Polypropylene film and preparation method thereof, metallized film and film capacitor

Abstract

The invention relates to the technical field of polypropylene films, in particular to a polypropylene film and a preparation method thereof, a metallized film and a film capacitor. The method comprises the following steps: carrying out melt extrusion and curtain coating cooling on polypropylene to obtain a polypropylene film; the polypropylene film is subjected to two-way stretching, and the polypropylene film is obtained; the temperature of a die head of an extruder for melt extrusion is T1, the temperature of a cold roller for curtain coating cooling is T2, and the condition that the difference value between the temperature of the die head and the temperature of the cold roller is the degree of supercooling delta T is met; the ratio of the supercooling degree delta T to the cold roller temperature T2 is greater than or equal to 1.9 and less than or equal to 2.25; wherein the delta T is equal to T1-T2. The polypropylene film prepared by the preparation method is low in corona surface fluctuation degree, and the corona surface topology meets the conditions that Sz is more than or equal to 300 nm and less than or equal to 700 nm, and SSK is more than or equal to-0.3 and less than or equal to 0. The capacitor is obtained by coating and winding the polypropylene film, the ESR of the device is low, the loss is low, and the polypropylene film is particularly suitable for high-frequency pulse capacitors and related fields.

Description

Technical Field

[0001] The present invention relates to the technical field of polypropylene films, and particularly to a polypropylene film, a preparation method thereof, a metallized film and a film capacitor. Background Art

[0002] Isotactic polypropylene, with a highly ordered isotactic optical structure and a non-polar chemical structure, has extremely strong crystallization ability. The ordered chain structure and crystallization behavior endow the polypropylene film with characteristics such as high breakdown field strength, low dielectric loss and high charge-discharge efficiency. Pulse capacitors, with characteristics such as high electromagnetic pulse tolerance, low inductance, fast charge and discharge, etc., are widely used in the fields of pulse power technology, lasers, radars, etc., and can withstand a relatively high charge-discharge frequency and high voltage in a short period. Polypropylene film is the most commonly used basic dielectric material in current pulse capacitors.

[0003] In the existing technical means, the polypropylene raw material is processed through processes such as melt extrusion, casting and cooling into a thick sheet, and biaxial stretching to complete the preparation of the base film. Through the complex evolution of the crystalline structure inside the base film, it has a uniform fibrous crystal network structure, enabling it to withstand a sufficiently high voltage; while a large number of roughening rings (topological structures) are generated on the surface of the base film, so that the film has a microscopic morphology with complex surface undulations and a certain roughness. Then, the base film undergoes a continuous post-treatment process, and corona treatment is performed on one of the two sides, introducing polar groups on this side to enhance its adhesion, and this side is the corona side. Finally, the base film after aging treatment is cut into film rolls with different sizes and shapes for subsequent metallization treatment.

[0004] Metallization treatment means that the above-mentioned base film passes through a coating machine to complete the evaporation coating treatment of the corona side. Through the vapor deposition of the metal, a metal coating is attached to the corona side, and the composite film containing the coating and the base film is the metallized film. After the metallized film is designed and repeatedly laminated and wound in multiple layers, the assembly of a capacitor is completed.

[0005] In the design of high-frequency pulse capacitors, the design of the metal coating is crucial. The extremely high number of pulses and the charge-discharge frequency require that the capacitor design must have low energy loss to ensure stable long-term operation. The deterioration of the performance of capacitor devices caused by increased loss is comprehensive, including not only the decrease in capacitance, but also the decrease in withstand voltage capacity caused by factors such as heat generation, etc., especially bringing many hidden dangers to the use of pulse capacitors in extreme environments. To alleviate the problems caused by excessive loss, first of all, the base film must have low loss, so the polypropylene material base film becomes the first choice; secondly, the metal coating should have low loss, which requires depositing a coating with low sheet resistance (sheet resistance, Ω / □). To achieve a low sheet resistance coating, existing inventions and solutions mainly focus on increasing the coating thickness, such as CN108497865A and CN103077821A; and some technical principles in the design of capacitors include considerations of increasing the coating thickness, such as CN114360904A, CN112271082A, and CN104078233A, etc. It can be understood that the voltage acting on the capacitor in the short term can be considered constant, so the current loss work is actually closely related to the physical shape of the coating. According to the resistance law R = ρ(L / S), where R is the sheet resistance of the coating, ρ is the density of the coating material, L is the total length of the coating (i.e., the total winding circumference), and S is the cross-sectional area of the coating. As the coating thickness increases, the actual S increases, and the overall sheet resistance of the coating decreases.

[0006] However, such methods have exposed many problems in practical applications. First, the increase in coating thickness does not improve the capacitance value, but increases the volume and mass of the capacitor, because the specific energy density is reduced. Second, generally in the industry, the thinner the coating, the better the withstand voltage of the capacitor device, that is, the "self-healing" efficiency is high. In high-frequency pulse capacitors, the high-energy and high-frequency current passing through the capacitor has a contradiction between the short-term withstand voltage time and the increase in coating thickness. Generally, the deposited coating is made as thin as possible to give priority to ensuring "self-healing" and stable withstand voltage. Moreover, the increase in coating thickness is also accompanied by a relatively high inductance, which also needs to be avoided.

[0007] From the above, how to achieve low charge-discharge energy loss on the basis of as thin a coating thickness as possible is an important technical problem faced in high-frequency pulse capacitors.

[0008] Currently, the technical means in this regard focus on considering from the perspective of the coating, and no one has provided a solution to reduce loss from the perspective of the structure of the polypropylene base film. This is because it is really difficult to reduce loss simply from the intrinsic crystal structure of the base film. The existing polypropylene base films used in pulse capacitors already have a dielectric loss factor that can be maintained in the order of 10 -4 ~10 -3 order of magnitude, which is already extremely low.

[0009] However, the base film and the coating of the metallized film are an integral whole. The physical structure at the joint between the coating and the film surface is actually determined by the topological structure of the film surface. Due to the presence of the roughening ring, the surface of the base film is always uneven. On these undulating topological structures, the metal coating attached to it will correspondingly have shape changes in the microscopic cross-section. The cross-sectional thickness of the coating is not uniform, and relatively thin coating areas will inevitably be generated. This part forms the high-resistance area of the narrow current path, such as Figure 1 . The influence of a single one or several high-resistance areas may be weak, but when magnified to the entire capacitor, these high-resistance areas are everywhere and the quantity base is huge, resulting in a great impact on the capacitor loss.

[0010] Therefore, controlling the topological structure of the base film surface of the thin film metallization surface and reducing its undulation degree can reduce the high-resistance areas of the subsequent metallized film and promote the uniformity of the thickness of the entire metal coating. On the basis of a coating as thin as possible, the energy loss of the pulsed capacitor during multiple charge and discharge applications can be reduced. Summary of the Invention

[0011] To solve the problem of excessive undulation of the topological structure of the base film surface of the polypropylene film in the above-mentioned prior art, the present invention provides a polypropylene film, a preparation method thereof, a metallized film and a film capacitor.

[0012] To solve the above technical problems, one of the technical solutions provided by the present invention is as follows: A preparation method of a polypropylene film, comprising the following steps: Melting and extruding polypropylene and casting and cooling to obtain a polypropylene film sheet; After biaxially stretching the polypropylene film sheet, a polypropylene film is obtained; The die head temperature of the melting and extrusion is T1, the cold roll temperature of the casting and cooling is T2, and the difference between the die head temperature and the cold roll temperature is the supercooling degree ΔT; the ratio of the supercooling degree ΔT to the cold roll temperature T2 satisfies 1.9≤ΔT / T2≤2.25; wherein, ΔT = T1 - T2.

[0013] In one embodiment, the isotactic index of the polypropylene is above 98.5%; and / or The molecular weight distribution index of the polypropylene is 5.0 - 6.5; and / or The melt flow index of the polypropylene is 3.0 - 3.5 g / 10 min; and / or The ash content of the polypropylene is below 20 ppm.

[0014] In one embodiment, the preparation method of the polypropylene film further includes pre-treating the polypropylene before the melting and extrusion; The pretreatment includes heat drying the polypropylene and / or adding a nucleating agent to the polypropylene that can reduce the β crystal form; Preferably, the temperature of the heat drying treatment is 30°C to 45°C, and the time of the heat drying treatment is 48 to 72 h; preferably, the nucleating agent is an α nucleating agent, and the mass percentage of the nucleating agent in the polypropylene film is 0.05% to 0.9%.

[0015] In one embodiment, the temperature of the extruder die head is 235 to 250°C, and the temperature of the cooling roll is 75 to 85°C.

[0016] In one embodiment, the biaxial stretching is synchronous biaxial stretching.

[0017] In one embodiment, the stretching section heating method of the synchronous biaxial stretching is wind-heat circulation heating, including preheating, stretching, and heat setting. Among them, the preheating temperature is 140 to 160°C, the stretching temperature is 157 to 166°C, the heat setting temperature is 163 to 170°C, the longitudinal draw ratio is 5.9 to 6.9 times, and the transverse draw ratio is 8.2 to 9.5 times.

[0018] In one embodiment, the method for preparing the polypropylene film further includes post-treatment of the obtained polypropylene film after the biaxial stretching; The post-treatment includes corona treatment and / or aging treatment; Preferably, the corona treatment is a single-sided treatment on the roll contact surface, and the intensity of the corona treatment is 10 to 30 W·min / m 2 ; Preferably, a preheating roll is provided before the corona treatment, and the temperature of the preheating roll is above 50°C and less than 100°C.

[0019] The second technical solution provided by the present invention is as follows: A polypropylene film is prepared by the method for preparing a polypropylene film as described above; Preferably, the thickness of the polypropylene film is 2.0 to 6.0 μm, and the vertical distance S between the highest peak and the lowest valley of the topological structure of the roll contact surface of the polypropylene film z satisfies 300 nm ≤ S z ≤ 700 nm, and the average deviation degree SSK of the concavity and convexity satisfies -0.3 ≤ SSK ≤ 0.

[0020] The third technical solution provided by the present invention is as follows: A metallized film is formed by providing a metal film on at least one side of the polypropylene film as described above.

[0021] The fourth technical solution provided by the present invention is as follows: A film capacitor is formed by using the metallized film as described above.

[0022] Based on the above, compared with the prior art, by coordinating the relationship between the melt supercooling degree ΔT and the cold roll temperature T2, the present invention enables the CR surface of the thick sheet to quickly crystallize into small-sized spherulites when it adheres to the roll, so as to reduce the surface undulation of the CR surface during the subsequent stretching process and be applicable to the field of capacitor films; if the ratio of the two is too low, since the melt cooling rate is proportional to the supercooling degree ΔT and the crystallization rate is inversely proportional to the cold roll temperature T2, the cooling is slow, the crystallization is slow, the spherulite size is too large, and the surface undulation cannot be reduced; if the ratio of the two is too high, the cooling is fast, the crystallization is fast, the crystal size is too small, it is too smooth and the crystallinity is not high, then the base film is too smooth, seriously affecting the metallization.

[0023] Other features and beneficial effects of the present invention will be described in the subsequent specification, and partly will become obvious from the specification, or will be understood by implementing the present invention. The objectives and other beneficial effects of the present invention can be achieved and obtained through the structures specifically pointed out in the specification, claims and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for describing the embodiments or the prior art. Obviously, the drawings in the following description are some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained according to these drawings; the positional relationships shown in the drawings in the following description, unless otherwise specified, are all based on the directions in which the components are shown in the drawings.

[0025] Figure 1 Shown is a schematic diagram of the corona surface topology of the base film and the high surface resistivity area of the coating layer; Figure 2 Shown is a process flow chart of the preparation method of the polypropylene film provided by the embodiment of the present invention; Figure 3 Shown are the surface contour diagrams of the corona surfaces of the base films in Example 1 and Comparative Example 1. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0026] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the following will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. The technical features designed in different embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts fall within the protection scope of the present invention.

[0027] In the description of the present invention, it should be noted that all terms used in the present invention (including technical terms and scientific terms) have the same meaning as commonly understood by those of ordinary skill in the art to which the present invention belongs, and should not be construed as a limitation of the present invention. It should be further understood that the terms used in the present invention should be understood to have a meaning consistent with their meaning in the context of this specification and the relevant art, and should not be understood in an idealized or overly formal sense, unless explicitly defined as such in the present invention.

[0028] An object of an embodiment of the present invention is to provide a production process for a biaxially stretched polypropylene-based film for capacitors. The roller-attached surface (corona surface) of the base film has a low undulation degree, which is beneficial to reducing the high sheet resistance regions in the metal coating. On the basis of vapor-depositing a low-thickness coating, it can reduce the energy loss of the capacitor during operation and is applicable to high-frequency pulse capacitors.

[0029] Please refer to Figure 2 , an embodiment of the present invention provides a preparation method for a polypropylene film, including the following steps: Step 1, pre-treat the polypropylene; Specifically, the polypropylene used in this embodiment is a commercial electrical-grade polypropylene resin. For example, the isotactic index of the polypropylene is above 98.5%; and / or the molecular weight distribution index of the polypropylene is 5.0 to 6.5, such as 5.6, 5.7, 5.8, 5.9, 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, etc., but not limited to the listed values. Other unlisted values within this numerical range are equally applicable. More preferably, the molecular weight distribution index of the polypropylene is 5.6 to 6.5; and / or the melt flow index of the polypropylene is 3.0 to 3.5 g / 10 min, such as 3.0 g / 10 min, 3.1 g / 10 min, 3.2 g / 10 min, 3.3 g / 10 min, 3.4 g / 10 min, 3.5 g / 10 min, or any value between them; and / or the ash content of the polypropylene is below 20 ppm, such as the ash content is below 15 ppm, 10 ppm.

[0030] The pretreatment in this embodiment includes heat drying the polypropylene and / or adding a nucleating agent to the polypropylene that can reduce the β crystal form; In a preferred embodiment of the present invention, the temperature of the heat drying treatment is 30°C to 45°C, such as 30°C, 31°C, 32°C, 33°C, 34°C, 35°C, 36°C, 37°C, 38°C, 39°C, 40°C, 41°C, 42°C, 43°C, 44°C, 45°C, etc., but not limited to the listed values, and other unlisted values within this numerical range are equally applicable; the time of the heat drying treatment is 48 to 72 h, such as 48h, 50h, 52h, 54h, 56h, 58h, 60h, 62h, 64h, 66h, 68h, 70h, 72h or any value between them; In a preferred embodiment of the present invention, the nucleating agent is an α nucleating agent, and the mass percentage of the nucleating agent in the polypropylene film is 0.05% to 0.9%, such as 0.05%, 0.1%, 0.15%, 0.2%, 0.25%, 0.3%, 0.35%, 0.4%, 0.45%, 0.5%, 0.55%, 0.6%, 0.65%, 0.7%, 0.75%, 0.80%, 0.85%, 0.9%, etc., but not limited to the listed values, and other unlisted values within this numerical range are equally applicable; the addition of a trace amount of α nucleating agent in the present invention will neither have an obvious impact on the ash content of the raw material, nor can effectively inhibit the generation of β crystals, thereby inhibiting the increase in surface undulation caused by the transformation of the β crystal form to the α crystal form, increasing the nucleation sites, supplemented by the control of supercooling degree and cold roll temperature, and reducing the crystal size.

[0031] Specifically, in this embodiment, a trace amount of heterogeneous α nucleating agent is added to the polypropylene, which has a weak impact on the ash content and can effectively reduce the content of the β crystal form; and the ratio of the supercooling degree ΔT to the cold roll temperature T2 is controlled to satisfy 1.9 ≤ ΔT / T2 ≤ 2.25, so that the melt is quickly cooled and crystallized into small-sized crystals. The two aspects cooperate to make the cavitation effect in the stretching process weak, achieving a low undulation on the cold roll surface, and subsequently, corona treatment of the cold roll surface can be optionally performed so that the coating is located on the side with the low undulation.

[0032] Further, in an embodiment of the present invention, the pretreatment sequentially includes heat drying the polypropylene; to make the effect more significant, a nucleating agent that can reduce the β crystal form is added to the polypropylene that has completed the heat drying treatment.

[0033] Step 2: Melt-extrude and cast-cool the polypropylene to obtain a polypropylene film sheet; Specifically, step 2 of this embodiment includes two steps: melt extrusion and cast cooling; Among them, the melt extrusion includes melting the raw materials in an extruder. The raw materials include polypropylene mixed with a nucleating agent or can also be pure polypropylene; the extruder is a single-screw extruder; the extrusion temperature is controlled at 230°C to 265°C, such as 230°C, 235°C, 240°C, 245°C, 250°C, 255°C, 260°C, 265°C, etc., but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable; the die temperature of the extruder is 235 - 250°C, such as 235°C, 236°C, 237°C, 238°C, 239°C, 240°C, 241°C, 242°C, 243°C, 244°C, 245°C, 246°C, 247°C, 248°C, 249°C, 250°C or any value between them; The casting and cooling is single-roll air knife cooling, which includes casting and forming the polypropylene melt obtained by melting in the extruder through the air knife and single-roll cooling; the cooling roll is cooled by water circulation, and the temperature of the cooling roll is 75 - 85°C. This cooling temperature can specifically be 75°C, 76°C, 77°C, 78°C, 79°C, 80°C, 81°C, 82°C, 83°C, 84°C, 85°C, etc., but is not limited to the listed values, and other unlisted values within this numerical range are equally applicable; The die temperature of the melt extrusion is T1, and the temperature of the cold roll in the casting and cooling is T2. The difference between the die temperature and the cold roll temperature is the supercooling degree ΔT, that is, ΔT = T1 - T2; the ratio of the supercooling degree ΔT to the cold roll temperature T2 satisfies 1.9 ≤ ΔT / T2 ≤ 2.25. ΔT / T2 can specifically be 1.9, 1.95, 2.0, 2.05, 2.1, 2.15, 2.2, 2.25 or any value between them. In the present invention, by coordinating the relationship between the melt supercooling degree ΔT and the cold roll temperature T2, the CR surface of the thick sheet adheres to the roll and quickly crystallizes into small-sized spherulites, so as to reduce the surface undulation of the CR surface during the subsequent stretching process and be applicable to the field of capacitor films; if the ratio of the two is too low, since the melt cooling rate is proportional to the supercooling degree ΔT and the crystallization rate is inversely proportional to the cold roll temperature T2, the cooling is slow, the crystallization is slow, the spherulite size is too large, and the surface undulation cannot be reduced; if the ratio of the two is too high, the cooling is fast, the crystallization is fast, the crystal size is too small, it is too smooth and the crystallinity is not high, then the base film is too smooth, seriously affecting metallization.

[0034] Step 3: After biaxially stretching the polypropylene film sheet, a polypropylene film is obtained; Specifically in implementation, the biaxial stretching in this embodiment is synchronous biaxial stretching; In a preferred embodiment of the present invention, the heating method in the stretching section of the synchronous biaxial stretching is hot air circulation heating. A synchronous double stretching process of non-contact stretching is adopted, without involving contact with the stretching roll, and the whole stretching process is hot air circulation. Compared with the conventional contact asynchronous stretching, it has a significantly lower undulation level (SSK).

[0035] The synchronous biaxial stretching includes preheating, stretching and heat setting. Among them, the preheating temperature is 140-160 °C, such as 140 °C, 141 °C, 142 °C, 143 °C, 144 °C, 145 °C, 146 °C, 147 °C, 148 °C, 149 °C, 150 °C, 151 °C, 152 °C, 153 °C, 154 °C, 155 °C, 156 °C, 157 °C, 158 °C, 159 °C, 160 °C, etc., but not limited to the listed values, and other unlisted values within this numerical range are equally applicable; the stretching temperature is 157-166 °C, such as 157 °C, 158 °C, 159 °C, 160 °C, 161 °C, 162 °C, 163 °C, 164 °C, 165 °C, 166 °C or any value between them; the heat setting temperature is 163-170 °C, such as 163 °C, 164 °C, 165 °C, 166 °C, 167 °C, 168 °C, 169 °C, 170 °C or any value between them; the longitudinal draw ratio is 5.9-6.9 times, such as 5.9, 6.0, 6.1, 6.2, 6.3, 6.4, 6.5, 6.6, 6.7, 6.8, 6.9 times or any value between them; the transverse draw ratio is 8.2-9.5 times, such as 8.2, 8.3, 8.3, 8.4, 8.5, 8.6, 8.7, 8.8, 8.9, 9.0, 9.1, 9,2, 9.3, 9.4, 9.5 times or any value between them.

[0036] It should be noted that the asynchronous stretching in the MD area is hot roll contact stretching, and the roll surface contact has a great influence on the surface structure of the film, and it is very easy to cause an increase in the surface undulation of the film. On the one hand, the smoothness of the roll surface has a great influence; on the other hand, under the same conditions, the surface undulation of contact asynchronous stretching is larger than that of non-contact synchronous stretching.

[0037] Step 4. Post-treat the obtained polypropylene film; Specifically, the post-treatment includes corona treatment and / or aging treatment; In a preferred embodiment of the present invention, the corona treatment is a single-sided treatment on the roll surface, and the intensity of the corona treatment is 10-30 W·min / m 2 , such as 10 W·min / m 2 , 11 W·min / m 2 , 12 W·min / m 2 , 13 W·min / m 2 , 14 W·min / m 2 , 15 W·min / m 2 , 16 W·min / m 2 , 17 W·min / m 2 , 18 W·min / m2 、 19 W·min / m 2 、 20 W·min / m 2 、 21 W·min / m 2 、 22 W·min / m 2 、 23 W·min / m 2 、 24 W·min / m 2 、 25 W·min / m 2 、 26 W·min / m 2 、 27 W·min / m 2 、 28 W·min / m 2 、 29 W·min / m 2 、 30 W·min / m 2 etc., but not limited to the listed values, and other unlisted values within this numerical range are equally applicable; the corona treatment in this embodiment occurs on the surface of the laminating roller (the CR surface of the laminating roller), that is, on the low-undulation surface, so that the coating of the subsequent coating adheres to the low-undulation corona surface, reducing the appearance of high sheet resistance regions. Preheating before corona enhances the efficiency of the corona treatment. Although the physical roughening degree of the corona surface is reduced, the polarity is increased and the metallization ability has no attenuation.

[0038] Preferably, a preheating roller is provided before the corona treatment, and the temperature of the preheating roller is above 50°C and less than 100°C to increase the grafting efficiency of polar groups during the corona treatment, such as 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, 85°C, 90°C, 95°C, etc., but not limited to the listed values, and other unlisted values within this numerical range are equally applicable.

[0039] The embodiment of the present invention starts from the low-loss design of high-frequency pulse capacitors and provides a base film process technical solution from the perspective of the polypropylene film structure design with the principle of reducing the high sheet resistance region of the coating. Instead of following the traditional coating process path, this provides a new method for the design of high-frequency pulse capacitors from the perspective of the base film.

[0040] In the specific implementation process of the present invention, the degree of supercooling ΔT is proportional to the cooling rate of the melt, that is, the greater the degree of supercooling, the faster the cooling rate, following the basic laws of thermodynamics; while T2 is inversely proportional to the crystallization rate. This is because, despite the problem of heat transfer efficiency, it is still considered that T2 is positively correlated with the actual crystallization temperature. The crystallization rate of the polymer shows a typical peak function curve with temperature, that is, as the temperature increases, the crystallization rate increases to a maximum value and then decreases. The temperature involved in the embodiments of the present invention is the crystallization nucleation control region. As the temperature increases, the crystallization rate decreases, that is, T2 is inversely proportional to the crystallization rate. Therefore, the larger ΔT / T2 is, the faster the cooling rate and the crystallization rate are, and the easier it is for crystal growth to approach the state corresponding to T2, that is, small-sized crystals with a low chill roll temperature. However, it cannot be too large, otherwise it is similar to a packaging film and the surface is too smooth. This ratio cannot be too small, otherwise it is easy to cause sufficient crystallization of the thick sheet, large crystal size, and unstable film drawing.

[0041] The transformation from β crystal form to α crystal form and the cavitation effect of spherulites are the two main reasons for the generation of the surface topological structure of the film. Adding a small amount of α nucleating agent to the polypropylene raw material can inhibit the generation of metastable β crystals, so as to weaken the increase in topological structure fluctuations caused by crystal form transformation during the stretching process.

[0042] The crystal size of the thick sheet affects the competition between the shear and cavitation effects of the lamellae during the stretching process. The relationship between this size and the roughening (topological) structure of the base film surface is: the larger the size, the more obvious the cavitation effect, and the easier the surface roughening is, and even through holes may be generated due to excessive cavitation. Therefore, small-sized crystals on the roll-contact surface of the thick sheet can achieve a low-undulation state of the topological structure on this surface.

[0043] In summary, in the field of the present invention, the larger ΔT / T2 is, the faster the cooling rate and the crystallization rate are, and the smaller the crystal size on the roll-contact surface of the thick sheet is, the easier it is to achieve a low undulation on the roll-contact surface of the film. However, ΔT / T2 should not be too large or too small. If it is too small, the roughness of the roll-contact surface will increase, which violates the principle of the present invention and the low-undulation state cannot be achieved; if it is too large, the surface is too smooth, which is not beneficial to the metallization process, thereby affecting the capacitor life and even directly affecting the film winding.

[0044] Furthermore, for the base film including corona treatment, corona is actually the key to enhancing the coating adhesion to enhance its surface polarity and adhesion, and the corona surface is the metallization surface. It should be understood that the corona surface needs to have a certain degree of roughening to increase the surface energy. For the embodiments of the present invention, the corona surface is not the smoother the better. For the non-corona surface, surface roughening is also important. Sufficient winding tension is required to ensure the high efficiency of die making and winding, and to ensure the dimensional stability of the capacitor components.

[0045] The embodiments of the present invention provide a polypropylene film, which is prepared by the preparation method of the polypropylene film as described above; In a preferred embodiment of the present invention, the thickness of the polypropylene film is 2.0 to 6.0 μm, such as 2.0 μm, 2.1 μm, 2.2 μm, 2.3 μm, 2.4 μm, 2.5 μm, 2.6 μm, 2.7 μm, 2.8 μm, 2.9 μm, 3.0 μm, 3.1 μm, 3.2 μm, 3.3 μm, 3.4 μm, 3.5 μm, 3.6 μm, 3.7 μm, 3.8 μm, 3.9 μm, 4.0 μm, 4.1 μm, 4.2 μm, 4.3 μm, 4.4 μm, 4.5 μm, 4.6 μm, 4.7 μm, 4.8 μm, 4.9 μm, 5.0 μm, 5.1 μm, 5.2 μm, 5.3 μm, 5.4 μm, 5.5 μm, 5.6 μm, 5.7 μm, 5.8 μm, 5.9 μm, 6.0 μm, etc., but not limited to the listed values, and other unlisted values within this numerical range are equally applicable; the vertical distance S between the highest peak and the lowest valley of the topological structure of the polypropylene film adhering to the roller surface z satisfies 300 nm ≤ S z ≤ 700 nm, and the average unevenness deviation degree SSK satisfies -0.3 ≤ SSK ≤ 0.

[0046] The metallized film of the present invention is formed by disposing a metal layer on at least one surface of the above polypropylene film.

[0047] The metal layer functions as an electrode. As the metal used in the metal layer, for example, metal elements such as zinc, lead, silver, chromium, aluminum, copper, nickel, etc., various mixtures thereof, alloys thereof, etc. can be used. Among these, zinc and aluminum are preferred in terms of excellent environmental load, economy, and capacitor performance.

[0048] As a method of laminating the metal layer on at least one side (one side or both sides) of the polypropylene film, there is no particular limitation, and for example, vacuum evaporation method, sputtering method, etc. can be cited. From the viewpoint of excellent productivity and economy, the vacuum evaporation method is preferred. As the vacuum evaporation method, a crucible method, a wire method, etc. can be cited, and the best one can be appropriately selected.

[0049] The edge pattern during vapor deposition and lamination of the metal layer is not particularly limited. From the viewpoint of further improving the safety of the capacitor and further suppressing capacitor breakdown and short circuit, it is preferred to apply a pattern including a so-called special edge such as a fishing net pattern, a T-shaped edge pattern, etc. to a single surface of the biaxially stretched polypropylene film.

[0050] As a method of forming the edge, there is no particular limitation, and it can be formed according to known methods such as the tape method, the oil method, etc.

[0051] The thickness of the metallized film of the present invention is not particularly limited, and it is preferably 2.0 μm or more and 6.0 μm or less, more preferably 4.0 μm or more and 5.8 μm or less.

[0052] The film capacitor of the present invention is formed using the above-mentioned metallized film. The metallized film of the present disclosure can be laminated or wound by a conventionally known method to form a film capacitor.

[0053] The above-mentioned film capacitor may have a structure in which a plurality of metallized films are laminated, or may have a wound metallized film. Such a film capacitor can be suitably used for capacitor for inverter power supply device for driving engines of electric vehicles, hybrid vehicles, etc. In addition, it can also be suitably used in applications such as for railway vehicles, for wind power generation, for solar power generation, and for general household appliances.

[0054] The experimental results showing the technical advantages of the present invention will be described below using examples and comparative examples. Example 1

[0055] This example provides a method for preparing a polypropylene film for pulse capacitor, and the steps are as follows: (a) Raw material treatment: The raw material is a commercial electrical-grade polypropylene raw material. Specifically, it is Borealis HC300BF, with an isotacticity of 98.5%, a distribution index of 6.0, a melt index of 3.4 g / 10min, an ash content ≤ 20 ppm, and is heat-dried in a constant-temperature room at 40 °C for 48 h.

[0056] (b) Melt extrusion: The extruder is a single-screw extruder. Equipped with a high-precision loss-in-weight feeder, add sorbitol α nucleating agent to the raw material at a mass ratio of 0.1%, melt in the extruder, and the average temperature of each extrusion section is controlled at 250 °C, and the die head temperature is 243 °C.

[0057] (c) Casting thick sheet: The casting cooling is single-roll air knife cooling, and the melt is cooled and cast into a sheet by single-roll cooling. The temperature of the cooling roll is 75 °C, and the relationship between the supercooling degree and the cold roll temperature is ΔT / T2 = 2.24.

[0058] (d) Biaxial stretching: Adopt a non-contact synchronous stretching process, and the heating method is hot air circulation heating, including preheating, stretching and heat setting. The preheating temperature rises in stages, from 140 °C to 160 °C; then, the thick sheet is drawn to the stretching section, and the temperatures of each stretching section are 159 °C, 163 °C, and 165 °C respectively, the longitudinal stretching ratio is 6 times, and the transverse stretching ratio is 8.5 times. Heat setting is carried out while maintaining the stretching tension, and the temperatures of each heat setting section are 165 °C, 167 °C, and 169 °C respectively, and the production line speed is 210 m / min.

[0059] (e) Post-treatment: The post-treatment section is divided into corona treatment and aging treatment. The corona treatment surface is the roll-contact surface (CR surface), the roll temperature before corona treatment is 80 °C, and the corona intensity is 15 W·min / m 2, and then wind up; Aging treatment, let the wound large coil stand still at room temperature for 72 h, and then take samples for testing. Example 2

[0060] This example provides a preparation method of a polypropylene film for pulse capacitors, and the steps are as follows: (a), The same as Example 1.

[0061] (b), Melting and extrusion: The extruder is a single-screw extruder. Equipped with a high-precision loss-in-weight feeder, add sorbitol α nucleating agent to the raw materials at a mass ratio of 0.5%. Melt in the extruder, and control the average temperature of each extrusion section at 250 °C, and the die head temperature is 247 °C.

[0062] (c), Casting thick sheet: The casting and cooling is single-roll air knife cooling. Cool the melt to form a single-roll cast sheet. The temperature of the cooling roll is 85 °C, and the relationship between the supercooling degree and the cold roll temperature is ΔT / T2 = 1.91.

[0063] (d), Biaxial stretching: The same as Example 1.

[0064] (e), Post-treatment: The post-treatment section is divided into corona treatment and aging treatment. Carry out heat setting and corona treatment under the condition of maintaining the stretching tension. The heat setting temperature is 165 °C; The corona treatment surface is the roll-contact surface (CR surface), the temperature of the front corona roll is 80 °C, and the corona intensity is 18 W·min / m 2 , and then wind up; Aging treatment, let the wound large coil stand still at room temperature for 72 h, and then take samples for testing. Example 3

[0065] This example provides a preparation method of a polypropylene film for pulse capacitors, and the steps are as follows: (a), The same as Example 1.

[0066] (b), Melting and extrusion: The extruder is a single-screw extruder. Equipped with a high-precision loss-in-weight feeder, add sorbitol α nucleating agent to the raw materials at a mass ratio of 0.5%. Put the raw materials into the extruder and melt them. Control the average temperature of each extrusion section at 250 °C, and the die head temperature is 240 °C.

[0067] (c), Casting thick sheet: The casting and cooling is single-roll air knife cooling. Cool the melt to form a single-roll cast sheet. The temperature of the cooling roll is 79 °C, that is, the relationship between the supercooling degree and the cold roll temperature is ΔT / T2 = 2.03.

[0068] (d), Biaxial stretching: The same as Example 1.

[0069] (e) Post-treatment: The post-treatment section is divided into heat setting, corona treatment, and aging treatment. Heat setting and corona treatment are carried out while maintaining the stretching tension. The heat setting temperature is 165 °C; the corona treatment surface is the roll contact surface (CR surface), the temperature of the corona preheating roll is 65 °C, and the corona intensity is 14 W·min / m 2 , and then winding; aging treatment, the wound large coil is left standing at room temperature for 72 h, and then samples are taken for testing. Example 4

[0070] This example provides a method for preparing a polypropylene film for pulse capacitors, and the steps are as follows: (a) Raw material treatment: The raw material is a commercial electrical-grade polypropylene raw material. Specifically, it is Borealis HC300BF, with an isotacticity of 98.5%, a distribution index of 6.0, a melt index of 3.4 g / 10min, an ash content of ≤20 ppm, and is heat-dried in a constant temperature room at 40 °C for 48 h.

[0071] (b) Melting and extrusion: The extruder is a single-screw extruder. The raw material does not add a nucleating agent and is directly melted in the extruder. The average temperature of each extrusion section is controlled at 250 °C, and the die head temperature is 243 °C.

[0072] (c) Casting thick sheet: The casting cooling is single-roll air knife cooling. The melt is cooled and cast into a sheet by single-roll cooling. The temperature of the cooling roll is 75 °C, and the relationship between the supercooling degree and the cold roll temperature is ΔT / T2 = 2.24.

[0073] (d) (e) Biaxial stretching and post-treatment: The same as Example 1. Comparative Example 1

[0074] This comparative example provides a method for preparing a polypropylene film for capacitors, and the steps are as follows: (a) The raw material is put into storage and heat-dried in a constant temperature room at 40 °C for 48 h. The raw material is HC300BF.

[0075] (b) Melting and extrusion: The extruder is a single-screw extruder. The commercial raw material is directly put into the extruder and melted. The average temperature of each extrusion section is controlled at 245 °C, and the die head temperature is 230 °C.

[0076] (c) Casting thick sheet: The casting cooling is single-roll air knife cooling. The melt is cooled and cast into a sheet by single-roll cooling. The temperature of the cooling roll is 95 °C, that is, the relationship between the supercooling degree and the cold roll temperature is ΔT / T2 = 1.52; (d) Biaxial stretching: The same as Example 1.

[0077] (e) Post-treatment: The same as Example 1. Comparative Example 2

[0078] This comparative example provides a method for preparing a polypropylene film for capacitors, and the steps are as follows: (a) The raw materials are put into storage and heat-dried in a constant-temperature room at 40 °C for 48 h. The raw material is HC300BF.

[0079] (b) Melting and extrusion: The extruder is a single-screw extruder. The commercial raw materials are directly put into the extruder for melting, and the average temperature of each extrusion section is controlled at 252 °C, and the die head temperature is 245 °C.

[0080] (c) Casting thick sheet: The casting and cooling is single-roll air knife cooling. The melt is cooled and cast into a sheet by a single roll, and the temperature of the cooling roll is 90 °C. That is, the relationship between the supercooling degree and the cold roll temperature is ΔT / T2 = 1.72; (d) Biaxial stretching: The same as in Example 1.

[0081] (e) Post-treatment: The same as in Example 1. Comparative Example 3

[0082] This comparative example provides a method for preparing a polypropylene film for capacitors, and the steps are as follows: (a) The raw materials are put into storage and heat-dried in a constant-temperature room at 40 °C for 48 h. The raw material is HC300BF.

[0083] The processes of (b), (c), (d), etc. are the same as those in Example 2.

[0084] (e) Post-treatment: The difference from Example 2 is only in the corona treatment; the corona treatment surface is the air knife surface (AK surface), the corona preheating roll is 50 °C, and the corona intensity is 14 W·min / m 2 . Comparative Example 4

[0085] This comparative example provides a method for preparing a polypropylene film for capacitors, and the steps are as follows: Stages (a), (b), and (c): All the same as in Example 1.

[0086] (d) Biaxial stretching: An asynchronous biaxial stretching process is adopted. The cast thick sheet enters the MD longitudinal stretching zone through a transition roll. The longitudinal stretching zone consists of 8 - 12 rolls, and the surface of the thick sheet directly contacts the roll surface. This zone includes three parts: preheating, stretching, and shaping. The hot rolls are all controlled by oil heat circulation. The average temperature range of the multi-roll in the MD zone is 125 - 155 °C, and the MD stretching ratio is 6 times. Then it enters the TD transverse stretching zone. The transverse stretching zone is controlled by air heat circulation and is divided into three parts: preheating, stretching, and shaping. The average temperature range of this zone is 150 - 170 °C, and the TD stretching ratio is 8.5 times. The production line speed is 210 m / min.

[0087] (e) The post-treatment process is the same as that in Example 1. Comparative Example 5

[0088] This comparative example provides a method for preparing a polypropylene film for capacitors, and the steps are as follows: The overall process is the same as that in Example 1, that is, a trace amount of nucleating agent is added and a synchronous biaxial stretching process is adopted. The differences are only in (b) and (c). The die head temperature is 255 °C and the cooling roll temperature is 70 °C, that is, the relationship between the degree of supercooling and the cold roll temperature is ΔT / T2 = 2.64.

[0089] The slippage during the post-treatment and winding stages of the film is very serious, and the production test cannot run normally and stably. Test Example

[0090] (1) 3D optical profiler The topological structure of the corona surface of the base film is measured by an optical profiler of ComtourX-200 (Bruker, Germany). The vertical distance Sz and the average deviation degree of concavity and convexity SSK between the highest peak and the lowest valley of the corona surface of the base film are recorded.

[0091] (2) Broadband dielectric tester The tangent of the dielectric loss angle of the base film is measured by a broadband dielectric spectroscopy instrument of Concept-50 (Novocontrol, Germany). The electrode diameter is 25 mm. The two sides of the base film are sputtered with gold before testing, and the test frequency is 1000 Hz.

[0092] (3) Capacitor performance test The above-mentioned base films are respectively cut and drawn into the vacuum evaporation step, and a layer of aluminum or zinc is deposited on the surface of the base film to form a metallized polypropylene film. Then, the metallized polypropylene film is subjected to winding or lamination treatment, and assembled with leads and insulating packaging materials. Parameters such as the number of film layers and the winding tightness are precisely controlled. After processes such as welding and packaging, a capacitor with a rated capacitance of 35 μF is finally made. The equivalent series resistance ESR of the capacitor is measured by an LCR instrument of model EA980A (Agilent, USA), and the test temperature is 25 °C and 10 kHz.

[0093] The films prepared in the above Examples 1-4 and Comparative Examples 1-4 are respectively taken for physical property tests, and the test results are shown in Table 1.

[0094] Table 1 Test results of polypropylene films in Examples 1-4 and Comparative Examples 1-4

[0095] Please refer to Figure 3 , it can be found from Example 1, Comparative Example 1 and Comparative Example 3 that the undulation degree of the polypropylene film on the roll-attached surface prepared in Example 1 is significantly lower than that in Comparative Example 1.

[0096] Finally, it should be noted that the polypropylene film provided by the present invention and the capacitor using the same have wide applications in important high-frequency pulse capacitor fields such as military and industrial high-frequency high-voltage pulse capacitors, aircraft carrier electromagnetic guns, etc.

[0097] In addition, those skilled in the art should understand that although there are many problems in the prior art, each embodiment or technical solution of the present invention can be improved in only one or several aspects, and it is not necessary to solve all the technical problems listed in the prior art or the background art at the same time. Those skilled in the art should understand that the content not mentioned in a claim should not be regarded as a limitation to that claim.

[0098] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements on some or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for preparing a polypropylene film, characterized in that it includes the following steps: Melting and extruding polypropylene, and casting and cooling to obtain a polypropylene film sheet; After biaxially stretching the polypropylene film sheet, a polypropylene film is obtained; The die head temperature of the melt extrusion is T1, the cold roll temperature of the casting and cooling is T2, and the difference between the die head temperature and the cold roll temperature is the supercooling degree ΔT; the ratio of the supercooling degree ΔT to the cold roll temperature T2 satisfies 1.9 ≤ ΔT / T2 ≤ 2.25; wherein, ΔT = T1 - T2.

2. The method for preparing a polypropylene film according to claim 1, characterized in that the isotactic index of the polypropylene is above 98.5%; and / or the molecular weight distribution index of the polypropylene is 5.0 - 6.5; and / or the melt flow index of the polypropylene is 3.0 - 3.5 g / 10 min; and / or the ash content of the polypropylene is below 20 ppm.

3. The method for preparing a polypropylene film according to claim 1, characterized in that the method for preparing the polypropylene film further includes pretreating the polypropylene before the melt extrusion; the pretreatment includes heat drying treatment of the polypropylene and / or adding a nucleating agent capable of reducing the β crystal form to the polypropylene; the temperature of the heat drying treatment is 30°C - 45°C, and the time of the heat drying treatment is 48 - 72 h; the nucleating agent is an α nucleating agent, and the mass percentage of the nucleating agent in the polypropylene film is 0.05% - 0.9%.

4. The method for preparing a polypropylene film according to claim 1, characterized in that the die head temperature of the extruder is 235 - 250°C, and the temperature of the cooling roll is 75 - 85°C.

5. The method for preparing a polypropylene film according to claim 1, characterized in that the biaxial stretching is synchronous biaxial stretching.

6. The method for preparing a polypropylene film according to claim 5, characterized in that the heating method of the stretching section of the synchronous biaxial stretching is wind - heat circulation heating, including preheating, stretching and heat setting. Among them, the preheating temperature is 140 - 160°C, the stretching temperature is 157 - 166°C, the heat setting temperature is 163 - 170°C, the longitudinal draw ratio is 5.9 - 6.9 times, and the transverse draw ratio is 8.2 - 9.5 times.

7. The method for preparing a polypropylene film according to claim 1, characterized in that the method for preparing the polypropylene film further includes post - treatment of the obtained polypropylene film after the biaxial stretching; the post - treatment includes corona treatment and / or aging treatment; The corona treatment is a single-sided treatment on the roller surface, and the intensity of the corona treatment is 10~30 W·min / m 2 ; a preheating roll is provided before the corona treatment, and the temperature of the preheating roll is above 50°C and less than 100°C.

8. A polypropylene film prepared by the method for preparing a polypropylene film according to any one of claims 1 - 7; The thickness of the polypropylene film is 2.0 to 6.0 μm, and the vertical distance S between the highest peak and the lowest valley of the topological structure of the polypropylene film sticking to the roller surface z satisfies 300 nm ≤ S z ≤ 700 nm, and the average deviation degree SSK of the concavity and convexity satisfies -0.3 ≤ SSK ≤ 0.

9. A metallized film formed by disposing a metal film on at least one side of the polypropylene film according to claim 8.

10. A film capacitor formed by using the metallized film according to claim 9.

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