MXPA00003230A - Method of making electret articles and filters with increased oily mist resistance - Google Patents

Abstract

An electret article can be made by quenching a molten blend containing a polymer and a performance-enhancing additive. The resulting low crystallinity material can be annealed and charged to produce a material for making electret filters having superior oily mist loading performance properties.

Description

METHOD FOR ELBOWING ELECTROTE ARTICLES AND FILTERS WITH INCREASED RESISTANCE TO OLIVE LLOVIZNA Technique of the Invention This invention pertains to a method for making electret articles, electret filters, respirators employing electret filters, and the use of electret filters to remove particles from a gas, especially removing aerosols from the air. This invention is especially concerned with methods that produce electret filters that have improved properties such as electret stability in the presence of oily drizzles (ie, liquid aerosols).

BACKGROUND OF THE INVENTION Scientists and engineers have long sought to improve the filtering performance of the filters. Some of the most effective air filters use electret items. Electret items show a persistent or almost permanent electrical change. See G. M. Sessler, De electret, Springer Verlag, New York, 1987. Researchers have expended considerable effort to improve the properties of electret articles for use in REF .: 119259 filters. Despite extensive research aimed at producing improved electret articles, the effects of processing variables are not well understood, and, in general, the effects of varying processing conditions are difficult if not impossible to predict. Electret items have special properties requirements such as load stability, load execution, moisture resistance and oil exposure, etc., which can be significantly affected by the processing steps that will generally be harmless or beneficial to fabrics of non-woven fabrics. fabrics and materials of similar fabrics. Thus, in the absence of extensive empirical data, it is often very difficult to understand the effects that a particular stage of processing (e.g., cooling) may or may not have the electret properties of the resulting product. One method that has been reported to improve the performance of the electret filter is by mixing an additive that improves the performance in a polymer that is used to form electret fibers. For example, Jones et al., In U.S. Patent Nos. 5,411,576 and 5,472,481 disclose electret filters that are made by extruding a polymer blend and a processible melt fluorochemical to form a microfibrous fabric that is subsequently quenched and corona treated. Lifshutz et al. WO 96/26783 (corresponding to US Pat. No. 5,645,627) reports electret filters that are made by extruding a polymer and a fatty acid amide or a fluorochemical oxazolidinone fluorochemical to form a microfibrous tissue that is subsequently tunes and treated with crown. Other techniques have been reported that improve the loading properties of an electret article. For example, Klaase et al. In U.S. Patent No. 4,588,537 report that it uses a corona treatment to inject charge into an electret filter. Angadjivand et al. In U.S. Patent No. 5,496,507 found that bumping water droplets into a microfibrous fabric of the non-woven fabric imparted to a load to the fabric, and Rousieau or co-workers in WO 97/07272 discloses electret filters which are made by extruding the mixtures of a polymer and a fluorochemical or an organic triazine compound to form a microfibrous fabric which is subsequently collided with droplets of water to impart the charge and whereby the performance of the filtration of the hydrocarbon fabric is improved. Matsuura and co-workers in US Patent No. 5,256,176 describe a process of making stable electrets by exposing an electret to alternating cycles of applying electric charges and subsequently heating the article. Matsuura et al. Does not describe electretos that have additives that improve the execution of the charge of oily drizzle. In addition, electret articles and methods of preparation such as articles are described in DE-A-20 35 383.

Brief Description of the Invention This invention provides a method for making an electret article in which a molten mixture is formed of a polymer and an additive that improves performance (other ingredients may also be added as will be described later). The molten mixture can be converted into a desired form, such as a film or fiber and cooled. The cooled material can then be quenched and charged to produce an electret article. The electret article may, for example, be in the form of a fiber or film, or it may be in the form of a nonwoven fabric, especially when used as a filter. Cooling reduces the order of a material (for example the crystallinity) how the order of the material was compared without the cooling process. The cooling step occurs concurrently with or briefly after converting a molten material into a desired shape. Normally the material is formed by extruding it through a die hole and cooling it (usually by applying a cold fluid to the extrudate) immediately after it leaves the hole. The inventors discovered that improved filtration performance can be imparted to the cooled electret material, where the material contains a molten polymer blend and an additive that improves performance. The cooling step tends to freeze the polymer in an amorphous state and thus reduce the extent of crystallinity as compared to the crystallinity of the polymer without cooling. The crystallized material is a valuable intermediate that can also be converted by the known processes to form an electret article. The invention also provides a single electret article containing a polymer and an enhancement additive that can be characterized through certain characteristics in a Thermally Stimulated Discharge Current (TSDC) spectrum. Electrolytic filters incorporate electret items that exhibit the unique TSDC spectral characteristics that can show surprisingly superior filtering performance. The invention includes articles incorporating electret articles made according to the methods described above, and also includes methods of removing liquid aerosol or particulate solid from a gas using the inventive electret articles. The invention further provides electret filters that exhibit superior properties not achieved in similarly constructed filters that do not use the inventive electret articles. These filters contain fibers made from a mixture of polymer and an additive that improves the performance and exhibit superior performance of liquid dioctyl phthalate (DOP) aerosol loading. The DOP liquid aerosol charging run is defined in relation to the particular tests in the Examples section. Preferred filters show an oily drizzle loading performance and decreased penetration of aerosols or particulates while at the same time showing a pressure drop across the filter.

The present invention can provide numerous advantages over known electret filters including improved performance of oily spray aerosol charge, charge stability in the presence of liquid aerosol, and decreased penetration of aerosols or particulates with a small pressure drop through the filter. The electret articles of the present invention can find their use in numerous filtration applications, including respirators such as face masks, home and air conditioners for industry and the home, furnaces, air purifiers, vacuum purifiers, filter medical and air line, and air purification systems in vehicles and electronic equipment such as computers and disk drives BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a flow map illustrating a process for an electret filter medium according to the invention. Figure 2 shows a plane of the "minimum to test" (ie the mass, in milligrams (mg), of dioctyl phthalate (DOP) that has been incident on a filter tissue at the point where the DOP Penetration Percentage reaches a minimum value, then "MinTChl") of the sample cut from an electret filter tempered, not cooled vs. the Crystallinity Index of the sample before annealing. As explained in detail in the Examples section, these data were obtained by exposing the filter tissues to a liquid DOP aerosol in an instrument that measures the concentration of the DOP liquid aerosol upstream and downstream of the filter. Percent Penetration is calculated by dividing the concentration of the downstream aerosol by the upstream concentration and multiplying by 100. Figure 3 shows a Min @ chl plane of the cut of the samples of a tempered electret filter, not cooled . the Crystallinity Index of the samples before annealing. Figure 4 shows a Min @ chl plane of the cut of the samples of a tempered electret filter, not cooled vs. the crystallinity index of the samples before annealing. Figure 5 shows a Min @ chl plane of the cut of the samples of tempered, uncooled or cooled electret filters vs. the Crystallinity Index of the samples before annealing. Figure 6 shows a MinTchl plane of the cut of the samples of tempered electret filters, not cooled and cooled vs. the Crystallinity Index of the samples before annealing. Figure 7 shows a Min @ chl plane of the cut of the samples of tempered, uncooled and cooled electret filters vs. the Crystallinity Index of the samples before annealing. Figure 8 shows a Min @ chl plane of the cut of the samples of tempered, cooled and cooled electret filters vs. the crystallinity index of the samples before annealing. Figure 9 shows a Mingchl plane of the cut of the samples of tempered electret filters, not cooled and cooled vs. the crystallinity index of the samples before annealing. Figure 10 shows a respirator or mask for the filtering face 10 incorporating an electret filter of the invention. Figure 11 shows a cross-sectional view of the body of the respirator 17. Figure 12 shows a thermally stimulated discharge current (TSDC) spectrum of the non-charged polymer and an enhancement enhancer containing fabrics that have been stimulated in a field 2.5 kilovolts per millimeter (kV / mm) at 100 ° C for one minute. The tissues were produced using the following four processing conditions; a) cooled, not tempered, b) not cooled, not tempered, c) cooled, tempered, and d) uncooled, quenched. Figure 13a shows a plane of the crystallinity index of 6 uncharged and unhardened polymers and an additive that improves the performance containing the tissue samples Vs. the charge density of the samples after annealing (without loading) that have been driven in an electric field of 2.5 kilovolts per millimeter (kV / mm) at 100 ° C for 1 minute. Figure 13b shows a plane of the DOP charge execution (in MinTChl) of 6 uncharged and unhardened polymers and an additive that improves the performance containing the tissue samples Vs. the charge density of the samples after annealing (without loading) that have been driven in an electric field of 2.5 kilovolts per millimeter (kV / mm) at 100 ° C for 1 minute. Figure 14 shows the TSDC spectrum of the tempered, non-driven and corona-loaded polymer without the additive that improves the performance containing the tissues. Samples a and b were cooled during processing while samples a 'and b' did not cool. The A side refers to the side of the tissue that contacts the upper electrode when a positive current is discharged while the B side refers to the opposite side of the tissue which, when the upper electrode contacts, discharges a negative current. Figure 15 shows the TSDC spectrum of the tempered, non-driven and corona-loaded polymer and the additive that improves the performance containing the tissues. Samples a and b were cooled during processing while samples a 'and b' did not cool. Side A refers to the same side of the tissue as side A of Figure 14 with respect to contacting the upper electrode, and side B refers to the opposite side of the fabric. Figure 16a shows the TSDC spectrum of the cooled, quenched and corona-loaded polymer, and the performance enhancing additive containing the fabrics that have been driven in an electric field of 2.5 kV / mm at 100 ° C for a) 1 minute, b) 5 minutes, c) 10 minutes and d) 15 minutes. Figure 16b shows the TSDC spectrum of the cooled, quenched and corona-loaded polymer, and the performance-enhancing additive containing the fabrics that have been driven in an electric field of 2.5 kV / mm at 100 ° C for a) 1 minute, b) 5 minutes, c) 10 minutes and d) 15 minutes.

Figure 17 shows a plane of charge density vs. the time that it drives for polymer loaded with corona and tempered, cooled (dotted line) and uncooled (solid line) and an additive that improves the execution that contains the tissues.

DETAILED DESCRIPTION OF THE PREFERRED MODALITIES The electret articles of the invention contain a polymer and an additive that improves performance. The polymer can be a non-conductive thermoplastic resin that is, a resin having a strength greater than 1014 ohm-cm, more preferably 10lQ ohm-cm. The polymer must have the ability to possess a trapped charge of long and non-transient life. The polymer can. be a homopolymer, copolymer or polymer mixture. As reported by Klaase and collaborated on the North American Patent No. 4,588,537, the preferred polymers include polypropylene, poly (methyl-l-pentene), linear low density polyethylene, polystyrene, polycarbonate and polyester. The major component of the polymer is preferably polypropylene because of the high strength of the polypropylene, it has the ability to form meltblown fibers with useful diameters for air filtration, satisfactory filler stability, hydrophobicity, and moisture resistance. On the other hand, polypropylene is not normally oleophobic. The electret articles of the invention preferably contain about 90 to 99.8 weight percent polymer; more preferably about 95 to 99.5 weight percent; and more preferably about 92 to 99 weight percent, based on the worst article. Additives that improve performance, as defined in the present invention, are additives that improve the filterability of the oily aerosol of the electret article after it has been formed in an electret filter. The filtration capacity of the oily aerosol is measured by the DOP loading tests described in the Examples section. Additives that enhance the particular performance include those described by Jones et al., US Patent No. 5,472,481 and Rousseau et al., WO 97/07272. Performance-enhancing additives include fluorochemical additives such as fluorochemical oxazolidinones such as those described in U.S. Patent No. 5,025,052 (Crater et al.), Fluorochemical piperazines and stearate esters of perfluoroalcohols. In view of its demonstrated efficacy in improving the properties of the electret, the performance enhancing additive is preferably a fluorochemical, more preferably a fluorochemical oxazolidinone. Preferably the fluorochemical has a melting point above the melting point of the polymer and below the extrusion temperature. For processing considerations, when using polypropylene, fluorochemicals preferably have a melting point above 160 ° C and more preferably a melting point of 160 ° C to 290 ° C. Particularly preferred fluorochemical additives include Additives A, B and C of U.S. Patent No. 5,411,576 having the respective structures.

C8F1_ / SO ¿_N (CH i_ (CH3) S 02C8 F17 The electret article of the invention preferably contains about 0.2 to 10 weight percent of the additive that improves performance; more preferably about 0.5 to 5 percent by weight; and still more preferred about 1.0 to 2.0 weight percent, based on the weight of the article.

The polymer and additive that improves performance can be mixed as solids before melting, but the components are preferably melted individually and mixed together as liquids. Alternatively, the fluorochemical additive and a portion of the polymer can be mixed as solids and melted to form a relatively rich fluorochemical melt mixture which is subsequently combined with the non-fluorochemical polymer. The molten mixture is then formed into a desired form as a film or fiber . Normally the molten mixture is formed by extruding through a die, but in less preferred modalities the mixture can be formed by alternative processes such as extraction in an electrostatic field (see, for example, Y. Trouilhet, "New Method of Manufacturing Nonwovens by Electrostatic Laying," in Index 81 Congress Papers, Advances In Web Forming European Disposable and Nonwovens Association, Amsterdam, May 5 to 7, 1981. A preferred extrusion process uses two extruders, in this process about 10 to about 20 weight percent of the fluorochemical additive and about 80 to about 90 per cent. 100 weight percent of the polymer are mixed in a first extruder and this has a molten mixture of relatively high flourochemical content that is fed in a second extruder with the molten polymer (which does not contain a fluorodichic) to form a mixture that is extruded through the extruder. a hole in the die The molten mixture of high fluorochemical content is preferably combined with the A polymer that does not contain fluorochemical only before the extrusion of the molten material through a die. This minimizes the time that the fluorochemical is exposed to high temperature. The temperature should be controlled during extrusion to provide the desired extrusion rheology and avoid thermal degradation of the fluorochemical. Different extruders usually require different temperature profiles, and some experimentation may be required to optimize the extrusion conditions for a particular system. For the polypropylene / fluorochemical mixture the temperature during extrusion is preferably kept below approximately 90 ° C to reduce the thermal degradation of the fluorochemical. If the extruders are used, they will preferably be of the double screw type for a better mix, and the extruders may be those commercially available as those of Werner &; Pfleiderer or Berstorff. The molten mixture is preferably extruded through a die, and more preferably the mixture is extruded through a die under the melt blowing conditions. Blow molding is known to offer numerous advantages, especially fabrics that produce non-woven fabrics, and articles of the invention can be made using blow molding processes and apparatus that are well known in the art. The meltblown fiber was initially described by Van Wente, "Superfine Thermoplastic Fibers," Jn. Eng. Chem. , vol. 48, pp. 1342-46, (1956). In general, the blowing melt in the present invention is conducted using conventional procedures with the modification that the material is cooled (cooled) when it leaves the die. Convenient cooling techniques include spraying water, spraying with a volatile liquid, or contacting with cooled air or cryogenic gases such as carbon dioxide or nitrogen. Normally the cold fluid (liquid or gas) is sprayed from the nozzles located within approximately 5 centimeters (cm) of the holes of the die. In the case of materials extruded through a die, the cold fluid impacts the molten extrudate immediately after it is extruded from the die (and well before the material is collected). For example, in the case of blown melt fibers, the molten extrudate must be cooled before being collected in the form of a nonwoven fabric. The cold fluid is preferably water. The water can be tap water but preferably it is distilled water or deionized water. The object of the cooling step is to minimize the crystallinity of the polymer in the resulting article. The inventors discovered that electret filters made of cooled materials show an unexpectedly good liquid aerosol filtration performance when they are subsequently annealed and charged. The cooling step reduces the crystallinity content of the polymer as compared to the uncooled polymer extruded under the same conditions. The cooled material has a low degree of crystallinity preferably as determined by the diffraction of the x-rays. Preferably, the polymer in the cooled material has a crystallinity index of less than 0.3, more preferably less than 0.25, still more preferably less than 0.2, and still more preferably less than 0.1, as measured by the intensity ratio of the critical point of crystallinity at the total scattered intensity above 6 to 36 degrees of the dispersion angle range. Thus, a preferred intermediate composition for making an electret filter is made by mixing and extruding a mixture of 90 to 99.8 percent by weight of the organic polymer and 0.2 to 10 percent by weight of an additive that improves the performance; where the material is extruded through a die under blown melt conditions to form fibers that are collected as a nonwoven fabric. The fibers are cooled, before being collected, by a cooling process such as spraying water, spraying with a volatile liquid, or contacting with cooled air or cryogenic gases; as carbon dioxide or nitrogen.

After cooling, the material is collected. If the material is in the form of fibers, they can be collected, cut and carded into a nonwoven fabric. The blowing melt fibers can normally be collected as a fabric of the non-woven fabric in a rotary drum or in a movable belt. Preferably the cooling and collection steps are conducted such that there is no excess cooling fluid (if there is a residual fluid that is normally water) remaining from the collected material. The remaining fluid in the collected material can cause problems with storage and may require additional heating during tempering to drive the cooling fluid. Thus, collected material preferably contains less than 1 weight percent of the cooling fluid, and more preferably does not contain residual cooling fluid. The cooled material is tempered to increase the stability of electrostatic charge in the presence of oily drizzles. Preferably, the performance enhancing additive is a substance having low energy surfaces such as a fluorochemical, and the tempering step is directed at a sufficient temperature and for a time sufficient to cause the additive to diffuse to the interfaces (e.g. the air interface of the polymer, and the boundary between the crystalline and amorphous phases) of the material. Generally, the high tempering temperatures allow for shorter times. To obtain the desirable properties for the final product, tempering of polypropylene materials should be conducted at about 100 ° C. Preferably, tempering is directed from about 130 to 155 ° C for about 2 to 20 minutes; more preferably from about 140 to 150 ° C for about 2 to 10 minutes, and even more preferably at about 150 ° C for about 4.5 minutes. The tempering must be conducted under conditions that do not substantially degrade the structure of the tissue. For polypropylene fabrics, tempering at temperatures substantially above about 155 ° C may not be desired because the material may be damaged. Fabrics that have not generally been tempered do not show acceptable oily drizzle loading performance. Unhardened fabrics usually show a Min @ Chl of zero. The inventors assume that the improved performance of the tempered fabrics is due to an increase in the interfacial area and / or an increase in the number of sites that trap the stable charge. Thus, alternative methods of increasing interfacial area can be used instead of annealing. Tempering increases the crystallinity of the polymer in the material. Tempering is also known to increase the stiffness of the material and to decrease elongation, softness and tear strength. However, the decrease in softness and tear resistance is irrelevant since the goal of the invention is to improve the performance of the electret filter. With or without cooling, the tempering step is usually a ratio that limits the stage that makes the electret filter fabrics resistant to the liquid aerosol. In one embodiment, the fabric is formed in a melt blow process at a rate of approximately 2.48 to 6.94 g / h / m (0.5 to 1.4 lbs./hr / inch) of die. The inventive method further includes the step of electrostatically charging the material after it has cooled. Examples of useful electrostatic charging methods of the invention include those described in U.S. Patent Nos. Re. 30,782 (van Turnhout), Re. 31,285 (van Turnhout), 5,401,446 (Tsai, et al.), 4,375,718 (Wadsworth et al. ), 4,538,537 (Klaase et al.), And 4,592,815 (Nakao). The electret materials may also be hydrocarbon (see U.S. Patent No. 5,496,501 to Angadjivand et al.). The staple fibers can be tribocharged by rubbing or shaking with dissimilar fibers (see US Pat. No. 4)., 798,850 by Brown et al.). Preferably, the precess for charging involves subjecting the material to a corona discharge or pulsed high voltage as described in some of the patents mentioned above. The fibers may be of a central coating configuration and, if also, the coating must contain the additive that improves performance as described in the mixtures discussed above. Preferably, the extrudate is in the form of microfibers having an effective diameter of about 5 to 30 micrometers (μm), preferably about 6 to 10 μm as calculated according to the method indicated in Davies, CN, "The Separation of Airborne Dust and Particulates, "Inst. Of Mech Eng., London, IB Proceedings, 1952. The electret articles of the invention can be characterized by the studies of TSDC. In TSDC a sample is placed between two electrodes, heated to a constant proportion, and the current discharged from the sample is measured by an ammeter. The TSDC is well known in the art. For example, see U.S. Patent No. 5,256,176, Lavergne et al. "Review of Thermo-Stimulated Current" IEEE Electrical Insulation Magazine, vol. 9, no. 2, 5-21, 1993, and Chen et al. "Analysis of Thermally Stimulated Processes" Pergamon Press, 1981. The current discharged from the sample is a function of the polarizability and charge traps of the article to be tested. The loaded items can be tested directly. Alternatively, loaded and uncharged articles can first be driven in an electric field at an elevated temperature and then can quickly be cooled below the crystallinity transition temperature (Tg) of the polymer with the polarized field in the polarization induced "in freezing. " The sample is then heated to a constant speed and the resulting discharged current is measured. In the process of polarization, charge injection, dipole alignment, load redistribution or some combination of those that may occur. During a thermally stimulated discharge, the charges stored in an electrode are mobile and neutralized either at the electrodes or in the volume sample by recombination with opposite signal charges.

This generates an external current that shows a number of critical points when it is recorded as a temperature function that is planned in a graph (determined a TSDC spectrum). The shape and location of these critical points depend on the energy levels that trap the load and the physical location of the sites that trap the load. As indicated by several investigators (see, for example, Sessler, ed., "Electrets," Springer-Verlag, 1987 and Van Turnhout, "Thermally Stimulated Discharge of Polymer Electrets," Elsevier Scientific Publishing Co, 1975), are typically stored electret loads in structural anomalies, such as impurities, defects of monomer units, chain irregularities, etc. The amplitude of a TSDC peak is influenced by the distribution of the levels that trap the charge in the electret levels. In semicrystalline polymers, charges are often either accumulated or emptied near the amorphous crystalline interfaces due to the difference in phase conductivity (the Maxwell-Wagner effect). These trapping sites are normally associated with different trapping energies where a continuous distribution of activation energies will be expected and the TSDC critical points will be expected to overlap and unite at a broad peak.

In a series of TSDC measurements described in the Examples section, it has been surprisingly discovered that several features in the TSDC spectrum correlate with the superior charging performance of oily drizzle. The spectral characteristics of TSDC are correlated with the superior execution that includes the features discussed below as preferred modalities. In a preferred embodiment, an intermediate composition for making an electret filter, the composition comprises a nonwoven fabric of fibers having a charge density of at least about 10 microcolombos per square meter (μC / m2) when tested according to the TSDC Test Procedure 1 (as indicated in the Examples section) In another preferred embodiment an electret article has a TSDC spectrum that exhibits a peak at approximately 15 ° C to 30 ° C, more preferably from about 15 ° C to 25 ° C, below the mng temperature of the article, as measured by the TSDC Test Procedure 2. When the polymer is polypropylene, the TSDC exhibits a peak at approximately 130 to 140 °. C.

In yet another preferred embodiment, an electret article having a TSDC spectrum showing a peak having a mid-height amplitude of less than about 30 ° C, more preferably a half-height amplitude of less than about 25 ° C, and still more preferably a half height amplitude less than about 20 ° C, as measured by the TSDC Test Procedure 3. In cases where the polymer is polypropylene, the narrow peak described above has its maximum at about 138 to 142 ° C . In another preferred embodiment an electret article shows an increasing charge density for 1 to 5 minutes, and / or 5 to 10 minutes, of driving time, as measured by the TSDC Test Procedure 4. The electret article can be in the form of a fiber and a multitude of these fibers can be formed in an electret filter. An electret filter may take the form of a nonwoven fabric containing at least some electret fibers combined with a support structure. In any case, the electret article can be combined with some material without electret. For example, the support structure may be of fibers without electret or a support without electret, fabrics of the non-woven fabric. The electret filter is preferably an electret fabric of the non-woven fabric containing electrically charged blowing mmicrofibers. Electret filter fabrics may also include main fibers that provide less dense tissue, high. The methods of incorporating main fibers into the nonwoven fabric may be those performed as described in U.S. Patent No. 4,118,531 to Hauser. If the main fibers are used, the fabric preferably contains less than 90 weight percent of main fibers, more preferably less than 70 weight percent. For the reasons of simplicity and optimization of the execution, the electret fabric may in some instances consist essentially of blowing melt fibers and not contain the main fibers. The electret filter may contain additional absorbent particulates such as alumina or activated carbon. The particulates can be added to the filter to assist in the removal of gaseous pollutants from air stream passing through the filter. Particle-laden fabrics are described, for example, in U.S. Patent Nos. 1,971,373 to Braun, 4,100,324 to Anderson and 4,429,001 to Kolpin et al. If the particulate material is added, the fabric preferably contains less than 80 volume percent of the particulate material, more preferably less than 60 volume percent. In embodiments where the electret filter does not need to remove the gaseous contaminants, the filter may contain only the melt blowing fibers. The material used forms the electret filter which is desirably substantially free of materials such as antistatic agents which could increase the electrical conductivity or otherwise interfere with the ability of the article to accept and retain the electrostatic charge. Additionally, the electret article should not be subjected to unnecessary treatments such as exposure to gamma rays, UV irradiation, pyrolysis, oxidation, etc., which could increase electrical conductivity. Thus, in a preferred embodiment the electret article is made and used without exposure to gamma ray irradiation or other ionized irradiation. The electret filters are made of blowing melt fibers which normally have a basis weight of about 10 to 500 grams per square meter (g / m2), more preferably about 10 to 100 g / pr. Filters that are too dense may be difficult to load while those that are too light or too thin may be brittle or may have insufficient filtration capacity. For several applications the electret filters are approximately 0.25 to 20 millimeters (mm) thick, and commonly approximately 0.5 to 2 mm thick. The electret filters of these weights and base thicknesses can be particularly useful in a respirator. Filters of the invention preferably show an initial DOP penetration of less than 5% and an average of Min @ Chl greater than 200 mg of DOP, more preferably greater than 400 mg of DOP, as measured by the Load Test Procedure of the DOP Filter Fabric 1 described in the Examples section. The "average" as used in the Tables and Examples e means the measurements made from 4 to 6 samples cut from the equally spaced parts that traverse the total width of the filter fabric. For any other group of samples, the average is defined as the means of the Min @ Chl value of an appropriate number of samples that are selected and the load was tested using the "t-test" as described in Devore, "Probability and Statistics for Engineering and the Sciences, "Brooks / Cole Publishing Co. (1987) to determine a statistically significant average within a normal deviation. The performance of the upper filtration is achieved by the preferred inventive filters in which filter taken separately without averaging (afterwards, simply " filtrate" is indicated) shows a MinTChl greater than 500 mg of DOP, more preferably greater than about 600, and still more preferably from about 800 to 1000 mg of DOP. These filters preferably show a pressure drop of less than 13 mm (127 Pa) of H20, more preferably less than 10 mm (98 Pa) of H 0, and even more preferably less than 8 mm (78 Pa) of H 20, as measured by the Load Test Procedure 1 method as described in the section of Examples The penetration of DOP is usually measured in an instrument known as an Automated Filter Tester (AFT). An initiation period is required for the DOP aerosol to reach the filter and for the electronics in the AFT to establish. DOP penetration refers to the% DOP that penetrates the tissue during the initial exposure, typically 6 to 40 seconds, while the testing apparatus is balanced. The initial DOP penetration is the first reading presented by the AFT using the established program. The filters of the present invention have at least a perceptible penetration (ie, a penetration about 0.001 ° or for the AFT instruments described in the Examples section). In respirators, fibrous electret tissues can be specially formed or housed; for example, in the form of molding or folded half-face masks, filter elements for replaceable cartridges or receptacles, or pre-filters. An example of a respirator 10 of the present invention is shown in Figures 10 and 11. The respirator mask of the body 17 can be curved, hemispherical or it can take other forms as desired (see, for example, US Pat. Nos. 5,307,796 and 4,827,924). In the respirator 10, the electret filter 15 is sandwiched between cover fabric 11 and the inner molded layer 16. The molded layer 16 provides the structure to the mask 10 and is supported for the filtration of the layer 18. The molded layer 16 it can be located inside and / or outside the filtration layer 18 and can be made, for example, from a non-woven fabric of thermally bonded fibers molded in a cup-like configuration. The molded layer can be molded according to known procedures (see, for example, US Patent No. 5,307,796). The molded layer or layers are usually made of bicomponent fibers having a center of a high melting material such as polyethylene terephthalate surrounded by a coating of low melting material so that when heated in a mold, the layer molded according to mold shape and retain this shape when cooled to room temperature. When pressed together with another layer, such as the filter layer, the low melting coating material can also serve to join the layers together. To hold the mask on the user's face, the body of the mask may have straps 12, lasso strands, mask harnesses, etc. attached to it. A flexible soft band 13 of metal like aluminum can be provided in the body of the mask 17 to enable it to be formed to hold the mask of the face in a desired adjustment on the user's nose (see, for example, US Patent No. 5,558,089 ). Respirators may also have additional features such as additional layers, valves (see, for example, U.S. Patent No. 5,509,436), molded face pieces, etc. Examples of respirators that can incorporate the improved electret filters of the present invention include those described in U.S. Patent Nos. 4,536,440, 4,827,924, 5,325,892, 4,807,619 4,886,058 and U.S. Patent Application no. 08 / 079,234. The respirators of this invention have a surface area of approximately 180 square centimeters (cirr) preferably show a Min @ Chl greater than 400 milligrams (mg) of DOP, more preferably greater than 600 mg of DOP, when tested using the National Insistute for Occupational Safety and Health (NIOSH) Particulate Filter Penetration Procedure for Test Negative Pressure Respirators Against Liquid Particles (Procedure APRS-STP-0051-00, Morgantown WV, NIOSH Division of Saftey Research, May 31, 1995). The respirators preferably show an initial DOP penetration of less than 5%. Test respirators according to this Procedure preferably show a pressure drop of less than 13 mm (127 Pa) of H20, more preferably less than 10 mm (98 Pa) of H20, and still more preferably less than 8 mm (78 Pa) of H20. Respirators of the greater surface area are tested according to these standards by reducing the surface area exposed to 180 cirr. The smallest respirators are tested according to this standard by adapting a fastener for several respirators that have a total exposed area of approximately 180 cm2. Filter elements of this invention having a surface area of about 150 crrr preferably exhibit a MingChl greater than 300 mg of DOP, more preferably greater than 450 mg of DOP, when tested using NIOSH Method APRS-STP4051-00. The filters used as pairs on a respirator are tested using a single torque filter. The filters were preferably tested according to this procedure showing an initial DOP penetration of less than 5%. The filters preferably show a pressure drop of less than 13 mm (127 Pa) of H20, more preferably less than 10 mm (98 Pa) of H20, and still more preferably less than 8 mm (78 Pa) of H20. The prefilters of this invention have a surface area of about 65 cm2 preferably show a MinTChl greater than 170 mg of DOP, more preferably greater than 255 mg of DOP, when tested using NIOSH Method APRS-STP-0051-00. The pre-filters were used, as pairs in a respirator were tested using a single filter of the pair. Prefilters preferably show an initial DOP penetration of less than 5%. The prefilters were tested according to this Procedure preferably showing a pressure drop of less than 17 mm (167 Pa) of H20, more preferably less than 14 mm (137 Pa) of H20, and still more preferably less than 12 mm (118 Pa) of H20.

EXAMPLES PREPARATION AND TESTING OF THE GENERAL SAMPLE Extrusion of the Fabrics The following descriptions exemplify certain preferred embodiments of the methods of making electret articles containing a polymer and an additive that improves performance. The articles in these examples are nonwoven fabric filter fabrics made of a mixture of polypropylene and a fluorochemical that is extruded under blown melt conditions and collected to form a blown microfiber fabric (BMF). The fluorochemical melt additive was fed into the throat of a twin screw extruder along with polypropylene to produce a melt stream containing about 11 weight percent of the fluorochemical. The volume of the polypropylene was added in the throat of a second twin screw extruder. In some cases a peroxide is also measured to reduce the viscosity. The extruder performance containing a fluorochemical is pumped into the extruder containing polypropylene at a rate to manufacture the total yield of about 1.1 percent by weight of the fluorochemical melt additive. The temperature of the melt flow stream containing the fluorochemical melt additive was kept below 290 ° C of all points. The tissue itself was produced in a conventional manner similar to that described in Van Went, et al., Except that a die with a punched hole was used.

Cooling The two cooling methods were used and described below, Method A A spray bar containing thirteen Single Flat Ventilation Nozzles with UniJet Spray Nozzle Tips No. 9501 spaced 10.16 cm (four inches) apart were mounted 49.05 mm (0.75 inches) from the surface of the die and 63.5 mm (2.5 inches) below the molten polymer streams that exit of the die Each nozzle was turned at 10 ° from the direction of transverse tissue so that the fans of the water droplets do not interfere with each other and the water pressure was set at the minimum level that would maintain a uniform spray.

Method B A Sonic Spray System spray bar with 15 spray nozzles Model No. SDC 035, available from Sonic Environmental Corp. of Pennsauken, NJ, was mounted approximately 178 mm (7 inches) below the center line and approximately 25.4 mm (one inch) downstream of the tip of the die. The air pressure was fixed at 345 kPa (50 pounds per square inch (psi)) and the water pressure was fixed at 207 kPa (30 psi). The water flow was measured, unless otherwise specified, adjusted so that each nozzle released 30 ml / min of water. Each nozzle releases a cone of water droplets to the molten polymer streams that exit the die.

Tempered The extruded fabrics were further treated by passing them through a heated oven at an average temperature of about 150 ° C at a rate such that the time it dwells in the oven was about 4.5 minutes. This tempering process causes the additional crystallization of the polymer and causes the fluorochemical molten additive to diffuse at the fiber interfaces.

Loading After tempering the tissues were further treated with corona charging using a high voltage electric field provided between 30 linear transverse tissue crown sources and an established electrode with a corona current of 2.6 10"3 milliamps / cm length crown source and a residence time of approximately 15 seconds.

Fabric Specifications The thickness of the fabric was measured according to ASTM D1777-64 using a weight of 230 g on a disc of cm in diameter. The pressure flow can be measured according to ASTM F778. The basis weight was calculated from the weight of a 13.3 cm (5.25 in) diameter disc.

DOP Load Test The dioctyl phthalate (DOP) loading measurements were performed by monitoring the penetration of the DOP aerosol through a sample during prolonged exposure to a controlled DOP aerosol. The measurements were made using an Automated Filter Tester (A POPA) model # 8110 or # 8130 (available from TSI Incorporated, St. Paul, Minnesota) adapted for the DOP aerosol. The D0% Penetration% is defined as: DOP Penetration% = 100 (DOP Conc. Downstream / DOP Conc. Upstream), where the upstream and downstream concentrations were measured by A light scatter and Percent Penetration of DOP was calculated automatically by the AFT. The DOP aerosol generated by the 8110 and 8130 AFT instruments was nominally a 0.3 micrometer monodispersor of medium diameter mass that has an upstream concentration of 100 milligrams per cubic meter as measured by a standard filter. The test samples were all tested with the aerosolized ionizer being returned and at a flow rate through the filter fabric sample of 85 liters per minute (LPM).

DOP Filter Fabric Loading Test Procedure 1 The measurements were made using an adapted AFT model # 8110 for the DOP aerosol. The extruded fabric was cut into the 17.15 cm (6.75 inch) diameter discs. Two of the disks were stacked directly on top of each other, and the disks were mounted on a sample handle such that a 15.2 cm (6.0 inch) diameter circle was exposed to the aerosol. The surface velocity was 7.77 centimeters per second (cm / sec). Samples were weighed before inserting them into the test handle. Each test is continued until it is a clear trend to increase the PDO Penetration Percent with continuous DOP aerosol exposure or at least up to a 200 milligram DOP exposure. The percentage of PDO Penetration and the data of the Pressure Drop corresponding to a linked computer where they are stored were transmitted. After the completion of the DOP load test, the load samples were again weighted to monitor the amount of DOP collected in the fibrous tissue samples. This served as a cross-check of the extrapolated DOP exposure of the measured DOP concentration incident on the fibrous tissue and the flow velocity of the aerosol measured across the tissue.

The resulting load data was imported into an extended sheet to calculate the minimum tested (Min @ Chl). The Min @ Chl is defined to be the total DOP proof or the DOP mass that has been incident (ie the DOP mass in and through the sample) in the filter fabric to the point where the Percent Penetration of DOP reaches its minimum value. This Min @ Chl is used to characterize the development of the fabric against the load of DOP, the largest of Min @ Chl is the best of the processing of DOP load.

Test Procedure 2 Loading the DOP filter fabric Procedure 2 is the same as 1 except that the samples were cut to 13.34 cm (5.25 inches) in diameter and placed on the sample handle that leaves a 11.4 cm circle (4.5 inch) in exposed diameter, and the surface velocity was 13.8 cm / sec. In any procedure, the tests can be conducted using equivalent filter testers. One could also test simple layers instead of the double layers of filter cloth if an instantaneous filtration of the single layer is made such that there is a pressure drop of 8 to 20 mm H20 and a perceptible penetration of less than 36% DOP penetration as measured in an exposed area of 102.6 cpr at a flow rate of 85 LPM using an AFT model no. TSI 8110 that has an ionizer. Any procedure includes testing the smaller surface area filters using a sample holder that would have mounted a filter medium with an equivalent exposed area (ie 102.6 cm2 for Procedure 2).

Determination of the Crystallinity Index of the Polymer Crystallinity data were collected using a Philips vertical X-ray diffractometer, a Ka-radiation of copper and recorded proportional scattered radiation detector. The diffractometer was adjusted with the different opening apertures of the inconsistent opening, fixed to receive the opening, and the diffracted beam monochromator. The ray generator was directed to dispersions of 45 kV and 35 mA. The analysis stage was conducted from 5 to 40 degrees (2?) Using a particle size of 0.05 degrees and a continuous time of 5 seconds. The samples were assembled from aluminum handles using a double-coated tape without the support plate or support used under the fabric.

The scattered data observed was reduced to the x-y pairs of the scattering angle and the intensity values and subjected to adjusting the profile using the set of Origin ™ programs to analyze the data (available from Microcal Software Inc., Northhampton MA). A model of the critical point shape was used to describe the six critical points of polypropylene in the form of alpha and the amorphous critical point contributions. For some data groups, an amorphous critical point does not adequately act for the dispersion intensity that does not have an alpha shape. In these cases, the maximum additional amplitude was used to consider it completely for the intensity observed. These broad inflections were mainly due to the mesomorphic shape of polypropylene (for a discussion of mesomorphic polypropylene see Krueger et al., US Patent No. 4,931,230 and references cited herein). The contribution of the dispersion due to the mesomorphic form of the polypropylene was combined with the amorphous dispersion. The crystallinity indices were calculated as the velocity of the crystalline critical point area at the total dispersion intensity (crystalline + amorphous) within 6 to 36 degrees (2?) Range of the scattering angle. A value of 100 percent represents the crystallinity and zeros represent no crystallinity.

Thermally Stimulated Discharge Current (TSDC) TSDC studies were conducted using a Solo at TSC / RMA model 91000 with a rotating electrode, available from TherMold Partners, L.P., Stanford Thermal Analysis Instruments, CT. The tissue samples were cut and placed between the electrodes on the Solomat TSC / RMA. In the Solomat instrument, a thermometer is arranged adjacent to, but not in contact with, the sample. Woven samples should be optically dense, there should be no visible holes through the sample tissue. The samples must be large enough to cover the upper contact electrode. Since the electrode is approximately 7 mm in diameter, the samples were cut larger than 7 mm in diameter. To ensure good electrical contact with the electrodes, the tissue samples were compressed by a factor of 10 in thickness. The air is evacuated from the sample chamber and replaced with helium at a pressure of approximately 1100. Liquid nitrogen is used to cool.

TSDC Test Procedure 1 An article is stimulated at 100 ° C for 1 minute in an applied electric field of 2.5 kilovolts per minute (kV / mm) in the apparatus described above. With the field in, the sample is rapidly cooled (at the maximum speed of the instrument) to -50 ° C. The sample is held at -50 ° C for 5 minutes with the field, then heated to 3 ° C / min while the discharge current is measured. The loading densities can be calculated from each peak of the TSDC spectra by extracting a basic between the minima of each side of a selected peak and integrating the area under the peak.

TSDC Test Procedure 2 The discharge current of an unstimulated article is measured starting at 25 ° C and heating at a rate of 3 ° C / mm. The two samples of the article are tested identically except for the samples that are oriented in opposite directions when placed between the electrodes. The position (s) of the peak is measured for the article that was oriented to produce a positive discharge current at temperatures above about 110 ° C (for example side B in Figure 15).

The melting temperature of the article is determined by a differential analyzer calorimeter (DSC) directed at a heating rate of 10 ° C / min, and defined as the maximum peak caused by a melt as observed in the second DSC heating cycle ( that is, the peak observed after heating to about the melting temperature, the article is cooled to freezing and overheated).

TSDC Test Procedure 3 A sample is studied by the TSDC method of Procedure 2 to determine the correct orientation of the sample. The articles are then oriented in the TSC Solomat in the direction that produces a positive discharge current the peak of the lowest temperature in Procedure 2. The items are then tested according to Procedure 1 except that each sample is stimulated to 100 ° C for or 1, 5, 10 or 15 minutes. The amplitude value of the peak at half height of each peak is calculated by drawing a baseline, based on the slope of the curve from 0 to about 30 ° C, and measuring the amplitude of the peak at half the height.

TSDC Test Procedure 4 This procedure is identical to procedure 3 except that the loading density of the article at each moment of stimulation is calculated by drawing a basic line between the minimum on each side of a selected peak, or if there is no minimum the side of the high temperature of a peak, where the curve crosses or extrapolates to cross the current to zero, and integrate the area under the peak.

Comparative Examples 1-3 Examples 1-3 demonstrate that the performance of the improved filler can be achieved by tempering the polymer and the performance enhancing additive containing the compositions having a relatively low crystallization index.

EXAMPLE 1 A nonwoven fabric filter fabric was prepared from Exxon Escorene 3505G, available from Exxon Chemical Company, and the fluorochemical at a rate of 23 kilograms per hour (kg / hr) (80 pounds per hour lb / hr) and a melting temperature of 288 ° C using 121.9 cm (48 inches) by drilling the die hole. The fabric had a basis weight of 71 grams per square meter, a thickness of 1.3 millimeters (mm) and a pressure drop of 6.6 mm (65 Pa) of H20 measured at a surface velocity of 13.8 cm / s. After hardening and loading the tissue as described above, the DOP loading test was performed in 13.34 cm (5.25 inch) diameter double layer samples taken from six positions across the tissue width. The crystallization index of polypropylene was determined for the samples cut from the six tissue positions before (positions 1, 4 and 6) and after annealing (positions 1-6). The loading data (in Min @ Chl) and the crystallization indexes for the six positions in Table 1, and the unhardened crystallization indexes vs. the MinTChl values for positions 1, 4 and 6 are plotted in Figure 2.

Table 1 As the values are shown in Table 1 for positions 1, 4 and 6 and the plane in Figure 2, there is a correlation between the DOP load execution (in Min @ Chl) and the crystallization index of the tissue before tempering . The lowest crystallization before tempering, is the highest value of the Min @ Chl. On the other hand, as shown in Table 1, they do not pursue a correlation between the crystallization index of the fabric after annealing and the DOP load execution (in Min @ Chl).

EXAMPLE 2 The BMF fabric was prepared and treated as described in Example 1. The fabric has a basis weight of 74 grams per square meter, a thickness of 1.4 mm and a pressure drop of 7.0 mm (68 Pa) of H20 measured at a surface speed of 13.8 cm / s. The DOP loading fabric was tested and analyzed by the crystallization index as in Example 1 and the resulting data are given in Table 2 and Figure 3.

Table 2 Again, the values in Table 2 and Figure 3 show the general trend that lower the crystallization indexes of the unhardened composition in correlation with the best load performance while no correlation is observed for the tempered filters.

EXAMPLE 3 The BMF fabric was prepared and treated as described in Example 1 except that the polypropylene resin Fina 3860, available from Fina Oil and the Chemical Company, and a peroxide concentrate containing 2,5-dimethyl were used. -2, 5-di (tert-butylperoxy) hexane were co-fed into the extruder to control the melting rheology of the polypropylene and the physical parameters of the meltblown fabric. The fabric had a basis weight of 73 grams per square meter, a thickness of 1 4 mm and a surface pressure of 7.0 mm (69 Pa) of H ?0 measured at 85 liters per minute. The loading fabric was tested and analyzed by the crystallization index as in Example 1 and the resulting data are presented in Table 3 and Figure 4.

Table 3 Again, the values in Table 3 and Figure 4 show the general trend that the lowest crystallization rates of the unhardened composition correlate with the best load performance while no correlation is observed for the tempered filters.

Examples 4-8 Examples 4-8 illustrate that the cooling or low crystallinity of the unhardened fibers (ie the intermediate composition) is correlated with the charging properties of the upper oily drizzle of the electret filter fabrics tempered.

EXAMPLE 4 The BMF fabric was prepared and treated as in Example 1. The fabric had a basis weight of 69 grams per square meter, a thickness of 1.3 mm and a pressure drop of 6.2 mm (61 pa) of H20 measured at a surface speed of 13.8 cm / s. After sufficient tissue was collected for an additional process and test, the extrudate is roelated with water using Method A described above. The water was purified by reverse osmosis and a deionization was used. In this experiment the spray bar only reached approximately 2/3 of the amplitude of the die. The collector was moved from about 30 to about 20 cm (12 to about 8 inches) to maintain the desired tissue parameters. The tissues are the PDO test load and analyzed by the Crystallinity Index as in Example 1, and the resulting data are provided in Tables 4A and 4B and Figure 5.

Table 4A Uncooled, Comparative Examples Table 4B With Chilled 3? The data in Tables 4A and 4B shows that cooling reduces the Crystallinity Index of extruded fibers. The tempering of the composition of the low crystallinity index improves the charging performance of the hardened filter and load fabric. The data further shows that tempered compositions having a low crystallinity index of about 0.3 result in electret filters having a higher loading performance. More particularly, tempered fabrics having a low Crystallinity Index of about 0.3 result in fibers having an average of MinTChl greater than 200 mg while tempered fabrics having a Crystallinity Index of about 0.3 result in filters having an average of Min @ Chl less than 200 mg.

EXAMPLE 5 BMF fabric was prepared and treated as in Example 1 except that the extrusion rate was 45 gk / h (100 pounds per hour) and the peroxide was added as in Example 3 to control the melting rheology of the polypropylene and the physical parameters of the meltblown fabric. The fabric had a basis weight of 73 grams per square meter, a thickness of 1.3 mm and a pressure drop of 6.6 mm (65 pa) of H20 measured at a surface velocity of 13.8 cm / s. After sufficient tissue was gathered for an additional process and test (see examples in Table 5A) the extrudate was sprayed with water using Method B described above. The spray bar reached the entire fabric having a basis weight of 74 grams per square meter, a thickness of 1.3 mm and a pressure drop of 6.2 mm (64 pa) of H? 0 measured at 85 liters per minute. The collector was moved from 30 to 28 cm (12 to 11 inches) to maintain tissue parameters. Water from the tap was used without purification. The DOP loading fabrics were tested and analyzed by the Crystallinity Index as in Example 1 except that 17.15 cm (6.75 inch) circles were used for the test load and the resulting data are given in Tables 5A and 5B and Figure 6.

Table 5A Without Off, Comparative Table 5B With Chilled As in Example 4 the data in Tables 5A and 5B show that cooling reduces the Crystallinity Index of the unhardened tissue and improves loading performance of the hardened and loaded tissue. Additional data shows that tempered fabrics have a low crystallinity index of about 0.3 resulting in filters that have an average of Min @ Chl of greater than 200 mg while tempered fabrics have a Crystallinity Index of about 0.3 resulting in filters that they have an average of Min @ Chl less than 200 mg. The data also shows that low crystallinity compositions, such as those having a Crystallinity Index of about 0.1 can be conducted to further improve charge performance. For example, some electret filters may have a Min @ Chl greater than 500 mg.

EXAMPLE 6 The BMP fabric was prepared and treated as described in Example 1. The fabric has a basis weight of 73 grams per square meter, a thickness of 1.3 mm and a pressure drop of 7.0 mm (69 pa) of H20 measured at a surface speed of 13.8 cm / s. After enough tissue was collected for an additional process and test the extrudate was sprayed with water as in Example 5 using Method B described above. The collector was moved from 25 to 24.6 cm (10 to 8.5 inches) to maintain tissue parameters. The fabric was sprinkled with water having a basis weight of 71 grams per square meter, a thickness of 1.4 mm and a pressure drop of 6.6 mm (65 pa) of H20 measured at 85 liters per minute. The DOP loading fabrics were tested and analyzed by the crystallinity index as in Example 5 and the resulting data are given in Tables 6A and 6B and Figure 7.

Table 6A Uncooled, Comparative Table 6B With Chilled As in Examples 4-7, the data in Tables 6A and 6B shows that cooling reduces the Crystallinity Index of the unhardened tissue and improves loading performance of the hardened and loaded tissue. The data further show that temperate fabrics having a low cpstatinity index of about 0.3 resulting in filters that have an average of Min @ Chl of greater than 200 mg while tempered fabrics having a crystallinity index of about 0.3 resulting in filters that have an average of Min @ Chl less than 200 mg. The data also shows that some electret filters were made from chilled materials that can have a Min @ Chl greater than 500 mg and some with a Min @ Chl greater than 800 mg.

EXAMPLE 7 The BMF tissues were made and treated as in Example 6 with and without water spraying using Method B. For this example the water was purified by reverse osmosis and deionization. The tissue specification was similar to those in Example 6. The loading tissues were tested and analyzed by the crystal index as in Example 6 and the resulting data are given in Tables 7A and 7B and Figure 8.

Table 7A No Water Spray Table 7B With Water Spray As in Examples 4-6, the data in Tables 7A and 78 show that cooling reduces the Crystallinity Index of the unhardened tissue and improves loading performance of the hardened and loaded tissue. The data further shows that tempered fabrics having a low crystallinity index of about 0.3 result in filters having an average of MinßChl greater than 200 mg while tempered fabrics having a crystallinity index of about 0.3 result in filters having a average of Min @ Chl less than 200 mg. The data also shows that some electret filters were made from chilled materials that can have a Min @ Chl greater than 500 mg and some with a Min @ Chl greater than 800 mg.

EXAMPLE 8 The BMF fabrics were made and treated as in Example 7 with and without water spray using Method B. The fabrics have tissue specifications similar to those in Example 7. The load tissues were tested and analyzed by the crystallinity index as in the previous examples and the resulting data are given in Tables 8A and 8B and Figure 9.

Table 8A Without Chilling, Comparison Table 8B With Chilled As in Examples 4-7, the data in Tables 8A and 8B show that the cooling reduces the crystallinity index of the unhardened tissue and improves the charging performance of the hardened and loaded tissue. The data further shows that tempered fabrics having a Crystallinity Index less than about 0.3 result in filters having an average of MinSChl greater than 200 mg while tempering fabrics having a crystallinity index of about 0.3 result in filters having an average of MinSChl less than 200 mg. The data also show that some electret filters were made from the cooled materials that a Min2Chl greater than 500 mg can have. Tables 9A and 9B show an average of MinTChl of data for Examples 4-8 for the uncooled and cooled samples.

Table 9A Average Min @ Chl Data (mg) - Without Chilled, Comparative Table 9B Average Min @ Chl Data (mg) - Chilled The data averaged in Tables 9A and 9B, combined with the values of crystallinity shown in the above Tables demonstrate that cooling can reduce the index of crystallinity of the unhardened tissue by approximately 0 3 and in addition to the tempered tissue having an Index of low crystallinity under about 0.3 resulting in filters that have an average MingChl greater than 200 mg while tempering fabrics that have a Crystallinity Index over about 0.3 resulting in filters that have an average of Min @ Chl less than 200 mg.

Examples 9 and 10 Examples 9 and 10 show that the addition of an additive that improves the performance causes a strong signal in the TSDC spectrum. A nonwoven fabric was prepared as described in Example 4 (including cooling). A second sample was prepared identically except that without an additive that improves execution. Both tissue samples were studied by the method of Test Procedure 1 of TSDC. The sample containing the additive that improves the performance showed a significant discharge peak at approximately 110 ° C. In comparison, tissue that does not have an additive that improves performance did not show a significant peak. This observation suggests that the discharge current generated by the sample containing the additive that improves the performance is due to the depolarization of the additive that improves the execution when heated. It is believed that the additive that improves performance is polarized in the stimulation stage.

Examples 11-15 Examples 11-15 show that the cooled fabrics, after stimulation, have a higher charge density than the uncooled fabrics. Test tissues (cooled, unhardened) and c (cooled, tempered) are the same as those described in Example 4, position 4 (except without corona loading). Sample b (uncooled, unhardened) is present as described in Example 2, position 4 (except without corona loading) and sample d (uncooled, quenched) is as present as described in Example 2, position 6 (except without crown loading). All tissue samples were studied by the method of Test Procedure 1 of TSDC. The resulting TSDC spectrum is shown in Figure 12. The charge densities of each peak of the TSDC spectrum can be calculated by drawing a basic line between the minimum on each side of a selected peak and the integration of the area under the peak. As illustrated in Figure 12, the TSDC spectrum generally shows an excessive discharge current increasing as the temperature approaches the melting point of the test article. Multiple samples of the unloaded and tempered tissues as described in Example 7 were tested as described in Examples 11-15 for both tissues without cooling (positions 2 and 6) and cooled (positions 3, 4, 5 and 6). ). None of the uncooled tissues had a charge density over 10 microcolombs per square meter (μC / irr). The indexes of crystallinity of the untouched tissues vs. The charge density of the tempered, unloaded fabrics are plotted in Figure 13a. Figure 13a shows that unhardened fabrics have a relatively low crystallinity index generally have a higher charge density as determined by SDC Test Procedure 1. DOP load execution (in Min @ Chl) of tempered and loaded tissues vs. load density of the hardened, unburdened fabrics are plotted in Figure 13b. Figure 13b shows the really surprising result that was tempered, the uncharged fabrics having a charge density value of about 10 μC / pr, as measured by the TSDC Test Procedure 1 having a top loading run of DOP after loading.

Examples 17 and 18 Examples 17 and 18 illustrate the TSDC spectrum of corona-laden, chilled and uncooled fabrics, made without an additive that improves performance. The cooled (a, b) and uncooled tissues (a ', b') were prepared as described in Example 4 except that no additive without fluorochemical was present in the tissues. The TSDC spectrum of the tissues without stimulation was acquired using Test Procedure 2 and are shown in Figure 14. The discharge current signal (positive or negative) is a function of tissue orientation in the TSC instrument with regard to to the orientation during crown loading.

Examples 19 and 20 Examples 19 and 20 illustrate the TSDC spectrum of corona-loaded, quenched, cooled and uncooled fabrics, made without an additive that improves performance. The cooled (a, b) and uncooled tissues (a ', b') were prepared as described in Example 8, position 1. The tissues were studied by TSDC as described in Test Procedure 2 of TSDC. The results of the TSDC study are shown in Figure 15. As part of the test procedure, the melting point of the article is tested to be determined by DSC, and in this case it was found to be 159 ° C.

As shown in Figure 15, when oriented to show a positive discharge current of about 110 ° C, the spectrum of the cooled tissue, a, shows a relatively narrow peak of about 137 ° C. This spectrum indicates that the cooling causes a narrowing of the energy distribution of the sites that trap the charge in the hardened and load-bearing tissue. In comparison, the uncooled fabric spectrum, a ', shows only the amplitude of the peak centered at a significantly lower temperature (approximately 120 ° C), indicating a relatively wide distribution of site energy levels that traps the charge. Thus, the articles of the invention can show the characteristics that distinguish them from a peak current centered at approximately 15 to 30 ° C below the melting point of the article when measured by TSDC Test Procedure 2. As shown by the DOP load test results discussed above, the fabrics made from the cooled intermediates (or the relatively low crystallinity) have a much improved DOP loading performance as compared to the fabrics made from the uncooled intermediates. (or the relatively high crystallinity). Thus, the inventors have surprisingly discovered a characteristic spectral feature (ie, the peak current described above) that correlates with the improved DOP charge execution.

Examples 20 and 21 Examples 20 and 21 show the TSDC spectrum of the cooled (Figure 16a) and uncooled articles (Figure 16b) and illustrate the spectral features that may characterize certain articles of the invention. These examples were the tissues described in Example 8, position 3 (cooled and uncooled). The TSDC studies are conducted as described in Test Procedure 3 of ISDC. The items in Figure 16a differ only in their stimulation times: a - 1 minute, b - 5 minutes, c - 10 minutes, and d - 15 minutes. Similarly, the articles in Figure 16b differ only in their stimulation times: a '- 1 minute, b' - 5 minutes, c '- 10 minutes, and d' - 15 minutes. The TSDC spectrum in Figure 16a shows the broad average peak that increases from 18 (b), 14 (c), and 19 (d) for the stimulation times of 5, 10 and 15 minutes respectively. These three peaks have a maximum at 140 or 141 ° C. In comparison, the uncooled comparative examples in Figure 16b show the mean peak amplitudes increasing from 40 (b '), 32 (c'), and 34 (d ') for the stimulation times of 5, 10 and 15 minutes ° C, respectively, and the maximum peak at 121, 132 and 136 ° C, respectively. The performance of the top load of chilled items is discussed above in relation to the DOP load test. Thus, Figures 16a and 16b and the DOP load test show the surprising discovery of articles characterized by the amplitude of the TSDC peak below 30 ° C (as measured by Test Procedure 3) by correlating it with the Loading charge of the upper oily drizzle. These results suggest that articles that have a narrow distribution of energy levels that trap the load in the stimulated state correlate with the performance of improved loading. Thus, most preferred articles have a peak amplitude less than 25 ° C, and still more preferably less than 20 ° C. The data also shows that, at least for articles containing polypropylene, there is a correlation between peak position and load execution with preferred articles having peak positions at about 138 to 142 ° C.

Examples 22 and 23 Other TSDC data from the group were acquired to prepare the samples identically and tested as described in Examples 20 and 21. Charge densities were calculated for each test condition as described in Test Procedure 4 of TSDC and were classified in Table 10 and plotted in Figure 17.

Table 10 Load Density (μC / m2) vs. Stimulation Time Comparing the loading densities of the chilled and uncooled articles, as measured by Test Procedure 4, with the corresponding DOP load test surprisingly shows a correlation between the charge density loading while the article is stimulated and Execute the load. As can be seen in Figure 17, the cooled items (top load execution) (dotted lines) show the increase in charge density while the article is stimulated for 1 to 10 minutes. In contrast, articles (poorer load execution) (continuous lines) show the decrease in charge density over the same period of stimulation. Thus, a feature of the preferred articles of the invention is to increase the charge density by 1 to 5 and / or 5 to 10 minutes of the stimulation time, as measured by the TSDC test 4 procedure. All patents and patent applications mentioned herein are incorporated by reference as indicated in full. The invention may have several modifications and alterations. Therefore, this invention will not be limited to the foregoing examples but will be controlled by the limitations set forth in the following claims and any equivalents herein.

It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is the conventional one for the manufacture of the objects or products to which it refers. Having described the invention as above, property is claimed as contained in the following:

Claims (19)

1. A method of making an electret article, characterized in that the method comprises: forming a molten, hot material comprising a mixture of a polymer and an additive that improves the performance; wherein the polymer is a non-conductive thermoplastic resin having resistivity greater than 101"chm / cm and wherein the performance enhancing additive improves the filterability of the oily aerosol of the electret article to form the heated, melted material to form a heated, formed material, rapidly cooling the heated material, formed to form a cooled material, and quenching and loading the cooled material to form the electret article.

2. The method according to claim 1, characterized in that the step of forming the molten material comprises extruding the molten material through a hole in the die or die to form extrudate, and wherein the step of cooling comprises cooling the molten material when it emerges of the hole of the die; and wherein the additive that improves the performance comprises a fluorochemical.

3. The method according to claims 1-2, characterized in that the article is electret fibers.

4. The method according to claims 1-3, characterized in that the extrusion step comprises extruding the mixture under conditions of blowing the melt.

5. The method according to claims 1-4, characterized in that the step of cooling results in cooled fibers and additionally where the cooled fibers are gathered as a network of the non-woven fabric.

6. The method according to claims 1-5, characterized in that the polymer is polypropylene.

7. The method according to claims 1-6, characterized in that the additive that improves the performance is selected from the group consisting of r v SO N 'CW' C? óFxl2 - * "w 2-; CH 2 CH3) SO, C F 17

8. The method according to claims 1-7, characterized in that the mixture comprises 95 to 99 5 weight percent polypropylene and 0.5 to 5 weight percent fluorochemical.

9. The method according to claims 3-8, characterized in that the cooled fibers, prior to quenching, have a Crystallinity Index less than 0.3.

10. The method according to claims 1-9, characterized in that the elctret article is a nonwoven fabric containing meltblown fibers.

11. The method according to claims 1-10, characterized in that the cooling step comprises spraying water.

12. The method according to claims 1-10, characterized in that the tempering step is conducted from about 130 to 150 ° C, and the mixture is extruded at a die speed of about 2.48 to 6.94 g / h / cm. (0.5 to 14 Ib. / Hr / pulagadas).

13. The method according to claims 5-12, characterized in that the step of loading the fabric comprises the corona treatment, and wherein the tempering step is subsequently directed to the loading stage.

14. The method according to claims 6-8, characterized in that the fluorochemical has a melting point before the melting point of the polypropylene and below the extrusion temperature.

15. The method according to claims 1-14, characterized in that the polymer is selected from the group consisting of polypropylene, poly (4-methyl-1-pentene), linear low density polyethylene, polystyrene, polycarbonate, polyester, and combinations of the same.

16. The method according to claims 1-15, characterized in that the electret article has a thermally stimulated discharge current (TSDC) the spectrum shows a peak having an amplitude at an average height of less than about 25 ° C, as measured by the TSDC test procedure.

17. The electret article prepared in accordance with the method of claims 1 to 16.

18. The article according to claim 17, characterized in that the article is in the form of a fiber, a filter, or a fabric of the non-woven fabric.

19. Electret felts comprise a multitude of filters prepared according to the method of claims 1 to 16. METHOD FOR ELBORATING ELECTROTE ARTICLES AND FILTERS WITH INCREASED RESISTANCE TO OLIVE LLOVIZNA SUMMARY OF THE INVENTION An electret article can be made by cooling a molten mixture containing a polymer and an additive that improves performance. The resulting low crystallinity material can be quenched and charged to produce a material for making electret filters that have superior oily drizzle charge performance properties.