Meltblown nonwoven webs including nanofibers and apparatus and method for forming such meltblown nonwoven webs

Apparatus and method for forming nanofibers and nonwoven webs formed of such nanofibers. The nanofibers are formed by changing the rheology in at least first and second flows of a liquid material such that the changed rheology of the first and second flows differs by an amount sufficient to produce a phase separation when the first and second flows are combined to define meltblown fibers. Each meltblown fiber has lengthwise first and second cross-sectional regions formed of the liquid material from the first and second flows, respectively. These regions are separated along a length of at least a majority of the meltblown fibers to form a plurality of nanofibers, which are collected together with any unsplit meltblown fibers to form the nonwoven web. The difference in the changed rheology of the flows may be produced by using two extruders with different barrel diameters to create differ shear histories for the two flows.

FIELD OF THE INVENTION

The present invention generally relates to nonwoven webs and, more particularly, to nonwoven webs formed from a majority of meltblown nanofibers, and to apparatus and methods for forming these nonwoven webs.

BACKGROUND OF THE INVENTION

Melt spun nonwoven webs may be made by a number of processes. The most popular processes are meltblowing and spunbonding, both of which involve melt spinning of thermoplastic material. Meltblowing is a manufacturing process for nonwoven webs in which a molten thermoplastic material is extruded from a row of outlets in a die tip. The streams of thermoplastic material exiting the die tip are immediately contacted with sheets or jets of hot air to attenuate the fibers. The fibers are then deposited onto a collector in a random manner and form a nonwoven web used in such products as diapers, surgical gowns, carpet backings, filters and many other consumer and industrial products.

Generally, meltblown fibers are formed by extruding a low viscosity (i.e., high melt flow rate) thermoplastic material through an array of holes in a meltblown die and impinging the extruded material with high velocity heated air. The resulting fibers have an averaging diameter of between two and five microns. Meltblown fibers are commonly formed from multiple components in which each component may include a unique thermoplastic material having a different chemical composition.

Nonwoven webs of meltblown nanofibers may be made by a process known as electro-spinning that generally involves spinning a solvent-diluted low viscosity polymer in the presence of a directional electric field. Such nonwoven webs, which are characterized by nanofibers of a sub-micron fiber diameter, are known to have utility in a number of applications, such as filtering of particles from fluid streams, for example from air streams and liquid (e.g. non-aqueous and aqueous) streams. In such filtration applications, the interstitial spaces between the nanofibers define small pores that increase the filtration efficiency of the nonwoven. Nonwovens formed from nanofibers also permit the use of lower basis weight, which reduces the cost of products constructed from those nonwovens.

Electro-spinning processes suffer from multiple disadvantages, including the need to remove the solvent from the deposited fibers and an inherently low production rate. Moreover, electro-spinning is not practical on a commercial scale for thermoplastic material since commercially used thermoplastic materials cannot be diluted with a solvent without detrimental consequences to the nonwoven web. The high electric fields required to electro-spin undiluted thermoplastic materials are susceptible to breakdown in air and result in unwanted electrical discharges.

For these reasons, it is desirable to provide apparatus and methods for forming nonwoven webs comprising a majority of meltblown nanofibers that overcome the various problems associated with conventional meltblowing methods for forming such nonwoven webs.

SUMMARY OF THE INVENTION

In accordance with an embodiment of the present invention, a method of forming a nonwoven web includes establishing a first and second flow of liquid material and changing the rheology of the liquid material in the first and second flows. The changed rheology of the second flow differs from the changed rheology of the first flow by an amount sufficient to produce a phase separation between the liquid material in the first and second flows when combined. The method further includes combining the first flow of the liquid material with the changed rheology and the second flow of the liquid material with the changed rheology to form a plurality of meltblown fibers. Each of the meltblown fibers has a length, a first cross-sectional region formed of the liquid material from the first flow, and a second cross-sectional region formed of the liquid material from the second flow. The first and second cross-sectional regions extend along the length of each fiber. The first cross-sectional region is separated from the second cross-sectional region along the length of at least a majority of the meltblown fibers to form a plurality of nanofibers and the nanofibers are then collected to form the nonwoven web. Any un-separated meltblown fibers are collected in the nonwoven web along with the nanofibers.

In yet another aspect of the present invention, a melt spinning apparatus includes a first extruder providing the first flow of a liquid material and a second extruder providing a second flow of the liquid material. The first extruder is configured to change the rheology of the liquid material in the first flow and the second extruder is configured to change the rheology of the second flow to differ from the rheology of the first flow sufficient to produce a phase separation between the liquid material in the first and second flows when combined. The melt spinning apparatus further includes a spinpack coupled with the first and second extruders for receiving the first flow of the liquid material with the changed rheology and the second flow of the liquid material with the changed rheology. The spinpack combines the first flow and the second flow to form a plurality of meltblown fibers each having a length, a first cross-sectional region formed of the liquid material from the first flow, and a second cross-sectional region formed of the liquid material from the second flow. The first and second cross-sectional regions extend along the length of the fiber. The spinpack directs air toward the meltblown fibers with a velocity effective to attenuate and split at least a majority of the meltblown fibers into nanofibers. A substrate collects nanofibers and any unsplit meltblown fibers to form a nonwoven web.

The nanofibers of the nonwoven webs of the present invention have a significantly reduced average diameter as compared with conventional meltblown fibers. Such sub-micron diameters are unachievable with conventional meltblowing processes. For example, the discharge outlet diameter in the die tip of conventional melt spinning apparatus cannot be simply scaled downward without limitation for reducing the fiber diameter. The nanofibers of the present invention provide an enhanced surface area to mass ratio as compared with larger diameter conventional meltblown fibers.

These and other advantages of the present invention shall become more apparent from the accompanying drawings and description thereof.

DETAILED DESCRIPTION

For purposes of this description, words such as “vertical”, “horizontal”, “bottom”, “right”, “left” and the like are applied in conjunction with the drawings for purposes of clarity and for purposes of defining a frame of reference. As is well-known, melt spinning devices may be oriented in substantially any orientation, so these directional words should not be used to imply any particular absolute directions for a melt spinning assembly or apparatus.

With reference toFIG. 1, a melt spinning assembly10constructed in accordance with the inventive principles includes a manifold assembly12for supplying liquid material to liquid inputs14,16of a spinpack18. The inputs14and16are sealed to the manifold assembly12such as by static seals retained within recesses (not shown) around each input14,16. The manifold assembly12includes first and second outer manifold elements20,22coupled by an intermediate manifold element24. An upper surface of intermediate manifold element24includes first and second liquid supply inlets25,26. Gear pumps150,151(FIG. 8) each pump a respective flow of a chemically-identical liquid material from one of first and second extruders202,206(FIGS. 8-10) to a corresponding one of the first and second liquid supply inlets25,26. Such chemically-identical solid source materials are characterized by the same composition and identical physical characteristics, such as intrinsic viscosity, melt flow rate, melt viscosity, die swell, density, crystallinity, and melting point or softening point.

Supply inlet25communicates with a coat-hanger shaped recess (not shown) defined between outer manifold element20and intermediate manifold element24. The recess provides a first manifold liquid passage to provide liquid material to at least a portion of the longitudinal length of liquid input14of the spinpack18. Similarly, supply inlet26communicates with another coat-hanger shaped recess (not shown) defined between outer manifold element22and intermediate manifold element24that provides a second manifold liquid passage to provide liquid material to at least a portion of the longitudinal length of liquid input16of the spinpack18. The manifold assembly12may include a plurality of supply inlets25,26and corresponding first and second manifold liquid passages defined by coat-hanger shaped recesses along its longitudinal length depending on the length of the spinpack18.

Holes28and30located along the length of each outer manifold element20,22each receive a heating device, such as an electrical heater rod32, for independently heating the liquid material in the first and second manifold liquid passages and the process air to an appropriate application temperature. Temperature sensing devices (not shown), such as resistance temperature detectors (RTD's) or thermocouples are also placed in outer manifold elements20,22to independently control the temperature of each flow of liquid material. It should be appreciated by those skilled in the art that various heating systems consistent with aspects of the invention may be appropriately used in different applications.

Outer manifold elements20,22further include a plurality of air supply passages34,36for supplying pressurized air (i.e., process air) to air passage inputs38,40of the spinpack18. Fibers42are extruded along the longitudinal length of the spinpack18from a row of discharge outlets44(seeFIGS. 3-5) and are attenuated by the process air emitted from air supply passages34,36. The attenuated fibers42form a nonwoven web46upon a collector or substrate48that generally is moving transverse to the melt spinning assembly10, such as shown by arrow50.

With reference toFIG. 2, the spinpack18includes the fiber producing features of the melt spinning assembly10. In particular, spinpack18includes a transfer block52and a die tip block58, attached below the transfer block52to form a die tip. The transfer block52includes longitudinal side recesses54,56for mounting the spinpack18to the manifold assembly12, the liquid inputs14,16and air passage inputs38,40. The die tip block58includes first and second rows of air passages60,62and first and second rows of liquid passages64,66. Attached below the die tip block58ais pair of air knife plates68,70.

With reference toFIGS. 3-5, the spinpack18is depicted in assembled condition showing how the process air and the two streams110,112of liquid material are brought together at each discharge outlet44. First and second flows80,90of the liquid material are kept separate from one another in respective flow paths throughout the entire spinpack18and are extruded separately as streams110,112. In particular, one of the streams110of liquid material is extruded at a plurality of first outlets76and the other of the streams112of liquid material is extruded at a plurality of second outlets78, each second outlet78adjacent to a corresponding one of the first outlets76.

In particular, the liquid material supplied from the manifold assembly12enters the first liquid input14in the transfer block52of the spinpack18to form the first flow80. Liquid material in the first flow80encounters a first filter82disposed within a first filter recess84for entrapping contaminants. The first flow80continues through a first liquid transfer passage86, which may be a single longitudinal slot or a series of passages each longitudinally aligned with one of the first outlets76. The die tip block58has a longitudinally aligned row of first die tip liquid passages88communicating between the first liquid transfer passage86in the transfer block52and with a respective one of the first outlets76in the die tip block58.

Similarly, another supply of the liquid material from the manifold assembly12enters the second liquid input16in the transfer block52of the spinpack18to form the second flow90. Liquid material in the second flow90encounters a second filter92disposed within a second filter recess94for entrapping contaminants. The second flow90continues through a second liquid transfer passage96, which may be a single longitudinal slot or a series of passages each longitudinally aligned with one of the second outlets78. The die tip block58has a longitudinally aligned row of second die tip liquid passages98communicating between the second liquid transfer passage96in the transfer block52and with a respective one of the second outlets78in the die tip block58.

The transfer block52includes a first air transfer passage99that communicates with the first air passage input38and a second air transfer passage100that communicates with the second air passage input40. The die tip block58includes a first die tip air passage102that communicates between the first air transfer passage99and a converging air channel104formed between the air knife plate68and the die tip block58. Similarly, the die tip block58includes a second die tip air passage106that communicates between the second air transfer passage100and a converging air channel108formed between the air knife plate70and the die tip block58. The air channels104,108may be mutually aligned symmetrically relative to the first and second outlets76,78and with an included angle of, for example, between about 60° and about 90°.

With particular reference toFIG. 4, the first flow80is extruded from one of the first outlets76as a single-component strand or stream110and the second flow90is extruded from one of the second outlets78as a single-component strand or stream112. The first and second streams110,112thereafter combine into a fiber42having a side-by-side cross-sectional configuration. Bonding or combining is promoted by the proximity of the first and second outlets76,78and the converging orientation of the first and second die tip liquid passages88,98.

With particular reference toFIG. 5, each pair of adjacently positioned first and second outlets76,78are shown to tangentially meet. Consequently, the streams110,112of liquid material do not contact one another until after extrusion. Each outlet76,78is oblong due to the non-perpendicular orientation of the corresponding die tip liquid passages88,98with respect to a bottom, external surface of the die tip block58.

A first air jet114exits air channel104at a first spin slot116and is directed at each fiber42. A converging, second air jet118exits air channel108at a second spin slot120and is directed at the fiber42. Generally, the air temperature of the air flow from air jets114,118is approximately equal to the temperature of the material constituting the fibers42. The high velocity air flow from the air jets114,118impinges and attenuates the fibers42.

Spinpack18provides two flows80,90of liquid material ultimately forming individual streams110,112at discharge outlets44that are combined post-extrusion into fiber42. There is substantially no physical interaction or contact between the two flows80,90of liquid material before extrusion. The two individual streams110,112are urged together by the momentum of extrusion to define fibers42. However, the invention contemplates that the spinpack18may have a different configuration in which the flows80,90of liquid material are combined before fibers42are extruded from discharge outlets44. Specifically, any spinpack18capable of forming multicomponent fibers in a meltspinning apparatus may be used in the present invention. Melt spinning assembly10is further described in U.S. Pat. No. 6,565,344, the disclosure of which is hereby incorporated by reference herein in its entirety.

With reference toFIGS. 6 and 7in which like reference numerals refer to like features inFIGS. 1-5, a portion of a different spinpack18afor use with melt spinning assembly10is described in which the two flows80,90of liquid material flowing in liquid passages88,98intersect and become merged inside of the spinpack18a. In other aspects, the spinpack18ais substantially identical to spinpack18(FIGS. 1-5). Downstream of the intersection between liquid passages88,98, the merged flow is directed into one of a plurality of passageways122. Each passageway122emerges from the spinpack18aat a corresponding one of a plurality of discharge outlets124, which extend in a row across the width of the spinpack18a. The flows80,90are separated in the spinpack18and are combined only immediately prior to reaching the discharge outlets124. The merged flows80,90extruded from each discharge outlet124define one of a plurality of fibers42subsequently collected on substrate28(FIG. 1) to form the nonwoven web46(FIG. 1).

Flanking the discharge outlets124are spin slots116,120that emerge from respective air channels104,108of the spinpack18a. The air jets114,118of pressurized process air, typically heated, emitted from these spin slots116,120impinge the fiber42, which attenuates and splits the fiber42consistent with the principles of the present invention. The air channels104,108ofFIG. 6are angled with a different included angle than shown inFIGS. 3 and 4, so that the corresponding air jets114,118converge at a different inclination relative to the fibers42but, nevertheless, split and attenuate fibers42. Spin pack18a, as well as spinpack18(FIGS. 1-5), is configured to produce fibers42consistent with the principles of the invention. Accordingly, spinpack18amay be substituted for spinpack18in the melt spinning assembly10.

With reference toFIG. 8, melt spinning assembly10, including the spinpack18or optionally spinpack18a, is installed in a meltspinning apparatus200, which may be any suitable conventional meltspinning apparatus or, for example, the apparatus disclosed in U.S. Pat. No. 6,182,732, the disclosure of which is hereby fully incorporated by reference herein. The apparatus200generally includes a first extruder202with a feed line204for feeding a first flow of the liquid material to the melt spinning assembly10and a second extruder206with a feed line208for feeding a second flow of the liquid material to the melt spinning assembly10. The spinpack18is configured to thermally isolate the two flows80,90(FIG. 3) of liquid material from each other while inside spinpack18. The melt spinning assembly10is supported by columns198,199of a support structure and suspended above substrate48so that the fibers42deposit on substrate48to form nonwoven web46.

Melt spinning apparatus200further includes a pair of gear pumps150,151each of which receives liquid material from one of the feed lines204,208and pumps the received liquid material to one of the first and second liquid supply inlets25,26(FIG. 1) in the respective outer manifold elements20,22for delivery to the liquid inputs14,16(FIG. 1) of the spinpack18. Branching from a single inlet duct156is a pair of air supply ducts152,154that deliver process air to the air supply passages34,36in the outer manifold elements20,22, respectively. The various other details of the meltspinning apparatus200, such as, for example, a system controlling the operation of the apparatus200and quench air outlets for cooling the fibers42after forming, are not described herein as these details will be readily understood by those of ordinary skill in the art.

With reference toFIG. 9in which like reference numerals refer to like features inFIG. 8, the first extruder202includes a cylinder or barrel210, a screw212stationed within the barrel210, and a hopper214that receives and melts amounts of a solid source material to provide molten liquid material. The barrel210, which is heated along its length across four separate zones by heaters216,218,220,222, defines a cylindrical housing within which the screw212rotates. The temperature of the liquid material advancing in the barrel210incrementally increases across the zones of barrel210associated with heaters216,218,220,222, respectively. The screw212is powered by a motor213and includes a helically flighted shaft that rotates within the barrel210to advance liquid material delivered to the barrel210from hopper214to feed line204. The space between the flight bounded by the screw212and the cylindrical bore of the barrel210defines a channel for fluid transport in the first extruder202to the first feed line204. Operation of the first extruder202changes the rheology of the liquid material in the first flow80.

With reference toFIG. 10in which like reference numerals refer to like features inFIG. 8, the second extruder206is similar in construction to the first extruder202. The second extruder206includes a cylinder or barrel230, a screw232stationed within the barrel230, and a hopper234that receives and melts amounts of a solid source material to provide molten liquid material. Barrel230, which is heated along its length across five separate zones by heaters236,238,240,242,244, defines a cylindrical housing within which the screw232rotates. The temperature of the liquid material advancing in the barrel230incrementally increases across the zones of barrel230associated with heaters236,238,240,242,244, respectively. The screw232is a helically flighted shaft, which is powered by a motor233, that rotates within the barrel230to advance liquid material delivered to the barrel230from hopper234to feed line208. The space between the flight bounded by the screw232and the cylindrical bore of the barrel230defines a channel for fluid transport in the first extruder202to the first feed line204. Operation of the second extruder206changes the rheology of the liquid material in the second flow90; however, the changed rheology of the second flow differs from the changed rheology of the first flow by an amount sufficient to produce a phase separation between the liquid material in the first and second flows80,90when combined.

The invention contemplates that the first and second hoppers214,234may constitute a single hopper (not shown) into which the chemically-identical solid source material is added and initially melted for subsequent extrusion from the first and second extruders202,206. This sharing is possible because the same liquid material is provided in the streams80,90but with different shear histories.

The first and second extruders202,206differ in a manner that causes the liquid material delivered to the spinpack18by the first extruder202to experience a different shear history (i.e., rheology) than the chemically-identical liquid material delivered to the spinpack18by the second extruder206. The different shear histories in the extruders202,206differentially changes a rheological property of the liquid material, such as viscosity, in each of the two flows80,90in liquid transfer passage86,96, respectively. The liquid material in flows80,90, which are subjected to different shear histories in the extruders202,206, are also subjected to different thermal histories while inside the extruders202,206. Shear history is related to thermal history by shear heating, which inherently results from friction caused by fluid flow through passages. As used herein, the differentially change in rheology between the two flows80,90may be provided by mechanical approaches that provide different shear histories and by thermal approaches that use differential heating.

With regard to the specific embodiment of the present invention depicted inFIGS. 9 and 10, a diameter, D2, of the barrel230of the second extruder206is larger than a diameter, D1, of the barrel210of the first extruder202. As a result, the two flows80,90(FIG. 3) defining streams110,112(FIG. 4) of liquid material that ultimately form fibers42are composed of the same liquid material (i.e., chemically identical liquid materials) but have a different rheology due to the difference in shear history inside the extruders202,206. The flow paths in the spinpack18are identical for the two flows80,90of liquid material, although the invention is not so limited as will be described below.

The shear history of each flow80,90of liquid material is a function of the shear rate experienced by the liquid material in each flow over its individual flow path. The shear rate is the overall velocity across the cross section of the barrels210,230with which the individual liquid material layers constituting each of the flows80,90are gliding along each other or along the wall of the barrels210,230in laminar flow. Among other variables, the difference in shear history may depend upon the different surface area of the barrels210,230, different residence times in the respective one of the extruders202,206, and different pressure drops during the extrusion process. The stream of liquid material advanced in the smaller-diameter barrel210of the first extruder202has a different shear history than the stream of liquid material advancing in the larger-diameter barrel230of the second extruder206. The differences in shear history will also inherently result in different thermal histories for the two flows80,90of liquid material due to differences in shear heating inside the extruders202,206.

The liquid material forming fibers42may be any thixotropic liquid material exhibiting non-Newtonian rheological flow behavior where viscosity depends on the shear history. An amount of solid source material is added to hopper214, melted, and supplied in molten form to first extruder202. Another amount of a chemically-identical solid source is added to hopper234, melted, and supplied in molten form to the second extruder206. As mentioned above, the chemically-identical solid source materials added to hoppers214,234have the same composition and identical physical characteristics, such as intrinsic viscosity, melt flow rate, melt viscosity, die swell, density, crystallinity, and melting point or softening point.

The solid source material may be any melt-processable thermoplastic polymer selected from among any commercially available meltspun grade of a wide range of thermoplastic polymer resins, copolymers, and blends of thermoplastic polymer resins including, but not limited to, polyolefins, such as polyethylene and polypropylene, polyesters, nylons, polyamides, polyurethanes, ethylene vinyl acetate, polyvinyl chloride, polyvinyl alcohol, and other melt processable polymers. The constituent thermoplastic polymer resin may also be blended with additives such as surfactants, colorants, anti-static agents, lubricants, flame retardants, antibacterial agents, softeners, ultraviolet absorbers, polymer stabilizers, and the like.

As shown inFIGS. 11 and 11A, the combined streams110,112(FIG. 4) define two distinct cross-sectional regions41a,41bcoextensive along an interface43extending axially along the length of the fiber42. The differing shear histories of the two flows80,90(FIG. 3) defining streams110,112cause a phase separation to occur between regions41,41b. Due to this phase separation, the regions41a,41bare weakly bonded along interface43so that a sufficient force acting on the fiber42is capable of splitting the fiber42along the interface43. The phase separation of the two regions41a,41band the consequential presence of interface43results from the inability of the liquid material in region41ato intermix and chemically react with the liquid material in region41b. If the liquid material in the two flows80,90were to have an identical rheology, which they do not, the resulting regions41a,41bwould intermix and bond to an extent sufficient to prevent splitting when fiber42is exposed to a high velocity streams of process air.

As best shown inFIG. 11, the flow of process air from the air jets114,118(FIG. 4) attenuates the fiber42and causes the two regions41a,41bto split apart or divide along the axial interface43, which defines two smaller diameter daughter fibers42a,42beach corresponding to one of the regions41a,41b. Preferably, the high velocity air flow from air jets114,118attenuates the parent fiber42to a smaller diameter than the initial extruded diameter before splitting occurs along interface43. After splitting, the average fiber diameter of the daughter fibers42a,42bis smaller than the average diameter of each parent fiber42. For the illustrated side-by-side fiber configuration in which each region41a,41bconstitutes half of the total fiber42, the diameter of the split fibers42a,42bis approximately one-half of the original fiber diameter. As used herein, the diameter of a noncircular cross-section fiber42is determined as the equivalent diameter of a circle having the same cross-sectional area.

After the larger parent fibers42are split, the properties (e.g., orientation, crystallinity) of the constituent liquid material of the individual split daughter fibers42a,42bare not significantly altered. After splitting, the resulting daughter fibers42a,42bare smaller in diameter than the parent fiber42but retain some of the same mechanical properties. Constructing the extruders202,206so that the liquid material forming each of the regions41a,41bhas a differential rheology causes relatively weak bonding along the interface43. Because of this phase separation between the regions41a,41b, the fibers42are more susceptible to splitting longitudinally along the length of the interface43when exposed to the high-velocity flow of process air. Small diameter fibers42a,42bmay be produced with greater attenuation than fibers of the same liquid material extruded directly to equivalent diameters due to the larger effective surface area before splitting. A majority of the parent fibers42are split into daughter fibers42a,42b, which are nanofibers having a submicron diameter. Fibers42a,42band any of the unsplit parent fibers42are subsequently deposited as nonwoven web46(FIG. 1).

Each fiber42is illustrated inFIGS. 11 and 11Aas constituted by side-by-side regions41a,41bthat are approximately equal in volume and cross-sectional area. However, the invention is not so limited as regions41a,41bmay be divided unequally, such as 30% and 70% of the total cross-sectional area. In addition, each fiber42may have a different multi-component configuration, such as a segmented pie, with more than two distinct regions each weakly bonded along an interface created by phase separation, such that more than two individual daughter fibers42a,42bare formed from the larger parent fiber42after splitting. The different components of such fibers42are arranged in substantially distinct regions, like regions41a,41b, across the cross-section of the fiber and extend continuously along the length of the fiber42. Adjacent regions in such fibers42are formed from liquid material of a different shear history so that these regions are weakly bonded and splittable.

As another example and with reference toFIG. 12, each fiber42may have a circular cross-section and, before splitting into four smaller fibers while in flight from the die tip block58to the substrate48, include four distinct cross-sectional layers or regions140a,140b,140c,140dextending along the length of fiber42. Adjacent regions140aand140bare formed from first and second flows of liquid material having differing rheology, region140cis formed from a third flow of the liquid material having a different rheology than adjacent region140b, and region140dis formed from a fourth flow of the liquid material having a different rheology than the third liquid material flow. The additional liquid material flows for the two added regions may be supplied from two additional extruders (not shown) like extruders202,206but with each additional extruder capable of imparting a unique shear history to the liquid material flow. Alternatively, the second and fourth liquid material flows may have the same the rheology because regions140band140dare not adjacent, and the first and third liquid material flows may have the same rheology because regions140aand140care not adjacent. In this alternative embodiment, each of the liquid material flows80,90from the first and second extruders202,206may be split for defining the regions140a-d. The present invention contemplates that the number of individual regions is not limited to two regions or four regions as in the illustrated embodiments. Instead, fiber42may embrace any number of regions of the liquid material arranged such that adjacent regions have been subjected to corresponding shear histories that differ to an extent sufficient to produce splitting in accordance with the principles of the invention.

Because of mutual phase separation between regions140aand140b, regions140band140c, and regions140cand140d, weakly bonded interfaces141a,141b,141care defined between adjacent pairs of regions140a-d. As a result, the larger parent fiber42will split along each of these interfaces141a-cto define four smaller diameter daughter fibers (not shown) that deposit on substrate48to form nonwoven web46. A majority of the parent fibers42subsequently deposited as the nonwoven web46(FIG. 1) are split into daughter fibers each corresponding to one of the four regions140a-d, in which each of the split regions140a-dconstitutes a nanofiber having a submicron diameter. Fibers42with this configuration, but formed from chemically-different liquid materials, are disclosed in U.S. Pat. No. 5,207,970, the disclosure of which is hereby incorporated by reference herein in its entirety.

In alternative embodiments of the invention and with renewed reference toFIGS. 11 and 11A, the differences in the changed rheologies or shear histories of the flows80,90of the liquid material may be created in the melt spinning apparatus200by other approaches capable of that differentially changing the shear histories of the two flows80,90by an amount sufficient to cause phase separation between the regions41a,41b. The differential shear history may result from exposing the two flows80,90(FIG. 3) to different shear rates for the same length of time, the same shear rate for different lengths of time, or different shear rates for different lengths of time. For example, the spinpack18may be configured to present a path length for the flow80of liquid material forming region41athat differs from the path length for the flow90of liquid material forming region41b. Another approach is to differentially shear the two flows80,90of liquid material at the gear pumps150,151by suitably adjusting the operation of the gear pumps150,151. In addition to configuring the extruders202,206with different barrel diameters, other approaches for imparting a differential change in shear history is to operate extruders202,206of equal barrel diameter at different pressures, to provide extruders202,206of equal different length, to operate identical extruders202,206with different rates, or a combination of these configurations. The heating inside extruders202,206of equal diameter and length may be adjusted so that the flows80,90have different shear histories. Persons of ordinary skill will appreciate that the various approached for differentially changing the shear history may be combined.

The nonwoven webs46of the invention may be further processed after collection to enhance the degree of fiber splitting for any fibers42not split by the impinging process air from the air jets114,118. The nonwoven webs46of the invention may have a wide variety of uses where high surface area is important including, but not limited to, filtration media and filtration devices, medical fabrics, sanitary products, apparel fabrics, and thermal or acoustical insulation.

Further details and embodiments of the invention will be described in the following example.

EXAMPLE

Thermoplastic fibers of the configuration shown inFIG. 11Awere produced by a melt spinning apparatus200configured as described with regard toFIGS. 1-8and collected to form a nonwoven web46. The solid source material used in this example was PF017 (2000 MFR) polypropylene, which is commercially available from Basell North America Inc. (Elkton, Md.). Amounts of the solid polypropylene were supplied to the hoppers214,234of the respective extruders202,206and melted. Power to heaters216,218,220,222of extruder202and power to heaters236,238,240,242,244of extruder206were adjusted such that the temperature of the liquid polypropylene supplied to each of the feed lines204,208was about 485° F. The pressure at the outlet of each of the extruders202,206was about 900 psi. The gear pumps150,151were operated at 6.7 revolutions per minute (rpm) and 10 rpm, respectively (or 30 cc/rev and 20 cc/rev, respectively) to provide flows80,90of polypropylene. The melt density of the polypropylene was 0.75 g/cc and the throughput for each stream110,112of polypropylene was 0.135 grams per hole per minute (ghm). The temperature of the process air for air jets114,118exiting air channels104,108was about 500° F. The included angle of the air channels104,108was about 60° and the air gap was about 1.016 mm. The number of first and second outlets76,78was 50 holes per inch with a 0.318 mm hole diameter. The polypropylene streams110,112from the outlets76,78were combined to form fibers42, as described herein, having side-by-side cross-sectional regions41a,41bof approximately equal area, as shown inFIG. 11A. Nonwoven web46was formed by collecting the fibers42on a substrate48moving at about fifty-five (55) meters per minute relative to the stationary spinpack18.

The nonwoven web46had an average basis weight of 4.6 gsm, an average air permeability of 92.5 cfm at 125 PA, and an average hydrohead of 17.6 mbar at 60 mbar/min, but samples with layer of screen protection exhibited 30 mbar at 60 mbar/min. Due to the difference in the diameter of the barrels210,230of the extruders202,206, the polypropylene in the two regions41a,41bare subjected to different shear histories. When exposed to the high velocity process air of air jets114,118, the polypropylene fibers42are attenuated and also tend to split along the interface43between the cross-sectional regions41a,41b. As a result, a majority of the polypropylene fibers42splits or divides into smaller daughter fibers42a,42bbefore collection on substrate48so that the nonwoven web46is formed primarily from the daughter fibers42a,42bof polypropylene.

FIG. 13presents the results of measurements of fiber or fiber diameter made at various locations across the width of the nonwoven web46. As is apparent fromFIG. 13, the average fiber diameter was measured to be about 0.94 micron, which is significantly smaller than the average fiber diameter of conventional meltblown nonwoven webs.FIG. 13indicates that about seventy (70) percent of the nonwoven web46was formed from the individual daughter fibers42a,42bresulting from split fibers42and having a diameter of less than or equal to one (1) micron.

While the present invention has been illustrated by a description of various preferred embodiments and while these embodiments have been described in considerable detail in order to describe the best mode of practicing the invention, it is not the intention of applicants to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications within the spirit and scope of the invention will readily appear to those skilled in the art. The invention itself should only be defined by the appended claims, wherein we claim: