Patent Application
20060012084
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.
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.
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.
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.
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.
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 (
It would be desirable to have an improved electroblowing process which would avoid these disadvantages.
In the process of the present invention, referring to
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
In another embodiment of the invention, as shown in
In another embodiment, a nonconductive moving collection substrate 118 according to
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
It has further been found that when the tip of the spinning nozzle or die tip protrudes from the spinneret by a distance e (
The voltage applied to the collection means, either to the moving conductive belt 110 as in
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.
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.
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.
