Method for improving performance of high molecular weight polyethylene by using polypropylene

Abstract

The invention provides a method for improving the performance of high molecular weight polyethylene by using polypropylene. The method comprises the following steps: melting, blending and banburying polyethylene and polypropylene through a modified grafting agent; the polypropylene accounts for 1.5-10% of the weight of the polyethylene, and the modified grafting agent accounts for 1-15% of the weight of the polyethylene; the modified grafting agent is a cyclic carboxylic acid derivative coupling olefine acid ester compound. Polyethylene is modified, so that the melting point, wettability, tensile strength and other properties of the polyethylene diaphragm material are effectively improved, and the safety, reliability and applicability of the lithium battery diaphragm are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of polyethylene preparation, and particularly relates to a method for improving the performance of high molecular weight polyethylene with polypropylene. Background Art

[0002] As a core component of new energy vehicles and energy storage systems, the installed capacity of lithium-ion batteries has shown exponential growth in recent years. However, the potential safety hazards caused by the inherent thermodynamic instability of the electrochemical system have become the key bottleneck restricting its development. According to statistics, 78.6% of global lithium battery safety accidents in 2022 were caused by separator failure. As a physical barrier between the positive and negative electrodes inside the battery, the separator needs to maintain efficient Li+ transmission during charge and discharge while preventing internal short circuits caused by the piercing of lithium dendrites. However, the formation of lithium dendrites is driven by multiple factors such as cell design defects, cycle life attenuation, and non-standard charge and discharge operations. Their cumulative growth is likely to exceed the critical mechanical strength of the separator, leading to the risk of thermal runaway.

[0003] In the prior art, the strategies for improving the safety of separators focus on material optimization, including increasing the separator thickness to enhance the mechanical barrier ability, or improving performance parameters such as puncture resistance, melting point, and electrolyte wettability. However, the rapid charge and discharge requirements of high-power density batteries in downstream application scenarios require the separator to have both ultrathin and high Li+ migration efficiency characteristics, which is fundamentally contradictory to the traditional thickening scheme. High molecular weight or ultra-high molecular weight polyethylene has currently become the preferred material for high-power battery separators. However, the performance of existing high molecular weight polyethylene still needs to be further improved to meet the increasingly high performance requirements of lithium batteries for separators. The prior art has modified polyethylene with polypropylene. However, due to the poor interfacial compatibility between polypropylene (PP, non-polar) and polyethylene (PE, semi-crystalline non-polar), phase separation is likely to occur, and it is necessary to rely on highly efficient compatibilizers or complex process optimization. Moreover, the block copolymers of the two (such as PP-b-PE) are costly, and large-scale industrial production still faces challenges in catalyst efficiency and process stability. In the field of lithium battery separators, how to achieve performance balance so that it can not only meet the high safety of the separator but also have excellent mechanical properties still requires further research. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for improving the performance of high molecular weight polyethylene with polypropylene to solve the above problems.

[0005] The present invention is implemented by adopting the following technical scheme:

[0006] A method for improving the properties of high molecular weight polyethylene with polypropylene, which comprises melt-blending and kneading polyethylene with polypropylene through a modified grafting agent; the polypropylene accounts for 1.5-10% of the weight of the polyethylene, and the modified grafting agent accounts for 1-15% of the weight of the polyethylene; the modified grafting agent is a cyclic carboxylic acid derivative-coupled acrylic acid ester compound.

[0007] Preferably, the polypropylene accounts for 1.5-3% of the weight of the polyethylene, and the modified grafting agent accounts for 5-15% of the weight of the polyethylene; further preferably, the polypropylene accounts for 1.5-3% of the weight of the polyethylene, and the modified grafting agent accounts for 8-12% of the weight of the polyethylene. Most preferably, the polypropylene accounts for 2% of the weight of the polyethylene, and the modified grafting agent accounts for 10% of the weight of the polyethylene.

[0008] The modified grafting agent is a cyclic carboxylic acid derivative-coupled acrylic acid ester compound having the general formula structure of formula B, wherein Y is an acrylic acid ester group; R is selected from H, halogen, C1-C3 alkyl, C1-C3 alkoxy, phenyl or Y;

[0009]

[0010] In the present invention, the properties of polyethylene are improved by a small amount of polypropylene. To solve the problems of poor interfacial compatibility and easy phase separation between the two, a modified grafting agent is added. The modified grafting agent of the present invention is a cyclic carboxylic acid derivative-coupled acrylic acid ester compound, which uses a cyclic carboxylic acid derivative as the main framework and simultaneously introduces an acrylic acid ester group. The cyclic carboxylic acid derivative can open the ring, and the acrylic acid ester group has a double bond that can be grafted onto the main chains of polyethylene and polypropylene. Moreover, both the acrylic acid ester end and the cyclic carboxylic acid derivative have polar groups, which can improve the hydrophilicity of the polyethylene main chain and improve the processing performance of polyethylene. When used as a lithium battery separator, it helps to improve the puncture resistance and wettability of the lithium battery separator.

[0011] The acrylic acid ester group Y coupled to the cyclic carboxylic acid derivative framework can be represented by the following structural formula: -C=C-(CH2) n -COOR1, where n = 0-3, and R1 is selected from C1-C3 alkyl. The carbon chain length of the acrylic acid ester group Y should not be too long, otherwise the grafting rate of the modified polyethylene will decrease and the hydrophilicity will also decrease. Preferably, n = 0-1, and R1 is selected from methyl or ethyl. The acrylic acid ester group Y is more preferably a methyl acrylate group, i.e., -C=C-COOCH3.

[0012] The substituent R on the cyclic carboxylic acid derivative framework is preferably a group with a small steric hindrance, which is more conducive to the grafting modifier being grafted onto the polyethylene main chain. Preferably, R is selected from H, methyl, ethyl, methoxy or ethoxy.

[0013] As a preferred embodiment of the present invention, the modified grafting agent is selected from any one of the following compounds of formula B1 to B4:

[0014]

[0015] Further, the preparation method of the cyclic carboxylic acid derivative-coupled enoate compound is to react a halogenated cyclic carboxylic acid derivative with an enoate under the action of a catalyst.

[0016] Specifically, the preparation method of the cyclic carboxylic acid derivative-coupled enoate compound includes the following steps: Dissolve the halogenated cyclic carboxylic acid derivative and the enoate in an organic solvent at a molar ratio of 1:1.1 - 2, add a catalytic amount of palladium salt, react at 110 - 130 °C for 3 - 12 hours, remove the solvent after the reaction, and purify the solid product to obtain the cyclic carboxylic acid derivative-coupled enoate compound.

[0017] The halogenated cyclic carboxylic acid derivative is preferably a bromide, such as but not limited to 2-bromomaleic anhydride, 3-bromo-4-methylmaleic anhydride, 2,3-dibromomaleic anhydride.

[0018] The enoate is represented by the following structural formula: C=C-(CH2) n -COOR1, where n = 0 - 3, and R1 is selected from C1-C3 alkyl groups. Preferably, n = 0 - 1, and R1 is selected from methyl or ethyl. The enoate is preferably methyl acrylate, i.e., C=C-COOCH3.

[0019] The catalyst is selected from palladium salts commonly used in the art, such as but not limited to palladium acetate and palladium triphenylphosphine. The organic solvent can be a solvent commonly used in the art, such as but not limited to DMF. The steps of removing the solvent after the reaction, purifying the product, etc. are all conventional operations in the art. For example, the solvent is removed by vacuum distillation, and the product is purified by column chromatography. Details are not described here.

[0020] The following takes the compound of formula B1 as an example to specifically illustrate the preparation method of the cyclic carboxylic acid derivative-coupled enoate compound. Dissolve 2-bromomaleic anhydride and methyl acrylate in an organic solvent at a molar ratio of 1:1.1 - 2, add a catalytic amount of palladium salt, react at 110 - 130 °C for 3 - 12 hours, remove the solvent after the reaction, and purify the solid product to obtain the compound of formula B1.

[0021] As a preferred embodiment of the present invention, the method for improving the performance of high molecular weight polyethylene with polypropylene specifically includes the following steps:

[0022] (1) Add polyethylene to a kneader for pretreatment;

[0023] (2) Then add a modified grafting agent to graft polyethylene;

[0024] (3) Finally, add polypropylene for kneading, and cool after kneading to obtain the product.

[0025] Among them, in step (1), after adding polyethylene into the internal mixer, it is pre-treated by internal mixing at 180 - 200 °C and an rpm of 5 - 15 for 1 - 10 min. For example, in a specific embodiment, polyethylene powder is added into the internal mixer and pre-treated by internal mixing at 190 °C and an rpm of 10 for 5 min.

[0026] In step (2), after adding the modified grafting agent into the internal mixer, it is internally mixed at 180 - 200 °C and an rpm of 5 - 15 for 1 - 10 min. For example, in a specific embodiment, after adding the modified grafting agent, it is internally mixed at 190 °C and an rpm of 10 for 5 min.

[0027] In step (3), after adding polypropylene into the internal mixer, it is internally mixed at 180 - 200 °C and an rpm of 50 - 70 for 5 - 15 min. For example, in a specific embodiment, after adding polypropylene, it is internally mixed at 190 °C with an rpm of 60 for 10 min.

[0028] The polyethylene is preferably high molecular weight polyethylene or ultra-high molecular weight polyethylene, with an average molecular weight of 500,000 - 2,000,000. For example, high molecular weight polyethylene or ultra-high molecular weight polyethylene of models such as LPF - 8008V, LPF - 8010U, LPF - 8009V, LPF - 8006V, LPF - 8020U of Parkene New Energy Materials (Shanghai) Co., Ltd. is used.

[0029] The modified polyethylene prepared by the above method can be used to prepare a high-safety lithium battery separator. The high-safety lithium battery separator can be prepared by conventional methods in the art, such as a dry unidirectional stretching process, a dry bidirectional stretching process, or a wet bidirectional stretching process.

[0030] Compared with the prior art, the present invention has the following beneficial effects:

[0031] The present invention effectively improves the properties such as the melting point of polyethylene by using polypropylene to improve the properties of high molecular weight polyethylene. To solve the problems of poor interfacial compatibility between polyethylene and polypropylene and easy phase separation, the present invention designs and synthesizes a cyclic carboxylic acid derivative-coupled acrylate-based grafting agent. The grafting agent has more polar groups and can effectively improve the properties such as the wettability and tensile strength of polyethylene. Specific Embodiments

[0032] The following description is used to disclose the present invention so that those skilled in the art can implement the present invention. The preferred embodiments in the following description are only examples and do not limit the scope of the present invention. Those skilled in the art can think of other obvious variations. The basic principles of the present invention defined in the following description can be applied to other embodiments, variants, improvements, equivalent schemes, and other technical solutions without departing from the spirit and scope of the present invention.

[0033] For the instruments or reagents in the embodiments of the present invention that are not indicated with the manufacturer, they are all conventional commercial instruments or reagents. The main reagents used in the examples are specifically as follows:

[0034] 2-Bromo-maleic anhydride (CAS: 5926-51-2 Sigma-Aldrich)

[0035] 3-Bromo-4-methylmaleic anhydride (CAS: 59107-74-3 Shanghai Macklin Biochemical Co., Ltd.)

[0036] 2,3-Dibromomaleic anhydride (CAS: 1122-12-9 Shanghai Macklin Biochemical Co., Ltd.)

[0037] Methyl acrylate (CAS: 96-33-3 Sigma-Aldrich)

[0038] Methyl 3-butenoate (CAS: 3724-55-8 Sigma-Aldrich)

[0039] Cyclobutane-1,2-dicarboxylic anhydride (CAS: 4462-96-8 Aladdin Reagent Co., Ltd.)

[0040] Palladium(II) acetate (CAS: 3375-31-3 Sigma-Aldrich)

[0041] Polyethylene: Puxijing New Energy Materials (Shanghai) Co., Ltd. LPF-8010U

[0042] Polypropylene: (CAS: 9003-07-0 Exxon mobil)

[0043] All solvents involved in the present invention need to be treated without water unless otherwise specified.

[0044]

Preparation Example

[0045] Preparation of the modified grafting agent.

[0046] Prepare the modified grafting agents B1 to B4 according to the cyclic carboxylic acid derivatives and acrylic esters listed in Table 1 respectively.

[0047] Table 1 Structure and preparation raw materials of the modified grafting agent

[0048]

[0049] Preparation of Modified Grafting Agent B1:

[0050] Dissolve 2-bromomaleic anhydride (0.99 g, 5 mmol) and methyl acrylate (0.52 g, 6 mmol) in 100 ml of DMF solution in a 250 ml three-necked flask, and add catalytic amount of palladium acetate (2.25 μg, 10 μmol). Carry out water separation and reflux at 120 °C for 12 h. After the reaction is completed, remove the solvent DMF by means of vacuum distillation, and purify the solid product by column chromatography (eluent = EA: HEX = 1:5), with a final yield of 88%. (1H 300 MHz, DMSO): 7.50 (s, 1H, C═C-H); 7.13 (s, 1H, ring H); 5.38 (s, 1H, C═C-H); 3.94 - 3.71 (m, 3H, -CH3). C8H6O5: Calculated values of elemental analysis (%): C, 52.76; H, 3.32; O, 43.92. Experimental measured values (%): C, 52.90; H, 3.39; O, 44.03.

[0051] Preparation of Modified Grafting Agent B2:

[0052] Dissolve 3-bromo-4-methylmaleic anhydride (0.95 g, 5 mmol) and methyl acrylate (0.52 g, 6 mmol) in 100 ml of DMF solution in a 250 ml three-necked flask, and add catalytic amount of palladium acetate (2.25 μg, 10 μmol). Carry out water separation and reflux at 120 °C for 12 h. After the reaction is completed, remove the solvent DMF by means of vacuum distillation, and purify the solid product by column chromatography (eluent = EA: HEX = 1:5), with a final yield of 76%. (1H 300 MHz, DMSO): 7.52 (s, 1H, C═C-H); 5.33 (s, 1H, C═C-H); 3.86 - 2.61 (m, 6H, CH3). C8H8O5: Calculated values of elemental analysis (%): C, 55.11; H, 4.11; O, 40.78. Experimental measured values (%): C, 55.20; H, 4.23; O, 40.83.

[0053] Preparation of Modified Grafting Agent B3:

[0054] In a 250 ml three-necked flask, 2,3-dibromomaleic anhydride (1.27 g, 5 mmol) and methyl acrylate (0.52 g, 6 mmol) were dissolved in 100 ml of DMF solution, and a catalytic amount of palladium acetate (2.25 μg, 10 μmol) was added. Water was separated and refluxed at 120 °C for 12 h. After the reaction was completed, the solvent DMF was removed by means of vacuum distillation, and the solid product was purified by column chromatography (eluent = EA: HEX = 1:1), and the final yield was 27%. (1H 300 MHz, DMSO): 7.51 (s, 2H, C═C-H); 5.35 (s, 2H, C═C-H); 3.81 (s, 6H, -CH3). C 12 H 10 O7: Calculated values (%) for elemental analysis: C, 54.14; H, 3.79; O, 42.07. Experimental values (%): C, 54.14; H, 3.80; O, 42.11.

[0055] Preparation of modified grafting agent B4:

[0056] In a 250 ml three-necked flask, 2-bromomaleic anhydride (0.88 g, 5 mmol) and methyl 3-butenoate (0.60 g, 6 mmol) were dissolved in 100 ml of DMF solution, and a catalytic amount of palladium acetate (2.25 μg, 10 μmol) was added. Water was separated and refluxed at 120 °C for 12 h. After the reaction was completed, the solvent DMF was removed by means of vacuum distillation, and the solid product was purified by column chromatography (eluent = EA: HEX = 1:5), and the final yield was 74%. (1H 300 MHz, DMSO): 8.37 (s, 1H, Ar-H); 7.93 (s, 1H, Ar-H); 7.74 (s, 1H, Ar-H); 7.52 (s, C═C-H); 6.32 (s, C═C-H); 3.82 (s, 3H, -CH3). C 12 H8O5: Calculated values (%) for elemental analysis: C, 55.11; H, 4.11; O, 40.78. Experimental values (%): C, 55.22; H, 4.26; O, 40.93.

[0057] Testing of grafting rate of modified grafting agent

[0058] According to the raw materials and dosages in Table 2, the obtained modified grafting agent (about 1 g) was grafted with high molecular weight polyethylene (about 10 g). The specific steps were as follows: High molecular weight polyethylene and xylene (50 ml) were added to a container with stirring and N2 was introduced. It was heated to 140 °C. After complete melting, the modified grafting agent was added for grafting modification. The reaction was carried out for 1 h. After completion, the mixture was poured into a large amount of ethanol and stirred.

[0059] The grafting ratio was determined by infrared spectroscopy. Since the cyclic carboxylic acid derivative has distinct characteristic peaks (C=O, 1780 cm -1 ), the infrared spectra of the original polyethylene and the modified polyethylene were collected, and the grafting ratio was calculated according to the Lambert-Beer law based on the intensities of the characteristic peaks.

[0060] Table 2 Proportions of Different Modified Grafting Agents and Polyethylene and Their Grafting Ratios

[0061] High molecular weight polyethylene (g) Modified grafting agent (g) Grafting rate (%) 10 B1 1g 5.4 10 B2 1g 2.6 10 B3 1g 0.3 10 B4 1g 2.7

[0062] As can be seen from Table 2, the modified grafting agent provided by the present invention can graft with high-molecular-weight polyethylene. Among them, the grafting ratio of the modified grafting agent B1 is the highest, reaching 5.7%.

[0063]

Examples and Comparative Examples

[0064] For the specific components, refer to Table 3.

[0065] The high-molecular-weight polyethylene (LPF-8010U) powder was added to an internal mixer, and pre-treated at 190 °C for 5 min at rpm = 10; subsequently, a grafting agent was added to graft the polyethylene under the reaction conditions of 190 °C for 5 min; finally, polypropylene powder was added, and it was kneaded at 190 °C at rpm = 60 for 10 min.

[0066] Melting point analysis was performed using DSC. The parameter settings were as follows: heating from room temperature to 180 °C at 10 °C / min, cooling to room temperature at 20 °C / min, then heating to 180 °C at 10 °C / min, and finally rapidly cooling to room temperature.

[0067] Table 3 Melting Point Tests of Examples and Comparative Examples

[0068]

[0069]

[0070] Note: In Comparative Example 4, the grafting agent D is cyclobutane-1,2-dicarboxylic anhydride.

[0071] As can be seen from Table 3, the embodiments of the present invention can effectively increase the melting point of polyethylene by adding appropriate amounts of modified grafting agents and polypropylene. In particular, Example 1 shows the most significant melting point increase effect, which may be due to the unique functional group design of B1, which can promote interfacial bonding without excessively interfering with the crystallization process. The increase in the melting point of polyethylene helps to improve the safety, reliability and applicability of lithium battery separators through enhanced thermal stability, optimized mechanical properties, and pore structure regulation, and is suitable for battery technology requirements in high energy density, fast charging and extreme environments. Comparative Example 1 shows that the addition of polypropylene alone has a weak effect on increasing the melting point; Comparative Example 2 shows that an excessive amount of grafting agent may destroy the crystal structure of the material; Comparative Example 3 has less polypropylene, reflecting the insufficient synergistic effect of the grafting agent and polypropylene.

[0072]

Application performance testing

[0073] Taking Example 1 as an example, the present invention makes the polyethylene samples of Example 1 and Comparative Example 4 into diaphragms to test their application performance.

[0074] The specific preparation method is as follows: polyethylene and white oil are mixed evenly (the mass fraction of polyethylene is 40%), and the oil-containing cast sheet is obtained through a twin-screw extruder (extrusion temperature 160°C), and the oil-containing cast sheet is longitudinally stretched and transversely stretched to obtain a diaphragm with a thickness of about 12 μm. The diaphragm is dried and evaporated, and cut into suitable sizes for standby use.

[0075] The diaphragm determination standard is tested by the following method:

[0076] 1. Puncture strength determination: Puncture strength determination refers to standard GB / T 36363-2018;

[0077] 2. Determination of tensile yield strength: The tensile yield strength is determined according to the standard GB / T1040.2-2006;

[0078] 3. Wettability determination: refer to standard GB / T 30447-2013.

[0079] The performance test results of the polyethylene diaphragm samples are shown in Table 4.

[0080] Table 4 Performance determination of polyethylene diaphragm samples

[0081] Sample number Puncture strength (N / μm) Tensile yield strength (MPa) Wetting angle (°) Blank group 0.339 162 125 Example 1 0.390 183 93 Comparative example 4 0.359 171 115

[0082] As can be seen from Table 4, the embodiments of the present invention help to improve the properties of polyethylene, significantly increase the puncture strength and tensile yield strength of the polyethylene diaphragm, and significantly improve the wettability of the diaphragm. The inventor speculates that the modified grafting agent of the present invention contains a large number of polar carboxylic acid groups, which can form a network structure, improve the structural strength of polyethylene to a certain extent, resulting in an increase in puncture strength and tensile strength. In particular, compared with Comparative Example 4, the introduction of polar groups greatly improves the hydrophilicity of polyethylene and reduces the wetting angle of polyethylene.

Claims

1. A method for improving the properties of high molecular weight polyethylene with polypropylene, characterized in that, Polyethylene is melt-blended and kneaded with polypropylene through a modified grafting agent; The polypropylene accounts for 1.5 - 10% of the weight of the polyethylene, and the modified grafting agent accounts for 1 - 15% of the weight of the polyethylene; The modified grafting agent is a cyclic carboxylic acid derivative-coupled acrylate compound.

2. The method for improving the properties of high molecular weight polyethylene with polypropylene according to claim 1, characterized in that, The modified grafting agent is a cyclic carboxylic acid derivative-coupled acrylate compound having the general formula B structure, where Y is an acrylate group; R is selected from H, halogen, C1-C3 alkyl, C1-C3 alkoxy, phenyl or Y; 3. The method for improving the properties of high molecular weight polyethylene with polypropylene according to claim 2, characterized in that, The acrylic ester group Y is represented by the following structural formula: -C=C-(CH2) n -COOR1, where n = 0-3 and R1 is selected from C1-C3 alkyl groups.

4. The method for improving the properties of high molecular weight polyethylene with polypropylene according to claim 2, characterized in that, The acrylate group Y is a methyl acrylate group.

5. The method for improving the properties of high molecular weight polyethylene with polypropylene according to claim 2, characterized in that, R1 to R5 are each independently selected from H, methyl, ethyl, methoxy or ethoxy.

6. The method for improving the properties of high molecular weight polyethylene with polypropylene according to claim 2, characterized in that, The modified grafting agent is selected from any one of the following compounds of formula B1 to B4:

7. The method for improving the properties of high molecular weight polyethylene with polypropylene according to claim 2, characterized in that The preparation method of the cyclic carboxylic acid derivative-coupled acrylate compound is obtained by reacting a halogenated cyclic carboxylic acid derivative with an acrylate in the presence of a catalyst.

8. The method for improving the properties of high molecular weight polyethylene with polypropylene according to claim 1, characterized in that, It includes the following steps: (1) Add polyethylene to a kneader for pretreatment; (2) Then add a modified grafting agent to graft the polyethylene; (3) Finally, add polypropylene for kneading, and cool after kneading to obtain the product.

9. The method for improving the properties of high molecular weight polyethylene with polypropylene according to claim 8, characterized in that, In step (1), after adding polyethylene to the kneader, knead at 180 - 200 °C and 5 - 15 rpm for 1 - 10 min for pretreatment; In step (2), after adding the modified grafting agent to the kneader, knead at 180 - 200 °C and 5 - 15 rpm for 1 - 10 min; In step (3), after adding polypropylene to the kneader, knead at 180 - 200 °C and 50 - 70 rpm for 5 - 15 min.

10. The method for improving the properties of high molecular weight polyethylene with polypropylene according to claim 1, characterized in that, The polyethylene includes high molecular weight polyethylene or ultra-high molecular weight polyethylene, and its average molecular weight is 500,000 - 2,000,000.

Images