阅读说明：本技术 一种低闭孔高破膜的聚烯烃隔膜及其制备方法 (Low-closed-pore high-rupture-membrane polyolefin diaphragm and preparation method thereof ) 是由 陶晶 王志豪 李伟 陈强 袁其振 杨影杰 于 2019-10-21 设计创作，主要内容包括：本发明公开了一种低闭孔高破膜的聚烯烃隔膜及其制备方法；所述聚烯烃隔膜主要由聚乙烯、聚丙烯和聚丙烯接枝支化聚乙烯共混组成；所述聚乙烯包含低熔点聚乙烯和高熔点聚乙烯。本发明解决了聚乙烯与聚丙烯的相容性问题,提高了两者在熔融状态下的均相能力,使得聚丙烯分子链穿插在聚乙烯分子链之间,起到刚性骨架支撑作用,防止隔膜破裂,从而获得了较高的破膜温度和优异的热收缩性能。本发明还通过高/低熔点聚乙烯共混,使共混聚乙烯起熔温度降低,熔程加长,增加闭孔过程持续时间,并形成连续闭孔特点。(The invention discloses a polyolefin diaphragm with low closed pores and high rupture strength and a preparation method thereof; the polyolefin diaphragm is mainly formed by blending polyethylene, polypropylene and polypropylene grafted branched polyethylene; the polyethylene comprises a low melting point polyethylene and a high melting point polyethylene. The invention solves the compatibility problem of polyethylene and polypropylene, improves the homogeneous phase capability of the polyethylene and the polypropylene in a molten state, enables the polypropylene molecular chains to be inserted between the polyethylene molecular chains, plays a role of rigid framework support, prevents the diaphragm from cracking, and thus obtains higher film breaking temperature and excellent heat shrinkage performance. The invention also reduces the melting temperature of the blended polyethylene, prolongs the melting process, increases the duration time of the hole closing process and forms the characteristic of continuous hole closing by blending the high/low melting point polyethylene.)

1. A polyolefin separator, characterized in that: the polyolefin diaphragm is mainly formed by blending polyethylene, polypropylene and polypropylene grafted branched polyethylene.

2. The polyolefin separator according to claim 1, characterized in that: the polypropylene grafted branched polyethylene is prepared by the following method: mixing branched polyethylene with perfluorophenyl azide containing a propylene functional group, activating azide by ultraviolet radiation, connecting the generated active azide to the branched polyethylene through a covalent bond, dissolving the perfluorophenyl azide grafted branched polyethylene, adding an initiator and a propylene monomer for reaction, and obtaining the polypropylene grafted branched polyethylene.

3. The polyolefin separator according to claim 2, characterized in that: the branched polyethylene has a branching degree of between 2.8 and 5.69.

4. The polyolefin separator according to claim 2, characterized in that: the propylene functional group is methacrylate, propylene isocyanate and acrylamide functional group.

5. The polyolefin separator according to claim 2, characterized in that: the initiator is azobisisobutyronitrile, potassium persulfate, dibenzoyl peroxide, cumene hydroperoxide, triethyl aluminum or titanium tetrachloride.

6. The polyolefin separator according to claim 1, characterized in that: the polyethylene comprises low-melting point polyethylene and high-melting point polyethylene, the melting point range of the low-melting point polyethylene is between 105 ℃ and 110 ℃, the melting point range of the high-melting point polyethylene is between 135 ℃ and 140 ℃, and the mass ratio of the low-melting point polyethylene to the high-melting point polyethylene is 0-10: 0-10.

7. The polyolefin separator according to claim 6, characterized in that: the melting point range of the polypropylene grafted branched polyethylene is between 120 ℃ and 125 ℃.

8. The polyolefin separator according to claim 1, characterized in that: the melting point of the polypropylene ranges between 155 ℃ and 165 ℃.

9. The polyolefin separator according to any one of claims 1 to 8, characterized in that: the mass ratio of the polyethylene to the polypropylene grafted branched polyethylene is 1-10:5-15: 1-10.

10. The method for preparing a polyolefin separator according to any one of claims 1 to 9, wherein: the method comprises the following steps: mixing polyethylene, polypropylene grafted branched polyethylene and a plasticizer, melting, extruding, and then at least carrying out the steps of sheet casting, longitudinal stretching, transverse stretching, extraction and heat setting to obtain the polyolefin diaphragm.

Technical Field

The invention belongs to the field of lithium battery diaphragm materials, and particularly relates to a low-closed-pore high-rupture-membrane polyolefin diaphragm and a preparation method thereof.

Background

Polyolefin microporous membranes are used for microfiltration membranes, battery separators, capacitor separators, fuel cell materials, and the like. Among these applications, when used as a battery separator, particularly a lithium ion battery separator, the polyolefin microporous membrane is required to have excellent ion permeability, excellent mechanical strength, and the like.

In order to ensure the safety of batteries, separators for high-capacity batteries in recent years are required to have "low closed-cell temperature characteristics", "high rupture temperature characteristics", and "low heat shrinkability". In addition, in order to reduce variations in battery characteristics, it is also required to reduce variations in film thickness.

The "low closed cell temperature characteristic" is a function of ensuring the safety of the battery by melting the separator to form a film covering the electrode and blocking the current when the inside of the battery is overheated due to an overcharge state or the like. It is known that in the case of a polyethylene microporous membrane, the closed cell temperature, i.e., the temperature at which the melt characteristics are exhibited, is about 140 ℃. However, in order to prevent runaway reaction and the like in the battery as early as possible, it is preferable that the melting temperature is lower.

The "high rupture temperature characteristic" means a property of the separator that the separator does not crack even when heated to a temperature higher than the melting temperature. Further, "low heat shrinkability" means a property that the heat shrinkability is small even when heated to a temperature equal to or higher than the melting temperature. Both of these are necessary in order to maintain the shape even after melting and to maintain the insulation between the electrodes.

In order to ensure the safety of the battery at 150 ℃, the battery diaphragm is required to reach the American standard UL1642

The performance of the battery safety evaluation criteria specified in "Standard for Lithium B atteries". The evaluation was performed by keeping the separator in an oven at 150 ℃ for 10 minutes. To meet this standard, it is desirable that the membrane melt at 130 ℃ - & 140 ℃ without porosity, and that no rupture of the membrane occurs and that thermal shrinkage is minimized to maintain the shape even when heated above 150 ℃.

"the deviation of the film thickness is small" is an important property for stably obtaining the above-mentioned properties. The low closed-cell temperature characteristic, the high rupture temperature characteristic, and the heat shrinkability may not be stably obtained due to variation in the membrane thickness. Further, when the distance between the positive and negative electrode plates is changed, various performances of the battery vary, and for example, when the battery is used as a built-up battery in a notebook computer, the variation further increases, and the yield is reduced. In addition, film formation itself is difficult to perform, and this may cause a decrease in productivity.

Disclosure of Invention

In view of the above, the present invention provides a polyolefin separator with low closed cell and high rupture strength and a preparation method thereof, which can solve the problem of compatibility between polyethylene and polypropylene, and achieve better low closed cell temperature characteristics, high rupture strength temperature characteristics and thermal shrinkage.

In order to achieve the purpose, the invention provides the following technical scheme:

the invention provides a polyolefin diaphragm which is mainly formed by blending polyethylene, polypropylene and polypropylene grafted branched polyethylene.

As a preferred technical scheme, the polypropylene grafted branched polyethylene is prepared by the following method: mixing branched polyethylene with perfluorophenyl azide containing a propylene functional group, activating azide by ultraviolet radiation, connecting the generated active azide to the branched polyethylene through a covalent bond, dissolving the perfluorophenyl azide grafted branched polyethylene, adding an initiator and a propylene monomer for reaction, and obtaining the polypropylene grafted branched polyethylene.

As a preferred technical scheme, the branching degree of the branched polyethylene is between 2.8 and 5.69.

Preferably, the propylene functional group is a methacrylate, a propylene isocyanate or an acrylamide functional group.

As a preferable technical scheme, the initiator is azobisisobutyronitrile, potassium persulfate, dibenzoyl peroxide, cumene hydroperoxide, triethyl aluminum or titanium tetrachloride.

As a preferable technical scheme, the polyethylene comprises low-melting point polyethylene and high-melting point polyethylene, the melting point range of the low-melting point polyethylene is between 105 ℃ and 110 ℃, the melting point range of the high-melting point polyethylene is between 135 ℃ and 140 ℃, and the mass ratio of the low-melting point polyethylene to the high-melting point polyethylene is 0-10: 0-10.

As a preferable technical scheme, the melting point range of the polypropylene grafted branched polyethylene is between 120 ℃ and 125 ℃.

As a preferred technical scheme, the melting point range of the polypropylene is between 155 ℃ and 165 ℃.

As a preferable technical scheme, the mass ratio of the polyethylene to the polypropylene grafted branched polyethylene is 1-10:5-15: 1-10.

The invention also provides a preparation method of the polyolefin diaphragm, which comprises the following steps: mixing polyethylene, polypropylene grafted branched polyethylene and a plasticizer, melting, extruding, and then at least carrying out the steps of sheet casting, longitudinal stretching, transverse stretching, extraction and heat setting to obtain the polyolefin diaphragm.

The invention has the beneficial effects that:

1. according to the invention, the polypropylene grafted and branched polyethylene is blended with the polyethylene and the polypropylene, so that the compatibility problem of the polyethylene and the polypropylene is solved, the homogeneous phase capability of the polyethylene and the polypropylene in a molten state is improved, polypropylene molecular chains are inserted between the polyethylene molecular chains to play a rigid framework supporting role, and the diaphragm is prevented from cracking, so that higher film breaking temperature and excellent heat shrinkage performance are obtained.

2. The invention forms an average random interpenetrating model structure in the melt extrusion process by blending high/low melting point polyethylene, so that the crystal regularity of the high/low melting point polyethylene and the low/average random interpenetrating model structure is reduced, and the formation of crystal boundary defects is reduced. Due to the interaction of molecular chains, the melting temperature of the blended polyethylene is reduced, the melting range is lengthened, the duration time of the hole closing process is prolonged, and the characteristic of continuous hole closing is formed.

Drawings

In order to make the object, technical scheme and beneficial effect of the invention more clear, the invention provides the following drawings for explanation:

fig. 1 is a graph of closed cell temperature and rupture temperature test data of a diaphragm.

Detailed Description

The present invention is further described with reference to specific examples to enable those skilled in the art to better understand the present invention and to practice the same, but the examples are not intended to limit the present invention.