| Microporous polyolefin film with improved meltdown property and preparing method thereof -> Monitor Keywords |
|
|
 
Microporous polyolefin film with improved meltdown property and preparing method thereofRelated Patent Categories: Stock Material Or Miscellaneous Articles, Web Or Sheet Containing Structurally Defined Element Or Component, Composite Having Voids In A Component (e.g., Porous, Cellular, Etc.)Microporous polyolefin film with improved meltdown property and preparing method thereof description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070190303, Microporous polyolefin film with improved meltdown property and preparing method thereof. Brief Patent Description - Full Patent Description - Patent Application Claims
TECHNICAL FIELD [0001] The present invention is related to microporous polyolefin films having superior melt-down property and methods of manufacturing the same. More concretely, the present invention is related to microporous polyolefin films that can enhance the performance and stability of batteries using these films in that they have superior extrusion compoundability and physical properties as well as superior melt-down property and high productivity. The present invention is also related to the methods of manufacturing the same. [0002] Microporous polyolefin films have been used widely for various battery separators, separation filters, microfiltration membranes, etc. owing to their superior chemical stability and superior physical properties. Among them, secondary battery separators require for the highest-level quality along with a high stability. Recently, it has been required to have thermal stability for separators in accordance with the trend of the high capacity and high output of secondary batteries. Particularly, in case of lithium secondary batteries, there is a danger of explosion due to melt-down of separators coming from overheating of batteries if the thermal stability of separators is lowered. [0003] General methods of manufacture of porous films are introduced in U.S. Pat. No. 4,247,498. Disclosed in this patent is the technology of manufacturing microporous polyolefin films by making a thermodynamically single-phase solution by blending polyethylene and a compatible liquid compound at a high temperature, cooling the solution, and performing solid-liquid or liquid-liquid phase separation of polyethylene and the compatible solvent during the process of cooling. [0004] Also disclosed in U.S. Pat. No. 4,539,256 is the basic method of manufacturing microporous films through extrusion processing of polyethylene and a compatible liquid compound, and stretching and extracting them. [0005] The typical method of improving the strength of microporous films is to increase the molecular weight of a composition by using or blending ultrahigh-molecular-weight polyolefins (UHMWPO) having a weight average molecular weight of about 1,000,000. [0006] Further, disclosed in U.S. Pat. No. 5,051,183 are microporous polyolefin films using a composition containing 10.about.50 weight % of polyolefin containing greater than 1% of ultrahigh-molecular-weight polyolefin having a weight average molecular weight of greater than 700,000 and 90.about.50 weight % of a solvent such as a mineral oil, etc., and having a polydisperse index (weight average molecular weight/number average molecular weight) of 10.about.300. The method of forming cells is to form porous films by extruding the above composition to make gel-phase sheets, stretching the sheets at a temperature between the melting point of the composition and the melting point +10.degree. C. and extracting the solvent. However, this method leads to blending of ultrahigh-molecular-weight polyolefins as well as a wide distribution of molecular weights and an excessive amount of polyolefins having large molecular weights. This further leads to lowering of stretchability since chain entanglement among molecules may occur seriously. That is, melt-down at a high stretching rate and high stretching speed or non-stretching phenomenon at a low stretching rate may occur. [0007] The methods of solving the above problems include making the composition soft by increasing the stretching temperature during stretching or obtaining the same effect as that of increasing the temperature of the composition by slowing down the stretching speed. Still, to the contrary, there occurs a problem of lowering physical properties of the final porous films as the orientation of the resin becomes minor during stretching and stretching effects are lowered. Also, films that are made of resins having a wide distribution of molecular weights generally have many defects due to molecules having small molecular weights compared to the films made of resins having a narrow distribution of molecular weights, thus lowering impact strength and puncture strength. These phenomena are not exceptional for microporous films, and puncture strength, which is one of important physical properties of microporous films, is not sufficiently high if the distribution of molecular weights becomes wide. That is, the effects of ultrahigh-molecular-weight polyolefins added to improve physical properties are not shown sufficiently. Such problems appear in Japanese Laid-Open Patent No. H06-234876, Japanese Laid-Open Patent No. H06-212006, and U.S. Pat. No. 5,786,396 that disclose similar technologies. [0008] Such problems in processing according to the use of ultrahigh-molecular-weight polyolefins are general and induce problems such as increase in extrusion load, lowering of extrusion compounding with a compatible compound, increase in the load of a stretching machine during stretching, occurrence of non-stretching, lowering of productivity according to lowering of the stretching speed and stretching ratio, etc. [0009] Further disclosed in U.S. Pat. Nos. 4,588,633 and 4,873,034 are processes of manufacturing microporous films by using ultrahigh-molecular-weight polyolefins having a weight average molecular weight of greater than 500,000 and an excessive amount of a solvent that can dissolve polyolefin at a high temperature, and going through 2-step solvent extraction process and stretching process. However, these methods are disadvantageous in that an excessive amount of a solvent should be used during the process of extrusion in order to improve compoundability and extrudability with the solvent, which is a disadvantage of ultrahigh-molecular-weight polyolefins, and this solvent should be extracted in the first step and extracted again after stretching. [0010] The thermal stability of separators in batteries is determined according to the closing temperature and melt-down temperature. The closing temperature is a temperature at which no more current can flow since minute pores of separators are closed when the inner temperature of batteries is increased abnormally. The melt-down temperature is a temperature at which current flows again owing to melt-down of separators when the temperature of batteries is increased continuously higher than the closing temperature. It is preferable that the closing temperature is low but the melt-down temperature is high for the stability of batteries. Particularly, the melt-down temperature is a temperature at which current can be cut off when it is likely to induce the explosion of batteries, and is very closely related to the stability of batteries. [0011] Efforts to improve the thermal stability of separation membranes have been developed in three directions largely: a method of cross-linking separation membranes, a method of adding inorganic compounds, and a method of using heat-resistant resins. [0012] Among them, the method of cross-linking separation membranes is shown in U.S. Pat. Nos. 6,127,438 and 6,562,519. This method is a method of electronic line cross-linking or chemical cross-linking of films. However, in case of line cross-linking, this method is disadvantageous in that it is necessary to install line cross-linking equipment using radiation, the speed of production is restricted, and there is a deviation in quality coming from uneven cross-linking. Whereas, in case of chemical cross-linking, this method is disadvantageous in that the process of extrusion compounding is complicated, it is likely to have gels generated in the films due to uneven cross-linking, and it is necessary to perform long-time high-temperature ageing. That is, the increase in heat resistance according to the method of cross-linking may bring about increase in inefficiency during the process of production and unevenness in quality. [0013] Still further, disclosed in U.S. Pat. No. 6,949,315 is a method of improving the thermal stability of separation membranes by compounding an inorganic material, such as 5-15 weight % of titanium oxide, to ultrahigh-molecular-weight polyethylene. However, this method is disadvantageous in that it is likely to have problems in using ultrahigh-molecular-weight resins as well as problems of lowering compoundability according to the input of inorganic materials, and uneven quality and generation of pinholes according to lowering of compoundability. And physical properties of films such as impact strength, etc. are lowered due to lack of compatibility of the interface between the inorganic material and the polymer resin. [0014] The typical method of using resins having a superior heat resistance in order to increase the thermal stability of separation membranes is to make multi-layered separation membranes through the lamination of resins having a high melting point. [0015] Disclosed in U.S. Pat. No. 5,691,077 is a method of manufacturing 3-layered separation membranes through the lamination of a polypropylene resin having a superior thermal stability (having a high melting point) to polyethylene having a superior closing property (having a low melting point). These separation membranes produced according to the drying method (a method of making pores by stretching the resin without a diluent) have not been used widely due to disadvantages such as uneven stretching, generation of pinholes, increased deviation in thickness, etc. during the process of manufacture of the original films along with the problems of lowered productivity due to the addition of the lamination process progressed in the separate process as well as the problem of delamination coming from inferior lamination although they have superior thermal characteristics. In spite of a superior heat resistance, the films manufactured according to this method have lowered strength, permeability, evenness in quality, and productivity that are essential for separation membranes for the secondary batteries. [0016] Another method of increasing the thermal stability of separation membranes is to compound and use resins having a superior heat resistance, which is disclosed in U.S. Pat. No. 5,641,565. In this method, separation membranes are made by mixing the resin mixture, in which polyethylene comprised of greater than 10 weight % of molecules having a weight average molecular weight of greater than 1,000,000 but greater than 5 weight % of molecules having a weight verage molecular weight of less than 100,000 and 5-45 weight % of polypropylene are mixed, with 30-75 weight % of an organic liquid compound and 10-50 weight % of an inorganic material, and extracting the organic liquid. compound and the inorganic material. This technology needs to have an inorganic material, polyethylene, and ultrahigh-molecular-weight molecules in order to prevent lowering of physical properties coming from the addition of polypropylene, which is a heterogeneous resin. However, this method may still have problems of an excessive number of ultrahigh-molecular-weight molecules as well as problems of lowering of compoundability according to the input of an inorganic material and uneven quality and generation of pinholes according to lowering of the compoundability. At the same time, the existence of low-molecular-weight molecules may lead to widening of the distribution of molecular weights and lowering of physical properties. Such example may be found in Japanese Laid-Open Patent No. H09-259858. It is seen in the preferred embodiment of that patent that the tensile strength of porous polyethylene films manufactured according to such method is at a comparatively low level. Also, this method is disadvantageous in that it has complicated processes due to the addition of the processes for extracting and removing the inorganic material used, and a comparatively large amount of polypropylene is required in order to obtain sufficient effects. [0017] The essential characteristics of separation membranes of the secondary batteries are strength, permeability, even quality, and productivity, and additionally, thermal stability. However, prior art described in the above use ultrahigh-molecular-weight resins in order to increase physical properties or use inorganic materials in manufacturing separation membranes, and have problems in processing such as the addition of further processes, etc., and therefore, it is difficult to meet required characteristics of the separation membranes of the secondary batteries simultaneously. SUMMARY OF THE INVENTION [0018] Accordingly, the inventors of the present invention repeated extensive studies in order to solve problems with prior art described in the above, and completed the present invention in light of the fact that it is possible to manufacture separation membranes having superior thermal stability (melt-down property) as well as strength, permeability, even quality, and productivity by maximizing compounding of a small amount of a propylene resin having a superior thermal stability without using ultrahigh-molecular-weight polyethylene or inorganic materials while maintaining the contents of low-molecular-weight polyethylene molecules and high-molecular-weight polyethylene molecules contained in general-molecular-weight polyethylene to be less than specific contents. [0019] It is, therefore, an object of the present invention to provide microporous polyethylene films having a high thermal stability and superior extrusion compoundability and physical properties and enabling increase in the performance and stability of batteries using such microporous membranes and increase in the productivity of microporous films themselves. [0020] Microporous polyolefin films according to the present invention to fulfill the above object are characterized by being manufactured in a method comprising the steps of melt-extruding a composition, comprised of 20-50 weight % of a resin composition, comprised of 90-98 weight % of polyethylene (Component I) having a weight average molecular weight of 2.times.10.sup.5.about.4.times.10.sup.5 and less than 5 weight % of molecules of which molecular weight is less than 1.times.10.sup.4 and less than 5 weight % of molecules of which molecular weight is greater than 1.times.10.sup.6, and 2-10 weight % of polypropylene (Component II) of which weight average molecular weight is 3.0.times.10.sup.4.about.8.0.times.10.sup.5 and the peak of the melting point is higher than 145.degree. C. and 80-50 weight % of a diluent (Component III), to mold in the form of sheets; stretching the above sheets to mold in the form of films; extracting the diluent from the above films; and heat-setting the above films. They are also characterized by having a puncture strength of greater than 0.14 N/.mu.m, Darcy's permeability constant of greater than 1.5.times.10.sup.-5 Darcy, closing temperature of microporous films of lower than 140.degree. C. and melt-down temperature of higher than 160.degree. C. [0021] Hereinafter, the method of manufacture of microporous polyolefin films from polyolefin used in the present invention is illustrated in more detail below: [0022] A low-molecular-weight organic material (hereinafter referred to as a diluent) having a similar molecular structure to that of polyolefin forms a thermodynamically single phase with polyolefin at the melting temperature of polyolefin. Phase separation between polyolefin and the diluent occurs during the process of cooling if polyolefin and the diluent solution in the thermodynamically single phase are cooled to a room temperature. Each phase which is subject to phase separation is comprised of a polyolefin-rich phase centered on the lamella which is the crystal portion of polyolefin and a diluent-rich phase comprised of a small amount of polyolefin melted in the diluent even at a room temperature and the diluent. Porous polyolefin films are made by extracting the diluent with an organic solvent after cooling. Continue reading about Microporous polyolefin film with improved meltdown property and preparing method thereof... Full patent description for Microporous polyolefin film with improved meltdown property and preparing method thereof Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Microporous polyolefin film with improved meltdown property and preparing method thereof patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Microporous polyolefin film with improved meltdown property and preparing method thereof or other areas of interest. ### Previous Patent Application: Filter for purifying hydrogen and method for manufacture thereof Next Patent Application: Upholstery part, in particular equipment for the interior of a motor vehicle for producing said part and seat Industry Class: Stock material or miscellaneous articles ### FreshPatents.com Support Thank you for viewing the Microporous polyolefin film with improved meltdown property and preparing method thereof patent info. IP-related news and info Results in 0.13123 seconds Other interesting Feshpatents.com categories: Accenture , Agouron Pharmaceuticals , Amgen , AT&T , Bausch & Lomb , Callaway Golf 174 |
 * Protect your Inventions * US Patent Office filing 
PATENT INFO |
|
