Patent Description:
Recently, energy storage technology has been given an increasing attention. Efforts into research and development for electrochemical devices have been actualized more and more, as the application of energy storage technology has been extended to energy for cellular phones, camcorders and notebook PC and even to energy for electric vehicles. In this context, electrochemical devices have been most spotlighted. Among such electrochemical devices, development of rechargeable secondary batteries has been focused. More recently, active studies have been conducted about designing a novel electrode and battery in order to improve the capacity density and specific energy in developing such batteries.

Among the commercially available secondary batteries, lithium secondary batteries developed in the early <NUM>'s have been spotlighted, since they have a higher operating voltage and significantly higher energy density as compared to conventional batteries, such as Ni-MH, Ni-Cd and sulfuric acid-lead batteries using an aqueous electrolyte.

Such a lithium secondary battery includes a positive electrode, a negative electrode, an electrolyte and a separator. Particularly, it is required for the separator to have insulation property for separating the positive electrode and the negative electrode from each other and high ion conductivity for increasing lithium ion permeability based on high porosity.

In addition, it is required for the separator to have a broad interval between a shutdown temperature and a meltdown temperature so that a lithium secondary battery including the separator may ensure safety. To broaden the interval between a shutdown temperature and a meltdown temperature, it is required that the shutdown temperature is controlled to be decreased and the meltdown temperature is controlled to be increased. As the meltdown temperature is increased, the separator has a higher melting temperature to provide improved stability. As the shutdown temperature is decreased, pore blocking may occur at a lower temperature. The shutdown temperature may be determined by the light transmittance of the separator. The method for manufacturing a crosslinked polyolefin porous membrane according to the related art shows an insufficient effect of increasing the light transmittance.

<CIT> relates to a polymer solid electrolyte layer integrated with a polymer, the organic electrolyte solution containing γ-butyronolactone, and the polymer solid electrolyte layer having a light transmittance of <NUM>% or more.

<CIT> describes the formation of a porous membrane by using a polyolefin, a diluent, an alkoxy group-containing vinylsilane, and an initiator to prepare a polyethylene solution grafting silane agent, comprising extruding a sheet to form the silane grafted polyolefin solution, preparing a porous film by extracting the diluent from the sheet, and crosslinking the porous film in presence of moisture.

<CIT> is concerned with a method of preparing a crosslinked polyolefin separator comprising preparing a silane grafted polyolefin solution using a polyolefin having a weight average molecular weight higher than or equal to <NUM>,<NUM>, a diluent, an alkoxy group containing vinylsilane, and an initiator; forming the silane grafted polyolefin solution in a sheet shape and stretching; extracting the diluent from the stretched sheet to produce a porous membrane; and crosslinking the porous membrane in the presence of water.

The present disclosure is designed to solve the problems of the related art, and therefore the present disclosure is directed to providing a method for manufacturing a crosslinked polyolefin separator which has a high meltdown temperature and shows improved processability by preventing a die-drool phenomenon, and a separator obtained by the method.

The present disclosure is also directed to providing a method for manufacturing a crosslinked polyolefin separator having high light transmittance and ensuring safety, and a separator obtained by the method.

In one aspect of the present disclosure, there is provided a crosslinked polyolefin separator according to the following embodiment.

According to the first embodiment of the present disclosure, there is provided a crosslinked polyolefin separator, which shows an average value of light transmittance of <NUM>% or more in a region of <NUM> to <NUM>, after four sides of the separator are fixed and allowed to stand at <NUM> for <NUM> minutes, the light transmittance being measured as described later, wherein.

In another aspect of the present disclosure, there is provided a crosslinked polyolefin separator according to any one of the following embodiments.

According to the second embodiment of the present disclosure, there is provided a method for manufacturing the crosslinked polyolefin separator of the first embodiment, the method including the steps of:.

According to the third embodiment of the present disclosure, there is provided the method for manufacturing a crosslinked polyolefin separator as defined in the second embodiment, wherein step (S1) includes:.

According to the fourth embodiment of the present disclosure, there is provided the method for manufacturing a crosslinked polyolefin separator as defined in the third embodiment, wherein the first alkoxysilane containing a carbon-carbon double bonded group has a boiling point of <NUM>-<NUM>, and the second alkoxysilane containing a carbon-carbon double bonded group has a boiling point of <NUM>-<NUM>.

According to the fifth embodiment of the present disclosure, there is provided the method for manufacturing a crosslinked polyolefin separator as defined in the third embodiment, wherein the weight ratio of the first alkoxysilane containing a carbon-carbon double bonded group to the second alkoxysilane containing a carbon-carbon double bonded group is <NUM>:<NUM>-<NUM>:<NUM>.

According to the sixth embodiment of the present disclosure, there is provided the method for manufacturing a crosslinked polyolefin separator as defined in the third embodiment, wherein the ratio of the elapsed time from the introduction of the first alkoxysilane containing a carbon-carbon double bonded group to the extrusion based on the elapsed time from the introduction of the polyolefin to the extrusion is <NUM>-<NUM>.

According to the seventh embodiment of the present disclosure, there is provided the method for manufacturing the crosslinked polyolefin separator as defined in the third embodiment, wherein step (S5) is carried out at <NUM>-<NUM> under a relative humidity of <NUM>-<NUM>% for <NUM>-<NUM> hours.

According to the eighth embodiment of the present disclosure, there is provided the method for manufacturing a crosslinked polyolefin separator as defined in the second embodiment, wherein step (S1) is carried out by introducing and mixing the initiator to the extruder in divided doses at a predetermined time interval, and
the weight ratio (A/B) of the total content (A) of the alkoxysilane containing a carbon-carbon double bonded group to the total content (B) of the initiator is <NUM>-<NUM>.

According to the ninth embodiment of the present disclosure, there is provided the method for manufacturing a crosslinked polyolefin separator as defined in the eighth embodiment, wherein step (S1) includes:.

According to the tenth embodiment of the present disclosure, there is provided the method for manufacturing a crosslinked polyolefin separator as defined in the ninth embodiment, wherein the weight ratio (c/d) of the alkoxysilane (c) containing a carbon-carbon double bonded group to the initiator (d) in the post-introduced composition is <NUM>-<NUM>.

According to the eleventh embodiment of the present disclosure, there is provided the method for manufacturing a crosslinked polyolefin separator as defined in the ninth embodiment, wherein the weight ratio (a/b) of the alkoxysilane (a) containing a carbon-carbon double bonded group to the initiator (b) in the pre-introduced composition is <NUM>-<NUM> times higher than the weight ratio (c/d) of the alkoxysilane (c) containing a carbon-carbon double bonded group to the initiator (d) in the post-introduced composition.

According to the twelfth embodiment of the present disclosure, there is provided the method for manufacturing a crosslinked polyolefin separator as defined in the ninth embodiment, wherein the ratio (t2/t1) of the elapsed time (t2) from the introduction of the post-introduced composition to the extrusion based on the elapsed time (t1) from the introduction of the pre-introduced composition to the extrusion is <NUM>-<NUM>.

According to the thirteenth embodiment of the present disclosure, there is provided the method for manufacturing a crosslinked polyolefin separator as defined in the ninth embodiment, wherein the weight ratio (a/b) of the alkoxysilane (a) containing a carbon-carbon double bonded group to the initiator (b) in the pre-introduced composition is <NUM>-<NUM>.

According to the fourteenth embodiment of the present disclosure, there is provided the method for manufacturing a crosslinked polyolefin separator as defined in the ninth embodiment, wherein the weight ratio (c/d) of the alkoxysilane (c) containing a carbon-carbon double bonded group to the initiator (d) in the post-introduced composition is <NUM>-<NUM>.

The method for manufacturing a crosslinked polyolefin separator according to an embodiment of the present disclosure can provide a crosslinked polyolefin separator having a high meltdown temperature through a silane aqueous crosslinking process.

In the method for manufacturing a crosslinked polyolefin separator according to an embodiment of the present disclosure, the first alkoxysilane containing a carbon-carbon double bonded group, which has a higher boiling point, is introduced previously and the second alkoxysilane containing a carbon-carbon double bonded group, which has a lower boiling point, is introduced subsequently. In this manner, it is possible to improve heat resistance characteristics and processability, and to provide a crosslinked polyolefin separator prevented from die-drooling.

In the method for manufacturing a crosslinked polyolefin separator according to an embodiment of the present disclosure, the total content of alkoxysilane containing a carbon-carbon double bonded group based on the total content of the initiator is controlled so that silane grafting may be formed to an adequate Si grafting ratio. Thus, as can be seen from an increase in light transmittance of a separator, it is possible to provide improved safety and processability to the separator. In addition, it is possible to inhibit a die-drool phenomenon caused by excessive use of alkoxysilane containing a carbon-carbon double bonded group.

In the method for manufacturing a crosslinked polyolefin separator according to an embodiment of the present disclosure, the weight ratio of the alkoxysilane containing a carbon-carbon double bonded group to the initiator and the introduction time point thereof are controlled so that silane grafting may be formed at an adequate Si grafting ratio. In this manner, as can be seen from an increase in light transmittance of a separator, it is possible to provide a crosslinked polyolefin separator ensuring safety, and to improve processability. It is also possible to inhibit side reactions resulting from direct polyolefin crosslinking caused by the initiator.

As used herein, the expression 'one portion is connected to another portion' covers not only 'a portion is directly connected to another portion' but also 'one portion is connected indirectly to another portion' by way of the other element interposed between them. Further, the connection refers to not only a physical connection, but it also includes an electrochemical connection.

Throughout the specification, the expression 'a part ┌includes┘ an element' does not preclude the presence of any additional elements but means that the part may further include the other elements.

In addition, it will be understood that the terms "comprises" and/or "comprising", or "includes" and/or "including" when used in this specification, refer to the presence of any stated shapes, numbers, steps, operations, members, elements and/or groups thereof, but do not preclude the addition of one or more other shapes, numbers, steps, operations, members, elements and/or groups thereof.

As used herein, the terms 'approximately', 'substantially', or the like, are used as meaning contiguous from or to the stated numerical value, when an acceptable preparation and material error unique to the stated meaning is suggested, and are used for the purpose of preventing an unconscientious invader from unduly using the stated disclosure including an accurate or absolute numerical value provided to help understanding of the present disclosure.

As used herein, the term 'combination(s) thereof' included in any Markush-type expression means a combination or mixture of one or more elements selected from the group of elements disclosed in the Markush-type expression, and refers to the presence of one or more elements selected from the group.

As used herein, the expression 'A and/or B' means 'A, B or both of them'.

In one aspect of the present disclosure, there are provided a method for manufacturing a crosslinked polyolefin separator and a crosslinked polyolefin separator obtained thereby.

A separator used for a lithium secondary battery shows excellent safety, when it has a large difference between the shutdown temperature and meltdown temperature. Herein, in order to broaden the interval between them, it is required that the shutdown temperature is controlled to be decreased and the meltdown temperature is controlled to be increased.

Methods for increasing the meltdown temperature include adding alkoxysilane containing a carbon-carbon double bonded group to polyolefin and a diluting agent to prepare a silane-grafted polyolefin composition from which a separator is obtained.

However, when the above-mentioned materials are introduced to and mixed in an extruder at once, it is difficult to control the process due to a die-drool phenomenon, etc. during the manufacture, thereby providing a separator having non-uniform appearance.

To solve the above-mentioned problem, an embodiment of the present disclosure is directed to providing a crosslinked polyolefin separator, which ensures processability and workability by reducing a die-drool phenomenon, as well as has a high meltdown temperature.

Meanwhile, as the shutdown temperature is decreased, pore blocking may occur at a lower temperature. When pore blocking occurs at a lower temperature, it is possible to improve the safety of an electrochemical device. Under these circumstances, it is required to provide a separator having a low shutdown temperature. The shutdown temperature may be determined through light transmittance. However, when polyolefin, a diluting agent, an initiator, a crosslinking agent and a crosslinking catalyst are introduced at once according to the related art, the resultant separator still shows low light transmittance. Thus, it is required to solve the above-mentioned problem.

Therefore, one aspect of the present disclosure is directed to providing a method for manufacturing a crosslinked polyolefin separator having high light transmittance and improved safety, and a crosslinked polyolefin separator obtained thereby.

In one aspect of the present disclosure, there is provided a method for manufacturing a crosslinked polyolefin separator, including the steps of:.

Hereinafter, the method for manufacturing a crosslinked polyolefin separator according to the present disclosure will be explained in detail.

According to an embodiment of the present disclosure, in step (S1), at least one of the initiator or the alkoxysilane containing a carbon-carbon double bonded group is introduced and mixed in divided doses at a predetermined time interval. Herein, when the alkoxysilane containing a carbon-carbon double bonded group is introduced in divided doses, the alkoxysilane containing a carbon-carbon double bonded group introduced previously to the extruder has a higher boiling point as compared to the alkoxysilane containing a carbon-carbon double bonded group introduced subsequently to the extruder. Meanwhile, when the initiator is introduced in divided doses, the weight ratio of the total content of the initiator based on the total content of the alkoxysilane containing a carbon-carbon double bonded group is controlled to a predetermined range.

According to an embodiment of the present disclosure, the alkoxysilane containing a carbon-carbon double bonded group may be introduced in divided doses at a predetermined time interval. This embodiment will be explained in detail hereinafter.

When the alkoxysilane containing a carbon-carbon double bonded group is introduced in divided doses, first, polyolefin, the diluting agent, alkoxysilane containing a carbon-carbon double bonded group, initiator and the crosslinking agent may be introduced to and mixed in the extruder. In a variant, polyolefin and the first diluting agent may be introduced to the extruder first (S11), and then the second diluting agent, the first alkoxysilane containing a carbon-carbon double bonded group, initiator and the crosslinking catalyst may be introduced to and mixed in the extruder (S12). According to an embodiment of the present disclosure, in order to effectively blend the alkoxysilane containing a carbon-carbon double bonded group with the molten materials in the extruder, polyolefin and the first diluting agent may be introduced first to the extruder, and then the first alkoxysilane containing a carbon-carbon double bonded group may be introduced thereto, or polyolefin and the diluting agent are introduced first to the extruder, and then the diluting agent, alkoxysilane containing a carbon-carbon double bonded group, initiator and the crosslinking catalyst may be introduced thereto, followed by mixing.

According to the related art, a diluting agent, initiator, alkoxysilane containing a carbon-carbon double bonded group (crosslinking agent) and a crosslinking catalyst are introduced to and mixed in an extruder at once, and then reactive extrusion is carried out to obtain a silane-grafted polyolefin composition.

However, according to the related art, there is a problem related with long-term processability due to die-drool during the manufacture of a separator.

Thus, the inventors of the present disclosure have conducted intensive studies to improve the processability of a crosslinked polyolefin separator. It has been found that when the retention time of the alkoxysilane containing a carbon-carbon double bonded group is controlled, considering the boiling point thereof, it is possible to provide a separator with a high meltdown temperature and to improve the appearance-related problem caused by a die-drool phenomenon. The present disclosure is based on this finding.

According to an embodiment of the present disclosure, step (S1) may include:.

Herein, polyolefin is the same as described hereinafter.

In addition, the diluting agent is the same as described hereinafter.

Meanwhile, according to an embodiment of the present disclosure, description of the first diluting agent, the second diluting agent and the third diluting agent may refer to the following description of the diluting agent. Herein, the first diluting agent, the second diluting agent and the third diluting agent may be the same as or different from one another.

According to an embodiment of the present disclosure, the first alkoxysilane containing a carbon-carbon double bonded group is a crosslinking agent inducing silane crosslinking, is grafted to polyolefin through vinyl groups, and functions to perform crosslinking of polyolefin through aqueous crosslinking caused by alkoxy groups.

According to the present disclosure, the first alkoxysilane containing a carbon-carbon double bonded group has a higher boiling point as compared to the second alkoxysilane containing a carbon-carbon double bonded group described hereinafter. As used herein, the first alkoxysilane containing a carbon-carbon double bonded group refers to the alkoxysilane containing a carbon-carbon double bonded group introduced previously to the extruder. As used herein, the second alkoxysilane containing a carbon-carbon double bonded group refers to the alkoxysilane containing a carbon-carbon double bonded group introduced subsequently to the extruder.

When using a vinylsilane having a high boiling point, such as the first alkoxysilane containing a carbon-carbon double bonded group, it is possible to reduce the amount of alkoxysilane containing a carbon-carbon double bonded group which evaporates under the high-temperature condition of the extruder. Therefore, it is possible to reduce the problem of die-drool caused by fume generation during the extrusion of a sheet, thereby providing improved processability. However, there is a problem in that the first alkoxysilane containing a carbon-carbon double bonded group has a high molecular weight to cause degradation of reactivity with polyolefin.

On the contrary, when using the second alkoxysilane containing a carbon-carbon double bonded group as described hereinafter, the grafting ratio of polyolefin with silane is high. It seems that such a high grafting ratio results from high mobility of molecules derived from the low molecular weight of the second alkoxysilane containing a carbon-carbon double bonded group. However, in the case of the second alkoxysilane containing a carbon-carbon double bonded group, it may cause an appearance-related problem, such as die-drool, when it has a long retention time in the extruder.

Under these circumstances, the boiling point of the alkoxysilane containing a carbon-carbon double bonded group and the retention time thereof in the extruder are controlled at the same time according to the present disclosure to reduce a die-drool phenomenon and to provide a crosslinked polyolefin separator having improved processability and workability.

According to an embodiment of the present disclosure, the first alkoxysilane containing a carbon-carbon double bonded group may have a boiling point of <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>.

According to an embodiment of the present disclosure, the first alkoxysilane containing a carbon-carbon double bonded group may include vinyltriethoxysilane (b. <NUM>), vinyltriisopropoxysilane (b. <NUM>), vinyltriacetoxysilane (b. <NUM>), or a mixture of at least two of them.

According to an embodiment of the present disclosure, the content of the first alkoxysilane containing a carbon-carbon double bonded group may be <NUM>-<NUM> parts by weight, <NUM>-<NUM> parts by weight, or <NUM>-<NUM> parts by weight, based on <NUM> parts by weight of the total content of the polyolefin, the first diluting agent, the second diluting agent and the third diluting agent. When the content of the first alkoxysilane containing a carbon-carbon double bonded group satisfies the above-defined range, it is possible to prevent the problems of a decrease in crosslinking degree, caused by a low silane content and reduced graft ratio, and degradation of the appearance of an extruded sheet, caused by a high silane content and residue of unreacted silane.

According to an embodiment of the present disclosure, the ratio of the elapsed time from the introduction of the first alkoxysilane containing a carbon-carbon double bonded group to the extrusion based on the elapsed time from the introduction of the polyolefin to the extrusion may be <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>. When the retention time of the first alkoxysilane containing a carbon-carbon double bonded group in the extruder satisfies the above-defined range, it is possible to reduce the problem of poor grafting of the first alkoxysilane containing a carbon-carbon double bonded group to the polyolefin, caused by an excessively short extrusion time, and to blend the first alkoxysilane containing a carbon-carbon double bonded group with the polyolefin more effectively by preliminarily blending the polyolefin with the first diluting agent.

Then, the third diluting agent and the second alkoxysilane containing a carbon-carbon double bonded group may be introduced to and mixed in the extruder, and reactive extrusion of a silane-grafted polyolefin composition may be carried out (S13).

According to an embodiment of the present disclosure, description of the third diluting agent may be the same as the description of the first diluting agent.

According to an embodiment of the present disclosure, the third diluting agent may be the same as or different from each of the first diluting agent and the second dilution agent as described above.

According to an embodiment of the present disclosure, the total content of the first diluting agent, the second diluting agent and the third diluting agent may be <NUM>-<NUM> parts by weight, <NUM>-<NUM> parts by weight, or <NUM>-<NUM> parts by weight based on <NUM> parts by weight of the polyolefin. When the total content of the first diluting agent, the second diluting agent and the third diluting agent satisfies the above-defined range, it is possible to reduce the problems caused by an increase in polyolefin content, such as a decrease in porosity and pore size, reduced interconnection among pores and a significant decrease in permeability, an increase in viscosity of a polyolefin composition and extrusion load, and a difficulty in processing. It is also possible to reduce the problems caused by a decrease in polyolefin content, such as breakage upon stretching and non-uniform thickness, caused by a decrease in compatibility of polyolefin with the first diluting agent, the second diluting agent and the third diluting agent and extrusion of polyolefin in the form of gel without thermodynamic blending of polyolefin with the first diluting agent, the second diluting agent and the third diluting agent.

According to an embodiment of the present disclosure, the weight ratio of the first diluting agent to the second diluting agent may be <NUM>:<NUM>-<NUM>:<NUM>. The weight ratio of the first diluting agent to the third diluting agent may be <NUM>:<NUM>-<NUM>:<NUM>. When the weight ratio of the first diluting agent, the second diluting agent and the third diluting agent satisfies the above-defined ranges, it is possible to preliminarily blend the polyolefin having high viscosity with the first diluting agent and to introduce the crosslinking additives smoothly into the extruder together with the second diluting agent and the third diluting agent.

According to the present disclosure, the first diluting agent may be introduced simultaneously with the polyolefin in order to increase the compatibility with the polyolefin. Then, the second diluting agent and the first alkoxysilane containing a carbon-carbon double bonded group may be introduced to the extruder, and then the third diluting agent and the second alkoxysilane containing a carbon-carbon double bonded group may be further introduced to the extruder at the time interval described hereinafter.

According to the present disclosure, the second alkoxysilane containing a carbon-carbon double bonded group is a crosslinking agent inducing silane crosslinking, is grafted to polyolefin through vinyl groups, and functions to perform crosslinking of polyolefin through aqueous crosslinking caused by alkoxy groups. However, according to an embodiment of the present disclosure, the second alkoxysilane containing a carbon-carbon double bonded group has a lower boiling point as compared to the first alkoxysilane containing a carbon-carbon double bonded group as described above.

According to an embodiment of the present disclosure, the second alkoxysilane containing a carbon-carbon double bonded group may have a boiling point of <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>.

According to an embodiment of the present disclosure, the second alkoxysilane containing a carbon-carbon double bonded group may include vinyltrimethoxysilane (b. <NUM>), vinyl(chlorodimethyl)silane (b. <NUM>), or a mixture thereof.

According to an embodiment of the present disclosure, the content of the second alkoxysilane containing a carbon-carbon double bonded group may be <NUM>-<NUM> parts by weight, <NUM>-<NUM> parts by weight, or <NUM>-<NUM> parts by weight, based on <NUM> parts by weight of the total content of the polyolefin, the first diluting agent, the second diluting agent and the third diluting agent. When the content of the second alkoxysilane containing a carbon-carbon double bonded group satisfies the above-defined range, it is possible to prevent the problems of a decrease in crosslinking degree, caused by a low silane content and reduced graft ratio, and degradation of the appearance of an extruded sheet, caused by a high silane content and residue of unreacted silane.

According to an embodiment of the present disclosure, the weight ratio of the first alkoxysilane containing a carbon-carbon double bonded group to the second alkoxysilane containing a carbon-carbon double bonded group may be <NUM>:<NUM>-<NUM>:<NUM>, <NUM>:<NUM>-<NUM>:<NUM>, or <NUM>:<NUM>-<NUM>:<NUM>. When the weight ratio of the alkoxysilanes containing a carbon-carbon double bonded group satisfies the above-defined range, both the first alkoxysilane containing a carbon-carbon double bonded group having a relatively high boiling point and the second alkoxysilane containing a carbon-carbon double bonded group having a relatively low boiling point can contribute to the crosslinking of the polyolefin, thereby further improving the meltdown temperature.

According to an embodiment of the present disclosure, the ratio of the elapsed time from the introduction of the second alkoxysilane containing a carbon-carbon double bonded group to the extrusion based on the elapsed time from the introduction of the polyolefin and the first diluting agent to the extrusion may be <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>. When the retention time of the second alkoxysilane containing a carbon-carbon double bonded group satisfies the above-defined range, it is possible to avoid the problem of degradation of appearance, caused by an excessively long retention time of the second alkoxysilane containing a carbon-carbon double bonded group, and to reduce the problem of poor grafting to the polyolefin, caused by an excessively short retention time of the second alkoxysilane containing a carbon-carbon double bonded group in the extruder.

As described above, since the first alkoxysilane containing a carbon-carbon double bonded group is introduced previously to the extruder according to the present disclosure, the retention time of the first alkoxysilane containing a carbon-carbon double bonded group in the extruder is controlled to be longer than the retention time of the second alkoxysilane containing a carbon-carbon double bonded group in the extruder.

In other words, according to the present disclosure, the retention time of the first alkoxysilane containing a carbon-carbon double bonded group having lower reactivity with the polyolefin is controlled to a longer time in order to allow the first alkoxysilane to sufficiently react with the polyolefin. On the other hand, the retention time of the second alkoxysilane containing a carbon-carbon double bonded group is controlled to a shorter time, since the second alkoxysilane having a lower boiling point may cause more fume generation when it has a long retention time in the extruder, resulting in the problem of an appearance-related problem.

According to an embodiment of the present disclosure, the initiator may be any initiator, as long as it can generate radicals. Non-limiting examples of the initiator include <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di(tert-butylperoxy)hexane (DHBP), benzoyl peroxide, acetyl peroxide, dilauryl peroxide, di-tert-butyl peroxide, dicumyl peroxide, cumyl peroxide, hydrogen peroxide, potassium persulfate, or the like.

According to an embodiment of the present disclosure, the content of the initiator may be <NUM>-<NUM> parts by weight, <NUM>-<NUM> parts by weight, or <NUM>-<NUM> parts by weight, based on <NUM> parts by weight of the total content of the first alkoxysilane containing a carbon-carbon double bonded group and the second alkoxysilane containing a carbon-carbon double bonded group. When the content of the initiator satisfies the above-defined range, it is possible to prevent the problems of a decrease in silane graft ratio, caused by a low content of initiator, and crosslinking between polyethylene molecules in an extruder, caused by a high content of initiator.

According to another embodiment of the present disclosure, the initiator is introduced and mixed in divided doses in step (S1), wherein the weight ratio (A/B) of the total content (A) of the alkoxysilane containing a carbon-carbon double bonded group to the total content (B) of the initiator is <NUM>-<NUM>.

When polyolefin, an initiator, alkoxysilane containing a carbon-carbon double bonded group and a crosslinking catalyst are introduced to an extruder at once according to the related art, the initiator may work on the alkoxysilane containing a carbon-carbon double bonded group and polyolefin. Particularly, radicals generated in the alkoxysilane containing a carbon-carbon double bonded group by the initiator may react with the polyolefin to cause silane grafting. In addition, radicals generated in the polyolefin by the initiator may react with another polyolefin to cause direct crosslinking between one polyolefin and another polyolefin. The silane grafting and direct polyolefin crosslinking occur competitively in the extruder.

It is required to accelerate the silane grafting, since it reduces the melting point of the silane-grafted polyolefin composition, resulting in a decrease in shutdown temperature and an increase in light transmittance. On the contrary, it is required to minimize the direct crosslinking, since it hardly provides an effect of increasing light transmittance.

Under these circumstances, according to the present disclosure, the initiator provided to the extruder is introduced and mixed in divided doses at a predetermined interval, wherein the weight ratio (A/B) of the total content (A) of the alkoxysilane containing a carbon-carbon double bonded group to the total content (B) of the initiator is controlled to <NUM>-<NUM>.

Particularly, it is preferred that the diluting agent and the alkoxysilane containing a carbon-carbon double bonded group are introduced and mixed in divided doses with a view to accelerating the silane grafting and minimizing the direct polyolefin crosslinking.

According to an embodiment of the present disclosure, the weight ratio (A/B) of the total content (A) of the alkoxysilane containing a carbon-carbon double bonded group to the total content (B) of the initiator may be <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>.

When the weight ratio (A/B) of the total content (A) of the alkoxysilane containing a carbon-carbon double bonded group to the total content (B) of the initiator is less than <NUM>, it is not possible to provide a sufficient effect of increasing light transmittance. When the weight ratio (A/B) of the total content (A) of the alkoxysilane containing a carbon-carbon double bonded group to the total content (B) of the initiator is larger than <NUM>, alkoxy group-containing vinylsilane present in the extruder may be increased, or drooling may occur at a die or lip.

According to an embodiment of the present disclosure, the initiator may be introduced to the extruder in divided doses at a predetermined time interval, together with at least one of the diluting agent, aloxysilane containing a carbon-carbon double bonded group or the crosslinking catalyst, in step (S1).

Herein, the weight ratio (a/b) of the alkoxysilane (a) containing a carbon-carbon double bonded group to the initiator (b) in the pre-introduced composition may be <NUM>-<NUM>, and the weight ratio (c/d) of the alkoxysilane (c) containing a carbon-carbon double bonded group to the initiator (d) in the post-introduced composition may be smaller than the weight ratio (a/b) of the alkoxysilane (a) containing a carbon-carbon double bonded group to the initiator (b) in the pre-introduced composition.

According to the present disclosure, the ratio of the alkoxysilane containing a carbon-carbon double bonded group to the initiator may be controlled by using at least two inlet ports in the extruder. Herein, it is possible to maximize the silane grafting and to minimize the direct polyolefin crosslinking by controlling the weight ratio (a/b) of the alkoxysilane (a) containing a carbon-carbon double bonded group to the initiator (b) in the composition introduced previously to the first hopper to be larger than the weight ratio (c/d) of the alkoxysilane (c) containing a carbon-carbon double bonded group to the initiator (d) in the composition introduced subsequently to the second hopper.

According to an embodiment of the present disclosure, the weight ratio (a/b) of the alkoxysilane (a) containing a carbon-carbon double bonded group to the initiator (b) in the pre-introduced composition may be <NUM>-<NUM> times, <NUM>-<NUM> times, or <NUM>-<NUM> times higher than the weight ratio (c/d) of the alkoxysilane (c) containing a carbon-carbon double bonded group to the initiator (d) in the post-introduced composition.

Within the above-defined range, it is possible to control a change in pressure during extrusion processing and to optimize the transportability of the blended molten material.

According to an embodiment of the present disclosure, the weight ratio (a/b) of the alkoxysilane (a) containing a carbon-carbon double bonded group to the initiator (b) in the pre-introduced composition may be <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>. Within the above-defined range, it is possible to maximize the grafting ratio of the alkoxysilane containing a carbon-carbon double bonded group.

According to an embodiment of the present disclosure, the weight ratio (c/d) of the alkoxysilane (c) containing a carbon-carbon double bonded group to the initiator (d) in the post-introduced composition may be <NUM>-<NUM>, <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>. Within the above-defined range, it is possible to maximize the grafting ratio of the alkoxysilane containing a carbon-carbon double bonded group.

According to an embodiment of the present disclosure, the ratio (t2/t1) of the elapsed time (t2) from the introduction of the post-introduced composition to the extrusion based on the elapsed time (t1) from the introduction of the pre-introduced composition to the extrusion may be <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>. Within the above-defined range of elapsed time, it is possible to control the grafting rate of the alkoxysilane containing a carbon-carbon double bonded group and to inhibit side reactions.

When the above-mentioned processing conditions (such as a/b, c/d, t2/t1) are satisfied, the extruded sheet obtained through the extruder may include <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM> units to which alkoxysilane containing a carbon-carbon double bonded group is grafted, based on <NUM> carbon atoms in the polyolefin backbone. When the number of units to which alkoxysilane containing a carbon-carbon double bonded group is grafted is within the above-defined range, it is possible to increase the light transmittance of a separator.

According to an embodiment of the present disclosure, the polyolefin may be polyethylene; polypropylene; polybutylene; polypentene; polyhexene; polyoctene; a copolymer of at least two of ethylene, propylene, butene, pentene, <NUM>-methylpentene, hexene, heptene and octene; or a mixture thereof.

Particularly, the polyethylene includes low-density polyethylene (LDPE), linear low-density polyethylene (LLDPE), high-density polyethylene (HDPE), or the like. Among those, high-density polyethylene having a high crystallization degree and a high resin melting point is most preferred.

According to an embodiment of the present disclosure, the polyolefin may have a weight average molecular weight of <NUM>,<NUM>-<NUM>,<NUM>,<NUM>, <NUM>,<NUM>-<NUM>,<NUM>, or <NUM>,<NUM>-<NUM>,<NUM>. According to the present disclosure, it is possible to obtain a separator having excellent strength and heat resistance, while ensuring separator film uniformity and film-forming processability, by using high-molecular weight polyolefin having a weight average molecular weight of <NUM>,<NUM>-<NUM>,<NUM>,<NUM> as a starting material.

According to an embodiment of the present disclosure, the diluting agent may include liquid or solid paraffin oil, mineral oil, wax, soybean oil, or the like, used currently for manufacturing a separator through a wet process.

According to an embodiment of the present disclosure, the diluting agent may be one capable of liquid-liquid phase separation against polyolefin. For example, the diluting agent may be a phthalic acid ester, such as dibutyl phthalate, dihexyl phthalate, dioctyl phthalate, or the like; aromatic ether, such as diphenyl ether, benzyl ether, or the like; C10-C20 fatty acid, such as palmitic acid, stearic acid, oleic acid, linolic acid, linolenic acid, or the like; C10-C20 fatty acid alcohol, such as palmitic acid alcohol, stearic acid alcohol, oleic acid alcohol, or the like; and a fatty acid ester such as palmitic acid mono-, di- or tri-ester, stearic acid mono-, di- or tri-ester, oleic acid mono-, di- or tri-ester, linoleic acid mono-, di- or tri-ester, or the like, resulting from an esterification between saturated or unsaturated fatty acid whose fatty acid group has <NUM>-<NUM> carbon atomsor one or at least two fatty acids having an epoxy-substituted double bond of unsaturated fatty acid, and a C1-C10 alcohol having <NUM>-<NUM> hydroxyl groups.

The diluting agent may include any one of the above-described ingredients or a mixture of at least two of them.

According to an embodiment of the present disclosure, the content of the diluting agent may be <NUM>-<NUM> parts by weight, <NUM>-<NUM> parts by weight, or <NUM>-<NUM> parts by weight, based on <NUM> parts by weight of polyolefin. When the total content of the diluting agent satisfies the above-defined range, it is possible to reduce the problems caused by an increase in polyolefin content, such as a decrease in porosity and pore size, reduced interconnection among pores and a significant decrease in permeability, an increase in viscosity of a polyolefin composition and extrusion load, and a difficulty in processing. It is also possible to reduce the problems caused by a decrease in polyolefin content, such as breakage upon stretching and non-uniform thickness, caused by a decrease in compatibility of polyolefin with a diluting agent and extrusion of polyolefin in the form of gel without thermodynamic blending of polyolefin with a diluting agent.

According to an embodiment of the present disclosure, the alkoxysilane containing a carbon-carbon double bonded group is a crosslinking agent inducing silane crosslinking, is grafted to polyolefin through vinyl groups, and functions to perform crosslinking of polyolefin through aqueous crosslinking caused by alkoxy groups.

According to an embodiment of the present disclosure, the alkoxysilane containing a carbon-carbon double bonded group may include a compound represented by the following Chemical Formula <NUM>:
<CHM>.

Meanwhile, R may further include an amino group, epoxy group or an isocyanate group.

According to an embodiment of the present disclosure, the alkoxysilane containing a carbon-carbon double bonded group may include vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane, (<NUM>-methacryloxypropyl)trimethoxysilane, (<NUM>-methacryloxypropyl)triethoxysilane, vinylmethyl dimethoxysilane, vinyl-tris(<NUM>-methoxyethoxy)silane, vinylmethyldiethoxysilane, or a mixture of at least two of them.

According to an embodiment of the present disclosure, the crosslinking catalyst is added to accelerate silane crosslinking.

According to an embodiment of the present disclosure, the crosslinking catalyst may be a carboxylate of metal, such as tin, zinc, iron, lead or cobalt, organic base, inorganic acid or an organic acid. Non-limiting examples of the crosslinking catalyst include metal carboxylates, such as dibutyltin dilaurate, dibutyltin diacetate, stannous acetate, stannous caprylate, zinc naphthenate, zinc caprylate and cobalt naphthenate, organic bases, such as ethylamine, dibutylamine, hexylamine and pyridine, inorganic acids, such as sulfuric acid and hydrochloric acid, and organic acids, such as toluene sulfonic acid, acetic acid, stearic acid and maleic acid. Such crosslinking agents may be used alone or in combination.

According to an embodiment of the present disclosure, the content of the crosslinking catalyst may be <NUM>-<NUM> parts by weight, <NUM>-<NUM> parts by weight, or <NUM>-<NUM> parts by weight, based on <NUM> parts by weight of the alkoxysilane containing a carbon-carbon double bonded group. When the content of the crosslinking catalyst satisfies the above-defined range, it is possible to carry out silane crosslinking to a desired level and to prevent undesired side reactions in a lithium secondary battery. In addition, there is no cost-related problem, such as waste of the catalyst.

According to an embodiment of the present disclosure, the content of the alkoxysilane containing a carbon-carbon double bonded group may be <NUM>-<NUM> parts by weight, or <NUM>-<NUM> parts by weight, based on <NUM> parts by weight of the total content of the polyolefin and the diluting agent. When the content of the alkoxysilane containing a carbon-carbon double bonded group satisfies the above-defined range, it is possible to prevent the problems of a decrease in crosslinking degree, caused by a low silane content and reduced graft ratio, and degradation of the appearance of an extruded sheet, caused by a high silane content and residue of unreacted silane.

According to an embodiment of the present disclosure, if necessary, the silane-grafted polyolefin composition may further include conventional additives for improving specific functions, such as an antioxidant, UV stabilizer, antistatic agent, nucleating agent, or the like.

According to an embodiment of the present disclosure, the reactive extrusion may be carried out by using a uniaxial extruder or biaxial extruder.

Next, the reactive extruded silane-grafted polyolefin composition is molded in the form of a sheet and stretched (S2).

For example, the reactive extruded silane-grafted polyolefin composition is extruded by using an extruder equipped with a T-die, or the like, and then cooled extrudate may be formed by using a general casting or calandering process using water-cooling or air-cooling mode.

According to an embodiment of the present disclosure, it is possible to provide a separator having improved mechanical strength and puncture strength through the stretching step as mentioned above.

According to an embodiment of the present disclosure, the stretching may be carried out by sequential or simultaneous stretching using a roll or tenter. The stretching ratio may be <NUM> times or more, or <NUM>-<NUM> times, each in the machine direction and transverse direction. When the stretching ratio satisfies the above-defined ratio, it is possible to prevent the problem of insufficient stretching in one direction, a failure in balance of physical properties between the machine direction and the transverse direction, and degradation of tensile strength and puncture strength. When the total stretching ratio satisfies the above-defined range, it is possible to prevent the problems of non-stretching or a failure in pore formation.

According to an embodiment of the present disclosure, the stretching temperature may be varied with the melting point of polyolefin and concentration and type of the diluting agent.

According to an embodiment of the present disclosure, when the polyolefin is polyethylene and the diluting agent is liquid paraffin, the stretching temperature may be <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM> in the machine direction (MD), and may be <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM> in the transverse direction (TD). When carrying out stretching in both directions at the same time, the stretching temperature may be <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>.

When the stretching temperature satisfies the above-defined range, it is possible to prevent the problem of breakage or non- stretching caused by the lack of softness resulting from a low stretching temperature. In addition, it is possible to prevent overstretching or a difference in physical properties, caused by a high stretching temperature.

Then, the diluting agent is totally extracted from the stretched sheet to obtain a porous membrane (S3).

According to an embodiment of the present disclosure, the diluting agent may be extracted from the porous membrane by using an organic solvent, and then the porous membrane may be dried.

According to an embodiment of the present disclosure, the organic solvent is not particularly limited, as long as it is capable of extraction of the diluting agents. However, it is suitable to use methyl ethyl ketone, methylene chloride or hexane having high extraction efficiency and showing a high drying rate.

According to an embodiment of the present disclosure, the extraction may be any conventional solvent extraction process, such as an immersion process, solvent spray process or ultrasonication process, and such processes may be used alone or in combination. After carrying out extraction, the content of residual diluting agents should be preferably <NUM> wt% or less. When the content of residual diluting agents is larger than <NUM> wt%, physical properties are degraded and permeability of the porous membrane is reduced. The content of residual diluting agents may be affected by extraction temperature and extraction time. To increase the solubility of diluting agents in the organic solvent, a higher extraction temperature is preferred. However, considering the safety problem caused by boiling of the organic solvent, the extraction temperature is preferably <NUM> or lower. When the extraction temperature equals to or is lower than the solidifying point of the diluting agent, extraction efficiency may be decreased significantly. Thus, the extraction temperature should be higher than the solidifying point of the diluting agent.

In addition, the extraction time may be varied with the thickness of a porous membrane. In the case of a porous membrane having a thickness of <NUM>-<NUM>, the extraction time may be <NUM>-<NUM> minutes suitably.

Then, the porous membrane is thermally fixed (S4).

The thermal fixing is a process of fixing and heating the porous membrane so that the porous membrane to be shrunk may be fixed forcibly to remove residual stress.

According to an embodiment of the present disclosure, the thermal fixing temperature may be <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>.

When the polyolefin is polyethylene and the thermal fixing temperature satisfies the above-defined range, polyolefin molecules may be rearranged to remove residual stress of the porous membrane, and the problem of pore blocking of the porous membrane caused by partial melting may be reduced.

According to an embodiment of the present disclosure, the thermal fixing time may be <NUM>-<NUM> seconds, <NUM>-<NUM> seconds, or <NUM>-<NUM> seconds. When the thermal fixing is carried out for the above-defined range of time, polyolefin molecules may be rearranged to remove residual stress of the porous membrane, and the problem of pore blocking of the porous membrane caused by partial melting may be reduced.

After that, the thermally fixed porous membrane is crosslinked in the presence of water (S5).

According to an embodiment of the present disclosure, the crosslinking may be carried out at <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>.

According to an embodiment of the present disclosure, the crosslinking may be carried out under a humidity of <NUM>-<NUM>% for <NUM>-<NUM> hours.

Meanwhile, according to an embodiment of the present disclosure, when the alkoxysilane containing a carbon-carbon double bonded group is introduced in divided doses, step (S5) may be carried out at a temperature of <NUM>-<NUM> under a relative humidity of <NUM>-<NUM>% for <NUM>-<NUM> hours.

Particularly, the crosslinking may be carried out at a temperature of <NUM>-<NUM>, <NUM>-<NUM>, or <NUM>-<NUM>. Herein, the crosslinking may be carried out under a humidity of <NUM>-<NUM>% for <NUM>-<NUM> hours, <NUM>-<NUM> hours, or <NUM>-<NUM> hours. Thus, it is possible to reduce the shutdown temperature by controlling the temperature, humidity and time in the crosslinking step after silane grafting is carried out. In this manner, it is possible to provide a crosslinked polyolefin separator having high light transmittance. Herein, it is required to totally satisfy the above-defined ranges of temperature, humidity and time in order to provide such a crosslinked polyolefin separator having high light transmittance.

In another aspect of the present disclosure, there is provided a crosslinked polyolefin separator obtained by the above-described method.

According to an embodiment of the present disclosure, there is provided a crosslinked polyolefin separator which shows an average value of light transmittance of <NUM>% or more, <NUM>% or more, <NUM>% or more, or <NUM>% or more, in a region of <NUM> to <NUM>, after four sides of the separator are fixed and allowed to stand at <NUM> for <NUM> minutes, the light transmittance being measured as described below, wherein the crosslinked polyolefin separator has <NUM>-<NUM> grafted units based on <NUM> carbon atoms in the polyolefin backbone forming the crosslinked polyolefin separator as measured as described later, and the grafted units are silane-grafted units.

As used herein, 'light transmittance' may be calculated by the formula of {(Amount of light before passing through separator - Amount of light after passing through separator) / Amount of light before passing through separator} x <NUM>, when horizontal light is allowed to pass through a separator in the thickness direction.

According to an embodiment of the present disclosure, the average value of light transmittance is <NUM>% or more, <NUM>% or more, <NUM>-<NUM>%, <NUM>-<NUM>%, or <NUM>-<NUM>%, when light in a region of <NUM> to <NUM> passes through the separator. When the light transmittance satisfies the above-defined range, it is possible to improve the safety of a lithium secondary battery through pore blocking caused by shutdown. Particularly, when pores are present in a separator, the separator itself seems opaque due to a difference in refractive index between polyolefin and air. However, when the temperature increases above the shutdown temperature, air cannot pass through the separator due to pore blocking, and thus air is interrupted. Therefore, it is possible to obtain a separator including polyolefin only and interrupted from air, and to provide a separator having high light transmittance.

According to an embodiment of the present disclosure, the light transmittance may be determined by using a hazemeter according to ASTM-D1003. Particularly, light transmittance may be determined by measuring light transmittance as a function of wavelength in the visible light range of <NUM>-<NUM>, followed by averaging.

According to an embodiment of the present disclosure, the separator may have a thickness of <NUM>-<NUM>, or <NUM>-<NUM>.

According to an embodiment of the present disclosure, the separator may be applied to a lithium secondary battery.

First, <NUM> of high-density polyethylene (Korea Petrochemical Ind. , VH035) having a weight average molecular weight of <NUM>,<NUM> as polyolefin, and <NUM> of liquid paraffin oil (Kukdong Oil & Chem. LP350F, <NUM> cSt) as the first diluting agent were introduced to an extruder and mixed therein.

Next, <NUM> of liquid paraffin oil (Kukdong Oil & Chem. LP350F, <NUM> cSt) as the second diluting agent, <NUM> of vinyltriethoxysilane (b. <NUM>) (as the first alkoxysilane containing a carbon-carbon double bonded group), <NUM> of dibutyltin dilaurate (as a crosslinking catalyst), and <NUM> of <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di(tert-butylperoxy)hexane (DHBP) (as an initiator) were further introduced to and mixed in the extruder.

Then, <NUM> of liquid paraffin oil (Kukdong Oil & Chem. LP350F, <NUM> cSt) as the third diluting agent and <NUM> of vinyltrimethoxysilane (b. <NUM>) (as the second alkoxysilane containing a carbon-carbon double bonded group) were introduced to and mixed in the extruder to carry out reactive extrusion of the silane-grafted polyolefin composition.

Therefore, in Example <NUM>, the weight ratio of the first alkoxysilane containing a carbon-carbon double bonded group to the second alkoxysilane containing a carbon-carbon double bonded group was <NUM>:<NUM>.

In addition, in example <NUM>, the weight ratio of the first diluting agent to the second diluting agent was <NUM>:<NUM>, and the weight ratio of the first diluting agent to the third diluting agent was <NUM>:<NUM>.

Meanwhile, the ratio of the elapsed time from the introduction of the first alkoxysilane containing a carbon-carbon double bonded group to the extrusion based on the elapsed time from the introduction of the polyolefin to the extrusion was <NUM>%. In other words, the elapsed time from the introduction of the first alkoxysilane containing a carbon-carbon double bonded group to the extrusion was about <NUM> times of the elapsed time from the introduction of the polyolefin to the extrusion. Particularly, the elapsed time from the introduction of the polyolefin to the extrusion was <NUM> seconds, and the elapsed time from the introduction of the first alkoxysilane containing a carbon-carbon double bonded group to the extrusion was <NUM> seconds.

In addition, the ratio of the elapsed time from the introduction of the second alkoxysilane containing a carbon-carbon double bonded group to the extrusion based on the elapsed time from the introduction of the polyolefin to the extrusion was <NUM>%. In other words, the elapsed time from the introduction of the second alkoxysilane containing a carbon-carbon double bonded group to the extrusion was about <NUM> times of the elapsed time from the introduction of the polyolefin to the extrusion. Particularly, the elapsed time from the introduction of the polyolefin to the extrusion was <NUM> seconds, and the elapsed time from the introduction of the second alkoxysilane containing a carbon-carbon double bonded group to the extrusion was <NUM> seconds.

After that, reactive extrusion was carried out at a temperature of <NUM> to obtain a silane-grafted polyethylene composition.

The resultant silane-grafted polyethylene composition was molded into a sheet-like shape through a T-die and cold casting roll, and then biaxial stretching was carried out by using a tenter-type sequential stretching machine performing MD stretching and then TD stretching. Both the MD stretching ratio and the TD stretching ratio were <NUM> times. The stretching temperature was <NUM> in MD and <NUM> in TD.

Then, the first diluting agent, the second diluting agent and the third diluting agent were extracted from the stretched sheet by using methylene chloride and thermal fixing was carried out at <NUM> to obtain a porous membrane. The porous membrane was subjected to crosslinking at <NUM> under a relative humidity condition of <NUM>% for <NUM> hours to obtain a crosslinked polyethylene separator. The resultant crosslinked polyolefin separator had a thickness of <NUM>.

A crosslinked polyolefin separator was obtained in the same manner as Example <NUM>, except that the crosslinking was carried out for <NUM> hours. The resultant crosslinked polyolefin separator had a thickness of <NUM>.

A separator was obtained in the same manner as Example <NUM>, except that the materials introduced to the extruder were controlled as follows. The resultant polyolefin separator had a thickness of <NUM>.

Particularly, <NUM> of high-density polyethylene (Korea Petrochemical Ind. , VH035) having a weight average molecular weight of <NUM>,<NUM> as polyolefin, and <NUM> of liquid paraffin oil (Kukdong Oil & Chem. LP350F, <NUM> cSt) as a diluting agent were introduced to an extruder and mixed therein.

In Comparative Example <NUM>, the second alkoxysilane containing a carbon-carbon double bonded group and the third diluting agent were not introduced, unlike Example <NUM>.

Particularly, a separator was obtained as follows:
First, <NUM> of high-density polyethylene (Korea Petrochemical Ind. , VH035) having a weight average molecular weight of <NUM>,<NUM> as polyolefin, and <NUM> of liquid paraffin oil (Kukdong Oil & Chem. LP350F, <NUM> cSt) as the first diluting agent were introduced to an extruder and mixed therein.

In Comparative Example <NUM>, the weight ratio of the first diluting agent to the second diluting agent was <NUM>:<NUM>.

Then, the first diluting agent and the second diluting agent were extracted from the stretched sheet by using methylene chloride and thermal fixing was carried out at <NUM> to obtain a porous membrane. The porous membrane was subjected to crosslinking at <NUM> under a relative humidity condition of <NUM>% for <NUM> hours to obtain a crosslinked polyethylene separator. The resultant crosslinked polyolefin separator had a thickness of <NUM>.

In Comparative Example <NUM>, the first alkoxysilane containing a carbon-carbon double bonded group and the second diluting agent were not introduced, unlike Example <NUM>.

Next, <NUM> of liquid paraffin oil (Kukdong Oil & Chem. LP350F, <NUM> cSt) as the third diluting agent, <NUM> of vinyltrimethoxysilane (b. <NUM>) (as the second alkoxysilane containing a carbon-carbon double bonded group), <NUM> of dibutyltin dilaurate (as a crosslinking catalyst), and <NUM> of <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di(tert-butylperoxy)hexane (DHBP) (as an initiator) were further introduced to and mixed in the extruder to carry out reactive extrusion of the silane-grafted polyolefin composition. Herein, the weight ratio of the first diluting agent to the third diluting agent was <NUM>:<NUM>.

The ratio of the elapsed time from the introduction of the second alkoxysilane containing a carbon-carbon double bonded group to the extrusion based on the elapsed time from the introduction of the polyolefin to the extrusion was <NUM>%. In other words, the elapsed time from the introduction of the second alkoxysilane containing a carbon-carbon double bonded group to the extrusion was about <NUM> times of the elapsed time from the introduction of the polyolefin to the extrusion. Particularly, the elapsed time from the introduction of the polyolefin to the extrusion was <NUM> seconds, and the elapsed time from the introduction of the second alkoxysilane containing a carbon-carbon double bonded group to the extrusion was <NUM> seconds.

Then, the first diluting agent and the third diluting agent were extracted from the stretched sheet by using methylene chloride and thermal fixing was carried out at <NUM> to obtain a porous membrane. The porous membrane was subjected to crosslinking at <NUM> under a relative humidity condition of <NUM>% for <NUM> hours to obtain a crosslinked polyethylene separator. The resultant crosslinked polyolefin separator had a thickness of <NUM>.

A separator was obtained in the same manner as Example <NUM>, except that polyolefin, the first diluting agent, the first alkoxysilane containing a carbon-carbon double bonded group and the second alkoxysilane containing a carbon-carbon double bonded group were introduced at once to the extruder.

Particularly, the above-mentioned materials were introduced to the extruder as follows:
First, <NUM> of high-density polyethylene (Korea Petrochemical Ind. , VH035) having a weight average molecular weight of <NUM>,<NUM> as polyolefin, <NUM> of liquid paraffin oil (Kukdong Oil & Chem. LP350F, <NUM> cSt) as the first diluting agent, <NUM> of vinyltriethoxysilane (b. <NUM>) (as the first alkoxysilane containing a carbon-carbon double bonded group), <NUM> of vinyltrimethoxysilane (b. <NUM>) (as the second alkoxysilane containing a carbon-carbon double bonded group), <NUM> of dibutyltin dilaurate (as a crosslinking catalyst), and <NUM> of <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di(tert-butylperoxy)hexane (DHBP) (as an initiator) were introduced to and mixed in the extruder to carry out reactive extrusion of the silane-grafted polyolefin composition. Herein, the weight ratio of the first alkoxysilane containing a carbon-carbon double bonded group to the second alkoxysilane containing a carbon-carbon double bonded group was <NUM>:<NUM>.

The resultant crosslinked polyolefin separator had a thickness of <NUM>.

In Table <NUM>, each evaluation item is determined by the following methods.

The thickness of a separator was determined by using a thickness measuring system (VL-<NUM>-B available from Mitutoyo Co.

Air permeability was determined by using a Gurley type air permeability tester according to JIS P-<NUM>. Herein, the time required for <NUM> of air to pass through a diameter of <NUM> and an area of <NUM><NUM> was determined.

The meltdown temperature of a crosslinked polyethylene separator was determined by using a thermomechanical analyzer (TMA). Particularly, a load of <NUM>. 01N was applied to the separator, and how much it was deformed was observed while the temperature was increased at a rate of <NUM>/min. The temperature at which point the separator was shrunk and then elongated and broken was determined as 'meltdown temperature of separator'. As the meltdown temperature is increased, it can be stated that the separator retains melt integrity at high temperature and has dimensional stability.

After carrying out extrusion for <NUM> hours or more during the manufacture of a crosslinked polyethylene separator, it was visually determined whether any defect, such as a die-drool phenomenon, appeared or not at the T-die. The results are shown in Table <NUM>.

The Si grafting ratio means the quantitatively determined number of silane-grafted units based on <NUM> carbon atoms in the polyolefin backbone. The Si grafting ratio was obtained by measuring Si content through the inductively coupled plasma (ICP) analysis of a separator and converting the measurement into a Si grafting ratio.

The light transmittance was determined by using a hazemeter (available from Gardner BYK) based on ASTM-D1003.

Particularly, each of the separators according to Examples <NUM> and <NUM> and Comparative Examples <NUM>-<NUM> was fixed at its four sides and allowed to stand at <NUM> for <NUM> minutes. Then, the light transmittance, when light in a region of <NUM>-<NUM> was allowed to pass through each separator, was measured three times and the average value was calculated.

Herein, the light transmittance can be calculated by the formula of {(Amount of light before passing through separator - Amount of light after passing through separator) / Amount of light before passing through separator} x <NUM>.

As can be seen from Table <NUM>, in the case of Examples <NUM> and <NUM> in which both the introduction point of the first alkoxysilane containing a carbon-carbon double bonded group and that of the second alkoxysilane containing a carbon-carbon double bonded group are controlled, no die-drool phenomenon occurs and each separator shows a high meltdown temperature. In addition, in the case of Examples <NUM> and <NUM> in which the crosslinking time, temperature and relative humidity during crosslinking are controlled, the light transmittance is higher as compared to Comparative Examples and the silane grafting ratio is also high.

On the contrary, in the case of Comparative Example <NUM> in which the first alkoxysilane containing a carbon-carbon double bonded group having a high boiling point is introduced alone, the separator has good appearance, but shows a meltdown temperature of <NUM>, which is lower than a desired meltdown temperature according to the present disclosure.

In the case of Comparative Example <NUM> in which the second alkoxysilane containing a carbon-carbon double bonded group having a low boiling point is introduced alone, no die-drool phenomenon occurs by virtue of a decreased retention time in the extruder, but the separator shows a meltdown temperature of <NUM>, which is lower than a desired meltdown temperature according to the present disclosure, due to such a decreased retention time in the extruder.

Meanwhile, in the case of Comparative Example <NUM>, no crosslinking additives (alkoxysilane containing a carbon-carbon double bonded group, initiator, crosslinking catalyst, or the like) are introduced, and thus shows the highest air permeability and the lowest meltdown temperature. Thus, Comparative Example <NUM> provides a separator having physical properties opposite to desired physical properties according to the present disclosure.

In the case of Comparative Example <NUM>, polyolefin, a diluting agent and alkoxysilane containing a carbon-carbon double bonded group are introduced at once to the extruder according to the related art, and thus the boiling point of alkoxysilane containing a carbon-carbon double bonded group and the retention time thereof in the extruder are not considered. In this case, a die-drool phenomenon occurs to provide poor appearance. Thus, the separator according to Comparative Example <NUM> is not suitable as a separator for use in an electrochemical device.

First, <NUM> of high-density polyethylene (Korea Petrochemical Ind. , VH035) having a weight average molecular weight of <NUM>,<NUM> as polyolefin was introduced to an extruder hopper.

Next, <NUM>/hr of liquid paraffin oil (Kukdong Oil & Chem. LP350F, <NUM> cSt) (as a diluting agent), <NUM>/hr of <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di(tert-butylperoxy)hexane (DHBP) (as an initiator (b)), <NUM>/hr of vinyltrimethoxysilane (as alkoxysilane (a) containing a carbon-carbon double bonded group), and <NUM>/hr of dibutyltin dilaurate (as a crosslinking catalyst) were introduced at once to the first port of the extruder, as a pre-introduced composition, and then mixed therein. In the pre-introduced composition, the weight ratio (a/b) of the alkoxysilane (a) containing a carbon-carbon double bonded group to the initiator (b) was <NUM>.

Then, <NUM>/hr of liquid paraffin oil (Kukdong Oil & Chem. LP350F, <NUM> cSt) (as a diluting agent), <NUM>/hr of <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di(tert-butylperoxy)hexane (DHBP) (as an initiator (d)), and <NUM>/hr of vinyltrimethoxysilane (as alkoxysilane (c) containing a carbon-carbon double bonded group) were introduced at once to the second port of the extruder, as a post-introduced composition, and then mixed therein. In the post-introduced composition, the weight ratio (c/d) of the alkoxysilane (c) containing a carbon-carbon double bonded group to the initiator (d) was <NUM>.

Herein, the ratio (t2/t1) of the elapsed time (t2) from the introduction of the post-introduced composition to the extrusion based on the elapsed time (t1) from the introduction of the pre-introduced composition to the extrusion was <NUM>. Particularly, the elapsed time (t1) from the introduction of the pre-introduced composition to the extrusion was <NUM> seconds, and the elapsed time (t2) from the introduction of the post-introduced composition to the extrusion was <NUM> seconds. In addition, the total content of the alkoxysilane containing a carbon-carbon double bonded group was <NUM> times of the total content of the initiator.

Then, the diluting agent was extracted from the stretched sheet by using methylene chloride and thermal fixing was carried out at <NUM> to obtain a porous membrane. The porous membrane was subjected to crosslinking at <NUM> under a relative humidity condition of <NUM>% for <NUM> hours to obtain a crosslinked polyethylene separator. The resultant crosslinked polyolefin separator had a thickness of <NUM>.

A crosslinked polyolefin separator was obtained in the same manner as Example <NUM>, except that the content of each ingredient in the compositions introduced to the extruder and time were controlled as shown in the following Table <NUM>.

A crosslinked polyolefin separator was obtained in the same manner as Example <NUM>, except that the weight ratio (A/B) of the total content (A) of the alkoxysilanes containing a carbon-carbon double bonded group to the total content (B) of the initiators introduced to the first port and the second port of the extruder was controlled to <NUM>.

Particularly, <NUM>/hr of liquid paraffin oil (Kukdong Oil & Chem. LP350F, <NUM> cSt) (as a diluting agent), <NUM>/hr of <NUM>,<NUM>-dimethyl-<NUM>,<NUM>-di(tert-butylperoxy)hexane (DHBP) (as an initiator (b)), <NUM>/hr of vinyltrimethoxysilane (as alkoxysilane (a) containing a carbon-carbon double bonded group), and <NUM>/hr of dibutyltin dilaurate (as a crosslinking catalyst) were introduced at once to the first port of the extruder, as a pre-introduced composition, and then mixed therein. In the pre-introduced composition, the weight ratio (a/b) of the alkoxysilane (a) containing a carbon-carbon double bonded group to the initiator (b) was <NUM>.

A crosslinked polyolefin separator was obtained in the same manner as Example <NUM>, except that the initiator was introduced merely to the first port of the extruder.

Then, <NUM>/hr of liquid paraffin oil (Kukdong Oil & Chem. LP350F, <NUM> cSt) (as a diluting agent), and <NUM>/hr of vinyltrimethoxysilane (as alkoxysilane (c) containing a carbon-carbon double bonded group) were introduced at once to the second port of the extruder, as a post-introduced composition, and then mixed therein. In the post-introduced composition, the weight ratio (c/d) of the alkoxysilane (c) containing a carbon-carbon double bonded group to the initiator (d) was ∞.

A crosslinked polyolefin separator was obtained in the same manner as Example <NUM>, except that merely polyolefin and the diluting agent were introduced to the extruder. Herein, polyolefin was introduced to the extruder hopper and the diluting agent was introduced through the first port.

Particularly, the content of each ingredient in the compositions introduced to the extruder and time were controlled as shown in the following Table <NUM>.

Polyolefin was introduced to the extruder hopper, and a part of the diluting agent, initiator, alkoxysilane containing a carbon-carbon double bonded group and the crosslinking catalyst were introduced at once through the first port. However, another part of the diluting agent was introduced in a divided dose at a predetermined time interval through the second port.

Particularly, a crosslinked polyolefin separator was obtained in the same manner as Example <NUM>, except that the content of each ingredient in the compositions introduced to the extruder and time were controlled as shown in the following Table <NUM>.

A crosslinked polyolefin separator was obtained in the same manner as Example <NUM>, except that the weight ratio (A/B) of the total content (A) of the alkoxysilane containing a carbon-carbon double bonded group to the total content (B) of the initiator introduced to the first port and the second port of the extruder was controlled to <NUM>.

It can be seen from the foregoing that each of the separators according to Examples <NUM> and <NUM> of the present disclosure shows silane grafting formed at an adequate Si grafting ratio, thereby providing excellent processability as well as high light transmittance.

On the contrary, each of the separators according to Comparative Examples <NUM>-<NUM> shows a Si grafting ratio of <NUM> or low Si grafting ratio, thereby providing significantly low light transmittance.

Claim 1:
A crosslinked polyolefin separator having an average value of light transmittance of <NUM>% or more in a region of <NUM> to <NUM>, after four sides of the separator are fixed and allowed to stand at <NUM> for <NUM> minutes, the light transmittance being measured as described in the specification, wherein
the crosslinked polyolefin separator has <NUM>-<NUM> grafted units based on <NUM> carbon atoms in the polyolefin backbone forming the crosslinked polyolefin separator as measured as described in the specification, and
the grafted units are silane-grafted units.