Electroblowing web formation process

Abstract

An improved electroblowing process is provided for forming a fibrous web of nanofibers wherein polymer stream is issued from a spinning nozzle in a spinneret with the aid of a forwarding gas stream and a resulting nanofiber web is collected on a collection means. The process includes applying a high voltage to the collection means and grounding the spinneret such that an electric field is generated between the spinneret and the collection means of sufficient strength to impart an electrical charge on the polymer as it issues from the spinning nozzle.

Description

FIELD OF THE INVENTION

The present invention relates to a process for forming a fibrous web wherein a polymer stream is spun through a spinning nozzle into an electric field of sufficient strength to impart electrical charge on the polymer and wherein a forwarding gas stream aids in transporting the polymer away from the spinning nozzle.

BACKGROUND OF THE INVENTION

PCT publication no. WO 03/080905A discloses an apparatus and method for producing a nanofiber web. The method comprises feeding a polymeric solution to a spinning nozzle to which a high voltage is applied while compressed gas is used to envelop the polymer solution in a forwarding gas stream as it exits the nozzle, and collecting the resulting nanofiber web on a grounded suction collector.

There are several disadvantages to the process disclosed in PCT publication no. WO 03/080905A, particularly if the process is carried out on a commercial scale. For one, the spinning nozzle, and the spinneret and spin pack of which the nozzle is a component and all of the associated upstream solution equipment is maintained at high voltage during the spinning process. Because the polymer solution is conductive, all of the equipment in contact with the polymeric solution is brought to high voltage, and if the motor and gear box driving the polymeric solution pump are not electrically isolated from the pump, a short circuit will be created which will reduce the voltage potential of the pack to a level insufficient to create the electric fields required to impart charge on the polymer solution.

Another disadvantage of the process disclosed in PCT publication no. WO 03/080905A is that the process solution and/or solvent supply must be physically interrupted in order to isolate it from the high voltage of the process. Otherwise, the solution and/or solvent supply systems would ground out the pack and eliminate the high electric fields required for imparting charge on the polymeric solution.

Additionally, all of the equipment in contact with the electrified polymer solution must be electrically insulated for proper and safe operation. This insulation requirement is extremely difficult to fulfill as this includes large equipment such as spin packs, transfer lines, metering pumps, solution storage tanks, pumps, as well as control equipment and instrumentation such as pressure and temperature gauges. A further complication is that it is cumbersome to design instrumentation and process variable communication systems which can operate at high voltages relative to ground. Furthermore, all exposed sharp angles or corners that are held at high voltage must be rounded, otherwise they will create intense electric fields at those points which may discharge. Potential sources of sharp angles/corners include bolts, angle irons, etc. Moreover, the high voltage introduces a hazard to those persons providing routine maintenance to electrified equipment in support of an on-going manufacturing process. The polymeric solutions and solvents being processed are often flammable, creating a further potential danger exacerbated by the presence of the high voltage.

SUMMARY OF THE INVENTION

The invention relates to an electroblowing process for forming a fibrous web comprising:

(a) issuing a polymer stream from a spinning nozzle in a spinneret whereupon a fibrous web is formed, the web having an electric charge of a positive or negative polarity, and

(b) collecting the fibrous web on a collection means,

wherein a voltage is applied to the collection means, the voltage having a polarity opposite that of the fibrous web, and wherein the spinneret is substantially grounded, such that an electric field is generated between the spinneret and the collection means of sufficient strength to impart an electrical charge on the polymer stream as it issues from the spinning nozzle.

DEFINITIONS

The terms “electroblowing” and “electro-blown spinning” herein refer interchangeably to a process for forming a fibrous web by which a forwarding gas stream is directed generally towards a collection means, into which gas stream a polymer stream is injected from a spinning nozzle, thereby forming a fibrous web which is collected on the collection means, wherein a voltage differential is maintained between the spinning nozzle and the collection means and the voltage differential is of sufficient strength to impart charge on the polymer as it issues from the spinning nozzle.

The term “nanofibers” refers to fibers having diameters of less than 1,000 nanometers.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate the presently contemplated embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 is an illustration of the prior art.

FIG. 2 is a schematic of a process according to the present invention.

FIG. 3A is a schematic of an alternative process according to the present invention.

FIG. 3B is a detail from FIG. 3A of the collection means.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Throughout the drawings, like reference characters are used to designate like elements.

An electroblowing process for forming fibrous web is disclosed in PCT publication number WO 03/080905A (FIG. 1), the contents of which are hereby incorporated by reference. There are several disadvantages to this process, as already described herein in the Background of the Invention.

It would be desirable to have an improved electroblowing process which would avoid these disadvantages.

In the process of the present invention, referring to FIG. 2, according to one embodiment of the invention, a polymer stream comprising a polymer and a solvent, or a polymer melt, is fed from a storage tank, or in the case of a polymer melt from an extruder 100 to a spinning nozzle 104 (also referred to as a “die”) located in a spinneret 102 through which the polymer stream is discharged. Compressed gas, which may optionally be heated or cooled in a gas temperature controller 108, is issued from gas nozzles 106 disposed adjacent to or peripherally to the spinning nozzle 104. The gas is directed generally downward in a forwarding gas stream which forwards the newly issued polymer stream and aids in the formation of the fibrous web.

While not wishing to be bound by theory, it is believed that the forwarding gas stream provides the majority of the forwarding forces in the initial stages of drawing of the fibers from the issued polymer stream and in the case of polymer solution, simultaneously strips away the mass boundary layer along the individual fiber surface thereby greatly increasing the diffusion rate of solvent from the polymeric solution in the form of gas during the formation of the fibrous web.

At some point, the local electric field around individual fibers is of sufficient strength that the electrical force becomes the dominant drawing force which ultimately draws the individual fibers to diameters measured in the hundreds of nanometers or less.

It is believed that the geometry of the tip of the spinning nozzle, also referred to as the “die tip,” creates an intense electric field in the three-dimensional space surrounding the tip which causes charge to be imparted to the web. The die tip may be in the form of a cylindrical capillary or in the form of a linear array of cylindrical capillaries. In the embodiment in which the die tip is a linear array, the forwarding gas stream is issued from gas nozzles 106 on each side of the spinneret 102. The gas nozzles are in the form of slots formed between elongated knife edges, one on each side of the spinneret, along the length of the linear array, and the spinneret. Alternately, in the embodiment in which the die tip is in the form of a cylindrical capillary, the gas nozzle 106 may be in the form of a circumferential slot surrounding the spinneret 102. It is believed that the electric field combined with the charge on the fibrous web provides spreading forces which act on the fibers and fibrils of the web, causing the web to be better dispersed and providing for very uniform web laydown on the collection surface of the collection means.

The velocity of the compressed gas issued from gas nozzles 106 is advantageously between about 10 m/min and about 20,000 m/min, and more advantageously between about 100 and about 3000 m/min.

Advantageously, the polymeric solution is electrically conductive. Examples of polymers for use in the invention may include polyimide, nylon, polyaramide, polybenzimidazole, polyetherimide, polyacrylonitrile, PET (polyethylene terephthalate), polypropylene, polyaniline, polyethylene oxide, PEN (polyethylene naphthalate), PBT (polybutylene terephthalate), SBR (styrene butadiene rubber), polystyrene, PVC (polyvinyl chloride), polyvinyl alcohol, PVDF (polyvinylidene fluoride), polyvinyl butylene and copolymer or derivative compound thereof. The polymer solution is prepared by selecting a solvent suitable to dissolve the polymer. The polymer solution can be mixed with additives including any resin compatible with an associated polymer, plasticizer, ultraviolet ray stabilizer, crosslink agent, curing agent, reaction initiator, etc. Any polymer solution known to be suitable for use in a conventional electrospinning process may be used in the process of the invention.

In another embodiment of the invention, the polymer stream fed to the spin pack and discharged through the nozzle in the spinneret is a polymer melt. Any polymer known to be suitable for use in a melt spinning process may be used in the process in the form of a polymer melt.

Polymer melts and polymer-solvent combinations suitable for use in the process are disclosed in Z. M. Huang et al., Composites Science and Technology, volume 63 (2003), pages 2226-2230, which is herein incorporated by reference.

Located a distance below the spinneret 102 is a collection means for collecting the fibrous web produced. In one embodiment of the invention, as shown in FIG. 2, the collection means comprises a moving conductive belt 110 connected to high voltage onto which the fibrous web is collected. The belt 110 is advantageously made from a porous conductive material such as a metal screen so that a vacuum can be drawn from beneath the belt through gas collecting tube 114 by blower 112. In this embodiment of the invention, the collection belt must be isolated from ground by any known means. The collected fibrous web of nanofibers is sent to a wind-up roll, not shown.

In another embodiment of the invention, as shown in FIGS. 3A and 3B, the moving collection substrate 118 (FIG. 3B) is a nonconductive substrate superposed over a conductive element 120 connected to high voltage, itself superposed on a nonconductive support material 122. The conductive element 120 and/or the nonconductive support material 122 can be stationary. The moving collection substrate 118 is supplied from a supply roll 124 and the combined collected fibrous nanofiber web and collection substrate 118 are sent to a wind-up roll 126. In one embodiment of the invention, nanofibers and the forwarding gas stream are directed toward the collection substrate 118, where the nanofibers are deposited and collected into a fibrous nanofiber web superposed on the nonconductive collection substrate 118. The collection substrate 118, conductive element 120 and support material 122 are each highly breathable, so that the gas from the forwarding gas stream as it impinges the collection substrate may be exhausted through the collection substrate 118, conductive element 120 and support material 122 using vacuum. The vacuum can be drawn from beneath the support material 122 through gas collecting tube 114 by blower 112. The collection substrate 118 can be any of a number of substantially nonconductive breathable materials such as woven fabrics, nonwoven fabrics, scrims, etc. When the forwarding gas stream gases are exhausted by vacuum, the conductive element 120 is a porous material, and more advantageously a metal screen, for example a fine mesh screen having a mesh greater than about 50. In this embodiment, the high voltage conductive screen 120 must be isolated from ground by any known means.

In another embodiment, a nonconductive moving collection substrate 118 according to FIG. 3B can be supplied from a supply roll and fed over the moving conductive belt 110 of FIG. 2. In this manner, a fibrous web containing nanofibers is deposited onto the collection substrate, the combination of nanofiber web and nonconductive moving collection substrate are separated from the moving conductive belt by conventional means and are forwarded to a wind-up roll.

It has been found that the distance between the spinneret and the collection surface (also referred to as the “die to collector distance” or “DCD”; illustrated in FIGS. 2 and 3A) is in the range of about 1 to about 200 cm, and more advantageously in the range of about 10 to about 50 cm.

It has further been found that when the tip of the spinning nozzle or die tip protrudes from the spinneret by a distance e (FIGS. 2 and 3A), such that the distance between the nozzle and the collection surface is less than the distance between the spinneret and the collection surface, a more uniform electric field results. Not wishing to be bound by theory, it is believed that this is because the protruding nozzle establishes a sharp edge or point in space for electric field lines to concentrate around.

The voltage applied to the collection means, either to the moving conductive belt 110 as in FIG. 2 or the stationary conductive screen 120 as in FIG. 3, is in the range of about 1 to about 500 kV, and more advantageously about 10 to about 100 kV.

The process of the invention avoids the necessity of maintaining the spin pack including the spinneret, as well as all other upstream equipment, at high voltage, as described in the Background of the Invention. By applying the voltage to the collection means, the pack, the spinneret and all upstream equipment may be grounded or substantially grounded. By “substantially grounded” is meant that the spinneret may be held at a low voltage level, i.e., between −100 V and +100 V.

Advantageously, the polymer discharge pressure is in the range of about 0.01 kg/cm² to about 200 kg/cm², more advantageously in the range of about 0.1 kg/cm² to about 20 kg/cm², and the polymer solution throughput per hole is in the range of about 0.1 cc/min to about 15 cc/min.

EXAMPLE 1

A test was run with a 0.1 meter spin pack to demonstrate the process while applying high voltage to the collector. In this test, the collector consisted of a rectangular metal screen supported by a frame. The collector was stationary and electrically insulated from ground with the use of Teflon® supports. A voltage of −60 kV was applied to the collector and the spin pack was connected to ground.

A 22 wt % solution of nylon 6 (type BS400N obtained from BASF Corporation, Mount Olive, N.J.) in formic acid (obtained from Kemira Industrial Chemicals, Helsinki, Finland) was electroblown through a spinneret of 100 mm wide, having 11 nozzles at a throughput rate of 1.5 cc/hole. A forwarding air stream was introduced through air nozzles at a flow rate of 4 scfm (2 liters per second). The air was heated to about 70° C. The distance from the spinneret to the upper surface of the collector was approximately 300 mm. The process ran for about 1 minute.

Nineteen fibers from the product collected were measured for fiber diameter. The average fiber size was 390 nm with a standard deviation of 85.

COMPARATIVE EXAMPLE

The test was repeated as described above with the negative voltage supply attached to the spin pack. All other process settings were the same.

The process ran equally well as the process in tests conducted in which the high voltage was applied to the spin pack and the collection surface was grounded. Fine fibers were produced which pinned well to the collector.

Nineteen fibers from the product collected were measured for fiber diameter. The average fiber size of the Comparative Example was 511 nm with a standard deviation of 115.

Claims

1. An electroblowing process for forming a fibrous web comprising:

(a) issuing a polymer stream from a spinning nozzle in a spinneret whereupon a fibrous web is formed, the web having an electric charge of a positive or negative polarity, and

(b) collecting the fibrous web on a collection means, wherein a voltage is applied to the collection means, the voltage having a polarity opposite that of the fibrous web, and wherein the spinneret is substantially grounded, such that an electric field is generated between the spinneret and the collection means of sufficient strength to impart an electrical charge on the polymer stream as it issues from the spinning nozzle.

2. The process of claim 1 wherein the polymer stream is a stream of polymeric solution.

3. The process of claim 1 wherein the polymer stream is a stream of molten polymer.

4. The process of claim 1 wherein the polymer stream is electrically conductive.

5. The process of claim 1 wherein the collection means is a conductive moving collection belt having a collection surface.

6. The process of claim 5 wherein the conductive moving collection belt is porous and wherein the process further comprises applying suction to the collection belt on the side opposite the collection surface.

7. The process of claim 1 wherein the collection means is a nonconductive moving collection substrate superposed over a conductive element supported by a nonconductive support structure and wherein the voltage is applied to the conductive element.

8. The process of claim 7 wherein the nonconductive moving collection substrate is breathable and the conductive element is porous and wherein the process further comprises applying suction to the nonconductive support structure on the side opposite the collection substrate.

9. The process of claim 1 wherein the distance from the spinneret to the collection surface is between about 1 and about 200 cm.

10. The process of claim 1 wherein the distance from the spinneret to the collection surface is between about 10 and about 50 cm.

11. The process of claim 1 wherein the voltage is between about 1 and about 500 kV.

12. The process of claim 1 wherein the voltage is between about 10 and about 100 kV.

13. The process of claim 1 wherein the polarity of the charged web is negative and the polarity of the voltage applied to the collection means is positive.

14. The process of claim 1 wherein the polarity of the charged web is positive and the polarity of the voltage applied to the collection means is negative.

15. The process of claim 1 wherein the distance between the spinning nozzle and the collection surface is less than the distance between the spinneret and the collection means.

16. The process of claim 7, wherein the conductive element is stationary.

17. The process of claim 5, wherein the collection means further comprises a nonconductive moving collection substrate supplied from a supply roll which is fed over the conductive moving collection belt.