Classifications

• • D—TEXTILES; PAPER
  • D01—NATURAL OR MAN-MADE THREADS OR FIBRES; SPINNING
  • D01D—MECHANICAL METHODS OR APPARATUS IN THE MANUFACTURE OF ARTIFICIAL FILAMENTS, THREADS, FIBRES, BRISTLES OR RIBBONS
  • D01D5/00—Formation of filaments, threads, or the like
  • D01D5/08—Melt spinning methods
  • D01D5/098—Melt spinning methods with simultaneous stretching
  • D01D5/0985—Melt spinning methods with simultaneous stretching by means of a flowing gas (e.g. melt-blowing)
• • D—TEXTILES; PAPER
  • D01—NATURAL OR MAN-MADE THREADS OR FIBRES; SPINNING
  • D01D—MECHANICAL METHODS OR APPARATUS IN THE MANUFACTURE OF ARTIFICIAL FILAMENTS, THREADS, FIBRES, BRISTLES OR RIBBONS
  • D01D1/00—Treatment of filament-forming or like material
  • D01D1/06—Feeding liquid to the spinning head
• • D—TEXTILES; PAPER
  • D01—NATURAL OR MAN-MADE THREADS OR FIBRES; SPINNING
  • D01D—MECHANICAL METHODS OR APPARATUS IN THE MANUFACTURE OF ARTIFICIAL FILAMENTS, THREADS, FIBRES, BRISTLES OR RIBBONS
  • D01D4/00—Spinnerette packs; Cleaning thereof
  • D01D4/02—Spinnerettes
  • D01D4/025—Melt-blowing or solution-blowing dies

Definitions

• This invention relates to devices and methods for preparing melt blown fibers.
• Nonwoven webs typically are formed using a meltblowing process in which filaments are extruded from a series of small orifices while being attenuated into fibers using hot air or other attenuating fluid.
• the attenuated fibers are formed into a web on a remotely-located collector or other suitable surface.
• a spun bond process can also be used to form nonwoven webs.
• Spun bond nonwoven webs typically are formed by extruding molten filaments from a series of small orifices, exposing the filaments to a quench air treatment that solidifies at least the surface of the filaments, attenuating the at least partially solidified filaments into fibers using air or other fluid and collecting and optionally calendaring the fibers into a web.
• Spun bond nonwoven webs typically have less loft and greater stiffness than melt blown nonwoven webs, and the filaments for spun bond webs typically are extruded at lower temperatures than for melt blown webs.
• Web uniformity typically is evaluated based on factors such as basis weight, average fiber diameter, web thickness or porosity. Process variables such as material throughput, air flow rate, die to collector distance, and the like can be altered or controlled to improve nonwoven web uniformity. In addition, changes can be made in the design of the meltblowing or spun bond apparatus. References describing such measures include U.S. Patent Nos. 4,889,476, 5,236,641, 5,248,247, 5,260,003, 5,582,907, 5,728,407, 5,891,482 and 5,993,943.
• An extruder and one or more metering gear pumps generally are used to supply fiber- forming material to a meltblowing die.
• the gear pump typically has two counter- rotating meshed gears.
• Wide melt blown nonwoven webs have been formed by arranging a plurality of meltblowing dies in a side-by-side array, and by using a plurality of such gear pumps to deliver molten polymer to the array of dies, see U.S. Patent Nos. 5,236,641 and 6,182,732.
• the '641 patent utilizes sensors and a feedback system to measure a physical property (e.g., thickness or basis weight) of strips of the web, and then alters the speeds of the gear pumps to maintain uniformity of the selected property within the strips or across the width of the web.
• a physical property e.g., thickness or basis weight
• FIG. 1 is a schematic top sectional view of a planetary gear metering pump.
• FIG. 2 is a schematic side view of a planetary gear metering pump.
• FIG. 3 is a schematic perspective view, partially in section, of a meltblowing die incorporating a planetary gear metering pump and a multiple-inlet tee slot meltblowing die cavity.
• Fig. 3a is a schematic side view of the outlet region of the meltblowing die of
• FIG. 4 is a schematic perspective view, partially in section, of a meltblowing die incorporating a planetary gear metering pump and an array of fish tail meltblowing die cavities in a side-by-side relationship.
• Fig. 5 is a schematic perspective view, partially in section, of a meltblowing die incorporating a planetary gear metering pump and an array of coathanger meltblowing die cavities in a side-by-side relationship.
• Fig. 6 is a schematic perspective view, partially in section, of a meltblowing die incorporating a planetary gear metering pump and an array of substantially uniform residence time meltblowing die cavities in a side-by-side relationship.
• Fig. 7a is top sectional view of a die cavity of Fig. 6.
• Fig. 7b is a side sectional view of the die of Fig. 7a, taken along the line 7b-
• Fig. 7c is a schematic perspective sectional view of the die of Fig. 7a.
• Fig. 8 is an exploded view of another meltblowing die incorporating a planetary gear metering pump.
• Fig. 9 is a schematic perspective view, partially in phantom, of a meltblowing die incorporating a planetary gear metering pump connected to an array of meltblowing die cavities in a vertically stacked relationship.
• meltblowing requires particularly high temperatures. These high temperatures can be very hard on meltblowing dies and other associated equipment, including the above-described gear pumps. Occasionally pump breakdowns will occur. Periodic pump maintenance is required in any event. When a set of gear pumps is employed, it is difficult to maintain them so that they all have the same tolerances and operating conditions. For these and other reasons it can be very difficult to obtain uniform nonwoven webs in a factory setting, especially when forming wide melt blown nonwoven webs using a multiple metering pump system, and whether or not a pump feedback system is employed. [0019] Although useful, macroscopic nonwoven web properties such as basis weight, average fiber diameter, web thickness or porosity may not always provide a sufficient basis for evaluating nonwoven web quality or uniformity.
• a more uniform web could be obtained if each stream of fiber-forming material delivered to a meltblowing die cavity or array of such die cavities had the same or substantially the same physical or chemical properties as it entered the die cavity or array. Uniformity of such physical or chemical properties can be facilitated by subjecting the ' fiber-forming material streams to the same or substantially the same pumping conditions, thereby exposing the fiber- forming material to a more uniform thermal history before it reaches the die or array.
• the extruded filaments that later exit the die or array may have more uniform physical or chemical properties from filament to filament, and after attenuation and collection may form higher quality or more uniform melt blown nonwoven webs.
• the desired filament physical property uniformity preferably is evaluated by determining one or more intrinsic physical or chemical properties of the collected fibers, e.g., their weight average or number average molecular weight, and more preferably their molecular weight distribution.
• Molecular weight distribution can conveniently be characterized in terms of polydispersity.
• the present invention provides, in one aspect, a method for forming a fibrous web comprising supplying fiber-forming material to a planetary gear metering pump having a plurality of outlets, flowing fiber-forming material from the pump outlets through a plurality of inlets in one or more die cavities, and meltblowing the fiber-forming material to form a nonwoven web.
• the method employs a plurality of such die cavities arranged to provide a wider or thicker web than would be obtained using only a single such die cavity.
• the invention provides a meltblowing apparatus comprising a planetary gear metering pump having a plurality of fiber-forming material outlets connected to a plurality of fiber-forming material inlets in one or more die cavities of one or more meltblowing dies.
• the meltblowing die comprises a plurality of die cavities arranged to provide a wider or thicker web than would be obtained using only a single such die cavity.
• nonwoven web refers to a fibrous web characterized by entanglement, and preferably having sufficient coherency and strength to be self-supporting.
• meltblowing means a method for forming a nonwoven web by extruding a fiber-forming material through a plurality of orifices to form filaments while contacting the filaments with air or other attenuating fluid to attenuate the filaments into fibers and thereafter collecting a layer of the attenuated fibers.
• meltblowing temperatures refers to the meltblowing die temperatures at which meltblowing typically is performed. Depending on the application, meltblowing temperatures can be as high as 315°C, 325°C or even 340°C or more.
• meltblowing die refers to a die for use in meltblowing.
• melt blown fibers refers to fibers made using meltblowing.
• the aspect ratio (ratio of length to diameter) of melt blown fibers is essentially infinite (e.g., generally at least about 10,000 or more), though melt blown fibers have been reported to be discontinuous.
• the fibers are long and entangled sufficiently that it is usually impossible to remove one complete melt blown fiber from a mass of such fibers or to trace one melt blown fiber from beginning to end.
• the phrase "attenuate the filaments into fibers” refers to the conversion of a segment of a filament into a segment of greater length and smaller diameter.
• polydispersity refers to the weight average molecular weight of a polymer divided by the number average molecular weight of the polymer, with both weight average and number average molecular weight being evaluated using gel permeation chromatography and a polystyrene standard.
• fibers having substantially uniform polydispersity refers to melt blown fibers whose polydispersity differs from the average fiber polydispersity by less than ⁇ 5%.
• shear rate refers to the rate in change of velocity of a nonturbulent fluid in a direction perpendicular to the velocity. For nonturbulent fluid flow past a planar boundary, the shear rate is the gradient vector constructed pe ⁇ endicular to the boundary to represent the rate of change of velocity with respect to distance from the boundary.
• the phrase “residence time” refers to the flow path of a fiber-forming material stream through a die cavity divided by the average stream velocity.
• substantially uniform residence time refers to a calculated, simulated or experimentally measured residence time for any portion of a stream of fiber- forming material flowing through a die cavity that is no more than twice the average calculated, simulated or experimentally measured residence time for the entire stream.
• planetary gear metering pump 1 employs a so-called planetary or epicyclic gearset inside the pump.
• a rotating driving or sun gear 2 is surrounded by and engaged with a plurality of driven or planet gears 3 through 6.
• Fig. 2 shows a side view of pump 1 of Fig. 1.
• Rotating driveshaft 9 passes through seal 10 into the interior of pump 1.
• Fiber-forming material enters pump 1 through inlet port 11, and exits pump 1 through outlets such as outlets 12.
• the body of pump 1 may be made from a plurality of machined plates such as plates 13 through 15.
• a variety of planetary gear metering pumps may be employed in the invention.
• the pump preferably should withstand exposure to fiber-fomiing material at meltblowing temperatures. For some meltblowing applications this will require a relatively robust planetary gear metering pump capable of operating at temperatures as high as 350°C, and may require special pump materials and hardened components.
• Suitable planetary gear metering pumps may have a variety of configurations, with, for example 2, 3, 4, 6, 8 or more outlets per pump, and with various arrangements of the inlet and outlet ports on one or two sides of the pump.
• the pumps can employ static mixer elements at or near one or both of the pump inlet and pump outlet. Use of such static mixers can facilitate mixing and distribution of the fiber-forming material.
• Preferred planetary gear metering pumps are described in, for example, “Feinpruef Spinning Pumps” (brochure from Mahr GmbH; The “F 16" alloy Feinpruef pumps are particularly preferred); “Planetary Polymer Metering Pumps” (web page of Slack & Parr, Ltd. at http://www.slack-parr.com/meter_pumps/polymer.htm); “Zenith® Pumps Planetary Gear Pumps” (brochure from the Zenith Pumps Division of Parker Hannifin Corporation). More general disclosure of planetary gear metering pumps can be found in, for example, U.S. Patent Nos. 3,498,230; 5,354,529; 5,637,331 and 5,902,531; and U.K.
• meltblowing the fiber- forming material exiting the die outlet typically has a much higher temperature, a much lower molecular weight and a significantly lower viscosity than molten material exiting a melt-spun die.
• meltblowing the extruded fibers are attenuated in thickness (and thereby lengthened in the extrusion direction) by the action of a high velocity air stream.
• melt-spinning an attenuating air stream typically is not employed.
• the fiber-forming material may be significantly thinned or even thermally degraded by passage through the pumps, by passage through the meltblowing die, by the high temperatures required to reach the desired low melt viscosity or by the stream of air or other attenuating fluid.
• melt-spinning the extent of thinning or thermal degradation is believed to be much less extensive. The temperatures and forces associated with meltblowing thus tend to magnify nonuniformities in the final nonwoven product, especially when there are differences in the fiber-forming material thermal history at various parts of the meltblowing process.
• the fiber product obtained by melt-spinning is believed to be much more uniform.
• a planetary gear metering pump to supply one or more meltblowing dies may help reduce variation in the collected product, because the pump supplies each fiber- forming material inlet in a die or array of dies with a fiber- forming material stream having a similar flow rate and thermal history. Because the nature of the melt-blown process magnifies any differences that may be present in the fiber-forming material supply streams, the use of a planetary gear metering pump can provide product uniformity advantages that might not be observed or might not be significant in melt-spun fiber manufacturing. [0038] Fig.
• Fig. 3 shows a meltblowing apparatus 20 of the invention that includes a planetary gear metering pump 21 whose four outlets 22a through 22d supply fiber- forming material via conduits 23a through 23d to inlets 24a through 24d of tee slot die cavity 25 in die body 26.
• Die cavity 25 includes manifold 27 and slot 28.
• Fig. 3a is sectional side view of the outlet region of die cavity 25 of Fig. 3, taken along the line 3a-3a ⁇ As shown in Fig.
• the fiber-forming material (which undergoes considerable heat-induced viscosity reduction or even thermal degradation and usually a molecular weight change due to passage through the die cavity) exits die cavity 25 at die tip 27 through a row of side-by-side orifices such as orifice 29 drilled or machined in die tip 27 to produce a series of filaments 31.
• High velocity attenuating fluid e.g., air
• orifices 32a and 32b is supplied under pressure to orifices such as orifices 32a and 32b from plenums 33a and 33b adjacent die tip 27.
• the fluid attenuates the filaments 31 into elongated and reduced diameter fibers 34 by impinging upon, drawing down and possibly tearing or separating the filaments 31.
• the fibers 34 are collected at random on a remotely-located collector such as a moving screen 36 or other suitable surface to form a coherent entangled nonwoven web 38.
• the fiber-forming material streams delivered to inlets 24a through 24d of die cavity 25 all have a similar thermal history, thus promoting the formation of fibers 34 having substantially uniform fiber physical or chemical properties. Further details regarding the manner in which meltblowing would be carried out with such an apparatus can be found, for example, in Wente, Van A., "Superfine Thermoplastic Fibers" in Industrial Engineering Chemistry, Vol. 48, p. 1342 et seq. (1956), or in Report No. 4364 of the Naval Research Laboratories, published May 25, 1954, entitled “Manufacture of Superfine Organic Fibers," by Wente, V.
• Fig. 4 shows a meltblowing apparatus 40 of the invention that includes a planetary gear metering pump 41 whose three outlets 42b, 42d and 42f located on the top of pump 41 and three further outlets located at the bottom of pump 41 (not shown in Fig. 4) supply fiber-forming material via conduits 43a through 43f to inlets 44a through 44f of an array of six fish tail die cavities 45a through 45f arranged in a side-by-side relationship in die body 46.
• Each fish tail die includes a manifold such as manifold 47a. The dies share a common slot 48.
• Fig. 5 shows a meltblowing apparatus 50 of the invention that includes a planetary gear metering pump 51 whose three outlets located at the bottom of pump 51 (not shown in Fig. 5) supply fiber-forming material via conduits 53a through 53c to inlets 54a through 54c of three coathanger die cavities 55a through 55c arranged in a side-by- side relationship in die body 56.
• Each die cavity includes a manifold such as manifold 57a.
• Fig. 6 shows a top sectional view of a substantially uniform residence time meltblowing apparatus 60 that has particular utility for use in a meltblowing system of the invention.
• Apparatus 60 includes a planetary gear metering pump 61 whose four outlets 62a through 62d located at the top of pump 61 supply fiber-forming material via conduits 63a through 63d to inlets 64a through 64d of four die cavities 66a through 66d arranged in a side-by-side relationship in die body 66. Fiber-forming material flows from the outlets of pump 61 through the die body inlets and thence through each die cavity as described in more detail below.
• Fig. 7a shows a schematic top sectional view of die cavity 66a of Fig. 6.
• Fiber- forming material enters die body 66 via inlet 64a and flows through manifold 72 along manifold arm 72a or 72b.
• Manifold arms 72a and 72b preferably have a constant width and variable depth.
• Some of the fiber- forming material exits die cavity 66a by passing through manifold arm 72a or 72b and through orifices such as orifice 78a or 78b machined or drilled in die tip 77.
• the remaining fiber- forming material exits die cavity 66a by passing from manifold arm 72a or 72b into slot 73 and through orifices such as orifice 78 in die tip 77.
• the exiting fiber-forming material produces a series of filaments 67.
• a plurality of high velocity attenuating fluid streams supplied under pressure from orifices (not visible in Fig. 3) near die tip 77 attenuate the filaments 67 into fibers 68.
• the fibers 68 are collected at random on a remotely-located collector such as a moving screen 69 or other suitable surface to form a coherent entangled nonwoven web 69a.
• Fig. 7b shows a cross-sectional view of the die 48 of Fig. 3, taken along the line 7b-7b'.
• Manifold arm 72a has a variable depth H that ranges from a maximum near inlet 64a to a minimum near the ends of manifold arms 72a and 72b.
• Slot 73 has fixed depth h. Fiber- forming material passes from manifold arm 72a into slot 73 and exits die cavity 66a through orifice 78 in die tip 77 as filament 67. Air knife 74 overlays die tip 77. Die tip 77 is removable and preferably is split into two matching halves 77a and 77b, permitting ready alteration in the size, arrangement and spacing of the orifices 78. A pressurized stream of attenuating fluid can be supplied from plenums 79a and 79b in the exit face of die cavity 66a through orifices 79c and 79d in air knife 74 to attenuate the extruded filaments 67 into fibers. [0045] Fig.
• FIG. 7c shows a perspective sectional view of meltblowing die 48. For clarity, only the lower half 77b of die tip 77 is shown, and air knife 74 has been omitted from Fig. 7c. The remaining elements of Fig. 7c are as in Fig. 7a and Fig. 7b.
• Die cavities such as die cavity 66a may be designed with the aid of equations discussed in more detail below and in copending Application Serial No. 10/177,446 entitled "NONWOVEN WEB DIE AND NONWOVEN WEBS MADE THEREWITH", filed June 20, 2002.
• the equations can provide an optimized nonwoven die cavity design having a uniform residence time for fiber-forming material passing through the die cavity.
• the filaments exiting such a die cavity preferably have uniform physical or chemical properties after they have been attenuated, collected and cooled to form a nonwoven web.
• die 66a of Fig. 7a is much deeper from the fiber-forming material inlet to the filament outlet for a given die cavity width.
• Die cavities such as die cavity 66a may be scaled to a variety of sizes to form nonwoven webs of various desired web widths
• forming wide webs e.g., widths of about one-half meter or more
• Wide webs of the invention preferably have widths of 0.5, 1, 1.5 or even 2 meters or more and preferably are formed using a plurality of die cavities arranged to provide a wider web than would be obtained using only a single such die cavity.
• a nonwoven die of the invention that is substantially planar
• a plurality of die cavities preferably are arranged in a side-by-side relationship as shown, for example, in Fig. 6.
• a die such as that shown in Fig.
• the die cavity outlet is angled away from the plane of the die slot.
• Die 80 includes upright base 81 which is fastened to die body 82 via bolts (not shown in Fig. 8) through bolt holes such as hole 84a.
• Die body 82 and base 81 are fastened to air manifold 83 via bolts (also not shown in Fig. 8) through bolt holes such as holes 84b and 84c.
• Die body 82 includes a contiguous array of eight die cavities 85a through 85h like that shown in Fig. 3, each of which preferably is machined to identical dimensions. Die cavities 85a through 85h share a common die land 89.
• Die cavity 85a includes manifold 86a, slot 87a and inlet port 88a.
• Air manifold 83 includes inlet ports 94a and 94b through which air can be conducted via internal passages (not shown in Fig. 8) to plenums 95a and 95b and thence to air knife 92. Insulation pads 96a and 96b help maintain apparatus 80 at a uniform temperature.
• two 4- port planetary gear metering pumps 97a and 97b supply fiber-forming material through distribution chamber 98.
• the use of two pumps facilitates conversion of apparatus 80 to other configurations, e.g., as a die for extrusion of multilayer webs or for extrusion of bicomponent fibers.
• the fiber-forming material is conducted via internal passages (not shown in Fig. 8) in base 81 through ports such as port 99a and then through ports such as port 88a into die cavities 85a through 85h.
• the fiber-forming material passes over die land 89 and makes a right angle turn into a slit (not shown in Fig. 8) in air manifold 83.
• die cavities 85a through 85h are surrounded by machined metal surfaces of ample width that can be firmly clamped to base 81 and air manifold 83.
• heat input devices it would be difficult to place heat input devices in some regions of a die design like that shown in Fig. 8.
• such a die design preferably can be operated with reduced reliance on such heat input devices. This provides greater flexibility in the overall die design and enables the major components, machined surfaces and parting lines in the die to be arranged in a configuration that can be repeatedly assembled and disassembled for cleaning while reducing the likelihood of wear-induced leakage.
• the slit in air manifold 83 conducts the fiber-forming material to orifices drilled or machined in tip 90 whereupon the fiber-forming material exits die 80 as a series of small diameter filaments. Meanwhile, air entering air manifold 83 through ports 94a and 94b impinges upon the filaments, attenuating them into fibers as or shortly after they pass through slit 100 in air knife 92.
• Die cavities having shapes like the tee slot, coathanger and fishtail die cavities shown above or die cavities such as die cavity 66a of Fig. 7a may also be arranged to provide a thicker web than would be obtained using only a single such die cavity.
• a plurality of such die cavities preferably are arranged in a stack to form thick webs.
• Fig. 9 illustrates a meltblowing system 110 of the invention incorporating a vertical stack of die cavities 111, 112 and 113.
• System 110 includes a planetary gear metering pump 51 whose three outlets located at the bottom of pump 51 (not shown in Fig.
• die tips 114, 115 and 116 are shown without the overlying air knives that would direct attenuating fluid from orifices such as orifice 119 onto the filaments exiting orifices such as orifice 118 in die tip 114.
• Die 110 may be used to form three contiguous nonwoven web layers each containing a layer of entangled, attenuated melt blown fibers.
• meltblowing die does not need to be planar.
• a meltblowing apparatus of the invention can employ an annular die having a central axis of symmetry, for forming a cylindrical areay of filaments.
• a die having a plurality of nonplanar (curved) die cavities whose shape if made planar would be like that shown in Fig. 7a can also be arranged around the circumference of a cylinder to form a larger diameter cylindrical array of filaments than would be obtained using only a single annular die cavity of similar die depth.
• a plurality of nested annular nonwoven dies of the invention can also be arranged around a central axis of symmetry to form a multilayered cylindrical array of filaments.
• Preferred meltblowing dies for use in the invention can be designed using fluid flow equations based on the behavior of a power law fluid obeying the equation:
• an x-y coordinate axis has been overlaid upon die cavity 66a, with the x-axis corresponding generally to the die cavity outlet edge (or in other words, the inlet side of die tip 77) and the y-axis corresponding generally to the centerline of die cavity 66a.
• Die cavity 66a has a half width of dimension b and an overall width of dimension 2'b.
• the fluid flow rate Q m (x) in the manifold at position x can be assumed for mass balance reasons to equal the flow rate of material exiting the die cavity between positions x and b, and can also be assumed to equal the average velocity of the fluid in the manifold times the cross-sectional area of the manifold arm:
• Q m (x) is the fluid flow rate in the manifold arm at position x
• v m is the average fluid velocity in the manifold arm
• b is the half width of the die cavity
• v s is the average fluid velocity in the slot h is the slot depth
• H(x) is the manifold arm depth at position x
• W is the manifold arm width.
• the manifold arm width is assumed to be some appreciable dimension, e.g., a width of 1 cm, 1.5 cm, 2 cm, etc.
• a value for the slot depth h can be chosen based on the range of rheologies of the fiber-forming fluids that will flow through the die cavity and the targeted pressure drop across the die.
• the fluid flow in the manifold is assumed to be nonturbulent and occurring in the direction of the manifold arm.
• the fluid flow in the slot is assumed to be laminar and occurring in the -y direction.
• the dotted lines A and B in Fig. 7a represent lines of constant pressure, normal to the fluid flow direction.
• the pressure gradient in the slot is related to the pressure gradient in the manifold arm by the equation: (3) d Yy 'slot ⁇ dt / manifold arn ⁇ where ⁇ is the hypotenuse of the triangle formed by ⁇ x and ⁇ y , shown in Fig. 7a where dotted lines A and B intersect the contour line C between right-hand manifold arm 72b and slot 73.
• the fluid pressure gradient ⁇ p and shear ⁇ w at the die cavity wall can be calculated by assuming steady flow in both the slot and manifold, and neglecting the influence of any fluid exchange. Assuming that the fluid obeys the power law model of viscosity:
• Equation (12) can be used to design the contour of the manifold arm.
• the manifold arm depth H(x) can be calculated using the equation:
• a die cavity designed using the above equations can have a uniform residence time, as can be seen by dividing the numerator and denominator of equation (3) by ⁇ t to yield the equation:
• Equation (14) can be manipulated to give:
• the residence time in the manifold is accordingly the same as the residence time in the slot.
• the fluid experiences not only the same shear rate but also experiences that rate for the same length of time. This promotes a relatively uniform thermal and shear history for the fiber-forming material stream across the width of the die cavity.
• the value for y(x) provided by equation (12) may vary, e.g., by about ⁇ 50%, more preferably by about ⁇ 25%, and yet more preferably by about ⁇ 10% across the die cavity.
• the die cavity manifold arms and die slot can meet within curves defined by the equation:
• residence time does not need to be perfectly uniform across the die cavity.
• the residence time of fiber- forming material streams within the die cavity need only be substantially uniform. More preferably, the residence time of such streams is within about ⁇ 50% of the average residence time, more preferably within about ⁇ 10% of the average residence time.
• a tee slot die or coathanger die typically exhibits a much larger variation in residence time across the die.
• the residence time may vary by as much as 200% or more of the average value, and for coathanger dies the residence time may vary by as much as 1000% or more of the average value.
• meltblowing systems incorporating die cavities like the design shown in Fig. 7a the shear rate at the die cavity wall and the shear stress experienced by the flowing fiber-forming material can be the same or substantially the same for any point on the wetted surface of the die cavity wall.
• meltblowing systems incorporating a planetary gear metering pump and such die cavities relatively insensitive to alteration in the viscosity or mass flow rate of the fiber-forming material, and can enable such meltblowing systems to be used with a wide variety of fiber-forming materials and under a wide variety of operating conditions. This also can enable such meltblowing systems to accommodate changes in such conditions during operation of the system.
• Preferred meltblowing systems of the invention can be used with viscoelastic, shear sensitive and power law fluids.
• Preferred meltblowing systems of the invention may also be used with reactive fiber-forming materials or with fiber-forming materials made from a mixture of monomers, and may provide uniform reaction conditions as such materials or monomers pass through the die cavity.
• the constant wall shear stress provided by such preferred meltblowing systems may promote a uniform scouring action throughout the die cavity, thus facilitating thorough and even cleaning action.
• It may be prefe ⁇ ed to supply identical streams of attenuating fluid to each extruded filament.
• the attenuating fluid preferably is supplied using an adjustable attenuating fluid manifold as described in copending Application Serial No.
• Preferred meltblowing systems of the invention may be operated using a flat temperature profile, with reduced reliance on adjustable heat input devices (e.g., electrical heaters mounted in the die body) or other compensatory measures to obtain uniform output. This may reduce thermally generated stresses within the die body and may discourage die cavity deflections that could cause localized basis weight nonuniformity. Heat input devices may be added to the dies of the invention if desired. Insulation may also be added to assist in controlling thermal behavior during operation of the die. [0064] Preferred meltblowing systems of the invention can produce highly uniform webs.
• adjustable heat input devices e.g., electrical heaters mounted in the die body
• Heat input devices may be added to the dies of the invention if desired. Insulation may also be added to assist in controlling thermal behavior during operation of the die.
• preferred meltblowing systems of the invention may provide nonwoven webs having basis weight uniformities of ⁇ 2% or better, or even ⁇ 1% or better.
• preferred meltblowing systems of the invention may provide nonwoven webs comprising at least one layer of melt blown fibers whose polydispersity differs from the average fiber polydispersity by less than ⁇ 5%, more preferably by less than ⁇ 3%.
• a variety of synthetic or natural fiber- forming materials may be made into nonwoven webs using the meltblowing systems of the invention.
• Preferred synthetic materials include polyethylene, polypropylene, polybutylene, polystyrene, polyethylene terephthalate, polybutylene terephthalate, linear polyamides such as nylon 6 or nylon 11, polyurethane, poly (4-methyl pentene-1), and mixtures or combinations thereof.
• Preferred natural materials include bitumen or pitch (e.g., for making carbon fibers).
• the fiber- forming material can be in molten form or carried in a suitable solvent.
• Reactive monomers can also be employed in the invention, and reacted with one another as they pass through the pump or into or through the die.
• the nonwoven webs may contain a mixture of fibers in a single layer (made for example, using two closely spaced die cavities sharing a common die tip), a plurality of layers (made for example, using a die such as shown in Fig. 7), or one or more layers of multicomponent fibers (such as those described in U.S. Patent No. 6,057,256).
• the fibers in nonwoven webs made using the meltblowing systems of the invention may have a variety of diameters.
• the fibers may be ultrafine fibers averaging less than 5 or even less than 1 micrometer in diameter; microfibers averaging less than about 10 micrometers in diameter; or larger fibers averaging 25 micrometers or more in diameter.
• the nonwoven webs made using the meltblowing systems of the invention may contain additional fibrous or particulate materials as described in, e.g., U.S. Patent Nos. 3,016,599, 3,971,373 and 4,111,531.
• Other adjuvants such as dyes, pigments, fillers, abrasive particles, light stabilizers, fire retardants, absorbents, medicaments, etc., may also be added to the nonwoven webs.
• the addition of such adjuvants may be carried out by introducing them into the fiber-forming material stream, spraying them on the fibers as they are formed or after the nonwoven web has been collected, by padding, and using other techniques that will be familiar to those skilled in the art.
• the completed nonwoven webs may vary widely in thickness. For most uses, webs having a thickness between about 0.05 and 15 centimeters are preferred.
• two or more separately or concurrently formed nonwoven webs may be assembled as one thicker sheet product.
• a laminate of spun bond, melt blown and spun bond fiber layers (such as the layers described in U.S Patent No. 6,182,732) can be assembled in an SMS configuration.
• Nonwoven webs may also be prepared using the meltblowing systems of the invention by depositing the stream of fibers onto another sheet material such as a porous nonwoven web that will form part of the completed web.
• the nonwoven webs may be further processed after collection, e.g., by compacting through heat and pressure to cause point bonding, to control sheet caliper, to give the web a pattern or to increase the retention of particulate materials.
• the nonwoven webs may be electrically charged to enhance their filtration capabilities as by introducing charges into the fibers as they are formed, in the manner described in U.S. Pat. No. 4,215,682, or by charging the web after formation in the manner described in U.S. Pat. No. 3,571,679.
• the nonwoven webs made using the meltblowing systems of the invention may have a wide variety of uses, including filtration media and filtration devices, medical fabrics, sanitary products, oil adsorbents, apparel fabrics, thermal or acoustical insulation, battery separators and capacitor insulation.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Textile Engineering (AREA)
• Nonwoven Fabrics (AREA)
• Spinning Methods And Devices For Manufacturing Artificial Fibers (AREA)
• Rotary Pumps (AREA)
• Lasers (AREA)
• Semiconductor Lasers (AREA)

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