阅读说明：本技术 一种聚丙烯隔膜及其制备方法 (Polypropylene diaphragm and preparation method thereof ) 是由 张金辉 王珑 安佳琳 张晓楠 王博 丰华 王国辉 王为陶 于 2018-07-19 设计创作，主要内容包括：本发明涉及一种聚丙烯隔膜及其制备方法,属于电池隔膜技术领域。本发明的聚丙烯隔膜,主要由结晶度为60～75％、等规度为97～99％、重均分子量为30万～50万的聚丙烯制成。本发明的聚丙烯隔膜具有较高的抗拉强度,厚度为14～40μm的聚丙烯隔膜根据GB/T 1040.3-2006进行强度测试时,纵向5％形变位置对应的纵向抗拉强度≥40MPa,横向5％形变位置对应的横向抗拉强度≥16MPa；本发明的聚丙烯隔膜能够减少隔膜在注液过程中产生的褶皱,提高电池的安全性能。(The invention relates to a polypropylene diaphragm and a preparation method thereof, belonging to the technical field of battery diaphragms. The polypropylene diaphragm is mainly prepared from polypropylene with the crystallinity of 60-75%, the isotacticity of 97-99% and the weight-average molecular weight of 30-50 ten thousand. The polypropylene diaphragm has high tensile strength, when the polypropylene diaphragm with the thickness of 14-40 mu m is subjected to strength test according to GB/T1040.3-2006, the longitudinal tensile strength corresponding to the longitudinal 5% deformation position is more than or equal to 40MPa, and the transverse tensile strength corresponding to the transverse 5% deformation position is more than or equal to 16 MPa; the polypropylene diaphragm can reduce the wrinkles generated in the liquid injection process of the diaphragm and improve the safety performance of the battery.)

1. A polypropylene separator characterized by: the polypropylene is mainly prepared from polypropylene with the crystallinity of 60-75%, the isotacticity of 97-99% and the weight-average molecular weight of 30-50 ten thousand.

2. The polypropylene separator according to claim 1, wherein: is prepared from the polypropylene and an antioxidant; the mass ratio of the polypropylene to the antioxidant is 1000: 1-2.

3. The polypropylene separator according to claim 2, wherein: the antioxidant consists of a phosphite antioxidant and a hindered phenol antioxidant; the mass ratio of the phosphite antioxidant to the hindered phenol antioxidant is 0.8-1.3: 1.

4. The polypropylene separator according to claim 1, wherein: the thickness of the polypropylene diaphragm is 10-40 mu m.

5. A method for preparing the polypropylene separator according to claim 1, wherein: the method comprises the following steps: providing a processing material with polypropylene as a main component, sequentially carrying out melt extrusion and casting on the processing material to form a thick sheet, and crystallizing and stretching the thick sheet.

6. The method for producing a polypropylene separator according to claim 5, wherein: the processing temperature of the melt extrusion is 150-240 ℃.

7. The method for producing a polypropylene separator according to claim 5, wherein: the crystallization ensures that the crystallinity of the thick plate after crystallization is more than 80 percent, and the content of alpha crystal in a crystal region is more than 90 percent.

8. The method for producing a polypropylene separator according to claim 5 or 7, wherein: the crystallization temperature is 80-120 ℃, and the crystallization time is 8-24 h.

9. The method for producing a polypropylene separator according to claim 5, wherein: the stretching is longitudinal stretching and transverse stretching in sequence; the stretching ratio of the longitudinal stretching is 2-6 times; the stretching ratio of the transverse stretching is 1.2-5 times.

10. The method for producing a polypropylene separator according to claim 9, wherein: the temperature of longitudinal stretching is 90-130 ℃; the temperature of the transverse stretching is 100-135 ℃.

Technical Field

The invention relates to a polypropylene diaphragm and a preparation method thereof, belonging to the technical field of battery diaphragms.

Background

The polypropylene diaphragm is mainly used for isolating the anode and the cathode of the battery and conducting lithium ions through electrolyte. When the existing polypropylene diaphragm is applied to a lithium ion battery, a problem commonly encountered by a battery core manufacturer is the problem of liquid injection wrinkles. The electrolyte injection wrinkle problem occurs after the battery is injected with electrolyte, which is manifested as dendritic wrinkles of the separator in the transverse or longitudinal direction. The electrolyte injection wrinkle problem has a negative effect on the cyclicity of the battery, because the existence of the membrane wrinkle affects the conduction efficiency of lithium ions in the membrane, particularly the position with obvious wrinkles is easy to form a poor liquid area of electrolyte, lithium ions can be continuously gathered at the position in the circulation process, and finally lithium dendrites are formed, and the existence of the lithium dendrites can cause the micro short circuit problem of the battery in use, thereby increasing the safety risk of the battery in use.

In order to solve the problem of liquid injection wrinkles of polypropylene diaphragms, various methods are adopted in the industry, and a battery core factory usually reduces the wrinkle degree of the liquid injection wrinkles by adjusting the proportion of different components of electrolyte or adjusting the tension during winding of a battery core, but the method cannot fundamentally solve the problems and needs to consider the problems from the characteristics of the diaphragm material.

Disclosure of Invention

The invention aims to provide a high-strength polypropylene diaphragm, which can reduce wrinkles generated by the diaphragm in the liquid injection process and improve the safety performance of a battery.

The invention also provides a preparation method of the high-strength polypropylene diaphragm.

In order to achieve the above purpose, the technical scheme adopted by the high-strength diaphragm of the invention is as follows:

a polypropylene diaphragm is mainly prepared from polypropylene with the crystallinity of 60-75%, the isotacticity of 97-99% and the weight-average molecular weight of 30-50 ten thousand.

The polypropylene diaphragm provided by the invention adopts polypropylene raw materials with more long-chain structures and higher regularity, and materials with high alpha crystal content can be obtained more easily in the preparation process by adopting a conventional method, so that the prepared polypropylene diaphragm has higher tensile strength. When the polypropylene diaphragm with the thickness of 14-40 mu m is subjected to strength test according to GB/T1040.3-2006, the longitudinal tensile strength corresponding to the longitudinal 5% deformation position is more than or equal to 40MPa, and the transverse tensile strength corresponding to the transverse 5% deformation position is more than or equal to 16 MPa. Due to the fact that the tensile strength of the diaphragm is improved, the stiffness of the diaphragm can be improved, the deformation resistance of the diaphragm is improved, and wrinkles generated in the liquid injection process of the diaphragm are reduced.

The polypropylene diaphragm has the advantage of high strength, and can reduce liquid injection wrinkles of the diaphragm and improve the safety performance of a battery.

Preferably, the polypropylene diaphragm is made of the polypropylene and an antioxidant; the mass ratio of the polypropylene to the antioxidant is 1000: 1-2.

Preferably, the antioxidant consists of a phosphite antioxidant and a hindered phenol antioxidant; the mass ratio of the phosphite antioxidant to the hindered phenol antioxidant is 0.8-1.3: 1. The phosphite antioxidant is antioxidant 168; the hindered phenol antioxidant is antioxidant 1010.

Preferably, the thickness of the polypropylene diaphragm is 10-40 μm.

Preferably, the polypropylene comprises a first polypropylene and a second polypropylene; the crystallinity of the first polypropylene is 60-70%, the isotacticity is 97-99%, and the weight average molecular weight is 38-50 ten thousand; the second polypropylene has a crystallinity of 65 to 75%, an isotacticity of 97 to 98%, and a weight average molecular weight of 30 to 50 ten thousand. When two kinds of weight average molecular weight polypropylenes are used, generally, the difference between the two molecular weights is within 20 ten thousand, the high molecular weight polypropylene provides strength, the low molecular weight polypropylene ensures processability, and the use of two kinds of polypropylenes can provide wider processability than that of a single polypropylene.

Preferably, the mass ratio of the first polypropylene to the second polypropylene is 1: 0.8-3.

The preparation method of the polypropylene diaphragm adopts the technical scheme that:

the preparation method of the polypropylene separator comprises the following steps: providing a processing material with polypropylene as a main component, sequentially carrying out melt extrusion and casting on the processing material to form a thick sheet, and crystallizing and stretching the thick sheet.

The preparation method of the polypropylene diaphragm provided by the invention is simple in process, low in cost and convenient to popularize and apply, and the obtained polypropylene diaphragm has high tensile strength and can reduce wrinkles generated in the battery liquid injection process.

Preferably, the processing material further comprises an antioxidant. The mass ratio of the polypropylene to the antioxidant is 1000: 1-2.

Preferably, the antioxidant consists of a phosphite antioxidant and a hindered phenol antioxidant; the mass ratio of the phosphite antioxidant to the hindered phenol antioxidant is 0.8-1.3: 1. The phosphite antioxidant is antioxidant 168; the hindered phenol antioxidant is antioxidant 1010.

Preferably, the processing temperature of the melt extrusion is 150-240 ℃.

The thickness of the slab has an effect on the crystallinity, which is higher if the slab is heated more uniformly at the same temperature. Preferably, the thickness of the thick sheet is 0.1-1.1 mm. More preferably, the thickness of the slab is 1 to 1.1 mm.

The high alpha crystal proportion in the thick plate crystal area after crystallization can generate the micro defect quantity generated by the sliding of the plate crystal in the stretching process to meet the requirement of pore forming. Preferably, the crystallization is performed such that the degree of crystallization of the slab after crystallization is 80% or more and the content of α -crystal in the crystal region is 90% or more. Preferably, the content of alpha crystals in the crystal region is 95% or more. The content of α crystal in the crystal region can be measured by DSC method.

The polypropylene crystals are divided into alpha crystals (platelets) and beta crystals (spherulites); compared with the alpha crystal, the beta crystal is easy to tear and convert into the alpha crystal due to the poor stability of the beta crystal when being subjected to external force or external temperature. After the crystallinity of the thick sheet and the content of alpha crystals in a crystal region are improved, due to the close arrangement of the alpha crystals, the consistency of the structure between wafers and the binding force between the wafers is high, the stretching inconsistency caused by crystal form conversion in the stretching process is reduced/avoided, the uniformity of the stretching process is improved, the uniform tearing between the wafers is ensured, the silver veins/micropores with high consistency are formed, the orientation consistency of polypropylene fibers/molecular chains in the final diaphragm is ensured, and the higher tensile strength of the diaphragm is ensured.

Preferably, the crystallization temperature is 80-120 ℃. The crystallization time is 8-24 h. Because the content of alpha crystal in the slab crystal area is increased by high-temperature aging during slab processing.

During the stretching process, the alpha crystal generates wafer slip to form micropores. Preferably, the stretching is longitudinal stretching and transverse stretching in sequence. The stretching ratio of the longitudinal stretching is 2-6 times. The stretching ratio of the transverse stretching is 1.2-5 times. Firstly, longitudinal stretching is carried out to enable alpha crystals to slide in the longitudinal direction, silver grains are generated to form micropores, and then, transverse stretching is carried out to carry out hole pattern modification to ensure that round-like holes are obtained.

The stretching temperature is selected to make the membrane have better orientation degree when oriented, thereby obtaining better tensile strength. Preferably, the temperature of the transverse stretching is 100-135 ℃. The temperature of longitudinal stretching is 90-130 ℃.

Further preferably, the temperature of the transverse stretching is 105-130 ℃.

Further preferably, the temperature of the longitudinal stretching is 95-120 ℃.

Preferably, the preparation method of the polypropylene separator further comprises winding the stretched material on a winding mechanism.

Detailed Description

The technical solution of the present invention will be further described with reference to the following embodiments.

Example 1

The polypropylene diaphragm of the embodiment is prepared from the following raw materials in parts by weight: 998.5 parts of polypropylene and 1.5 parts of antioxidant; the crystallinity of the adopted polypropylene is 70 percent, the isotacticity is 98 percent, and the weight-average molecular weight is 45 ten thousand; the antioxidant adopted consists of an antioxidant 1010 and an antioxidant 168, and the mass ratio of the antioxidant 1010 to the antioxidant 168 is 1: 1; the thickness of the polypropylene separator was 18 μm.

The preparation method of the polypropylene separator of the embodiment comprises the following steps:

1) taking the polypropylene particles and the antioxidant in the formula ratio, putting the mixture into a high-speed mixer, uniformly mixing the mixture in a solid state, and granulating the mixture again by using a double-screw extruder to prepare a special material (namely a processing material) for the high-strength polypropylene diaphragm;

2) conveying the special material for the high-strength polypropylene diaphragm to a bin of a single-screw extruder through a feeding pipeline, and carrying out melt extrusion in the extruder, wherein the processing temperature of the extruder is controlled to be 200 ℃ during melt extrusion;

3) uniformly applying the melt after the melt extrusion on a casting sheet roller through a die head to form a thick sheet with the thickness of 1 mm;

4) winding the prepared thick sheet on a paper tube by using an auxiliary winding machine to form a thick sheet mother roll, then placing the obtained mother roll on an aging vehicle, and pushing the aging vehicle into a high-temperature aging chamber with the set temperature of 90 ℃ to perform standing for 12 hours; after the standing is finished, the crystallinity of the thick sheet is 88 percent, and the content of alpha crystal in a crystal region is 97 percent;

5) respectively stretching the obtained thick sheet with the crystallinity of 88 percent and the alpha crystal content of 97 percent in a crystal region in two directions by a longitudinal stretching machine and a transverse stretching machine, wherein the stretching ratio of longitudinal stretching is 4 times, and the stretching ratio of transverse stretching is 1.8 times; the temperature of longitudinal stretching is 95 ℃; the temperature of transverse stretching is 124 ℃;

6) and (3) uniformly coiling the polypropylene diaphragm formed after longitudinal and transverse stretching on a coiling mechanism through a traction mechanism to finish the production process of the high-strength polypropylene diaphragm.

Example 2

The polypropylene diaphragm of the embodiment is prepared from the following raw materials in parts by weight: 998.5 parts of polypropylene and 1.5 parts of antioxidant; the adopted polypropylene consists of a first polypropylene and a second polypropylene in a mass ratio of 1:1, wherein the crystallinity of the first polypropylene is 65 percent, the isotacticity is 97 percent, the weight-average molecular weight is 38 ten thousand, the crystallinity of the second polypropylene is 75 percent, the isotacticity is 98.5 percent, and the weight-average molecular weight is 50 ten thousand; the antioxidant adopted consists of an antioxidant 1010 and an antioxidant 168, and the mass ratio of the antioxidant 1010 to the antioxidant 168 is 1: 1; the thickness of the polypropylene separator was 16 μm.

The preparation method of the polypropylene separator of the embodiment comprises the following steps:

1) taking the first polypropylene particles, the second polypropylene particles and the antioxidant in the formula ratio, putting the first polypropylene particles, the second polypropylene particles and the antioxidant into a high-speed mixer, uniformly mixing the first polypropylene particles, the second polypropylene particles and the antioxidant in a solid state, and granulating the mixture again by using a double-screw extruder to prepare a special material (namely a processing material) for the high-strength polypropylene diaphragm;

2) conveying the special material for the high-strength polypropylene diaphragm to a bin of a single-screw extruder through a feeding pipeline, and carrying out melt extrusion in the extruder, wherein the processing temperature of the extruder is controlled to be 215 ℃ during melt extrusion;

3) uniformly applying the melt after the melt extrusion on a casting sheet roller through a die head to form a thick sheet with the thickness of 1 mm;

4) winding the prepared thick sheet on a paper tube by using an auxiliary winding machine to form a thick sheet mother roll, then placing the obtained mother roll on an aging vehicle, and pushing the aging vehicle into a high-temperature aging chamber with the set temperature of 100 ℃ to perform standing for 16 hours; after the standing is finished, the crystallinity of the thick sheet is 85 percent, and the content of alpha crystal in a crystal region is 95 percent;

5) the obtained thick sheet with the crystallinity of 85 percent and the content of alpha crystals in a crystal region of 95 percent is respectively stretched in two directions by a longitudinal stretching machine and a transverse stretching machine, wherein the stretching ratio of the longitudinal stretching is 4.7 times, and the stretching ratio of the transverse stretching is 1.8 times; the temperature of longitudinal stretching is 97 ℃; the temperature of transverse stretching is 123 ℃;

6) and (3) uniformly coiling the polypropylene diaphragm formed after longitudinal and transverse stretching on a coiling mechanism through a traction mechanism to finish the production process of the high-strength polypropylene diaphragm.

Example 3

The polypropylene diaphragm of the embodiment is prepared from the following raw materials in parts by weight: 1000 parts of polypropylene and 1 part of antioxidant; the adopted polypropylene consists of a first polypropylene and a second polypropylene in a mass ratio of 1.1:1, wherein the crystallinity of the first polypropylene is 65 percent, the isotacticity is 98 percent, the weight average molecular weight is 40 ten thousand, the crystallinity of the second polypropylene is 70 percent, the isotacticity is 98.5 percent, and the weight average molecular weight is 43 ten thousand; the antioxidant adopted consists of an antioxidant 1010 and an antioxidant 168, and the mass ratio of the antioxidant 1010 to the antioxidant 168 is 1.3: 1; the thickness of the polypropylene separator was 10 μm.

The preparation method of the polypropylene separator of the embodiment comprises the following steps:

1) taking the polypropylene particles and the antioxidant in the formula ratio, putting the mixture into a high-speed mixer, uniformly mixing the mixture in a solid state, and granulating the mixture again by using a double-screw extruder to prepare a special material (namely a processing material) for the high-strength polypropylene diaphragm;

2) conveying the special material for the high-strength polypropylene diaphragm to a bin of a single-screw extruder through a feeding pipeline, and carrying out melt extrusion in the extruder, wherein the processing temperature of the extruder is controlled to be 150 ℃ during melt extrusion;

3) uniformly applying the melt after the melt extrusion on a casting sheet roller through a die head to form a thick sheet with the thickness of 1 mm;

4) winding the prepared thick sheet on a paper tube by using an auxiliary winding machine to form a thick sheet mother roll, then placing the obtained mother roll on an aging vehicle, and pushing the aging vehicle into a high-temperature aging chamber with the set temperature of 80 ℃ to perform standing for 24 hours; after the standing is finished, the crystallinity of the thick sheet is 84%, and the content of alpha crystals in the crystal region is 96%;

5) respectively stretching the obtained thick sheet with the crystallinity of 84% and the content of alpha crystals in a crystal region of 96% in two directions by a longitudinal stretching machine and a transverse stretching machine, wherein the stretching ratio of the longitudinal stretching is 2 times, and the stretching ratio of the transverse stretching is 1.2 times; the temperature of longitudinal stretching is 95 ℃; the temperature of transverse stretching is 120 ℃;

6) and (3) uniformly coiling the polypropylene diaphragm formed after longitudinal and transverse stretching on a coiling mechanism through a traction mechanism to finish the production process of the high-strength polypropylene diaphragm.

Example 4

The polypropylene diaphragm of the embodiment is prepared from the following raw materials in parts by weight: 1000 parts of polypropylene and 2 parts of antioxidant; the adopted polypropylene consists of a first polypropylene and a second polypropylene in a mass ratio of 1.2:1, wherein the crystallinity of the first polypropylene is 65 percent, the isotacticity is 97 percent, the weight average molecular weight is 38 ten thousand, the crystallinity of the second polypropylene is 75 percent, the isotacticity is 98.5 percent, and the weight average molecular weight is 50 ten thousand; the antioxidant adopted consists of an antioxidant 1010 and an antioxidant 168, and the mass ratio of the antioxidant 1010 to the antioxidant 168 is 0.8: 1; the thickness of the polypropylene separator was 40 μm.

The preparation method of the polypropylene separator of the embodiment comprises the following steps:

1) taking the polypropylene particles and the antioxidant in the formula ratio, putting the mixture into a high-speed mixer, uniformly mixing the mixture in a solid state, and granulating the mixture again by using a double-screw extruder to prepare a special material (namely a processing material) for the high-strength polypropylene diaphragm;

2) conveying the special material for the high-strength polypropylene diaphragm to a bin of a single-screw extruder through a feeding pipeline, and carrying out melt extrusion in the extruder, wherein the processing temperature of the extruder is controlled to be 240 ℃ during melt extrusion;

3) uniformly applying the melt after melt extrusion on a casting sheet roller through a die head to form a thick sheet with the thickness of 1.1 mm;

4) winding the prepared thick sheet on a paper tube by using an auxiliary winding machine to form a thick sheet mother roll, then placing the obtained mother roll on an aging vehicle, and pushing the aging vehicle into a high-temperature aging chamber with the set temperature of 120 ℃ to perform standing for 8 hours; after the standing is finished, the crystallinity of the thick sheet is 91 percent, and the content of alpha crystal in a crystal region is 98 percent;

5) respectively stretching the obtained thick sheet with the crystallinity of 91 percent and the alpha crystal content of 98 percent in a crystal area in two directions by a longitudinal stretching machine and a transverse stretching machine, wherein the stretching ratio of longitudinal stretching is 6 times, and the stretching ratio of transverse stretching is 5 times; the temperature of longitudinal stretching is 97 ℃; the temperature of transverse stretching is 120 ℃;

6) and (3) uniformly coiling the polypropylene diaphragm formed after longitudinal and transverse stretching on a coiling mechanism through a traction mechanism to finish the production process of the high-strength polypropylene diaphragm.

Example 5

The polypropylene separator of the present example differs from the polypropylene separator of example 2 only in that: the polypropylene adopted in the preparation process consists of a first polypropylene and a second polypropylene according to the mass ratio of 1:3, the longitudinal stretching temperature is 130 ℃, and the transverse stretching temperature is 135 ℃; the rest is the same as the example 2.

Comparative example 1

The polypropylene separator of comparative example 1 was prepared in exactly the same manner as in example 2, except that the first polypropylene having a weight average molecular weight of 10 ten thousand and the second polypropylene having a weight average molecular weight of 20 ten thousand were used.

Comparative example 2

The polypropylene separator of comparative example 2 was prepared in exactly the same manner as in example 2, except that the first polypropylene having a weight average molecular weight of 80 ten thousand and the second polypropylene having a weight average molecular weight of 100 ten thousand were used.

Comparative example 3

The polypropylene separator of comparative example 3 was prepared in exactly the same manner as in example 2, except that the first polypropylene having a crystallinity of 50% and the second polypropylene having a crystallinity of 50% were used.

Comparative example 4

The polypropylene separator of comparative example 4 was prepared in exactly the same manner as in example 2, except that the first polypropylene having a crystallinity of 80% and the second polypropylene having a crystallinity of 80% were used.

Comparative example 5

The polypropylene separator of comparative example 5 was prepared except in step 4): the standing time is 4h, the content of alpha crystal in the crystal region of the slab after the completion of standing is 70%, and the rest is completely the same as that of the example 2.

Examples of the experiments

The polypropylene diaphragms of examples 1 to 5 and comparative examples 1 to 5 were subjected to strength performance tests according to GB/T1040.3-2006, respectively, wherein the width and length of the diaphragm sample used in the test process were 20mm and 40mm, the tensile speed was 50mm/min, and the longitudinal tensile strength corresponding to the longitudinal 5% deformation position and the transverse tensile strength corresponding to the transverse 5% deformation position are shown in Table 1.

Table 1 polypropylene separator performance test results

	Longitudinal tensile strength/MPa	Transverse tensile strength/MPa
			Example 1	50	18
Example 2	55	18.5
			Example 3	56	18
Example 4	56	17.5
			Example 5	58	19.5
Comparative example 1	45	15
			Comparative example 2	50	15
Comparative example 3	36	13
			Comparative example 4	44	14
Comparative example 5	40	12

As can be seen from the data in table 1, the polypropylene diaphragms of examples 1 to 5 have significantly improved effects in both the longitudinal 5% deformation corresponding strength and the transverse 5% deformation corresponding strength of the diaphragms, as compared to the polypropylene diaphragms of comparative examples 1 to 5.