The present invention provides a bag filter comprising a melt-blown fibrous nonwoven web formed into a bag configuration with a closed end, an open end, an inside surface, and an outside surface. The bag filter is constructed such that (a) the fibrous nonwoven web comprises fibers such that 90% of the fibers have a diameter ranging from a minimum fiber diameter to a maximum fiber diameter which is no more than about three times the minimum fiber diameter, (b) the fibrous nonwoven web as oriented in the bag filter has a tensile strength in the circumferential direction at least about 1.5 times the tensile strength in the longitudinal direction, and/or (c) there is no side seam and an injection-molded thermoplastic elastomer end closure. The present invention also provides a method of treating a fluid by passing the fluid through such a bag filter.
Various techniques of preparing melt-blown fibrous nonwoven webs have been disclosed. For example, in U.S. Pat. No. 3,825,380, the Exxon melt-blowing system is described; however, this system results in fibrous nonwoven webs with undesirable fiber roping and twinning. Both twinning and roping cause the fibrous nonwoven web to have a relatively high pressure drop and low filtration efficiency. The presence of shot, i.e., small pellets of unfiberized resin interspersed in the web, can also induce irregular pore size and be a problem with such fibrous nonwoven webs. U.S. Pat. No. 4,021,281 describes a method in which attenuated fibers are produced by a system which bears similarities to the Exxon system. The resulting fibrous nonwoven web contains many of the same deficiencies present in the fibrous nonwoven web prepared using the Exxon system.
The fibrous nonwoven webs are generally in the forms of sheets which are formed into a bag filter by folding the sheet into a tubular form, sewing together the adjoining portions and one end of the tube, typically turning the resulting bag inside-out, and then heat-sealing the seams with thermoplastic tape. The bag filters thus produced have two seams, namely a side seam and an end seam. For instance, U.S. Pat. No. 5,156,661 discloses a bag filter comprising at least one sheet of a filter medium, preferably a polypropylene high dirt capacity filter medium, formed into a bag configuration with an opening, an inside surface, an outside surface, and at least one seam formed at adjoining filter medium portions which have been sewn together, wherein the seams are sealed with a thermoplastic tape that has been heat-sealed to the adjoining filter medium portions.
Similarly, U.S. Pat. No. 5,205,938 discloses a bag filter comprising at least one sheet of a filter, preferably a polyester high dirt capacity filter medium having a graded pore structure, formed into a bag configuration with an opening, an inside surface, an outside surface, and at least one seam formed at adjoining filter medium portions which have been sewn together. The seams are optionally sealed with a thermoplastic tape that has been heat-sealed to the adjoining filter medium portions.
Bag filters produced by sewing and heat-sealing have certain deficiencies. Since the seams have been punctured by the sewing needle and are held together by the thread, the structural integrity of the seam is weakened and hence that of the bag filter itself. Moreover, sewn seams can provide fluid leakage pathways during use. In addition, the method of producing such bag filters is time-consuming and costly because of the number of steps involved when the seams are sewn and heat-sealed with a thermoplastic tape.
Some bag filters are formed by merely thermally sealing the seams, i.e., bonding the fibrous web to itself without any sewing. Although such a sealing technique avoids the need to puncture the fibrous nonwoven web with a sewing needle, the thermal sealing technique also results in two seams (namely, a side seam and an end seam) and suffers from other deficiencies. In particular, the thermal sealing technique requires the partial melting of the fibrous nonwoven web, thereby adversely affecting the structural integrity and filtering characteristics of the bag filter.
Bag filters free of a side seam have been disclosed. For example, U.S. Pat. No. 4,021,281 discloses the production of nonwoven tubular webs from which bag filters may be produced. Although the bag filters are free of side seams, one end of the tube is still subject to the usual closing steps, namely, sewing and heat-sealing with a thermoplastic tape or thermally sealing the fibrous nonwoven web to itself. Thus, there is created an end seam which has the same deficiencies as the side seam, namely, the weakened structural integrity of the bag filter, possible fluid leakage pathways, and the time-consuming steps involved in the closing.
Many bag filters have a collar attached to the open end so that the filter may be suitably positioned in the filter holder during use. The collar is generally stitched and heat-sealed to the opening of the bag filter, or merely thermally sealed directly to the bag filter at its opening, thereby creating a collar seam. For the same reasons indicated above with respect to the side seam and end seam, a collar seam can be similarly problematic.
Accordingly, there remains a need for improved bag filters. In particular, there is a need for bag filters which avoid some or all of the problems associated with nonuniform filter media and sewn or thermally sealed seams. The present invention seeks to provide such a bag filter. These and other objects and advantages of the present invention, as well as additional inventive features, will be apparent from the description of the invention provided herein.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a bag filter comprising a melt-blown fibrous nonwoven web formed into a bag configuration with a closed end, an open end, an inside surface, and an outside surface. The bag filter is constructed such that (a) the fibrous nonwoven web comprises fibers such that 90% of the fibers have a diameter ranging from a minimum fiber diameter to a maximum fiber diameter which is no more than about three times the minimum fiber diameter, (b) the fibrous nonwoven web as oriented in the bag filter has a tensile strength in the circumferential direction at least about 1.5 times the tensile strength in the longitudinal direction, and/or (c) there is no side seam and an integral injection-molded thermoplastic elastomer end closure. The present invention also provides a method of treating a fluid by passing the fluid through such a bag filter.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a cross-sectional view of a conventional fiberizing orifice.
FIG. 2 is a cross-sectional view of a preferred fiberizing nozzle.
FIG. 3 is a perspective view of a melt-blowing apparatus with a single row of fiberizing nozzles.
FIG. 4A is an end view of a melt-blowing apparatus with four rows of fiberizing nozzles.
FIG. 4B is an enlarged plan view of the same apparatus depicted in FIG. 4A as seen along line 4B--4B of FIG. 4A.
FIG. 5 is an end view of a melt-blowing apparatus with two rows of angled and offset fiberizing nozzles.
FIG. 6A is an end view of another melt-blowing apparatus with two rows of angled and offset fiberizing nozzles, while FIG. 6B is a top view of the same apparatus as seen along line 6B--6B of FIG. 6A. FIG. 6C is a side view of a melt-blowing apparatus showing the translation of the collecting cylinder.
FIG. 7 is a scanning electron micrograph (220.times.) of a melt-blown fibrous nonwoven web prepared in accordance with the present invention.
FIG. 8 is a side view of a melt-blowing apparatus useful in the preparation of laminates in accordance with the present invention.
FIG. 9 is a scanning electron micrograph (220.times.) of a melt-blown fibrous nonwoven web prepared in accordance with the present invention.
FIG. 10 is a scanning electron micrograph (500.times.) of a melt-blown fibrous nonwoven web prepared in accordance with the present invention.
FIG. 11 is a scanning electron micrograph (220.times.) of a typical commercially available melt-blown fibrous nonwoven web.
FIG. 12 depicts a .beta.-ray backscatter record in the cross-machine direction for a melt-blown fibrous nonwoven web prepared in accordance with the present invention.
FIG. 13 depicts a .beta.-ray backscatter record in the machine direction for the same melt-blown fibrous nonwoven web prepared in accordance with the present invention as is the subject of FIG. 12.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present inventive bag filter comprises a filter medium formed into a bag configuration with a closed end, an open end, an inside surface, and an outside surface. The filter medium from which the bag filter is formed comprises a melt-blown fibrous nonwoven web. The filter medium is generally in a tubular configuration which can be effected either by forming the nonwoven web into a tube during the melt-blowing process (e.g., melt-blowing the fibers onto a rotating mandrel) which avoids a side seam or by taking one or more flat sheets of the nonwoven web, forming the sheet(s) into a tube with a side seam, and sealing the seam. When a flat sheet is converted into a bag filter, a side seam, an end seam, and a collar seam (to the extent the bag filter comprises a collar) need to be sealed. When a tubular web is converted into a bag filter, there exists only an end seam and a possible collar seam which need to be sealed. Such a bag filter can be prepared from a variety of melt-blown fibrous nonwoven webs and utilize a variety of seam closure techniques.
The present inventive bag filter is desirably constructed such that (a) the fibrous nonwoven web comprises fibers such that 90% of the fibers have a diameter ranging from a minimum fiber diameter to a maximum fiber diameter which is no more than about three times the minimum fiber diameter, (b) the fibrous nonwoven web as oriented in the bag filter has a tensile strength in the circumferential direction at least about 1.5 times the tensile strength in the longitudinal direction, and/or (c) there is no side seam and an integral injection-molded thermoplastic elastomer end closure. The present inventive bag filter preferably is the result of the combination of these three features.
Fibrous Nonwoven Web
While any suitable melt-blown fibrous nonwoven web can be used to prepare a bag filter, the present inventive bag filter is preferably prepared from a melt-blown fibrous nonwoven web which is highly uniform. Such a highly uniform fibrous nonwoven web is described in U.S. patent application Ser. No. 08/281,772.
The filter medium of the present inventive bag filter preferably comprises a melt-blown fibrous nonwoven web of fibers having an average fiber diameter of less than about 2 .mu.m, preferably less than about 1.5 .mu.m, and more preferably less than about 1 .mu.m. Moreover, the melt-blown fibrous nonwoven web can have a narrow distribution of fiber diameters, preferably such that 90% of the fibers of the web have a diameter ranging from a minimum fiber diameter to a maximum fiber diameter which is no more than about three times, preferably no more than about two times, and more preferably no more than about 1.5 times, the minimum fiber diameter.
The preferred melt-blown fibrous nonwoven web can be, and desirably is, characterized by a weight distribution varying by less than 1% when measured in both the longitudinal and transverse directions, with such weight distribution measured along 0.64.times.13 cm areas and on 2.54 cm squares. The preferred melt-blown fibrous nonwoven web is also substantially free of roping, twinning, and shot.
The preferred present inventive melt-blown fibrous nonwoven web can be, and desirably is, further characterized by a tensile strength in a first direction (e.g., the cross-machine direction (CMD) or circumferential direction of the resulting bag filter) at least about 1.5 times, preferably at least about 2 times, and more preferably at least about 4 times, the tensile strength in a second direction 90.degree. to the first direction (e.g., the machine direction (MD) or longitudinal direction of the resulting bag filter). It is particularly advantageous to use such a melt-blown fibrous nonwoven web when the fibrous nonwoven web is oriented in the bag filter having a circumferential direction and a longitudinal direction such that the first direction (e.g., CMD) of the fibrous nonwoven web is in the circumferential direction of the bag filter and the second direction (e.g., MD) of the fibrous nonwoven web is in the longitudinal or axial direction of the bag filter. This is because the stress on the walls of the resulting bag filter due to internally applied pressure is twice as high in the tangential direction as compared with the stress in the axial direction. Thus, if the web tensile strengths are equal in both the CMD and MD directions, a failure due to internal pressure would be tangential, i.e., by a split along the length of the bag filter parallel to its axis. By using a melt-blown fibrous nonwoven web in which the fibers are oriented such as to provide a web with tangential strength about twice the axial strength, a bag filter prepared therefrom uses all of the fibers of the web to maximum advantage, will not fail prematurely, and will resist higher internal pressure.
A method of preparing the preferred melt-blown fibrous nonwoven web is disclosed in U.S. patent application Ser. No. 08/281,772 and preferably comprises extruding the molten resin from two parallel rows of linearly arranged, substantially equally spaced nozzles to form fibers onto the surface of a cylindrical collector having a longitudinal axis arranged parallel to the rows of nozzles, wherein the rows of nozzles are offset from each other and are angled toward each other. The rows of nozzles are preferably offset from each other by about one-half the spacing between the nozzles within each row and the rows of nozzles are preferably angled toward each other by substantially equal but opposite angles, e.g., each of the rows of nozzles is angled by about 25.degree. or less, preferably about 5.degree. to about 20.degree., more preferably about 10.degree. to about 16.degree., from a vertical plumb line originating at the center of the cylindrical collector. The cylindrical collector can be rotated at any suitable surface velocity, generally at least about 20 m/min and preferably not exceeding about 600 m/min, although a higher surface velocity (e.g., about 1000 m/min or higher which can be obtained by rotating a 35 cm diameter cylindrical collector at about 900-1000 rpm) may produce a fibrous nonwoven web which is superior for some applications. The cylindrical collector can have any suitable diameter, preferably about 5 cm to about 150 cm, more preferably about 10 cm to about 50 cm, and most preferably about 10 cm to about 35 cm. The nozzles can be spaced any suitable distance from the cylindrical collector, preferably about 2 cm to about 10 cm, more preferably about 2 cm to about 8 cm, and most preferably about 2 cm to about 5 cm. The cylindrical collector is preferably translated at a rate not exceeding about 2 cm/revolution, more preferably at a rate not exceeding about 1 cm/revolution, and most preferably at a rate not exceeding about 0.75 cm/revolution. Within each of the rows, the nozzles can be spaced apart any suitable distance, generally about 2 cm or less, preferably about 0.25 cm to about 2 cm, more preferably about 0.1 cm to about 1.5 cm, and most preferably about 0.37 cm to about 1.2 cm, such as about 0.5 cm to about 1 cm. The parallel rows can be spaced apart any suitable distance, preferably such that the nozzle tip to nozzle tip separation between rows is about 1 to 2 cm. Moreover, the present inventive method is preferably carried out while a negative pressure is maintained between the rows of the nozzles.
Suitable methods of modifying the preferred melt-blown fibrous nonwoven web are also disclosed in U.S. patent application Ser. No. 08/281,772 and preferably comprise modifying the melt-blown fibrous nonwoven web to alter the critical wetting surface tension (CWST) of the web to between about 73 and about 100 dynes/cm. The bag filters of the instant invention can be made of such modified webs, as well as the unmodified webs.
The CWST of the web may be modified by a novel grafting procedure, in which the CWST is raised to above 73 dynes/cm by a two or three step process comprising (a) exposing the porous medium for about 10 to 30 seconds to a plasma of an inert gas, such as helium, neon, or argon at about 20 to 700 .mu.m Hg, (b) optionally evacuating the chamber to a pressure of less than about 5 .mu.m Hg, and then (c) introducing into the chamber liquid hydroxypropyl methacrylate or other unsaturated monomer presenting a hydroxyl, carboxyl, amine, or amide group and holding for a period of about 30 seconds or more. After filling the chamber with air, the grafted polymer may be removed and used without further processing or may be subjected to washing, e.g., water washing, to remove any unbound residual material (e.g., contaminants which were present in the monomer composition). Weight gain depends on the surface area of the porous medium. A typical value is in the range of up to about 5 to 10%. The temperature during the operation remains essentially at the ambient. Preferred monomers include hydroxypropyl acrylate (HPA) and hydroxy ethyl methacrylate (HEMA), along with other similarly functional monomers which are known to those familiar with the art of grafting. In a variation of the above described procedure the unsaturated monomer may be used to form a plasma in step (a) in place of the inert gas, with similar end results.
A remarkable feature of this grafting procedure is that, unlike other grafting procedures known to those familiar with the art, the CWST produced with HPA and HEMA is 74 to 76 dynes/cm over a wide range of concentrations and times of exposure when used to treat hydrophobic polyester substrates. Other resins can be similarly converted to form fibrous porous media and then hydrophilized as described above.
Other means to achieve a permanent graft include cobalt 60 irradiation, UV exposure, or electron beam, in each case followed by exposure to an aqueous solution of a suitable monomer, which could for example be an acrylic alcohol, which is then typically followed by washing and drying.
The melt-blown fibrous nonwoven web can be unsupported or supported, e.g., as part of a porous composite structure comprising at least one porous sheet containing thereon a melt-blown fibrous nonwoven web, preferably wherein no more than about 10%, more preferably no more than about 5%, of the pores of the porous sheet are blocked by the melt-blown fibrous nonwoven web. The melting point of the fibrous melt-blown nonwoven web is preferably lower than that of an adjoining porous sheet to assist in securing the composite together. Similarly, two porous sheets can be bound together by the melt-blown fibrous nonwoven web.
The bag filters of the present invention are made preferably from fibrous nonwoven webs which can be produced by any of the methods described in U.S. patent application Ser. No. 08/281,772. The web may be formed, as briefly described below, from a single row of individual nozzles, from a multiple row arrays of fiberizing nozzles, by the crossed fiber system, or by, most preferably, the scanning system.
Fiberizing Nozzles
In the widely practiced Exxon melt-blowing process, molten resin, for example molten polypropylene, is extruded through a row of linearly disposed holes of diameter about 0.35 to 0.5 mm. The holes are drilled in linear array on about 1 to 2 mm centers into a flat surface about 1 to 2 mm wide, with the surface being located as shown in FIG. 1 at the apex of a member having a triangular cross section, and with the angles at the apex being about 45.degree. to 60.degree. to the center line. Surrounding the apex 11 as shown in FIG. 1 are two slots 12, 13, one on each side, through which is delivered heated air, which attenuates the molten resin extruded through the holes, thereby forming a stream of fibers. The fibers are collected on one side of a moving screen which is separated from the nozzle tips by about 10 cm or more, with the other side of the screen being connected to a suction blower. In operation, most of the fibers are collected on the screen to form a low density web with a rough surface; however, a significant proportion of the fibers escape into the surroundings, and a suction hood into which they are collected and sent to waste is provided.
An improved fiberizing nozzle is depicted in FIG. 2, wherein the fiberizing nozzle 21 contains a capillary 22 through which the resin is pumped and a circular annulus 23 through which hot air is delivered. The pumped resin exits the capillary 22 into the resin disruption zone 24 and then into the nozzle channel 25 where the resin, now fragmented into tiny droplets, is carried in the air stream out of the nozzle tip 26.
Because the air supply is used more efficiently and is correspondingly less in proportion to the weight of the product web, the fiberized product of the present invention can be collected as a web by impinging it on a solid collecting surface, as opposed to the vacuum backed screen of the Exxon apparatus. In another marked improvement on the prior art, the DCD (distance between the nozzle tip 26 in FIG. 2 and the target collecting surface) may be shortened to under about 2.8 to 5.5 cm, i.e., about one half or less than used for the Exxon system, thereby reducing the width of the fiber stream and further improving fiber collection efficiency.
A Single Row Of Individual Nozzles
An arrangement for collecting a fibrous web from a single row of individual fiberizing nozzles (of the type described above and depicted in FIG. 2) is shown in perspective view in FIG. 3, with each fiberizing nozzle 31 being connected to a double manifold 32, one portion of which is arranged to supply molten resin to the nozzles from an extruder and the other portion to supply heated air at controlled temperature and pressure. The nozzles 31 are arranged in a single line, spaced apart from each other by a distance which is preferably in the range between about 0.4 and 1.5 cm and more preferably in the range of about 0.6 to 1.2 cm. This arrangement, as one might expect, yields a striped product, which nevertheless has properties substantially superior to the products of the Exxon system with respect to better fiber conformation, and has the ability to make finer fibers which in use remove smaller particles and have longer life in service.
The webs formed by the apparatus of FIG. 3 may be thick enough and sufficiently coherent to permit formation directly onto the outer surface of cylinder 33, from which they can be withdrawn continuously in the manner of U.S. Pat. No. 4,021,281; however, it may be expedient, particularly when the web weight is less than about 3 to 10 milligrams per square cm, to have the web collected on the surface of a supporting fabric 34 which may for example be an inexpensive nonwoven, permitting the product to be collected, stored, and later used with the fabric in place or separated from the fabric prior to use.
Multiple Row Arrays Of Fiberizing Nozzles
An apparatus which was used in an attempt to reduce or to eliminate striping is shown in FIGS. 4A and 4B, where FIG. 4A is an end view of an apparatus differing from that of FIG. 3 only in the substitution of a fiberizer assembly with four rows of fiberizing nozzles, and FIG. 4B is an upwardly facing section along the line 4B--4B of FIG. 4A. In FIG. 4B, 41 refers to the collection surface, while 42-43 delineates a row of 106 linearly located fiberizing nozzles in which the fiberizing nozzles are spaced apart by 0.25 cm, 42-43 being one of four such rows, the rows being spaced apart from each other by 1.27 cm. In FIG. 4A, 44 is a rotating cylinder, around 180.degree. of which travels a 110 cm wide smooth surfaced nonwoven fabric 45. The fabric is impinged upon by the fiber streams 46 from the 4.times.106=424 fiberizing nozzles 42-43 in the manner shown in FIGS. 4A and 4B, thus forming a porous medium 47 carried by the fabric 45, from which the porous medium may be removed and separately rerolled.
As fabric 45 travelled over the rotating cylinder 44, it was impinged upon by 424 fiber streams 46, each stream originating from a nozzle spaced 0.25 cm from its diagonal neighbor. Since each fiber stream could be seen visually to lay down a swath about 0.5 cm wide as it struck the surface of the fabric 45, and since the nozzles were on 0.25 cm centers, it was anticipated that a uniform or nearly uniform fiber distribution would be obtained, thus diminishing or eliminating striping; instead the striping was accentuated from that obtained with a single row die. The stripes were spaced 1 cm apart, with the more transparent portions containing about one half the quantity of fiber contained in the less transparent portions.
Careful visual observation of the fiber streams while the apparatus was being operated revealed that as the fabric 45 to which the fiber streams were directed moved over the die in the direction of the arrows depicted in FIGS. 4A and 4B, the fiber stream originating from the first row of 106 dies 42-43 impinged on the fabric and formed 106 ridges of fibers, each of which caused the 106 fiber streams from the following three rows to be deflected from the vertical direction of the nozzle from which they originated in a manner such as to deposit a proportion of their fibers on the ridges made by the first row, thus enlarging the already deposited ridges of fibers rather than starting new ridges. To review, referring again to FIGS. 4A and 4B, with the fabric moving in the direction of the arrows, nozzles 42-43 deposited ridges which were located where one would expect, i.e., in line with the nozzles; however, the fiber streams from all of the other nozzles were visibly deflected towards the ridges made by the first row of nozzles 42-43, the last of the four rows surprisingly being deflected a full 0.75 cm. In this manner a product web was obtained which was heavily striped on 1 cm centers.
The aerodynamics which might account for this unexpected behavior have not been explained quantitatively, but qualitatively a consequence of Bernoulli's theorem can be applied, i.e., a rapidly moving stream of gas is deflected towards an adjacent solid surface, in this case toward the ridges formed by the leading row of fiberizing nozzles.
Crossed Fiber Streams
An end view of a configuration of a crossed fiber stream melt-blowing system of the present invention is shown in FIG. 5, in which 54 is a rotating cylinder over which a fabric 53, typically a disposable nonwoven is drawn, moving counterclockwise around the metal cylinder 54 towards a rewind station, which is not shown. Double manifold 51, of length a few centimeters less than the width of web 53, feeds hot air and molten resin to a row 56 of fiberizing dies tilted towards the right relative to vertical plumb line 55 drawn from the center of cylinder 54, generating fiber streams which strike the collector cylinder at 59. A matching set comprising manifold 52 and nozzles 57 is tilted towards the left and deposits resin on the collector surface at 58; if the axial spacing (perpendicular to the paper) between adjacent nozzles is distance D, then the two rows of nozzles are offset from each other by distances 0.5 D, thus the fiber streams cross each other. The distance 58-59 by which they overlap may in the practice of the invention be as much as 1 cm or more. The distance 58-59 may be zero, and a limited negative lap (separation) may be acceptable in some circumstances. Over the whole range of overlap, from 1 cm or more to a negative lap, no interference between adjacent fiber streams can be visually detected, an observation very much contrary to the results described above for multiple rows of nozzles. As a result, media made using the crossed fiber stream system are more uniform, and, while not eliminated, striping is reduced.
The degree of overlap is determined in part by the DCD. With a 15 cm diameter collection cylinder and an angle between the two sets of nozzles of 26.degree., and the distance between the nozzle tips 56-57 set to 1.4 cm, a preferred overlap is about 0.5 cm for a relatively large DCD of 6 to 7 cm, and between about 0.23 cm to zero overlap for relatively smaller DCDs of about 4 to 2.8 cm.
In general, the DCD is smaller when a porous medium of higher density with lower voids volume and higher tensile strength is desired. The DCD of the processes of the invention ranges from about 2.5 cm to about 7.5 cm. Parameters other than DCD which can be varied to produce a desired product include the angles of tilt, the distance from die tip 56 to die tip 57, the offset if any from the center line of the matched fiberizer set to the center vertical plumb line 55 of the collection cylinder, and the temperature, flow rates, and fiberizing characteristics of the resin which is being fiberized, as well as the volume, temperature, and pressure of the air delivered to the fiberizing nozzles.
Throughout the many variations of operating conditions described above, the crossed fiber system has been consistent in showing no interaction between neighboring product streams; the fibers generated by this system collect on the target surfaces in exactly the manner expected for a system of a given geometry.
The Scanning System
FIGS. 6A-6C depict the scanning system of the present invention. In FIG. 6A the manifolds 61 are located similarly to and have the same function as manifolds 51 and 52 of FIG. 5. The area between the two manifolds has been enclosed at the bottom and at both ends to form a cavity 62 fitted at its lower end with a cylindrical opening 63. Cylinder 54 of FIG. 5 has been replaced by cylinder 64, and fabric 53 has been removed. FIG. 6B is a partial view along line 6B--6B in FIG. 6A, showing tilted nozzles 65 located on P--P centers, the nozzles of the one row offset by 0.5 P from those of the other row. FIG. 6C shows in elevation view a crossed stream fiberizer assembly 66 located near to the right end of collector cylinder 64.
In use the fiberizer assembly 66 is stationary while collector cylinder 64 is rotated, for example at a surface velocity in the range of about 20 to 600 meters per minute, and may be simultaneously translated in the range of about 0.01 to 0.1 cm per revolution. The rotation and translation rates are maintained constant while collector cylinder 64 is moved across the fiberizer to position 67 shown in broken lines, in the course of which a fibrous web 68 is formed by the impinging fibers. The web grows in length until the translation is complete and the whole surface of the collector cylinder is covered. The cylinder of porous medium may then be slit along its length, and its ends trimmed. The so formed sheet may be inspected on a light box where it is seen to be uniform and free of any visually detectable striping.
If while using the crossed fiber streams of the invention the translation per revolution (hereinafter T/R) is increased above 0.1 cm per revolution in about 0.04 cm or smaller increments while holding constant a given combination of fiberizing nozzle dimensions, nozzle placement, DCD, mandy. el diameter, mandrel rotation rate, and resin composition, and each so made specimen is then examined sequentially on a light box, a T/R will be reached at which the existence of parallel stripes in the product becomes readily apparent. By then backing off from that T/R by about 0.04 cm, a product of excellent uniformity is produced, and such a product is encompassed by the present invention. Products made using the crossed fiber streams of the invention which show faint or moderate striping may still be superior with respect to uniformity when compared with products of any previous melt-blowing method; such products are also encompassed by the present invention.
The magnitude of the T/R which produces a strip-free product is influenced by factors including the nozzle-nozzle spacing, which is preferred to be as small as is practical; fiberizing die assemblies with a 0.76 cm nozzle center to nozzle center spacing have been used to produce the examples of the invention, as preceding tests using similar apparatus spaced on 1.02 cm spacing were less successful. Under some circumstances, for example when operating with very large DCD's, stripe-free products may be obtained with nozzle spacing well over 1 to 2 cm, and such products fall within the scope of the present invention. Spacing less than 0.76 cm is desirable and may be possible, albeit such reduction would be somewhat restricted by design considerations such as the dimension of air and resin flow passages. Other criteria for achieving perfect uniformity are that rates of revolution, translation, and resin delivery must be constant throughout the formation of the entire length of the sheet.
Most of the examples of the present invention were performed using convenient T/R values which were a fraction, for example less than one quarter to one half of the maximum T/R. In an experiment dedicated to exploring the maximum T/R possible, in which the air nozzle diameter was a relatively large 0.17 cm, the air temperature was 310.degree. C., 305.degree. C. resin was delivered at 0.51 g/min/nozzle and DCD was 4.1 cm, excellent uniformity on light box inspection was obtained in the T/R range from 0.12 to 0.44 cm, and uniformity remained almost as good up to T/R of 0.63 cm, with visible stripes appearing in the product at 0.76 cm.
In other experiments at various fiberizing conditions the onset of stripy conditions was seen to occur at much lower values. Occasionally conditions were such that striping appeared at a lower value, and then disappeared as the T/R was further increased up to about 0.29 cm (0.375.times. the 0.76 cm nozzle spacing), and a further "node" at which the product improved has been observed at 0.48 cm (0.625.times. the 0.76 cm nozzle spacing).
Prior to the conception of the crossed fiber stream system for delivering fibers to a translating cylinder, attempts were made to use the translating cylinder with other types of fiber delivery systems, including single and double rows of nozzles (not crossed) with the same fiberizing nozzles and with alternate nozzles. None of these yielded other than clearly striped product.
Other fiberizing systems, for example those based on the Exxon process, may yield unstriped product but such products are inferior with respect to uniformity of fiber diameter, weight distribution, and freedom from shot, twinning and roping, and are incapable of making media with average fiber diameters below about 3 to 5 .mu.m.
By changing orientation, location, and geometry of the crossed stream fiberizers, changing resin flow rate, air flow rate, and temperature, and using nozzles with larger or smaller orifices, media can be made which as taken off of the machine have an average fiber diameter from less than 1 .mu.m to more than about 20 to 50 .mu.m, with a range of voids volumes from about 60% to about 94% and a range of thicknesses from less than 0.008 cm to 0.5 cm or more, all with good tensile properties, controlled pore size, thickness, fiber orientation, and voids volume. When used as filters, so made media provide particle removal ratings as measured by the OSU (Oklahoma State University) test from 1 .mu.m to 200 .mu.m or more. Long life in service is obtained using these media due to their high voids volume and resistance to compression as the collected solids cause pressure to build up across the filter.
Referring to FIG. 6A, the locations of the fiberizing nozzles is preferred to be such that the distance N--N between the nozzle tips is in the range from about 0.5 to 3 cm, and more preferred to be in the range of about 1 to 2 cm, and still more preferred to be in the range of about 1.2 to 1.6 cm, and it is preferred that the angle .theta. between the nozzle and the vertical plumb line from the center of the collector cylinder be within about 5.degree. or less of equal but of opposite direction for both dies, and it is further preferred that the angle .theta. be in the range of about 3.degree. to 25.degree., and more preferred to be in the range of about 5.degree. to 20.degree., and still more preferred to be in the range of about 10.degree. to 16.degree.. The volume and type of fibers issuing from each side is usually preferred to be equal; however, products of interest for special purposes may be made by operating each side using different conditions, for example to combine high mechanical strength with very small fiber diameter.
The deposited porous medium may be calendared to reduce the pore size to provide a filter medium with an absolute removal rating of less than 0.5 .mu.m as measured by the methods described in the U.S. Pat. No. 4,340,479.
The collector cylinder 64 of FIGS. 6A and 6C may be surfaced by a suitable release coating. Depending on the thickness, voids volume, and other characteristics of the porous medium, a tubular cylinder of porous medium may then be withdrawn from the collector cylinder and used for example as a filter with the flow from inside to out, or it may be pleated to form a seamless pleated filter element.
The crossed fiber stream arrangement is preferred to be used as the fiber generator with the scanning system of the invention because it permits high translation rates together with high fiber deposition rates while minimizing fiber loss due to overspray, and, because unlike arrays in which the nozzles are parallel, it can be used to make very uniform fibrous products with a very wide range of characteristics with precise lot to lot reproducibility. The media so made are uniform within the sensitivity of the tests which can be applied, such as weight per unit of area, thickness, voids volume, pore size, wicking rate, and particle removal capability.
Referring again to FIG. 6A, a useful mode of operation is achieved by attaching at connection 63 means to generate within chamber 62 a negative pressure in the range from zero to about 3" of water column, thereby achieving a more uniform product (as may be seen in example 44). Further, while it is an advantage of the crossed fiber stream system that both sets of fiber streams impinge on the collector cylinder on or close to a straight line, thus helping to minimize fibers which are not collected, nevertheless, when operating at relatively high air flow rates, the volume of air reaching the cylinder may be so high that some of the fibers bypass the collector cylinder, and are lost into the exhaust duct. Negative pressure applied to chamber 62 acts to prevent or diminish bypassing and to reduce loss of fibers to waste.
The melt-blown fibrous nonwoven media used to prepare the present inventive bag filters can comprise a wide variety of polymers including polyethylene terephthalate, polybutylene terephthalate (PBT), polypropylene, polyethylene, polymethylpentene, polychlorotrifluoroethylene, polyphenylsulfide, poly(1,4-cyclohexylene dimethylene terephthalate), PETG, a polyester polymerized with an excess of glycol, nylon 6, nylon 66, nylon 612, nylon 11, and a nylon 6 copolymer described as "80% nylon 6 with 20% polyethylene-oxide-diamine."
EXAMPLES
Fibrous Nonwoven Media
The following examples illustrate the method of preparation of the fibers and nonwoven webs from which the bag filters of the present invention are preferably prepared as described above.
Examples 1-6
In order to prepare the porous medium of example 1, the scanning system was operated with a fiberizer assembly comprising two fiberizers each with 21 fiberizing nozzles with air apertures 0.13 cm in diameter supplied with air at 304.degree. C. and 0.39 kg/cm.sup.2 pressure. The two fiberizers, each with 21 nozzles on 0.76 cm centers were offset axially from each other by 0.38 cm, and were angled towards each other at an inclination of 13.degree. from the vertical, with the distance N--N of FIG. 6B set at 1.42 cm. The two sets of intersecting fiber streams delivered polybutylene terephthalate (hereinafter PBT) resin at 293.degree. C. at the rate of 0.44 grams per minute per nozzle. The fiber streams impinged over a distance of 4.1 cm (i.e., DCD=4.1 cm) on a 15 cm diameter by 137 cm long collection cylinder which was rotated at 512 rpm while it was simultaneously translated axially at the rate of 0.2 cm per revolution for the length of a single 124 cm stroke, thereby depositing on the surface of the collector cylinder in 1.2 minutes 0.0043 grams per cm.sup.2 of fibrous porous medium which was then slit lengthwise, trimmed at both ends, and then removed from the cylinder, thereby forming a sheet 47.5 cm wide by 102 cm long.
The air pressure and the DCD were then varied to produce five additional sheets with results as presented as examples 1-6 in Table I. All of the so formed sheets were examined on a light box, and were seen to be stripe-free and uniform. The sheets were very easy to manipulate, and could be stacked and removed repeatedly with no pilling or other visible surface disruption. No dimensional changes occurred during a three month storage period in a mixed stack of 24 sheets. This degree of dimensional stability is quite remarkable for a plastic product in some of which the solids content is as low as 8% by volume, with the remainder being air.
TABLE I
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Product Characteristics
Test Conditions Average
Air Voids
Fiber
Tensile Properties
Example
pressure
DCD Thickness
Volume
Diameter
Strength (glc).sup.1
Elongation (%)
No. (kg/cm.sup.2)
(cm)
(cm) (%) (.mu.m)
CMD.sup.2
MD.sup.3
CMD MD
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1 0.39 4.1 0.0185
82.5 4.2 230 90 5.0 5.0
2 0.79 4.1 0.029 89 1.1 510 130 9.7 14.7
3 1.06 4.1 0.054 94 1.0 440 110 8.5 13.5
4 1.06 3.6 0.042 92 0.9 500 150 8.0 13.2
5 1.06 3.0 0.027 88 1.0 440 200 6.8 11.3
6 1.06 2.8 0.021 84.2 1.2 390 240 6.0 84
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.sup.1 grams per linear centimeter.
.sup.2 cross machine direction perpendicular to the length of the sheet
.sup.3 machine direction parallel to the length of the sheet
All of the examples 1 to 6 exhibit the low lateral flow times which are a desirable feature of the invention. Examples 3, 4 and 5 have RMS average fiber diameters respectively of 1.0, 0.9 and 1.0 .mu.m (arithmetic average
