Polypropylene film based on branched structure, preparation method and application

Abstract

The application discloses a kind of polypropylene film based on branched structure, preparation method and application, preparation method includes the following steps: step 1: linear polypropylene and branched polypropylene are mixed, and the obtained stretching sheet is obtained by melt blending, hot pressing molding;Linear polypropylene is 80~99 parts, and branched polypropylene is 1~20 parts according to weight fraction;Step 2: the stretching sheet obtained in step 1 is stretched by bidirectional synchronous biaxial stretching, and the required polypropylene film can be obtained;The application constructs specific entanglement mode in situ in polymer melt by introducing branched structure, realizes the synergistic optimization of chain segment stretching and entanglement control while maintaining low melt viscosity;Combined with the biaxial stretching process of segmented strain rate control and isothermal heat relaxation, the polymer chain sequentially experiences orientation, slip and conformation rearrangement at different stages, significantly improves the efficiency of bidirectional stress transmission, and inhibits local stress concentration, hole formation and early membrane failure.

Description

Technical Field

[0001] This invention relates to the field of polymer film technology, specifically to a branched polypropylene film, its preparation method, and its application. Background Technology

[0002] Polymer film capacitors, represented by biphase stretched polypropylene (BOPP), are fundamental components of energy storage devices and are widely used in electric vehicles, underground oil exploration, and implantable medical devices. As electronic devices and energy systems become increasingly miniaturized, polymer electrolyte films face the dual requirements of high energy density (Ue) and high volumetric capacitance (Cp).

[0003]

[0004] In the formula: ε0 and ε r These are the vacuum permittivity (8.85 × 10⁻⁶). - ¹² F / m) and relative permittivity, E b Let d be the electric field strength and d be the film thickness. From the above relationships, it can be seen that the key to improving energy density lies in increasing the dielectric constant and breakdown strength; while improving volumetric capacitance requires minimizing the film thickness while maintaining the dielectric constant.

[0005] To achieve high energy density, much research has focused on developing high-dielectric-constant polymers with both high breakdown strength and high dielectric constant. Key approaches include introducing wide-bandgap nanofillers, high-electron-affinity semiconductors, and surface modification techniques. However, progress on design strategies to improve device-level performance by increasing volumetric capacitance and reducing film thickness has been relatively slow. Polymer dielectric films are typically fabricated using biaxial stretching and other processing techniques. However, existing approaches present significant contradictions: on the one hand, advanced nanofillers or semiconductor components used to improve dielectric constant and breakdown strength often severely compromise the stretchability of capacitor films. High-surface-energy fillers tend to aggregate in the polymer matrix, hindering stable stress transfer and causing stress concentration, leading to rupture of the composite film at relatively low stretch ratios. On the other hand, when films are stretched to micrometer-scale thicknesses, they face the risk of cohesive fracture. This is because at extremely thin scales, the lack of sufficient molecular chain connections and weak interchain interactions makes it difficult to maintain the continuity of the film structure and counteract the scale effects brought about by high surface energy.

[0006] From the perspective of the intrinsic molecular structure of polymers, a widely adopted strategy to improve stretchability is to reduce the degree of molecular chain entanglement. Biaxial stretching essentially reflects the dynamic structural evolution of polymer chains under tensile loads, corresponding to the classical stress-strain response, and involves the linear elastic region, the yield initiation stage, the plastic deformation region dominated by chain segment slip, and the strain hardening stage of overall stretching of the macromolecular network. The release of entanglement points can encourage more polymer chains to participate in slip, delay strain hardening, and thus significantly expand the plastic deformation region. However, while this de-entanglement theory can effectively prepare fibers with ultra-high draw ratios in uniaxial stretching, it is essentially ineffective or may even have negative effects on biaxially stretched ultrathin films made of non-ultra-high molecular weight polymers. This is because de-entanglement disrupts the macromolecular network required to maintain coordinated deformation and weakens inter-chain interactions in orthogonal directions.

[0007] Therefore, there is an urgent need to develop a polypropylene-based dielectric film and its preparation method that does not rely on high filler content and can achieve ultra-high biaxial stretch ratio at low melt viscosity. This would effectively suppress defect formation during stretching while endowing the film with ultra-thin thickness, high volumetric capacitance, and extremely low power consumption, thus promoting the application of polymer film capacitors in miniaturization scenarios. Summary of the Invention

[0008] This invention addresses the problems existing in the prior art by providing a branched polypropylene film, its preparation method, and its application.

[0009] The technical solution adopted in this invention is: a method for preparing a branched polypropylene film, comprising the following steps: Step 1: Mix linear polypropylene and branched polypropylene, melt blend and hot press to obtain the sheet to be stretched; the linear polypropylene is 80-99 parts and the branched polypropylene is 1-20 parts by weight. Step 2: The sheet to be stretched obtained in Step 1 is subjected to biaxial synchronous stretching to obtain the desired polypropylene film.

[0010] Furthermore, the bidirectional synchronous biaxial stretching in step 2 is a three-stage stretching; Primary tension: Preheat at 155~170 ℃ for 30~120 s, then stretch at a stretching rate of 20~40% / s to 400%~500%; Secondary tension: Stretching to 550%~650% at a stretching rate of 1~4% / s at 155~170 ℃; Level 3 stretching: At 155~170 ℃, it was stretched to 860 %×860 at a stretching rate of 0.1~1 % / s.

[0011] Furthermore, after the secondary stretching is completed, the sample is allowed to stand for 30-60 seconds for isothermal heat relaxation.

[0012] Furthermore, the melt blending temperature in step 1 is 170~200 ℃.

[0013] Furthermore, the melt blending is performed using a twin-screw extruder or internal mixer with a screw speed of 30-80 rpm.

[0014] Furthermore, in step 1, the hot pressing process is first carried out at 180~200℃, and then cold pressing is performed at 20~40℃.

[0015] Furthermore, the pressure in the hot pressing process is 3~10 MPa, the hot pressing time is 3~8 min, and the cold pressing time is 2~5 min.

[0016] A polypropylene film based on a branched structure.

[0017] Furthermore, the thickness of the film is 1–2 μm.

[0018] An application of a branched polypropylene film, wherein the polypropylene film is used as a dielectric film in a film capacitor.

[0019] The beneficial effects of this invention are: This invention introduces branched structures to construct specific entanglement modes in situ within polymer melts, achieving synergistic optimization of chain extension and entanglement control while maintaining low melt viscosity. By introducing specific branching densities and short-chain branched polypropylene into a linear polypropylene matrix, under low content conditions, the short chains preferentially form one-to-one couplings with the main chain, rather than self-entanglement or multiple entanglements between branches, thereby constructing a uniform "single entanglement" structure.

[0020] This invention combines a biaxial stretching process with segmented strain rate control and isothermal thermal relaxation, allowing the polymer chain to undergo orientation, slip, and conformational rearrangement in different stages, significantly improving bidirectional stress transfer efficiency and suppressing local stress concentration, void formation, and early film rupture failure.

[0021] Based on the synergy of special structure design and process, this invention achieves an ultra-high biaxial stretch ratio of 860%×860% and obtains an ultrathin film with a thickness of 1.2μm, providing a structural design and process route basis for the preparation of ultrathin high-performance films from non-ultra-high molecular weight polymers. Attached Figure Description

[0022] Figure 1 This is a cross-sectional view of the thin film obtained in Example 1 of the present invention.

[0023] Figure 2 The ¹³C-NMR spectra of the thin films obtained in Examples 1, 1, and 2 of this invention are shown.

[0024] Figure 3 The results show the statistical results of the number of branches per 1000 carbon atoms in the branched polypropylene used in Examples 1, 1, and 2 of this invention.

[0025] Figure 4 The radii of rotation of the thin films were obtained in Examples 1, 2, and 3 of this invention.

[0026] Figure 5 The tensile curves of the films obtained in Examples 1, 2, and 3 of this invention are shown.

[0027] Figure 6 This is a schematic diagram of the mechanism by which the thin films obtained in Embodiment 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 of the present invention are connected by linear chains.

[0028] Figure 7 These are the stress-strain curves of the thin film obtained in Example 1 and Comparative Example 3 of the present invention during the biaxial stretching process.

[0029] Figure 8 This is an optical image of the thin film obtained in Comparative Example 3 of the present invention.

[0030] Figure 9 The stress-strain curves of the thin film during the biaxial stretching process were obtained for Comparative Examples 1 and 2 of this invention.

[0031] Figure 10 This is an optical image of the thin film obtained in Comparative Example 1 of the present invention.

[0032] Figure 11 This is an optical image of the thin film obtained in Comparative Example 2 of the present invention.

[0033] Figure 12 The optical fracture image of the thin film obtained in Example 1 of the present invention is shown.

[0034] Figure 13 The breakdown strength of the film at 25 °C was obtained for Examples 1 and 3 of the present invention, according to Weibull statistics.

[0035] Figure 14 The breakdown strength of the film at 120 °C was obtained for Examples 1 and 3 of the present invention, according to Weibull statistics.

[0036] Figure 15 The calculated discharge energy density of the thin film at 25 °C is obtained for Examples 1 and 3 of the present invention.

[0037] Figure 16 The DE cycling curves of the thin films obtained in Example 1 and Comparative Example 3 of this invention are shown below at 120 °C.

[0038] Figure 17 The calculated discharge energy density of the thin film at 120 °C was obtained for Examples 1 and 3 of the present invention. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0040] A method for preparing a branched polypropylene film includes the following steps: Step 1: Mix linear polypropylene and branched polypropylene, melt blend and hot press to obtain a sheet to be stretched; the linear polypropylene is 80-99 parts by weight and the branched polypropylene is 1-20 parts by weight; wherein the linear polypropylene is a conventional homopolymer polypropylene industrial raw material, preferably capacitor grade polypropylene.

[0041] Branched polypropylene is short-chain polypropylene, such as MFX-3 type branched polypropylene, in which only about 21 short branches per 1000 carbon atoms and no long branches. The choice of branched polypropylene is the basis for the differentiation of entanglement modes and the difference in biaxial tensile properties. In the ¹³C-NMR spectrum, a weak resonance peak at about 30.1 ppm indicates short branches, and a weak resonance peak at about 32.1 ppm indicates long branches. This invention specifically selects polypropylene with only short branches and a low content of short branches.

[0042] The content of branched polypropylene has a significant impact on chain entanglement topology and biaxial tensile stability. When the amount of branched polypropylene added is less than 20 parts, the extensional freedom of the linear polypropylene chain can be preserved to the greatest extent, which is beneficial to chain segment orientation, uniform stress transmission, and ultra-high biaxial tensile strength. However, when the amount of branched polypropylene added is more than 20 parts, excessive short or long branches will induce the formation of a large number of inter-chain entanglements and local entanglement aggregation structures, resulting in restricted chain slip, increased local stress concentration, and weakened cooperative load-bearing capacity during biaxial synchronous deformation. Ultimately, it is more likely to cause local fracture and film rupture failure during the high strain tensile stage. Therefore, the preferred amount of branched polypropylene added in this invention is 1 to 20 parts.

[0043] The melt blending temperature is 170~200 ℃, and the melt blending is carried out using a twin-screw extruder or internal mixer with a screw speed of 30~80 rpm. In hot pressing, the material is first hot-pressed at 180~200 ℃, followed by cold pressing at 20~40 ℃. The pressure in hot pressing is 3~10 MPa, the hot pressing time is 3~8 min, and the cold pressing time is 2~5 min.

[0044] Step 2: The sheet to be stretched obtained in Step 1 is subjected to biaxial synchronous stretching to obtain the desired polypropylene film.

[0045] Bidirectional synchronous biaxial tension is a three-stage tension; Primary tension: Preheat at 155~170 ℃ for 30~120 s, then stretch at a stretching rate of 20~40% / s to 400%~500%; Secondary tension: At 155~170 ℃, stretch to 550%~650% at a stretching rate of 1~4% / s; after the second stretching, let stand for 30~60 s for isothermal heat relaxation.

[0046] Level 3 stretching: At 155~170 ℃, it was stretched to 860 %×860 at a stretching rate of 0.1~1 % / s.

[0047] Biaxial stretching is performed under thermal conditions with segmented, controlled stretching rates. Initially, a higher stretching rate is used, followed by a reduction in the stretching rate. After isothermal relaxation to dissipate accumulated internal stress, the stretching rate is further reduced to a slower pace, ultimately achieving an ultra-high biaxial stretch ratio for the polypropylene capacitor film. This "fast-slow-slowest" stretching rate, combined with a specific entanglement mode, synergistically achieves the ultra-high biaxial stretch ratio of the film.

[0048] Example 1 A method for preparing a branched polypropylene film includes the following steps: Step 1: By weight, 80 parts of linear polypropylene and 20 parts of low-content short-chain polypropylene MFX-3 are melt-blended in a twin-screw extruder at a temperature of 190 ℃ and hot-pressed to obtain a sheet to be stretched; wherein the screw speed is 50 rpm.

[0049] First, hot pressing is performed at 190℃, followed by cold pressing at 30℃. The pressure during hot pressing is 6 MPa, the hot pressing time is 5 min, and the cold pressing time is 3 min. The most significant innovation of this invention is not in melt blending and hot pressing, so the parameters can be selected within a given range; however, these process conditions are still an indispensable part.

[0050] Step 2: The sheet to be stretched obtained in Step 1 is subjected to the following biaxial synchronous stretching to obtain the desired polypropylene dielectric film.

[0051] Primary tension: Preheat at 165 °C for 80 s, then stretch at a stretching rate of 30 % / s to 450 %×450 % (the given range is the biaxial stretching rate, which can be 400 %×400 %~500 %×500 %, and the same applies below).

[0052] Secondary tension: The temperature was maintained at 165 ℃, and the material was stretched to 600 %×600 % at a stretching rate of 2 % / s. After the two-stage stretching technique, the material was subjected to isothermal thermal relaxation for 50 s to dissipate the accumulated internal stress.

[0053] Level 3 stretching: While maintaining the temperature at 165 ℃, the film was stretched to 860 % × 860 % at a rate of 0.7 % / s, resulting in a film thickness of approximately 1.2 μm. The image of the film is shown below. Figure 1 As shown, the resulting film is simply referred to as 20M3 or M3.

[0054] Example 2 A method for preparing a branched polypropylene film includes the following steps: Step 1: According to the weight parts, 90 parts of linear polypropylene and 10 parts of low-content short-chain polypropylene MFX-3 are melt-blended in a twin-screw extruder at a temperature of 190 ℃ and hot-pressed to obtain the sheet to be stretched; wherein the screw speed is 50 rpm.

[0055] Step 2: The sheet to be stretched obtained in Step 1 is subjected to the following biaxial synchronous stretching to obtain the desired polypropylene dielectric film.

[0056] Primary tension: Preheat at 170℃ for 30 s, then stretch to 400%×400 at a stretching rate of 20% / s.

[0057] Secondary tension: The temperature was maintained at 170 ℃, and the material was stretched to 550 %×550 % at a stretching rate of 1 % / s. After the two-stage stretching technique, isothermal thermal relaxation was performed for 30 s to dissipate the accumulated internal stress.

[0058] Level 3 stretching: While maintaining the temperature at 170 °C, the film was stretched to 860% × 860% at a rate of 0.1% / s, resulting in a film thickness of approximately 1.0 μm.

[0059] Example 3 A method for preparing a branched polypropylene film includes the following steps: Step 1: According to the weight percentage, 99 parts of linear polypropylene and 1 part of low-content short-chain polypropylene MFX-3 are melt-blended in a twin-screw extruder at a temperature of 190 ℃ and hot-pressed to obtain the sheet to be stretched; wherein the screw speed is 50 rpm.

[0060] Step 2: The sheet to be stretched obtained in Step 1 is subjected to the following biaxial synchronous stretching to obtain the desired polypropylene dielectric film.

[0061] Primary tension: Preheat at 155℃ for 80 s, then stretch to 500%×500 at a stretching rate of 40% / s.

[0062] Secondary tension: The temperature was maintained at 155℃, and the material was stretched to 650%×650% at a stretching rate of 4% / s. After the two-stage stretching technique, the material was subjected to isothermal thermal relaxation for 60 seconds to dissipate the accumulated internal stress.

[0063] Level 3 stretching: While maintaining the temperature at 155℃, the film was stretched to 860%×860% at a rate of 1% / s, resulting in a film thickness of approximately 1.0 μm.

[0064] Comparative Example 1 All other steps in this comparative example are the same as in Example 1, except that the branched polypropylene used is MFX-6, which is a high-content short-chain polypropylene; the film obtained in this comparative example is simply referred to as 20M6 or M6. The film in this comparative example ruptured prematurely during biaxial stretching before reaching a stretch ratio of 770% × 770%, as shown in the rupture diagram below. Figure 10 As shown. The films used in this comparative example were films formed before they ruptured.

[0065] Comparative Example 2 The other steps in this comparative example are the same as in Example 1, except that the branched polypropylene used is WB140, which contains both high levels of short-chain and long-chain polypropylene. The resulting film is simply referred to as WB140, 20WB140, or 20LCB. The film in this comparative example ruptured prematurely during biaxial stretching before reaching a stretch ratio of 840% × 840%, as illustrated in the rupture diagram below. Figure 11 As shown. The films used in this comparative example were films formed before they ruptured.

[0066] Comparative Example 3 All other steps in this comparative example are the same as in Example 1, except that branched polypropylene (PP) is not included in step 1. In this comparative example, localized chain segment breakage and voids occurred before reaching a 750% × 750% stretch ratio during biaxial stretching, eventually leading to rupture. A schematic diagram of the rupture is shown below. Figure 8 As shown. The films used in this comparative example were films formed before they ruptured.

[0067] The branched structure of the branched polypropylene used in Examples 1, 1, and 2 of this invention was characterized by ¹³C-NMR spectra, as shown below. Figure 2 As shown in the figure, three strong resonance peaks appear at chemical shifts of 43.4, 25.8, and 21.3 ppm, which are attributed to the methylene hydrocarbon, methyl carbon, and methyl carbon in the branched PP polymer backbone, respectively. Two weak resonance peaks, absent in linear PP, are also observed at 32.1 and 30.1 ppm, originating from the tertiary carbons connecting the main chain. These two weak resonance peaks (labeled d and e) also originate from the tertiary carbons connecting the main chain, and their chemical shifts vary with the branch length, indicating that "lower chemical shifts correspond to shorter branches." In other words, a weak resonance peak at approximately 30.1 ppm in a ¹³C-NMR spectrum indicates a short branch, while a weak resonance peak at approximately 32.1 ppm indicates a long branch. Therefore, M3 and M6 only have a weak resonance peak at 30.1 ppm, indicating that they contain only short branches. WB140, on the other hand, has weak resonance peaks at both 30.1 and 32.1 ppm, indicating that it contains both short and long branches.

[0068] The branching number per 1000 carbon atoms of the branched polypropylene used in Examples 1, 1, and 2 of this invention is as follows: Figure 3 As shown in the figure, the low-content short-branched polypropylene M3 used in Example 1 contains only about 21 short branches per 1000 carbon atoms and no long branches. The high-content short-branched polypropylene M6 used in Comparative Example 1 contains about 28 short branches per 1000 carbon atoms and no long branches. The polypropylene WB140 used in Comparative Example 2 contains both short and long branches, with 29 short branches and 26 long branches per 1000 carbon atoms. Low-content branched polypropylene is defined as branched polypropylene containing a maximum of 21 short branches per 1000 carbon atoms and no long branches, which is the branched polypropylene used in this invention. High-content branched polypropylene is defined as branched polypropylene containing 28 or more short branches per 1000 carbon atoms, or branched polypropylene containing a large number of short and long branches, which is the polypropylene in the comparative examples. The precise quantification of the branch structure constitutes the molecular basis for the subsequent differentiation of entanglement modes and differences in biaxial tensile properties.

[0069] The radii of gyration of the thin films obtained in Embodiment 1, Comparative Example 1, and Comparative Example 2 of the present invention are as follows: Figure 4 As shown in the figure, the radius of gyration Rg of 20M3 reaches a maximum of 30.1 nm, while the Rg of 20M6 and 20WB140 are significantly reduced. This directly confirms that excessive entanglement inhibits the full extension of the molecular chain.

[0070] The tensile curves of the films obtained in Examples 1, 2, and 3 of this invention are as follows: Figure 5 As shown in the figure, 20M3 exhibits the highest elongation at break (1333%) in uniaxial tension, while the elongation at break of pure PP, 20M6 and 20WB140 decreases in that order, to approximately 600%, 950% and 800%, respectively.

[0071] The thin films obtained in Examples 1, 2, and 3 of this invention are connected by linear chains via the following mechanism: Figure 6 As shown in the figure, pure PP has only linear chains with small molecular chain size, few entanglement points, and the smallest strain hardening region. 20M3 achieves a large molecular chain size by coupling branched PP with multiple linear PP chains through single entanglement on short branch branches, which maximizes chain extension while maintaining an appropriate entanglement density to enhance inter-chain interactions in orthogonal directions. 20M6, due to its excessively high short branch density, causes inter-chain entanglement between linear PP chains, limiting the degree of molecular chain extension, and the entanglement points become stress concentration points. 20WB140 forms multiple entanglements on long chain branches, exacerbating stress concentration and shortening the effective chain length, similarly limiting the degree of molecular chain extension.

[0072] according to Figure 6 The linear chain connection mechanism, combined with the quantitative results of the radius of gyration Rg and the mechanical response of the tensile curve, shows that the three corroborate each other, indicating that only low-content short-branched polypropylene can construct a "single entanglement" mode, precisely achieving the optimal match between entanglement density and chain extension degree in promoting biaxial tensile properties.

[0073] The stress-strain curves of the thin film obtained in Example 1 and Comparative Example 3 of this invention during biaxial stretching are shown below. Figure 7 As shown, the stress-strain curves of the film during biaxial stretching in Example 1 and Comparative Example 2 are as follows: Figure 9 As shown in the figure, pure linear polypropylene, due to its low entanglement density and small molecular cluster size, has a narrow strain hardening region and poor stretchability during biaxial stretching, making it difficult to prepare ultrathin films. It exhibits localized chain segment breakage and voids before reaching a stretch ratio of only 750% × 750%, ultimately leading to rupture. According to... Figure 6The entanglement mechanism shown indicates that pure PP possesses only small-sized linear chain conformations, lacking sufficient inter-chain connections to maintain biaxial coordinated deformation. Comparative Example 1 shows that the film ruptured prematurely before reaching a stretch ratio of 770% × 770%. According to... Figure 6 The entanglement mechanism shown indicates that the excessively high density of short branches causes interchain entanglement between linear PP chains, rather than forming a single "one-point-multiple-chain" connection with the branches. This restricts the extension of the molecular chains, and the entanglement points become stress concentration sources, weakening the chain's extension capacity and reducing its tensile strength. The film obtained in Comparative Example 2 cracked before reaching a stretch ratio of 840% × 840%. According to... Figure 6 The entanglement mechanism shown indicates that its failure originates from multiple entanglements formed along the chain body by long branches. This topological structure exacerbates stress concentration and significantly shortens the effective chain length, thus limiting the extent of molecular chain extension and severely restricting the chain's extension potential. Benefiting from this, its plastic deformation capacity at room temperature shows a significantly improved performance compared to the control group, and it achieves an ultra-high elongation ratio of 860% × 860% in biaxial tensile testing.

[0074] Biaxial stretching profoundly affects the chain conformational evolution, entanglement topology, and stress transfer behavior between branched and linear polypropylene, making it a key step in achieving ultra-high draw ratios and ultrathin films. By combining segmented strain rate control with isothermal thermal relaxation, sufficient slippage and rearrangement of polymer chains are achieved, thereby significantly improving the stretchability and thickness control of the film. Figure 4 It can be seen that the low-content short-branched system 20M3 has the largest molecular chain radius of rotation Rg (30.1 nm); combined with Figure 6 As shown, its short side chains preferentially form a "one-point-multiple-chain" single entanglement structure with multiple linear polypropylene chains, which enhances the interaction between orthogonal chains while maximizing the extensional freedom of the linear chains. In the initial stage of biaxial stretching, a relatively high stretching rate of 20-40% / s is used to rapidly orient and extend the linear polypropylene main chain, and to gradually straighten the short side chains, thereby quickly constructing a highly oriented chain network. Figure 6 As shown in the figure, a high stretching rate can suppress premature chain relaxation and improve the overall orientation and initial stress bearing capacity. Figure 5As shown, the 20M3 system exhibits the highest elongation at break (1333%), indicating that this single entangled structure can effectively promote chain extension and maintain inter-chain synergy. When the stretch ratio reaches 400%~500%, the stretching rate is reduced to 1~4% / s, allowing the pre-oriented chain segments sufficient time for slip and conformational rearrangement. The short branches, acting as dynamic connecting nodes, maintain the continuity of the chain network, thereby promoting uniform stress distribution and reducing local hardening, allowing the film to be stably stretched to 550%~650% strain. Furthermore, an isothermal thermal relaxation step is introduced before entering the 860% high-strain hardening stage, allowing the residual internal stress to gradually dissipate. While the linear chains further complete local slip and orientation optimization, the short branches continue to maintain inter-chain coupling in the biaxial direction, enabling the film to maintain a smooth and stable stress response in the high-strain range. Figure 7 (As shown). The stretching rate was then further reduced to 0.1–1% / s. This extremely low stretching rate provided sufficient time for local relaxation and coordinated rearrangement of the highly oriented chains, allowing the single entangled network to gradually transform into a stable load-bearing skeleton. This prevented the highly oriented chain segments from breaking due to instantaneous stress concentration and further extended the strain-hardening region. In contrast, the excessively high short-branch density in 20M6 induced a large number of inter-chain entanglements, while the long branches in 20WB140 formed multiple entanglements. Both exacerbated local stress concentration and restricted chain extension, ultimately leading to rupture before 770% × 770% and 840% × 840%, respectively. Furthermore, in pure linear polypropylene, the original entanglements gradually unraveled during stretching, and the inter-chain interactions in the orthogonal directions rapidly weakened, making it difficult to maintain the large-scale coordinated load-bearing network required for biaxial synchronous deformation. This ultimately easily induced local chain breakage and void formation, resulting in film failure before 770% × 770%.

[0075] The Weibull distribution statistical function was used for analysis. The Weibull statistical results of the breakdown strength of Example 1 and Comparative Example 3 under the conditions of room temperature and 120 °C are as follows: Figure 13 and 14 As shown in the figure, there was no significant degradation despite the substantial reduction in film thickness.

[0076] The calculated discharge energy density values ​​of the thin film at 25 °C obtained in Examples 1 and 3 are as follows: Figure 15 As shown in the figure, the ultrathin film obtained in Example 1 has a discharge energy density of about 3 J / cm³ at room temperature with an electric field strength of 500 MV / m, which is comparable to that of traditional BOPP film.

[0077] The DE cycling curves of the thin films obtained in Example 1 and Comparative Example 3 at 120 °C are shown below. Figure 16As shown in the figure, under high temperature conditions of 120 °C, the ultrathin film exhibits significantly better performance than the traditional BOPP, with the ultrathin film displaying a smaller hysteresis loop and a lower residual polarization DE loop.

[0078] The calculated discharge energy density values ​​of the thin film at 120 °C obtained in Examples 1 and 3 are as follows: Figure 17 As shown in the figure, the discharge efficiency of the thin film obtained in Example 1 is as high as 83%, which is far superior to the 73% of the pure BOPP film. This high-temperature performance advantage is attributed to the extreme stretching causing the molecular units to be highly oriented, which effectively restricts the movement of thermally activated charge carriers, thereby reducing the conductivity loss and polarization loss at high temperatures. At the same time, the film only requires an operating voltage of 605V to achieve an electric field strength of 500MV / m.

[0079] This invention introduces a method for in-situ constructing specific entanglement patterns in polymer melts by introducing branched structures, achieving synergistic optimization of chain extension and entanglement control while maintaining low melt viscosity. On the one hand, the branched structure promotes the unfolding of molecular chains in the flow field, improving stretchability; on the other hand, the introduction of moderate entanglement enhances interchain interactions, providing a stable network basis for biaxial stretching. For the first time, a significant trade-off between branching-induced chain extension and entanglement density is revealed: enhanced entanglement improves network stability, but excessive entanglement inhibits chain extension, thereby reducing biaxial orientation efficiency. Based on this principle, low-content short-branched polypropylene is identified as the optimal structural system, achieving a balanced match between chain extension and entanglement. Introducing specific branching densities and short-branched polypropylene into a linear polypropylene matrix, under low-content conditions, short branches preferentially form one-to-one couplings with the main chain, rather than self-entanglement or multiple entanglements between branches, thus constructing a uniform "single entanglement" structure. This structure achieves a precise match between entanglement density and chain extension. Moderate entanglement maintains the integrity of the molecular network, ensuring biaxial synergistic deformation, while avoiding excessive entanglement that restricts chain movement. This allows the linear chain to fully extend at low viscosity, achieving a molecular chain gyration radius Rg of 30.1 nm and an elongation at break of 1333%. In terms of fabrication, combining segmented strain rate control with isothermal thermal relaxation biaxial stretching allows the polymer chain to sequentially undergo orientation, slip, and conformational rearrangement at different stages, significantly improving biaxial stress transfer efficiency and suppressing localized stress concentration, void formation, and early film failure. Based on this synergistic structure and process, an ultra-high biaxial stretching ratio of 860% × 860% was achieved, yielding an ultrathin film with a thickness of 1.2 μm. This provides a structural design and process pathway for preparing ultrathin, high-performance films from non-ultra-high molecular weight polymers.

Claims

1. A method for preparing a polypropylene film based on a branched structure, characterized in that, Includes the following steps: Step 1: Mix linear polypropylene and branched polypropylene, melt blend and hot press to obtain the sheet to be stretched; the linear polypropylene is 80-99 parts and the branched polypropylene is 1-20 parts by weight. Step 2: The sheet to be stretched obtained in Step 1 is subjected to biaxial synchronous stretching to obtain the desired polypropylene film.

2. The method for preparing a branched polypropylene film according to claim 1, characterized in that, In step 2, the bidirectional synchronous biaxial stretching is a three-stage stretching process. Primary tension: Preheat at 155~170 ℃ for 30~120 s, then stretch at a stretching rate of 20~40% / s to 400%~500%; Secondary tension: Stretching to 550%~650% at a stretching rate of 1~4% / s at 155~170 ℃; Level 3 stretching: At 155~170 ℃, it was stretched to 860 %×860 at a stretching rate of 0.1~1 % / s.

3. The method for preparing a branched polypropylene film according to claim 2, characterized in that, After the secondary stretching is completed, the body is allowed to stand for 30-60 seconds for isothermal heat relaxation.

4. The method for preparing a branched polypropylene film according to claim 1, characterized in that, The melt blending temperature in step 1 is 170~200 ℃.

5. The method for preparing a branched polypropylene film according to claim 4, characterized in that, The melt blending is performed using a twin-screw extruder or internal mixer with a screw speed of 30-80 rpm.

6. The method for preparing a branched polypropylene film according to claim 1, characterized in that, In step 1, the hot pressing process is first carried out at 180~200℃, and then cold pressing is performed at 20~40℃.

7. The method for preparing a branched polypropylene film according to claim 6, characterized in that, The pressure in the hot pressing process is 3~10 MPa, the hot pressing time is 3~8 min, and the cold pressing time is 2~5 min.

8. A branched polypropylene film obtained by any of the preparation methods described in claims 1 to 7.

9. A polypropylene film based on a branched structure according to claim 8, characterized in that, The thickness of the film is 1 to 2 μm.

10. The application of the branched polypropylene film as described in any one of claims 8 and 9, characterized in that, The polypropylene film is used as a dielectric film in film capacitors.

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