NEWMAN, Marc, Steven (102 Vicksburg Court, Simpsonville, SC, 29681, US)
GILLESPIE, Jay, Darrell (502 Jackson Downs Boulevard, Nashville, TN, 37214, US)
NEWMAN, Marc, Steven (102 Vicksburg Court, Simpsonville, SC, 29681, US)
| CLAIMS 1. A meltblown fiber spinning device comprising: a spin block having an inlet side; an outlet side opposite the inlet side; a discharge surface disposed at the outlet side of the spin block; a flow channel formed at the inlet side of the spin block for receiving molten polymer from a polymer source; an inlet surface formed in the flow channel towards the outlet side of the spin block; and a plurality of spaced apart capillaries formed in the inlet surface of the spin block, the capillaries having an inlet end in fluid communication with the flow channel and a discharge end at the discharge surface of the spin block, wherein the capillaries have a diameter (D) in the range of from 2-10 mils and a length (L), wherein the L/D ratio is at least 1 and less than 20, and wherein the number of capillaries is in the range of from 20-200 capillaries per inch. 2. The spinning device of claim 1 , wherein the capillaries have a diameter in the range of from 2-6 mils. 3. The spinning device of claim 1 or 2, wherein the capillaries have an L/D ratio which is at least 2 and less than 20. 4. The spinning device of any one of claims 1-3, wherein the number of capillaries is in the range of from 50-200 capillaries per inch. 5. The spinning device according to any one of claims 1-4, wherein the capillaries are cone-shaped capillaries. 6. The spinning device of any one of claims 1-5, wherein the discharge surface has a width extending laterally of the spin block that is at least 3 mils. 7. The spinning device of claim 1 , wherein the spin block has a one-piece construction. 8. The spinning device of any one of claims 1-7, wherein the discharge surface has a width extending laterally of the spin block that is from about 5 to 10 mils. 9. The spinning device of any one of claims 1-8, wherein the inlet end of each capillary is spaced apart from each of the inner side walls of the spin block by at least 1 mil. 10. The spinning device of any one of claims 1-9, wherein the spin block is an elongate, longitudinally extending spin block having two opposing ends; the inlet side extends longitudinally from one of said ends to the other end; the outlet side extends longitudinally from one of said ends to the other end of the spin block; the spin block has a pair of convergent outer side walls extending longitudinally of the spin block and defining a portion of the outlet side of the spin block having a longitudinally extending triangular cross-section; the discharge surface is a longitudinally extending discharge surface which extends between the pair of convergent outer side walls; the flow channel is a longitudinally extending flow channel which includes a pair of opposing inner walls disposed in an interior space of the spin block, the pair of opposing inner walls each extending longitudinally of the spin block; and the inlet surface extends longitudinally of the spin block; 11. The spinning device of claim 10, wherein the inner side walls converge towards each other in the direction of polymer flow through the spin block. 12. The spinning device of claim 11 , wherein an angle defined by each of the inner side walls and an axis extending between the inlet and outlet sides of the spin block is between about 18 and 24 degrees. 13. The spinning device of claim 12, wherein the angle is about 20 degrees. 14. The spinning device of any one of claims 10-13, wherein the discharge end of each capillary is spaced apart from each of the convergent outer walls of the spin block by at least 1 mil. 5. The spinning device of any one of claims 10-14, wherein the inner walls of the spin block extend longitudinally from one of said opposing ends of the spin block to the other end of the spin block. 16. The spinning device of any one of claims 10-15, wherein the discharge end of each capillary is spaced apart from each of the convergent outer side walls of the spin block by at least 4 mils. 17. The spinning device of claim 16, wherein the discharge end of each capillary is spaced apart from each of the convergent outer side walls of the spin block by at least 8 mils. 18. The spinning device of any one of claims 1-17, further comprising air knifes for blowing gas from a location downstream of said capillaries to attenuate fibers discharged therefrom. 19. The spinning device of y one of claims 1-18, wherein the number of capillaries is between about 80 and 100 capillaries per inch. 20. A method of forming a meltblown web formed of nanofibers comprising the steps of: introducing a molten polymer into a flow channel of a spin block, the flow channel being formed in an inlet side of the spin block; feeding the molten polymer through a plurality of spaced apart capillaries that have an inlet end in fluid communication with the flow channel and a discharge end at an outlet side of the spin block, wherein the capillaries have a diameter (D) in the range of from 2-10 mils and a length (L), wherein the L/D ratio is at least 1 and less than 20, and wherein the number of capillaries is in the range of from 20- 200 capillaries per inch; discharging strands of polymer through the discharge ends of each of the capillaries; and collecting the strands of polymer on a collection surface to produce a coherent fabric. 21. The method of claim 20, wherein the capillaries are cone-shaped capillaries. 22. The method of claim 20 or 21 , wherein the flow channel is a longitudinally extending flow channel of a spin block; the discharge ends of the capillaries are formed in a longitudinally extending discharge surface disposed at the outlet side of the spin block, the discharge surface being substantially planer and disposed between a pair of convergent outer side walls extending longitudinally of the spin block, and wherein the discharge end of each capillary is spaced apart from each of the convergent outer side walls. 23. The method of any one of claims 20-22, wherein the polymer is fed at a throughput of between about 2 and 60 kg/meter/hour. 24. The method of claim 22 wherein the polymer is fed at a throughput of between about 10 and 30 kg/hour. 25. The method of any one of claims 20-24, wherein the strands have a diameter that is less than 1 ,200 nanometers. 26. The method of claim 25, wherein the strands have a diameter that is less than 500 nanometers. 27. The method of any one of claims 20-26, wherein the capillaries have a length L, and wherein the capillaries have an L/D ratio that is at least 5 and less than 20. 28. The method of any one of claims 20-27, wherein the capillaries have a length L that is between about 20 and 30 mils. 29. The method of any one of claims 20-28, wherein the converging outer side walls defining a portion of the outlet side of the spin block having a longitudinally extending triangular cross-section. 30. The method of any one of claims 20-29, further comprising the step of subjecting the strands to a stream air prior to being collected on the collecting surface to produce fibers having discrete lengths. 31. A web obtainable by a method according to any one of claims 20-30. 32. The web according to claim 31 having a mean pore size in the range of from 5-15 μιη. 33. The web according to claim 31 or 32 having a TSI efficiency in the range of from 20-60%. 34. The web according to any one of claims 31-33 having a Textest air permeability in the range of from 40-100%. 35. The web according to claim 31 having a mean pore size in the range of from 5-15 Mm, a TSI efficiency in the range of from 20-60%, and a Textest air permeability in the range of from 40-100%. 36. A product comprising a web according to any one of claims 31-35. 37. A product according to claim 36 which product is a filter. |
FIELD OF THE INVENTION
The present invention relates generally to dies for preparing meltblown fibers.
BACKGROUND OF THE INVENTION
Meltblown fibers, owing to their fine size, are used in a wide variety of applications including filtration, clothing, medical garments and fabrics, sanitary products, and oil absorbents, to name but a few. Meltblown fibers are prepared in a process in which strands of molten polymer are extruded from a die and then subjected to a stream of high velocity air that attenuates the molten polymer strands and forms fibers. The resulting meltblown fibers have diameters that generally range from 1 to 15 μιη. The meltblown fibers are then randomly deposited on a collection surface to produce a nonwoven sheet material.
A conventional meltblowing system includes an extruder, a polymer distribution system, and meltblowing die block. A typical melt-blowing die is disclosed in US
3,825,380 and comprises an elongate die nosepiece having a hollow interior defining an elongate polymer distribution channel that extends longitudinally along the length of the die. The polymer distribution system introduces molten polymer along the length of the distribution channel. Exterior surfaces of the die define a generally triangular cross- sectional configuration forming an apex edge where a series of longitudinally spaces apart die orifices are formed connecting with the elongate polymer distribution channel. The orifices extend from the distribution channel to the outlet side of the die block from which the molten polymer streams are discharged. Typically, the orifices have a diameter that can range from 12 to 15 mils in diameter and have a density that is between about 30 to 35 orifices per inch. An air manifold, also referred to as an air knife, is positioned adjacent to the outlet side of the orifices to direct a stream of high velocity air against the molten polymer filaments being discharged from the orifices.
Increasingly, there is a desire to produce fibers having a diameter even finer diameter than meltblown fibers. In particular, there is a desire to produce nanofibers having a diameter less than 1 ,200 nm. Nanofibers are known to have many useful properties making them desirable for use in a variety of different applications. For example, nanofibers have been shown to provide increases in filtration efficiency without sacrificing permeability.
One common technique of producing nanofibers is electrospinning. In
electrospinning, a polymer is typically dissolved in a solvent and is placed in a small capillary tube. Voltage is applied between one end of the tube and a grounded collection surface which acts as a pulling force to pull a jet of the polymer solution in the direction of the collection surface. The jets form fine polymeric filaments having diameters less than 500 nm. Electrospinning can be used to produce nanofibers with a diameter as low as 50 nanometers. However, the production rate for electrospinning is generally very low, making this technique less desirable for commercial production. For example, in electrospinning the polymer throughput is typically less than 0.09 kg/hr./meter. Further, electrospinning does not lend itself to easily switching between different polymers. As a result, electrospinning provides a less than desired solution for the production of nanofibers.
Most of the prior art manufacturing devices for producing meltblown fibers do not allow the formation of nanofibers as their spinning orifices are too coarse. Therefore fibers with an average diameter below about 1 m can not be produced with these conventional devices.
Other prior art manufacturing devices for preparation of melt blown fibers do allow the formation of nanofibers. For example, U.S. Patent Publication No. 2008/0023888, entitled Method and Apparatus for Production of Meltblown Nanofibers, describes a die for preparing meltblown nanofibers. The die utilizes a pair of cooperating plates having spin holes that are formed by grooves in the surface of each plate. The die uses a polymer distribution system in which molten polymer is fed into the plates through a die manifold that laterally feeds the polymer between the plates. This system requires a large L/D (length divided by diameter) ratio, preferably between 100 and 1000, to ensure adequate bonding between the plates during operation. Moreover, the system uses a relatively large meltblowing die that cannot be used with melt distribution systems in conventional meltblowing dies. As a result, this system requires a significant capital investment in order to produce meltblown nanofibers. Moreover, said device is operated with a throughput or polymer flow rate that is very low.
Accordingly, there exists a need for a spinning device that can be used with existing conventional meltblown systems, which is capable of producing excellent meltblown nanofibers, and which can be operated with high polymer throughputs. BRIEF SUMMARY OF THE INVENTION
The present invention provides a meltblown fiber spinning device comprising: a spin block having an inlet side; an outlet side opposite the inlet side;
a discharge surface disposed at the outlet side of the spin block;
a flow channel formed at the inlet side of the spin block for receiving molten polymer from a polymer source;
an inlet surface formed in the flow channel towards the outlet side of the spin block; and a plurality of spaced apart capillaries formed in the inlet surface of the spin block, the capillaries having an inlet end in fluid communication with the flow channel and a discharge end at the discharge surface of the spin block, wherein the capillaries have a diameter (D) in the range of from 2-10 mils and a length (L), wherein the L/D ratio is at least 1 and less than 20, and wherein the number of capillaries is in the range of from 20- 200 capillaries per inch.
The present a melt spinning device is capable of producing meltblown nanofibers having excellent performance properties and that can advantageously be used in a preexisting conventional meltblowing apparatus. In one embodiment, the present invention provides a melt spinning device comprising an elongate, longitudinally extending spin block having two opposing ends, an inlet side extending longitudinally from one of the ends to the other end, and an outlet side opposite the inlet side that extends longitudinally from one end of spin block to the other end of the spin block. The inlet side of the spin block suitably includes a longitudinally extending flow channel formed therein for receiving molten polymer from a polymer source. The spin block suitably includes an interior space having a pair of opposing inner walls disposed therein that extend in the longitudinal direction of the spin block and define the flow channel. A longitudinally extending inlet surface is suitably formed in the flow channel towards the outlet side of the spin block. Preferably, the inlet surface is relatively planar and includes a width that extends laterally of the spin block between the pair of opposing inner walls.
The outlet side of the spin block suitably includes a pair of convergent outer side walls that extend longitudinally of the spin block and define a portion of the outlet side of the spin block having a longitudinally extending triangular cross-section. The outer side walls suitably converge toward each other and terminate in a longitudinally extending discharge surface that is disposed at the outlet side of the spin block. The discharge surface preferably includes a width that extends between the pair of convergent outer side walls, and extends laterally of the spin block. A plurality of longitudinally spaced- apart capillaries are suitably formed in the inlet surface of the spin block for the formation of fine filaments. Each capillary includes an inlet end in fluid communication with the flow channel and a discharge end at the outlet side of the spin block. The present invention also provides method of forming a meltblown web formed of nanofibers comprising the steps of:
introducing a molten polymer into a flow channel of a spin block, the flow channel being formed in an inlet side of the spin block;
feeding the molten polymer through a plurality of spaced apart capillaries that have an inlet end in fluid communication with the flow channel and a discharge end at an outlet side of the spin block, wherein the capillaries have a diameter (D) in the range of from 2-10 mils and a length (L), wherein the L/D ratio is at least 1 and less than 20, and wherein the number of capillaries is in the range of from 20-200 capillaries per inch;
discharging strands of polymer through the discharge ends of each of the capillaries; and
collecting the strands of polymer on a collection surface to produce a coherent fabric.
Molten polymer introduced into the flow channel flows into the capillaries and is then extruded from the discharge ends of the capillaries as strands of fine filaments. The strands can then be collected on a collection surface to produce a nonwoven web. In a preferred embodiment, a stream of attenuating heated air is applied to the thus formed strands to form meltblown nanofibers having a diameter of less than 1 ,200 nanometers, preferably a diameter less than 1 ,000 nanometers, more preferably a diameter in the range of from 25-500 nanometers, and most preferably a diameter in the range of from 50-250 nanometers.
The discharge end of the capillaries are suitably spaced apart from the convergent side walls on the outlet side of the spin block by at least 1 mil (1 mil =0.001 inch), with a spacing of 2 to 10 mils being more preferred. The discharge surface is disposed downstream of the inlet surface of the flow channel. Preferably, the discharge surface and the inlet surface have substantially planar surfaces that are parallel to each other.
In accordance with the present invention the capillaries have a diameter that is in the range of from 2-10 mils. In the context of the present invention the diameter of the capillaries is defined as the defined as the diameter of the discharge ends of the capillaries. In the production of meltblown nanofibers, the capillaries preferably have a diameter in the range of from 2 to 6 mils. Preferably, the diameter of the capillaries is in the range of from 2-5 mils. The length of the capillaries is selected so that the ratio (L/D) of the length L of each capillary to the diameter D of the capillary is at least 1 and less than 20. Preferably, the capillaries have an L/D ratio which at least 2 and less than 20. More preferably, the L/D ratio of the capillaries is at least 5 and less than 20. In one preferred embodiment, the L/D ratio is at least 8 and less than 20. The capillaries can suitably be arranged in an array in the longitudinal direction of the spin block. The number of capillaries ranges from 20 to 200 capillaries per inch. Suitably, the number of capillaries is more than 35 and up to 200. Preferably, the number of capillaries ranges of from 40-200 capillaries per inch. More preferably, 50-200. Even more preferably 60-120, and most preferably of from 80-100 capillaries per inch. The use of capillaries having a small diameter, particularly in combination with a high density, result in very attractive performances of the spinning die.
The present invention also provides a web obtainable with the method according to the present invention. Suitably, the web according to the present invention has a mean pore size in the range of from 5-15 μιη, preferably 5-13 pm, and more preferably 7-13 pm. The present invention also provides a web formed of nanofibers, which web has a mean pore size in the range of from 5-15 μητι, preferably 5-13 pm, and more preferably 7-13 pm. The present invention also relates to a web obtainable by the method according to the present invention and having a TSI efficiency in the range of from 20-60%, preferably 25- 50%. The present invention also relates to a web formed of nanofibers, which web has a TSI efficiency in the range of from 20-60%, preferably 25-50%. In addition, the present invention also relates to a web obtainable by the method according to the present invention and having a Textest air permeability in the range of from 40-100%, preferably in the range of from 50-90%. The present invention also relates to a web formed of nanofibers, which web has a Textest air permeability in the range of from 40-100%, preferably in the range of from 50-90%. In addition, the present invention provides a web obtainable by the method according to the present invention and having a mean pore size in the range of from 5-15 μιτι, preferably 5-13 pm, and more preferably 7-13 pm, and a TSI efficiency in the range of from 20-60%, preferably 25-50% . The present invention also relates to a web formed of nanofibers, which web has a mean pore size in the range of from 5-15 μητι, preferably in the range of from 5-13 pm, and more preferably in the range of from 7-13 pm, and a TSI efficiency in the range of from 20-60%, preferably 25- 50%. The present invention also provides a web obtainable by the method according to the present invention and having a mean pore size in the range of from 5-15 pm, preferably in the range of from 5-13 pm, and more preferably in the range of from 7-13 pm, and a Textest air permeability in the range of from 40-100%, preferably in the range of from 50-90%. The present invention also provides a web formed of nanofibers, which web has a mean pore size in the range of from 5-15 pm, preferably in the range of from 5- 13 pm, and more preferably in the range of from 7-13 pm , and a Textest air permeability in the range of from 40- 00%, preferably in the range of from 50-90%. The present invention also relates to a web obtainable by the method according to the present invention and having a Textest air permeability in the range of from 40-100%, preferably in the range of from 50-90%, and a TSI efficiency in the range of from 20-60%, preferably 25-50%. The present invention also provides a web formed of nanofibers, which web has a TSI efficiency in the range of from 20-60%, preferably 25-50%, and a Textest air permeability in the range of from 40-100%, preferably in the range of from 50-90%. The present invention also provides a web obtainable by the method according to the present invention and having a mean pore size in the range of from 5-15 pm, preferably in the range of from 5-13 pm, and more preferably in the range of from 7-13 μητι, a TSI efficiency in the range of from 20-60%, preferably 25-50%, and a Textest air permeability in the range of from 40-100%, preferably in the range of from 50-90%. The present invention also provides a web formed of nanofibers, which web has a mean pore size in the range of from 5-15 pm, preferably in the range of from 5-13 pm, and more preferably in the range of from 7-13 pm, a TSI efficiency in the range of from 20-60%, preferably 25-50%, and a Textest air permeability in the range of from 40-100%, preferably in the range of from 50-90%. In accordance with the present invention the TSI efficiency is measured at a flow rate of 32 liter per minute utilizing 0.3 micron salt particles that are introduced into the air stream, whereby the test is performed with a TSI 8130 Automatic Filter Tester. In accordance with the present invention, the Textest air permeability is determined according to ASTM D737-04, and the mean pore size is determined according to ASTM E1294-89 with test instrument CFP-1200A manufactured by Porous Materials
Incorporated.
The web in accordance with the present invention has preferably a basic weight in the range of from 5-11 grams/m 2 . Such a basic weight has a positive impact on the uniformity of the web resulting in improved TSI efficiencies and Textest air permeabilities. The present invention further provides a product comprising a web according to the present invention. Such a product can suitably be a filter. Therefore, the present invention also provides a filter comprising a web according to the present invention. The webs in accordance with the present invention display unique properties in terms of drapability and stickiness. The present webs display an improved drapability when compared with webs obtained in a conventional meltblown method as for instance described in US 2008/0023888. Moreover, the present webs are less sticky than those obtained in known electrospinning methods.
As discussed above, the present invention provides a melt spinning device for preparing meltblown nanofibers, a method for forming a meltblown web formed of nanofibers, a web formed of nanofibers, a meltblown web formed of nanofibers obtainable by said method, and a product, in particular a filter, comprising a web according to the present invention. Advantageously, the melt spinning device can be used in combination with conventional meltblown equipment and thus reduces or removes the need for replacing existing metlblown equipment. Moreover, the present invention allows for high polymer throughputs.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S) Having thus described the invention in general terms, reference will now be made to the accompanying drawings, which are not necessarily drawn to scale, and wherein:
FIG. 1 is a perspective top view of a spinning device having a spin block that is in accordance with the present invention;
FIG. 2 is a perspective bottom view of the spinning device of FIG. 1 ;
FIG. 3 is a cross-sectional perspective view of the spin block of FIG. 1 in which the interior space of the spin block can be seen;
FIG. 3A is a magnified partial front view of a portion of the spin block depicted in
FIG. 3;
FIG. 4 is a cross-section side view of a portion of the spin block along line 4-4 of FIG. 1 ;
FIG. 4A is a magnified view of a portion of the outlet side of the spin block depicted in FIG. 4;
FIG. 5 a cross-sectional view of an alternative embodiment of the spin block; FIG. 6 is a cross-sectional view of the spin block of FIG. 1 that includes an air knife disposed adjacent to the outlet side of the spin block;
FIG. 7 is a partial cross-section view of a spin block that includes a filter assembly that is depicted in an exploded state;
FIG. 8 depicts the spin block of FIG. 7 in which a portion of the filter assembly is positioned within the spin block; and
FIG. 9 is a cross-sectional view of a spin assembly that includes the spin block of
FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
The present invention now will be described more fully hereinafter with reference to the accompanying drawings, in which some, but not all embodiments of the invention are shown. Indeed, the invention may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these
embodiments are provided so that this disclosure will satisfy applicable legal
requirements. Like numbers refer to like elements throughout.
With reference to FIGS. 1 and 2, a melt spinning device that can be used for preparing meltblown nanofibers is illustrated and broadly designated by reference number 10. Melt spinning device 10 includes an elongate, longitudinally extending spin block 12 (also known as a die tip) having two opposing ends 14, 16, an inlet side 18 extending longitudinally from one of the ends to the other end, and an outlet side 20 opposite the inlet side and extending longitudinally from one of the ends to the other end of the spin block. Preferably, the spin block has a configuration and shape similar to that of a conventional meltblown spinning die so that it can be used in conjunction with existing equipment for making meltblown fibers. As discussed in greater detail below, the outlet side 20 of the spin block includes a plurality of capillaries from which strands of molten polymer are extruded as fine filaments. The thus extruded filaments may then be subjected to a stream of air that attenuates the filaments to provide meltblown fibers. In some embodiments, the stream of air breaks the filaments to provide meltblown fibers of discrete lengths.
In a preferred embodiment, the outlet side 20 of the spin block includes a pair of outer side walls 22, 24 that converge towards each other in the direction of polymer flow through the spin block. The pair of converging outer side walls 22, 24 defines a portion of the outlet side of the spin block having a generally triangular cross-section 26 that extends longitudinally of the spin block. The angle formed by convergence of the two outer side walls can suitably range between 30° to 90°, with an angle of between 40° to 60° being preferred.
Preferably, the two outer side walls 22, 24 converge at a discharge surface (see briefly FIG. 4A, reference number 28) disposed adjacent to the outlet side 20 of the spin block on the triangular cross-section 26. The discharge surface 28 includes a width that extends laterally between outer side walls 22, 24, and a length that extends longitudinally of the spin block. Preferably, discharge surface 28 has a relatively planar surface.
As shown in FIG. 3, the spin block suitably includes a longitudinally extending flow channel 30 formed at the inlet side 18 of the spin block for receiving molten polymer from a polymer source. As discussed in greater detail below, the spin block includes a plurality of capillaries that extend from the flow channel 30 towards the outlet side of the spin block. Molten polymer introduced into the flow channel flows through the capillaries and is discharged from the spin block through the capillaries as polymeric strands of filaments.
In the illustrated embodiments, the spin block is depicted as including a recess/channel 29 that extends from end 14 to end 16 of the spin block. As discussed in greater detail below, recess/channel 29 can optionally be present for receiving a filter for filtering the molten polymer material prior to introduction into the flow channel. In the case of capillaries having relatively small discharge ends, such as less than 8 mils, it may be desirable to filter the molten polymer with a 200 mesh screen or greater. The flow channel 30 is disposed within the spin block in an interior space 34 that is defined by a pair of longitudinally extending opposing inner walls 36. In FIG. 3 only one of the opposing walls is illustrated so that the internal structure of the spin block can be seen by the reader. In the illustrated embodiment, the spin block includes a longitudinally extending cavity 38 defining the flow channel 30, interior space 34 and the pair of opposing inner walls 36. The flow channel can extend only partially between the opposing ends 14, 16 of the spin block (as depicted in FIG. 3), or may alternatively extend from one end 14 of the spin block to the opposite end 16.
As can best be seen in FIG. 3A, a longitudinally extending inlet surface 32 is disposed in the interior of the spin block adjacent to the flow channel 30. The inlet surface 32 has a width that extends laterally between the opposing inner walls.
Preferably, inlet surface 32 includes a generally planar surface that is oriented
perpendicular to the direction of polymer flow through the spin block. A plurality of longitudinally spaced apart capillaries 40 are disposed along the inlet surface 32. Each capillary 40 includes an inlet end 44 at the inlet surface 32 that is in fluid communication with the flow channel 30 and an opposite discharge end 46 at the outlet side 20 of the spin block. The capillaries 40 define spinning orifices from which strands of polymeric filaments are discharged from the outlet side 20 of the spin block. Preferably, the capillaries 40 extend in a substantially straight manner from the flow channel towards the outlet side of the spin block. That is, the capillaries channels preferably extend perpendicular to the longitudinal dimension of the spin block. In one embodiment, the capillaries preferably extend in a linear array in the longitudinal direction of the spin block.
Referring to FIGS. 4 and 4A, a cross section of the spin block taken along lines 4- 4 of FIG. 1 is illustrated. As briefly noted above, the spin block includes a flow channel 30, an inlet surface 32 and a plurality of capillaries 40 that extend from the flow channel 30 towards the outlet side 20 of the spin block. Each capillary 40 includes an inlet end 44 at the inlet surface 32 of the spin block and a discharge end at an opposite end. In this regard, FIG. 4A illustrates that the inlet ends 44 of the capillaries are formed in the inlet surface 32 of the spin block and the discharge ends 46 are formed in the discharge surface 28 of the spin block so that the capillaries extend from the flow channel 30 to the outlet side 20 of the spin block.
As can best be seen in FIG. 4A, each of the inlet ends 44 of the capillaries are formed in the inlet surface of the spin block and defines an origin of one of the capillaries 40 through which molten polymer flows from the flow channel, through the capillaries, and is discharged as fine strands of filament. The inner side walls of the spin block extend in the direction of polymer flow from the inlet side 18 of the spin block to the inlet surface 32 disposed in the interior space of the spin block. Preferably, each of the inlet ends of the capillaries are spaced apart from the inner side walls 36 by at least 1 mil, and more preferably from 2 to 5 mils.
The discharge ends of the capillaries are formed in discharge surface 28 of the spin block and are spaced apart from the outer side walls 22, 24 defining the triangular cross-section 26 of the spin block. Preferably, the discharge ends of the capillaries 40 are spaced apart from each of the outer side walls 22, 24 by at least 1 mil. In one embodiment, the capillaries are spaced apart from the outlet sides of the side block from 2 to 10 mils, including by at least one of 4, 5, 6, 7, or 8 mils. The discharge surface 28 of the spin block preferably has a lateral width extending laterally of the spin block that is in the range of from 3-16 mils. Preferably, the width is at least 4 mils. More preferably, the width is in the range of from 5-10 mils. In one embodiment, the discharge surface has a width extending laterally of the spin block that is less than 10 mils.
In the production of meltblown nanofibers, the discharge ends of the capillaries generally have a diameter (i.e. the discharge ends have a diameter) that is in the range of from 2-10 mils. In a preferred embodiment, the capillaries have a diameter that is in the range of from 3-8 mils, and more preferably in the range of from 4-6 mils. The length of the capillaries is selected so that the ratio (L/D) of the length L of the capillaries to the diameter D of the capillaries is at least 1 and less than 20. Preferably the L/D ratio of the capillaries is at least 2 and less than 20. More preferably, the L/D ratio is at least 5 and less than 20. In a particularly preferred embodiment, the L/D ratio is between at least 8 and less than 20. In the context of the L/D ratio, the diameter of each capillary is again measured at the discharge end of the capillary. The lengths of the capillaries can suitably range from 510 to 50 mils in length, with a length in the range of from 20-30 mils being preferred.
The spin block typically has a density in the range of from 20-200 capillaries per inch. Suitably, the number of capillaries is more than 35 and up to 200. Preferably, the number of capillaries ranges of from 40-200 capillaries per inch. More preferably, 50-200 capillaries per inch. Even more preferably 60-120, and most preferably of from 80-100 capillaries per inch.
FIG. 5 illustrates an embodiment of the invention in which the inner side walls 36 of the spin block converge towards each other in the flow direction of polymer flow through the spin block. In one embodiment, each of the inner side walls converge towards each other at an angle a that is between 18 and 40 degrees, with an angle a that is between 18 and 24 degrees being preferred. In the illustrated embodiment, angle a is defined by an angle that is between each of the inner side walls 36 and a central axis (identified by reference character X) that extends from inlet and outlet sides of the spin block, through the depth of the spin block so that it passes through the capillaries. In a preferred embodiment, the angle a between the inner side wall 36 and central axis X is 20 degrees. Generally, it is desirable for the inner side walls 36 to have a relatively flat or smooth surface so that the presence of "dead spaces" in the flow channel 30 can be avoided.
FIG. 6 illustrates an air knife 70 disposed adjacent to the outer side walls 22, 24 of the spin block to define air gap 72 between the air knife and outer side walls. Air gap 72 is present in the space between the air knife and the outlet side of the spin block, and is used to direct high velocity heated air against the molten polymer strands as they are discharged from the outlet end of the spin block. The thus attenuated fibers are then collected on a collection surface to produce a nonwoven web. Although FIG. 6 shows the bottom surface of the air knife 70 being partially disposed below the discharge surface of the spin block, it should be understood that the air knife can be disposed in different arrangements relative to the spin block. For example, the air knife can be positioned so that its lower surface is disposed beneath or aligned with the discharge surface of the spin block.
Preferably, the spin block has a one-piece construction in which the flow channel 30 is formed in the spin block using a milling technique. The use of a one-piece construction allows operation at high pressures resulting in attractive high polymer throughputs.The capillaries are preferably formed using a micro-milling process in which laser milling is used to bore the capillaries. Preferably, the capillaries are cone-shaped capillaries. This means that the diameter of the inlet end of the capillaries is larger than the diameter of the discharge end of the capillaries. Preferably, the ratio (Di/Dd) of the diameters of the inlet end (Di) and the discharge end (Dd) is in the range of from 2-20, more preferably this ratio is in the range of from 6-8 The cone-shaped capillaries have the advantage that the spin block is less prone to contamination and can more easily be cleaned than conventional spin blocks.
With reference to FIGS. 7 and 8, an embodiment of the spin block is illustrated in which the spin block includes a filter assembly for removing particulate matter from the molten polymer prior to introducing the polymer into the capillaries. As shown, the spin block includes a first filter 80 that is received in recess/channel 29, and a second filter 82 that is disposed between first filter 80 and flow channel 30. Preferably, second filter 82 has a finer filter than first filter s 80 so that progressively finer materials are filtered as the polymer flows through the spin block, ensuring that less contamination takes place and that less frequent maintenance should take place.
In one embodiment, the spin block 12 includes a cavity 83 formed towards the upstream end of the flow channel 30 and extends around the periphery of the flow channel. Cavity includes a shoulder 84 and an inner wall surface 85. The shoulder 84 provides a surface for receiving and supporting the second filter thereon. Preferably, the shoulder is provided adjacent to the entrance to the flow channel so that the second filter is disposed downstream of the first filter. Shoulder 84 can be formed in the spin block with convention milling techniques.
The first filter 80 comprises a longitudinally extending screen support plate 86 that includes an opening 88 through which polymer can be introduced into the flow channel 30. A filter screen 90, such as a mesh or surface having a plurality of openings (e.g., pores or capillaries) is positioned in the screen support plate 86 between opening 88 and flow channel 30. In one embodiment, filter screen 90 can be hand cut to fit within screen support plate 86 with no sealing around the edges.
The second filter 82 is preferably able to filter finer materials than the first filter 80 and is positioned downstream of the first filter. As shown in FIG. 8, the second filter can include a plurality of filter screens (collectively identified by reference number 92) that are stacked on top of each other so as to form a gradient filter. The plurality of filter screens 92 can be held together with a binder ring 94 comprising a flexible material that extends around the periphery of the filter screens and is bent or formed around the collective edges of the screen. For example, an aluminum binder ring can be used to form the second filter 82 and to hold the filters together. As shown, the binder ring includes an upstream surface 96 that wraps over the first upstream filter 100 and a downstream surface 98 that wraps about the downstream filter 102. Preferably, the binder ring, and hence, the second filter are configured and arranged to be supported on shoulder 84. In one embodiment, the peripheral dimensions of the second filter are selected so that a frictional seal is created between the inner wall 85 and the outer surface 95 of binder ring 94.
In one embodiment, the second filter includes at least 6 individual filter screens of the following mesh in the order depicted: 100 mesh, 180 mesh, 280 mesh, 320 mesh, 100 mesh, and 20 mesh to create a gradient filter. It should be recognized that other filter configurations and sizes can be used in the practice of the invention including sintered metals and laser etched screens. For example, the die assembly can include a second filter screen having as little as one individual screen in which the individual screen is at least 320 mesh or higher.
Although the FIGS, generally illustrate the spin block including recess/channel 29 for receiving a first filter, it should be understood that some embodiments may not include such a first filter. For example, in one embodiment the spin block may only include filter 82. In this embodiment, the top surface of filter 82 can be coplanar with the inlet side of the spin block. As noted above, the spin block 12 can be used in conventional meltblown equipment and thus helps reduce or remove the need for replacing existing meltblown equipment. In this regard, FIG. 9 illustrates a cross-section schematic of the spin block 12 being used in combination with a conventional meltblown system 110. The die system 110 includes body members (114 and 116), an elongate die spinblock 12 secured to the die body 114 and air knives ( 18 and 120). The spinblock 12 has converging outer sidewalls (22, 24) of triangular cross section terminating at a discharge surface 28. As discussed above, a flow channel 30 is formed in the spin block 12 and a plurality of side- by-side capillaries are laser drilled in the discharge surface. The die components are generally manufactured from high quality steel to provide durability. Molten polymer is delivered from the extruder through the die passages 124, 126 (coat hanger
configuration) to flow channel 30.
The air knives (118, 120) and the body members (114 and 116) define a plurality of air passages (128, 130, 132, 134). The air knives (118, 120) have tapered inwardly facing surfaces which in combination with the tapered surfaces of the nosepiece define converging air passages 132 and 134. As illustrated, the flow area of each air passage 132 and 134 is adjustable. Air is delivered from an air source via lines 112 through the air passages and is discharged onto opposite sides of the molten fibers as converging sheets of hot air. The converging sheets of hot air draw or attenuate the fibers exiting from the discharge end of the spin block.
In one aspect, the present invention also provides a method for preparing webs of meltblown nanofibers. In particular, meltblown spin blocks in accordance with the present invention can be used to prepare nanofibers having a diameter of less than 1 ,200 nanometers, and preferably less than 1 ,000 nanometers. In one embodiment, the present invention can be used to prepare meltblown nanofibers having a diameter in the range of from 25-500 nanometers, preferably in the range of from 50-250 nanometers.
More particularly, the present invention provides a method for preparing nanofiber meltblown webs at relatively high polymer throughput (e.g., polymer flow rate). For example, nanofiber meltblown webs can be produced at a throughput of the molten polymer that is between 2 and 60 kg/meter/ hour. In a preferred embodiment, nanofiber meltblown webs can be produced at a polymer throughput that is between 5 and 30 kg/meter/ hour, and more preferably between 10 and 30 kg/hour. In contrast, prior art methods of preparing meltblown nanofibers, such as electrospinning, have generally been limited to relatively modest throughput rates, (e.g., less than 0.09 kg/hour/meter) per spinning beam. In terms of grams/hole/minute the present invention enables a flow rate of 0.067 to 0.272 grams per spinning orifice per minute as is clear from the Examples in accordance with the present invention, whereas prior art devices such as described in US 2008/0023888 enable a flow rate of only approximately 0.01 grams per spinning orifice per minute or less. Moreover, the present invention has the advantage that meltdown fabrics can be prepared that display an improved filtration efficiency and an air permeability when compared with conventional meltdown fiber spinning devices.
In addition, the high polymer throughput ranges can be achieved with capillaries having relatively low L/D (length divided by diameter) ratios. As discussed above, the capillaries have an L/D ratio that at least 1 but less than 20.
The present invention, can be used with a variety of different polymer materials including polyolefins (e.g., polyethylenes, polypropylenes, and polybutylenes), polyesters (e.g., polyethylene terephthalate, polybutylene terephthalate, and polylactic acids) polyamides (e.g., nylons), for example but not limited to. The present invention can also be used advantageously to prepare composite structures in which a web of meltblown nanofibers is formed directly onto a substrate, such as nonwoven or woven substrate. EXAMPLES
In the following Examples, meltblown fabrics prepared using a spinning die in accordance with the present invention were compared to meltblown fabrics prepared with a conventional meltblown spinning die. In particular, filtration efficiency, air permeability, and mean pore size of the meltblown fabrics were compared.
The following test procedures were used in the Examples.
Basis Weight: ASTM D 3776-96 or ASTM D6242-98(04).
Textest air permeability: ASTM D737-04.
TSI efficiency: Filtration efficiency is tested at a flow rate of 32 liter per minute utilizing 0.3 micron salt particles that are introduced into the air stream. The test was performed with a TSI 8130 Automatic Filter Tester.
Mean Pore Size: ASTM E1294-89 with test instrument CFP-1200A manufactured by Porous Materials Incorporated.
Example 1
In Example 1 , filtration efficiency, air permeability, and mean pore size of meltblown webs prepared with conventional and inventive spinning dies at different throughput rates were compared. The conventional spinning die had one-piece construction and a conventional die tip structure with capillaries arranged in a longitudinal array along the length of the die. The discharge ends of the capillaries had a diameter of approximately 14 mils and a hole density of 32 holes per inch. The L/D for the die was 10. The capillaries were drilled in the die using conventional milling techniques. In the inventive samples (Samples 1-3), the conventional spinning die was replaced with a spinning die in accordance with the invention. The spinning die according to the invention had also a one-piece construction. In this spinning die the discharge ends of the capillaries had a diameter of approximately 5 mils, and the hole density was 80 holes per inch. The LJD for the die was 6. The capillaries were cone-shaped and were formed with laser milling techniques.
In Samples 1-3, the meltblown fabrics were prepared using the inventive meltblown spinning die. In Comparative Examples 1-3, the meltblown fabrics were prepared using the conventional meltblown spinning die. The meltblown fabrics were prepared using a single screw extruder. The meltblown fabrics were formed from polypropylene homopolymer available from LyondellBasell under the tradename
Metocene MF650Y. The results are summarized in Table 1 below.
Table 1 : Com arison of Meltblown Fabrics
From Table 1 , it can be seen that the meltblown fabrics prepared with the inventive spinning die exhibit significant improvements in TSI Efficiency as well as air permeability and mean pore size over fabrics prepared with conventional meltblown spinning dies. In particular, the resulting properties indicate that meltblown fabrics prepared in accordance with present invention provide meltblown webs having a more uniform structure. Clearly, the use of capillaries having a small diameter, particularly in combination with a high hole density (number of capillaries per inch) provides a denser web in which the individual fibers are more tightly and uniformly packed together. As a result, webs prepared in accordance with the present invention are particularly useful in filtration applications such as liquid and air filtration but not limited to.
Example 2
In the samples and comparative examples of Example 2, the meltblown fibers were deposited on a polypropylene spunbond substrate having a basis weight of 34 gsm. The resulting composites were then tested filtration efficiency, air permeability, and mean pore size. The same polymers and extrusions conditions were used as in Example 1 above. The results are summarized in Table 2 below.
Table 2: Composite fabric comprising Meltblown web and spunbond polypropylene substrate
From Table 2, it can be seen that the improvements in filtration efficiency, air permeability, and mean pore size are present in composite structures that utilize the meltblown fibers prepared in accordance with the present invention.
Many modifications and other embodiments of the invention set forth herein will come to mind to one skilled in the art to which the invention pertains having the benefit of the teachings presented in the foregoing descriptions and the associated drawings. Therefore, it is to be understood that the invention is not to be limited to the specific embodiments disclosed and that modifications and other embodiments are intended to be included within the scope of the appended claims. Although specific terms are employed herein, they are used in a generic and descriptive sense only and not for purposes of limitation.
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