Cross-linked polyolefin separator and manufacturing method thereof

Abstract

A method for manufacturing a crosslinked polyolefin separator and the crosslinked polyolefin separator obtained therefrom are provided. The method includes non-grafted polyolefin having a weight average molecular weight of 300,000 or more and silane-grafted polyolefin having a weight average molecular weight of 300,000 or more. The method minimizes gel formation, a side reaction occurring in an extruder during the manufacture of the separator, and provides the separator having a uniform surface.

Claims

1. A method for manufacturing a crosslinked polyolefin separator, comprising: (S1) mixing non-grafted polyolefin having a weight average molecular weight of 300,000 or more, silane-grafted polyolefin having a weight average molecular weight of 300,000 or more, a diluting agent, an initiator, an alkoxysilane compound containing a carbon-carbon double bonded group and a crosslinking catalyst to an extruder and then carrying out reactive extrusion at 200° C. or higher to obtain a silane-grafted polyolefin composition; (S2) molding and orienting the reactive extruded silane-grafted polyolefin composition in the form of a sheet; (S3) extracting the diluting agent from the oriented sheet to obtain a porous membrane; (S4) thermally fixing the porous membrane; and (S5) crosslinking the porous membrane in the presence of water, wherein a content of the alkoxysilane compound containing the carbon-carbon double bonded group is 0.01-2 parts by weight based on 100 parts by weight of a total weight of the non-grafted polyolefin, silane-grafted polyolefin and the diluting agent.

2. The method for manufacturing the crosslinked polyolefin separator according to claim 1, wherein a weight ratio of the non-grafted polyolefin to the silane-grafted polyolefin is 90:10-20:80.

3. The method for manufacturing the crosslinked polyolefin separator according to claim 1, wherein the non-grafted polyolefin has a weight average molecular weight of 300,000-1,500,000.

4. The method for manufacturing the crosslinked polyolefin separator according to claim 1, wherein the silane-grafted polyolefin has a weight average molecular weight of 300,000-1,000,000.

5. The method for manufacturing the crosslinked polyolefin separator according to claim 1, wherein the reactive extrusion is carried out at a temperature of 200-250° C.

6. The method for manufacturing the crosslinked polyolefin separator according to claim 1, wherein the alkoxysilane compound containing the carbon-carbon double bonded group includes vinyltrimethoxysilane, vinyltriethoxysilane, vinyltriacetoxysilane, (3-methacryloxypropyl)trimethoxysilane, (3-methacryloxypropyl) triethoxysilane, vinylmethyl dimethoxysilane, vinyl-tris(2-methoxyethoxy)silane, vinylmethyldiethoxysilane or a mixture of at least two thereof.

7. The method for manufacturing the crosslinked polyolefin separator according to claim 1, wherein a content of the alkoxysilane compound containing the carbon-carbon double bonded group is 0.05-1.2 parts by weight based on 100 parts by weight of the total weight of the non-grafted polyolefin, silane-grafted polyolefin and the diluting agent.

8. The method for manufacturing the crosslinked polyolefin separator according to claim 1, wherein the non-grafted polyolefin has a weight average molecular weight of 300,000-1,000,000, the silane-grafted polyolefin has a weight average molecular weight of 300,000-1,000,000, a weight ratio of the non-grafted polyolefin to the silane-grafted polyolefin is 90:10-50:50, the reactive extrusion is carried out at a temperature of 200-230° C., and a content of the alkoxysilane compound containing the carbon-carbon double bonded group is 0.1-0.5 parts by weight based on 100 parts by weight of the total weight of the non-grafted polyolefin, silane-grafted polyolefin and the diluting agent.

9. The method for manufacturing the crosslinked polyolefin separator according to claim 1, wherein the thermal fixing is carried out at a temperature of 100-230° C.

10. The method for manufacturing the crosslinked polyolefin separator according to claim 1, further comprising applying and drying slurry for forming a porous coating layer including inorganic particles, a binder polymer and a solvent, after (S5).

11. The method for manufacturing the crosslinked polyolefin separator according to claim 1, wherein a phosphorus compound containing a carbon-carbon double bonded group is further introduced to the extruder in (S1).

12. The method for manufacturing the crosslinked polyolefin separator according to claim 11, wherein (S1) comprises mixing the non-grafted polyolefin having a weight average molecular weight of 300,000 or more, the silane-grafted polyolefin having a weight average molecular weight of 300,000 or more, the diluting agent, the initiator, alkoxy group-containing vinylsilane, the phosphorus compound containing the carbon-carbon double bonded group and a crosslinking catalyst to the extruder, and then carrying out reactive extrusion at 200° C. or higher to obtain the polyolefin composition having the silane compound and the phosphorus compound grafted to a backbone of polyolefin.

13. The method for manufacturing the crosslinked polyolefin separator according to claim 11, wherein the phosphorus compound containing the carbon-carbon double bonded group includes diphenylvinylphosphine oxide, diphenylvinylphosphine, dimethyl vinyl phosphonate, diethyl vinyl phosphonate, diphenylvinyl phosphate, dimethylvinyl phosphate, diethylvinyl phosphate, ethenyl dihydrogen phosphate, isopropenyl dihydrogen phosphate, vinylphosphonic acid or a mixture of at least two thereof.

14. The method for manufacturing the crosslinked polyolefin separator according to claim 11, wherein a total content of the alkoxysilane compound containing the carbon-carbon double bonded group and the phosphorus compound containing the carbon-carbon double bonded group is 0.01-2 parts by weight based on 100 parts by weight of the total weight of the non-grafted polyolefin, silane-grafted polyolefin and the diluting agent.

15. The method for manufacturing the crosslinked polyolefin separator according to claim 11, wherein a weight ratio of the alkoxysilane compound containing the carbon-carbon double bonded group to the phosphorus compound containing the carbon-carbon double bonded group is 90:10-30:70.

16. The method for manufacturing the crosslinked polyolefin separator according to claim 11, wherein the separator has a shutdown temperature of 135° C. or lower and a meltdown temperature of 185° C. or higher.

17. The method for manufacturing the crosslinked polyolefin separator according to claim 11, wherein the separator has a temperature difference of 30° C. or higher between the shutdown temperature and meltdown temperature.

18. A crosslinked polyolefin separator obtained by the method as defined in claim 11, wherein the separator comprises a silane compound and a phosphorus compound grafted to a backbone of polyolefin, and has a silane-derived crosslinking structure.

19. A crosslinked polyolefin separator obtained by the method as defined in claim 1.

Description

EXAMPLE 1

Including Non-Grafted Polyolefin and Silane-Grafted Polyolefin

(1) First, 24 kg/hr of non-grafted high-density polyethylene (Korea Petrochemical Ind. Co. Ltd., VH035) having a weight average molecular weight of 300,000 and a melting point of 135° C. as non-grafted polyolefin, 24 kg/hr of silane-grafted polyethylene (Hyundai EP, X650) having a weight average molecular weight of 300,000 as silane-grafted polyolefin, and 112 kg/hr of liquid paraffin oil (Kukdong Oil & Chem. LP350F, 68 cSt) as a diluting agent were introduced to an extruder and mixed therein.

(2) Herein, the weight ratio of the non-grafted polyethylene:silane-grafted polyethylene:diluting agent was 15:15:70. Meanwhile, 0.5 parts by weight of vinyltrimethoxysilane (as an alkoxysilane compound containing a carbon-carbon double bonded group) based on 100 parts by weight of the total weight of the non-grafted polyolefin, silane-grafted polyolefin and the diluting agent; 2 parts by weight of dibutyltin dilaurate (as a crosslinking catalyst) based on 100 parts by weight of the alkoxysilane compound containing a carbon-carbon double bonded group; and 2 parts by weight of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DHBP) (as an initiator) based on 100 parts by weight of the alkoxysilane compound containing a carbon-carbon double bonded group were further introduced to and mixed in the extruder.

(3) Then, reactive extrusion was carried out at a temperature of 200° C. to obtain a silane-grafted polyethylene composition.

(4) The resultant silane-grafted polyethylene composition was molded into a sheet-like shape through a T-die and cold casting roll. Then, biaxial orientation was carried out by using a tenter-type sequential orienting machine performing MD orientation and then TD orientation. The MD orientation ratio and the TD orientation ratio were 5.5 times and 5.0 times, respectively. The orientation temperature was 105° C. in MD and 125° C. in TD.

(5) After that, the diluting agent was extracted from the oriented sheet by using methylene chloride, and the sheet was thermally fixed at 126° C. with an orientation ratio from 1.3 to 1.1 times to obtain a porous membrane. The porous membrane was crosslinked at 85° C. under a humidity condition of 85% for 48 hours to obtain a crosslinked polyethylene separator. The resultant crosslinked polyethylene separator had a thickness of 12 μm.

EXAMPLE 2

Including Non-Grafted Polyolefin and Silane-Grafted Polyolefin+SRS Coating Layer

(6) Slurry for forming a porous coating layer was applied to both surfaces of the crosslinked polyethylene separator obtained from Example 1. Herein, the slurry for forming a porous coating layer was prepared in such a manner that the weight ratio of alumina particles:cyanoethyl polyvinyl alcohol:polyvinylidene fluoride-co-hexafluoropropylene (PVDF-HFP):acetone might be 16.0:0.2:3.8:80. The slurry for forming a porous coating layer was coated on both surfaces of the crosslinked polyethylene separator under a humidity of about 65% to a thickness of 4.0 μm on each surface, and then dried at 60° C. The resultant composite membrane had a total thickness of 20.0 μm.

EXAMPLE 3

Controlling Silane Content

(7) A separator was obtained in the same manner as Example 1, except that the composition introduced to the extruder was changed as shown in the following Table 1.

(8) Particularly, 0.01 parts by weight of vinyltrimethoxysilane (as an alkoxysilane compound containing a carbon-carbon double bonded group) based on 100 parts by weight of the total weight of the non-grafted polyolefin, silane-grafted polyolefin and the diluting agent; 2 parts by weight of dibutyltin dilaurate (as a crosslinking catalyst) based on 100 parts by weight of the alkoxysilane compound containing a carbon-carbon double bonded group; and 2 parts by weight of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DHBP) (as an initiator) based on 100 parts by weight of the alkoxysilane compound containing a carbon-carbon double bonded group were further introduced to and mixed in the extruder.

EXAMPLE 4

Controlling Silane Content

(9) A separator was obtained in the same manner as Example 1, except that the content of the alkoxysilane compound containing a carbon-carbon double bonded group introduced to the extruder was changed as shown in the following Table 1.

EXAMPLE 5

Introducing Phosphorus Compound

(10) Example 5 further includes introducing a phosphorus compound containing a carbon-carbon double bonded group, as compared to Example 1.

(11) First, 24 kg/hr of non-grafted high-density polyethylene (Korea Petrochemical Ind. Co. Ltd., VH035) having a weight average molecular weight of 300,000 and a melting point of 135° C. as non-grafted polyolefin, 24 kg/hr of silane-grafted polyethylene (Hyundai EP, X650) having a weight average molecular weight of 300,000 as silane-grafted polyolefin, and 112 kg/hr of liquid paraffin oil (Kukdong Oil & Chem. LP350F, 68 cSt) as a diluting agent were introduced to an extruder and mixed therein.

(12) Herein, the weight ratio of the non-grafted polyethylene:silane-grafted polyethylene:diluting agent was 15:15:70. Meanwhile, 0.25 parts by weight of vinyltrimethoxysilane (as an alkoxysilane compound containing a carbon-carbon double bonded group) based on 100 parts by weight of the total weight of the non-grafted polyolefin, silane-grafted polyolefin and the diluting agent; 0.25 parts by weight of diphenylvinyl phosphine oxide (as a phosphorus compound containing a carbon-carbon double bonded group) based on 100 parts by weight of the total weight of the non-grafted polyolefin, silane-grafted polyolefin and the diluting agent; 2 parts by weight of dibutyltin dilaurate (as a crosslinking catalyst) based on 100 parts by weight of the total weight of the alkoxysilane compound containing a carbon-carbon double bonded group and the phosphorus compound containing a carbon-carbon double bonded group; and 2 parts by weight of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DHBP) (as an initiator) based on 100 parts by weight of the total weight of the alkoxysilane compound containing a carbon-carbon double bonded group and the phosphorus compound containing a carbon-carbon double bonded group were further introduced to and mixed in then extruder.

(13) Then, reactive extrusion was carried out at a temperature of 200° C. to obtain a polyethylene composition to which the silane compound and phosphorus compound are grafted.

(14) The resultant silane- and phosphorus compound-grafted polyethylene composition was molded into a sheet-like shape through a T-die and cold casting roll. Then, biaxial orientation was carried out by using a tenter-type sequential orienting machine performing MD orientation and then TD orientation. The MD orientation ratio and the TD orientation ratio were 7.0 times and 7.0 times, respectively. The orientation temperature was 110° C. in MD and 125° C. in TD.

(15) After that, the diluting agent was extracted from the oriented sheet by using methylene chloride, and the sheet was thermally fixed at 126° C. with an orientation ratio from 1.3 to 1.1 times to obtain a porous membrane. The porous membrane was crosslinked at 85° C. under a humidity condition of 85% for 48 hours to obtain a crosslinked polyethylene separator. The resultant crosslinked polyethylene separator had a thickness of 12 μm.

COMPARATIVE EXAMPLE 1

Controlling Alkoxysilane Content

(16) A separator was obtained in the same manner as Example 1, except that the content of the alkoxysilane compound containing a carbon-carbon double bonded group and that of the crosslinking catalyst introduced to the extruder were changed as shown in the following Table 1.

(17) Particularly, 2.5 parts by weight of vinyltrimethoxysilane (as an alkoxysilane compound containing a carbon-carbon double bonded group) based on 100 parts by weight of the total weight of the non-grafted polyolefin, silane-grafted polyolefin and the diluting agent; 1 parts by weight of dibutyltin dilaurate (as a crosslinking catalyst) based on 100 parts by weight of the alkoxysilane compound containing a carbon-carbon double bonded group; and 2 parts by weight of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DHBP) (as an initiator) based on 100 parts by weight of the alkoxysilane compound containing a carbon-carbon double bonded group were further introduced to and mixed in the extruder.

COMPARATIVE EXAMPLE 2

Controlling Alkoxysilane Content

(18) A separator was obtained in the same manner as Example 1, except the following.

(19) First, 24 kg/hr of non-grafted high-density polyethylene (Korea Petrochemical Ind. Co. Ltd., VH035) having a weight average molecular weight of 300,000 and a melting point of 135° C. as non-grafted polyolefin, 24 kg/hr of silane-grafted polyethylene (Hyundai EP, X650) having a weight average molecular weight of 300,000 as silane-grafted polyolefin, and 112 kg/hr of liquid paraffin oil (Kukdong Oil & Chem. LP350F, 68 cSt) as a diluting agent were introduced to an extruder and mixed therein.

(20) In addition, 3 parts by weight of vinyltrimethoxysilane (as an alkoxysilane compound containing a carbon-carbon double bonded group) based on 100 parts by weight of the total weight of the non-grafted polyolefin, silane-grafted polyolefin and the diluting agent; 1 parts by weight of dibutyltin dilaurate (as a crosslinking catalyst) based on 100 parts by weight of the alkoxysilane compound containing a carbon-carbon double bonded group; and 2 parts by weight of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane (DHBP) (as an initiator) based on 100 parts by weight of the alkoxysilane compound containing a carbon-carbon double bonded group were further introduced to and mixed in the extruder.

COMPARATIVE EXAMPLE 3

Using Low-Molecular Weight Non-Grafted Polyolefin Alone, Reactive Extrusion Temperature 180° C.

(21) A separator was obtained in the same manner as Example 1, except the following.

(22) Non-grafted polyolefin (LG Chem., XL1800) having a weight average molecular weight of 200,000 was used alone and introduced to the extruder, and reactive extrusion was carried out at a temperature of 180° C.

COMPARATIVE EXAMPLE 4

Using Low-Molecular Weight Non-Grafted Polyolefin Alone, Reactive Extrusion Temperature 200° C.

(23) A separator was obtained in the same manner as Example 1, except the following.

(24) Non-grafted polyolefin (LG Chem., XL1800) having a weight average molecular weight of 200,000 was used alone and introduced to the extruder, and reactive extrusion was carried out at a temperature of 200° C.

COMPARATIVE EXAMPLE 5

Using Low-Molecular Weight Silane-Grafted Polyolefin Alone, Reactive Extrusion Temperature 180° C.

(25) A separator was obtained in the same manner as Example 1, except the following.

(26) Silane-grafted polyolefin (Hyundai EP, X460) having a weight average molecular weight of 200,000 was used alone and introduced to the extruder, while not introducing any crosslinking additives. Then, reactive extrusion was carried out at a temperature of 180° C.

COMPARATIVE EXAMPLE 6

Using Low-Molecular Weight Silane-Grafted Polyolefin Alone, Reactive Extrusion Temperature 200° C.

(27) A separator was obtained in the same manner as Example 1, except the following.

(28) Silane-grafted polyolefin (Hyundai EP, X460) having a weight average molecular weight of 200,000 was used alone and introduced to the extruder, while not introducing any crosslinking additives. Then, reactive extrusion was carried out at a temperature of 200° C.

COMPARATIVE EXAMPLE 7

Reactive Extrusion Temperature 180° C.

(29) A separator was obtained in the same manner as Example 1, except that the reactive extrusion temperature was controlled to 180° C.

COMPARATIVE EXAMPLE 8

Using Vinyl Group-Containing Phosphorus Compound Alone with No Vinyl Group-Containing Alkoxysilane

(30) A separator was obtained in the same manner as Example 1, except that a phosphorus compound containing a carbon-carbon double bonded group was used alone instead of the alkoxysilane compound containing a carbon-carbon double bonded group.

COMPARATIVE EXAMPLE 9

Using Phosphorus Compound Containing No Vinyl Group Alone with No Vinyl Group-Containing Alkoxysilane

(31) A crosslinked polyolefin separator was obtained in the same manner as Example 1, except that trimethyl phosphate, a phosphorus compound containing no vinyl group, was used alone instead of the alkoxysilane compound containing a carbon-carbon double bonded group.

TEST EXAMPLES

(32) Each of the separators according to Examples and Comparative Examples was evaluated. The results are shown in the following Table 1.

(33) TABLE-US-00001 TABLE 1 Ex. Ex. Ex. Ex. Ex. Comp Comp Comp. Comp Comp Comp Comp Comp Comp. 1 2 3 4 5 Ex. 1 Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6 Ex. 7 Ex. 8 Ex. 9 Content of 0.5 0.5 0.01 2 0.25 2.5 3.0 0.5 0.5 — — 0.5 — — alkoxysilane compound containing carbon- carbon double bonded group (parts by weight) (based on 100 parts by weight of total weight of non-grafted polyolefin, silane- grafted polyolefin and diluting agent) Content of — — — — 0.25 — — — — — — — 0.5 0.5 phosphorus parts compound by containing weight carbon- of carbon phos- double phorus bonded com- group (parts pound by weight) contain- (based on ing 100 parts by no weight of vinyl total weight group of non-grafted was contain used polyolefin, silane- grafted polyolefin and diluting agent) Content of 2 2 2 2 2 2 2 2 2 — — 2 2 2 initiator (parts by weight) (based on 100 parts by weight of alkoxysilane compound containing carbon- carbon double bonded group) Content of 2 2 2 2 2 1 1 2 2 — — 2 2 2 crosslinking catalyst (parts by weight) (based on 100 parts by weight of alkoxysilane compound containing carbon- carbon double bonded group) Die-drool No No No No No Yes Yes Yes Yes No No Yes Yes No Appearance Good Good Good Good Good Poor Poor Poor Poor Poor Poor Poor Poor Good of T-die sheet Thermal 126 126 126 126 126 126 126 126 126 126 126 126 126 126 fixing temperature (° C.) Total 12 20 12 12 12 12 Sep- 12 12 12 12 12 12 12 thickness of arator separator was (μm) broken Thickness ±0.5 ±0.7 ±0.5 ±0.6 ±0.5 ±0.9 during ±0.8 ±0.8 ±1.2 ±1.2 ±0.9 ±1.0 ±0.9 uniformity second- in width ary direction orien- (300 mm) tation Air 151 315 131 119 123 118 126 128 131 132 130 138 133 permeation time (sec/100 cc) Meltdown 190 198 174 188 185 190 171 178 199 198 177 174 148 temperature (° C.) Shutdown 138 141 139 136 135 135 139 139 136 136 139 135 140 temperature (° C.) Heat MD 66 48 68 64 60 60 64 65 64 65 63 66 65 shrink- TD 62 39 64 61 58 59 62 62 62 62 60 61 63 age (150° C./ 30 min) Gel content 60.1 23.4 12.9 85.3 75.1 88.6 54.1 58.6 77.9 90.1 91.3 31.3 45.2 0 (%) Reactive 200 200 200 200 200 200 200 180 200 180 200 180 200 200 extrusion temperature (° C.)

(34) As shown in Table 1, in the case of Examples 1-5, since the content of alkoxy group-containing vinylsilane is significantly low and non-grafted polyolefin and silane-grafted polyolefin having a controlled weight average molecular weight are used at the same time, no die-drool phenomenon occurs even when the reactive temperature is increased to 200° C., and the separators retain thickness uniformity in the width direction.

(35) On the contrary, when the content of alkoxy group-containing vinylsilane is increased in the case of Comparative Example 1 or 2, die-drooling occurs and the sheet appearance is poor. In addition, in the case of Comparative Example 1, the gel content in the extruder is increased significantly. Meanwhile, in the case of Comparative Example 2, the separator is broken during the secondary orientation.

(36) Then, Comparative Examples 3-6 in Table 1 will be compared with one another.

(37) First, in terms of a degree of die-drool generation, die-drool generation becomes severe in the order of Comparative Example 4 (reactive extrusion of low-molecular weight non-grafted polyolefin alone at 200° C.)>Comparative Example 3 (reactive extrusion of low-molecular weight non-grafted polyolefin alone at 180° C.)>Comparative Example 6 (reactive extrusion of low-molecular weight silane-grafted polyolefin alone at 200° C.)>Comparative Example 5 (reactive extrusion of low-molecular weight silane-grafted polyolefin alone at 180° C.). In the case of non-grafted polyolefin, it is thought that die-drool generation is more severe, since vinylsilane monomer does not react with polyolefin but may be evaporated by itself with high possibility. Such a die-drool phenomenon is increased, as the reactive extrusion temperature is increased. Meanwhile, when using non-grafted polyolefin, which has a lower molecular weight as compared to an embodiment of the present disclosure, alone (Comparative Examples 3 and 4), the low-molecular weight non-grafted polyolefin has low viscosity and shows higher volatility after silane grafting, and thus causes more die-drool generation.

(38) Next, in terms of the appearance of a T-die sheet, the sheet appearance becomes poor in the order of Comparative Example 3 (reactive extrusion of non-grafted polyolefin alone at 180° C.)>Comparative Example 5 (reactive extrusion of silane-grafted polyolefin alone at 180° C.)>Comparative Example 7 (the same as Example 1, except that reactive extrusion is carried out at 180° C.)>Comparative Example 4 (reactive extrusion of non-grafted polyolefin alone at 200° C.)>Comparative Example 6 (reactive extrusion of silane-grafted polyolefin alone at 200° C.).

(39) Particularly, in the case of reactive extrusion carried out at 180° C. under the conditions of an extrusion amount of 150 kg/hr or more and a retention time in the extruder of 5 minutes of less (Comparative Example 3), it is more difficult to prepare a homogeneous polyethylene solution as compared to reactive extrusion carried out at 200° C. (Comparative Example 4). Therefore, it seems that the compatibility of polyolefin, the diluting agent and crosslinking additives (alkoxy group-containing vinylsilane, initiator, crosslinking catalyst, etc.) is degraded.

(40) In addition, in the case of grafted polyolefin, it is more likely that the polyolefin is crosslinked in the extruder to cause an increase in viscosity. Thus, reactive extrusion carried out at 180° C. (Comparative Example 5) is relatively disadvantageous as compared to reactive extrusion carried out at 200° C. (Comparative Example 6). Meanwhile, when viscosity is increased, drooling at a T-die, or the like, is relatively decreased, but thickness uniformity in the width direction is degraded as compared to Examples.

(41) Meanwhile, in the case of Comparative Examples 5 and 6, silane-grafted polyolefin is used and evaporation of unreacted silane is less likely, and thus a relatively high grafting conversion ratio is provided. Therefore, it is thought that gel content in the case of Comparative Examples 5 and 6 is higher as compared to the embodiments in which non-grafted polyolefin is used (i.e. Comparative Examples 3 and 4).

(42) Meanwhile, Example 5 includes further introducing a phosphorus compound containing a carbon-carbon double bonded group together with a silane compound. On the contrary, Comparative Examples 8 and 9 use a phosphorus compound alone. Each of Comparative Examples 8 and 9 shows a meltdown temperature of 174° C. and 148° C., respectively, and thus has lower safety as compared to Examples. On the contrary, Example 5 shows a lower shutdown temperature as compared to the other Examples and maintains a relatively higher meltdown temperature, thereby providing significantly improved safety.

(43) In Table 1, each evaluation item is determined by the following methods.

(44) 1) Method of Observing Die-Drool Phenomenon

(45) It is judged that a die-drool phenomenon occurs, when 3 or more foreign materials having a diameter of 1.0 mm or more are detected in a T-die after extrusion is carried out by the method for manufacturing a crosslinked polyolefin separator according to each of Examples and Comparative Examples.

(46) 2) Method for Determining Thickness of Separator

(47) The thickness of a separator was determined by using a thickness measuring system (VL-50S-B available from Mitutoyo Co.).

(48) 3) Method for Determining Air Permeability

(49) Air permeability was determined by using a Gurley type air permeability tester according to JIS P-8117. Herein, the time required for 100 mL of air to pass through a diameter of 28.6 mm and an area of 645 mm.sup.2 was measured.

(50) 4) Method for Evaluating T-Die Sheet Appearance and Thickness Uniformity in Width Direction (300 mm)

(51) The appearance and thickness profile of a casting sheet ejected from a T-die were evaluated by using a radiation gauge (Eurotherm Gauging System Inc./ASC-190), when the sheet passed through the casting roll in the T-die.

(52) 5) Method for Determining Heat Shrinkage

(53) Heat shrinkage was calculated by the formula of (Initial length−Length after carrying out heat shrinking treatment at 120° C. for 1 hour)/(Initial length)×100

(54) 6) Method for Determining Gel Content in Separator

(55) A sheet was obtained right after it was ejected from an extruder T-die, was allowed to stand for 7 days or more, and then liquid paraffin oil contained in the sheet was extracted with methylene chloride. The sheet from which liquid paraffin oil was removed was dipped in 1,2,4-trichlorobenezene solution and heated at 160° C. for 12 hours, and then the dry weight of residue was measured. The dry weight of residue was converted into percentage (%) based on the initial weight.

(56) 7) Method for Determining Shutdown Temperature

(57) First, each of the separators according to Examples and Comparative Examples was introduced to an electrolyte (1M LiPF.sub.6 in ethylene carbonate:ethylmethyl carbonate=5:5).

(58) Then, the shutdown temperature was defined by the point where ion conductivity becomes 0, while increasing the temperature from 30° C. at a rate of 5° C./min, and the temperature at this point was measured.

(59) 8) Method for Determining Meltdown Temperature

(60) The meltdown temperature is determined by taking a sample each in the machine direction (MD) and the transverse direction (TD) perpendicular to MD and analyzing the same through thermomechanical analysis (TMA). Particularly, a sample having a length of 10 mm is introduced to a TMA instrument (TA instrument, Q400) and exposed to an increasing temperature condition (heating rate 5° C./min. from 30° C.), while applying a tension of 19.6 mN thereto. As the temperature is increased, the sample undergoes a change in length, and the temperature at which point the sample length is increased rapidly to cause fracture is measured. The temperature is measured each in MD and TD, and the higher temperature is defined as the meltdown temperature of the corresponding sample.