the above steps 1) to 6), the temperature distribution curve T2 of the film in the transverse direction during the secondary transverse stretching is determined according to the temperature distribution curve T1 of the film in the transverse direction during the primary transverse stretching, and the film is heated according to the temperature distribution curve T2 of the film in the transverse direction during the secondary transverse stretching. During the secondary transverse stretching, the temperature of the center point of the middle area is 163.2°C, and the temperature distribution of the thinnest parts of the film thickness in the two side areas is 167.1°C and 167.2°C, and the stretching ratio is 3 times;

[0138] S5, secondary synchronous stretching: the film after secondary transverse stretching is subjected to secondary synchronous stretching, the temperature of the secondary synchronous stretching is 165-180°C, the stretching ratio of the longitudinal stretching is about 4 times, and the stretching ratio of the transverse stretching is about 1.3 times;

[0139] S6, post-processing: After the secondary synchronous stretching, the film is heat-set at 155°C, cooled, trimmed and corona treated to obtain a finished ultra-wide polypropylene film for film capacitors, with a width of 7.3m.

[0140] Example 3

[0141] A method for preparing an ultra-wide polypropylene film for capacitors comprises the following steps:

[0142] S1, preparation of casting sheet: 100 parts of linear polypropylene, 30 parts of long-chain branched polypropylene, 3 parts of nano-silicon dioxide, and 10 parts of low-density linear polyethylene were dried and mixed evenly, and then added into a twin-screw extruder, melt-treated at 260°C to form a uniform melt, and then continuously extruded through a T-die at 250°C, and cooled through a cooling roller at 90°C to obtain a casting sheet;

[0143] S2, one-time synchronous stretching: after the formed casting sheet is preheated at 145-160°C for 120s, it is subjected to one-time synchronous stretching, the temperature of one-time synchronous stretching is 160-178°C, the stretching ratio of the longitudinal stretching is about 6 times, and the stretching ratio of the transverse stretching is about 2.5 times;

[0144] S3, primary transverse stretching: determining the temperature distribution curve T1 of the film in the transverse direction during the primary transverse stretching according to the thickness distribution of the film in the width direction before the primary transverse stretching according to the method described in the aforementioned steps (I) to (IV), and heating the film according to the temperature distribution curve T1 of the film in the transverse direction during the primary transverse stretching, wherein the temperature of the center point of the middle area during the primary transverse stretching is 160.3°C, the temperature distribution of the thinnest parts of the film thickness in the two side areas is 155.4°C and 155.6°C, and the stretching ratio is 1.3 times;

[0145] S4, secondary transverse stretching: according to the method described in the above steps 1) to 6), the temperature distribution curve T2 of the film in the transverse direction during the secondary transverse stretching is determined according to the temperature distribution curve T1 of the film in the transverse direction during the primary transverse stretching, and the film is heated according to the temperature distribution curve T2 of the film in the transverse direction during the secondary transverse stretching. During the secondary transverse stretching, the temperature of the center point of the middle area is 161.8°C, and the temperature distribution of the thinnest parts of the film thickness in the two side areas is 166.8°C and 167°C, and the stretching ratio is 4 times;

[0146] S5, secondary synchronous stretching: the film after secondary transverse stretching is subjected to secondary synchronous stretching, the temperature of the secondary synchronous stretching is 165-180°C, the stretching ratio of the longitudinal stretching is about 4 times, and the stretching ratio of the transverse stretching is about 2 times;

[0147] S6, post-processing: After the secondary synchronous stretching, the film is heat-set at 160°C, cooled, trimmed and corona treated to obtain a finished ultra-wide polypropylene film for film capacitors, and the width of the finished product is 6.5m.

[0148] Comparative Example 1

[0149] A method for preparing an ultra-wide polypropylene film for capacitors, which differs from the above-mentioned embodiment 2 only in that: in step S3, a uniform heat source is used to heat the film to maintain the temperature of the film in the width direction at 162.7±0.5°C.

[0150] Comparative Example 2

[0151] A method for preparing an ultra-wide polypropylene film for capacitors, which differs from the above-mentioned embodiment 2 only in that: in step S4, a uniform heat source is used to heat the film to maintain the temperature of the film in the width direction at 163.2±0.5°C.

[0152] Comparative Example 3

[0153] A method for preparing an ultra-wide polypropylene film for capacitors, which differs from the above-mentioned embodiment 2 only in that: in step S2, a uniform heat source is used to heat the film, and the temperature of the film in the width direction is maintained at 162.7±0.5°C; in step S3, a uniform heat source is used to heat the film, and the temperature of the film in the width direction is maintained at 163.2±0.5°C.

[0154] Performance Testing:

[0155] The polypropylene films obtained in the above Examples 1 to 3 and Comparative Examples 1 to 3 were tested, and the results were as follows:

[0156]

[0157] The embodiments of the present application are described above in conjunction with the accompanying drawings. In the absence of conflict, the embodiments in the present application and the features in the embodiments can be combined with each other. The present application is not limited to the above-mentioned specific implementation methods. The above-mentioned specific implementation methods are merely illustrative and not restrictive. Under the guidance of the present application, ordinary technicians in this field can also make many forms without departing from the purpose of the present application and the scope of protection of the claims, all of which are within the protection of the present application.

Claims

1. A method for preparing a polypropylene film for a film capacitor, characterized in that: Includes steps: S1, casting sheet preparation: the polypropylene film raw material is melted, extruded, cooled and formed to obtain a casting sheet; S2, one-time synchronous stretching: after the formed casting sheet is preheated, it is subjected to one-time synchronous stretching. During the one-time synchronous stretching, the longitudinal stretching ratio is 1.4-2.5 times the transverse stretching ratio; S3, one transverse stretching: the intermediate film after the one synchronous stretching is subjected to one transverse stretching. During the one transverse stretching process, the temperature distribution of the film in the transverse direction is as follows: the film temperature in the middle region is higher than the temperature in the two side regions; S4, secondary transverse stretching: the film after the primary transverse stretching is subjected to secondary transverse stretching, and the temperature distribution of the film in the transverse direction during the secondary transverse stretching is as follows: the film temperature in the middle area is lower than the temperature in the areas on both sides; S5, secondary synchronous stretching: performing secondary synchronous stretching on the film after secondary transverse stretching; S6, post-processing: the film after secondary synchronous stretching is subjected to heat setting, cooling, edge trimming and corona treatment to obtain a finished product of ultra-wide polypropylene film for film capacitors; The temperature distribution curve T1 of the film in the transverse direction during the first transverse stretching process is a curve that is positively correlated with the thickness of the film before the first transverse stretching. The temperature distribution curve T1 of the film in the transverse direction is obtained by fitting according to the thickness distribution of the film in the width direction before the first transverse stretching. The process is as follows: (I) First, the film is divided into several equal parts in the width direction before the first transverse stretching, and the thickness data of the film in each width region is measured in turn to obtain the distribution data of the thickness of the film in the width direction before the first transverse stretching, and a scatter plot is drawn based on the data, the abscissa of the scatter plot is the width of the film, the center point of the width of the film is the 0 point, the left side of the 0 point is the negative direction, and the right side is the positive direction, and the ordinate of the scatter plot is the film thickness data corresponding to each width partition; (II) Fitting and connecting the film thickness data corresponding to each width partition to obtain the film thickness distribution curve L1 in the width direction; (III) Dividing the film into a middle area and two side areas according to the thickness distribution of the film in the width direction; (IV) determining the temperature distribution curve T1 of the film in the transverse direction according to the thickness distribution of the film in the width direction; In the step (IV), the process of determining the temperature distribution curve T1 of the film in the transverse direction according to the thickness distribution of the film in the width direction is as follows: (a) First, select a distance of 40% to 80% of the total width of the film from the center point in the width direction of the film to both sides as the reference distance, and use the film thickness data corresponding to each width partition within the reference distance as the sample for calculating the thickness reference value; (b) Perform Anderson-Darling normality calculation on the thickness values ​​of each sample selected for calculation to obtain its overall mean value h0 and standard deviation δ1; (c) The standard deviation δ1 is used to measure the discreteness of the array, and N1 times δ1 is used as the discrete standard level. The allowable deviation range of the film thickness before the first transverse stretching is obtained as: h0±N1·δ1; (d) determining the temperature value of each point in the width direction of the film according to the allowable deviation range of the film thickness before the first transverse stretching, and then obtaining the temperature distribution curve T1 by fitting the temperature values ​​of each point in the width direction of the film; In the step (d), the temperature value at each point in the width direction of the film is determined as follows: First, determine whether the thickness value of the point is within the allowable deviation range of the film thickness h0±N1·δ1: If so, the temperature value of this point is the reference temperature value h 基准 ; If not, the temperature value of this point is the reference temperature value h 基准 Correction value h 校正 .

2. The method for preparing a polypropylene film for a film capacitor according to claim 1, characterized in that: In the step S2, the stretching ratio of the longitudinal stretching is 6 to 8 times, and the stretching ratio of the transverse stretching is 2.5 to 4 times.

3. The method for preparing a polypropylene film for a film capacitor according to claim 1, wherein in step S2, the temperature of the preheating treatment of the casting is 145-160°C, the preheating treatment time is 80-120s, and the temperature of the first synchronous stretching is 160-178°C.

4. The method for preparing a polypropylene film for a film capacitor according to claim 1, characterized in that: The correction value h of the reference temperature value 校正 The calculation process is as follows: Firstly, determine the correction amplitude △h of the film temperature value; The reference temperature value h is determined according to the deviation between the actual thickness of the point and the overall average value h0 and the correction amplitude △h of the film temperature value. 基准 Correction value h 校正 , where the correction value h 校正 = reference temperature value h 基准 ±α·correction amplitude △h, where the value range of α is 0~1.

5. The method for preparing a polypropylene film for a film capacitor according to claim 4, characterized in that: In the step S4, the temperature distribution curve T2 of the film in the transverse direction is determined according to the temperature distribution curve T1 of the film in the transverse direction in the step S3. The process of determining the temperature distribution curve T2 of the film in the transverse direction is as follows: 1) First, the film is divided into several equal parts in the width direction before the secondary transverse stretching, and the thickness data of the film in each width area is measured in turn to obtain the distribution data of the thickness of the film in the width direction before the secondary transverse stretching, and a scatter plot is drawn accordingly. The abscissa of the scatter plot is the width of the film, where the center point of the width of the film is the 0 point, the left side of the 0 point is the negative direction, and the right side is the positive direction. The ordinate of the scatter plot is the film thickness data corresponding to each width partition; 2) Fitting and connecting the film thickness data corresponding to each width partition to obtain the film thickness distribution curve L2 in the width direction; 3) Divide the edge area of ​​the film according to the thickness distribution of the film in the width direction; 4) Obtain the total width of the middle area and the areas on both sides of the curve L1 and the curve L2 respectively, record the total width of the middle area and the areas on both sides of the curve L1 as W1, record the total width of the middle area and the areas on both sides of the curve L2 as W2, and calculate the ratio between W1 and W2; 5) According to the ratio of W1 and W2, the temperature distribution curve T1 of the film in the transverse direction during the first transverse stretching is proportionally stretched to obtain a temperature distribution curve T1', wherein the ratio of the distance X1 between the two ends of the curve T1 and the distance X2 between the two ends of the curve T1' is equal to the ratio of W1 and W2; 6) Establish a temperature baseline B, where the temperature baseline B is a horizontal straight line, and the vertical coordinate temperature value of the temperature baseline B is 0.5~1℃ higher than the average temperature of the film during a transverse stretching process; 7) The temperature distribution curve T1' is flipped mirror-symmetrically relative to the temperature baseline B to obtain the temperature distribution curve T2 of the film in the transverse direction during the secondary transverse stretching.

6. The method for preparing a polypropylene film for a film capacitor according to claim 1, characterized in that: In the step S5, the stretching ratio of the longitudinal stretching is 2 to 4 times, and the stretching ratio of the transverse stretching is 1.3 to 2 times.

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