JP2541551C - - Google Patents

Description

The present invention relates generally to melt-blown materials, and more particularly, to the depth direction, ie, Z
It relates to a melt-blown material having a fineness gradient in the axial direction. BACKGROUND OF THE INVENTION Materials produced by melt blowing are well known and are widely used in various aspects for commercial, industrial, and household products. Melt blowing is used to form thermoplastic fibers in the form of a web, which melts by heating a polymer resin and extruding the melt through a die orifice in a die head. Blowing a stream of heated air, usually consisting of air, onto the melt extruded from the die orifice to form discontinuous and fibrillated filaments or fibers which are collected on a drum or porous belt. At the time of collection, the fibers retain their tackiness and thus bind together to form an integrated web. Such melt-blown methods are known and have been described in various patents and publications, for example, NRL Report 4364 "Production of Ultrafine Organic Fibers" by VA Vent, EL Boone and CD Fulherty, KD Lawrence , RT Lucas and JA Young et al., NRL Report 5265, "Improved Apparatus for Forming Ultrafine Thermoplastic Fibers", and 19
U.S. Pat. No. 3,849,241 issued Oct. 19, 74 and the like. [Problems to be Solved by the Invention] The fine fiber melt-blown web is useful as a filter medium, an absorbent material, a moisture-proof layer, an insulator, a wiper and the like. In particular, the molten material appears to be a potential filter media application for HEPA and near HEPA filters. HEPA filters have a filtration efficiency of at least 99.97% for 0.3 micron particles. Efficiency of HEPA filter is US military standard 282, test method 10
According to the test procedure described in 2.1, 10.4 to 10 feet per minute (approximately 316.9 c
It is measured using a dioctyl phthalate particle with an average diameter of 0.3 micron at a face velocity of (m to 32.0 cm). The efficiency of the filter is defined as the percentage of particles filtered from the air stream by the HEPA filter (
Percent). As long as the required filtration efficiency reaches 99.97%, HEP
Classified as A filter. A filter having a filtration efficiency of about 90% to 99.97% is called a near HEPA filter. HEPA filters and near-HEPA filters are used for air filtration in clean rooms for manufacturing integrated circuits and precision equipment. HEPA filters and near-HEPA filters are also used in air filtration in operating rooms to remove bacteria and other foreign materials that are present in the air and harmful to patients. Not only must the HEPA and near-HEPA filters achieve the required filtration efficiency, but it is equally important that the pressure drop across the filter be as low as possible for any filtration efficiency. HEPA filter or near HEP
If the pressure drop across the A-filter becomes excessive, a larger and more powerful fan is required to compensate for this excessive pressure drop, resulting in increased power and noise. It is therefore important that HEPA filters and near-HEPA filters keep the pressure drop as low as possible at any filtration efficiency rating over the useful life of the filter media. Usually, the HEPA filter and the near HEPA filter have a filament diameter of 0.
Manufactured from glass filaments of 0.3 microns to 2.0 microns. The glass filter medium is formed into a sheet by a wet (papermaking) method. HEP made of glass fiber
In order for the A filter and the near HEPA filter to achieve the required filtration efficiency, 0
The filament size must be such that a pore size such that 0.3 micron diameter particles do not completely pass through the filter medium can be realized in the filter. Such a filter medium is usually formed in a single sheet in which glass fibers are uniformly arranged in the depth direction (in the Z-axis direction). The melt-blown filter media, which approximates the near-HEPA filtration efficiency, has a melt-blown extruder output of approximately 4 per hour per die width (about 2.54 cm).
The conventional polypropylene throughput of about 5 to 5 pounds (PIH) is reduced to about 1 pound and the fluid used to finely cut and cut the polymer stream. The flow rate is reduced to about 100-150 standard cubic feet per minute (
By increasing the SCFM) (approximately 2.8 m ³ to 4.2 m ³⁾ to about 250 to 325SCFM (about 7 to 9.1 m ^3), can be produced. (Standard cubic feet / minute is for a 20 inch (about 50.8 cm) wide die head. Therefore, the flow rate for cubic feet / minute per inch of die head width is SCFM.
It is determined by dividing the value by 20. As a result, the average fineness of the melt-blown fibers is about 5 to 6 microns (the fineness range is about 0.5 to 10 microns), which is about the same value as normal melt-blown fibers, but the overall melt-blown web Over the conventional melt-blown web. The filtration efficiency of conventional melt-blown webs is well below 70% at a pressure drop of about 0.1 inches (about 0.254 cm) of water, while an improved melt-blown web containing fibers of similar fineness. Has a filtration efficiency of about 0.25 to 0.36 inches (about 0.
It is about 95-99.26% at the pressure drop of the water column. Such melt blown filter media can be about 100 pounds per square inch (about 7.03 kg / cm ^2).
) To can be further improved by cold rolling meltblown material at a pressure of about 300 lbs / square inch (about 21.09kg / cm ^2). When the cold-rolled melt-blown material thus obtained is used as a filter medium, the filtration efficiency is about 97.7 at a pressure drop of about 0.32 to 0.65 inches (0.81 to 1.65 cm) of water. 0%
To 99.57%. A HEPA filter consisting of a single sheet of 0.3 to 2.0 micron diameter glass fiber or 0.5 to 10 micron diameter cold rolled melt blown polypropylene fiber uses a near HEPA filter to produce fine particles of the order of 0.3 microns. When filtering air containing particles and particles having a much larger diameter, a filter material for a single sheet glass fiber or a melt-blown polypropylene fiber made of a HEPA filter or a near HEPA filter has a large particle diameter. Tend to attach particulate matter rapidly as they become trapped on the upstream surface of As a result, the upstream side of the filter is rapidly blocked by particulate matter, which greatly increases the pressure drop across the filter and reduces the useful life of the filter. To overcome such premature blockage and filter replacement, a depth filter may be used instead of a single sheet filter having a uniform fiber distribution in the Z-axis direction. The depth filter contains layered fibers having different filtration efficiencies in the Z-axis direction. These layers may be laminated separation layers, may have different densities,
It may be obtained from different types of fibers. In the latter category, U.S. Pat. No. 3,073,375 to Till et al. Discloses a method for making depth filter media. The process includes a first delicate formation for depositing fine plastic fibers of 0.5 micron to about 10 micron diameter on a collection belt by means of a spray tube and an air nozzle. In the second forming section, in order to deposit rayon fibers having a diameter of 10 microns or more on the fine plastic fibers adhered by the spray tube and the air nozzle, a fan is provided through a duct separated from the spray tube and the air nozzle. Blow out the staple rayon fibers with. The two fiber deposits form a superposed conical fiber pattern, thereby forming a mixture of fibers in the center of the web. The fineness gradient of the web is said to function as a depth filter because one side is made of small diameter thermoplastic fibers and the other side is made of large diameter staple fibers. Paul U.S. Pat. No. 4,032,680 discloses a method of forming a meltblown filter media on a rotating mandrel. By inclining the filter media with respect to the mandrel,
The resulting web has a high fiber density near one surface and a low fiber density near the opposing surface. However, the diameter average of the fibers themselves appears to be uniform across the depth of the filter media. As mentioned above, depth filter media can also be manufactured by laminating web-like materials having different filtration efficiencies or characteristics. For example, U.S. Pat. No. 4,011,067 to Carrie discloses a filter medium consisting of fine fibers by melt or solution blowing. The filter medium is a layered filter medium having a porous base web, a fine fiber intermediate layer, and a porous upper web. The multiple layers provide only a small portion of the pressure drop, typically less than 20%, and are generally nonwoven webs such as polyethylene terephthalate. The fine fiber interlayer is thick enough for the manufacture of HEPA filters. U.S. Pat. No. 4,375,718 to Wadworth et al. Discloses a method for making charged filter media. The fibers in the filter media may be polypropylene or may be formed by melt blowing techniques. The fineness of the meltblown fibers is disclosed to be 0.3 to 0.5 micron diameter. The meltblown filter media is then formed into a layer on one side using a contact web of nonwoven cellulosic fibers such as cotton, rayon, wood pulp, or hemp, or a mixture of these fibers. Nonwoven contact webs have certain electrical properties that accept charge. [Means for Solving the Problems] Accordingly, an object of the present invention is to provide a method for producing a single melt-blown thermoplastic web that can be used as a depth filter medium having a fineness gradient in the depth direction of the filter medium. It is in. The above objective is accomplished by a general melt blowing method performed on a forming line having a number of die heads spaced along a collection belt. By varying the operating parameters for each die head to produce fine fibers of varying average diameter, a web can be formed of continuous layers, each having a generally different predetermined fineness. As a result, the first (upstream) layer of the web has a high fineness (ultra coarse), and thus the pore size is large enough to capture large particles. The intermediate layer has a smaller fineness (coarse and medium fineness) that the medium-sized particles can capture. The final (downstream) layer has a smaller fineness (fineness) and a smaller pore size, and can capture the minimum number of particles that have passed through a layer having a larger pore size. As a result, the large- and medium-sized particles are trapped by the first and intermediate layers in the depth direction of the filter, so that early blocking of the upstream surface of the filter medium and the fine final layer can be avoided. More specifically, the method of the present invention provides a method for reducing the throughput of a polypropylene melt to about 1 pound per hour (PIH) per inch of die head width (about 2.54 cm).
And an air flow rate of 250 per minute for a 20 inch (about 50.8 cm) wide die head.
~ 325 standard cubic feet (SCFM) (approximately 7-9.1 m ³ ) to form a fine layer, an intermediate layer with an output of more than 1 PIH and an air flow of less than 250 SCFM, and a coarse layer And an air flow rate below 250 SCFM. Since the filter medium of the present invention is formed on the same forming line by thermoplastic fines of the same composition, the binding between the fines in each layer is ensured, and the filter medium guarantees a predictable filtering action and performs delamination. There will be uniformity to prevent throughout. or,
Cold rolling of the formed web improves filtration efficiency. Other objects and features of the present invention will become apparent with reference to the following detailed description and drawings. EXAMPLES Hereinafter, the present invention will be described based on preferred examples and procedures, but it will be understood that the present invention is not limited to the examples and processing. On the contrary, the intent is to cover all alternatives, modifications and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. FIG. 1 shows a web forming reload 10 for forming a melt-blown web 12 having a fineness gradient in a depth direction. Apparatus 10 has six identical extruders 14A to 14F, each having a hopper 16A to 16F containing thermoplastic resin pellets. The extruders 14A to 14F have built-in screw conveyors driven by drive motors 15A to 15F, respectively. Each of the extruders 14A through 14F is heated along its length to the melting temperature at which the thermoplastic resin pellets become a melt. The screw conveyor is driven by motors 15A to 15F and extrudes thermoplastic material from the extruder into discharge pipes 20A to 20F attached to each. The discharge pipes 20A to 20F are connected to die heads 22A to 22F each having a die width 25. As an example, FIG. 2 shows a cross section of the die head 22A. Die head 22 shown
A includes a die opening or die tip 24 having an orifice 26. Heated fluid (usually air) is supplied to the die tip via tubes 32 and 34 (FIG. 1) terminating in channels 28 and 30 adjacent the die tip outlet 26. As the thermoplastic polymer is extruded from the tip openings 26 of each die head, the high pressure air fines and cuts the polymer stream to form fibers at each die head. The fibers are deposited in layers on a movable porous belt 38 to form a layered web 12. During the melt blowing process, a vacuum is drawn behind the porous belt 38 and the fibers are drawn onto the belt 38. A vacuum chamber behind the porous belt may be provided separately for each die head to provide an appropriate vacuum for each die head. Once the fiber layer is applied on the movable belt 38 by the plurality of die heads 22A to 22F, the web 1
2 is withdrawn from belt 38 by withdrawal rolls 40 and 42. The cold rolling rolls 44 and 46 engage the web 12 after passing through the drawer roll, and roll the web, thereby increasing the filtration efficiency of the layered web 12 as a filter medium. The above description of the meltblowing apparatus 10 is generally known in the prior art. The properties of the meltblown web 12 can be adjusted by manipulating various operating parameters used for each extruder and die head when performing the meltblown process on the meltblown apparatus 10. The following parameters can be adjusted and changed for each extruder and die head to change the properties of the resulting fiber layer. 1. Kind of polymer 2. Polymer extrusion amount (pound / hour per 1 inch die width (approximately 2.54 cm ² ) ・ ・ ・ P
IH) 3. Polymer melting temperature (° F or ° C) 4. Air temperature (° F or ° C) 5. Air flow rate (standard cubic feet / minute ... SCFM ... 20 inches (about 50.8 cm)
Calibration with width die head 6. Distance between die tip and forming belt (inch or centimeter) 7. Vacuum (inch (cm)) water column under forming belt Die heads 22A to 22C to form depth filter media of the present invention Deposits three layers of fine fibers in a layered fashion on a porous belt, where multiple layers of fine fibers are usually required due to limitations in the amount of extrusion involved in making a fine fiber layer. The die head 22D deposits a single layer of medium fineness fibers on the fine fibers.
2E deposits a single layer of coarse fibers on the medium size fiber layer of the web 22. The die head 22F deposits a single layer of ultra coarse fibers on the layer of coarse fibers of the web 12. As a result, the composite depth filter
The fibers are formed so as to gradually change from fine fibers on one side of the web 12 to ultra coarse fibers on the other side of the web 12. When such a layered web 12 is used as a filter medium, it becomes a depth filter medium having a fineness gradient in the depth direction, that is, the Z-axis direction. Since the fine layer on the depth filter achieves the highest filtration efficiency as a depth filter, the filtration efficiency required for a HEPA filter or a near HEPA filter can be achieved. Further, 100 to 300 ps by the cold rolling rolls 44 and 46
Rolling the web 12 at a pressure of i tends to increase the filtration efficiency by densifying the web in the Z-axis direction, thereby sealing the observable surface pores of the fine layer. It is clear that sealing the surface pores indicates that the pore size in the fine filter layer is smaller. When the web 12 is used as a filter medium, the fine fiber side of the web 12 is used downstream of the filter, and the ultra coarse fiber side of the web 12 is used upstream of the filter medium. Since the large-diameter particles are captured by the super-coarse layer and the coarse layer of the depth filter, the particles do not move to the fine layer and do not block the fine filter layer at an early stage. Similarly, the medium-sized particles are trapped in the medium fineness layer, so that they do not move to the fine layer and block it early. A depth filter medium having two layers of fine fibers and one layer of each of medium fineness, coarse and ultra coarse fibers was produced using five of the six die heads shown in FIG. 1 according to the following working parameters. . Experimental example 1 1. Polymer 3145 is a polypropylene resin manufactured by Exxon, Des Plaines, Illinois. 2. 1.04 OZ / yd ² (about 35.26 g / cm ² ) is the combined basis weight of both microlayers. Calibrated at an inch (about 50.8 cm) wide die head, but also as cubic feet per minute (SCFM) per inch of die head width
It may be expressed by dividing the SCFM value by 20 inches. 4. Rolling was performed on 10 inch (about 25.4 cm) square test pieces. Therefore,
The rolling pressure of 10 tons is (10 tons × 2000 lbs / ton) / 100 in ² = 200 lbs / in ² (about 10 tons
X about 9030 kg / ton) / 645 cm ² = about 145 kg / cm ^{2 As} shown in Experimental Example 1, the filtration efficiency of the composite web is 53%. This filtration efficiency is equal to the filtration efficiency of the two fine layers of the composite laminated web. To increase the filtration efficiency of the composite laminate into or near the HEPA zone or near HEPA zone, 0.3 to 0.3.
It is necessary to increase the basis weight of the total fine layer by increasing the number of die heads for extruding 5 micron diameter fine fibers. For example, by further adding a die head for obtaining fine fibers, the total basis weight of the entire fine fiber layer is adjusted to 1.04 to 2.0 OZ / yd ² (about 3
Increasing from 5.26 to about 67.8 g / cm ² ) increases the filtration efficiency to 71%.
As the basis weight is further increased in connection with the additional rolling pressure, it is believed that the filtration efficiency of the fine fiber layer will be a higher percentage. Air flow between 250 and 325
Increasing to SCFM increases the filtration efficiency of the fine layer. Also, the use of other polymers, such as polypropylene resin PC973 manufactured by Himont USA Inc. of Wilmington, Del., Can provide a web with higher filtration efficiency. Experimental Example 1 shows a composite web in which the fineness gradient decreases throughout the depth of the web (from upstream to downstream). The present invention also provides a fineness gradient throughout the depth of the composite web. It is also intended for a composite web in which the number can be increased or decreased.
For example, the present invention also contemplates a composite web having a layer of fineness (from upstream to downstream) comprising coarse, medium, fine, medium and coarse fibers. Of the above layers, the last two downstream layers (medium fineness and coarseness) may be added to protect the fine layer during handling or to obtain a filter medium that can be attached regardless of upstream or downstream. Good. FIG. 3 is a micrograph showing the fine fiber layer of Experimental Example 1 at a magnification of 500 times. This layer has fibers of 0.3 to 5.0 microns diameter, and in Example 1 the average fineness is 2.5 microns. 4, 5 and 6 are similarly micrographs showing each of the medium fineness, coarse and ultra coarse fibers of Experimental Example 1 at a magnification of 500 times. These micrographs qualitatively illustrate the nature of the fineness gradient across the depth of the composite filter media.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic view showing an apparatus for producing a melt-blown material according to the present invention; FIG. 2 is a cross-sectional view of a die head used for carrying out a melt-blown manufacturing method according to the present invention; The figure is a photomicrograph (500) showing the fiber shape of the fine layer of the filter medium according to the present invention.
×), FIG. 4 is a micrograph (50) showing the fiber shape of the medium fineness layer of the filter medium according to the present invention.
0 ×), FIG. 5 is a micrograph (500) showing the fiber shape of the coarse layer of the filter medium according to the present invention.
×), and FIG. 6 is a micrograph (50) showing the shape of the fibers of the ultra coarse layer of the filter medium according to the present invention.
0 ×). 10 Web forming apparatus 12 Melt blown webs 22A to 22F Die heads 44, 46 Cold rolls

Claims (1)

(1) In a method for forming a composite web composed of thermoplastic fibers formed by melt blowing having a fineness gradient in a depth direction of the web, at least three layers of thermoplastic fibers having the same fiber composition are melted. A method for forming a composite web, comprising: depositing on a collecting device and joining fibers of adjacent layers together so that fibers of adjacent layers have different densities by blowing. (2) The method for forming a composite web of melt-blown thermoplastic fibers according to claim 1, wherein the web is cold-rolled at a pressure of 100 to 300 psi. (3) said layer comprises at least one fine fiber layer and at least one intermediate fiber layer;
The fine fiber layer has an extrusion rate of the polypropylene melt of less than 0.454 kg (1 pound) per hour per 2.5 cm (1 inch) die head width and an air flow rate of 7.0 per minute for the fine fiber. From 9.1 standard cubic meters (250 to 325 standard cubic feet per minute) by depositing polypropylene fibers through a fine fiber die head, wherein the intermediate fiber layer determines the extrusion rate of the polypropylene melt by the die head width. By depositing polypropylene fibers through an intermediate fiber die head at an amount exceeding 0.454 kg / h per 2.5 cm and an air flow rate for the intermediate fibers of less than 7.0 standard cubic meters per minute. The method for forming a composite web of melt-blown thermoplastic fibers according to claim 2, wherein the composite web is formed. (4) The layer further comprises a polypropylene melt through the coarse fiber die head at a polypropylene melt extrusion rate above the intermediate fiber layer and a coarse fiber air flow below 7.0 standard cubic meters per minute. 4. A method according to claim 3 including at least one coarse fiber layer formed by applying. (5) A filter medium formed of a composite web composed of thermoplastic fibers formed by melt-blowing with a fineness gradient in all directions of the depth of the web, wherein thermoplastic fibers having the same fiber composition are adjacently melt-blown. A filter medium formed by a composite web formed by depositing on a collecting device and joining fibers of adjacent layers together so that fibers of different layers have different densities. (6) The filter medium according to claim 5, wherein the web is cold-rolled at a pressure of 100 to 300 psi. (7) said layer comprises at least one fine fiber layer and at least one intermediate fiber layer;
The fine fiber layer has a polypropylene melt extrusion rate of less than 0.454 kg (1 pound) per hour per 2.54 cm (1 inch) die head radiation and a fine fiber air flow rate of 7.0 per minute. To 9.1 standard cubic meters (250 to 325 standard cubic feet / minute) by depositing polypropylene fibers through a fine fiber die head, wherein the intermediate fiber layer controls the polypropylene melt extrusion rate by die head radiation. A polypropylene fiber is applied through an intermediate fiber die head at an amount exceeding 0.454 kg per hour per 2.54 cm and an air flow rate for the intermediate fiber of less than 7.0 standard cubic meters per minute. The filter medium according to claim 6, wherein the filter medium is formed by the following steps. (8) The layer further comprises a polypropylene melt extruding through the coarse fiber die head at a polypropylene melt extrusion rate above the intermediate fiber layer and a coarse fiber air flow below 7.0 standard cubic meters per minute. 8. The filter media according to claim 7, comprising at least one coarse fiber layer formed by applying.